Carbon Materials for Advanced Technologies Episode 9 pdf

11. 12 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. Mullhaupt, J.T., BeVier, W.E., McMahon, K.C., Van Slooten, R.A., Lewis, I.C., Grienke, R.A., Strong, S.L., Ball, D.R., and Steele, W.E., Carbon '92 p. 367 (1992) Remick, R.J., & Tiller, A.J., Advanced Methods for Low Pressure Storage of CNG, Non-petroleum Vehicular Fuels Conference, Washington, April 1985 Otto, K., Alternative Energy Sources IV, Vo16 p241, Ann Arbor Science, MI Barton, S.S., Holland, J.A. & Quinn, D.F., "The Development of Adsorbent Carbon for Storage of Compressed Natural Gas, Report AF-85-01, Ontario Ministry of Transportation, 1985 1201 Wilson Ave, Downsview, Ontario, Canada M3M 1J8 Hayhurst, D.T. & Lee, J.C., J. Coll. 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COX Intefuce Sci. 1976,56, 83 Cracknell, R.F., Gordon, P. and Gubbins, K.E., J Phys. Chem. 1993,97,494 Cracknell, R.F. and Gubbins, K.E., J. Mol. Liquids 1992, 54,261 Adsorption Science and 301 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59 60. 61. 62. 63. 64 65. 66. 67. 68. Ventura, S. C., Hum, G.P. and Narang, S.C., “Novel Strategies for the Synthesis of Methane Adsorbents with Controlled Porosity and High Surface Area”, Gas Research Institute Report GRI-93/0018 , 1993 Laine, J., Calafat, A. and Labady, M., Carbon 22 191 (1989) Jagtoyen, M. and Derbyshire, F., Carbon Botha, F.D. and McEnaney, B., Adsorption Science and Technology (1 993) MacDonald, J.A.F. and Quinn, D.F., Carbon 34 11 03 (1996) Barton, S.S Evans, M.J.B., and MacDonald, J.A.F., Carbon 2.9 1099 Alcaniz-Monge, J., dela Casa-Lillo, M.A., Cazorla-Amoros, D. and Linares- Solano, A., Carbon 1997,35, 291 Lopez, M., Labady, M. and Laine, J., Carbon 1996,34,825 Quinn, D.F. and MacDonald, J.A.F., US Patent 5071820 Quinn, D.F. and MacDonald, J.A.F., “Natural Gas Adsorbents” Report to Ministry of Transportation, Ontario, 1987, 1201 Wilson Ave, Downsview, Ontario, Canada M3M 158 Chaudron, G., “Natural Gas for Vehicles Adsorption Storage Tanks” Intercom, Belgium 1989 Petersen, A.S. and Larsen, B., Riso National Laboratory Report M-2781, Denmark, 1989 Lin, Y.C. and Huff, G.A., “Adsorbed Natural Gas” SAE Future Transportation Conference, San Antonio, Texas, 1993 Chen, X. and McEnaney, B., Carbon ‘95 Abstracts p 504, San Diego 1995 Manzi, S., Valladares, D., Marchese, J. and Zgrablich, G., Adsorption Science and Technology 1997,15, 301 Berl, E., Trans. Farad. Soc. 1938, 34, 1040 Wennerberg, A.N. and O’Grady, T.M., US Patent 4082694 (1978) Otowa, Y., US Patent 5064805 (1991) Lewis, I.C., Greinke, R.A. and Strong, S.L., Carbon ‘93 Abstracts p490, Buffalo, 1993 Kaneko, K. and Murata, K., Adsorption 1997,3, 197 Chaffee, A. and Pandolfo, A., Carbon ‘90 Abstracts p246, Paris 1990 also presentation to Gas Utilisation Research Forum, London, 1990 Verheyen, V., Jagtoyen, M. and Derbyshire, F., Carbon ‘93 Abstracts p 474 1993 AGLARG Report to US Dept. of Energy, Contract 466590,1997 Private Communication, J. Wegrzyn, Brookhaven National Laboratory Allied Signal, US Patent 5292706,5292707,5308821 and 5461023 Westvaco, US Patent 5416056,5626637, Euro Pat. Application 649815, Canadian Pat. Application 2134160 Quinn, D.F., Report to AGLARG, September 1993 Elliott, D. .and Topaloglu, T., Gaseous Fuels for Transportation I, p489 B.C. Research, Vancouver (1 986) Golovoy, A. & Blais, E.J., SAE Conference Proc., Pittsburgh, p47, (1983) Chaffee, A.L., Loeh, H.J. and Pandolfo, A.G., “Methane Adsorption on High Surface Area Carbons” CSIRO, Division of Fuel Technology, Investigation ReportFT/IR031R(1989) 1 185 (1 993) 18 1 (1991) 302 69. 70. 71 72. 73. 74. 75. Getman, R. Atlanta Gas Light Co. R&D Report #9 1 4- 10 (1 99 1) Fricker, R.N. and Parkyns, N.D., "Adsorbed Natural Gas Road Vehicle" NGV92, Gothenberg, Sweden. Sept. 1992 Valenzuela, D. and Myers, A.L., "Adsorption Equilibrium Data Handbook", Prentice Hall, (New Jersey) 1989 ISBN 0- 13-003815-3 Ritter, J.A. and Yang, R.T., lad. Eng. Chem. Res. 1987,26, 1679 Urbanic, J.E. et al. Paper 890621 SAE Conf. Proc., Detroit, (1989) Critoph, R.E. and Turner, L., Int. J Heat &Mass Transfer, 1995,38(9), 1577) "Cyclic Test Unit 62 Filter Evaluations", Report to Future Fuels Inc., Oct. 1987, Alcohol Energy Systems, California 303 CHAPTER 10 Adsorption Refrigerators and Heat Pumps Dr. R.E. CRITOPH Engineering Department University of Warwick Coventry CY4 7AL, UK 1 Why Adsorption Cycles ? Active carbons can be used in both refrigeration and heat pumping cycles, but their potential for use in these applications does not necessarily merit the development of such systems. Before devoting research and development effort into active carbon-based thermodynamic cycles, the interest in both heat-driven cycles in general, and adsorption cycles in particular, must be justified. A major reason for the interest in heat-driven cycles is that they offer better utilisation of primary energy. Conventional vapour compression cycles used for