Detailed Exergy Analysis Research Articles

Integration of a turbine expander with an exothermic reactor loop—Flow sheet development and application to ammonia production

This paper investigates the direct integration of a gas turbine power cycle with an ammonia synthesis loop. Such a loop represents a typical reactor–separator system with a recycle stream and cold separation of the product from the recycle loop. The hot reaction products are expanded directly instead of raising steam in a waste heat boiler to drive a steam turbine. Two new combined power and chemicals production flow sheets are developed for the process. The flow sheets are simulated using the flow sheet simulator AspenPlus (licensed by Aspen Technology, Inc.) and compared to a simulated conventional ammonia synthesis loop. The comparison is based on energy as well as exergy analysis. It was found that the pressure ratio over the turbine expander plays an important role in optimisation of an integrated system, specifically due to the process comprising an equilibrium reaction. The inlet temperature to the reactor changes with changing pressure ratio, which in turn determines the conversion and consequently the heat of reaction that is available to produce power. In terms of the minimum work requirement per kg of product a 75% improvement over the conventional process could be obtained. The work penalty due to refrigeration needed for separation was also accounted for. Furthermore this integrated flow sheet also resulted in a decrease in exergy loss and the loss was more evenly distributed between the various unit operations. A detailed exergy analysis over the various unit operations proved to be useful in explaining the overall differences in exergy loss between the flow sheets.

Exergy simulation and optimization of adiabatic and diabatic binary distillation

A detailed exergy analysis of a distillation system has been conducted. This analysis is divided in four parts: (1) adiabatic rectification column; (2) adiabatic stripping column; (3) diabatic rectification column; and (4) diabatic stripping column. The results of all cases are presented in this paper. This analysis concerns the mixture water–ethanol working at 1 bar. The main objective is to determine the distribution of exergy losses inside the column and the optimal distribution of heat to be transferred inside the column in order to produce the minimum overall exergy losses.

Exergy Analysis of a Reactive Distillation MTBE Unit

In this paper application of exergy analysis to the reactive distillation system of a Methyl TerButyl Ether (MTBE) production unit of a crude oil refinery is presented. In a refinery, the MTBE is obtained from methanol, and butanes (isobutylenes) produced in the fluidized catalytic cracking (FCC) unit. The reactives (isobutylenes) after purification are introduced to the main reaction system, the products of which are sent to the reactive distillation system to complete the reaction. A top rectification section, a bottom stripping section and a medium reaction section, compose the reactive column. The results of the exergy analysis of the unit indicate that the main exergy losses (about 63%) of the MTBE plant occur in the reactive distillation system, particularly in the distillation column itself and in its associated condenser. A detailed exergy analysis of the system, is presented in this paper. This paper was presented at the ECOS'00 Conference in Enschede, July 5-7, 2000

Exergetic criteria in process optimisation and process synthesis: Opportunities and limitations

The exergy concept is a well established way to express the quality of any kind of process stream, be it energy or matter. Thus detailed exergy analysis of chemical processes can in general be performed and several such applications have been reported in the literature. However, systematic methods for exergetic process optimisation only exist for the comparably less complex area of power systems. For chemical processes a solid optimisation approach based on exergy analysis has yet to be derived. This paper presents a thorough investigation of opportunities and limitations of the exergy concept in both process optimisation and process synthesis. The two tasks are treated separately since their specific requirements and solution strategies are different. As a result of the presented evaluation, concepts for the efficient use of exergetic criteria are proposed.

