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Carrol solutions
Applied Thermal Engineering 20 (2000) 269±284
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An experimental integrated absorption heat pump euent puri®cation system. Part II: operating on water/Carrol solutions S. Santoyo-GutieÂrrez a,*, J. Siqueiros b, C.L. Heard c, E. Santoyo d, F.A. Holland e a
Unidad Geotermia, Instituto de Investigaciones EleÂctricas, Av. Reforma 113, Col. Palmira, CP 62490, Temixco, Mor., Mexico b Centro de InvestigacioÂn en IngenierõÂa y Ciencias Aplicadas, Universidad AutoÂnoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, CP 62210, Cuernavaca, Mor., Mexico c Uso de la EnergõÂa EleÂctrica, Instituto de Investigaciones EleÂctricas, Av. Reforma 113, Col. Palmira, CP 62490, Temixco, Mor., Mexico d Centro de InvestigacioÂn en EnergõÂa, Universidad Nacional AutoÂnoma de MeÂxico, Apartado Postal 34, CP 62580, Temixco, Mor., Mexico e Overseas Educational Development Oce, University of Salford, Salford, M4 5WT, UK Received 2 March 1999; accepted 2 March 1999
Abstract An experimental integrated absorption heat pump euent puri®cation system (IAHPEPS) was built and originally operated with water-lithium bromide as a working mixture. The experimental results of IAHPEPS operated with water-Carrol as a working mixture are presented. Tap water was used as euent and distilled water was obtained after puri®cation. Pure euent production rates ranged between 1.2 and 4 kg hÿ1. The actual coecient of performance (COP)A varied from 1.35 to 1.55. The heat pump eectiveness (HPE) varied from 0.74 to 0.82. The results from the small scale system indicate the likely results from industrial scale units which could be operated with low quality heat such as waste heat, solar or geothermal resources. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Absorption; Heat pumps; Puri®cation systems; Distillation; Water puri®cation; Euent concentration
* Corresponding author. Tel.: +52-7318-3811; fax: +52-7318-2526. E-mail address: [email protected] (S. Santoyo-GutieÂrrez) 1359-4311/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 9 9 ) 0 0 0 2 5 - 3
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Nomenclature COP FR GTL H HPE M Q T X
coecient of performance (dimensionless) ¯ow ratio (dimensionless) gross temperature lift (TCOÿTEV) (K) enthalpy per unit mass (kJ kgÿ1) heat pump eectiveness (dimensionless) mass ¯ow rate (kg sÿ1) heat duty (kW) absolute temperature (K) mass fraction (dimensionless)
Subscripts A measured or real AB absorber or from absorber to generator C Carnot CO condenser EV evaporator GE generator EH enthalpy based R refrigerant 1 into absorber from evaporator 2 out of generator to absorber 3 into generator from absorber 4 out of generator to condenser 5 into evaporator from condenser 1. Introduction An experimental integrated absorption heat pump euent puri®cation system (IAHPEPS) was built and originally operated with water-lithium bromide as a working mixture [1]. Dierent working mixtures and dierent euent types can be studied with the IAHPEPS. In the present work the IAHPEPS was operated using water-Carrol as a working mixture. Tap water was used as euent to be puri®ed. Carrol is a mixture of lithium bromide and ethylene glycol in a 1:4.5 weight mixture [2]. It has a wider solubility range than pure lithium bromide. It allows higher temperature lifts and better eciencies than pure lithium bromide with water as an absorbent in absorption heat pumps [3]. 2. Absorption heat pump Absorption heat pumps allow heat to drive a heat recovery process with few moving parts and a high primary energy ratio [4]. Simple distillation processes for solvent recovery and
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water puri®cation are highly eective but also energy intensive [5]. A suitable absorption heat pump could greatly reduce the energy intensity of such a process [6±8]. Fig. 1 shows the basic process in two parts. The upper diagram is the absorption heat pump and the lower part the simple distillation system. The driving heat is supplied to the generator at the highest temperature in the thermodynamic cycle. Heat from the absorber and the condenser is supplied to the distillation process and heat is recovered from the distillation process in the evaporator. The energy eciency of a heat pump most frequently used is the coecient of performance (COP). This is de®ned in dierent ways depending on the type of heat pump and the use to
Fig. 1. Schematic diagram of an absorption heat pump assisted euent puri®cation system.