refrigeration, air conditioning and heat pumping use electricity to drive a mechanical compressor. The efficiency of conversion from mechanical work to cooling or heating can be high. For example, the COP (Coefficient of Performance, equal to cooling power divided by input power) may be 3 in an air conditioning application. However, the conversion of primary fuel (oil, gas, coal or nuclear) to electricity at the power station, followed by transmission losses on route to the consumer may only be 25% efficient. Thus the overall conversion of primary energy to cooling is about 75% efficient. A heat-driven air conditioner using gas as its energy source might have a COP slightly greater than 1.0, but this is the overall conversion efficiency from primary energy, which is considerably better than that of the conventional electrically driven machine. The COP'S of specific air conditioners will vary widely with both manufacturer and application. Electricity utility efficiencies will also differ between countries. However, the reason for the economic interest in heat-driven cycles remains clear. Given that prirnary fuels can cost the consumer approximately 25% of the cost of electricity and that electricity frequently costs more at times of peak demand, there is justification for considering alternative systems. The use of a primary fuel at the point of use can also reduce CO, and other emissions. Another reason for the interest in heat-driven cycles is their ability to produce higher temperature outputs than vapour compression cycles. There are industrial heat pump or thermal transformer applications where the ability to pump heat at several hundred degrees Celsius is required. This is generally beyond the capability of the refrigerants and compressors used in conventional vapour compression systems. A further application of heat-driven systems is in places where there is no electrical energy supply available. An example is the refrigeration of vaccines and other medicines in remote areas of developing countries. The World Health Organisation has evaluated a number of solar adsorption refrigerators designed for this purpose. They have to compete with vapour compression refrigerators powered by photo-voltaic panels. The inherent simplicity of solar thermal- powered refrigerators makes them ideal in these applications. There is also a need for larger thermal refrigerators for food preservation in remote areas. There is a particular need for local ice production in fishing villages, where a large proportion of the catch is often spoilt before it can be transported to market or be preserved elsewhere. Machines of up to 1 tonnelday of ice production are required for this application. They need not be solar powered, which is an expensive option in this size range, but could be driven by heat derived from locally available fuels such as agricultural waste, wood, charcoal, etc. Heat-driven cycles can be split into two broad categories: engine-dnven cycles and sorption cycles. The former use some sort of engine to produce work which then powers a conventional refrigeration cycle. Stirling engines, gas turbines, and conventional reciprocating engines have all been used. The refrigeration cycle is normally a vapour compression cycle, but Brayton cycles and Ericsson cycles have both been used experimentally. Engine-dnven cycles have been built and operated successfully but have potential problems with noise and maintenance requirements I reliability. These problems can be minimised in an industrial or large commercial environment and hence most of the successful applications have been in 100 -I- kW sizes. Sorption cycles do not have a mechanical compressor and need little or no mechanical work input. Consequently they have few or no moving parts. This makes them particularly attractive for smaller applications, although it should be mentioned that the biggest existing market is for Lithium Bromide - Water absorption air conditioners which provide cooling in the MW range. All sorption (absorption and adsorption) cycles can be thought of as using a ‘chemical compressor’ rather than a mechanical one. In its simplest form an adsorption refrigerator consists of two linked vessels, both of which contain refrigerant and one of which is also filled with adsorbent as shown in Fig. 1. 