VIETNAM NATIONAL UNIVERSITY HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY FACULTY OF CHEMICAL ENGINEERING PROCESSES AND EQUIPMENT DEPARTMENT ACKNOWLEDGEMENT First of all, I am extremely grateful to my ins[.]
THEORY
Distillation definition
Distillation is a purifying process which is used to separate components in a liquid mixture or liquid-gas mixture, basing on the volatility of each component This technique can be used to either increase the concentration of a particular component in a mixture or obtain pure components from the mixture
Instead of introducing a new substance into the mixture in order to create the second phase, as is done in absorption or desorption, the new phase is created from the original solution by vaporization or condensation.
Considering a simple mixture with 2 components, we have
The top product consists of a large amount of most volatile component and very small amount of less volatile component/
The bottom product consists of a large amount of less volatile component and a very small amount of most volatile component.
In distillation, if we consider only binary system for Benzene-Toluene, distillate products are mostly benzene and a small fraction of toluene, whereas the bottom products are mainly toluene and some remaining benzene.
Distillation method classification
Basing on working pressure: low-pressure distillation, normal-pressure distillation and high-pressure distillation The basic fundamental is based on the boiling point of the components, if the Boling point is too high, we need to reduce the working pressure to decrease the Boling point of the components
Distillation equipment
In manufacturing, different types of distillation equipment are used However, they all have basic requirement that is the area of contacted phase surface must be large, depending on the dispersion of this fluid into the other fluid
There are two main types of columns, batch and continuous distillation column We investigate two types of distillation towers commonly used in continuous distillation, which are Tray towers and packed towers In term of tray column, sieve trays and bubble cap trays are mostly widespread.
Tray tower: The cylindrical vertical body of the tower, which sticks Trays having different structures in the interior to split the body of the tower into equal segments, on the Trays liquid phase and the vapor phase are exposed to each other Depending on the composition of the plates, includes:
Bubble cap trays column: on Trays having bubble cap having form of circle, S,… The structure is surrounded by grooves to pass the gas through and the transmission pipeline
Sieve Tray column: on Tray having holes having diameter of (3-12 mm) distribution Packed column: A cylindrical tower, consisting of several segments connected by flanges or welding Packings are placed in the tower in one of two methods: random or sequential.
In this case, bubble caps tray is chosen for Benzene-Toluene mixture distillation This type consists of some advantages: High efficiency, stable operation, less energy consumption, so there are few trays required.
Benzene
Benzene (C6H6) is a clear, colorless, highly flammable and volatile, liquid aromatic hydrocarbon with a gasoline-like odor Benzene is non-polar, so it dissolves greatly in organic solvent and very weekly in water Benzene is found in crude oils and as a by- product of oil-refining processes In industry benzene is used as a solvent, as a chemical intermediate, and is used in the synthesis of numerous chemicals However, nowadays, benzene is no longer using widely, because it is found that small concentration benzene in air can cause some blood diseases
Table 1 Physical properties of benzene
Toluene
Toluene is a colorless, water-insoluble liquid with the smell associated with paint thinners It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) attached to a phenyl group Toluene is non-polar, so it is greatly soluble in benzene Toluene has solvent properties like benzene but less toxic, so it widely is used as solvent in labs and industries
Table 1.2 Physcial properties of toluene
Vapor-Liquid Diagram
Table 1.3 Tx-y data for benzene-toluene binary mixture x 0 0.05 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 y 0 0.118 0.214 0.38 0.511 0.619 0.712 0.79 0.854 0.91 0.959 1
Figure 1.4 VLE diagram of benzene-toluene tixture
Figure 1.5 Tx-y diagram of benzene-toluene binary mixture
MASS BALANCE
Plug-flow diagram
Continuous distillation is selected to achieve high concentration of top product The benzene-water mixture containing 15% mole fraction of Benzene presents at Feed tank with temperature about 20 Then, the liquid is moved to Heater to be heated to obtain bubble point temperature (approximately 104.15) The mixture is introduced into the column through feed tray The liquid falls and being contact with the vapor counter- currently up The concentration of the downward liquid decreases due to volatility affected by the steam derived from the Reboiler The temperature declines when the vapor flows up higher Therefore, Benzene with higher boiling point is condensed in the first instance by the way vapor rising from bottom to top As the result, at the top,
Benzene dominates in concentration (97% mole fraction) In the next step, the Benzene vapor passes through the Condenser and is condensed completely in form of liquid The liquid is divided into two fractions One keeps going to the Heat exchanger to cool down to temperature 40 and be collected and stored in Overhead product tank Other is recycled back to the column at the top tray In contrast, in the liquid at the bottom, Toluene (high boiling point constituent) occupies on account of volatile component continuously being elevated The liquid is released out of the column and brought to the Reboiler where an amount of the liquid being volatilized and reintroduced into the column as the steam The residue of the liquid progresses to eventually being contained in the Bottom product tank. The continuous distillation process produces top product of Benzene and bottom product of Toluene is disposed.
