Modelling of an ethanol–water distillation column with vapour recompression

This paper presents a simulation study of an ethanol–water distillation column with vapour recompression. The compression of the overhead vapour from the distillation column and the use of condensing vapours to reboil the bottom products were modelled. Predicted results were compared with those from a simulation model for an ethanol–water distillation column assisted by an external heat pump, as previously developed by the authors. Copyright © 2001 John Wiley & Sons, Ltd.     

Heat pump assisted distillation. V: A feasibility study on absorption heat pump assisted distillation systems

The problems in matching a heat driven absorption heat pump to a distillation process in heat pump assisted distillation are discussed. The performance of an absorption system is a function of the temperatures in the evaporator, the condenser, the absorber and the generator and the ratio of the mass flow rate in the secondary circuit to the mass flow rate in the primary circuit. In absorption systems design choices are limited by the Gibbs phase rule. Plots are given of the coefficient of performance against the temperatures of the top and bottom products and also against the energy saved.     

An economic comparison of a fixed speed,a two speed,and a variable speed vapour compression heat pump

The seasonal performances, accounting for steady state conditions and on/off cycling losses, of a conventional fixed speed, a two speed, and a variable speed vapour compression heat pump have been determined for the annual heating demand of a large detached house. An economic analysis of the energy savings shows that, compared with a conventional fixed speed heat pump, the two speed, simple variable speed, and microprocessor-controlled heat pumps have pay-back periods of two, six and five years respectively.     

Optimum performance of a heat engine‐driven combined vapour compression–absorption–ejector heat pump

Optimum performance of an endoreversible heat engine‐driven heat pump cycle, based on a combination of an absorption cycle with a vapour and ejector compression cycles is investigated. This combination increases the performance of the conventional ejector and absorption cycles and provides high performance for heating. The analysis show that the combined heat pump cycle has a significant increase in system performance over the heat engine‐driven vapour compression or absorption heat pump cycle and heat engine‐driven combined vapour compression and absorption heat pump cycle. Copyright © 2000 John Wiley & Sons, Ltd.     

Experimental study of the operating characteristics of a heat pump assisted distillation system using R11 as the external working fluid

Heat pump assisted distillation with an external working fluid is one of the most obvious methods to reduce the energy consumption in a distillation process. The heat pump working fluid extracts heat from the top of the column, increases the temperature of the recovered heat and recycles it to provide the heat input to the reboiler.The interaction between the external parameters and the internal parameters for a specially designed heat pump assisted distillation system has been studied experimentally. The external parameters were mass flow rate, temperature and concentration of the feed, the concentration of the top and bottom products and the mass flow rate of the working fluid. The effects of the variations of these external parameters on the internal parameters such as the energy (steam) consumption, the actual coefficient of performance and the temperatures at the top and bottom of the column, together with the condensation and evaporation temperatures, are presented.     

Heat pump assisted distillation. II: Matching an external heat pump to a fractional distillation system

The problems in matching an external mechanical vapour compression heat pump to a distillation process in heat pump assisted distillation are discussed. There are four main design parameters to consider when selecting a working fluid for the heat pump, of which only two are independent. Various arrangements for imperfectly matched systems are presented, including the use of a two stage heat pump. A number of other factors which affect the choice of a heat pump assisted distillation system, including economic factors, are also considered.     

Heat pump assisted distillation. VII: A feasibility study on heat transformer assisted distillation systems

The problems in matching a heat driven heat transformer to a distillation process in heat pump assisted distillation are discussed. The performance of such a system is a function of the temperature difference across the distillation column. Plots are given of the calculated coefficients of performance against the temperatures of the top and bottom products. In addition plots are given of the calculated coefficients of performance against the generator temperature when the generator is independently heated. The percentage of energy saved is also plotted against coefficient of performance.     

Heat pump assisted distillation. VIII: Design of a system for separating ethanol and water

A heat pump assisted distillation system has been designed for the separation of ethanol from 7 per cent aqueous mixtures to produce 93 per cent ethanol by weight. The distillation column has been designed on the basis of conventional design procedures. Valve trays were chosen to provide operational flexibility. R114 was chosen as the working fluid for the mechanical vapour compression heat pump. The heat pump system has been designed to match the heat loads determined for the column. Auxiliary heat exchangers have been provided to aid flexibility and control of column operation.     