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which it is put. For example, in the case of a mechanical vapour compression heat pump for cooling the real COP is the energy extracted from the material to be cooled, via the evaporator, divided by the work energy input to the compressor [9,10]. However for a given heat pump the maximum theoretical COP is that of the Carnot cycle. The Carnot cycle COP is solely dependant on the temperatures of the heat supplied and rejected in each component of the cycle. Eq. 1 gives the Carnot coecient of performance (COP)C for an absorption heat pump for heating. TEV TGE ÿ TAB COPC 1 1 TGE TCO ÿ TEV The Carnot cycle is not a useful yardstick for evaluating real heat pump systems, since it includes assumptions of reversable isothermal heat transfer with in®nitesimal temperature dierences and expansion and compression of wet vapour. In practice the maximum attainable COP is enthalpy based as described in Eq. 2. COPEH
H1 FR ÿ 1H2 ÿ FRH3 H4 ÿ H5 H4 FR ÿ 1H2 ÿ FRH3
2
The comparison of this with the experimental or real COP is the Heat Pump Eectiveness (HPE) [Eq. 3]. HPE
COPA COPEH
3
One of the other important measures of the performance of an absorption heat pump is the ¯ow ratio (FR). In a simple single eect absorption heat pump for heating this is the ratio of the mass ¯ow rate of the solution from the absorber to the mass ¯ow rate of refrigerant into the evaporator (Eq. 4). FR
MAB XGE MR XGE ÿ XAB
4
The higher the ¯ow ratio the greater the internal heat transfer necessary in the economiser and the larger the mass ¯ow rates in the absorber and generator. This will be re¯ected in larger equipment, higher capital costs, more opportunities for heat losses from the system, a slower startup and a larger inventory of absorbent. Carrol with its higher solubility should allow systems to be operated with lower ¯ow ratios than pure lithium bromide. The application of a heat pump to a simple euent distillation process is attractive because of the small temperature dierence between the euent and the pure solvent. The impurities in the euent seldom lower the vapour pressure greatly relative to that of the pure solvent. To equalise or slightly exceed the vapour pressure of the pure solvent it is only necessary to raise the euent evaporator temperature a little above that of the solvent condenser. The higher the temperature dierence, the greater the driving force for the process and the greater capacity for a given size of heat exchangers. However the energy eciency of the heat pump is adversely aected by raising this temperature dierence. In practice a balance between thermal energy
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eciency and equipment size and sophistication will determine the optimum temperature dierence for a particular application. Fig. 2 shows how an absorption heat pump system can be integrated into a simple distillation system. The auxiliary condenser balances the heat ¯ows in the system. In a large scale installation this heat might be recovered to preheat the euent entering the system, increasing the energy eciency. The pressure levels refer to the heat pump working ¯uid. 3. Experimental equipment The experimental equipment was designed for use with water as a working ¯uid and salt solutions as an absorbent. Most of the heat exchange surfaces were of stainless steel and the vessels were made of carbon steel. To simplify the construction the main heat exchangers were made from identical stainless steel coils from 6.1 m lengths of 19 mm (3/4 in) outside diameter tubing which were mounted in adapted 20 kg domestic gas cylinders. The economiser was an available spiral tube heat exchanger. The auxiliary puri®ed euent condenser was made from a length of 12.7 mm (1/2 in) outside diameter tubing coiled in a small carbon steel shell. Positive displacement sliding vane pumps were used with inverters to control the pump motor speeds. This allowed calibration of the pumping rate with respect to motor speed to be
Fig. 2. Pressure and temperature levels for an integrated absorption heat pump euent puri®cation system.
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Fig. 3. Schematic diagram of the experimental integrated absorption heat pump assisted euent puri®cation system (IAHPEPS).
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used to measure ¯ow rates. Other ¯ow rates were measured by rotameters. The production of distillate and the steam consumption were measured by weighing condensates over time. Temperatures were measured with type E thermocouples. Pressures were measured with Validyne dierential pressure transducers with respect to ambient pressure which was measured in the same laboratory with a mercury column barometer. Solution concentrations were measured from samples extracted from the generator and absorber solutions exits. The refractive index was measured in an Abbe refractometer whose heating plate was pre-heated at 408C. The solution concentrations were inferred from a previously established concentration/
Fig. 4. Photograph of the experimental integrated absorption heat pump assisted euent puri®cation system (IAHPEPS).
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refractive index relationship at a constant temperature of 408C using a numerical correlation derived from experimental data [11]. Fig. 3 is a schematic diagram of the equipment showing how the heat pump and euent circuits were integrated. The heat pump circuit is a simple single stage absorption system with low pressure steam used as the heat source. Fig. 4 is a front view of the experimental integrated absorption heat pump euent puri®cation system (IAHPEPS) showing the physical distribution of components. The euent circuit was a forced circulation system with liquid euent pumped through the heat pump absorber coil and then through the condenser coil. The resulting two phase mixture was separated for recirculation of the liquid phase and the vapour phase passed to the heat pump evaporator for subsequent condensation. To close the heat balance an auxiliary puri®ed euent condenser was placed in series with the heat pump evaporator. The refrigerant circuit also used a forced circulation system on the evaporator. Forced circulation for ¯uids boiling inside tubes with condensation on the shell side was used to improve the heat transfer and thus the capacity of the equipment. The equipment was manually controlled through the steam pressure of the heat supply to the generator and the pump speeds. The cooling water temperature in the auxiliary puri®ed
Fig. 5. Enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the gross temperature lift GTL=(TCOÿTEV).
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euent condenser was as supplied from the water main. The equipment took from 30±40 min to reach steady state from startup. The IAHPEPS can be simply described as follows. The procedure was initiated when the vacuum requirements were achieved using vacuum pumps connected to the system (Fig. 3). These conditions had to be obtained in the generator, evaporator, absorber and the auxiliary condenser. Once the IAHPEPS was under adequate vacuum conditions, steam was supplied to the generator. Simultaneously the solution circulation system was started. Afterwards the euent circulation was initiated. The euent circulated inside the absorber and the condenser (Fig. 3). From these components the euent was fed to a centrifugal separator from where the vapour phase was sent as a heating medium to the evaporator. The partial euent vapour condensation provided the heat of vaporization for the liquid refrigerant. The separated liquid euent was recirculated. The refrigerant from the evaporator was fed to a centrifugal separator, the vapour phase being fed to the absorber and the liquid phase being recirculated to the evaporator. The startup procedure terminated in the supply of cooling water to the auxiliary condenser to complete the euent vapour condensation. Once the system had been started up, steady state conditions needed to be achieved for each experimental run. This required that the steam supply to the generator and euent, refrigerant
Fig. 6. Gross temperature lift GTL=(TCOÿTEV) against the evaporator temperature TEV.
