Study Of Cryogenic Process Based On The T-H Diagram

Refrigeration process optimization is important to the industrial sectors including chemical engineering. Taking cascade refrigeration systems, liquefied natural gas system, air separation system as example,the paper finds out the unreasonable energy utilization of process and puts forward corresponding solutions to achieve the integration and optimization of the system energy basing on the T-H diagram. Through the continuous improvement of the process,improve the relative position of logistics composite curve in the T-H diagram to form a "parallel" matching, in order to achieve the purpose of energy saving. The specific content and research contents are as follows:1. Conducted a study on a cascade refrigeration system with three different temperature out-put.By up-regulation of the degree of suction superheat and adjusting the condenser depression of three refrigerants in the cascade refrigeration system, designed a cascade refrigeration,resulting in a most efficient way of using refrigeration capacity during the regeneration of refrigerant, meanwhile latent heat generated from the evaporation of the refrigerant consumed by system itself was reduced, realized decrease of the refrigerant and reduction of the power dissipation. Energy matching condition was under the hot and cold logistics curves, equations of the refrigeration cyclic undercooling, the suction temperature of the compressor and the refrigerant use level were raised to calculate. Solving the equations can obtain the design parameters of a new process, which is applicable to different process conditions.2.Conducted a study on propane pre-cooled mixed refrigerant cycle LNG process.The energy integration and system optimization were carried out by application of T-H diagram. Changed the composition of the mixed refrigerants, the low boiling point of nitrogen was removed and reduced the low-temperature region of heat exchanging temperature difference. Improved the matching condition of hot and cold logistics curves in T-H flow diagram, reclaimed internal energy and exported high grade refrigeration capacity of low temperature by adjusting two logistics cooling temperature and the refrigerant throttle pressure. Through optimization and adjustment, total amount of cold was increased by10.8%and liquefaction of natural gas was increased by3.9%without fluctuation of power consumption3.Analyzed a200000Nm3/d air separation and liquefaction process by using LNG refrigeration capacity, assayed current T-H flow diagram, and fully utilized refrigeration capacity of low temperature which was not reasonably used in system. Increased condenser depression of liquid nitrogen at the top of rectifying column, so as to decrease the needs of refrigeration capacity produced by nitrogen refrigeration cycle, and the liquefaction process of nitrogen gas was changed, which was liquefied only in first heat exchanger and reduced consumption of refrigeration capacity at low temperature. In addition, throttle and compressor were added to vaporization process, the pressure and temperature of the LNG into the main heat exchanger were changed so that the consumption of LNG was reduced by10.2%. Although added a compressor, for the entire system, the total energy consumption is reduced and energy consumption of unit product was decreased by3.7%.The hot and cold logistics curve position in T-H diagram tends to be relatively parallel by adjusting the production process. Temperature difference between the hot and cold logistics was reduced, energy loss process was cut down, and cascade utilization of energy was realized. Besides these, the recovery of system internal energy was maximized, and improved the efficiency of overall system as well as the consumption of external energy was reduced. Our research has provided a new thinking for energy-saving during low-temperature process, with certain applying value.