Impact of vapor recompression in batch distillation on energy consumption,cost and CO2 emission: Open-loop versus closed-loop operation

This paper aims at analyzing the thermal integration of direct vapor recompression system with a distillation column operated in batch mode. The unsteady state nature of the batch processing makes the use of vapor recompression a challenging task. For a meaningful comparison, it is attempted to run the vapor recompressed batch distillation (VRBD) column at same dynamics with its conventional counterpart. With this operating objective, the manipulation of a few process variables is proposed in open-loop fashion. Along with the energy savings and total annualized cost, the CO₂ emission level is also used for quantitative performance evaluation of the proposed VRBD column. Aiming to produce a constant and high-purity distillate product, a gain-scheduled proportional integral (GSPI) controller that targets to keep the stability margin constant is devised for the VRBD process. Finally, this heat integration mechanism is illustrated by a binary batch distillation example in both open-loop and closed-loop modes. It is investigated that the VRBD shows a slightly higher energy savings (62%) compared to its closed-loop version (59.13%) but at the expense of distillate purity. The heat integration also achieves a significant savings in cost and CO₂ emission.     

Use of heat pumps with solar collectors for domestic space heating in the United Kingdom

The performance of air to air heat pumps is discussed and it is shown that the machines which are commercially available in the UK have higher running costs when used for domestic space heating than gas or coal fires. It is technically feasible to improve the performance of a heat pump by supplying solar energy as a low temperature heat source, but a method developed for evaluating the financial advantage of such a scheme showed it to be unattractive. It is recommended that more effort should be made to improve the COP of air source heat pumps for domestic space heating in the United Kingdom.     

Economic analysis and comparison of chemical heat pump systems

Over the last years great interest has been shown in chemical heat pump systems. Chemical heat pumps represent a new technology with great potential to reduce the energy consumption in very different sectors. They can provide the ability to capture the rejected low-grade heat and to reuse it at increased temperature levels in various industrial processes. Heat can be removed from a heat source at low-temperature by an endothermic reaction and can be boosted to a heat sink at high-temperature by an exothermic reaction.Since chemical heat pumps can operate without compression, with less electrical power and at higher temperature levels compared to conventional heat pumps, they can afford high performance advantages. As an additional advantage, energy storage can also be accomplished so that intermittent energy sources can be utilized in a chemical heat pump system.The objective of this work was to study methanol–formaldehyde–hydrogen, ethanol–acetaldehyde–hydrogen, i-propanol–acetone–hydrogen and n-butanol–butyraldehyde–hydrogen chemical heat pump systems based on catalytic dehydrogenation of alcohols at low-temperature and hydrogenation of aldehydes and a ketone at high-temperature. On the base of economic analysis, the quantity of waste-heat that must be supplied to produce the benefits of the process heat and also the improvement in the net gain reached were determined and compared.     

Performance analysis of an irreversible cogeneration heat pump cycle

An irreversible heat engine-driven vapour compression and absorption heat pump system is considered as a cogeneration cycle. The effects of thermal resistances and internal irreversibilities on the coefficient of performance (COP) of this cogeneration cycle were investigated using finite-time thermodynamic approach. An improved equation for the COP of the system under consideration was obtained. The results obtained here may serve as a good guide for the evaluation of existing real cogeneration heat pumps or provide some theoretical bases for the optimal design of future cogeneration heat pumps. © 1998 John Wiley & Sons, Ltd.     

Experimental study of operating characteristics of compression/absorption high-temperature hybrid heat pump using waste heat

This research describes the development of a compression/absorption hybrid heat pump system that utilizes a mixture of NH₃ and H₂O as a working fluid. The heat pump cycle is based on a hybrid combination of vapor compression cycle and absorption cycle. The system consists of major components of two-stage compressors, absorbers, and a desorber. There are also auxiliary parts like a desuperheater, solution heat exchangers, a solution pump, a rectifier, and a liquid/vapor separator to support stable operation of the heat pump. This compression/absorption hybrid heat pump provides many advantages of performance over conventional vapor compression heat pumps including a large temperature glide, an improved temperature lift, a flexible operating range, and greater capacity control. These benefits are optimized by changing the composition of the mixture. In this study, the effect of the composition on the operating characteristics of the compression/absorption hybrid heat pump was experimentally observed.     

Techno-economic feasibility of high-temperature high-lift chemical heat pumps for upgrading industrial waste heat

This paper presents the results of a techno-economic feasibility study on two high-temperature high-lift chemical heat pumps for upgrading industrial waste heat. The study was set up in order to select the most promising heat pump concept for further development. First, a market study is performed to assess the amount of waste heat and the temperature dependence thereof. Based on the market potential, two heat pump concepts are selected for further detailed evaluation. These are an isopropanol heat pump and a salt/ammonia vapour heat pump. Both heat pump concepts are technically able to upgrade the waste heat to usable temperature levels. However, the salt/ammonia vapour heat pump has a much better technical performance. In addition, the economic analysis shows that this heat pump has a far better economic feasibility with a mean internal rate of return of 14%. Therefore, this heat pump is selected for further development.     

