Simulation studies of a vacuum and temperature swing adsorption process for the removal of VOC from waste air streams

A theoretical study of the cyclic combined vacuum and temperature swing adsorption (VTSA) process for removing of volatile organic compounds (VOC) from waste air streams is done on the basis of computer simulation. The VTSA system consists of two adsorption columns with a fixed bed of activated carbon. The adsorption cycle for each column is operated in four steps: adsorption, indirect adsorbent bed heating, vacuum desorption and cooling. A nonequilibrium, nonisothermal mathematical model of the VTSA process is developed and solved using the numerical method of lines. Exemplary simulation results are presented for the system: 2-propanol–activated carbon Sorbonorit 4. The effect of desorption temperature, pressure and purge gas use on 2-propanol desorption efficiency is investigated.     

Heat and mass transfer in adsorbent bed with consideration of non-equilibrium adsorption

In the solid adsorption refrigeration cycles, the actual adsorption processes are all non-equilibrium. To investigate the heat and mass transfer in adsorbent bed, mathematical model is established and solved by a numerical method. The relations between adsorption temperature, adsorption velocity, adsorption quantity, coefficient of performance (COP), specific cooling power (SCP) and time are discussed during the process of cooling the adsorbent bed. The relations between desorption temperature, desorption velocity, desorption quantity and time are discussed during the process of heating the adsorbent bed. It indicates that there is a peak value for adsorption velocity in the adsorption process and there is also a peak value for desorption velocity in the desorption process. It also shows that the changing rate of the adsorbent temperature tends to let up, and the coefficient of performance value grows nearly linearly in the adsorption process and there is a peak value of SCP in the adsorption process.     

The effect of microwave regenerated adsorbent bed on the performance of an adsorption heat pump

The heat transfer problem of an adsorption heat pump during the regeneration of adsorbent bed was investigated numerically. A numerical analysis of the heat and mass transfer in an adsorbent bed during an adsorption heat pump cycle, achieved with both conventional and microwave heating, was successfully simulated. The influence of the microwave heating on the performance criteria of an adsorption heat pump was investigated. The distributions of temperature, pressure and adsorbate concentration of adsorbent bed through the radius of the bed were analyzed. The Clausius–Clapeyron diagram was constructed for both cases. The period of the cycle was improved by about 20% with the microwave regeneration, since the period of the isobaric desorption process for a microwave heated cycle was enhanced by 51% relative to the period of the isobaric desorption process for a conventional heated cycle. The COP for the microwave heated cycle was improved by 61% according to the conventional heated cycle.     

Entropy generation and its relation to heating and cooling requirements of a metal hydride hydrogen storage reactor

In this paper a metal hydride hydrogen storage reactor is analyzed from heat and mass transfer and entropy generation points of view. A transient two dimensional energy equation along with suitable reaction kinetics and entropy balance equation is solved numerically. Results are obtained keeping hydrogen flow rates constant during absorption and desorption. For a fixed mass of metal hydride in the reactor the amount of hydrogen transferred and the time in which the transfer takes place are kept fixed. Using the mathematical model the entropy generated during the process and the external cooling and heating fluid requirements are obtained. Results show how improvement in the design and/or operating conditions leads to reduced cooling and heating requirements and lower entropy generation. For the system considered in the study the internal heat transfer characteristics of the hydride bed are seen to influence the reactor performance significantly. With improved bed heat transfer the required heat transfer fluid temperature during desorption can be reduced and that during absorption can be increased significantly. This automatically leads to lower entropy generation and a more economic system operation. It is expected that the methodology proposed and the results presented in this study will be useful in the optimal design of metal hydride reactors for a variety of practical applications, including hydrogen storage.     

Experimental study of a solidified activated carbon-methanol adsorption ice maker

In this paper, the experiments are performed on an adsorption ice maker driven by waste heat, which uses up to two beds. Each bed uses methanol as refrigerant and solidified activated carbon (120 kg adsorbent totally, 60 kg adsorbent per bed) as adsorbent. This system is designed to be driven by the waste heat of a 100 kW diesel engine. The experiments show that the cooling power could be enhanced by the mass recovery process up to 11%, and the heating power could be lowered by the heat recovery process up to 30%. The optimal cooling power of this prototype is about 2.0 kW and corresponds to a specific cooling power (SCP) is about 17 W/kg with both heat and mass recoveries between two beds. Considering the optimal adsorption time is much longer than optimal desorption time at the condition of ice making, the experiments are operated on a single bed (60 kg adsorbent per bed) and the adsorption time used in experiments is two times of desorption time, then the performance of a three-bed adsorption ice maker (120 kg adsorbent totally, 40 kg adsorbent per bed) is predicted by the results of experiments on this single bed. The results of prediction show that both COP and cooling power of three-bed operation could be enhanced greatly compared to the two-bed operation; optimal SCP and COP are respectively 22 W/kg and 0.239 when mass and heat recoveries proceed between three beds. Optimal ice productivity of this three-bed system is 21 kg/h when the water temperature is 25 °C and ice temperature is −7 °C.     

