The general framework for exergy analysis together with the three dimensional exergy analysis diagram and global energy costing model have been demonstrated as effective methods to optim
INTRODUCTION
The motivation of research
Energy consumption for human activity is increasing, while fossil fuel resources are depleting It is also a well acknowledged global problem that the more energy is consumed, the more emission of carbon dioxide is generated This damages natural environment, so that energy becomes global problem nowadays
In chemical industrial sector, separation processes are indispensable, in which distillation and absorption/desorption make up to 95% of all separation processes (Jana, 2010) In the U.S.A alone, there are over 40,000 distillation and desorption columns that consume 19% of energy usage in the manufacturing sector and 6% of the total national energy usage In Japan, 15% of all energy is consumed in the chemical industry, out of which distillation and desorption columns expend 40% of total energy usage (Ohe, 2006) For example, in acid gas removal by amines processes, specific energy consumption is around 1,300–4,500 kJ per kg of acid gas (Bullin & Polasek, 1982; National Renewable Energy Laboratory, 2009) In general, distillation and desorption columns consume 35–40% of energy in the chemical industry, make up around 3% of the total energy consumed in the world (National Research Council (U.S.), 2005) These numbers show that distillation and absorption/desorption are the most energy intensive processes in the chemical industry Therefore, energy savings for distillation and absorption processes are very significant in economics and environment conservation Consequently, this work is hoped to be established as an imperative need.
Exergy
Exergy is a concept that is based on the first and the second laws of thermodynamics
The first law is concerned with the conservation of energy in a process, from which enthalpy is defined as the total energy of a system comprising internal energy and produced work The second law further expresses the degradation of energy potential during heat transfer The significance of the second law relates to the degradation of the quality of energy when energy is used The quality degradation of energy is measured by entropy generation Consequently, exergy which means of quantity and quality of energy is a function of enthalpy and entropy
Perhaps energy analysis is the most widely used technique in energy audit Energy analysis is based on the first law of thermodynamics An energy analysis quantifies energy based on properties of system such as temperature, pressure and compositions (Dincer & Cengel, 2001; Wall & Gong, 2001; Dincer & Rosen, 2007) In energy analysis, the state of the system is only considered The effect of the surrounding environment is not considered in the analysis Therefore, the results of energy analysis on a system are always invariant to the changes in the surrounding environment
Figure 1.1: Power generation by steam turbine Steam inlet, h in
For example, power generation by steam turbine as shown in Figure 1.1 has the same energy efficiency when it is located in the cold climate like Alaska or tropical climate like Malaysia The reason is that energy efficiency is determined by enthalpy of steam inlet, condensate and shaft work, which are independent to the surrounding environment Energy audit performed by energy analysis technique may not be a realistic measure of energy performance because it does not reflect the impact of system and the surrounding environment The drawback of energy analysis may be overcome by using exergy analysis
The concept of exergy may be traced to a statement by Carnot in 1824 Carnot realised that requirements for power generation are consumption of heat and difference of body temperature (Demirel, 2004; Sciubba & Wall, 2007) In 1850, Clausius generalised Carnot’s statement by means of the second law of thermodynamics In 1873, Gibbs generated a new thermodynamic measure which is free energy (g) as a function of enthalpy (h) from the first law and entropy (s) from the second law Consequently, Gibbs became the pioneer to introduce explicitly the concept of available work Since then, “available work” was mentioned by many researchers in different terminologies; useful energy, utilisable energy, available energy, availability, work potential and essergy to name a few At a scientific meeting in 1953, Rant first suggested the word “exergy” to define the quality of energy (Wall
In 1970s, the OPEC oil embargo caused a drastic rise of crude oil price and plunge the world into an energy crisis Following the energy crisis, the concept of exergy has become the main interest of many researchers as a tool to improve energy efficiency performance, especially in oil refineries, power plants and chemical industries For example, Fonyo (1974a, 1974b) executed exergy analysis for distillation column sequence Riekert (1974) used the concept of work equivalent based on the first and the second laws as a measure of energy efficiency in ammonia and nitric acid production plants The essence of work equivalent is the same to exergy concept Bailie and Doner (1975) provided a systematic method for evaluating the energy efficiency of resource recovery processes Fitzmorris and Mah (1980) used exergy analysis for improving distillation design
Exergy is the maximum available work that can be extracted from a system as it is brought from an initial state to a final state The final state is in equilibrium with its surrounding environment (Wall, 1986; Winterbone, 1997; Kutz, 1998; Dincer &
Cengel, 2001; Gong & Wall, 2001; Wall & Gong, 2001; Annamalai & Puri, 2002;
Rivero, 2002; Wall, 2002; Horlock, 2003; Anon, 2004; Arons, Kooi, &
Sankaranarayanan, 2004; Demirel, 2004; Sato, 2004; Yantovski, 2004; Addington &
Schodek, 2005; Anon, 2005; Kreith, 2005; Szargut, 2005; Anon, 2006; Coatanea, Kuuva, Makkonnen, Saarelainen, & Castillon–Solano, 2006; Dincer & Rosen, 2007;
Hammond, 2007; Perry, Green, & Maloney, 2008) Exergy of the system depletes gradually to zero at final state
The quantity of exergy is measured as the difference of enthalpy change and entropy generation ex = 'h – T o 's (1–1)
In Equation (1–1), T o is ambient temperature Exergy equals to the Gibbs free energy change if the system and its surrounding environment have the same temperature
Exergy is a property of a system and its surrounding environment (Rosen, 2001, 2002a; Dincer & Rosen, 2007) It means a quantity of exergy for a given system depends on the state of both system and its surrounding environment Exergy calculations are based on both the first and the second laws of thermodynamics It not only considers quantity of energy, but also its quality through entropy generation For a system, exergy is an attribute that expresses the deviation of the system from its surrounding environment or reference state The further is the distance of a system from its reference state, the larger is the quantity of exergy (Dincer & Cengel, 2001;
Dincer & Rosen, 2007) In this case, the potential for the system to deliver useful work is higher When a system is in complete equilibrium with its surrounding environment, exergy is zero because there is no difference in temperature, pressure and compositions between the state of the system and the state of the surrounding environment
The process performance of a system depends on the driving force of the process
The driving force is the difference of potential necessary for the system to drive from initial state and final state For example, the driving force of heat transfer is temperature difference, while that one of mass transfer is concentration difference
The larger is the difference, the larger is the driving force and consequently the bigger is the exergy lost Exergy is only conserved in reversible processes and is always destroyed in any process that is irreversible In the real processes, energy always losses its quality Exergy lost is also known as anergy production Anergy is a part of energy that carries no available work Hence, it cannot be converted into work (Sato, 2004; Winter, 2007; Fischer, Schuller, Albrecht, & Faltenbacher, 2008) The results of exergy analysis may be presented in a Sankey diagram as shown in Figure 1.2b
The diagram shows that energy is degraded as it is being used by a process because of irreversibility Figure 1.2a expresses energy analysis result in which energy lost is only in emission streams Emission streams release to environment such as stack gas and condensed water
Figure 1.2: Sankey diagram for energy and exergy analysis
On the other hand, process performance can be evaluated by efficiency which is a ratio of outlet and inlet streams For energy analysis technique, energy efficiency is a ratio of energy outlet to energy inlet in emission lost in in out en Energy
For exergy analysis technique, exergy efficiency is the ratio of exergy outlet to exergy inlet as per Equation (1–4) in ility irreversib lost emission lost in in out ex Exergy
Exergy efficiency is only perfect in reversible processes due to conservation of exergy and decreases in any irreversible processes The reasons are exergy lost due to emission streams and irreversibility of process Opposite to exergy efficiency, energy efficiency only reflects the energy lost owing to emission streams Thus, exergy analysis technique is an approach to ideal state that is always missing in energy analysis technique In general, exergy is the part of energy which carries greater economic value than energy alone The relation between exergy and the environment makes it a more practical measure of how efficient energy is utilized in a given process system
Exergy can be divided into three broad classes Firstly, exergy of work is defined as the work produced by system: ex w = w (1–5)
In case of volume changed, the net exergy is then the difference of produced work and entropy generation by volume change at ambient pressure: ex net = w – p o 'v (1–6)
Secondly, exergy of a heat flow is proportional to Carnot factor as follows: áạ ă ã ©§
In Equation (1–7), q, T and T o are heat content, temperature of heat flow and ambient temperature, respectively
Finally, material stream may carry four types of exergy: physical, chemical, potential and kinetic (Wall & Gong, 2001; Dincer, Hussain, & Al–Zaharnah, 2004a;
Dincer & Rosen, 2007) Physical exergy of a material stream is the maximum work obtainable when it is brought from its initial state to the environment state (T o , p o ) As shown in Equation (1–8), 'h and 's are molar enthalpy and molar entropy changes of the material stream from initial state to the environment state ex ph = 'h – T o 's (1–8)
