Mixed Refrigerant Joule-Thomson Research Articles

Effect of Heat Exchanger and Capillary Geometry on the performance of Joule- Thomson Refrigerators operating with different mixtures

Abstract Joule-Thomson (J-T) refrigerators or J-T cryocoolers are extensively used in many low-temperature applications. J-T refrigerators operating with nitrogen-hydrocarbon (N2-HC) refrigerant mixtures offer several advantages, such as low operating pressures (<20 bar), high exergy efficiency, no moving parts in the cold section, and low cost. The cooling power or cooling capacity of the J-T refrigerator depends on the hardware used as well as the refrigerant composition. The proposed work focuses on estimating the cooling capacity of a mixed refrigerant J-T (MRJ-T) refrigerator of the given hardware and specified refrigerant. An iterative steady-state full-cycle simulation procedure has been presented in this work to simulate the complete system and estimate the cooling capacity, taking into account the possibility of choking of the expansion capillary. Some of the results have been validated against experimental results of an MRJT refrigerator available in the open literature. The details of the simulation model and the results of our studies on the prediction of stable operating range, maximum cooling capacity, the effect of heat exchanger geometry, expansion capillary geometry, mixture composition, and choking of the refrigerant mixture on the performance of an MRJ-T refrigerator are presented in this paper

Influences of distributed J-T effect on the performance of mixed-refrigerant Joule-Thomson refrigeration cycle

In a mixed-refrigerant Joule-Thomson (MRJT) cycle, the working fluid usually produces a throttling effect in the expansion element. However, the high-pressure flow in the recuperator could also generate a throttling effect due to the narrow channels, i.e., the distributed Joule-Thomson effect (DJTE). Previously published studies indicated that the cooling performance of cryocoolers would be improved by DJTE accompanying the heat recovery process. However, most of the current research on this phenomenon is focused on the heat transfer characteristics of the cryocooler itself, with a lack of research on the effect of DJTE on the cycle performance in much detail.Therefore, a detailed investigation of the influence pattern of DJTE on the cycle performance is carried out by combining with the exergy analysis. The simulation is conducted under three quintessential conditions of the recuperator. The results indicate that it is possible to manipulate the DJTE to maintain satisfactory cycle performance under specific conditions. In addition, at a constant minimum temperature difference in the recuperator, the maximum achievable DJTE is closely related to the isothermal throttling effect of the mixed refrigerant. The conclusions of this paper clarify the effect and the application value of DJTE on the cycle efficiency, which provide a reference for determining reasonable operating conditions and ensuring the cycle efficiency in practical applications.

Design of nonflammable mixed refrigerants based on the partial molar enthalpy difference in mixed-refrigerant Joule-Thomson refrigerators with/no pre-cooling stage

Mixed-refrigerant Joule-Thomson refrigeration (MJTR) is an important cooling method at temperatures from 80 to 230 K. It can be used in cryosurgery, high-temperature superconductivity and sensor cooling etc. However, most of the studies are on nitrogen-hydrocarbon refrigerants, which are not allowed in applications where there is a special need to avoid flammability risks. Therefore, non-flammable mixed refrigerants were investigated in this study.The composition of the mixed refrigerant is a key factor in the system performance (refrigeration temperature, refrigeration capacity, etc). However, the purely mathematical optimization methods lack system knowledge in the optimization of the process, and may have the disadvantages of being a time-consuming process. In this study, the isothermal throttling effect of mixed refrigerants is optimized, and the optimization process is based on the partial molar enthalpy difference of each component. The method is based on thermodynamic properties and is time-saving relative to purely mathematical optimization techniques.Non-flammable mixed refrigerants (NFMR) with refrigeration temperatures from 100 K to 140 K were designed in this study. The results show that the designed mixed refrigerant has a higher COP compared to the reference. Argon has an advantage at refrigeration temperatures from 120 K to 140 K, while nitrogen has an advantage from 100 K to 120 K. In addition, mixed refrigerants for systems with pre-cooling stage were optimized and the results showed that the highest exergy efficiency is achieved at a pre-cooling temperature of 250 K. The exergy efficiencies with pre-cooling stage are nearly twice as high as those without that. Therefore, a pre-cooling stage for nonflammable mixed refrigerants is necessary where there is no requirement for system size.