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and auxiliary condenser cooling water ¯ow rates be controlled manually with a frequency controller. Usually these steady state conditions were obtained when the temperature, pressure and ¯ow rate variations were very small and maintained for at least 30 min. A detailed description of the experimental run procedure for the IAHPEPS was presented by SantoyoGutieÂrrez (1997) [11]. 4. Results All the results were obtained under steady state conditions. The steady state was checked during each test and also from the recorded data series. Tap water, characterised by ion chromatography technique [12], was used as the euent. Chloride, calcium and sulphate concentrations were measured (Table 1). The working mixture was made up with distilled water, lithium bromide and ethylene glycol with sodium dichromate added as a corrosion inhibitor. The heat balances were determined from measured temperatures, concentrations and ¯ow rates. Eq. 5 gives the basic heat balance for the heat pump. QGE QEV QCO QAB
5
Eq. 6 gives the heat balance for the euent side, ignoring heat losses. QCO QAB QEV QAUX
6
where QAUX QGE
Table 1 Results of the chemical analyses of the euent supplied, euent treated and euent concentrated to the absorption heat pump assisted euent puri®cation system Concentration in mg kgÿ1 Test No.
1 2 3 4 5 6 7 8
Euent supplied
Euent treated
Euent concentrated
Cl
Ca
SO4
Cl
Ca
SO4
Cl
Ca
SO4
134.6 136.7 127.0 128.7 130.0 130.4 128.9 133.7
148.1 150.2 140.5 142.2 143.5 143.9 142.4 147.2
56.5 58.6 48.9 50.6 51.9 52.3 50.8 55.6
0.2 0.2 0.1 0.1 0.2 0.2 0.1 0.2
1.0 1.1 1.0 1.0 1.0 1.0 1.0 1.0
0.4 0.4 0.3 0.4 0.4 0.4 0.4 0.4
193.7 196.7 182.7 185.2 187.0 187.6 185.5 192.4
213.1 216.1 202.1 204.6 206.5 207.0 204.9 211.8
81.3 84.3 70.4 72.8 74.7 75.2 73.1 80.0
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The solution enthalpies were determined from numerical polynomial correlations obtained by Reimann and Biermann [13] and the water enthalpies from numerical polynomial correlations of the values tabulated by Mayhew and Rogers [14]. The ¯ow ratios were calculated from the measured concentrations.
5. Discussion The capacity of the system varied from 1.2 to 4 kg hÿ1 distilled water production. Twenty three experimental runs were made under dierent operation conditions. Generation temperatures ranged from 110±1208C with heat pump evaporation temperatures from 63±768C. Refrigerant poor solution concentrations (XGE) ranged from 56.5±59.2%. Fig. 5 shows the variation of the enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the gross temperature lift (TCOÿTEV). (COP)EH and (COP)A decreased when the gross temperature lift increased. The dispersion of the points is due to dierences in the conditions for the experimental test runs. This dispersion
Fig. 7. Enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the evaporator temperature TEV.
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was also found for compression heat pumps [10]. The COP values ranged from 1.35 to 1.55. The majority of the experimental test runs are comprised of gross temperature lifts (TCOÿTEV) between 28±328C. This range of gross temperature lift was easier to obtain because it is not near the evaporator and condenser temperature limits (see Fig. 6). Fig. 7 shows the variation of the enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the evaporator temperature. The (COP)EH and (COP)A increased when the evaporator temperature increased. This behaviour agrees with theory. Eq. (1) shows that when the evaporator temperature increases the numerator increases and the denominator decreases thus the COP must increase. Fig. 6 shows the variation of the gross temperature lift (TCOÿTEV) against the evaporator temperature TEV. Small gross temperature lifts corresponded to the highest evaporator temperatures and the lowest condenser temperatures. High values of gross temperature lift corresponded to low evaporator temperatures and high condenser temperatures. Operating near the limits of the range for evaporator and condenser temperatures it was not easy to maintain at steady state conditions. Small values of gross temperature lift are of more interest for this system since the COP values are highest for these conditions.
Fig. 8. Heat pump eectiveness HPE against the ¯ow ratio (FR).
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Fig. 8 shows the variation of the heat pump eectiveness (HPE) against the ¯ow ratio (FR). The HPE decreases very slowly when the FR values increase. The values of HPE were between 0.74 and 0.82. These results show that the system is working approximately at 80% of the maximum possible. That such a high HPE was obtained in a small laboratory sized apparatus augurs well for an industrial sized unit. An industrial unit should be able to operate at HPEs well above 80% due to lower heat losses, optimised design of heat and mass transfer equipment and automated control. Fig. 9 shows the variation of the enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the production rate of pure euent. For the range shown there was no appreciable trend. It is to be expected that for the highest values of (COP)A the distilled water production values would also be highest. Fig. 10 shows the variation of the gross temperature lift (TCOÿTEV) against the production rate of pure euent. In this ®gure it is noted that when the gross temperature lift decreases the production rate of the distilled water increases. Fig. 11 shows the variation of the evaporator temperature TEV against the production rate of distilled water. The production of distilled water increases when the evaporator temperature also increases. It has been shown that the COP values increase when TEV also increases. This
Fig. 9. Enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the production rate of pure euent.