Chemical exergy of a material stream is the maximum work obtainable when it is brought from the environment state (T o , p o ) to the dead state The dead state is in complete equilibrium with its surrounding environment as well as with its compositions Dead state can be atmosphere, hydrosphere or lithosphere In an open system where the material stream is released into the environment, such as flue gas emission, chemical exergy is expressed as per the following equation ¦ ¦ ' k k koo ko k k k ch x x ex P P P (1–9)
Separation process
Smith (2005) proposed the role of separation process in a process design hierarchy as a second layer after the synthesis of reaction system Separation process, which is used to break off a mixture into distinct products, may be applied to heterogenous and homogenous mixtures The separated products differ in properties such as formation, compositions, particle size or crystalline structure A heterogenous mixture can be effectively separated by physical processes such as decantation, sedimentation, flotation, centrifugation, cyclone separation and filtration For a homogenous mixture, physical separations are the first choice due to the lower operation cost as compared to chemical separations such as absorption, extraction, distillation and adsorption For instance, crude oil is a mixture of hydrocarbons with different molecular types and homogenous series In order to use the valuable natural resources efficiently, crude oil needs to be fractionated in a crude distillation column into fuel gases, gasoline, diesel, jet fuel, fuel oil, asphalt, and residues However, some mixtures that cannot be separated by purely physical means, e.g azeotropic mixtures, may be considered for chemical separation For example, purification of ethanol–water mixture, through distillation may only achieve 89.4% maximum ethanol purity by molar basis (Perry et al., 2008) In order to collect high purity ethanol for biofuel production, chemical separation may be used to separate the azeotropic mixture of ethanol and water One of chemical separation processes to purify ethanol is extractive distillation in the presence of acetate salt Acetate salt changes the relative volatility between ethanol and water In other words, acetate salt brings the ethanol–water mixture to “jump over” its azeotropic point Among the many techniques of separation processes, distillation and absorption are very popular in chemical process plant because they may be operated at high capacity
Distillation process had been used in early Greek and Arabian civilization to purify alcohol The principle of distillation is based on boiling point difference or volatilisation of components in a feed mixture Thermal energy is used as an agent to drive the separation A simple distillation column as shown in Figure 1.4 usually comprises a column with several stages, a reboiler at the bottom and a condenser at the top Heat is only supplied at the reboiler and extracted at the condenser There is no utility heater or cooler along the column Hence, this structure is known as an adiabatic column, and it is more practical in industry
Raw material is usually preheated to liquid saturated level before it is being fed into the column at the feed stage Large amount of heat is supplied at the reboiler to boil off liquid down flow in the column Heat supply generates vapour back into the column, viz boil up flow The functions of boil up flow are to distribute thermal energy provided at reboiler along the column and to boil off liquid down flow at every stage Hence, light components are vaporised and move to the top The overhead
Bottom product vapour is condensed to distillate product Part of the distillate is pumped back into the top of column as reflux, which helps to keep the product purity under control The liquid flow exits the reboiler as bottom product
The parameters of a distillation process are identified in a McCabe–Thiele diagram As shown in Figure 1.5, the McCabe–Thiele diagram provides a useful basic tool for design and operations analysis of a distillation column The diagram shows the variation in molar compositions of liquid and vapour phases along the height of the column For a binary system, the true equilibrium curve shows molar compositions of liquid and vapour phases in equilibrium state The true equilibrium curve is usually constructed from experimental data However, a pseudo equilibrium curve shows the actual molar compositions of liquid and vapour phases at every stage
The pseudo equilibrium curve reflects the actual happening in the column, while true equilibrium curve is the theoretical boundary of the process The pseudo equilibrium curve is redrawn between the operating lines and the true equilibrium curve at different stage efficiency
Figure 1.5: McCabe–Thiele diagram for distillation column
At constant pressure, the true and pseudo equilibrium curves of an ideal mixture can be expressed by Equation (1–15), in which x, y and D i are molar compositions of
Rectifying line q line (feed line)
Stripping line x B y light component in liquid phase, molar compositions of light component in vapour phase and relative volatilities between key components, respectively Relative volatility is a measure comparing volatilisation between the light and heavy components in a mixture x x y i i
The subscript ‘i’ in Equation (1–15) is index of equilibrium curves at different relative volatilities such that {i = 0, 1, 2… I} The true equilibrium curve in Figure 1.5 is labelled D 0 while the pseudo equilibrium curves at different relative volatilities are denoted by {D 1 , D 2 … D I } Relative volatilities of pseudo equilibrium curves are always lower than the relative volatility of the true equilibrium curve (D i R min,i , the column performs at actual reflux ratio as shown in Figure 3.6 The driving force goes up to maximum, because the distance of operating lines to the true equilibrium curve increases
Figure 3.6: McCabe–Thiele diagram for adiabatic column with finite number of stages, perfect efficiency and actual reflux ratio
Rectifying line q line (feed line)
Stripping line Pseudo equilibrium curve, Į i
Rectifying line q line (feed line)
Stripping line Pseudo equilibrium curve, Į i
Figure 3.7 shows the column performance at actual number of stages (N act ) with perfect efficiency (K0%) and the reflux ratio reaches theoretical value (R the )
There is a constraint in this operation state that is theoretical number of stages is equal to actual number of stages Comparing to an ideal adiabatic column as shown in Figure 3.3, the driving force in this case is higher because there is no pinch point in Figure 3.7 The reason is due to the operation state is at actual number of stages and the reflux ratio is higher than the minimum value (R the >R min,0 ) However, exergy lost at this operation state is lower than that of the column in Figure 3.6 due to increase number of stages As discussed in Figure 3.3, higher number of stages brings the column towards ideal adiabatic condition
Figure 3.7: The McCabe–Thiele diagram for adiabatic column with actual number of stages, perfect efficiency and theoretical reflux ratio
Exergoeconomic analysis
Exergoeconomic analysis is a combination of exergy analysis and economic analysis to evaluate techno economic feasibility of a project Economic analysis of a project consists of two parts: investment cost and operation cost The factorial method by Guthrie and Lang (Sinnott, 2005; Perry et al., 2008) is used for investment cost estimation Operation cost typically includes the cost of raw material, catalysts, chemicals, energy, labour and maintenance A new model of energy costing is proposed in this section to evaluate energy cost based on quality of energy or exergy
The factorial method by Guthrie and Lang (Sinnott, 2005; Perry et al., 2008) is used to calculate investment cost based on purchase cost of major equipment (PCE) and other costs which are considered as factors of the PCE The major equipment items in a plant concern reaction vessels, columns, heat exchangers, tanks and etc The PCE may usually be obtained from supplier or handbooks of engineering material costing (Douglas, 1988; Sinnott, 2005; Perry et al., 2008) The details of other costs and their factors are presented in Table 3.2
Table 3.2: Typical factors of investment cost estimation
Purchase cost of major equipment (PCE) f 1 Equipment erection f 2 Piping f 3 Instrumentation f 4 Electrical f 5 Buildings, process f 6 Utilities f 7 Storages f 8 Site development f 9 Ancillary buildings
Physical plant cost (PPC): á u ạ ă ã © §1 ¦ PCE
2 Indirect costs f 10 Design and engineering f 11 Contractor’s fee f 12 Contingency
3 Start up the plant f 13 Design and engineering 0.05
> Investment cost @ > Fixed cost @ u 1 f 13 > Fixed cost @ u 1.05
The investment cost includes fixed cost and start up cost which is the cost of starting up the plant until income is generated The start up cost is estimated to be at 5% of fixed cost The fixed cost is also classified by direct and indirect costs, in which direct costs consist of equipment contributing to the plant such as major equipment, piping, instrumentation, electrical and buildings Direct costs is approximately 2.4 times of the PCE Indirect costs which relate to engineering, documentation and virtual cost such as design, contractor’s fee and contingency, make up approximately one third of fixed cost Overall, the investment cost is calculated by the following equation:
> Investment cost @ 3 40 u 1 45 u 1 05 u > Purchase cost of equipment @ (3–4)
The exergy unit costs or exergy prices cannot be predicted because they are only available after economic calculations are completed due to lack of data This is a loop error in current model of exergy cost evaluation as highlighted in section 2.4 Because there is no standard measurement of exergy price for different resources, a new model is required to solve this problem
The quality of energy is measured by its temperature level Thermal energy consists of two parts The first part, which can be converted into work entirely, is referred as “exergy”, and the second one that cannot be converted into work is called
“anergy” It means anergy is always destroyed in the conversion of heat into work (Sato, 2004; Cziesla et al., 2006; Winter, 2007; Fischer et al., 2008) Therefore, the monetary rate of energy needs to be evaluated by combination of exergy and anergy values Figure 3.19 illustrates the new model of energy cost evaluation
