Secondary Refrigerant Research Articles

Study and design of a Hydrogen liquefaction process using ethane as refrigerant fluid

Hydrogen liquefaction is an essential process in cryogenics, facilitating the efficient storage and transport of hydrogen as a clean energy carrier. This study investigates the liquefaction of hydrogen at a flow rate of 100 kmol/h, with an energy consumption of 447.05 kW. The process relies on sophisticated equipment, including multi-stage compressors and expansion devices, which play key roles in cooling and compressing the hydrogen gas. One of the notable advantages of this system is the energy recovery achieved by an expansion device, which generates 262.1 kW, enhancing overall efficiency.The hydrogen compression stages use a multi-stage compressor that employs water as the primary coolant. With a flow rate of 389.4 kmol/h, water is chosen due to its high specific heat capacity and availability, making it an ideal coolant for this cryogenic process. Ethane, serving as a secondary refrigerant, operates at a flow rate of 32 kmol/h. Its non-corrosive and inert properties contribute to system durability, while a portion of the ethane is strategically recycled through heat exchangers to minimize consumption and optimize cooling efficiency. Hydrogen enters the system at 5×10⁵ Pa and 288 K, and is gradually cooled and compressed to 2×10⁵ Pa and 22.75 K, resulting in the final liquid hydrogen product. This significant temperature reduction, achieved through the combined cooling effects of water and ethane, is critical for the successful liquefaction of hydrogen. The optimized use of refrigerants and efficient energy recovery in the expansion stages reduce operational costs and enhance process sustainability.

Performance assessment of residential heat pumps operating in extreme cold climates using zeotropic mixtures

Extremely cold climates subject residential heat pumps to significant temperature differences between the heat source and the ambient being heated, which may lead to system failure and reduced compressor performance. The present study considers the possibility of improving the performance of heat pump systems that are simultaneously used for residential space and domestic water heating while subjected to climates varying from -25 °C to 5 °C by exploring the use of zeotropic mixtures. CO2-based binary mixtures composed of low-GWP (global warming potential) refrigerants are considered – R32, R1234yf, and R290 – aiming to benefit from environmentally friendly and flame suppressants characteristics of CO2, as well as the improved thermal efficiency granted by the addition of a secondary refrigerant. A thermodynamic model was developed for a standard vapor-compression heat pump cycle and used to maximize the coefficient of performance limited by the minimum pinch point in the heat exchangers. Our analysis explores the effect of several parameters, such as, the mixture components, mass fractions, space and water heating demands, and heat source temperature on the heat pump's performance. For cold climates, the mixture of R32 (90%)/CO2 (10%) yields the highest COP and CO2-rich mixtures exhibit the lowest. However, by increasing the mass fraction of CO2 within the zeotropic mixture, the pressure ratio of the heat pump was improved. When considering combined performance criteria, such as the volumetric heating effect and the total heat delivered related to heat pump size, CO2-rich mixtures tend to allow more compact systems, especially in colder climates.

Comparison and analysis on comprehensive performance of CO2 multiphase refrigeration systems using liquefied natural gas cold energy

