Thermodynamic design of a cascade refrigeration system of liquefied natural gas by applying mixed integer non-linear programming

Liquefied natural gas(LNG) is the most economical way of transporting natural gas(NG) over long distances. Liquefaction of NG using vapor compression refrigeration system requires high operating and capital cost. Due to lack of systematic design methods for multistage refrigeration cycles, conventional approaches to determine optimal cycle are largely trial-and-error. In this paper a novel mixed integer non-linear programming(MINLP)model is introduced to select optimal synthesis of refrigeration systems to reduce both operating and capital costs of an LNG plant. Better conceptual understanding of design improvement is illustrated on composite curve(CC) and exergetic grand composite curve(EGCC) of pinch analysis diagrams. In this method a superstructure representation of complex refrigeration system is developed to select and optimize key decision variables in refrigeration cycles(i.e. partition temperature, compression configuration, refrigeration features, refrigerant flow rate and economic trade-off). Based on this method a program(LNG-Pro) is developed which integrates VBA,Refprop and Excel MINLP Solver to automate the methodology. Design procedure is applied on a sample LNG plant to illustrate advantages of using this method which shows a 3.3% reduction in total shaft work consumption.

Performance Study of a Novel Ejector-Cascade Refrigeration System with Regenerator

In this paper,a novel NH_(3)/CO_(2)ejector-cascade refrigeration system with regenerator is proposed,which can recycle the waste heat at the outlet of the compressor.After establishing the mathematical model of the system,the theoretical energy and exergy analysis are carried out and compared with the conventional cascade refrigeration system.It is concluded that compared with the conventional cascade refrigeration system,the novel ejector-cascade refrigeration system with regenerator has the advantages of less power consumption of the compressor,less component exergy destruction,high system performance,and is more suitable for working at a lower temperature.Under the working conditions studied in this paper,compared with the conventional cascade refrigeration system,the COP of the novel ejector-cascade refrigeration system with regenerator is increased by9.58%;the exergy efficiency is increased by 9.50%,and the optimal evaporation temperature is about-45℃.

Thermodynamic Analysis and Comparison on Low Temperature CO_2-NH_3 Cascade Refrigeration Cycle

This paper is focused on the cascade refrigeration cycle using natural refrigerant CO 2-NH 3. The properties of refrigerants CO 2 and NH 3 are introduced and analyzed.CO 2 has the advantage in low stage of cascade refrigeration cycle due to its good characteristics and properties. The thermodynamic analysis results of the CO 2-NH 3 cascade refrigeration cycle demonstrates that the cycle has an optimum condensation temperature of low stage and also has an optimum flow rate ratio.By comparing with the R13-R22 and NH 3-NH 3 cascade refrigeration cycles, the mass flow rate ratio of CO 2-NH 3 is larger than those of R13-R22 and NH 3-NH 3, the theoretical COP of CO 2-NH 3 cascade refrigeration cycle is larger than that of the R13-R22 cascade cycle and smaller than that of the NH 3-NH 3 cascade cycle. But the real COP of CO 2-NH 3 cascade cycle will be higher than those of R13-R22 and NH 3-NH 3 because the specific volume of CO 2 at low temperature does not change much and its dynamic viscosity is also small.

Novel Cascade Refrigeration Cycle for Cold Supply Chain of COVID-19 Vaccines at Ultra-Low Temperature -80°C Using Ethane (R170) Based Hydrocarbon Pair