Find the molar capacity
The flowrate of feed stream is equal to the flowrate of overhead stream and bottom stream of the column.
Mass balance for distillation column: (1)
Mass flow rate of each stream
Feed stream Overheat product stream Bottom product stream
Based on this table we draw the equilibrium line y = f (x) and determine the molar and boiling point respectively
Reflux ratio
The minimum reflux ratio is the working number that is associated with the number of hypotheses As a result, the cost is fixed but the operating costs (fuel, water, and pump) are minimal.
The line which connects two points ( and (, intersects the horizontal line at
It can be observed from the chart that
We choose to use equation below to define R
Number of theorical tray
From the VLE diagram, we sketch the rectifying operation line as the equation:
Feed material at bubble state so the feed line is vertical
Number of theorical tray is 19 trays, 8 trays for rectifying section, 10 trays for stripping section and feed tray is 11 th trays.
Actual number of trays
The actual number of plates (Nr) is calculated based on the average efficiency of the tower ():
Nt: Theoretical number of plates
The average efficiency of the tower is calculated by:
Nt: Theoretical number of plates
: Efficiency of plate 1, plate 2, plate 3…
- In this case we have:
: Efficiency of distillation, feed and bottom respectively
The average efficiency () is a function of Volativity () and Viscosity ():
: Equilibrium mole fraction of Benzene in liquid and vapour phase.
Assume that the Benzene - Toluene mixture is ideal then the viscosity of the mixture is calculated by:
: Mole fraction of Benzene in liquid phase
From the product, look up Figure IX.11/171 to find out the efficiency of each plate. x (mol %) y
The average efficiency and the real number of trays of the column are
Number of trays for Rectifying section
Number of trays for Stripping section:
Number of real trays of the whole column
ENERGY BALANCE
Energy balance for Feed-preheating equipment
F: Flowrate of feed after heating f: flowrate of feed before heating
Heat carried by heating steam
: Specific heat of condensed water
Saturated steam at 4.15 and P=3.047 bar(g)
Heat carried by feed before heating
We’ve chosen that the storage temperature of feed is 28 o C
Assume that according to TI.153-154_P.172[3]
Heat amount brought out by condensed water (from IX.153_P196[1]):
: Feed specific heat capacity (J/kg)
Heat amount brought out by surrounding about 5% of total heat loss (from
The amount of steam needed to heat the initial feed mixture to bubble point temperature (from IX.155_P196[1])
Energy balance for overhead condenser
The amount of water supplied for condenser
Assume: obtained from T.I.153_P.172[3] and interpolation
Energy balance for bottom reboiler
Flowrate of saturated steam supplied for reboilerChoose
Energy balance in cooler for bottom product
Energy balance in overhead product cooler
CALCULATION FOR DISTILLATION COLUMN
Diameter of tower
Vavg: average volumetric flow rate of vapor stream inside the tower, m 3 /h : Average velocity of vapor stream inside the tower, m/s
: Average mass flow rate of vapor stream inside the tower, kg/h
: Average vapor flux inside the tower, kg/m 2 s
4.1.1 Rectifying section kg/h : Average vapor flow inside the Rectifying section, kg/h
: vapor flow out of the top plate of Rectifying section, kg/h
: vapor flow into the first plate (upward direction) of Rectifying section, kg/h
: The amount of liquid in the first tray of rectifying section (kg/h)
: The amount of vapour in the first tray of rectifying section (kg/h)
: The amount of vapour leaving the highest trays of column (kg/h)
: Latent heat of the inlet mixture in the rectifying section (kJ/kg)
: Latent heat of the outlet mixture in the rectifying section (kJ/kg)
: mass fraction of vapour in the 1 st trays of rectifying section
Solve the three equations above for and
Substituting the obtained latent heats to the Equations
Mole fraction of vapour in the 1 st trays of rectifying section:
The velocity of vapour in bubble cap tray column is calculated by:
: The average density of liquid mixture according to the average temperature (kg/m 3 ) : The average density of vapor mixture according to the average temperature (kg/m 3 ) : The distance between trays (m)
: The average mole fraction of benzene
:the average mass fraction of benzene in liquid phase.
