• Energy Research

Ethylene oxide production process is one of the highest energy consumers in chemical industry, and therefore even a slight improvement in its overall efficiency can have a significant impact on the sustainability of the process. Efficiency improvement can be carried out using the exergy-aided pinch analysis outlined in this paper. The overall exergy loss distribution in different unit operations of an ethylene oxide process was first evaluated and mapped out in the form of “visualized exergetic process flowsheet”. An initial analysis of the four main functional blocks of the process showed that the exothermic reaction block contained the largest exergy loss (6043 and 428 kJ/kg of internal and external losses, respectively) which can be reduced by isothermal mixing, as well as increasing reaction temperature and reduction in pressure drop. The absorption block was also estimated to have the second highest contribution with total exergy losses of 3640 kJ/kg which were mainly due to the cooling column. These losses were then recommended to be reduced by improvements in the concentration and temperature gradients along the tower. Following the block-wise analysis, exergy analysis was then carried out for individual unit operations in each block to pinpoint the main sources of thermal exergetic inefficiency. Thermal solutions to reduce losses were also proposed in accordance with the identified sources of inefficiency, leading to a comprehensive list of cold and hot process streams that could be introduced to reduce losses. Finally, pinch analysis was brought into action to estimate the minimum energy requirements, to select utilities, and to design heat exchanger network. Thus, the methodology used in this work took advantage of both exergy and pinch analyses. The combined thermal-exergy-based pinch approach helped to set energy targets so that all the thermal possible solutions supported by exergy analysis were considered, preventing exclusion of any hot or cold process stream with high potential for heat integration during pinch analysis. Results indicated that the minimum cold utility requirement could be reduced from 601.64 MW (obtained via conventional pinch analysis) to 577.82 MW through screening of streams by the combined methodology.

Exergetic‐based solutions are proposed in this work to reduce the energy footprint of an alternative chlorine production process. By presenting exergy analysis results in the form of visualized exergetic process flowsheets using a solution panel, this study does not only find the low‐exergy‐efficient unit operations, but also proposes ways to improve the exergy efficiency based on the major sources of irreversibility of individual unit operations. Based on the amount of exergy losses, the alternative chlorine production process is divided into three main functional blocks. The total internal and external exergy losses for the entire process are calculated as 14,523.5 and 286.1 kJ/kg, respectively. The electrolysis block shows the highest potential for improvements with the largest exergy loss (responsible for more than 96.0% of total losses; with 14,093.2 kJ/kg of internal losses and 146.5 kJ/kg of external losses). Isothermal mixing, lower reaction temperature, and reduced pressure drop are suggested as avenues for improvement. Exergy losses in the gas treatment block with total exergy losses of 411.7 kJ/kg (equivalent to 2.8% of total losses) are mainly due to the cooling column (241.9 kJ/kg), where modifications in column might be helpful as concentration and temperature gradients along the tower are sources of exergy loss. The highest exergy loss in the feed pretreatment block with the total exergy losses of 158.2 kJ/kg (i.e., 1.1% of total losses) belongs to the mixers (106 kJ/kg) where isothermal and also isobaric mixing is required. © 2016 American Institute of Chemical Engineers Environ Prog, 35: 1512–1520, 2016

Abstract This paper presents a case study on the improvement of energy integration in a chlorine-caustic soda process based on the main sources of thermal exergy losses. Exergy analysis has been performed to pinpoint the causes of thermal exergetic inefficiency. Thermal solutions have been then developed, leading to a comprehensive list of cold and hot process streams. Finally, pinch analysis has been brought into action to estimate the minimum energy requirement, to select utilities and to design heat exchanger network. As a result, the combined methodology followed here takes advantages of both exergy and pinch analyses. This bilateral thermal-exergy-based pinch approach helps to set energy targets in a way that all the possible thermal solutions supported by exergy analysis are considered, including all hot and cold process streams that have a high potential for heat integration during pinch analysis. To demonstrate this, energy targeting through conventional pinch analysis leads to 7.74 MW and 13.00 MW of hot and cold utility energy demand, respectively. These figures change to 8.17 MW and 0.40 MW of hot and cold utility energy demand, respectively through streams screening by the combined methodology.