Figure 3.19: New model of energy cost evaluation
Consider an infinitesimal quantity of thermal energy Gq at temperature T As shown in Equation (3–5), the quantity of energy is made up of infinitesimal amount of exergy, ex, and anergy, an, respectively
Exergy (ex) is maximum available work extracted from energy as shown in Equation (3–6) ááạ ăă ã © §
Anergy (an) is the remaining part of energy and can only be used as thermal energy or heat From Equations (3–5) and (3–6):
Therefore, the monetary rate of an infinitesimal quantity of thermal energy Gq at temperature T is:
Gc = c w ex + c h an (3–8) where, c w and c h are the prices of work and heat, respectively Combined with Equations (3–6) and (3–7), Equation (3–8) yields: q
The Equation (3–9) is the same form of the conventional description of the energy cost which is proportional to energy content and energy price This expression can be further expanded by manipulating Equation (3–9) into Equation (3–10) as shown below
In Equation (3–10), the first part expresses the conventional model of energy cost evaluation while the second part expresses the additional cost of energy due to its quality The price of work, c w , is taken as the equivalence of electrical power price
The price of heat, c h , is assumed equal to the fuel price The price of work is higher than that of heat The energy price, which is known as the cost per unit of energy content, is rearranged from Equation (3–9): ằẳ º ôơê
Taking a limit of T at critical value yields: h o h w T w
Hence, the higher the temperature is, the higher the energy price would be In quality term, the maximum energy price is the price of work at infinite temperature, and the minimum energy price is the price of heat w h o w w h c
This new model is named as the global energy costing model because it implies a total value of energy comprising exergy and anergy The global energy costing model discloses the contribution of quantitative and qualitative values of energy in energy cost
The example below shows how the new global energy costing model can be used to calculate steam price at different levels
Three levels of steam produced in a utility plant are medium pressure (MP) at 180 o C, high pressure (HP) at 250 o C and hyper high pressure (HHP) at 400 o C Fuel oil is used to supply energy in the plant The price of fuel oil and electrical power are 3.95 USD/GJ and 25.00 USD/GJ, respectively Ambient temperature is 25 o C
Determine the prices of steam at different levels
Application of Equation (3–11), the prices of steam levels are:
MP ằẳº ôơê ằẳ ôơ º ê ááạ ăă ã © §
HP ằẳº ôơê ằẳ ôơ º ê ááạ ăă ã © §
HHP ằẳº ôơê ằẳ ôơ º ê ááạ ăă ã © §
3.2.3 Application of global energy costing model
For isothermal process or energy supplied continuously from combustion at temperature T, an integration of Equation (3–9) gives the energy cost:
By definition of heat (Gq) is a function of molar heat capacity (c p ) and temperature difference (dT), the energy cost of a material stream at temperature T above from ambient is: ³ ôơ ê ằẳ º
The energy cost of a material stream in which temperature is changed from T 1 to
T 2 by a process is: ³ 2 ôơ ê ằẳ º
Where pressure change is significant in ideal gas, the energy costs in Equations (3–15) and (3–16) are pressure corrected and yield Equations (3–17) and (3–18): ³ ³ ôơ ê ằẳ º p ôơ ê ằẳ º p h h w o T
3.2.4 Comparison between conventional and global energy costing models
Equation (3–9) expresses the global energy costing model which is similar to the form of conventional model Energy cost is proportional to energy price and energy content The first part in Equation (3–9) expresses energy price in readily available prices of work (c w ) and heat (c h ) It also carries the advantage of expressing the quality of energy from the temperature level Rearrangement of Equation (3–9) into Equation (3–10) which shows two parts in energy cost including quantity and quality of energy The first term in Equation (3–10) is proportional to energy content with the price of heat that is the same to conventional model In addition, quality of energy is also proportional to energy content with the price as a function of standard prices (c w , c h ) and temperature T As opposed to conventional model of exergy cost evaluation, where exergy cost is expressed as a function of exergy content and exergy price, the global energy costing model in Equations (3–9) and (3–10) shows energy cost as a function of extensive (Gq) and intensive (T) variables A comparison between conventional model of energy cost evaluation and the new global energy costing model for energy and material streams is shown in Table 3.3
Table 3.3: Comparison between conventional and global energy costing models
Cases Conventional model Global energy costing model
An amount of heat Q is released by combustion at constant temperature
An amount of heat Q is absorbed by a material stream from ambient T o to T
C o o o ex ằằ ằằ ằ ẳ º ôô ôô ô ơ ê ááạ ăă ã © § ln 1
T T c c c C o o o h w w ằằ ằằ ằ ẳ º ôô ôô ô ơ ê ááạ ăă ã © § ln
An amount of heat Q is absorbed by a material stream from T 1 to T 2
C o ex ằằ ằằ ằ ẳ º ôô ôô ô ơ ê ááạ ăă ã © §
T T c c c C o h w w ằằ ằằ ằ ẳ º ôô ôô ô ơ ê ááạ ăă ã © §
For energy stream in the first case, an amount of heat Q is released continuously from the flame of fuel combustion at constant temperature T In the second case, a material stream is heated from ambient temperature T o to temperature T by an amount of heat Q The last case is similar to the second case, except for temperature increases from T 1 to T 2 The energy cost in every case is estimated by conventional model and global energy costing model which are shown in the last two columns
Process simulation
Process simulation software is a powerful tool for supporting design and operations engineers All complex calculations are performed by computers, that saves much time and causes design process to become more flexible This work is performed with the support of process simulation software, Aspen HYSYS version 7.2 (Aspen Technology, Inc., 2010) and ProMax version 3.2 (Bryan Research & Engineering, Inc., 2010) The softwares provide sufficient available components, property package, models of distillation and absorption columns Running simulation software is to simulate the real case in industry and ensure the feasibility of a process improvement earlier before engaging on actual project implementation By changing in design and operations parameters, the effects on the feasibility of distillation and absorption columns are evaluated by data collection from converged simulation cases These data are used to calculate exergy lost For distillation column, the used model is typical distillation column with a condenser and a reboiler In case of absorption column, the model used is a counter current column The tray structure is selected for both distillation and absorption columns because exergy lost profile in the column is shown stage by stage Furthermore, packed column can be converted to tray column by using height equivalent to a theoretical plate (HETP) As discussed in sections 3.1.1 and 3.1.2, some design and operations parameters are defined and while others are estimated by process simulator For all case studies, the heat loss is assumed negligible, the species behave as ideal mixture, efficiency is equal at every stage and is the same with the column The procedure for running simulation is shown in Figure 3.20
Figure 3.20: Procedure for running simulation
(Material & energy streams, unit operations)
(Compositions of products: distillates, bottoms, treated gas)
(Stage efficiency, number of stages, feed condition)
Record results of Base case
Calculate exergy lost of Base case
In range of upper and lower bound
Calculate exergy lost of Case #N
In the first step, all components are identified by user from database of simulator based on case study For example, there are 43 species existing in the xylene column such as: meta xylene, para xylene, ortho xylene, ethyl benzene and etc The suitable property package should be then selected based on property of species The property package is a thermodynamic models which allow estimating the thermodynamic parameters such as temperature, pressure, vapour ratio, enthalpy, entropy, and so on
Each property package is suitable to some specific systems Selection of property package affects the accuracy of estimations For instance, the Peng–Robinson property package (PR) is appropriate to hydrocarbon systems For the Selexol process, Soave–Redlich–Kwong (SRK) property package is used for the DEPG solvent (Burr & Lyddon, 1997)
In the second step, the process flow diagram is built including material streams, energy streams and unit operations For material streams, the properties need to be set initially such as temperature, pressure, compositions For the columns, number of stages and stage efficiency are predefined manually as per the actual industrial
Before running simulation base case, the specifications of columns are set such as compositions of distillates and bottoms For instance, the specifications of distillates and bottoms in xylene column are 0.05 wt% of C9+ and 0.50 wt% of xylene, respectively If the simulation does not converged, the process flow diagram and specifications are rechecked When simulation base case converges, estimated data are recorded for exergy calculation such as temperature, pressure, molar flow, enthalpy and entropy of all streams and stage by stage in the columns Besides, the variables of distillation column are also extracted namely reflux ratio, duties of condenser and reboiler
Studying the effects on exergy lost is performed in the next step by changing variables such as stage efficiency, number of stages and feed condition A new simulation case is created when there is any change in variables Each variable varies from lower to upper bounds For example, stage efficiency changes from 30% to 100% with 5% of step, number of stages is set from 8 to 30 with 2 of step in benzene– toluene distillation column Consequently, there are the thousand of simulation cases executed For every simulation case, data recording for exergy calculation are done repeatedly as same as the base case when the simulation case converges In case of non–convergent, the variables are rechecked for availability Running simulation only stops when all variables are out of range of study Many sets of data for operation states which comprises number of stages, stage efficiency, reflux ratio and exergy lost are collected and expressed in the three dimensional coordinate.