Several media report highlight on that the pharmaceutical companies require ultra-low temperatures -80°C to transport and store its COVID-19 vaccines. This research presents the thermodynamic analysis on cascade refrigeration system (CRS) with several refrigerant pairs which are R32/R170, R123/R170, R134a/R170, R404A/R170, R407c/R170, R410/R170, and the hydrocarbon (HC) refrigerant pair Propane/Ethane, namely R290/R170. Besides, the results of R22/R170 pair, which is not recommended to be used due to phase out of R22 as per Montréal Protocol, are included as base case to compare the novel hydrocarbon pairs in CRS and the old trend of refrigerant pairs. Thermodynamic properties of all these pairs were investigated and compared under different intermediate temperature used in CRS heat exchanger, which thermally connected both the Low and High temperature cycles (LTC) and (HTC). By applying the first law of thermodynamics, the coefficients of performance (COPs) and the specific power consumptions (SPC) in kW/TR are presented and compared. In addition, by applying the second law of thermodynamics the exergetic efficiencies were estimated. The results reveal the promising opportunity of using the HC pair (R290/R170). The minimum SPC in kW/TR is recorded for the pair R123/R170. One the other hand, the highest exegetic efficiency values are observed to be 40%, 38%, and 35% for the pairs R123/R170, R290/R170, and R134/R170, respectively. This research concludes that the HC pair (R290/R170) is highly recommended for CRS applications either to transport the COVID-19 or store it in cold storage rooms in hospitals and clinics. All precautionary measures should be carefully applied in design and operation of HC pair (R290/R170) due to its flammability hazard.

Energy and Exergy Analysis for Low-Temperature Refrigeration System for Biogas Upgrading

Low-temperature refrigeration system for biogas upgrading has been developed by the Cryo Pur company based on cooling biogas in three steps： Removing most of the water content at -40 ℃, removing siloxanes and SVOCs at -85 ℃ and frosting CO2 at temperatures varying from -90 ℃ to -120 ℃. This process transforms biogas containing typically 60% methane, 35% CO2, 5% water vapor in methane containing 2.5% of CO2. This paper studies how a single low-temperature refrigeration system is able to cool biogas with an indirect system using low-temperature heat-transfer fluids. The exergy study defines the exergy losses and served as guidance for the energy/pinch analysis that is used for the design of the heat-exchanger series and the appropriate heat recovery. An optimal system could save up to 40% of the electric consumption of the refrigeration system.

Experimental Investigation of the Ejector Refrigeration Cycle for Cascade System Application

Ejector refrigeration cycle(ERC)with advantages of simple structure and low cost holds great application potential in cascade/hybrid cycles to improve the overall system performance by removing or recovering the heat from the main cycle.In this paper,a theoretical and experimental investigation of the ERC as a part of a cascade system was carried out.The operating parameters were optimized.The experimental ERC test rig was designed,developed and investigated at high evaporating temperatures and wide ranges of operating conditions.The influence of operating conditions on the efficiency of the ejector and ERC was analyzed.Experimental results and analysis in this study can be helpful for the application and operating condition optimization of ERC in cascade/hybrid refrigeration systems.

Numerical investigation of condensation of gasoline vapor with turbulent flow in vertical tubes

To investigate the characteristics of the condensation in gasoline vapor condensation recovery,the condensation process of gasoline vapor with turbulent flow in a vertical tube is simulated based on the gas-liquid two-phase flow model.An effective diffusion coefficient is used to describe mass diffusion among the species of gasoline vapor.Several variables including temperature,pressure,liquid film thickness and the variation of the Nusselt number in the tube are simulated.The effects of the inlet-to-wall temperature difference and the Reynolds number on the condensation rate and the Nusselt number are obtained by modelling.The results show that heat transfer and condensation can be enhanced significantly by increasing the inlet Reynolds number.However,the increase in the inlet-to-wall temperature difference has little effect on the condensation rate.It is also found that the gasoline vapor condensation rate is influenced greatly by the mass transfer resistance.The comparison of results from the model with previous experiments shows a good agreement.

Experimental study on a -60 °C cascade refrigerator with dual running mode

In this study,an experimental setup is designed and built to investigate the feasibility and performance of the proposed dual-mode cascade refrigeration cycle.The apparatus can be operated in two modes:dual-stage mode and single-stage mode such that the low temperature cycle(LTC) can be operated together with the high temperature cycle(HTC) or can run independently.Experimental results validate the feasibility of independent operation of LTC.The performance of the independent operation of LTC mode is theoretically investigated using experimental data as bases.Detailed suggestions are made for the improvement of the coefficient of performance(COP) of the experimental system.Theoretically,high COP of the cycle provides excellent application for the presented refrigeration cycle.