线性唯象律下三热源热泵的生态学优化性能

基于生态学目标函数对线性唯象律下的三热源热泵进行优化分析,导出热泵的生态学优化性能,并与最大供热率工况进行比较,获得一些重要新结论,它对太阳能热泵、吸收式热泵等一类三热源热泵的优化设计和最佳工况选择等均有指导意义     

Comparative study of internal storage and external storage absorption cooling systems

This work presents a comparative study of the performance of absorption cooling systems with internal storage and with external storage. A full dynamic simulation model including the solar collector field, the absorption heat pump system and the building loads has been performed. The first system is composed by four heat pumps that store energy in the form of crystallized salts so that no external storage capacity is required. The second one is a conventional system composed of one liquid absorption pump and external storage in a water tank. Many batteries of simulations have been done to evaluate the performance of these cooling machines when varying solar field surface, solar collector’s efficiency curve and the storage capacity of the systems. Two different indices have been calculated to analyze the response of both systems: Solar Fraction and Primary Energy Ratio. The comparison between both absorption chillers indicates that in order to reach similar values of storage energy, conventional system has a greater room requirement than four units with internal storage working in parallel, requiring an external water tank of at least 15 m³.     

Reducing CO2 emissions of internally heat-integrated distillation columns for separation of close boiling mixtures

A model developed originally for crude oil distillation units has been applied to a standalone internally heat integrated distillation column (HIDiC) to evaluate emissions levels and to generate design options for direct carbon dioxide emissions reduction. Simulations indicate that for propylene–propane separation, an ideal (no reboiler) HIDiC enables a reduction in emissions of 83% and of 36%, compared to conventional and heat pump alternatives, respectively. Integrating a turbine to drive the compressor, in conjunction with a suitable fuel is the key to the minimization of the emissions associated with the operation of a HIDiC. Importantly, while substantial emission reductions are achieved, the process economics are improved.     

Analysis and simulation of a solar water pump for lift irrigation

A solar water pump for lift irrigation, which was shown to be economically viable, was proposed by Rao and Rao [5]. A “modified pump” is suggested, which is suitable for village water supply. The thermodynamic analysis of the pumps is presented. Though the solar water pump is intended to be operated with flat-plate collectors, it is analysed whether the pump could be run more efficiently when coupled with concentrating collectors. The analysis is also applicable for bellow actuated solar water pumps.Preliminary experimental studies showed that the heat losses are 2–3 times the theoretical energy requirement and the losses to the water tank shell accounted for a major part of the total heat losses. To reduce these losses, it is proposed that the inner surface of the water tank shell be lined with a resin bonded cork insulation. A method to evaluate the heat losses to the shell with insulation by solving the unsteady state heat conduction equation for a composite cylindrical body with time varying convective boundary conditions is presented. The heat losses are reduced to 5–15 per cent of the theoretical energy requirement with the use of the internal insulation. The major problem encountered in operating the pump was the inadequate condensation of the working fluid, which resulted in failure of the suction of water into the water tank. To ensure proper suction of water, the conditions to be maintained are analysed. The presence of water vapor and air in the spent vapor has to be taken into consideration while designing the condenser.An algorithm to evaluate the year round performance of the water-cooled pump for any location, given the lift and collector area, is presented. The cost of the water-cooled pump is compared with the costs of other types of solar water pumps and it is shown that the pump under consideration costs several times less than the other ones. The conditions at which the pump will be economical vis-a-vis diesel and electrical pumps are presented.     

Industrial heat recovery by absorption/compression heat pump using TFE–H2O–TEGDME working mixture

The thermodynamic performance of a single-stage absorption/compression heat pump using the ternary working fluid Trifluoroethanol–Water–Tetraethylenglycol dimethylether (TFE–H₂O–TEGDME) for upgrading waste heat has been studied. A simulation program has been developed using a mathematical model based on mass and energy balances in all components of the cycle and thermodynamic equilibrium considerations. In order to establish the optimum operating conditions of the cycle for various thermal conditions, sensitivity studies of the coefficient of performance (COP), the flow rate of the weak solution and the compressor volumetric displacement, both per unit of upgraded energy, were carried out versus of water content in the vapour phase.The results obtained show that the operation of the cycle with this ternary system is still more advantageous than the TFE–TEGDME binary working pair. So, it is possible to upgrade thermal waste heat from 80 to 120°C, with a COP of about 6.4, with a compression pressure ratio of 4 at a low pressure of 100 kPa, the water mole fraction in the vapour being 42%. At these operating conditions, the necessary weak solution mass flow rate is about three times lower than the corresponding binary one. The performance comparison of such a cycle with other absorption cycles like the heat transformer or the single-effect heat pump, both of them using the ternary system, shows its interest and potential.