RESULTS AND DISCUSSIONS
Comparison of simulation results
In this work, a benzene–toluene distillation column (Chang & Li, 2005) is selected to compare the simulation results and exergy lost calculations The feed stream is at 25 o C, 1.01 bars and 317.5 kmol/hr being composed of equimolecular mixture of benzene and toluene The objective of distillation is to separate benzene and toluene into pure products of 95% and 90% purity, respectively Design parameters and specifications of benzene–toluene distillation column are listed in Table 4.1 The column contains 17 stages, a condenser and a reboiler
Table 4.2 compares the simulation in this work using Aspen HYSYS version 7.2 to the results by Chang and Li (2005) using CHEMCAD TM version 5.06 with SRK package It compares the operating conditions between two simulations The simulation in this work is verified by comparing the negligible errors ranging from 0.96% to 1.17%
Table 4.1: Design parameters and specifications of benzene–toluene distillation column
Feed flow rate (kmol/hr) 317.5
Mole fraction of benzene in feed 0.50 Mole fraction of benzene in distillate 0.95 Mole fraction of toluene in bottom 0.90
Table 4.2: Validation of operating conditions in benzene–toluene distillation column
Specifications (Chang & Li, 2005) Aspen HYSYS Error (%)
Distillate flow rate (kmol/hr) 166.7 168.3 0.96
Bottom flow rate (kmol/hr) 150.8 149.2 1.06
Table 4.3 compares the exergy lost calculations in this work to the results by (Chang & Li, 2005) It compares the losses of column, condenser and reboiler between two simulations The error of calculations in this work are negligible within 1.22% Therefore, the calculations are sufficiently correct to represent the separation of benzene–toluene mixture
Table 4.3: Validation of exergy lost in benzene–toluene distillation column
Units Exergy lost (kW) by
Three dimensional exergy analysis diagram
As discussed in sections 3.1.1 and 3.1.2, the three dimensional exergy analysis diagrams for distillation and absorption columns are constructed based on the correlations between stage efficiency (K), number of stages (N) and reflux ratio (R) or (L m /G m ) ratio These parameters of an operation state are expressed simultaneously in a plot, and operation points are then located in three dimensional coordinates by combining with appropriate exergy lost
4.2.1 Three dimensional exergy analysis diagram for distillation column
A binary system is considered for the construction of the three dimensional exergy analysis diagram for distillation column In practice, feed stream is usually preheated to boiling point before being fed to the column Therefore, the feed is fixed at saturated liquid condition, while number of stages, reflux ratio and stage efficiency are varied
Figure 4.1: Correlations between stage efficiency, reflux ratio and number of stages in a binary distillation column
Figure 4.1 shows that when stage efficiency reduces, the reflux ratio increases at constant number of stages For fixed reflux ratio, the number of stages increases with the lower stage efficiency Increasing number of stages decreases the reflux ratio at constant stage efficiency Reflux ratio reaches minimum value at infinite number of stages Similarly, the number of stages reaches minimum value as reflux ratio goes to infinity
The correlations between exergy lost (ExL), reflux ratio (R), number of stages (N) and stage efficiency (K) are shown in Figure 4.2 For constant number of stages, exergy lost can be linearly correlated to reflux ratio Exergy lost is smallest at perfect efficiency (K0%) Decreasing stage efficiency increases reflux ratio and exergy lost There are many lines for different number of stages, which overlap together in one plot as shown in Figure 4.2
Figure 4.2: Correlations between exergy lost, reflux ratio, number of stages and stage efficiency in a binary distillation column
By combining the correlations between reflux ratio, number of stages, stage efficiency and exergy lost in Figures 4.1 and 4.2, operation points could be located in a three dimensional coordinate by raising up the operation points from R–N plot with
N0; K 0% the height of exergy lost value Consequently, the three dimensional exergy analysis diagram for the binary distillation column is plotted in Figure 4.3 All curves in Figure 4.1 are converted into the three dimensional curves as shown in Figure 4.3 Moreover, these curves gather in an inclined plane and the front view of them is a single line in the R–ExL plane as illustrated in Figure 4.2 Meanwhile, the top view of the inclined plane consists of curves expressing the correlations between reflux ratio, number of stages and stage efficiency in the R–N plane as shown in Figure 4.1
The three dimensional exergy analysis diagram for the binary distillation column as shown in Figure 4.3 visualised the design and operation states of column at fixed condition of feed and specifications of products For a new column, an optimum design with appropriate number of stages, reflux ratio and stage efficiency is selected from this diagram For an existing column, the operation state is located in the diagram and the improvement options are proposed due to practical condition For example, any operation point in Figure 4.3 may be moving down the ExL axis following the path of efficiency by increasing number of stages and is limited by the economics of the process modification
Figure 4.3: Three dimensional exergy analysis diagram for a binary distillation column
Reflux ratio (R) Number of stages (N)
E xe rgy l os t ( Ex L , kW )
In general, for fixed condition of feed and specifications of products, the three dimensional exergy analysis diagram for a distillation column is an inclined plane (M) as shown in Figure 4.4 The plane is of operation points expressing the reflux ratio, number of stages, stage efficiency, relative volatility, q value and exergy lost of column The coordinate of an operation point is specified by three axes: reflux ratio (R), number of stages (N) and exergy lost of column (ExL) The inclined plane (M) is limited by perfect efficiency curve (K0%) and perpendicular to R–ExL plane The K curve moves up the plane toward the direction of decreasing stage efficiency
Therefore, the lowest curve corresponds to perfect efficiency (K0%)
Figure 4.4: Three dimensional exergy analysis diagram for a general distillation column
The effect of relative volatility (D) on exergy lost of column is realised by increase exergy lost due to movement of the true equilibrium curve As discussed in McCabe–Thiele diagram, perfect efficiency refers to the true equilibrium curve (D 0 )
When stage efficiency decreases, equilibrium curve moves from the true equilibrium curve (D 0 ) to the pseudo equilibrium curve (D i ) There is a gap between the true
Reflux ratio (R) Number of stages (N)
ExL B ExL A R min, 0 R the R min, i R act
M equilibrium curve (D 0 ) and the pseudo equilibrium curve (D i ) This gap represents amount of increase in exergy lost Comparing two operation states with respect to the same reference of pseudo equilibrium curve (D i ), the amount of increase in exergy lost is constant because the distance of true equilibrium curve (D 0 ) to pseudo equilibrium curve (D i ) is the same Consequently, the D curves are all parallel in Figure 4.4 The D 0 curve which referring to the true equilibrium curve overlaps with the curve at perfect efficiency (K0%) The D curve moves up parallel in the plane (M) as relative volatility decreases The vertical distance of two D curves represents the exergy lost due to different relative volatilities
The theoretical operation states of a distillation column, which are identified in the second step of exergy analysis in Figures 3.3, 3.4, 3.5, 3.6, 3.7 and 3.8, can also be depicted by points A, F, C, D, B and E in Figure 4.4, respectively Points A and F have the largest number of stages while point D has the smallest number of stages
Points A, B, C and D have the highest efficiency while point F has the lowest efficiency Point A is at minimum of reflux ratio and exergy lost In contrast, points D and E have the same highest reflux ratio and exergy lost Although points E and B have the same number of stages, point B has lower reflux ratio and exergy lost due to perfect efficiency A similar case is presented by points A and F Point C has efficiency better than point F, but the same exergy lost owing to the same reflux ratio
Point G has the same stage efficiency to point E, but number of stages reaches infinite Therefore, the reflux ratio and exergy lost of point G are lower than point E
The front view of plane (M) is a single line in the R–ExL plane, which expresses the correlation between exergy lost (ExL) and reflux ratio (R) In addition, its top view expresses the relations between four parameters; reflux ratio (R), number of stages (N), relative volatility (D) and stage efficiency (K) in the R–N plane
When the thermal condition of feed changes, the exergy lost of column also changes, and the plane (M) moves downward in the direction of decreasing q value
This means that there are many parallel planes corresponding to different q values As product purity decreases, the reflux ratio and exergy lost of column also reduce In this case, the plane slides downward in the same slope of original plane
Overall, the inclined plane (M) is characteristic of a distillation column at given feed and product properties For fixed products specifications, an operation state of the distillation column is expressed by an operation point in this plane and resulted by three degrees of freedom One of them is q value The other two variables can be selected from any of four parameters, namely reflux ratio (R), number of stages (N), relative volatility (D) and stage efficiency (K) The exergy lost of column (ExL) is a function of q value and reflux ratio, where reflux ratio is an implicit function of stage efficiency and number of stages
The three dimensional exergy analysis diagram shows that increase in stage efficiency or number of stages will reduce the reflux ratio and exergy lost of column
Case study 1: Exergy analysis and improvements of xylene distillation
This case study demonstrates the application of three dimensional exergy analysis diagram for analysis and improvement of xylene distillation column