Compositions optimization method based on the effective refrigeration effect for Mixed-refrigerant Joule-Thomson refrigerators cooling distributed-temperature heat loads

Mixed-refrigerant Joule-Thomson refrigeration (MJTR) is the dominant technology for cooling distributed-temperature heat loads, such as natural gas liquefaction. The mixed refrigerant determines the refrigeration cycle's performance (cooling temperature and efficiency, etc.). However, the current mathematical methods for mixed refrigerants’ optimization are time-consuming. In this paper, a mixed refrigerant optimization method based on the effective refrigeration effect is proposed. The equivalent enthalpy (EH˙) of temperature difference is proposed to predict the temperature difference distribution in the recuperator without cycle simulation. The optimization objective is to optimizeEH˙much closer to zero. In each step, only one component is adjusted, which is determined through its’ effective refrigeration effect, i.e., the partial molar enthalpy difference, in the optimization process. This method is applied to natural gas liquefaction (LNG), nitrogen liquefaction (LN2) and Cryo-compressed hydrogen storage (CcH2) systems. The exergy efficiencies in LNG, LN2 and CcH2 systems are 0.462, 0.390 and 0.421, which increase about 23.9%, 22.4% and 23.5% compared to references, respectively. The optimization time is less than 10 min per case in this study. This method can optimize mixed refrigerants in MJTR cooling various distributed-temperature heat loads.

EXPERIMENTAL INVESTIGATION ON FLOW CONDENSATION HEAT TRANSFER OF METHANE/ETHYLENE MIXTURE INSIDE A HORIZONTAL SMOOTH TUBE

Methane (R50) and ethylene (R1150) are important components in mixed-refrigerant Joule-Thomson (MRJT) refrigeration systems. Among many efforts to improve MRJT refrigerators, the work focusing on the properties of condensing heat transfer in mixtures is essential. Therefore, this paper aims to investigate these characteristics of high-temperature glide mixture R50/R1150 (26.7/73.3% molar fraction) inside a horizontally placed smooth tube. The experiments were done for varying saturation pressures (1.7-2.3 MPa), mass fluxes (101-391 kg m<sup>-2</sup> s<sup>-1</sup>), and the entire range of vapor qualities. The results indicate that higher heat transfer coefficients (HTCs) occur at larger mass fluxes and smaller saturation pressures. In addition, the effect of temperature glide was discussed by comparing the HTCs of R50/R1150 with those of R50/R170 reported in the previous literature under the same test condition. It is indicated that higher temperature glide contributes to worse heat transfer performance. Moreover, six existing correlations combined with the equilibrium model were employed to predict the HTCs of R50/R1150. All predictions exhibit an underestimation of the HTCs, and only Dobson and Chato correlation and Shah correlation presented reasonable predictions with a mean absolute relative deviation (MARD) under 30%. Therefore, a new and improved heat transfer correlation was developed based on the pure fluid correlation proposed by Shah. The correlation takes into account the effects of temperature glide and mass transfer resistance and draws on experimental data for hydrocarbon mixtures reported in the existing literature. Comparison with the experimental data shows that the new correlation has good applicability to hydrocarbon mixtures, providing satisfactory predictive values of R50/R1150 with a MARD of 14% and 95.3% of the data within ± 30% deviation.

The effective refrigeration temperature zone expressed by partial molar enthalpy difference of each component in mixed-refrigerant Joule-Thomson refrigerators

Mixed-refrigerant Joule-Thomson refrigerators (MJTRs) are widely used in the fields of energy, chemical engineering and medical treatment. Mixed refrigerants generally employ a mixture of 4–8 components, which provide refrigeration effects in different temperature zones. It is essential to reveal the effective refrigeration temperature zones of different components for the selection of components and adjustment of their concentrations. In this study, the partial molar enthalpy difference is used to characterize the contribution of components to the isothermal throttling effect. The results show that there is an obvious peak in the graph of the contribution of each component to the isothermal throttling effect with temperature, whose height and width represent the refrigeration effect and the effective refrigeration temperature zone of the component, respectively. The temperature zone where the partial molar enthalpy difference of the component is greater than the isothermal throttling effect of the mixed refrigerant is defined as the effective refrigeration temperature zone. The effective refrigeration temperature zone of the component with highest boiling point lies between the high-pressure and low-pressure dew points, and that of the component with lowest boiling point lies between the high-pressure and low-pressure bubble points. Further, the other components provide the refrigeration effect in the temperature zone between low-pressure dew point and high-pressure bubble point, thereby playing the connecting role.