太阳能冷管的研究及其进展

太阳能冷管以沸石分子筛—水为工质对，在一根玻璃管内完成吸附式制冷循环，一根冷管即为一个制冷单元，成功地解决了太阳能吸附式制冷技术难以转化为成果的问题。本文综述了作者近几年来对太阳能冷管首创性提出，以及其结构性能的研制和改进情况。采用真空集热方式和选择性涂层加强冷管对太阳能的吸收，采用整体固化复合吸附剂提高吸附床的吸附和脱附性能。本文还介绍了已制作的三代太阳能冷管型制冷系统的试验样机，在单一提供制冷的基础上，提出了既可以制冷又可以供热水的多功能太阳能冷管。目前，实验结果表明，最新的多功能太阳能冷管COP可达0．268，太阳能制冷与供热的总效率可达87．7％。     

A new type adsorber for adsorption ice maker on fishing boats

A new type of adsorber for an adsorption ice maker on fishing boats, which uses a compound adsorbent (activated carbon and CaCl₂) and ammonia working pair, is designed. This type of heat pipe adsorber solves the problem of incompatibility between ammonia, copper, seawater and steel. The heating/cooling power for the adsorption/desorption process of the adsorbent, which is required to be transferred by one heat pipe in the adsorber, is computed by the test results of the adsorbent, and the heat transfer performance of one heat pipe in the adsorber is simulated according to the theory of the two phase closed thermosyphon. The heat transfer performance of the heat pipe can meet the heat demands for adsorption/desorption of the adsorbent when the evaporating temperature is −15 °C and the cycle time is 10 min. A test unit is set up to test the heating/cooling performance of the heat pipe type adsorber, and the experimental results are coincident with the simulation. The performance of a two bed adsorption ice maker with heat pipe adsorbers is predicted, and the cooling power is about 17.1–17.8 kW at the evaporating temperature of −15 °C and cycle time of 10 min with mass recovery between two 29 kg compound adsorbent beds.     

Zeolite-active carbon compound adsorbent and its use in adsorption solar cooling tube

The “zeolite-water” pair adsorption process has been extensively studied for its application in cooling system. This paper presents a compound zeolite-active carbon adsorbent (CZACA) which enhances the heat transfer in the adsorbent bed by absorbing solar energy directly and decreases the desorption cycle time in a solar cooling system. Experimental study of a special solar cooling tube using this adsorbent is discussed. The effects of some operating parameters on the system performance are also evaluated.     

Cycle Optimization on Reheat Adsorption Cycle Applying Fixed Chilled Water Outlet Temperature

This study investigated the cycle optimization of four-bed, silica gel–water adsorption with reheat cycle, where the desorber (upper bed) and adsorber (lower bed) always interact with the condenser and evaporator, to exploit a low heat-source temperature. In a previous study, the performance of a reheat cycle with chilled water outlet temperature fixed at 9°C was observed without considering the cycle optimization. Maintaining a constant chilled water outlet temperature is also of equal importance to improve the conversion efficiency so that maximum cooling capacity can be derived. In this paper, a simulation model of reheat adsorption cycles is developed to analyze the optimization of the cycle time, including adsorption/desorption time, mass recovery time, and preheating/precooling time, with chilled water outlet temperature fixed. The reheat working principle is also introduced. The proposed cycle is compared with the four-bed versison without reheat cycle in terms of coefficient of performance (COP) and cooling capacity. The result shows that the performance of a reheat cycle is superior to that of four-bed version without reheat, especially for low heat-source temperature. For low heat-source temperature (55–65°C) both COP and cooling capacity of the reheat cycle with optimization were raised significantly compared to the high heat-source temperature (70–80°C).     

