Å. Jernqvist

Design and experimental performance evaluation of an absorption heat transformer with self-circulation

Abstract A 10 kW experimental absorption heat transformer unit operating with self-circulation has been thoroughly tested. The self-circulation is obtained according to the thermosyphon principle. The pressure difference in the unit is achieved through a difference in hydrostatic pressures. Theoretical relationships for the pressure profiles within the different components of the heat transformer have been derived. Stabilising the self-circulation has been the primary objective in this work, rather than the optimisation of the efficiency of heating and cooling areas. A satisfactory stable operation with self-circulation has been achieved. A reference heat transformer plant, delivering 100 kW, has been designed and installed in a major pulp and paper mill. This unit is directly incorporated with one of the evaporation plants of the mill. Plant operation data obtained under real industrial conditions are presented.

On the efficiencies of absorption heat transformers

Abstract Although commonly used, the coefficient of performance (COP) is not always an adequate measure to describe the effectiveness of a sorption heat pump. Equations for four different efficiencies are derived, discussed and compared for absorption heat transformers. A flow-sheeting computer program, developed for both design and evaluation simulations of arbitrarily complex absorption cycles, is used to exemplify the derived equations. The working pair H2ONaOH is used in two heat transformer systems. The given examples clearly show that the COP can only be used to compare different heat transformers operated at the same circulation ratio. The COP can be considered as an indicator of the effectiveness of heat exchange within and thermal insulation of a heat transformer operated at a fixed circulation ratio. The thermodynamic efficiency, E th , is shown to be a more logical measure of the heat transformer efficiency, since it takes into account both heat losses, heat exchange and the temperature lift. The exergetic efficiency, E ex , is an alternative to E th since both are mathematically compatible. The possibility of taking into consideration the temperature level, at which heat energy may be considered economically worthless, is demonstrated to be a major advantage of the exergetic efficiency. However, its numerical value does not provide a clear interpretation of the importance of exergetic losses in the system. The exergetic index, I ex , is directly related to the exergetic efficiency but its numerical value is more significant for evaluating the performance of the heat transformer system.

Experimental and theoretical study of an open multi-compartment absorption heat transformer for different steam temperatures. Part III: application to process industry

Abstract The technical and economic feasibility of incorporating an absorption heat transformer was investigated, to increase the energy efficiency of an evaporation–crystallization plant in a sugar mill, having an intake capacity of 17.5 kton of sugar beets per day. A new design of this transformer was developed to cope with the common situation prevailing in many process industries where there is both a need for different steam temperatures and access to waste vapours of various temperatures. Both the absorber and generator are partitioned in a number of compartments which depend on the number of high quality steams and waste vapours, respectively. The circulation of the working medium is quite low which causes a fairly high concentration change and thereby different temperatures in the compartments. The low circulation ratio eliminates the need of heat exchange between the generator and the absorber. Four main process configurations were simulated to establish optimal energy conservation strategies for the evaporation plant and to determine the optimum location and size of the transformer. The crystallization plant, which is supplied with vapour extracted from the evaporation plant, had to be incorporated in the calculations. The absorber and generator are provided with the waste vapour leaving the last evaporator and waste vapour streams leaving the crystallization plant, respectively. These vapours are normally discharged to a dump condenser. The results of these simulations revealed that the total amount of live steam used in the evaporation plant can be reduced by 11.8–16.4%, depending on the temperature level allowable in the crystallization plant. The payback period of this type of investment depends mainly on the equipment cost, local steam cost, and annual operation time.

An energy efficient evaporation process for treating bleach plant effluents

Abstract Simulation results of an energy efficient evaporation process are reported for the treatment of bleach effluents in the pulp and paper industry. Due to the low concentration of the effluent stream, the evaporation process must have a high degree of energy efficiency in order to compete with other treatment alternatives, such as ultrafiltration, adsorption, ion exchange and biological treatment. For a pulp and paper mill with an annual capacity of 335,000 ton of bleached kraft pulp, the capacity of the spent black liquor evaporation plant is about 7.5 ton water/ton pulp if its concentration is increased from 16 to about 65 wt% dry substance. An evaporation plant for the bleach effluent was simulated for a flow rate corresponding to 7.2 m3/ton pulp and a concentration of 1.4 wt% dry substance. This stream is to be concentrated to 16 wt% resulting in an evaporation capacity of 245 ton water/h. The total evaporation capacity for both evaporation plants would be increased by 91%. Optimal energy conservation strategies were investigated where an absorption heat transformer unit (AHT) is integrated with both the existing black liquor evaporation plant and the proposed bleach effluent evaporation process. Different process configurations were simulated using a flow sheeting program, developed for simulation of general multiple-effect evaporation processes and absorption heat pump systems. Using real operating data from a major Swedish pulp and paper mill, simulation results are reported for the optimum location of the AHT unit. Consequently, the energy requirements for both evaporation plants would increase by only 6.6%. One of the main practical features of this process is that integrating the AHT unit would require minimum changes in the existing plant. An economic analysis resulted in a cost of 23 SEK/ton of pulp for the first alternative where the pre-concentrated 16 wt% effluent stream is concentrated further to 65 wt%, within the existing black liquor evaporation plant, and then recycled to the recovery boiler. The corresponding cost of 49 SEK/ton of pulp would result for the second alternative, where the 16 wt% effluent stream is sprayed on the waste from bark peeling and co-burned in the bark boiler.