Flow condensation pressure drop characteristics of zeotropic mixtures of tetrafluoromethane/ethane: Experimental and analytical investigation

Mixed-refrigerant Joule-Thomson (MRJT) refrigeration systems have distinct advantages in the temperature range from 80 to 230 K. Tetrafluoromethane (R14) and ethane (R170) are important components to improve the recuperation process in the middle and low temperature regions of a MRJT refrigeration system. In this paper, an experimental investigation on condensation pressure drop characteristics of R14/R170 mixtures (0.193/0.807, 0.437/0.563, 0.632/0.368 and 0.799/0.201 by mole) in a horizontal smooth tube was carried out. Experiments were implemented at mass fluxes from 100 to 350 kg m−2 s−1, saturation pressures from 1.5 to 2.5 MPa and heat flux from 8.4 to 44.22 kW m−2 over the entire range of vapor quality. The different contributions of frictional pressure drop and deceleration pressure drop were calculated and compared. The effects of concentration, mass flux, saturation pressure, heat flux and vapor quality were analyzed and discussed. Finally, a new flow pattern-based pressure drop correlation was developed with a good predictive ability for the present experimental data with a MARD of 8.63%, and 14.20% for the literature data. The model considers the combined effects of the individual pressure drops of liquid and vapor as well as the frictional pressure drop due to the liquid-vapor interaction.

Experimental and analytical investigation on flow condensation heat transfer of zeotropic mixtures of tetrafluoromethane/ethane

Tetrafluoromethane (R14) and ethane (R170) are important components to improve the recuperation process in the middle and low temperature regions of a mixed-refrigerant Joule-Thomson (MRJT) refrigeration system. In this paper, an experimental investigation on flow condensation heat transfer characteristics of R14/R170 mixtures (0.193/0.807, 0.437/0.563, 0.632/0.368 and 0.799/0.201 by mole) in a horizontal smooth tube was carried out. Experiments were implemented at mass fluxes from 100 to 350 kg m−2 s−1 and saturation pressures from 1.5 to 2.5 MPa over the entire range of vapor quality. The effects of concentration, mass flux, saturation pressure and vapor quality were analyzed and discussed. Compared with linear interpolation values of pure fluids, heat transfer degradation of mixtures increases with the rise of vapor quality. In addition, all experimental data were compared with sixteen well-known correlations combined with equilibrium method. It is found that all models underestimate the current experimental data. This should be due to the neglect of the intensifying effect of vapour-liquid interface fluctuation. Finally, an improved heat transfer correlation for zeotropic mixtures was proposed and predicted the experimental results well with a mean absolute relative deviation of 13.91%.

Binary mixture isothermal vapour-liquid equilibrium: Trifluoromethane (R23) + octafluoropropane (R218) at the temperature from 223.150 K to 263.150 K

The mixed-refrigerant Joule-Thomson refrigerator (MJTR) is simple in structure, safe and reliable, and is suitable for temperatures from 80 to 230 K. Trifluoromethane (R23) and octafluoropropane (R218) are satisfactory as non-flammable refrigerants in MJTR. In this paper, the gas-phase single-cycle method was used to conduct a vapor–liquid equilibrium (VLE) experiment on the R23 + R218 binary system in the temperature range from 223.150 K to 263.150 K. The VLE experimental data were correlated using the Peng-Robinson (PR) equation of state and the Huron-Vidal (HV) mixing rule combined with non-random two-liquid (NRTL) activity coefficient model. The maximum average absolute relative deviation of pressure (AARDp) and the maximum average absolute deviation of vapor phase composition (AADy) are 0.67% and 0.005, respectively.