4.3.1 Base case of xylene distillation column
The distillation column selected in this case study is a xylene column in an integrated refinery–aromatics plant The main feed for refinery is crude oil while that for aromatics plant is natural gas condensates Figure 4.6 illustrates the process flow diagram for xylene distillation column
Figure 4.6: Process flow diagram of xylene distillation column
The xylene distillation column is a unit operation in the aromatics plant producing at a rate of 333.88 t/hr The column consists of 115 sieve trays and two feed streams, stream 201 and stream 160, respectively The xylene column operates at reflux ratio of 2.63 and 85% stage efficiency The distillate product compositions have a specification of 0.05 wt% C 9+ On the other hand, the specification for the bottom product is set at 0.50 wt% xylene In Figure 4.6, the condenser system consists of three parallel coolers with a total load of 86,749 kW The overhead vapour, stream 209, is splitted into three streams, streams 212, 221 and 225, coming to condensers
After condensation, they are collected at receiver From receiver, a part of distillate product is pumped back to column as reflux stream, stream 239, and the remaining part is pumped to distillate product tanks Reboiler is a gas fired heater that supplied heat to boil off bottom liquid The fired heater delivers a load of 99,132 kW and comprises four cells vertical box and two overhead convection sections The compositions and properties of streams are listed in Appendix A and B, respectively
The xylene distillation column is simulated in Aspen HYSYS version 7.2 environment The column exergy lost profile as shown in Figure 4.7 is evident that the major exergy lost is located at two feed stages In addition, considerable amount of exergy lost is also found at the bottom section stages The reason for the huge losses at feed locations is due to mixing of streams with different compositions and temperature
Figure 4.7: Exergy lost profile of xylene distillation column in base case
Table 4.4 shows comparison between plant data and simulated data The errors between plant data and simulated data are negligible so that the simulation result is validated to provide the properties for exergy lost calculation
Table 4.4: Validation of simulated xylene distillation column
Specifications Plant data Simulated data Error (%)
Distillate flow rate (kg/hr) 292,599 293,172 0.20
Bottom flow rate (kg/hr) 41,278 40,704 1.39
Figure 4.8 further shows the exergy lost of the base case The total exergy lost of xylene distillation system is 26,482 kW The reboiler contributes to the largest amount of exergy lost which is around 65.6% Meanwhile the losses of column and condenser are 15.2% and 19.2%, respectively
Exergy lost (ExL, kW) kW kW kW kW
Figure 4.8: Exergy lost of xylene distillation system in base case
4.3.3 The effects of reflux ratio
The linear correlations between reflux ratio and exergy lost of distillation column, condenser and reboiler are shown in Figure 4.9
Figure 4.9: Correlations between exergy lost, reflux ratio and stage efficiency in xylene distillation system
E x er gy l os t (Ex L , kW )
E x er gy l os t (Ex L , kW )
As discussed in Figure 4.2, increasing stage efficiency decreases reflux ratio, and exergy lost of the distillation column, condenser and reboiler The summation of all losses reflects the exergy lost of the whole xylene distillation system Further in Figure 4.9, exergy lost is shown to be directly proportional to the reflux ratio This means although reflux stream is important to achieve the specification of products purity, it is actually disadvantageous to energy consumption As reflux rate goes up, heat requirement at reboiler also rises As the fulfilled product is circulated much more, increasing of exergy lost is inevitable Therefore, reflux ratio is considered as a main factor that affects exergy lost of the xylene system
For base case, the feed properties, stage efficiency (K) and number of stages (N) are fixed Then, change in reflux ratio impacts on compositions of products, which is shown in Figure 4.10
Figure 4.10: Correlations between reflux ratio and products purity in xylene distillation column
At constant distillates compositions, reduction of reflux ratio causes xylene compositions in bottoms to increase The reason is because there is inadequate liquid flow from the top of column to condense the vapour flow from the bottom This
Direction of decreasing xylene in bottom, x B
R=4.94 x B =0.05% causes the compositions of light components in the bottoms to increase Besides, reduction of reflux ratio also increases C9+ compositions in distillates at constant of bottoms composition In other words, reduction of reflux ratio decreases products purity while the compositions of distillates and bottoms are maintained constant
Reflux stream plays an important role to maintain products purity
Figure 4.9 also shows the interaction between reflux ratio and exergy lost of xylene distillation system to be linear at fixed products purity as base case These results are extended as change in products purity, in which the compositions of C 9+ in distillates is in the range of 0.005–0.100 wt%, while xylene compositions in bottoms is in the range of 0.05–5.00 wt% The interaction between reflux ratio and exergy lost of system is observed as change in distillates and bottoms compositions as shown in Figure 4.11
Figure 4.11: Correlations between exergy lost of system, reflux ratio and products purity in xylene distillation column
For each pair of distillates and bottoms compositions, this correlation is still linear because the relation between reflux ratio and exergy lost is invariant with the change in products purity These lines are plotted together in Figure 4.11, and all of them
E x er gy l os t (Ex L , kW )
C 9+ in distillate (wt%) overlap onto one another The line direction in Figure 4.11 is similar to the line referring to the system in Figure 4.9 In other words, the correlation between reflux ratio and exergy lost of the system is linear and unique when the products purity changes The reflux ratio and exergy lost reduce with the decrease of products purity or increase in stage efficiency
The reflux ratio is a function of design and operations parameters such as the stage efficiency, number of stages, products compositions and relative volatility As a result, the distillation column should be operated at reflux ratio as low as practically feasible while keeping the purity of main products (distillates or bottoms) at specified level
4.3.4 The effects of feed thermal condition
The effects of feed thermal condition on reflux ratio, stage efficiency and exergy lost of system are studied Figure 4.12 shows that thermal condition of feed, expressed as q value, affects the exergy lost of system Exergy lost of system include losses at condenser, reboiler and column For a constant q value, the exergy lost of system correlates to reflux ratio linearly The points on the line move in the direction of increasing stage efficiency as the reflux ratio decreases For base case, q value is 1.71 due to the feed into the xylene column is subcooled liquid At constant q value, the line of the base case in Figure 4.12 overlaps together with the lines in Figures 4.9 and 4.11 Increasing stage efficiency decreases reflux ratio and exergy lost of system
Reduction of q value decreases exergy lost of system at constant reflux ratio The reason is that heat requirement is distributed at feed stream and reboiler, which causes the column to reach closer towards idealisation As q value of feed is lowered, the whole line expresses the correlation between exergy lost of system and reflux ratio moves downward parallel in the direction of lower exergy lost as shown in Figure 4.12 Again, the correlation between exergy lost and reflux ratio is still linear
Therefore, this line is unique and specific for feed property The total heat requirement of a distillation system should then be redistributed to achieve lower exergy lost
Figure 4.12: Correlations between exergy lost of system, reflux ratio and q value in xylene distillation column
4.3.5 Process improvement options for xylene distillation column
Case study 2: Exergy analysis and improvements of sulphur dioxide
In this case study, the application of three dimensional exergy analysis diagram is demonstrated for the analysis and improvement of sulphur dioxde absorption column
4.4.1 Base case of sulphur dioxide recovery process
The absorption process selected in this case is sulphur dioxide absorption using water as solvent (Richardson et al., 2002; Sinnott, 2005) In the production of sulphuric acid, sulphur is first combusted in air to produce sulphur dioxide
Sulphur dioxide is subsequently recovered from the rich flue gas in a process system as shown in Figure 4.16 First, sulphur dioxide is absorbed by water as lean solvent in a counter current absorption column It is then liberated by heat supplied to the rich solvent in a desorption column
Figure 4.16: Process flow diagram of sulphur dioxide recovery in sulphuric acid production
In this case study, 3,716 (std.m 3 /hr) of flue gas (stream 101) containing 8% volume of sulphur dioxide is cooled to 25 o C and fed to the bottom of the absorption column Water (stream 110) is introduced at the top of the column and absorbs 95% of sulphur dioxide from the flue gas The rich solvent (stream 103) exits at the bottom and is heated to 80 o C before being fed to the desorption column Sulphur dioxide (stream 105) is recovered from the top of desorption column for oxidation to sulphur trioxide in the manufacture of sulphuric acid:
Water coming out at the bottom (stream 106) is used to heat rich solvent at pre– heater, supplemented by make up stream (stream 111), then cooled to 35 o C and recycled to the absorption column
For the base case, both the absorption and desorption columns are made up of 10 sieve trays The absorption column operates at (L m /G m ) ratio of 58.3 The stage efficiency for the absorption and desorption columns are 80% and 90%, respectively
