PROPOSAL FOR A NOVEL CHEMICAL HEAT PUMP DRYER

ABSTRACT

A new chemical heat pump (CHP) system for ecofriendly effective utilization of thermal energy in drying is proposed from the viewpoints of energy saving and environmental impact. CHPs can store thermal energy in the form of chemical energy by an endothermic reaction and release it at various temperature levels for heat demands by exo/endothermic reactions. CHPs have potential for heat recovery and dehumidification in the drying process by heat storage and high/law temperature heat release. In this study, we estimate the potential of the CHP application to drying systems for industrial use. Some combined systems of CHPs and dryers are proposed as chemical heat pump dryers (CHPD). The potential for commercialization of CHPDs is discussed.     

EXPERIMENTAL STUDIES ON A NOVEL CHEMICAL HEAT PUMP DRYER USING A GAS–SOLID REACTION

The authors have been studying Chemical Heat Pumps (CHP) from the viewpoints of energy saving and environmental impact. The CHP can store thermal energy in the form of chemical energy by an endothermic reaction, and release it at various temperature levels during heat demand periods by exo/endothermic reactions. The authors have proposed in an earlier study a novel chemical heat pump (CHP) system for environmentally-friendly effective utilization of thermal energy in drying as a chemical heat pump dryer (CHPD). In this exploratory study, we test the effectiveness of operating the proposed CHPDs experimentally. Basic experiments on the CHPDs such as hot dry air production for convective drying are performed on lab-scale CHPD apparatuses using gas–solid reactions in calcium oxide/calcium hydroxide reactant beds. The proposed CHPDs are found to produce hot air by CHP operation for drying. The temperature levels of the produced hot air and the reaction rates/conversions are as good as in the case of hot water supply system using basically same CHP operation. The cold heat for air dehumidification is also found to be generated/recovered by the same CHPD system. The generated heat amounts can be increased by changing the operating conditions although the heat recovery must be enhanced for practical application of CHPDs.     

Energy and Cost Estimation for Application of Chemical Heat Pump Dryer to Industrial Ceramics Drying

Abstract

In this article we estimate the potential of a new chemical heat pump dryer (CHPD) application to an industrial ceramics drying process from the viewpoints of energy and cost saving. A CaSO₄/H₂O/CaSO₄·1/2H₂O hydration/dehydration CHPD system and a CaO/H₂O/Ca(OH)₂ hydration/dehydration CHPD system were examined. The CHPD systems store heat and simultaneously release the increased amount of heat at different temperature levels by using two chemical heat pumps (CHP) in their heat-enhancement mode. Furthermore, we propose enhanced systems using chemical heat pipes (CHPipe) for their environmental and cost merits. As a result, the consumed energy and the cost of using the CHPD systems in the industrial ceramics drying process are found to decrease to less than half of the conventional drying process using gas-fired boilers. For example, the energy efficiency and the cost of the present drying process are 28.4% and 604 × 10³ (JPY/month) (JPY: Japanese Yen), respectively. The energy efficiency and the cost of the proposed CHPD system are found to be 79.7% and 216 × 10³ (JPY/month), respectively, based on our experiments.     

A NOVEL RADIO FREQUENCY ASSISTED HEAT PUMP DRYER

ABSTRACT

This paper compares an experimental heat pump batch dryer with the implementation of volumetric Radio Frequency (RF) heating, in the combination drying of crushed brick particulate. Results are presented showing overall improvements in drying

A simplified mathematical drying model including the RF energy source has been developed using mass and energy conservation, confirming the experimental results.     

EXPERIMENTAL STUDIES ON A NOVEL CHEMICAL HEAT PUMP DRYER USING A GAS-SOLID REACTION

The authors have been studying Chemical Heat Pumps (CHP) from the viewpoints of energy saving and environmental impact. The CHP can store thermal energy in the form of chemical energy by an endothermic reaction, and release it at various temperature levels during heat demand periods by exo/endothermic reactions. The authors have proposed in an earlier study a novel chemical heat pump (CHP) system for environmentally-friendly effective utilization of thermal energy in drying as a chemical heat pump dryer (CHPD). In this exploratory study, we test the effectiveness of operating the proposed CHPDs experimentally. Basic experiments on the CHPDs such as hot dry air production for convective drying are performed on lab-scale CHPD apparatuses using gas-solid reactions in calcium oxide/calcium hydroxide reactant beds. The proposed CHPDs are found to produce hot air by CHP operation for drying. The temperature levels of the produced hot air and the reaction rates/conversions are as good as in the case of hot water supply system using basically same CHP operation. The cold heat for air dehumidification is also found to be generated/recovered by the same CHPD system. The generated heat amounts can be increased by changing the operating conditions although the heat recovery must be enhanced for practical application of CHPDs.     