Ammonia production through more efficient technologies can be achieved using exergy analysis. Ammonia production is one of the most important but also one of most energy consuming processes in the chemical industry. Based on a panel of solutions previously developed, this study helps to identify potential areas of improvement using an exergy analysis that covers all aspects of conventional ammonia synthesis and separation. The total internal and external exergy losses are calculated as 3,152 and 6,364 kJ/kg, respectively. The process is then divided into five main functional blocks based on their exergy losses. The reforming block contains the largest exergy loss (3,098 kJ/kg) and thus the largest potential for improvement including preheating cold feed through an economizer, developing technology towards isobaric mixing, and pressure drop reduction in the secondary reformer as the main contributors to the irreversibility (1,302 kJ/kg) in this block. The second largest exergy loss resides in the ammonia synthesis block (3,075 kJ/kg) where solutions such as reduced temperature rise across the compressor, proper compressor isolation, reducing undesired components such as argon in the reactor feed, and using lower temperatures for reactor outlet streams, are proposed to decrease the exergy losses. Throttling process in the syngas separator is the key contributing mechanism for the irreversibility (1,635 kJ/kg exergy losses) in the gas upgrading block. The exergy losses in the residual ammonia removal block (833 kJ/kg exergy losses) are mainly due to the stripper and the absorber column where a modified column design might be helpful. The highest exergy loss in the preheating block belongs to the compressors (518 kJ/kg exergy losses) where a lower inlet temperature and better system isolation could help to reduce losses.

AbstractThis paper presents a case study on improving the energy integration of a gas‐based ethylene process by focusing on major sources of thermal exergy losses. Exergy analysis is used to find the main sources of thermal exergetic inefficiency. Thermal solutions are then developed, leading to a comprehensive list of cold and hot process streams that could potentially reduce exergy losses from these sources. Pinch analysis is then carried out to screen these streams so only those, which can minimize the energy requirement, are maintained. Besides this, pinch analysis is used to select utilities and to explore cogeneration potential. Thus, the methodology used in this paper takes advantage of both exergy and pinch analyses, in a way that all the possible thermal solutions supported by exergy analysis are considered, preventing exclusion of any hot or cold process stream with high potential for heat integration during pinch analysis.

The environmental burdens of the ethylene oxide production processes are becoming more and more important due to the release of very harmful chemical components as well as its high-energy demand. One way to moderate its environmental burdens within the energy transition period is the natural gas/biomass-based scenarios. However, this Life Cycle Assessment (LCA) study reports that natural gas is not a right alternative for this special case, where natural gas-based scenarios are less sustainable than the residual fuel oil-based scenarios particularly concerning fossil depletion (93%), freshwater ecotoxicity (76%), marine ecotoxicity (59%), human ecotoxicity (53%), terrestrial acidification (51%) and particulate matter formation (40%). On the other hand, the LCA study shows that without revamping the heart of the process technology, the reduction in the environmental burdens is possible through biomass. The biomass-based scenarios reduce the burdens from 4.40 to 4.36 MJ (equivalent of non-renewables) according to Cumulative Exergy Demand or from 2.18E-04 to 1.85E-04 (dimensionless normalized results) in accordance with ReCiPe, preparing the way to a sustainable ethylene oxide process within the energy transition period where revamping the heart of the process technology is not desired. Graphic abstract

Abstract High energy consumption is one of the main challenges of the chemical industry. The energy footprint of most processes can, however, diminish through the solutions presented in the current paper, leading to cleaner ways of producing chemicals. Ethylene production process is selected as a case study to demonstrate the approach as it is one of the most energy consuming processes in the chemical industry. The study involves an exergetic diagnosis and does not only find the low-exergy-efficient unit operations, but also proposes tools to improve these units based on their key sources of irreversibility. For the ethylene production process, this is conducted by first splitting the flowsheet into four main functional blocks (namely cracking, compression, refrigeration, and separation and purification) according to their exergy losses. This results in identifying the cracking block as the most inefficient block with more than 45% of total exergy losses and thus the first block to be improved so that overall losses reduce (examples of which include increasing the number of furnace tubes while reducing their lengths). Although the compression block is found to have the lowest contribution to internal exergy losses, the inefficient unit operations such as the water cooler (with an exergy loss of 214 kJ/kg) can still be improved through solutions such as system isolation. The refrigeration block is also shown to have the second highest exergy losses with its ethylene and propylene compressors being the main contributors. Solutions are again provided to improve the block performance with specific focus on intercooler design improvement and system isolation. Finally, exergy losses in the purification and separation block are identified to be mainly due to demethanator, deethanator, and ethylene column where modifications in column design might be helpful as concentration and temperature gradients along the towers are the main sources of exergy losses. The approach used in the current study can also be applied to other chemical processes and the findings suggest that even for a well-developed process technology, there is still opportunity for thermodynamically justifiable energy efficiency improvements. Therefore, it is important for process developers to continuously revisit existing processes, in order to ensure lessons learned in one area can be applied to another one. Using a panel of solutions, which has been constructed from a number of previous case studies helps to make this approach more systematic and user-friendly.