The sulphur dioxide recovery process is simulated in Aspen HYSYS version 7.2 environment Total calculated exergy lost of the system amounts to 1,507 kW Nearly 70% of the exergy lost is in the desorption column while auxiliary equipment contribute more than 25% of exergy lost in the process Auxiliary equipment refers to reboiler, heat exchangers and pump The exergy lost profile for the absorption column is shown in Figure 4.17 Stage 1 is at the top of column and stage 10 is at the bottom of column
Exergy lost in the absorption column is minimum at the second stage, N=2, and starts to increase steadily until stage N=8 The amount of exergy lost increases rapidly at the last two stages, N=9 and N It is also observed that exergy lost is significant at top and bottom stages due to mixing of different streams Exergy lost has decreasing direction from the bottom to the top because temperature difference decreases stage by stage Moreover, sulphur dioxide composition in gas flow reduces and closes to equilibrium compositions in direction from the bottom to the top
Figure 4.17: Exergy lost profile of absorption column in base case
The correlations between exergy lost, (L m /G m ) ratio and stage efficiency in absorption column is depicted in Figure 4.18 The relation between exergy lost of absorption column and (L m /G m ) ratio is practically linear at fixed number of stages Exergy lost is minimum when the absorption column is at perfect efficiency (K0%) As the stage efficiency decreases, both (L m /G m ) ratio and exergy lost of absorption column increases The plot of (L m /G m ) ratio versus exergy lost of desorption column and auxiliary equipment are illustrated in Figure 4.19 Auxiliary equipment refers to the reboiler, heat exchangers and pump The results show that exergy lost of desorption column and auxiliaries escalate linearly due to increasing (L m /G m ) ratio The reason is that (L m /G m ) ratio is proportional to rich solvent flow rate, which affects directly to the subsequent unit operations As (L m /G m ) ratio increases, the rich solvent flow rate also goes up This means the capacity of the subsequent unit operations which receives the rich solvent also increases such as pre–heater, desorption column, reboiler and cooler
Exergy lost (ExL, kW) kW kW kW kW kW kW kW kW kW kW
Therefore, the exergy lost also rises On the other hand, the slopes of these lines express sensitivity of exergy lost to (L m /G m ) ratio The slope is smallest for absorption column and increases in desorption column and auxiliaries For the whole system, a 10.0% increase in (L m /G m ) ratio increases exergy lost of system by 9.7%
Figure 4.18: Correlations between exergy lost, (L m /G m ) ratio and stage efficiency in absorption column
Figure 4.19: Correlations between exergy lost, (L m /G m ) ratio and stage efficiency in the whole system
E x er gy l os t (Ex L , kW )
E x er gy l os t (Ex L , kW )
Auxiliaries (reboiler, heat exchangers, pump) System
4.4.4 The effects of solvent temperature
Water is used as a solvent for absorption of sulphur dioxide from the rich flue gas
The effects of solvent temperature on (L m /G m ) ratio, stage efficiency and exergy lost in absorption column are studied The selected solvent temperature is from ambient temperature to base case, 25–35 o C All other parameters are fixed such as 10 numbers of stages, 95% of sulphur dioxide recovery and 80 o C of feed temperature into desorption column Figure 4.20 shows the correlations between exergy lost of absorption column to (L m /G m ) ratio at different solvent temperature At a fixed solvent temperature, the correlation between (L m /G m ) ratio and exergy lost is linear
Increasing stage efficiency decreases (L m /G m ) ratio and exergy lost Reduction of solvent temperature decreases (L m /G m ) ratio as well as exergy lost because absorption process is usually favourable at low temperature The solubility of gas in liquid increases as solvent temperature is lower When the solvent temperature decreases, the whole line expressing the correlation between exergy lost of absorption column and (L m /G m ) ratio moves down in parallel Again, this correlation is still linear It means that this line is unique and specific for solvent temperature
Figure 4.20: Correlations between exergy lost, (L m /G m ) ratio and solvent temperature in absorption column
4.4.5 Process improvement options for sulphur dioxide absorption column
Exergy lost of an absorption column depends on properties of feed gas, lean solvent and products, number of stages, stage efficiency and (L m /G m ) ratio Similar to distillation column, operation states of the absorption column are expressed by operation points which are collected on an inclined plane The three dimensional exergy analysis diagram for sulphur dioxide absorption column is illustrated in Figure 4.21 The diagram shows variation in number of stages from N=4 to N , and
(L m /G m ) ratio in the range of 38–80 The coloured planes refer to solvent temperature in the range of 25–35 o C Their projections on the (L m /G m )–ExL plane express linear correlations between (L m /G m ) ratio and exergy lost of absorption column In addition, their projections on the (L m /G m )–N plane result in the curves expressing correlations between (L m /G m ) ratio, number of stages and stage efficiency
Figure 4.21: Three dimensional exergy analysis diagram for sulphur dioxide absorption column
As shown in Figure 4.21, the absorption column should be operated at (L m /G m ) ratio as low as possible This causes an overall reduction in exergy lost of system
Direction of decreasing solvent temperature 35 o C
E xe rgy l os t ( Ex L , kW ) including absorption column, desorption column and auxiliaries As discussed in analysis of base case, the initial exergy lost is 1,507 kW In this study, three options in order of complexity for absorption column improvement are compared in Table 4.6 It compares the operating condition and energy requirement of base case and improvement options All options must achieve 95% of sulphur dioxide recovery and operates at 80% of stage efficiency
Table 4.6: Improvement options in sulphur dioxide absorption column
Specifications Base case Option 1 Option 2 Option 3
Lean solvent flow rate (kg/hr) 151,124 117,761 137,625 107,306
Make–up flow rate (kg/hr) 362 304 361 302
Solubility of sulphur dioxide in water at 35 o C and 28 o C are 0.64 gr and 0.80 gr (SO2/100 gr water), respectively (Perry et al., 2008) These numbers show that the absorption of sulphur dioxide in water is more effective at low temperature In option 1, when solvent temperature is cooled down to 28 o C, (L m /G m ) ratio also reduces to 45.5 Hence, exergy lost of system reduces from 1,507 kW in base case to 1,175 kW, resulting in 22.0% savings Option 1 is an operational modification that can be implemented at low or no cost Table 4.6 shows that savings of reboiler duty amounts to 1,013 kW and savings of cooler duty is 965 kW The pump duty reduces slightly due to lower lean solvent flow rate
Case study 3: Exergy analysis and improvements of Selexol process
This case study demonstrates the combined application of three dimensional exergy analysis diagram and global energy costing model in the analysis and improvement of the Selexol process The three dimensional exergy analysis diagram for distillation and absorption columns are constructed Improvement options of energy performance in the Selexol process are further proposed
Selexol process uses physical solvent for the selective removal of acid gases such as hydrogen sulphide and other sulphur compounds, as well as carbon dioxide from synthetic and natural gas The solvent used in Selexol process is dimethyl ether of polyethylene glycol (DEPG) which is an inert solvent The solvent has a wide range of operating temperature from –18 o C to 175 o C Vapour pressure of DEPG is low, around 0.00073 mmHg at 25 o C (Burr & Lyddon, 1997), hence the lost of solvent is minimal in the process Another advantage of DEPG is that it can remove some non acidic compounds of sulphur without converting them to hydrogen sulphide These compounds include carbonyl sulphide, mercaptans, organic sulphides and thiophenes (Burr & Lyddon, 1997; The Dow Chemical Company, 1997; Kohl & Nielsen, 1997;
Dewing, Waring & Burns, 1998; Breckenridge, Holiday, Ong & Sharp, 2000; Nassar, Bullin & Lyddon, 2000; Nexant Inc., 2006) However, Selexol process requires energy to regenerate the lean solvent
The acid gas content in feed stream to Selexol process may vary from 5% to more than 60% by volume at pressure range between 20 and 140 bars Products purity can be achieved down to parts per million levels by volume (Burr & Lyddon, 1997)
4.5.2 Base case of acid gas removal in the Selexol process
An industrial case study by Bryan Research & Engineering Inc is adopted as a base case for the Selexol process (Burr & Lyddon, 1997) Selexol process is applied for hydro desulphurisation of 226 t/hr inlet synthesis gas at 32 bars and 25 o C originated from a steam reformer The specification for sulphur concentration in the outlet gas is 2.3 ppm by volume The compositions of synthesis gas are detailed in Table 4.7, where hydrogen, carbon monoxide and carbon dioxide make up the largest compositions
Table 4.7: Compositions of synthesis gas in Selexol process
The Selexol process flow scheme as shown in Figure 4.23 comprises an absorption column, three flash drums for pressure level degradation at high pressure, 13.8 bars, medium pressure at 6.9 bars, low pressure at 1.4 bars and solvent regeneration by distillation column at atmospheric pressure Synthesis gas is fed to the bottom of absorption column, and lean solvent at the top to remove sulphur compounds Rich solvent is expanded at high pressure flash down to 13.8 bars
Vapour from high pressure flash drum is compressed and cooled by air coolers for recycling to the absorption column This vapour comprises a half of carbon dioxide and one third of hydrogen and carbon monoxide The liquid is stepped down at medium and low pressure flash drums to 6.9 bars and 1.4 bars, respectively The remaining liquid is heated to 121 o C before being fed to a distillation column Lean solvent out from the bottom of distillation column is used to heat the distillation inlet at pre–heater Solvent make up is introduced to the cooled lean liquid and returned to absorption column at 37.8 o C