In order to fully apply liquefied natural gas (LNG) cold energy to the refrigeration system, four different types of CO2 multiphase refrigeration systems using LNG cold energy are designed. In this paper, (1) CO2 single-stage compressed solid and gas phase refrigeration cycle (SSCC1), (2) CO2 single-stage compressed solid and solid phase refrigeration cycle (SSCC2), (3) CO2 double-stage compressed solid and gas phase refrigeration cycle (DSCC1), and (4) CO2 double-stage compressed solid and solid phase refrigeration cycle (DSCC2) are combined with CO2 liquid phase secondary refrigerant cycle (RC), respectively, to effectively use LNG cold energy. The performance analysis, exergy analysis, economic analysis, and CO2 emission analysis of the proposed systems are carried out by establishing the mathematical models. The results show that the intermediate pressure of DSCC1-RC and DSCC2-RC reaches the best performance at 0.3 MPa, and the system performance decreases with the increase in intermediate temperature. The refrigerating capacity of the CO2 liquid phase secondary refrigerant cycle, the COP, and the exergy efficiency of four kinds of CO2 multiphase refrigeration systems decrease with the increase in the refrigerating capacity of the CO2 refrigeration cycle, while the power consumption of SSCC2-RC and DSCC2-RC decreases, SSCC1-RC and DSCC1-RC increased. The system with the shortest exergy loss is DSCC2-RC at 654.01 kW, while the system with the shortest payback period is SSCC2-RC at 0.88 years, and DSCC2-RC has the smallest CO2 emission. Four CO2 multiphase refrigeration systems and the ammonia combined refrigeration system with the same total refrigerating capacity are compared and analyzed, respectively; the results show that the performance, economy, and CO2 emission of CO2 multiphase refrigeration system are better than those of ammonia combined refrigeration system; and the exergy loss of CO2 multiphase refrigeration system is generally larger than that of ammonia combined refrigeration system because of the large temperature difference in heat transfer.

Energy, exergy, economic, and environmental compromising performance of dual-stage evaporation-ammonia hybrid compression–absorption refrigeration system for the cooling supply of keto-benzene dewaxing process

Absorption refrigeration systems driven by low-temperature waste heat is one way to achieve “carbon neutrality”. Meanwhile, the keto-benzene dewaxing equipment needs cooling capacity of 5 MW which refrigeration temperature of –10 °C and –25 °C. This paper researches the feasibility of DSE-AHCARS (Dual-stage Evaporation-Ammonia Hybrid Compression-Absorption Refrigeration System) replacing the vapor compression refrigeration system for keto-benzene dewaxing process based on 4E (Energy, Exergy, Economic, and Environmental) analysis. At the primary and secondary stage evaporation temperature of 0 and –23 °C, COP reaches the maximum value of 0.85. However, COP-Electricity reaches the minimum value of 8.1. When the secondary stage refrigeration temperature is –23 °C, CO2 emission increases from 1150 t·a–1 to 3600 t·a–1, and Life Cycle Climate Performance increases from 3.29×104 tons to 7.7×104 tons with the primary stage refrigeration temperature is –15 °C to 0 °C. As well as matching three parameters to ensure the 4E compromising performance by the multi-objective optimization. To guarantee the Life Cycle Climate Performance is less than 5.5×104 tons, the payback period is less than 2 yr, and COP is greater than 0.6 at the optimal operation ranges that refrigeration temperature difference between primary stage and secondary stage is within 20 °C. The power of DSE-AHCARS was reduced by 77% compared with the vapor compression refrigeration system. Therefore, the DSE-AHCARS can reduce CO2 emissions by about 6250 t·a–1 and save 1.2×105 tons of CO2 in the Life Cycle Climate Performance term. This result shows that the DSE-AHCARS can completely replace the vapor compression refrigeration system.

EXPLORACIÓN DE LA MEZCLA RE170/R600: MEJORA DE LA EFICIENCIA COMO ALTERNATIVA AL R600a

The utilization of alternative mixtures to R600a has been extensively explored across various applications, showcasing their potential to enhance the energy efficiency of isobutane in domestic refrigerators and standalone commercial appliances while maintaining comparable thermodynamic characteristics and operational performance parameters. Among these mixtures, those composed of a significant proportion of R600 (butane) and a small portion of secondary refrigerant have emerged as the most promising. However, many secondary refrigerants in these blends are synthetic. RE170 (dimethyl ether or DME), a non-fluorinated and natural refrigerant, has emerged as a particularly promising candidate for blending with R600. This paper presents an experimental analysis of the RE170/R600 refrigerant mixture as an alternative to R600a. For that purpose, a simple compressor highly equipper test bench was used, in which the mixture was analyzed at a composition range varying from 2.5/97.5%w to 27.5/72.5%w at steps of 2.5%. Results show COP increments up to 19.3% respect R-600a and VCC and that low required DME matches the one obtained by R600a. RE170/R600 has been demonstrated to be a long-term superior efficiency mixture.