真空集热型太阳能固体吸附式制冷的理论研究

为提高太阳能吸附制冷系统的集热性能。提出了采用真空集热管式吸附床的太阳能固体吸附制冷系统,并对选择吸收式和直接吸热式的真空集热制冷系统分别进行了理论分析与计算模拟。这两种系统均具有较高的制冷性能,前者宜以沸石-水为制冷工质对,而后者则宜采用活性碳-甲醇工质对。分析了工作参数对这两种真空集热型制冷系统的影响,并对系统结构进行了优化研究。     

Experiments of a solar flat plate hybrid system with heating and cooling

Based upon the prior research of the solar hybrid water heater and refrigerator, a new flat plate solar hybrid system with heating and cooling was proposed and experimental prototype device was constructed. With this new hybrid system, the heat and mass transfer can be improved effectively both in desorption process and adsorption process. The conventional flat plate solar water heater collector absorber is immersed inside adsorbent bed in the new hybrid system. The experimental results show that not only the cooling effect can be obtained, but also both the sensible heat of the adsorbent bed and the adsorption heat can be recovered effectively to produce hot water for domestic use. The COP of this new flat plate hybrid system can reach 0.11 and the heat efficiency is about 0.45, this achievement has demonstrated an efficient way of the application of solar energy.     

The Numerical Comparison of Heat Transfer in a Coated and Fixed Bed of an Adsorption Chiller

Ecological adsorption technology is becoming a focus of attention by industry due to the utilization of low grade thermal energy sources for cooling production. It can be a promising part of sustainable development concept of the global economy. Therefore, research aiming at improving their performance i.e. Coefficient of Performance (COP) by optimizing the construction of sorption beds with a built in heat exchanger system is crucial. The heat transfer characteristics between the bed of porous media (sorbent) and surface of the heat exchanger system determine the heating power of an adsorption chiller. The HP increase can be obtained by heat transfer intensification due to the increase in the thermal conductivity of the sorbent layer in the vicinity of the heat exchanger’s surface. The novel modification of the sorbent layer structure is proposed in the paper in order to improve the heat transfer processes in the heat exchanger boundary layer. The analysis of desorption process conditions in the parametric model of a coated and fixed adsorption bed design is presented in the paper. The computational fluid dynamics (CFD) with conjugate heat transfer analysis is used to determine the crucial input parameters (temperature distribution in the sorbent bed) for further analytical calculations. The commercial code Ansys Fluent was used to perform numerical simulations. The developed computational model consisted of three subdomains representing heating water, heat exchanger material (copper) and sorbent (silica gel). The comparison of a novel coated design and a conventional fixed bed is discussed in the paper. The numerical analysis is based on experimental thermal conductivity measurements of the sorbent layer in different configurations, which were performed using Laser Flash Method.     

Numerical study of a combined heat and mass recovery adsorption cooling cycle

A new transient two-dimensional model for the simulation of a combined heat and mass recovery adsorption cooling cycle based on the zeolite NaX/water working pair is proposed in this paper. The model describes the transfer phenomena in the adsorber in detail and is solved by control volume method. Internal and external mass transfer limitations which are neglected by many researchers are considered in the model since they have significant effects on the performance of the adsorption cooling cycle. The numerical results show that the combined heat and mass recovery cycle between two adsorbent beds can increase the coefficient of performance (COP) of an adsorption cooling system by more than 47% compared to the single bed cycle. This numerical model can be used in system optimization and design of adsorption cycles.     

A two-stage deep freezing chemisorption cycle driven by low-temperature heat source

A two-stage chemisorption cycle suitable for deep-freezing application driven by low- temperature heat source was proposed. Through two-stage desorption processes, the two-stage cycle can break through the limitations of the heating temperature and ambient cooling temperature. The two-stage cycle based on CaCl₂/BaCl₂-NH₃ working pair can utilize the heat source with a temperature of above 75°C, and simultaneously realize deep-freezing all the year round. Experimental results and performance prediction show that the adsorption quantity of calcium, theoretical coefficient of performance (COP) and optimized specific cooling power (SCP) of the CaCl₂/BaCl₂-NH₃ chemisorption system are 0.489 kg/kg (salt), 0.24 and 120.7 W/kg, when the heating temperature, ambient cooling temperature, pseudo-evaporating temperature and mass ratio of reacting salt and expanded graphite are 85, 30, -20, and 4∶1, respectively.     