Adiabatic Two-Phase Flow Patterns for Zeotropic Mixtures of Tetrafluoromethane/Ethane in a Horizontal Smooth Tube

Mixed-refrigerant Joule-Thomson refrigeration (MJTR) systems have distinct advantages in the temperature range from 80 to 230 K. Tetrafluoromethane (R14) and ethane (R170) are essential components of mixed-refrigerants. Heat transfer and pressure drop for twophase flow are closely related to corresponding flow patterns. In this paper, an experimental investigation on adiabatic twophase flow patterns for R14/R170 mixtures in a horizontal smooth tube with inner diameter of 4 mm was presented. Experiments were carried out at mass fluxes from 100 to 350 kg m−2 s−1 and saturation pressures from 1.5 to 2.5 MPa over the entire range of vapor quality. The effects of concentration, mass flux, saturation pressure and vapor quality were analyzed and discussed. Furthermore, the observed flow patterns were compared with six existing flow pattern maps. Our previous model developed for pure R14 shows the best predictive ability with 80.16% of the data points can be accurately predicted.

Condensation two-phase flow patterns for zeotropic mixtures of tetrafluoromethane/ethane in a horizontal smooth tube

Mixed-refrigerant Joule-Thomson refrigeration (MJTR) systems have distinct advantages in the temperature range from 80 to 230 K. Tetrafluoromethane (R14) and ethane (R170) are essential components of mixed-refrigerants. In this paper, the adiabatic flow pattern map for R14/R170 mixtures was firstly developed based on previous flow pattern data. Then an experimental investigation on condensation two-phase flow patterns for R14/R170 mixtures (0.193/0.0.807, 0.437/0.563, 0.632/0.368 and 0.799/0.201 by mole) in a horizontal smooth tube with inner diameter of 4 mm was presented. Experiments were carried out at mass fluxes from 100 to 350 kg m−2s−1, saturation pressures from 1.5 to 2.5 MPa and heat fluxes from 8.4 to 42.2 kW m−2 over the entire range of vapor quality. The effects of mass flux, saturation pressure, heat flux and concentration on flow pattern evolutions were discussed and presented. Analysis results indicated that mass transfer resistance and heat flux account for the nonequilibrium effects of zeotropic mixture condensation, which leads to the different evolutionary trends between condensation and adiabatic two-phase flow. Finally, a new condensation flow pattern map was deduced by containing nonequilibrium effects into adiabatic flow pattern map.

Two-phase flow condensation pressure drop of R14 in a horizontal tube: Experimental investigation and correlation development

Tetrafluoromethane (R14) is an important fluid in three-stage-cascade refrigeration and mixed-refrigerant Joule-Thomson refrigeration (MJTR). In this work, an experimental investigation on condensation pressure drop of R14 in a horizontal smooth tube with inner diameter of 4 mm was carried out. Experiments were implemented at mass fluxes from 150 to 650 kg/(m2 s), saturation pressures from 1 to 3 MPa and heat fluxes from 8.3 to 28.2 kW/m2 over the entire range of vapor quality. The effects of heat flux, mass flux, saturation pressure and vapor quality were analyzed and discussed. In addition, the frictional pressure drops were compared with twenty-nine well known frictional pressure drop correlations. An improved frictional pressure drop correlation was deduced and predicted R14 experimental data well with a mean absolute relative deviation of 12.18%. Finally, 2317 data points from nineteen published literature were adopted to evaluate the predictive ability of this new model. It achieves a satisfactory predicting result with a mean absolute relative deviation of 17.39% and 87.26% of the experimental data within the deviation bandwidth of ±30%.