Growth Kinetics and Porous Structure of Surfactant-Promoted Gas Hydrate.

Surfactants present in tiny amounts in the aqueous phase are known to be efficient gas hydrate promoters; yet, the promotion mechanisms are still not fully understood. Understanding and directing those mechanisms is key to the implementation of gas-hydrate-based applications such as gas storage and separation, secondary refrigeration or water treatment, and desalination. In this work, the growth at the water/gas interface and the porous structure of surfactant-promoted methane hydrate are observed by optical microscopy and Raman imaging in glass capillaries used as optical cells. Hollow crystals are continuously generated and expelled from the methane/water meniscus into the water or surfactant solution, where they ultimately form the skeleton of a porous medium filled with the solution. Unprecedented information is gathered over a range of scales from the molecular scale (crystal structure and cage filling) to the mesoscale (crystal morphologies, growth habits and pore sizes) and macroscale (rates and amounts of water and gas converted into hydrate and hydrate porosity). Following an early steady-state growth regime, a sudden order-of-magnitude increase of the conversion rate occurs, which is related to gaseous methane microbubbles being directly incorporated across the meniscus in the aqueous solution and later converted to methane hydrate. An assessment and comparison are made of the mechanisms and performance of two common anionic surfactants known to be efficient gas hydrate promoters, SDS (sodium dodecyl sulfate) and AOT (dioctylsulfosuccinate sodium or AerosolOcTyl). AOT provides a quicker but more limited conversion into hydrate than SDS, suggesting that it is more appropriate for continuous flow processes while SDS is better suited for gas storage applications. Raman spectra reveal that cage filling by methane of structure I methane hydrate is not affected by surfactants.

Preparation, characterisation and property modification of a calcium chloride hexahydrate phase change material slurry with additives for thermal energy transportation

Efficient thermal energy distribution and rational energy management are effective solutions to address the significant and fast-ascending energy consumption in building heating, ventilation, and air conditioning (HVAC). Phase change material (PCM) slurries with excellent thermal energy transportation and storage characteristics have been recognised as promising secondary refrigerants to simultaneously facilitate efficient heat/cold transport and effective thermal energy storage in air conditioning systems. Among various PCM slurries, salt hydrate PCM slurries have attracted increasing attention in recent years due to their many inherent advantages. This paper presented the preparation, characterisation, and property modification of a CaCl2·6H2O slurry based on a methodology proposed for optimal salt hydrate PCM slurry development. Theoretical modelling was first carried out to assess the availability of salt hydrate PCM slurries based on the phase diagram and solid fraction, followed by a series of property measurements to reveal the mechanisms and address the problems behind the phase transition, crystal particle morphology, stability and rheologic characteristics, using nucleating agents, surfactants and stabilisers. It was found that the salt hydrate PCM slurry developed provides a unit volume enthalpy twice that of water over the phase change temperature range of 5°C, together with acceptable fluidity. The results also showed that the PCM particle size can be well controlled, and the slurry can be stabilised, with the assistance of surfactants and stabilisers. Compared to other types of PCM slurries, the proposed salt hydrate PCM slurry has the advantages of being cost-effectiveness, easy preparation, adjustable phase change temperature, etc., which is promising to facilitate energy saving and rationalisation in building HVAC.

Simulative and experimental research on the heat exchanger for cold energy recovery of liquefied natural gas