Exergy analysis of chemical-looping combustion systems

A power plant based on chemical-looping combustion offers both a possibility of high net power efficiency and separation of the greenhouse gas CO2. This is due to the way the oxidation of the fuel takes place. Instead of oxidizing the fuel with oxygen from the combustion air, the fuel is oxidized by an oxygen carrier, i.e., an oxygen-containing compound. The oxygen carriers that have been suggested in previous studies are metal oxides like NiO, Fe2O3 and Mn3O4. The reduced oxygen carrier is in the next step reoxidized by air in a second reactor and then recirculated to the first reactor. In this way, fuel and air are never mixed and the fuel oxidation products CO2 and water leave the system undiluted by air. All that is needed to get an almost pure CO2 product is to condense the water vapour and remove the liquid water.Chemical-looping combustion (CLC) is also claimed to reduce the fuel exergy destruction in the overall reaction of combustion of the fuel. This gives a possibility to increase the net power efficiency.This paper gives an introduction to chemical-looping combustion. Results from simulations and a detailed exergy analysis of two different CLC gas turbine (GT) systems are also presented. The first system utilizes methane as a fuel and NiO as oxygen carrier. The second system utilizes a fuel gas mixture consisting mainly of CO and H2, simulating a fuel gas from for instance coal gasification. Results for this system are given for simulations with both NiO and Fe2O3 as oxygen carrier. The two systems are compared to comparable simulated systems with conventional combustion of the same fuel. The exergy analysis shows that the irreversibilities generated upon combustion of the fuel are reduced. The net power efficiency of the CLC–GT systems is similar or higher than for the corresponding GT systems with conventional combustion. The net power efficiency of CLC systems could be even further increased if the exergy remaining in the exhaust could be utilized in an efficient way.

95/00885 IAPCS: A computer model that evaluates pollution control systems for utility boilers

Natural gas combined cycle (NGCC) power plants with pre-combustion CO2 capture called, integrated reforming combined cycle (IRCC) power plants could be operated in a flexible and environment friendly manner by incorporating hydrogen storage. The current article presents a thermodynamic model evaluation of such a flexible large scale power plant system with co-production of hydrogen and subsequent hydrogen storage with metal hydrides (MH). A comparative analysis has been presented using steady state ASPEN Plus models for a Reference NGCC system, an IRCC system and a Flexible IRCC system with MH (MgH2) based hydrogen storage. In addition, a detailed exergy analysis indicating irreversibilities in the system has been presented. The system study indicates that such Flexible IRCC systems could be operated with a time based average (considering charging and discharging) electrical efficiency above 45% depending on the amount of hydrogen stored, appropriate heat integration and choice of the metal hydride. Exergy analysis indicates that such flexible systems could still be operated at a relatively high average net exergy efficiency (considering charging and discharging). Addition of MH based H2 storage in an IRCC system does not lead to significant electrical/exergy efficiency penalty. Carbon capture and storage is a larger source of efficiency penalty than the MH based H2 storage unit. The largest exergy destruction is observed in the gas turbine combustor and reformer units, providing additional scope for efficiency enhancement. With a larger share of renewable power in the total electrical power production expected in future, Flexible IRCC systems as presented in this article could be used as an efficient solution to address the large fluctuations in power supply and demand. Additional investigations are however encouraged further toward optimizing the choice of the metal hydride, detailed design of a single heat integrated system (including part load behavior) and economic assessments based on the heat and mass balances developed in this study.

95/00867 Extraction behavior of polycyclic aromatic hydrocarbons adsorbed on waste incinerator fly ash

Natural gas combined cycle (NGCC) power plants with pre-combustion CO2 capture called, integrated reforming combined cycle (IRCC) power plants could be operated in a flexible and environment friendly manner by incorporating hydrogen storage. The current article presents a thermodynamic model evaluation of such a flexible large scale power plant system with co-production of hydrogen and subsequent hydrogen storage with metal hydrides (MH). A comparative analysis has been presented using steady state ASPEN Plus models for a Reference NGCC system, an IRCC system and a Flexible IRCC system with MH (MgH2) based hydrogen storage. In addition, a detailed exergy analysis indicating irreversibilities in the system has been presented. The system study indicates that such Flexible IRCC systems could be operated with a time based average (considering charging and discharging) electrical efficiency above 45% depending on the amount of hydrogen stored, appropriate heat integration and choice of the metal hydride. Exergy analysis indicates that such flexible systems could still be operated at a relatively high average net exergy efficiency (considering charging and discharging). Addition of MH based H2 storage in an IRCC system does not lead to significant electrical/exergy efficiency penalty. Carbon capture and storage is a larger source of efficiency penalty than the MH based H2 storage unit. The largest exergy destruction is observed in the gas turbine combustor and reformer units, providing additional scope for efficiency enhancement. With a larger share of renewable power in the total electrical power production expected in future, Flexible IRCC systems as presented in this article could be used as an efficient solution to address the large fluctuations in power supply and demand. Additional investigations are however encouraged further toward optimizing the choice of the metal hydride, detailed design of a single heat integrated system (including part load behavior) and economic assessments based on the heat and mass balances developed in this study.