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Fig. 10. Gross temperature lift GTL=(TCOÿTEV) against the production rate of pure euent.
®gure shows less dispersion than Fig. 9 where the variation of COP is shown as a function of the distilled water production rate. Table 1 shows the results of the chemical analyses of the tap water, distilled water and concentrated euent for eight test runs. The quality of the distilled water produced was comparable or better than distilled water produced by an electric resistance water distiller. The quality of the distilled water obtained is comparable with distilled water obtained using a compression heat pump assisted water puri®cation system [9,10] and when operating this system with a water/LiBr solution [1].
6. Conclusions The system operated well with the pair water/Carrol. HPE values of 0.74 to 0.82 were obtained showing that the operation of the experimental equipment was ecient and reliable. A distilled water production rate from 1.2 to 4 kg hÿ1 showed the ¯exibility of operation and stability under a range of conditions. The (COP)A ranged from 1.35 to 1.55. This was achieved
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Fig. 11. Evaporator temperature TGE against the production rate of pure euent.
in small-scale equipment that had the potential for large heat losses since the design was not at all compact and had relatively long pipe runs. These results indicate that industrial sized equipment should be highly ecient even with a simple single eect heat pump system. If relatively low grade heat such as solar or waste heat are to be used to drive a simple euent distillation process a single eect system is a good option. However, if a higher grade heat source is available and suitable working pair can be found, multiple stage absorption heat pumps oer the prospect of much higher COPs [15]. References [1] S. Santoyo-GutieÂrrez, J. Siqueiros, C.L. Heard, E. Santoyo, F.A. Holland, An experimental integrated absorption heat pump euent puri®cation system. Part I: operating on water/lithium bromide solutions, Applied Thermal Engineering 19 (5) (1999) 461±475. [2] W. Rivera, M.J. Cardoso, R.J. Romero, Theoretical comparison of single stage and advanced heat transformers operating with water/lithium bromide and water/Carrol mixtures, International Journal of Energy Research 22 (5) (1998) 427±442. [3] R. Best, W. Rivera, M.J. Cardoso, R.J. Romero, F.A. Holland, Modelling of single-stage and advanced
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[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
S. Santoyo-GutieÂrrez et al. / Applied Thermal Engineering 20 (2000) 269±284 absorption heat transformers operating with the water/Carrol mixture, Applied Thermal Engineering 17 (11) (1997) 1111±1122. F.A. Holland, J. Siqueiros, S. Santoyo-GutieÂrrez S, C.L. Heard, E. Santoyo, Water Puri®cation Using Heat Pumps. E & FN SPON/Routledge, London, UK, 1999. S. Supranto, I. Chandra, M.B. Unde, P.J. Diggory, F.A. Holland, Heat pump assisted distillation. I Alternative ways to minimize energy consumption in fractional distillation, Energy Research 10 (5) (1986) 145± 161. M.A.R. Eisa, R. Best, F.A. Holland, Heat pump assisted water puri®cation systems. 3rd International Symposium on the large scale applications of heat pumps, 35±43, Oxford, UK 1987. R. Best, C.L. Heard, F.A. Holland, Developments of geothermal energy in MexicoÐpart 16: the potential for heat pump technology, Heat Recovery Systems & CHP 8 (3) (1988) 185±202. D.L. Hodgett, Aplicaciones industriales de las bombas de calor por compresioÂn mecaÂnica de vapor, Manual sobre Tecnologias de Bombas de Calor, Programa de CooperacioÂn e InvestigacioÂn: Instituto de Investigaciones EleÂctricas, Cuernavaca, MeÂxico y Universidad de Salford, UK, 1990. J.L. Frias, J. Siqueiros, H. FernaÂndez, A. Garcia, F.A. Holland, Developments in geothermal energy in MexicoÐpart 36: the commissioning of a heat pump assisted brine puri®cation system, Heat Recovery Systems & CHP 11 (4) (1991) 297±310. J. Siqueiros, C.L. Heard, F.A. Holland, The commissioning of an integrated heat pump-assisted geothermal brine puri®cation system, Heat Recovery Systems & CHP 15 (7) (1995) 655±664. S. Santoyo-GutieÂrrez, Absorption heat pump assisted euent puri®cation. PhD Thesis, University of Salford, 1997. E. Santoyo, R.M. BarragaÂn, D. Nieva, S.P. Verma, Application of ion chromatography to the chemical characterization of brines produced from wells of the Los Azufres geothermal ®eld (Mexico), in: Proceedings of the International Ion Chromatography Symposium, San Diego, CA, USA, Special Issue, 1990, p. 10. R.C. Reimann, W.J. Biermann, Development of a single family absorption chiller for use in solar heating and cooling systems, Phase III Final Report, Prepared for the USA Department of Energy under contract EG/77/ C/03/1587, Carrier Corporation, October 1984. Y.R. Mayhew, G.F.C. Rogers, Thermodynamic and Transport Properties of Fluids (SI Units), Basil Blackwell Publishers, Oxford, UK, 1976. F. Ziegler, Absorption refrigeration, heat pump, and heat transformer cycles, Seminario: Recuperacion de Calor Industrial por Medio de Bombas de Calor, 441±468, Cuernavaca, Morelos, Mexico 1993.
