• Energy Research

The energy-intensive hydrotreating processes, which operate at high pressure and high temperature, are widely adopted in refineries to improve the quality of oil products. In this paper, the energy and emission performance of a vacuum gas oil hydrotreating unit is investigated by the exergy load distribution analysis incorporated with process simulations. The quantitative relationship among the exergy loads, exergy efficiencies of the subsystems and the exergy efficiency of the entire system is obtained. Two operational optimisation strategies and one retrofit scheme for the entire system are proposed by adjusting the exergy loads and exergy efficiencies of the subsystems. Results show that the exergy efficiency of the entire system increases by 1.23% through the proposed operational optimisation strategies, and the CO2 emission of the entire system can be reduced by 56.45% through the proposed retrofit scheme.

Abstract Aiming to minimize the total cost of hybrid power system (HPS), a mathematical model for the configuration of battery energy storage system (BESS) with multiple types of batteries was proposed. The effects of battery types and capacity degradation characteristics on the optimal capacity configurations of the BESS and power scheduling schemes of the HPS were investigated. The effectiveness of the proposed model was verified and illustrated through a case study of a HPS with photovoltaic-wind-biomass-batteries. Results show that the optimal configuration of the BESS can be obtained by solving the proposed model, including the types and the capacities of batteries, and the power scheduling schemes of the batteries. For the integration of the HPS, the BESS with multiple types of batteries has certain economic advantages when compared with the BESS with a single type of batteries. In addition, the types and capacities of batteries in the BESS and the relative power scheduling schemes would change with the electricity curtailment rate of the renewable energy supply of HPS. The total cost of the HPS can be reduced at the expense of a certain amount of renewable energy supply.

Abstract A systematic strategy for retrofit of the multi-period heat exchanger network (HEN) on the basis of the multi-objective optimization is developed. In this three-stage procedure, a simplified multi-objective optimization model of the multi-period HEN is first established and then solved to target the retrofit, aiming to minimizing the total annual cost and total annual CO 2 emissions. The obtained Pareto front represents series of retrofit targets under different emission limitations, from which the most desirable one can be selected. The matching of the existing and the required heat exchangers is further implemented to finalize the retrofit, which will meet the practical retrofit requirements and matching restrictions. The application of the proposed procedure is illustrated through a case study of a HEN in a vacuum gas oil hydro-treating unit.

Abstract In process industries, a steam system is usually constructed to satisfy the demands of steam and electricity in a plant, in which steam turbines and electric motors are commonly used to drive rotating equipment, such as pumps and compressors. In this paper, a multi-objective optimization model for the design of a steam system with drivers option for rotating equipment is proposed to simultaneously minimize the economic and environmental objectives. It features that the energy performances of steam turbines and electric motors are embedded in the model, and the environmental impacts caused by both the material of equipment and the utilities consumption in the system are evaluated by life cycle assessment method. A steam system of the hydrotreating units in a refinery is taken as a case study to demonstrate the application of the proposed method. The Pareto front of the economic and environmental objectives is obtained, and three scenarios on the Pareto front are analyzed and discussed. Results show that appropriate drivers option can effectively reduce both the economic and environmental impacts of the steam system. In addition, the preference of steam turbines and electric motors in the system is intensively dependent on the two objectives. Consequently, the drivers option for the power demand in a plant should be determined before the entire steam system is designed and optimized.

Abstract In chemical industries, many processes have high energy consumption but low energy efficiencies. Quantitative methods are required to seek the unified basis for evaluating the energy performance of these processes. In this work, a systematic and versatile approach is proposed for benchmarking the energy consumption of these processes. The benchmarking approach to energy consumption is analyzed in detail from the perspective of systematic engineering. By determining relevant definitions and energy consumption indicators of the basic system and the system to be evaluated, the basic system is updated continuously. Based on this, the benchmark system and benchmark energy consumption of the system can be identified; the specific energy consumption can be determined and used to evaluate the energy performance quantitatively. This approach can be used to evaluate energy efficiency and energy saving potential of the energy-intensive chemical production system. A natural gas purification plant is studied to demonstrate the major procedure of energy benchmarking. The calculated specific energy consumption of purified natural gas is 4.64 × 10 2 kJ/m 3 , 23.1% less than that calculated by the literature method; that of sulfur is −2.14 × 10 3 kJ/kg, 35% greater than that calculated by the proposed method. Hence, the proposed benchmarking approach can be used for reducing the energy consumption and identifying the state-of-the-art energy utilization system.

This paper addresses the design of flexible multiperiod heat exchanger networks (HENs) when the disturbances of operating parameters in subperiods occur. In this work, a three-step method is developed, where a multiperiod HEN synthesis step, a flexibility analysis step, and a subperiod debottlenecking step are involved. In the proposed method, a nominal multiperiod HEN is first determined through the representative subperiod method. The flexibility index of the nominal multiperiod HEN in each subperiod is then analyzed to identify the bottlenecks that restrict the flexibility of the HEN. Finally, a subperiod debottlenecking model is solved to finalize the design of flexible multiperiod HEN. The application of the proposed method is verified through a case study of a multiperiod HEN in a vacuum gas oil (VGO) hydrotreating unit. Results show that the proposed method is effective to obtain a cost-effective multiperiod HEN with sufficient operating flexibility.

Abstract It is of significant importance to evaluate energy consumption and to optimize configuration of energy utilization in process systems. A comprehensive approach based on material flow analysis (MFA) and exergy flow analysis (EFA) is proposed to evaluate energy consumption and integrate energy utilization of a process system, in which energy-consuming devices and exothermic devices exist simultaneously. By using the proposed approach, the energy consumption of streams in the process system is evaluated by energy consumption distribution, and the energy efficiency of the system is improved by both the improvement of local sub-systems and the integration of energy utilization of the entire system. A natural gas purification plant with a processing capacity of 200 × 104 m3/d is taken as the case study to demonstrate the detailed implementation of the proposed approach. Results show that the streams with high energy consumption intensity are indicators of energy-intensive sub-systems, and energy conservation of these sub-systems can dramatically reduce the energy consumption of the system. In this case study, after the process improvement, the energy consumption of the desulfurization sub-system reduces by 9.62%, and the energy consumption of the exhaust gas treatment sub-system reduces by 21.02%. Furthermore, after the integration of heat exchange network of the entire system, the energy consumption of the system reduces by 2.16 × 105 kW. By these from the part to the whole strategies, the total energy-saving can reach 1.07 × 106 kW in the system. The proposed approach can be used for effectively identifying the bottlenecks of the energy consumption and improving the energy utilization of process systems.

For integration of hydrogen networks by mathematical programing, the utilization of a dedicated compressor in each match of hydrogen source and hydrogen sink can avoid the non-linear terms arising from the mixing inlet streams and the unknown pressures of compressors, which greatly reduced the complexity in solving the hydrogen network optimization models. However, it may lead to large number of compressors and higher capital costs. To solve this problem, a method of compressor merging is proposed in this work, in which two strategies of merging compressors are proposed, namely, the hydrogen sources oriented compressors merging strategy (HSCM) and the hydrogen sinks oriented compressors merging strategy (HKCM). The proposed strategies consider the upper capacity bound of a compressor and the capital cost of the compressors, and do not change the optimal matches of hydrogen sources and hydrogen sinks. A case study is employed to exemplify the implementation of the proposed method. The results show that the proposed strategies can reduce the number and the capital cost of compressors effectively. And the operating flexibility of the hydrogen network is also improved.