Numerical simulation of tubes-in-tube heat exchanger in a mixed refrigerant Joule–Thomson cryocooler

Mixed refrigerant Joule-Thomson (MRJT) cryocoolers can produce cryogenic temperatures with high efficiency and low operating pressures. As compared to the high system pressures of around 150–200 bar with nitrogen, the operational pressures with non-azeotropic mixtures (e.g., nitrogen-hydrocarbons) come down to 10–25 bar. With mixtures, the heat transfer in the recuperative heat exchanger takes place in the two-phase region. The simultaneous boiling and condensation of the cold and hot gas streams lead to higher heat transfer coefficients as compared to single phase heat exchange. The two-phase heat transfer in the recuperative heat exchanger drastically affects the performance of a MRJT cryocooler. In this work, a previously reported numerical model for a simple tube-in-tube heat exchanger is extended to a multi tubes-in-tube heat exchanger with a transient formulation. Additionally, the J-T expansion process is also considered to simulate the cooling process of the heat exchanger from ambient temperature conditions. A tubes-in-tube heat exchanger offers more heat transfer area per unit volume resulting in a compact design. Also, the division of flow in multiple tubes reduces the pressure drop in the heat exchanger. Simulations with different mixtures of nitrogen-hydrocarbons are carried out and the numerical results are compared with the experimental data.

Design of non-flammable mixed refrigerant Joule-Thomson refrigerator for precooling stage of high temperature superconducting power cable

A cryogenic Joule-Thomson (JT) refrigerator has many advantages for large industrial applications, including easy cooling power adjustability and high reliability because there are no moving parts at low temperature. In this paper, a single stage and a cascade type of non-flammable (NF) mixed refrigerant (MR) JT refrigerators have been proposed for the precooling process of a neon/nitrogen mixed refrigerant JT refrigerator. The neon/nitrogen MR JT refrigerator is used to maintain a subcooled state of liquid nitrogen coolant for an HTS (high temperature superconductor) cable.Both selected MRs for the 1st stage (low temperature cycle) of the cascade MR JT refrigerator and the single MR JT refrigerator are composed of Nitrogen (N2), Argon (Ar), Tetrafluoromethane (CF4, R14) and Octafluoropropane (C3F8, R218). R410A is selected as the refrigerant for the 2nd stage (high temperature cycle) of the cascade MR JT refrigerator, of which the cooling temperature is approximately 240K. A commercial software with Peng-Robinson equation of state (EOS) is utilized to design the non-flammable MR JT refrigerator. The optimal design is discussed with consideration for various parameters such as the temperature staging, the operating pressure of the compressor and the mass flow rate of the working fluid. The maximum coefficient of performance (COP) and Carnot efficiency of the cascade MR JT refrigerator are obtained as 0.216 and 40%, respectively at 105K. Exergy analysis is also carried out in this paper to reveal the irreversibility of the refrigeration cycle.

Steady-state thermodynamic simulation and structural design of the dephlegmator used in mixed-refrigerant Joule-Thomson refrigerators

Dephlegmators can be used to reduce the energy consumption and simplify the layout of the mixed-refrigerant Joule–Thomson (MRJT) cycle. Heat-exchange characteristics and refrigeration design are currently based on highly simplified assumptions. Synthesis methods to efficiently solve all design issues of dephlegmators in MRJT cycle are insufficient. No suitable separation module is available for the simultaneous heat and mass transfer processes in Aspen Plus because the module should be programmed and incorporated into Aspen Plus as a user-defined unit. In this paper, a systematic steady-state method was proposed for the detailed design of dephlegmators for gas mixture separation, considering the simulation and heat exchanger design simultaneously. The material balance, vapor–liquid equilibrium, mole fraction summation and heat balance (MESH) model was programmed in FORTRAN language. Good agreements and the feasibility of the MESH model were found. Deviations between the simulation results and patent data were all within 5%. The errors in the predicted temperatures of vapor and liquid products were less than 2% and 10%, respectively. Fine applicability and low energy consumption of the dephlegmator were addressed. The mole fraction of n-butane in the liquid phase had high recovery ratio of 90%. The dephlegmator decreased more than 30% of energy consumption compared with the traditional distillation tower under similar separation effects. In the structural design process, the dephlegmator was divided into certain segments by baffle plates on the basis of segmented calculation. The heat transfer coefficient, heat transfer area, pressure drop, and structural parameters of the dephlegmator were evaluated. A clear and comprehensive three-dimensional dephlegmator model was shown.