The average flowrate of vapor in stipping section:
In which: the amount of vapor entering the first stray of stripping section
: the amount of vapor going out of stripping section
- The amount of vapor entering the stripping section , the amount of liquid and the mass fraction of liquid can be determine by using equation:
: The amount of liquid in the first tray of stripping section (kg/h)
: Latent heat of the inlet mixture in the stripping section (kJ/kg) and at
Velocity of vapour in stripping section is calculated by:
The velocity of vapor through each section
Hence, the column diameter of 1.6m and tray distance of 0.4 is chosen.
The height of distillation column
The height of the body
In which, : Height of the tower (m)
: The distance between two plates (m)
: allowable distance between top and bottom
The height of the bottom and the head
We choose the ellipsoidal head and bottom with welded body for the column which is located vertically under external or internal pressure greater than
Conduct the same calculation for the head and bottom.
With Dished height and from Table X111.10_P382[1]
Choose straight flange height of ellipsoidal head:
The height of the distillation tower is 16 m
Bubble cap calculation
The number of caps placed on the tray
With : The inner diameter of the distillation column (m)
We decide that caps/tray
Choosing the diameter of the riser we have the upper gap between the riser and the cap choose
The diameter of each cap: we choose
Distance from the plate to the cap,
The height of liquid above the bubble – cap slot,
The length from the cap’s slot to the bottom of cap is determined:
The height of the cap’slot obeys equation IX.215_P.236[1]
: the flowrate of the vapor ()
: the average density of vapor phase (kg/m 3 )
: the average density of liquid phase (kg/m 3 )
The number of slots in each cap according to IX.216_P236[1] with : The distance between each slot (m) and a: the width of the slot,
The height of the slot ranges from 20 to 50 mm Therefore, choose Choose
The minimum step of the cap on the tray from IX.220_P.237[1]
The openness of the slot from equation 5.2_P108[4]
In which, : the height of the slot (m)
The total area of slots on the tray (m 2 )
The percentage of the openness for bubble caps:
The height of the bubble cap is chosen:
Hence, the height of the riser:
The minimum step of the cap on the tray from IX.220_P.237[1]
With : The minimum distance between caps (m) choose
The weir calculation
The liquid level above the weir obtained from IX.219_P237[1]
With : volumetric flowrate of the liquid phase (m 3 /h)
: the average flowrate of liquid phase (kg/h)
The liquid level above the weir according to equation 5.3_P110[4]:
Surface tension resistance
According to [2] – Eq IX 141, surface tension resistance is calculated by:
In case the bubble caps’ slots are fully opened: , with
: surface area of bubble cap’s slot
: equivalent perimeter of the bubble cap’s slot
According to [2] – Eq IX 139, hydrostatic resistance is calculated by:
: density of foam varies in 0.4 – 0.6
The height of foam is calculated by:
F: area for installing bubble caps f: total area of bubble caps on each tray
: height of liquid on overflow weirs (m)
: height of no – foam liquid on trays (m)
The width of the weir:
According to [2] – Eq IX 92, the amount of reflux liquid:
The average amount of liquid in rectifying section:
The average liquid flowrate through rectifying section:
Based on [2] – Fig IX 22, we can calculate the adjust coefficient:
The height of liquid mixture above the weir :
From [2] – Eq IX 219, the weir’s height:
Height of no foam liquid on trays:
Total area of bubble caps on each tray:
Area for installing bubble caps:
The height of bubble cap:
The average amount of liquid in stripping section:
The average liquid flowrate through rectifying section:
Based on [2] – Fig IX 22, we can calculate the adjust coefficient:
The height of liquid mixture above the weir :
From [2] – Eq IX 219, the weir’s height:
Height of no foam liquid on trays:
Total resistance in rectifying section
Total resistance in stripping section
Total resistance in entire column
Flooding test
The height of the liquid level with no foam for downcomer : ( [3] – Eq 5.9 )
: The height of weir on plate (mm)
: The height of liquid above the weir (mm)
: Gradient of liquid’s height on the plate (mm)
: The pressure drop of vapor phase through plate (mm)
: The hydraulic losses due to liquid flow from the downcomer to plate (mm)
Gradient of liquid’s height on the plate
From [3] – Eq 5.5, the gradient of liquid’s height on the plate can be determined by: with number of bubble caps which liquid phase go through, gradient of liquid’s height through a row of caps
Adjust coefficient for vapor flowrate, can be determined by [3] – Fig 5.10.
The average width of the area between two downcomers:
The average area between two segments downcomers
The distance between two segments downcomers
The average liquid flowrate in the distillation column:
The average speed of vapor:
The average density of vapor:
From [3] – Fig 5.10, we have the value of
The height of liquid on the plate:
From [3] – Fig 5.13.(b), we can find the value of using the value
Hydraulic losses due to liquid flow from the downcomer to plate
With the area between the downcomer and plate
Total pressure drop of vapor phase through a plate
The pressure drop due to friction and the change of vapor phase velocity through the caps when no liquid;
The total of riser area on a plate:
The area between the caps and the riser:
The area of the cap’s riser:
From the value of and data from [3] – Fig 5.16, we can obtain the value of
The height from liquid above the slot to the weir:
Total pressure drop of vapor phase through a plate:
The height of the liquid level with no foam for downcomer:
The column will not have flood
Check the liquid flow in the downcomer
From [3] – Eq 5.12, the value to describe the liquid flow inside the downcomer:
The distance between each plate (m)
Assume the average width of the downcomer is 20% of the column diameter
MECHANICAL DESIGN
Material
Due to the cylindrical column for benzene-water distillation with maximum working temperature of , stainless steal X18H10T is decided for fabrication the column to maintain the quality of the product presented in T XII.46_P349[1].
The thickness of body of distillation column
Inner diameter of the column: D=1.6m
According to T XIII.8_P362[1], we fabricate the cylindrical body of the tower by electric arc welding, two-sided welding Flanges are used to link between parts of the column.
We obtain the weld joint strength coefficient
Assume that the column is entirely reinforced without any holes for observation
The calculated pressure of the column with operating environment of liquid-vapor mixture based on equation 1-1_P10[5]:
In which, : hydrostatic pressure of the column of liquid.
The column works under internal pressure
The thickness of cylindrical body under internal pressure is performed by formula (I)
In which, C: additional coefficient due to corrosion, abrasion, thickness tolerance. : strength coefficient for vertical equipment wall.
: inner diameter of the column.
Choose the speed of corrosion ranges from 0.05 to 0.1 mm per year The column is intended for utilization for 10 years Therefore,
The allowable stress for testing stability of the equipment
In which, : adjustable coefficient relied on the working condition of the equipment ranging from 0.9 to 1 Choose
The calculated temperature From diagram 1.2_P16[5], we achieve
The additional coefficient is calculated by equation XIII.17_P363[1]:
In which, : additional coefficient for chemical corrosion of environment, 1mm
: additional coefficient for mechanical abrasion of environment (there are solid particles with high rate of movement), mm Choose
: additional coefficient for tolerance in manufacture and assemble process, mm Choose according to T XIII.9_P364[1].