Natural gas, a widely used energy source, is transported as liquefied natural gas (LNG). During the gasification of LNG, a large amount of cold energy is released, which can be recycled to achieve energy savings. An LNG cold energy heat-exchanger (shell-and-tube type) was designed to recover cold energy for application in 0 ℃ ammonia cold storage. To conduct a more intuitive and detailed study of the effects of the tube space and flow rate on each heat-exchange tube in a shell-and-tube heat exchanger, a double-pipe heat exchanger was designed and manufactured. Liquid nitrogen and a secondary refrigerant (antifreeze) was used as the cold and heat source, respectively. Owing to the effects of the tube space and refrigerant flow velocity on the heat transfer coefficient, simulations and experiments were conducted on the antifreeze velocity and tube space. The results showed that increasing the antifreeze flow velocity and reducing the tube space increased the amount of cold energy recovered. Based on the physical parameters of liquid nitrogen and LNG, the cold energy recovery ratio was 1.3. The predicted amount of cold energy recovered from LNG in double-pipe and shell-and-tube heat exchangers were determined by the cold energy recovery coefficient, cold energy recovery ratio, and number of heat-exchange tubes. The recovery of cold energy from LNG in a shell-and-tube heat exchanger was applied to cold storage, which can improve the cold storage performance by 36.8 %.

Experimental monitoring of CO2 hydrate slurry crystallization by heat flux rate determination in a jacketed reactor

CO2 hydrate slurries are promising phase change materials for secondary refrigeration applications. However, the difficulty of experimentally evaluating the crystallization kinetic of slurries limits their industrial use. Hydrate crystallization kinetics monitoring performs traditionally by a reactor mass balance. However, this method requires assumptions on CO2 liquid phase concentration and the hydration number. This work outlines the development of a specific method to determine kinetics thanks to the direct measure of heat flow through the reactor jacket and its evaluation compared with the mass balance method. The results of each method are then obtained by testing the two kinetic parameters of stirring speed and propeller type. The final hydrate mass fractions obtained with both methods are in good agreement, but the kinetic obtained by the mass balance method is faster than by the heat balance method.

Cryogenic cold energy recovery in liquid hydrogen refueling station with double-pipe heat exchanger

Recovering the cryogenic cold energy of liquid hydrogen (LH2) for precooling high-pressure hydrogen gas before refueling can significantly reduce the electricity and energy consumption of liquid hydrogen refueling stations. Existing methods, such as blending, require continuous cryogenic pump operation and are not suitable for various operating conditions. This work proposes a novel method to recover LH2 cryogenic cold energy using a double-pipe heat exchanger, which can decouple the compression and refueling process and meet the fluctuating demand for the cryogenic cold energy required by the hydrogen dispenser. The lumped parameter method and temperature partition method were adopted to design the heat exchanger structure. Numerical simulations of a 2D axisymmetric swirl model were done to verify the accuracy of the temperature partition method applied to high-pressure cryogenic hydrogen. Due to the low temperature of LH2, the secondary refrigerant dichloromethane (CH2Cl2) risks freezing. Comparing the outer wall surface temperature of the inner pipe with the CH2Cl2 freezing point temperature, the optimal anti-freezing condition is that the outer pipe nominal diameter should be selected as 0.032 m and CH2Cl2 mass flow rate should be at least 1.72 kg s−1. Recovery efficiency can reach over 75.39% without freezing.

STUDI EKSPERIMEN KINERJA SISTEM REFRIGERASI SETENGAH SIKLUS PADA KENDARAAN BERBAHAN BAKAR LPG

The Air Conditioning (AC) system is an important requirement for vehicle passengers to get thermal comfort. However, the AC system draws power from the engine to drive the compressor. Meanwhile, vehicles driven by Liquefied Petroleum Gas (LPG) provide decent cooling potential to assist the main air conditioning system. Therefore, this study aims to examine the effect of cooling on LPG-fueled vehicles by utilizing LPG flow to be used as a secondary refrigerant. The research was carried out on a 1495 cc engine with a special evaporator which was tested at variations of blower speed 1, 2, and 3. The tests were carried out at engine speeds of 1000, 2000, and 3000 rpm. The results showed the lowest cooling effect at blower speed level 1 with engine speed of 1000 rpm and the highest at blower speed level 3 with engine speed of 3000 rpm. The greatest cooling effect is obtained at 373 Watts at 3000 rpm engine speed and level 3 blower speed.