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An experimental integrated absorption heat pump euent puri®cation system. Part II: operating on water/Carrol solutions S. Santoyo-GutieÂrrez a,*, J. Siqueiros b, C.L. Heard c, E. Santoyo d, F.A. Holland e a
Unidad Geotermia, Instituto de Investigaciones EleÂctricas, Av. Reforma 113, Col. Palmira, CP 62490, Temixco, Mor., Mexico b Centro de InvestigacioÂn en IngenierõÂa y Ciencias Aplicadas, Universidad AutoÂnoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, CP 62210, Cuernavaca, Mor., Mexico c Uso de la EnergõÂa EleÂctrica, Instituto de Investigaciones EleÂctricas, Av. Reforma 113, Col. Palmira, CP 62490, Temixco, Mor., Mexico d Centro de InvestigacioÂn en EnergõÂa, Universidad Nacional AutoÂnoma de MeÂxico, Apartado Postal 34, CP 62580, Temixco, Mor., Mexico e Overseas Educational Development Oce, University of Salford, Salford, M4 5WT, UK Received 2 March 1999; accepted 2 March 1999
Abstract An experimental integrated absorption heat pump euent puri®cation system (IAHPEPS) was built and originally operated with water-lithium bromide as a working mixture. The experimental results of IAHPEPS operated with water-Carrol as a working mixture are presented. Tap water was used as euent and distilled water was obtained after puri®cation. Pure euent production rates ranged between 1.2 and 4 kg hÿ1. The actual coecient of performance (COP)A varied from 1.35 to 1.55. The heat pump eectiveness (HPE) varied from 0.74 to 0.82. The results from the small scale system indicate the likely results from industrial scale units which could be operated with low quality heat such as waste heat, solar or geothermal resources. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Absorption; Heat pumps; Puri®cation systems; Distillation; Water puri®cation; Euent concentration
* Corresponding author. Tel.: +52-7318-3811; fax: +52-7318-2526. E-mail address: [email protected] (S. Santoyo-GutieÂrrez) 1359-4311/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 9 9 ) 0 0 0 2 5 - 3
270
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Nomenclature COP FR GTL H HPE M Q T X
coecient of performance (dimensionless) ¯ow ratio (dimensionless) gross temperature lift (TCOÿTEV) (K) enthalpy per unit mass (kJ kgÿ1) heat pump eectiveness (dimensionless) mass ¯ow rate (kg sÿ1) heat duty (kW) absolute temperature (K) mass fraction (dimensionless)
Subscripts A measured or real AB absorber or from absorber to generator C Carnot CO condenser EV evaporator GE generator EH enthalpy based R refrigerant 1 into absorber from evaporator 2 out of generator to absorber 3 into generator from absorber 4 out of generator to condenser 5 into evaporator from condenser 1. Introduction An experimental integrated absorption heat pump euent puri®cation system (IAHPEPS) was built and originally operated with water-lithium bromide as a working mixture [1]. Dierent working mixtures and dierent euent types can be studied with the IAHPEPS. In the present work the IAHPEPS was operated using water-Carrol as a working mixture. Tap water was used as euent to be puri®ed. Carrol is a mixture of lithium bromide and ethylene glycol in a 1:4.5 weight mixture [2]. It has a wider solubility range than pure lithium bromide. It allows higher temperature lifts and better eciencies than pure lithium bromide with water as an absorbent in absorption heat pumps [3]. 2. Absorption heat pump Absorption heat pumps allow heat to drive a heat recovery process with few moving parts and a high primary energy ratio [4]. Simple distillation processes for solvent recovery and
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271
water puri®cation are highly eective but also energy intensive [5]. A suitable absorption heat pump could greatly reduce the energy intensity of such a process [6±8]. Fig. 1 shows the basic process in two parts. The upper diagram is the absorption heat pump and the lower part the simple distillation system. The driving heat is supplied to the generator at the highest temperature in the thermodynamic cycle. Heat from the absorber and the condenser is supplied to the distillation process and heat is recovered from the distillation process in the evaporator. The energy eciency of a heat pump most frequently used is the coecient of performance (COP). This is de®ned in dierent ways depending on the type of heat pump and the use to
Fig. 1. Schematic diagram of an absorption heat pump assisted euent puri®cation system.