PROPOSAL FOR A NOVEL CHEMICAL HEAT PUMP DRYER

A new chemical heat pump (CHP) system for ecofriendly effective utilization of thermal energy in drying is proposed from the viewpoints of energy saving and environmental impact. CHPs can store thermal energy in the form of chemical energy by an endothermic reaction and release it at various temperature levels for heat demands by exo/endothermic reactions. CHPs have potential for heat recovery and dehumidification in the drying process by heat storage and high/law temperature heat release. In this study, we estimate the potential of the CHP application to drying systems for industrial use. Some combined systems of CHPs and dryers are proposed as chemical heat pump dryers (CHPD). The potential for commercialization of CHPDs is discussed.     

THERMAL PERFORMANCE AND EVALUATION OF SIPERHEATED STEM DRYING SYSTEM USING SOLAR-ASSISTED ABSORPTION HEAT TRANSFORMER

ABSTRACT

This paper summarizes the main results of our study carried out under the priority-area research “Energy Conversion and Utilization with High Efficiency” supported by the grant-in-aid of scientific research from the Ministry of Education, Science and Culture of Japan. In the research we focused on applications of solar energy utilization for industrial drying systems. In this paper a new concept of a closed drying system with superheated steam provided from a temperature boosting type heat pump (absorption type heat transformer) was proposed. The heat transformer is driven directly by heat from a solar collector. The performance for different three drying periods and a number of factors affecting it were predicted by a computer simulation. Furthermore the heat and mass transfer characteristics of water evaporation into superheated steam stream by radiative and convective heating were indicated experimentally. It was concluded that the system would be useful for industries where high temperature (over 100°C) drying is required.     

HEAT PUMP DRYING: RECENT DEVELOPMENTS AND FUTURE TRENDS

ABSTRACT

This review paper focuses on the recent progress made in heat pump drying (HPD). An introduction is given on the operating and working principle of the heat pump drying system. Advantages and limitations of HPD are outlined. Two industrial applications of HPD, food and timber, are discussed. New developments in HPD are presented. A section discussing the potential of incorporating advanced heat pump cycles for drying application is also presented. Some possible new hybrid HPD technologies, e.g., radio frequency and infrared assisted HPD are discussed in terms of their industrial potential. Opportunities for further R&D to achieve better product quality and energy efficiency are identified. A new heat pump dryer design is also described to demonstrate the new trend in versatile heat pump design.     

Enhancement of Energy Efficiency of a Chemical Heat Pump-Assisted Convective Dryer

In earlier studies we have shown by simulation and experimental studies that the proposed chemical heat pump (CHP) unit can be used to recover waste heat from dryers and reuse it by storing and releasing heat with upgrading the temperature or by dehumidification. However, the final thermal energy production efficiency of the CHP for drying was found to be low. In this paper we present experimental results to demonstrate the potential for improved heat-recovery/storage and the heat-release/production of hot dry air for batch drying applications using the heat enhancement mode of the CHP. A new laboratory scale experimental CHP dryer system was built utilizing the calcium oxide/calcium hydroxide hydration/dehydration reversible reaction. The aim of this study is to improve the efficiencies of the heat recovery from heat source in the heat-storage step and the hot dry air production in the heat-release step of the CHP for heating up the air to around 100°C. The results of this experimental study utilizing a new reactor design showed that the shallow reactor/heat exchanger could accomplish 94% chemical heat storage and produce 100°C air at better than 75% efficiency for the reaction heat by controlling the preheating condition. The reaction conversion reached 90% in these experiments. The proposed CHP-assisted convective dryer system is found to be energy-efficient over a wide temperature range of industrial interest.     