The process is divided into two sections as indicated by the dotted line in Figure 4.23 The first section consists of absorption column and auxiliary units such as flash drums, compressor and coolers The second section comprises a distillation column and auxiliary units like pre–heater, condenser, reboiler, and pump The first section is affected by (L m /G m ) ratio of absorption column The second section is affected by both the (L m /G m ) ratio of absorption column and the reflux ratio of distillation column For the base case, the absorption column consists of 8 stages at 70% efficiency, while the distillation column contains 6 stages at 90% efficiency
Figure 4.23: Process flow diagram of Selexol process for acid gas removal
A BS O RP T IO N C OL UM N D IS T IL L A T IO N C O L U M N
Solvent make up Treated gas
The Selexol process is simulated in ProMax version 3.2 simulation environment (Bryan Research & Engineering, Inc., 2010) due to availability of SRK property package The distribution of exergy lost in Selexol process is depicted in Figure 4.24
The absorption column operates at 1.78 of (L m /G m ) ratio, while distillation column is at 1.68 of reflux ratio Total calculated exergy lost of the whole system amounts to 16,980 kW, in which exergy lost at absorption and distillation columns are 6.1% and 62.4%, respectively The remaining exergy lost occurs at flash drums, compressor, reboiler, heat exchangers and pump, which make up auxillaries in sections 1 and 2
Figure 4.24: Exergy lost of Selexol process in base case
The exergy lost profile of absorption column is shown in Figure 4.25 For absorption column, minimum exergy lost is at the second stage Then exergy lost increases gradually until stage N=6 The lost increases rapidly in the last two stages,
N=7 and N=8 Similar observation of the exergy lost profile is made in case study 2
The exergy lost is significant at the top and bottom stages because the mixing of streams with different compositions and temperature
For the distillation column, the exergy lost profiles is similar to the xylene distillation column in case study 1 as shown in Figure 4.26 Maximum exergy lost in distillation column is at stage N=2 which is the feed stage The exergy lost at the
E x er gy l os t (Ex L , kW ) middle stages is low, and increases at the last two stages again, N=5 and N=6 In general, exergy lost is greater at stages where the inlet streams are located
Figure 4.25: Exergy lost profile of absorption column in base case of Selexol process
Figure 4.26: Exergy lost profile of distillation column in base case of Selexol process
In this case study, (L m /G m ) ratio is varied at constant reflux ratio of 1.68 Reduction of stage efficiency increases (L m /G m ) ratio and exergy lost as shown in Figure 4.27 With
Exergy lost (ExL, kW) kW kW kW kW kW kW kW kW kW kW kW kW kW kW
8 stages in the column, minimum exergy lost of absorption column is 969 kW at 100% of stage efficiency and (L m /G m ) ratio of 1.67 Total exergy lost of the whole system is 16,136 kW When all other parameters are kept constant, an increase in (L m /G m ) ratio raises exergy lost of all unit operations, resulting in an increase in exergy lost of the whole system This affirms the important role of (L m /G m ) ratio in the whole Selexol process
Figure 4.27: Correlations between exergy lost, (L m /G m ) ratio and stage efficiency of absorption column in Selexol process
4.5.5 The effects of reflux ratio
The (L m /G m ) ratio is fixed at 1.78 as per base case operations However, the reflux ratio of distillation column is varied while maintaining the properties of distillation inlet As mentioned above, (L m /G m ) ratio affects the whole system, while reflux ratio affects only distillation column and the subsequent unit operations Therefore, exergy lost of absorption column and auxiliaries in section 1, for example the flash drums, compressor and coolers, are constant at fixed (L m /G m ) ratio As reflux ratio varies, exergy lost of distillation column and auxiliaries in section 2 such as pre–heater, condenser, reboiler, and pump change together The reason is that flow rate and
E x er gy l os t (Ex L , kW )
Auxiliaries in section 2 Absorption column Auxiliaries in section 1 Whole system
Base case compositions of streams out from distillation column change as variation in reflux ratio These interactions are linear as shown in Figure 4.28
Reduction of stage efficiency increases reflux ratio, exergy lost of distillation column and auxiliaries in section 2 However, exergy lost of absorption column and auxiliaries in section 1 is invariant to reflux ratio as explained above With 6 stages of distillation column, reflux ratio reaches 1.35 with exergy lost amounting to 10,220 kW at 100% of stage efficiency The total exergy lost of the whole system reaches 16,492 kW
Figure 4.28: Correlations between exergy lost, reflux ratio and stage efficiency of distillation column in Selexol process
4.5.6 The effects of lean solvent temperature
CONCLUSIONS AND FUTURE WORKS
Conclusions
This work has systematically developed the relations between exergy lost and relative position of operating lines and equilibrium curve in a distillation column as realised by Zemp et al (1997) The position of operating lines and equilibrium curve is identified by design and operations parameters such as reflux ratio, relative volatilities and compositions of products The distance between operating lines and equilibrium curve represents the driving force of process Consequently it results in the exergy lost of process Based on this approach, a general framework is built up to construct exergy lost model of any process The framework shows its capability by the result of two graphical diagrams for exergy analysis of distillation and absorption columns
With the above framework, theoretical operation states of a distillation column are investigated to improve the two level idealisation concept by Chang and Li (2005)
This work proposes a new concept comprising four idealisation levels The new concept shows more detail about the limitation of operation states of distillation and absorption columns Moreover, it also explore full potential of energy savings performance for distillation and absorption processes
A major part of this work is occupied by simulation of distillation and absorption processes to establish the correlations between design and operations parameters to exergy lost in the processes Exergy lost of columns is directly proportional to reflux ratio in distillation process or (L m /G m ) ratio in absorption process Moreover, thermal condition of feed also affects to exergy lost of columns In distillation column, an increase in thermal condition of feed from subcooled to saturated liquid reduces exergy lost For absorption column, exergy lost decreases as lean solvent temperature is lowered The number of stages and stage efficiency affect exergy lost through reflux ratio or (L m /G m ) ratio The reflux and (L m /G m ) ratios are the decreasing functions of number of stages and stage efficiency In general, exergy lost of distillation or absorption column is a function of reflux ratio and q value or (L m /G m ) ratio and lean solvent temperature, while the reflux and (L m /G m ) ratios are implicit functions of number of stages and stage efficiency Therefore, the distillation or absorption column has three degrees of freedom
A graphical method for exergy analysis of distillation column, three dimensional exergy analysis diagram, was developed The application of the method was demonstrated using a case study of xylene distillation column The three dimensional exergy analysis diagram expresses the correlations between design and operations parameters to exergy lost in the column, and gives a holistic view of their effects on exergy lost The combination of feed pre–heater installation and bottoms purity adjustment decreases 15.5% of exergy lost in the whole system Condenser and reboiler duties also reduce by 11.3% and 26.9%, respectively Similar to distillation column, a three dimensional graphical method for exergy analysis of absorption column was also developed and demonstrated in a case study of sulphur dioxide recovery process The features of the diagram for absorption column are the similar to the diagram for distillation column except reflux ratio is replaced by (L m /G m ) ratio By addition of four new stages in absorption column and reduction of lean solvent temperature to 28 o C, the savings accounts for 27.1% of the total exergy lost In other words, reboiler and cooler duties savings are achieved 28.0% and 28.7%, respectively
For a complex separation system comprising an absorption column and a distillation column such as Selexol process, a combination of two exergy analysis diagrams for distillation and absorption columns helps to explore the potentials of energy savings performance The total exergy savings amounts to 19.4% when two new stages are added into both absorption and distillation columns, lean solvent temperature decreases to 30 o C Besides, electric power for compressor, air cooler and pump reduces by 10.2% While the duty of reboiler, condenser and cooler decrease by 24.4%, 28.7% and 10.8%, respectively Moreover, these methods can be used for existing plant as well as new design For an existing plant, such methods help to locate operation point of system, explore the potential of improvements and estimate amount of exergy lost for each improvement For a new design, they are useful methods to aid the search for optimum design and operations with minimum exergy lost
A new model of energy cost evaluation is also proposed in this work This model is called global energy costing, evaluates energy cost initially under exergy concept, in which energy price is a function of temperature level The expressions of global energy cost are evidence to prove the correctness and consistency of the new approach In this case, the temperature is the fundamental measurement of energy quality The model is effective for costing of heat source at different temperature such as multiple levels steam in utility system Besides, the model allows engineers do optimisation directly without complex procedures and iterations The global energy costing model demonstrates that the value in use of thermal energy is always higher than that one evaluated by the conventional energy costing model Thermal energy is appraised including the value of exergy and anergy.