272
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which it is put. For example, in the case of a mechanical vapour compression heat pump for cooling the real COP is the energy extracted from the material to be cooled, via the evaporator, divided by the work energy input to the compressor [9,10]. However for a given heat pump the maximum theoretical COP is that of the Carnot cycle. The Carnot cycle COP is solely dependant on the temperatures of the heat supplied and rejected in each component of the cycle. Eq. 1 gives the Carnot coecient of performance (COP)C for an absorption heat pump for heating. TEV TGE ÿ TAB COPC 1 1 TGE TCO ÿ TEV The Carnot cycle is not a useful yardstick for evaluating real heat pump systems, since it includes assumptions of reversable isothermal heat transfer with in®nitesimal temperature dierences and expansion and compression of wet vapour. In practice the maximum attainable COP is enthalpy based as described in Eq. 2. COPEH
H1 FR ÿ 1H2 ÿ FRH3 H4 ÿ H5 H4 FR ÿ 1H2 ÿ FRH3
2
The comparison of this with the experimental or real COP is the Heat Pump Eectiveness (HPE) [Eq. 3]. HPE
COPA COPEH
3
One of the other important measures of the performance of an absorption heat pump is the ¯ow ratio (FR). In a simple single eect absorption heat pump for heating this is the ratio of the mass ¯ow rate of the solution from the absorber to the mass ¯ow rate of refrigerant into the evaporator (Eq. 4). FR
MAB XGE MR XGE ÿ XAB
4
The higher the ¯ow ratio the greater the internal heat transfer necessary in the economiser and the larger the mass ¯ow rates in the absorber and generator. This will be re¯ected in larger equipment, higher capital costs, more opportunities for heat losses from the system, a slower startup and a larger inventory of absorbent. Carrol with its higher solubility should allow systems to be operated with lower ¯ow ratios than pure lithium bromide. The application of a heat pump to a simple euent distillation process is attractive because of the small temperature dierence between the euent and the pure solvent. The impurities in the euent seldom lower the vapour pressure greatly relative to that of the pure solvent. To equalise or slightly exceed the vapour pressure of the pure solvent it is only necessary to raise the euent evaporator temperature a little above that of the solvent condenser. The higher the temperature dierence, the greater the driving force for the process and the greater capacity for a given size of heat exchangers. However the energy eciency of the heat pump is adversely aected by raising this temperature dierence. In practice a balance between thermal energy
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eciency and equipment size and sophistication will determine the optimum temperature dierence for a particular application. Fig. 2 shows how an absorption heat pump system can be integrated into a simple distillation system. The auxiliary condenser balances the heat ¯ows in the system. In a large scale installation this heat might be recovered to preheat the euent entering the system, increasing the energy eciency. The pressure levels refer to the heat pump working ¯uid. 3. Experimental equipment The experimental equipment was designed for use with water as a working ¯uid and salt solutions as an absorbent. Most of the heat exchange surfaces were of stainless steel and the vessels were made of carbon steel. To simplify the construction the main heat exchangers were made from identical stainless steel coils from 6.1 m lengths of 19 mm (3/4 in) outside diameter tubing which were mounted in adapted 20 kg domestic gas cylinders. The economiser was an available spiral tube heat exchanger. The auxiliary puri®ed euent condenser was made from a length of 12.7 mm (1/2 in) outside diameter tubing coiled in a small carbon steel shell. Positive displacement sliding vane pumps were used with inverters to control the pump motor speeds. This allowed calibration of the pumping rate with respect to motor speed to be
Fig. 2. Pressure and temperature levels for an integrated absorption heat pump euent puri®cation system.
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Fig. 3. Schematic diagram of the experimental integrated absorption heat pump assisted euent puri®cation system (IAHPEPS).
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used to measure ¯ow rates. Other ¯ow rates were measured by rotameters. The production of distillate and the steam consumption were measured by weighing condensates over time. Temperatures were measured with type E thermocouples. Pressures were measured with Validyne dierential pressure transducers with respect to ambient pressure which was measured in the same laboratory with a mercury column barometer. Solution concentrations were measured from samples extracted from the generator and absorber solutions exits. The refractive index was measured in an Abbe refractometer whose heating plate was pre-heated at 408C. The solution concentrations were inferred from a previously established concentration/
Fig. 4. Photograph of the experimental integrated absorption heat pump assisted euent puri®cation system (IAHPEPS).
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refractive index relationship at a constant temperature of 408C using a numerical correlation derived from experimental data [11]. Fig. 3 is a schematic diagram of the equipment showing how the heat pump and euent circuits were integrated. The heat pump circuit is a simple single stage absorption system with low pressure steam used as the heat source. Fig. 4 is a front view of the experimental integrated absorption heat pump euent puri®cation system (IAHPEPS) showing the physical distribution of components. The euent circuit was a forced circulation system with liquid euent pumped through the heat pump absorber coil and then through the condenser coil. The resulting two phase mixture was separated for recirculation of the liquid phase and the vapour phase passed to the heat pump evaporator for subsequent condensation. To close the heat balance an auxiliary puri®ed euent condenser was placed in series with the heat pump evaporator. The refrigerant circuit also used a forced circulation system on the evaporator. Forced circulation for ¯uids boiling inside tubes with condensation on the shell side was used to improve the heat transfer and thus the capacity of the equipment. The equipment was manually controlled through the steam pressure of the heat supply to the generator and the pump speeds. The cooling water temperature in the auxiliary puri®ed
Fig. 5. Enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the gross temperature lift GTL=(TCOÿTEV).
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euent condenser was as supplied from the water main. The equipment took from 30±40 min to reach steady state from startup. The IAHPEPS can be simply described as follows. The procedure was initiated when the vacuum requirements were achieved using vacuum pumps connected to the system (Fig. 3). These conditions had to be obtained in the generator, evaporator, absorber and the auxiliary condenser. Once the IAHPEPS was under adequate vacuum conditions, steam was supplied to the generator. Simultaneously the solution circulation system was started. Afterwards the euent circulation was initiated. The euent circulated inside the absorber and the condenser (Fig. 3). From these components the euent was fed to a centrifugal separator from where the vapour phase was sent as a heating medium to the evaporator. The partial euent vapour condensation provided the heat of vaporization for the liquid refrigerant. The separated liquid euent was recirculated. The refrigerant from the evaporator was fed to a centrifugal separator, the vapour phase being fed to the absorber and the liquid phase being recirculated to the evaporator. The startup procedure terminated in the supply of cooling water to the auxiliary condenser to complete the euent vapour condensation. Once the system had been started up, steady state conditions needed to be achieved for each experimental run. This required that the steam supply to the generator and euent, refrigerant
Fig. 6. Gross temperature lift GTL=(TCOÿTEV) against the evaporator temperature TEV.