HEAT PUMP DRYERS: RESEARCH AND DEVELOPMENT NEEDS AND OPPORTUNITIES

ABSTRACT

As some previous research works on heat pump dryers (HPD) gave contradictory results, there is a need to review and identify R&D needs and opportunities in HPD. It was found that mathematical modeling cannot ignore the interdependence of the heat pump working fluid and the process air of the dryer. The performances of various HPD configurations with respect to all operating varibles need further investigation. A well defined mathematical model of combined dryer-drying material characteristic is required for the system modeling. The relative specific drying cost and relative useful energy were introduced as the dryer selection criteria. The role of the heat pump in the HPD system, CFC alternatives and non-conventional heat pump cycles using air or steam as working fluids deserve further investigation.     

MATHEMATICAL MODEL DEVELOPMENT AND SIMULATION OF HEAT PUMP FRUIT DRYER

ABSTRACT

A mathematical model of a heat pump fruit dryer was developed to study the performance of heat pump dryers. Using the moisture content of papaya glace' drying, the refrigerant temperature at the evaporator and condenser and the performance, was verified. It was found that the simulated results using closed loop heat pump dryer were close to the experimental results. The criteria for evaluating the performance were specific moisture extraction rate and drying rate. The results showed that ambient conditions affected significantly on the performance of the open loop dryer and the partially closed loop dryer. Also, the fraction of evaporator bypass air affected markedly on the performance of all heat pump dryers. In addition, it was found that specific air flow rate and drying air temperature affected significantly the performance of all heat pump dryers.     

Drying Kinetics of Tomato by Using Electric Resistance and Heat Pump Dryers

Abstract

Drying kinetics of tomato was studied by using heat pump dryer (HPD) and electric resistance dryers with parallel and crossed airflow. The performance of both systems was evaluated and compared and the influence of temperature, air velocity, and tomato type on the drying kinetics was analyzed. The use of HPD showed to be adequate in the drying process of tomatoes, mainly in relation to the conversion rate of electric energy into thermal energy. The heat pump effective coefficient of performance (COP_HT,EF) was between 2.56 and 2.68, with an energy economy of about 40% when compared to the drying system with electric resistance. The Page model could be used to predict drying time of tomato and statistical analysis showed that the model parameters were mainly affected by drying temperature.     

DRYING HEAT PUMP WITH WATER VAPOUR AS WORKING MEDIUM

ABSTRACT

A new steam drying technique is presented which is realized by a heat pump bsscd on the minimum energy cycles approximation. The main advantage of this heat pump. in comparison with the steam recompression drying technique, is the use of significantly smaller heat exchanging area which means considerably lower investment cost.     

Reducing Dryer Energy Use by Process Integration and Pinch Analysis

ABSTRACT

A systematic classification is presented for methods for reducing energy use and cost in dryers. Opportunities include reducing the inherent evaporation load and dryer heat duty, heat recovery, and more effective use of utility systems, including CHP (combined heat and power) and heat pumping. The technique of pinch analysis provides a powerful method to identify the various opportunities and select the most appropriate one to use. Reductions in energy use bring corresponding benefits in reduced emissions and conservation of natural resources. Some examples are given of successful applications of energy minimization techniques in industry.     

氯化钙─甲醇化学热泵换向循环过程理论研究

研究将一种可改善气固化学热泵循环性能的周期性强制流向变换的动态操作引入双反应器氯化钙─甲醇化学热泵系统，在文献［３～５］的基本循环过程研究的基础上，建立了描述这类换向循环过程的动态分布参数模型。由此通过数值模拟分析了固相床层内的热波现象。对不同换向操作方式的研究结果表明：周期反向操作确实可以改善常规的同向操作的循环性能，而半周期反向操作则比常规的性能还要差。     

IDENTIFICATION OF MATHEMATICAL MODEL COEFFICIENTS IN THE ANALYSIS OF THE HEAT AND MASS TRANSPORT IN WOOD

ABSTRACT

A method based on the concept of solving inverse heat and mass transport problems was proposed to identify wood physical properties, with the use of empirical data, a mathematical model corresponding to the direct problem, and an optimization procedure. The computer software was developed to solve transient, three-dimensional, quasi-linear direct and inverse problems of heat and mass transport in wood as an anisotropic body together with initial and boundary conditions of any kind. The software was adapted to data parallel processing environment of high performance computers. Identification of thermal conductivity was performed for beech wood in three principal anatomic directions, and accuracy of temperature distribution predictions was significantly increased (the global relative error of prediction was reduced to 1.0 - 1.9%).     