Prediction of two-phase pressure drop in heat exchanger for mixed refrigerant Joule-Thomson cryocooler

The overall efficiency of a mixed refrigerant Joule-Thomson (MR J-T) cryocooler is governed by the performance of the recuperative heat exchanger. In the heat exchanger, the hot stream of the mixed refrigerant undergoes condensation at high pressure while the cold stream gets evaporated at low pressure. The pressure drop in the low pressure stream is crucial since it directly influences the achievable refrigeration temperature. However, experimental and theoretical studies related to two-phase pressure drop in mixtures at cryogenic temperatures, are limited. Therefore, the design of an efficient MR J-T cryocooler is a challenging task due to the lack of predictive tools.In the present work, the existing empirical correlations, which are commonly used for the prediction of pressure drop in the case of pure refrigerants, evaporating at near ambient conditions, are assessed for the mixed refrigerants. Experiments are carried out to measure the overall pressure drop in the evaporating cold stream of the tube-in-tube helically coiled heat exchanger. The predicted frictional pressure drop in the heat exchanger is compared with the experimental data. The suggested empirical correlations can be used to predict the hydraulic performance of the heat exchanger.

Investigation of neon–nitrogen mixed refrigerant Joule–Thomson cryocooler operating below 70 K with precooling at 100 K

There has been two-stage mixed refrigerant (MR) Joule–Thomson (JT) refrigeration cycle suggested for cooling high temperature superconductor (HTS) electric power cable below 70K. As the continuation effort of realizing the actual system, we fabricated and tested a small scale neon and nitrogen MR JT cryocooler to investigate the refrigeration characteristics and performance. The compression system of the refrigeration circuit was accomplished by modifying commercially available air-conditioning rotary compressors. Compressors stably operated at the maximum compression ratio of 31 when the suction pressure was 77kPa. The achieved lowest temperature was 63.6K when the heating load was 35.9W. The measured Carnot efficiency of the present system was 6.5% which was lower than that of the designed goal of 17.4%. The low efficiency of compressor (34.5%), and the pressure drop at the compressor suction were the main reasons for this efficiency degradation. The feasibility and usefulness of neon and nitrogen MR JT refrigeration cycle was validated that the achieved minimum temperature was 63.6K even though the pressure after the expansion was maintained by 130kPa. The comparison between the measurement and calculation showed that each stream temperature of refrigeration cycle were predictable within 3% error by Peng–Robinson equation of state (EOS).

Prediction of temperature distribution in heat exchanger for mixed refrigerant Joule–Thomson cryocooler

The overall performance of a mixed refrigerant Joule-Thomson (MR J–T) cryocooler depends on the efficiency of the heat exchanger used to cool the refrigerant prior to J-T expansion. The multi-component refrigerant mixture gets condensed and evaporated simultaneously at different pressures in the recuperative heat exchanger which is responsible to increase its performance. However, at present, the design of such heat exchanger is difficult due to lack of experimental heat transfer data.

Pressure drop characteristics of cryogenic mixed refrigerant at macro and micro channel heat exchangers

Mixed Refrigerant-Joule Thomson (MR-JT) refrigerators are widely used in various kinds of cryogenic systems these days. The temperature glide effect is one of the major features of using mixed refrigerants since a recuperative heat exchanger in a MR-JT refrigerator is utilized for mostly two-phase flow. Although a pressure drop estimation for a multi-phase and multi-component fluid in the cryogenic temperature range is necessarily required in MR-JT refrigerator heat exchanger designs, it has been rarely discussed so far. In this paper, macro heat exchangers and micro heat exchangers are compared in order to investigate the pressure drop characteristics in the experimental MR-JT refrigerator operation. The tube in tube heat exchanger (TTHE) is a well-known macro-channel heat exchanger in MR-JT refrigeration. Printed Circuit Heat Exchangers (PCHEs) have been developed as a compact heat exchanger with micro size channels. Several two-phase pressure drop correlations are examined to discuss the experimental pressure measurement results. The result of this paper shows that cryogenic mixed refrigerant pressure drop can be estimated with conventional two-phase pressure drop correlations if an appropriate flow pattern is identified.