With any materials, we have Consequently, the equation (I) according to equation 5- 3_P96[5] becomes:
Choose additional coefficient for rounding number:
Testing for the thickness of the body of the column
Check for condition from equation 5-10_P97[5]:
Allowable pressure of the body of the column followed by equation 5.11_P97[5]:
Stress based on tested pressure is calculated by the formula XIII.26_P365[1]:
In which, : tested pressure determined by the formula XIII.27_P366[1]:
With : tested hydrostatic pressure as displayed in T XIII.5_P358[1]
The thickness of the cylindrical body of the column is accepted As a result, the outer diameter of the column
The thickness of the bottom and the head of distillation column
The thickness of head and bottom conforming to equation XIII.47_P385[1]:
In which, : the height of the convex part of the bottom.
In case of body thickness:
According to T XII.4_P310[1] with the thickness of steel sheet varied from 4-25mm, we achieve that
Therefore, the equation for calculation of the thickness of the bottom and the head changes into:
Choose the thickness according to table XIII.12_P385[1].
Testing for the wall stress of the head of the equipment based on tested hydrostatic pressure as mentioned in equation XIII.49_P386[1]:
The thickness of the head and the bottom of the column is accepted.
The diameter of the pipes:
We determine the pipes which connect to the equipment by means of removable joint and being created by stainless steel X18H10T The pipes include two major parts: a short tube and a flange or a screw thread The flange is also made of stainless steel with structure of blind flange () while the screw refers to or in some cases
The diameter of the pipe is conducted by equation II.36_P369[3]:
In which, : volumetric flow rate ()
Then, the formula (ii) becomes
Pipes used for , kg/m 3 G, kg/hr ω, m/s
Calculated diameter d, mm Overhead vapor 25 228
The value of is selected according to table XIII.32_P434[1].
Pipes used for Calculated diameter d, mm , mm ,mm
The flange
The flange is one of the essential compositions applied to connect between parts of the device as well as the device with other parts.
Proposing the structure of the flange is welding flange that the flange is linked directly to the equipment by welding, casting, forging This kind of flange is chosen in cases of low and medium pressure The figure XIII.20_P408[1] with type I and type IV are examples for this kind of flange.
5.4.1 The flange connected to the body of the column
The flange is made of stainless steel X18H10T and its structure is type 1.
Db: the length of two symmetric bolts h: the height of flange db: bolt dic c ameter
Figure 5 1The flange connected to the body
According to table IX.5_P170[1] with inner diameter of 1600 mm and distance between two trays of 400 mm, we obtain the number of trays between two flanges and distance between two flanges of 2000 mm.
The number of flanges used is equal flanges
Dimensions of the flange gasket is obtained by table XIII.31_P433[1]:
5.4.2 The flange connected between part of the device and the pipes
According to table XIII.26_P409-415[1] with type of flange is 1 (Z: number of bolts)
Figure 5.2 The flange connected between part
Subsequently, based on the table XIII.30_P432[1], we have that
The supports
The preliminary calculation for the total weight of the column:
Noted that the material is stainless steel X18H10T with its density followed table XII.7_P313[1].
The mass of the head and the bottom:
As shown in the table XIII.11_P384[1], the weights of the head and the bottom are similar With
The mass of the body of the column:
The mass of the tray:
Assumption: in each tray, the mass loss due to the hole relatively equals to the caps and risers The mass of weir and downcomer are the same
The mass of the flange:
The mass of the liquid:
In this case, the column is flooding.
The weight of the column:
Choose the 4-leg support which is built up by stainless steel X18H10T.
So that the allowable weight pressed on one leg:
The dimensions of the support are displayed in the table XIII.35_P437[1]
Choose (F: surface area of the support)
AUXILIARY EQUIPMENT
Feed preheater
In this case, we select horizontal shell-tube condenser
The material for the tubes of the heat exchanger is stainless steel X18H10T. Inner tube diameter: 21 (mm) = 0.021 (m) (Table VI.6_P80[1])
Outer tube diameter: 25 (mm) The saturated steam passes through the outer tube:
The outer diameter of the outer tube:
The inner diameter of the outer tube:
The feed stream goes inside the internal tube:
The outer diameter of the inner tube:
The inner diameter of the inner tube:
The average temperature of the feed stream:
Calculation for heat transfer coefficient accompanied by equation V.5_P3[1]:
In which, : Heat supply coefficient of the steam burner
: Heat supply coefficient of the feed stream
: Heat resistant through the tube walls and from the fouling factor
Velocity of the feed in inner tube:
Nusselt number as stated by equation 3.27_P110[6]:
Applying Table 3.1_P110[6], with , is chosen.