Exergic performance of plate evaporator coated with nanoparticles for fish preservation

ABSTRACT A theoretical analysis is conducted using a nano-coated plate evaporator surface for fish preservation. Copper and alumina nanoparticles mixed with the base material (Steel) are considered for the evaporator material. Different brines (ethylene glycol, propylene glycol, potassium acetate, and calcium chloride) act as secondary refrigerants. Various performance parameters (pumping power, exergy rate change, irreversibility, exergic efficiency, non-dimensional exergy, and irreversibility distribution ratio) based assessment has been performed. The maximum percentage reduction in non-dimensional exergy and irreversibility, and maximum percentage rise in exergy rate change, irreversibility distribution ratio, and exergic efficiency have been acquired for propylene glycol brine. The pumping power decreased by 2.5% for alumina-copper hybrid nanoparticle-based material. The irreversibility and non-dimensional exergy have been reduced by 1.5%, whereas the exergy change rate, exergic efficiency, and irreversibility distribution ratio enhanced by 0.5%, 0.5%, and 2.5%, respectively, for PG brine (percentage-wise). The study reveals that the surface coated with nanoparticles provides better exergic performance.

Characterization of Copper Oxide–Jatropha Oil Nanofluid as a Secondary Refrigerant

Nanofluid is a new fluid with special characteristics that can be used in various industrial heat transfer applications. The nanofluid was created using the two-step approach of dispersing nanoparticles into Jatropha oil. The experiments were conducted for four samples of mixture nanofluid at four different volume concentrations of 0.5%, 1.0%, 1.5%, and 2%. The nanoparticles were characterized by X-ray diffractometric analysis, X-ray fluorescence spectroscopy, and scanning electron microscopy. The greatest increase in thermal conductivity for CuO–Jatropha oil nanofluids was made possible by 0.5%, 1.0%, 1.5%, and 2% mixture was 4.5%, 6.01%, 7.6, and 9.1%. The study observed that a mixture achieved the highest thermal conductivity for nanofluid at 2%. At a mixture of 0.5%, the lowest value of thermal conductivity within the range under consideration, a “deeding” effect was seen. To determine the thermal conductivity of CuO–Jatropha oil nanofluids based on temperature, volume concentration, and nanoparticle mixture volume concentrations, the study suggested and compared four sample models. The effect of nanoparticles on the rise of thermal conductivity and viscosity was investigated at temperatures ranging from 298 to 323 K. The X-ray diffractometer is used to characterize nanoparticles. Viscosity and thermal conductivity have been tested in the lab. The findings show that the copper oxide nanoparticles improved the base fluid’s thermophysical abilities more than Jatropha oil alone. For the evaluation of the thermal conductivity and viscosity of nanofluid, new empirical correlations were presented based on experimental data. The thermal conductivity of the nanofluids increases as the temperature rises, while the viscosity of the nanofluids decreases.

Numerical study of ice slurry flow and heat transfer in successive U-bends as part of tubular heat exchangers

Using ice slurry can assist the energy saving in cooling systems. Ice slurries are used as secondary refrigerant in large cooling systems. They are mixtures of tiny ice particles in water and an antifreezing agent. The cooling can be stored in ice particles during off-peak conditions and released during peak condition. The stored cooling in ice slurry must be delivered to the desired system through a heat exchanger. Successive U-bend pipes commonly exist in tubular heat exchangers. However, this configuration has not been investigated yet for ice slurries. The present study aims to investigate the heat transfer of these mixtures in successive in-plane U-bends as part of heat exchangers. Mass transfer between ice slurry and fluid is the main important part of this problem which is considered in the present study. For the simulation of ice slurry, the Eulerian two-fluid numerical method is used along with the consideration of mass transfer between two phases and the kinetic theory of granular flow for ice particles viscosity. Nusselt number and pressure drops are mainly studied for different geometric configurations. The length between two bends are changed from 50 to 400 mm and the radius of curvatures of the bends are changed from 50 to 100 mm. The results of the study indicated that the average Nusselt number increases with the radius of curvature and distance between two U-bend pipes. Two successive U-bends neutralize each other's centrifigual effects, then ice particles have chances to rearrange and reduce the wall temperature in each bend. This effect is more prominent in higher distances between two bends and higher radii of curvature. At low radius of curvatures, the variation of distance between two bends has stronger effect on heat transfer of the system; in such a way that by doubling the distance between two bends, the Nusselt number increases 4.1% and 1.5% respectively for radius of curvatures 50 mm and 100 mm. Also it was shown that by increasing the radius of curvature and distance between two U-bend pipes the skin friction coefficient increases. It can be seen that by increasing the ratio of the distance between two bends to the radius, the performance evaluation criterion (PEC) increases.