Future works
The achievements of this work open the applications in industry and research in three main directions such as computational graphical method, application of the general framework to other separation processes and application of global energy costing model in exergoeconomic evaluation of natural resources
The three dimensional exergy analysis diagrams for distillation and absorption columns are attributes of the system Therefore a database can be built for common systems and is very advantageous for process engineers The relations between exergy lost, reflux ratio or (L m /G m ) ratio, number of stages, stage efficiency and thermal condition of feed can be modelled mathematically and imported to simulation software This work can be done by cooperation to simulation engineering groups to build a module compile in commercial simulation software
The other separation process which also has significant energy consumption is drying process The application of exergy analysis to drying process sought the interest of some researchers Dincer and Sahin (2004) and Dincer (2011) proposed a model of exergy efficiency evaluation for drying process The authors quantified the factors that affect exergy efficiency such as temperature of dry air, flow rate of dry air, humidity of dry air and moisture content of dry product Aghbashlo et al (2012) used the same model by Dincer and Sahin (2004) to do exergy analysis for spray drying process of fish oil microencapsulation Colak and Hepbasli (2007) performed exergy analysis for drying of green olive in a tray dryer, and evaluated the potential of exergy improvement in the process However, all these works have not brought forward the direction of optimisation in process design and operations, and the full potential of improvement options The general framework proposed in this work is the promise of optimisation in drying process Therefore, a good match of this work and previous works by stated authors may be achieved by a new method for exergy analysis of drying process
Exergy costing gained the interests from many researchers such as Goran Wall from Sweden, George Tsatsaronis from Germany, Ibrahim Dincer from Saudi Arabia and Marc Rosen from Canada Wall (1986) evaluated exergy price of resources through exergy factors Exergy factor is a ratio of exergy and energy content of resource Consequently, exergy price is proportional to energy price and exergy factor The model of exergy cost is still conventional model which is the same to energy cost model Tsatsaronis and his colleagues did exergoeconomic analysis for many power plants (Cziesla & Tsatsaronis, 2002; Tsatsaronis & Park, 2002; Cziesla et al., 2006; Paulus & Tsatsaronis, 2006; Morosuk & Tsatsaronis, 2008) They also used the conventional exergy cost model However, exergy price cannot be estimated initially, and requires a number of additional equations Therefore, procedure of optimisation is very complex Dincer and Rosen collaborated in exergoeconomic analysis for many energy conversion plants (Rosen & Dincer, 2003; Rosen & Dincer, method, nevertheless there are so many equations and variables need to be specified in the method The global energy costing model in this work opens a new trend of energy price evaluation One of standards for energy appraisement of resources should be originated from properties of the resources The selected property must reflect the quality of resource From the global energy costing model, flame temperature of resource can be considered as a standard measurement For a resource, the full energy potential can be extracted by combustion, and its quality is identified by flame temperature Therefore, the theoretical flame temperature of a resource can be considered as the typical parameter of its quality Combination of the theoretical flame temperature and global energy costing model has the potential to help engineers evaluate the energy price under exergy concept more quickly and practically
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APPENDIX A: COMPOSITIONS OF FEED STREAM IN XYLENE COLUMN
N o Compositions, kg/hr Feed 1 Feed 2
25 1–Methyl–3–n–Propyl Benzene – 410.710 26 1,2,3,4–Tetramethyl Benzene – 399.980 27 1,2,3,5–Tetramethyl Benzene – 373.100
Total mass flow, kg/hr 218,000 115,877
APPENDIX B: PLANT DATA OF PROCESS STREAMS IN XYLENE COLUMN
Stream N o Temperature ( o C) Pressure (bar) Flow rate (kg/hr)
APPENDIX C: HYSYS PROCESS MODEL AND STREAMS DATA
Figure C.1: HYSYS process model for xylene distillation column
Table C.1: Streams data for xylene column in base case Streams 285 0.00 283.47 9.20 11050.5 13.46E5 1531.24 42254.8 373.53 0.0012 0.0005 0.0065 0.0001 0.1649 0.0000 0.0000 0.0121 0.1251 0.0000 0.0000
Unit C bar kmol/hr kg/hr m3 /hr kJ/kmol kJ/kmolC Mol frac
Specifications Vapour fraction Temperature Pressure Molar flow Mass flow Volume flow Molar enthalpy Molar entropy Compositions Meta Xylene Para Xylene Ortho Xylene Ethyl Benzene 1,3,5–Trimethyl Benzene Toluene 3–Ethyl Heptane Naphthalene Meta Ethyl Toluene Iso Propyl Cyclo Pentane Benzene
Table C.1: Streams data for xylene column in base case (continued) Streams 285 0.0000 0.3408 0.0593 0.0677 0.0582 0.0407 0.0083 0.0142 0.0123 0.0158 0.0086 0.0085 0.0075 0.0080 0.0053 0.0062 0.0073 0.0059 0.0033
Specifications Compositions Cyclohexane 1,2,4–Trimethyl Benzene 1,2,3–Trimethyl Benzene n–Propyl Benzene Para Ethyl Toluene Ortho Ethyl Toluene C11 Aroma 4–Ethyl Ortho Xylene 5–Ethyl Meta Xylene Indane 1,2,4,5–Tetramethyl Benzene 2–Ethyl Para Xylene 4–Ethyl Meta Xylene 1–Methyl–3–n–Propyl Benzene 1,2,3,4–Tetramethyl Benzene 1,2,3,5–Tetramethyl Benzene Cumene n–Decane 1,2–Dimethyl–3–Ethyl Benzene
Table C.1: Streams data for xylene column in base case (continued) Streams 285 0.0021 0.0032 0.0023 0.0016 0.0009 0.0006 0.0005 0.0005 0.0000 0.0000 0.0000 0.0000 0.0000
Specifications Compositions C12 Aroma 1–Methyl–4–n–Propyl Benzene Meta Diethyl Benzene 1–Methyl–2–n–Propyl Benzene Para Diethyl Benzene Iso Butyl Benzene Sec Butyl Benzene n–Butyl Benzene n–Octane 3,4–Dimethyl Hexane 2–Methyl Heptane 4–Methyl Heptane 3–Methyl Heptane
Table C.2: Streams data for xylene column in option 1 Streams 285 1.00 282.76 9.20 9813.31 11.91E5 1355.34 42623.8 370.81 0.0050 0.0021 0.0255 0.0004 0.1589 0.0000 0.0000 0.0118 0.1220 0.0000 0.0000
Unit C bar kmol/hr kg/hr m3 /hr kJ/kmol kJ/kmolC Mol frac
Specifications Vapour fraction Temperature Pressure Molar flow Mass flow Volume flow Molar enthalpy Molar entropy Compositions Meta Xylene Para Xylene Ortho Xylene Ethyl Benzene 1,3,5–Trimethyl Benzene Toluene 3–Ethyl Heptane Naphthalene Meta Ethyl Toluene Iso Propyl Cyclo Pentane Benzene
Table C.2: Streams data for xylene column in option 1 (continued) Streams 285 0.0000 0.3322 0.0578 0.0662 0.0568 0.0397 0.0081 0.0139 0.0120 0.0154 0.0083 0.0083 0.0073 0.0078 0.0052 0.0060 0.0092 0.0057 0.0032
Specifications Compositions Cyclohexane 1,2,4–Trimethyl Benzene 1,2,3–Trimethyl Benzene n–Propyl Benzene Para Ethyl Toluene Ortho Ethyl Toluene C11 Aroma 4–Ethyl Ortho Xylene 5–Ethyl Meta Xylene Indane 1,2,4,5–Tetramethyl Benzene 2–Ethyl Para Xylene 4–Ethyl Meta Xylene 1–Methyl–3–n–Propyl Benzene 1,2,3,4–Tetramethyl Benzene 1,2,3,5–Tetramethyl Benzene Cumene n–Decane 1,2–Dimethyl–3–Ethyl Benzene
Table C.2: Streams data for xylene column in option 1 (continued) Streams 285 0.0020 0.0031 0.0022 0.0016 0.0009 0.0005 0.0005 0.0005 0.0000 0.0000 0.0000 0.0000 0.0000
Specifications Compositions C12 Aroma 1–Methyl–4–n–Propyl Benzene Meta Diethyl Benzene 1–Methyl–2–n–Propyl Benzene Para Diethyl Benzene Iso Butyl Benzene Sec Butyl Benzene n–Butyl Benzene n–Octane 3,4–Dimethyl Hexane 2–Methyl Heptane 4–Methyl Heptane 3–Methyl Heptane
Table C.3: Streams data for xylene column in option 2 Streams 285 1.00 283.47 9.20 9389.39 11.43E5 1301.06 42252.2 373.53 0.0013 0.0006 0.0064 0.0001 0.1650 0.0000 0.0000 0.0121 0.1251 0.0000 0.0000
Unit C bar kmol/hr kg/hr m3 /hr kJ/kmol kJ/kmolC Mol frac
Specifications Vapour fraction Temperature Pressure Molar flow Mass flow Volume flow Molar enthalpy Molar entropy Compositions Meta Xylene Para Xylene Ortho Xylene Ethyl Benzene 1,3,5–Trimethyl Benzene Toluene 3–Ethyl Heptane Naphthalene Meta Ethyl Toluene Iso Propyl Cyclo Pentane Benzene
Table C.3: Streams data for xylene column in option 2 (continued) Streams 285 0.0000 0.3408 0.0593 0.0677 0.0582 0.0407 0.0083 0.0142 0.0123 0.0158 0.0086 0.0085 0.0075 0.0080 0.0053 0.0062 0.0071 0.0059 0.0033
Specifications Compositions Cyclohexane 1,2,4–Trimethyl Benzene 1,2,3–Trimethyl Benzene n–Propyl Benzene Para Ethyl Toluene Ortho Ethyl Toluene C11 Aroma 4–Ethyl Ortho Xylene 5–Ethyl Meta Xylene Indane 1,2,4,5–Tetramethyl Benzene 2–Ethyl Para Xylene 4–Ethyl Meta Xylene 1–Methyl–3–n–Propyl Benzene 1,2,3,4–Tetramethyl Benzene 1,2,3,5–Tetramethyl Benzene Cumene n–Decane 1,2–Dimethyl–3–Ethyl Benzene
Table C.3: Streams data for xylene column in option 2 (continued) Streams 285 0.0021 0.0032 0.0023 0.0016 0.0009 0.0006 0.0005 0.0005 0.0000 0.0000 0.0000 0.0000 0.0000
Specifications Compositions C12 Aroma 1–Methyl–4–n–Propyl Benzene Meta Diethyl Benzene 1–Methyl–2–n–Propyl Benzene Para Diethyl Benzene Iso Butyl Benzene Sec Butyl Benzene n–Butyl Benzene n–Octane 3,4–Dimethyl Hexane 2–Methyl Heptane 4–Methyl Heptane 3–Methyl Heptane
Table C.4: Streams data for xylene column in option 3 Streams 285 1.00 282.76 9.20 8065.32 9.789E5 1113.91 42620.1 370.78 0.0054 0.0023 0.0251 0.0004 0.1591 0.0000 0.0000 0.0118 0.1220 0.0000 0.0000
Unit C bar kmol/hr kg/hr m3 /hr kJ/kmol kJ/kmolC Mol frac
Specifications Vapour fraction Temperature Pressure Molar flow Mass flow Volume flow Molar enthalpy Molar entropy Compositions Meta Xylene Para Xylene Ortho Xylene Ethyl Benzene 1,3,5–Trimethyl Benzene Toluene 3–Ethyl Heptane Naphthalene Meta Ethyl Toluene Iso Propyl Cyclo Pentane Benzene
Table C.4: Streams data for xylene column in option 3 (continued) Streams 285 0.0000 0.3322 0.0578 0.0661 0.0568 0.0397 0.0081 0.0138 0.0120 0.0154 0.0083 0.0083 0.0073 0.0078 0.0052 0.0060 0.0090 0.0057 0.0032