Synthesis of work exchange networks for gas processing applications `1

The fast depletion of oil reserves, rising demand of energy, uncertainties in energy prices, technologies barriers in renewable energy, concerns over energy security,... In addition, LNG

CHAPTER 1

INTRODUCTION

world’s population has seen great increases over the years For instance, in the past decade alone, energy consumption has increased from 2573.8 mtoe (million tonnes of oil

approximately 55% in a decade, averaging over 5% per annum Such energy trends are expected to further increase into the future as many developing countries will continue to industrialize and experience rapid growth in population as well as improving living

2004 to 2030 In the USA, the total primary energy consumption is expected to grow 0.5% annually from 2007 to 2030 The industrial sector was the largest consumer of

The fossil fuels (oil, coal, and NG) currently supply over 85% of the worldwide energy demand while the rest is supplemented by nuclear and green initiatives such as solar, wind, bioenergy, and hydroelectricity The use of fossil fuels is projected to rise by

environment is significant Our first line of defense against these emissions and the resulting climate change is energy conservation (or using these fuels as efficiently as possible) The fast depletion of oil reserves, rising demand of energy, uncertainties in energy prices, technologies barriers in renewable energy, concerns over energy security,

and strict environmental regulations on CO2 emissions are among the several driving forces underlying the need for energy conservation

1.1 LNG Processes

Among the aforementioned fossil fuels, NG seems to be the most promising source of energy to power the future economy NG consists predominantly of methane (approximately 90%) with small amounts of ethane, propane and nitrogen as well as trace quantities of heavier hydrocarbons such as butane and pentanes It is currently the cleanest burning fossil fuel, producing significantly lower amounts of carbon dioxide,

global warming and the push for greener initiatives, NG seems to be the ideal short term solution to the global pollution problems or at least until other greener initiatives become

the fastest growing energy source and its consumption is expected to increase over 48%

between year 2006 and 2030

Most NG reserves are stranded and located offshore, far away from any demand centers Hence, the transportation of NG from the point of production to the markets is a technical and commercial challenge A popular solution would be to transport compressed NG through pipelines However, such pipelines are geographically restricted and may not be economically feasible In addition, energy security may be easily compromised An attractive alternative would be to liquefy NG into LNG and transported

makes long distance transport of LNG technically and economically possible An LNG

tanker has the capacity of transporting an equivalent of several billion cubic feet of NG It was reported in year 2008 that at least 27.8% of international trade in NG is in the form

The adoption of LNG as a mode of transportation in the early 1960s and 1970s were slow due to high capital costs of LNG facilities which includes liquefaction trains, storage facilities, and loading/unloading jetties In addition, LNG production is an energy intensive process due to the need for large compressors to provide refrigerant between ambient temperature and cryogenic LNG temperature of 111K Therefore, energy efficiency of the liquefaction process is an immediate concern in any LNG project or operation Classical technologies include the cascade process by ConocoPhillips where refrigeration is provided through a cascade of pure refrigerants Other processes utilize

refrigeration system Technological and process advancements have driven down the energy requirements and improved energy efficiencies to approximately 5.5 to 6.0 kWh

still relatively more costly in comparison to other fossil fuels such as crude oil and coal With increasing energy costs and stricter environmental protection legislations, further improvements and optimization of LNG processes are necessary and has been gathering the attention of academic and industrial researchers all around the world

In view of this, it is, therefore, in the interest of this work to pursue the

unique opportunity to study the optimal operation of the system as it had not been the subject of extensive studies by academic researchers as compared to more established processes such as the cascade and C3MR processes As compressors are the preeminent consumers of energy, a systematic optimization approach to minimize compressor power

operations and power consumption A brief explanatory is provided to understand the basics of the refrigeration process

1.2 Refrigeration Process

Refrigeration is a process in which work is done to move heat from one stream to another It includes five basic components refrigerant fluid, evaporator, compressor, condenser, and expansion valve In order to operate the process successfully, each component must be optimally operated