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and auxiliary condenser cooling water ¯ow rates be controlled manually with a frequency controller. Usually these steady state conditions were obtained when the temperature, pressure and ¯ow rate variations were very small and maintained for at least 30 min. A detailed description of the experimental run procedure for the IAHPEPS was presented by SantoyoGutieÂrrez (1997) [11]. 4. Results All the results were obtained under steady state conditions. The steady state was checked during each test and also from the recorded data series. Tap water, characterised by ion chromatography technique [12], was used as the euent. Chloride, calcium and sulphate concentrations were measured (Table 1). The working mixture was made up with distilled water, lithium bromide and ethylene glycol with sodium dichromate added as a corrosion inhibitor. The heat balances were determined from measured temperatures, concentrations and ¯ow rates. Eq. 5 gives the basic heat balance for the heat pump. QGE QEV QCO QAB
5
Eq. 6 gives the heat balance for the euent side, ignoring heat losses. QCO QAB QEV QAUX
6
where QAUX QGE
Table 1 Results of the chemical analyses of the euent supplied, euent treated and euent concentrated to the absorption heat pump assisted euent puri®cation system Concentration in mg kgÿ1 Test No.
1 2 3 4 5 6 7 8
Euent supplied
Euent treated
Euent concentrated
Cl
Ca
SO4
Cl
Ca
SO4
Cl
Ca
SO4
134.6 136.7 127.0 128.7 130.0 130.4 128.9 133.7
148.1 150.2 140.5 142.2 143.5 143.9 142.4 147.2
56.5 58.6 48.9 50.6 51.9 52.3 50.8 55.6
0.2 0.2 0.1 0.1 0.2 0.2 0.1 0.2
1.0 1.1 1.0 1.0 1.0 1.0 1.0 1.0
0.4 0.4 0.3 0.4 0.4 0.4 0.4 0.4
193.7 196.7 182.7 185.2 187.0 187.6 185.5 192.4
213.1 216.1 202.1 204.6 206.5 207.0 204.9 211.8
81.3 84.3 70.4 72.8 74.7 75.2 73.1 80.0
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The solution enthalpies were determined from numerical polynomial correlations obtained by Reimann and Biermann [13] and the water enthalpies from numerical polynomial correlations of the values tabulated by Mayhew and Rogers [14]. The ¯ow ratios were calculated from the measured concentrations.
5. Discussion The capacity of the system varied from 1.2 to 4 kg hÿ1 distilled water production. Twenty three experimental runs were made under dierent operation conditions. Generation temperatures ranged from 110±1208C with heat pump evaporation temperatures from 63±768C. Refrigerant poor solution concentrations (XGE) ranged from 56.5±59.2%. Fig. 5 shows the variation of the enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the gross temperature lift (TCOÿTEV). (COP)EH and (COP)A decreased when the gross temperature lift increased. The dispersion of the points is due to dierences in the conditions for the experimental test runs. This dispersion
Fig. 7. Enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the evaporator temperature TEV.
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was also found for compression heat pumps [10]. The COP values ranged from 1.35 to 1.55. The majority of the experimental test runs are comprised of gross temperature lifts (TCOÿTEV) between 28±328C. This range of gross temperature lift was easier to obtain because it is not near the evaporator and condenser temperature limits (see Fig. 6). Fig. 7 shows the variation of the enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the evaporator temperature. The (COP)EH and (COP)A increased when the evaporator temperature increased. This behaviour agrees with theory. Eq. (1) shows that when the evaporator temperature increases the numerator increases and the denominator decreases thus the COP must increase. Fig. 6 shows the variation of the gross temperature lift (TCOÿTEV) against the evaporator temperature TEV. Small gross temperature lifts corresponded to the highest evaporator temperatures and the lowest condenser temperatures. High values of gross temperature lift corresponded to low evaporator temperatures and high condenser temperatures. Operating near the limits of the range for evaporator and condenser temperatures it was not easy to maintain at steady state conditions. Small values of gross temperature lift are of more interest for this system since the COP values are highest for these conditions.
Fig. 8. Heat pump eectiveness HPE against the ¯ow ratio (FR).
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Fig. 8 shows the variation of the heat pump eectiveness (HPE) against the ¯ow ratio (FR). The HPE decreases very slowly when the FR values increase. The values of HPE were between 0.74 and 0.82. These results show that the system is working approximately at 80% of the maximum possible. That such a high HPE was obtained in a small laboratory sized apparatus augurs well for an industrial sized unit. An industrial unit should be able to operate at HPEs well above 80% due to lower heat losses, optimised design of heat and mass transfer equipment and automated control. Fig. 9 shows the variation of the enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the production rate of pure euent. For the range shown there was no appreciable trend. It is to be expected that for the highest values of (COP)A the distilled water production values would also be highest. Fig. 10 shows the variation of the gross temperature lift (TCOÿTEV) against the production rate of pure euent. In this ®gure it is noted that when the gross temperature lift decreases the production rate of the distilled water increases. Fig. 11 shows the variation of the evaporator temperature TEV against the production rate of distilled water. The production of distilled water increases when the evaporator temperature also increases. It has been shown that the COP values increase when TEV also increases. This
Fig. 9. Enthalpic coecient of performance (COP)EH and the actual coecient of performance (COP)A against the production rate of pure euent.