On the efficiencies of absorption heat pumps

Abstract Although commonly used, the coefficient of performance COP is not always an adequate measure to describe the effectiveness of a sorption heat pump. Equations for four different efficiencies are derived, discussed and compared for absorption heat pumps (AHP). A flow-sheeting computer program, developed for both design and evaluation simulations of arbitrarily complex absorption cycles, is used to exemplify the derived equations. The working fluid pair H 2 OLiBr has been used in two different AHP configurations. The examples given clearly show that the COP can only be used to compare different AHPs operated at the same circulation ratio. The COP can be considered as an indicator of the effectiveness of heat exchange within and thermal insulation of a heat pump operated at a fixed circulation ratio. Nevertheless, it is an insufficient measure to compare different AHPs, even when they are operated at the same circulation ratio. On the other hand, the coefficient of performance for cooling Q e / Q g is better in this respect since it takes into account the real heat flow to the generator. The Carnot efficiency COP rev takes into consideration both the real heat outputs from the absorber and condenser, and the temperature of heat sources and heat sinks. The thermodynamic efficiency E th is shown to be a more logical measure of the heat pump efficiency, since it takes into account the real heat input the generator. The exergetic efficiency E ex can be considered as an alternative to the thermodyanamic efficiency E th but it offers a possibility to take into account any temperature level where heat energy may be considered worthless. However, both E ex and E th are not conventionally used since their numerical values are always less than 1.0. On the other hand, the exergetic index I ex is directly related to E ex but its numerical value shoul be considered as a more significant measure for evaluating the performance of AHP systems, since it properly takes into account the exergy losses which inevitably occur in the system. It may however be stressed that exergy analysis should be used as a compliment to the First Law analysis.

Experimental and theoretical study of an open multi-compartment absorption heat transformer for different steam temperatures. Part I: hydrodynamic and heat transfer characteristics

Abstract A novel multi-compartment absorption heat transformer for different steam temperatures (MAD) has been developed. The MAD-transformer was designed to cope with the common situation prevailing in many process industries where there is both a need for different steam temperatures and access to waste vapour at various temperatures. Both the absorber and generator are partitioned in a number of compartments which depend on the number of high quality steam demands and waste vapour sources, respectively. The circulation of the working medium is quite low which causes a fairly high concentration change and thereby different temperatures in the compartments. The low circulation ratio eliminates the need for heat exchange between the generator and the absorber. A pilot-plant unit consisting of an absorber and a steam generator was designed and installed. The absorber consists of four compartments confined between five vertical lamellas. A number of experiments were performed, to investigate the hydrodynamic and heat transfer characteristics of the multi-compartment absorber using three different working fluid pairs. The distance between lamellas, apparent refrigerant level inside a lamella, flow rate of inlet steam to the compartments, flow rate of refrigerant to the lamellas, solution viscosity, and boiling point elevation of the absorbent are the process variables investigated. In Part I of this work, the working principles, control strategies, hydrodynamic behavior as well as the heat transfer characteristics of this MAD-transformer are reported.

Boiling vertical two-phase flow at sub-atmospheric pressures

An experimental set-up for the measurement of steady-state two-phase flow data at sub-atmospheric pressures has been built. It has been used to provide data for boiling water. Void fraction data showed a distinct mass quality dependence but only a weak mass flux dependence. Friction pressure drop was calculated from the measured total pressure drop. A clear dependence was found on mass quality and on mass flux. Quasi-local heat transfer coefficients were calculated from temperature and heat flow measurements. Nucleate boiling was found to dominate the heat transfer, although the convective contribution was evident. Empirical correlations for all the quantities were derived.

Integration of Absorption Heat Transformers in the Process Industry — Applications in the Oleochemical, Pulp and Paper Industries

Incorporating a heat transformer unit in an existing process to increase its energy efficiency was studied in this work. Two different applications; an evaporation plant in the pulp and paper industry and a fat hydrolysis unit in the oleochemical industry, were investigated.