Figure 1.1 represents a simple (one cycle) refrigeration cycle Refrigerant removes heat from the product in the evaporator by vaporizing itself Then, refrigerant passes through compressor to obtain higher pressure so that it can remove heat to condenser utilizing cooling water This pressure is very important for the condensation of the refrigerant in the condenser and depends on the cooling water inlet temperature For example, if cooling water temperature increases, then high compressor outlet pressure is required for refrigerant and thus, increases power consumption by the overall refrigeration cycle Then, condensed refrigerant passes through a throttling valve where it undergoes further expansion to reduce its temperature These condensed, low temperature, and low pressure refrigerant enters evaporator to provide cooling duty to the process stream

The following sections discuss about the energy intensive compression and expansion process and the importance of LNG processes

1.3 Compression and Expansion Processes

Plants are required to compress gas to meet process conditions These compression works are done by different types of compressors such as reciprocating compressor, and centrifugal compressor Note that this work is intended to centrifugal compressors that are being mostly used in chemical industries and thus, we limit our discussion in this type only Compression process is highly energy intensive in many chemical plants (Table 1.1) such as gas processing and transportation, Liquefied Natural Gas (LNG), refineries, petrochemicals, air enrichment, ammonia, and fertilizer Due to its highly energy intensive nature, we should ensure optimal design, synthesize, and operation of the compressor network However, its performance depends on several factors such as stream

temperature, pressure, composition, compressor type, speed, availability etc which make its design and operation much more complex

Table 1.1 Industrial applications of centrifugal compressors

Syn-Gas Make-up/Recycling

On the other hand, expansion of gases can be executed in different ways such as valves or turbines If valves are utilized for expansion, energy cannot be utilized, whereas turbines facilitate utilization of available pressure energy However, utilization of any amount of work will not be feasible comparing to plant economics

Figure 1.2 Work exchange between a centrifugal compressor and turbine

It is understood from the abovementioned definition that compression and expansion processes are opposite to each other where compression demands work and expansion offers work This situation offers an integration of energy among streams utilized in compression and expansion processes This work can be exchanged by using a single shaft (Figure 1.2) where compressor is placed in one side and turbine in another Turbine converts pressure and temperature energy to mechanical energy and rotates shaft This shaft, then, rotates compressor where inlet stream to compressor is compressed and thus, converted to pressure and temperature energy to mechanical energy Note that these conversions of energy impose conversion losses in the process and make it more challenging with respect to plant economies Other than this, operational constraints of compressor and turbine force this exchange much more complex and challenging

1.4 Research Objectives

presents a unique opportunity to study the optimal operation of the system as it had not been the subject of extensive studies by academic researchers as compared to more established processes such as the cascade and C3MR processes Hence, the objective of

optimal set of operating conditions resulting in lower compressor power requirements shall be obtained

As it is understood that compressors are the major consumer of energy for most of the chemical and petrochemical processes, the subsequent work focuses on the optimization of energy intensive compressor networks to utilize available process energy This work focuses on the exchanging available process energy among different streams via compressors and turbines placed in a single shaft that ultimately reduces the utility consumption and thus total cost of the network For this purpose, we develop an MINLP that incorporates thermodynamic and operational constraints to exchange realistic work

by minimizing total cost of the network

This thesis includes five chapters The current section is Chapter 1, where a brief

such as heat exchange network, mass exchange networks, etc existing in the literature

This section also outlines the technical and commercial challenges of the aforementioned processes

Next, Chapter 3 describes the challenges, simulation, formulation, and

advance discussions and improvements Chapter 4 illustrates the challenges and development of model for WENS Moreover, solution strategy is presented in the corresponding section as commercial solvers failed to generate optimal solution The application of this work is demonstrated in several case studies

Finally, Conclusions and Future Works are highlighted in Chapter 5 to provide several ideas, challenges, and scopes for the extension of these studies