Prandtl number for the feed at temperature:
Heat supply coefficient of the feed presenting in the inner tube:
Heat load at the side of the feed:
Heat transfer through the wall of the tube and the fouling layer:
In which, : the temperature of the wall of the inner tube contacting with the steam outside,
: the temperature of the wall of the inner tube contacting with the feed inside,
Based on the Table XII.7_P313[1], thermal conductivity of the stainless steel Dirty resistant factor: ) with application of T.31_P.419[6]
Fouling resistant factor: ) with application of T.31_P.419[6]
Heat supply coefficient of steam outside the tubes:
Conduct integration to obtain and
Average temperature of condensate water:
Estimate that the heat loss is negligible Therefore,
Then, the average temperature of the tube wall,
Test That means is accepted.
In conclusion, the feed preheater is the double-pipe heat exchanger with the length of the tube is 83 m.
Reboiler
Select a separate Kettle boiler for the tower so that the steam it produces can help evaporate some of the liquid at the bottom of the tower so that steam can be produced for the lower part of the tower This steam then helps the boiler receive the liquid dripping from below the tower The need for this is to climb the tower, choosing tube-and-shell heat exchanger for reboiler Kettle reboiler with tubes material is X18H10T ( The hot steam flows inside the tube while the bottom product flow outside.
According to (ST2: Table V.11/49), we choose: The device consists of 173 tubes arranged in a circle, arranged in 7 circles (), the number of tubes in the outermost ring is 43
Choose hexagon arrangement Tubes on the outer 6-edge polygon: a = 7
Gap between tubes: t = (1.2 ÷ 1.5).do Choose t = 1.2
Heat transfer coefficient from saturated steam to tube wall: At 132.9 o C:
Average thermal conductivity coefficient of tubes:
Choose the number of pipes on a straight line n = 13, we have ε = 0.5
Heat transfer through tube wall and foul:
Heat resistance of foul on the inner tube: = 200* (m 2 K/W)
Heat resistance of foul on the inner tube: = 172.4*10 -6 (m 2 K/W)
Average temperature of bottom product in reboiler:
When the liquid boils in a big volume:
Choose working pressure in liquid surface is P = 1 atm = 101325 Pa
Absolute pressure on the surface:
Assume heat loss is not considerable: q = q1 8528.867
Heat transfer through wall and foul:
Specific heat load of bottom product :
Condenser
Choosing tube-and-shell heat exchanger with tubes material is X18H10T ( The top product flows inside the tube while the water stream is outside.
Some physical parameters of water at 32.5:
From the Newton-Fourier equation:
- : Average heat coefficient of the heat carrier both sides of the wall surface, )
- : Heat transfer coefficient of steam, W/
- Heat resistance through tube wall and layer
- Heat load through pipe wall and layer
- Heat conductivity coefficient of stainless steel:
- Thermistor of the deposit layer in the pipe:
- Thermistor of the scale layer in the tube:
Determine the heat transfer coefficient of the water in the pipe:
Choose the speed of water in the pipe: = 0.6 (m/s)
Number of pipes in a water line:
: The correction factor depends on Re and the pipe ratio L and diameter d of the pipe.
Pr: Prandtl number standard of water at
: Prandtl number standard of water at mean wall temperature.
Heat transfer coefficient of water inside the pipe:
Heat load on the chilled water side:
Determine the heat transfer coefficient of the condensate outside the tube:
Contains no non-condensable air
Condensation on the outside of the tube
Average temperature of the condenser film:
Length of a heat transfer pipe:
Choose 8 m is the pipe’s length of this equipment.