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Fig. 10. Gross temperature lift GTL=(TCOÿTEV) against the production rate of pure euent.
®gure shows less dispersion than Fig. 9 where the variation of COP is shown as a function of the distilled water production rate. Table 1 shows the results of the chemical analyses of the tap water, distilled water and concentrated euent for eight test runs. The quality of the distilled water produced was comparable or better than distilled water produced by an electric resistance water distiller. The quality of the distilled water obtained is comparable with distilled water obtained using a compression heat pump assisted water puri®cation system [9,10] and when operating this system with a water/LiBr solution [1].
6. Conclusions The system operated well with the pair water/Carrol. HPE values of 0.74 to 0.82 were obtained showing that the operation of the experimental equipment was ecient and reliable. A distilled water production rate from 1.2 to 4 kg hÿ1 showed the ¯exibility of operation and stability under a range of conditions. The (COP)A ranged from 1.35 to 1.55. This was achieved
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Fig. 11. Evaporator temperature TGE against the production rate of pure euent.
in small-scale equipment that had the potential for large heat losses since the design was not at all compact and had relatively long pipe runs. These results indicate that industrial sized equipment should be highly ecient even with a simple single eect heat pump system. If relatively low grade heat such as solar or waste heat are to be used to drive a simple euent distillation process a single eect system is a good option. However, if a higher grade heat source is available and suitable working pair can be found, multiple stage absorption heat pumps oer the prospect of much higher COPs [15]. References [1] S. Santoyo-GutieÂrrez, J. Siqueiros, C.L. Heard, E. Santoyo, F.A. Holland, An experimental integrated absorption heat pump euent puri®cation system. Part I: operating on water/lithium bromide solutions, Applied Thermal Engineering 19 (5) (1999) 461±475. [2] W. Rivera, M.J. Cardoso, R.J. Romero, Theoretical comparison of single stage and advanced heat transformers operating with water/lithium bromide and water/Carrol mixtures, International Journal of Energy Research 22 (5) (1998) 427±442. [3] R. Best, W. Rivera, M.J. Cardoso, R.J. Romero, F.A. Holland, Modelling of single-stage and advanced
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[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
S. Santoyo-GutieÂrrez et al. / Applied Thermal Engineering 20 (2000) 269±284 absorption heat transformers operating with the water/Carrol mixture, Applied Thermal Engineering 17 (11) (1997) 1111±1122. F.A. Holland, J. Siqueiros, S. Santoyo-GutieÂrrez S, C.L. Heard, E. Santoyo, Water Puri®cation Using Heat Pumps. E & FN SPON/Routledge, London, UK, 1999. S. Supranto, I. Chandra, M.B. Unde, P.J. Diggory, F.A. Holland, Heat pump assisted distillation. I Alternative ways to minimize energy consumption in fractional distillation, Energy Research 10 (5) (1986) 145± 161. M.A.R. Eisa, R. Best, F.A. Holland, Heat pump assisted water puri®cation systems. 3rd International Symposium on the large scale applications of heat pumps, 35±43, Oxford, UK 1987. R. Best, C.L. Heard, F.A. Holland, Developments of geothermal energy in MexicoÐpart 16: the potential for heat pump technology, Heat Recovery Systems & CHP 8 (3) (1988) 185±202. D.L. Hodgett, Aplicaciones industriales de las bombas de calor por compresioÂn mecaÂnica de vapor, Manual sobre Tecnologias de Bombas de Calor, Programa de CooperacioÂn e InvestigacioÂn: Instituto de Investigaciones EleÂctricas, Cuernavaca, MeÂxico y Universidad de Salford, UK, 1990. J.L. Frias, J. Siqueiros, H. FernaÂndez, A. Garcia, F.A. Holland, Developments in geothermal energy in MexicoÐpart 36: the commissioning of a heat pump assisted brine puri®cation system, Heat Recovery Systems & CHP 11 (4) (1991) 297±310. J. Siqueiros, C.L. Heard, F.A. Holland, The commissioning of an integrated heat pump-assisted geothermal brine puri®cation system, Heat Recovery Systems & CHP 15 (7) (1995) 655±664. S. Santoyo-GutieÂrrez, Absorption heat pump assisted euent puri®cation. PhD Thesis, University of Salford, 1997. E. Santoyo, R.M. BarragaÂn, D. Nieva, S.P. Verma, Application of ion chromatography to the chemical characterization of brines produced from wells of the Los Azufres geothermal ®eld (Mexico), in: Proceedings of the International Ion Chromatography Symposium, San Diego, CA, USA, Special Issue, 1990, p. 10. R.C. Reimann, W.J. Biermann, Development of a single family absorption chiller for use in solar heating and cooling systems, Phase III Final Report, Prepared for the USA Department of Energy under contract EG/77/ C/03/1587, Carrier Corporation, October 1984. Y.R. Mayhew, G.F.C. Rogers, Thermodynamic and Transport Properties of Fluids (SI Units), Basil Blackwell Publishers, Oxford, UK, 1976. F. Ziegler, Absorption refrigeration, heat pump, and heat transformer cycles, Seminario: Recuperacion de Calor Industrial por Medio de Bombas de Calor, 441±468, Cuernavaca, Morelos, Mexico 1993.