Fault detection and isolation for a secondary loop refrigeration system

Secondary loop refrigeration systems offer advantages over conventional system architectures: refrigerant charge reduction, maintenance simplification, and reduced safety hazards. Currently, no model-based fault detection for a secondary loop system has been reported. This article presents a fault detection and isolation scheme for a secondary loop refrigeration system where parametric faults (dirt/ice-buildup and secondary loop pump failure) and sensor faults (four temperature sensors in the secondary loop) are detected and isolated. The fault detection and isolation system is based on unknown input observers and on extended Kalman filters. The models are obtained through physical relations combined with experimental parameter determination (gray-box model) and are cross-validated against measurement data. The complete fault detection and isolation architecture is experimentally validated on a secondary loop refrigeration system mounted on a cooling box inside a climate chamber. Experimental results show that important parametric faults can be reliably detected and isolated, and the fault detection and isolation of sensor faults are effective.

Exergy and performance analysis of low GWP and Non-flammable HFO based refrigerant mixtures as alternatives to R134a

R1234yf and R1234ze(E) can be potential retrofits for the conventional refrigerants used in domestic refrigerator provided their flammability is controlled. In this study, the inerting effect of the dilutants R125, R134a, R227ea and R245fa on R1234yf and R1234ze(E) based ternary mixtures is studied theoretically. R152a, R161 and RE170 are used as the secondary refrigerants with R1234yf and R1234ze(E) based ternary mixtures. Minimum inerting concentration (most important parameter that decides the flammability of the mixture) is estimated with thermal balance method (TBM) and modified thermal balance method (MTBM) for the selected mixtures. The calculated MIC values are compared with 86 experimental data points (both binary and ternary) available in the literature. It is calculated that TBM and MTBM predicted the minimum inerting concentration values (for a majority of the mixtures) within ±30% and ±8% respectively when compared with the experimental data. The COP ratios of the non-flammable mixtures (that are proposed with MTBM) are estimated and is found to be maximum at a volumetric cooling capacity ratio close to 0.8. It is also calculated that the exergy efficiency varied between 0.54 to 0.64 for the low GWP mixtures (M1 to M5) proposed in this study.

Optimization of co-production process of cryogenic helium concentration and liquefied natural gas

Effective helium recovery is currently one of the major issues in the oil industry. To overcome the bottleneck of the existing single helium extraction process and provide novel coupling and control methods for the co-production process, a process for natural gas helium concentration and LNG coproduction (He-LNG) is proposed. Based on gas from a gas-processing plant in China, key parameters are optimized, and the adaptability is analyzed by the HYSYS and MATLAB software. The optimal pressure and theoretical tray number is 3.8 MPa and 16 respectively, according to an analysis of the two key parameters of the high-pressure concentrator, the key equipment of the concentration unit. When the cold box pinch temperature in the liquefaction unit exceeds 3.0 °C, the counter-current refrigeration process of secondary throttling mixed refrigerant is more cost-effective and energy-efficient than other similar processes. The He-LNG process has a good adaptability to feed gas with different N2 and CO2 contents, and different schemes can be adopted for different N2 contents. The results show that in the co-production process, the reflux flow at the top of the helium concentrator is controlled by temperature, which makes the helium concentration effect very well. The genetic algorithm-optimized mixed refrigerant refrigeration process produces cold energy for liquefaction units with LNG production up to 380 t/d and higher, which is more productive and cost-effective than the single process.