A NUMERICAL STUDY OF FLUID FLOW AND HEAT/MASS TRANSFER OVER A STACK OF PLANKS

ABSTRACT

Heat/mass transfer by air flow over a sample stack of planks is studied numerically. For the simulations, the low Re k-epsilon turbulence model and bounded QUICK scheme are used. The calculated Nusselt numbers are in good agreement with the experimental data

The results of our study show that the low Re turbulence models have advantages over the conventional high Re models for this type of industrial application. This is mainly due to the small height of separation bubbles resulting from the selected large blockage ratios (more than 50 percent) occurring in such flows

Numerical simulations were carried out to study the effect of the vertical air gap due to shrinkage and non uniform sawing as well as the non uniformity in the height of boards on the flow field and heat/mass transfer characteristics. The results show that the selected gap size significantly affects the local and average Nu numbers across the stack. We have suggested optimum gap sizes for maximum heat/mass for different flow velocities (Re numbers).     

Chemical and adsorption heat pumps: Cycle efficiency and boundary temperatures

The efficiency of an ideal three-temperature (3T) cycle of a chemical heat pump (CHP) is considered. For a reversible CHP 3T cycle, the maximal efficiency can be determined using the ratio of the heat of evaporation of the working fluid and the heat of the chemical reaction or using the boundary temperatures of the cycle. The boundary temperatures of the reversible CHP cycle are not independent variables relative to each other. As they are related by the Clausius-Clapeyron equation for the equilibrium of the pure fluid and by the van’t Hoff equation for the chemical reaction, the choice of one of these temperatures completely determines the two others. Comparison of the efficiencies of the CHP cycle and the cycle of an adsorption heat pump (AHP) shows that the CHP efficiency can theoretically reach the Carnot-cycle efficiency whereas the AHP efficiency is always less than the Carnot-cycle efficiency because of the generation of entropy due to the transfer of heat to (from) the adsorber at a finite temperature difference. The minimal temperature of the external heat source needed for the operation of the CHP (AHP) cycle, the minimal evaporator temperature, and the maximal condenser temperature are calculated. These temperatures are determined for CHP (AHP) applications such as heating, air conditioning, ice production, and deep freezing in different climatic zones. Original Russian Text ? Yu.I. Aristov, 2008, published in Teoreticheskie Osnovy Khimicheskoi Tekhnologii, 2008, Vol. 42, No. 6, pp. 676–685.     

CONJUGATE HEAT AND MASS TRANSFER IN CONVECTIVE DRYING OF MULTIPARTICLE SYSTEMS. PART I: THEORETICAL CONSIDERATIONS

Abstract

This is the first of two papers concerning conjugate transport in convective drying of multiparticle systems. In this study, a methodology for the solution of conjugate transport problems is proposed. The theoretical aspects and relevant assumptions are briefly discussed and the results for convective heat and mass transfer in assemblages of spheres are presented. It is shown that both heat and mass transfer rates for an assemblage of two spheres in tandem arragement are significantly different from those of a single sphere case, and that care should be taken when modeling multiparticle systems by means of single-particle analysis.     

Heat integration and analysis of decarbonised IGCC sites

Integrated gasification combined cycle (IGCC) power generation systems have become of interest due to their high combined heat and power (CHP) generation efficiency and flexibility to include carbon capture and storage (CCS) in order to reduce CO₂ emissions. However, IGCC's biggest challenge is its high cost of energy production. In this study, decarbonised coal IGCC sites integrated with CCS have been investigated for heat integration and economic value analyses. It is envisaged that the high energy production cost of an IGCC site can be offset by maximising site-wide heat recovery and thereby improving the cost of electricity (COE) of CHP generation. Strategies for designing high efficiency CHP networks have been proposed based on thermodynamic heuristics and pinch theory. Additionally, a comprehensive methodology to determine the COE from a process site has been developed. In this work, we have established thermodynamic and economic comparisons between IGCC sites with and without CCS and a trade-off between the degree of decarbonisation and the COE from the heat integrated IGCC sites. The results show that the COE from the heat integrated decarbonised IGCC sites is significantly lower compared to IGCC sites without heat integration making application of CCS in IGCC sites economically competitive.