固体吸附式冷管循环工作过程的动态模拟与分析

针对余热驱动吸附式冷管单元(直径16mm,总长1020mm)制冷循环过程的变压吸/脱附特性,采用线性驱动力(LDF)模型,建立了动态的传热传质数学模型。对吸附式冷管的主要部件——吸附器和冷凝/蒸发器在加热解吸和冷却吸附不同阶段的工作过程,分别建立了耦合的动态方程。对所提出的模型进行分析和合理简化后,利用数值方法对模型进行了求解,获得了冷管单元吸附器、冷凝/蒸发器的循环工作参数的动态变化规律,计算结果与实验结果较好地吻合。为吸附式冷管单元及其组合式制冷系统进一步的优化设计和实用化的改进研究提供了重要参考。     

Experimental investigation of a natural zeolite–water adsorption cooling unit

In this study, a thermally driven adsorption cooling unit using natural zeolite–water as the adsorbent–refrigerant pair has been built and its performance investigated experimentally at various evaporator temperatures. The primary components of the cooling unit are a shell and tube adsorbent bed, an evaporator, a condenser, heating and cooling baths, measurement instruments and supplementary system components. The adsorbent bed is considered to enhance the bed’s heat and mass transfer characteristics; the bed consists of an inner vacuum tube filled with zeolite (zeolite tube) inserted into a larger tubular shell. Under the experimental conditions of 45 °C adsorption, 150 °C desorption, 30 °C condenser and 22.5 °C, 15 °C and 10 °C evaporator temperatures, the COP of the adsorption cooling unit is approximately 0.25 and the maximum average volumetric cooling power density (SCP_v) and mass specific cooling power density per kg adsorbent (SCP) of the cooling unit are 5.2 kW/m³ and 7 W/kg, respectively.     

Utilizing Accessible Heat Enhancing Cooling Effect with Three Bed Solar Adsorption Chiller

ABSTRACT

Conventional solar heat-driven single stage two bed chillers demand a large area for installation of solar thermal collector to activate the chiller, but in a highly populated tropical country open spaces is insufficient. In the intention to utilize accessible solar energy with better performance, a mathematical investigation is carried out with a three bed adsorption cooling unit working with silica gel-water pair. The studied chiller is powered by direct solar heat collected by a series of compound parabolic concentrator solar thermal collectors without any heat or mass recovery. The working principal of the chiller is, in principle, the same as the conventional two-bed adsorption chiller. However, instead of two half cycles, there are three one third cycles in the proposed chiller in which at every cycle the former desorber is kept in the precooling mode and as an adsorber for the next two one third cycles, respectively. As desorption kinetic is faster than the adsorption kinetics, this longer precooling mode helps the silica gel granules to adsorb more water molecules and increase evaporation rate. Hence, a better cooling effect of at least 1°C can be observed, increases chiller working hour after sunset for almost a further one hour.     

新型平板式太阳能冷热联供装置

在积累了太阳固体吸附式制冷循环研究的基础上，与现有的平板式太阳热水器制造技术紧密结合，提出了平板式太阳冷热联供循环方式，并在实验室内成功地制作了实物样机。该装置能有效地回收太阳固体吸附式制冷不中吸附床的显热及吸附热，且操作简便。实验结果有效地支持了所提出的设想，为太阳固体吸附式制冷的实用化应用打下了良好基础。     

Activated carbon + HFC 134a based two stage thermal compression adsorption refrigeration using low grade thermal energy sources

A thermodynamic analysis is presented for the two stage thermal compression process for an adsorption refrigeration cycle with HFC-134a as the working fluid and activated carbon as the adsorbent. Three specimens of varying achievable packing densities were evaluated. The influence of evaporating, condensing/adsorption and desorption temperatures was assessed through three performance indicators, namely, the uptake efficiency, the coefficient of performance and the exergetic efficiency. Conditions under which a two stage thermal compression process performs better than the single stage unit are identified. It is concluded that two stage thermal compression will be a viable proposition when the heat source temperature is low or when adsorption characteristics are weak or when adequate packing densities are difficult to realize.     

System performance of a combined heat and mass recovery adsorption cooling cycle: A parametric study

Based on the previous work of the authors [K.C. Leong, Y. Liu, Numerical study of a combined heat and mass recovery adsorption cooling cycle, Int. J. Heat Mass Transfer 47 (2004) 4761–4770], a numerical study of the effects of system design and operation parameters on the performance of a combined heat and mass recovery adsorption cycle is presented in this paper. The effects of bed dimensions, bed thermal conductivity, heat exchange fluid velocity, driven temperature and the degree of the heat recovery on the system performance are investigated. It is found that an increase in the driven temperature results in the increase of both the coefficient of performance (COP) and specific cooling power (SCP) of the adsorption cycle. On the other hand, the system performance can be severely deteriorated for velocities of the heat exchange fluid smaller than a critical value. An increase in the bed thickness will result in an increase in COP and a decrease in the SCP. The results of our simulations will provide useful guidelines for the design of this type of advanced adsorption cooling cycle.