High-pressure rheological signatures of CO2 hydrate slurries formed from gaseous and liquid CO2 relevant for refrigeration, pipeline transportation, carbon capture, and geological sequestration

Carbon dioxide hydrates are solid crystalline compounds made of water and carbon dioxide gas. Rheological characterization of carbon dioxide (CO2) hydrates is vital for advancing, optimising, and developing CO2 hydrate-based technologies. In this work, the rheology of the CO2 hydrate slurry has been investigated with and without surfactant, sodium dodecyl sulfate (SDS), using a modified Couette geometry at a constant temperature of 274.55 K and varying pressures (3.5, 5, and 6.5 MPa). It was observed that at higher pressures, dissolved CO2 had been actively engaged in nucleation and growth, unlike free gas at lower pressures. Steady-state viscosities and flow curves vary proportionately with driving force , exhibiting shear thinning behaviour in good agreement with the Cross-model criterion. With the addition of SDS, both the peak and steady-state viscosities experience a drop for all the experimental pressures. Moreover, the flow curves and yield stress of hydrate slurries reduce with surfactant concentration, exhibiting pseudoplastic behaviour. Viscoelastic properties manifest solid behaviour for hydrate slurries formed at a higher pressure of 5 and 6.5 MPa whereas liquid behaviour dominates at lower pressure of 3.5 MPa. Eventually, the presence of SDS has been observed to delay the CO2 hydrate dissociation (3.54 ± 0.4 K/h for 2.5 h), signifying enhanced stability owing to surfactant blanketing/encircling the CO2 hydrate crystals. Thus, SDS facilitates CO2 hydrate flow under higher and lower pressures, administering an anti-agglomerating effect. This encourages its use in industrial applications like secondary refrigeration, cold storage, gas separation, and for improved hydrate slurry flowability in offshore pipelines.

Molecular dynamics simulations of CO2 clathrate hydrate in the presence of organic components

As the major greenhouse gas emission, releasing CO2 through human activities has already devastating consequences on the planet. In this context, hydrate-based (HB) techniques in favour of CO2 capture, sequestration, or utilization (CCSU) are perceived to be a novel option to arrest increasing concentrations of CO2 in the atmosphere. The end uses of captured CO2 encompass its utilization for different realms of industry such as food and beverage manufacturing plants; water desalination; metal fabrication plants; and secondary refrigeration. To offset the cost of CO2 capture as well as generating revenue, the increasing effectiveness of aforesaid techniques is crucial. Although HB approaches are faced with several limitations, the solution would be the inclusion of organic promoters which are classified as environmentally-friendly substances. However, the microscopic influences of such components on CO2 hydrates are mostly unexplored. This work highlights the CO2 clathrate hydrate stability and decomposition in the existence of organic additives through classical molecular dynamics (MD) simulations. The results can help to understand the molecular mechanisms involved in such CO2 hydrate systems which may also aid to find the more efficient organic promoters for HB applications.

An equation of state and a phase diagram for gas-hydrate-forming slurries in gas-shortage

Slurries of gas hydrates, especially mixed hydrates of CO2 and TBPB (Tetra-n-Butyl Phosphonium Bromide), are currently under consideration for secondary refrigeration. Secondary refrigeration circuits contain wide sections where the two-phase slurry (solid + liquid), separated from the gas phase, undergoes transformations that would if gas were present, lead to both hydrate formation and gas dissolution. The absence of gas hinders these processes: the slurry is in gas shortage. The analysis demonstrates that, although thermodynamically distant from three-phase equilibria, slurries in gas-shortage are in equilibrium. For describing their thermodynamic state, the notion of CO2 partial pressure in a gas mixture is introduced, which leads to an equation of state and a new phase diagram with temperature or enthalpy versus CO2 mass-fraction. This diagram complements those commonly used for three-phase systems. The thermal behavior of slurries in gas shortage is then investigated, leading to the possibility of future experimental validation of the present analysis.