3 05 (a) (b) Fig. 1. Simplified adsorption cycle schematic Initially the whole assembly is at low pressure and temperature, the adsorbent contains a large concentration of refrigerant within it and the other vessel contains refrigerant gas (a). The adsorbent vessel (generator) is then heated, driving out the refrigerant and raising the system pressure. The desorbed refrigerant condenses as a liquid in the second vessel, rejecting heat (b). Finally the generator is cooled back to ambient temperature, readsorbing the refrigerant and reducing the pressure. The reduced pressure above the liquid in the second vessel causes it to boil, absorbing heat and producing the refrigeration effect. The cycle is discontinuous since useful cooling only occurs for one half of the cycle. Two such systems can be operated out of phase to provide continuous cooling. The above description is of an adsorption cycle which might well use an active carbon adsorbent. However, it applies equally well to liquid sorbents used in absorption cycles. The thermodynamics of liquid absorption and solid adsorption cycles are very similar, although the practicalities are very different. The major, and obvious, difference is that it is not possible to pump the solid adsorbents around the system. Given that the whole machine is a heat transfer device, it would clearly be advantageous to pump the sorbent through a heat exchanger. There are ways in which a bed of a solid sorbent can be made to behave as if it has been pumped through a counterflow heat exchanger, but it is more complicated than if it could be truly pumped like a liquid. Available methods are discussed in Section 5.2. Whilst the heat and mass transfer limitations imposed by the use of a solid adsorbent are a problem, there are a number of advantages that solid adsorbents have over liquid absorbents. The fist advantage of solid adsorbents is that they are totally non-volatile unlike most liquid absorbents. One of the two conventional liquid absorption cycle pairs uses ammonia as the refrigerant and water as the absorbent. In the generation phase a-b above, when a concentrated ammonia - water solution is heated, the ammonia is driven off but the vapour contains a few percent of water. This must be removed in a rectifier which preferentially condenses most of the water vapour and returns it to the generator. Unfortunately this reduces the energy efficiency as well as requiring an additional heat exchanger within the system. The other commonly used pair uses water as the refrigerant and Lithium 306 Bromide as the absorbent in air conhtioning applications. It does not suffer from the same problem, since LBr is effectively non-volatile. However, the pair does have limitations due to the crystallisation limits of LBr in water. In very hot climates where heat rejection temperatures are higher than about 35°C the pair cannot be used unless additives are used to move the crystallisation boundary. The major advantage that solid sorbents have over liquid systems is the large range of suitable materials available and the ability to engineer them for a particular application. The number of liquid absorbent - refrigerant pairs that give reasonable performance is very limited and governed by unalterable chemistry and physics. When using physical adsorption, almost any refrigerant may be used and in principle an adsorbent can be manufactured with the optimal pore size distribution for the particular application. In summary, heat-driven cycles for cooling or heat pumping can have energy saving and environmental benefits. There are also niche applications in developing countries or remote areas. Adsorption cycles using active carbons are one of a number of approaches that might be economically viable. 2 The Basic Adsorption Cycle 2. I Introduction In order to understand the operation of the cycle and the ideas put forward later it is useful to look at the essential properties of adsorbent-adsorbate pairs and the way that they are used in the solar refrigerator. Adsorbents such as active carbons, zeolites or silica gels can adsorb large quantities (c. 30% by weight) of many gases within their micropores. The most widely used combinations are active carbons with ammonia or methanol, and zeolites with water, but the choice of which adsorbent and which refrigerant gas to use depends on the application. The quantity of refrigerant adsorbed depends on the temperature of the adsorbent and the system pressure. A good approximation to the form of the function is given by the Dubinin - Astakhov (D-A) equation which is illustrated graphically in Fig. 2 and is commonly referred to as a Clapeyron diagram. The following section may be omitted on first reading: In its original formulation, the D-A equation is 307 where : V is the micropore volume filed with the adsorbed phase. V, is the limiting micropore volume. B is a function of the micropore structure, decreasing as microporosity increases. T is the temperature (K). p is the affinity coefficient, which is a property of the adsorbate alone. It is approximated by the ratio of the adsorbate volume with the adsorbed volume of a reference substance (normally benzene) under the same conditions. n is a constant p is the system pressure. p* is the pressure of the adsorbed phase within the micropores. p' will vary within the micropores and is impossible to measure directly. However, the assumption is made that the adsorbed phase is analogous to saturated liquid at the same temperature, and pa may be replaced by psot the saturation pressure of the adsorbate at temperature T. At temperatures higher than the critical temperature, other estimates for p* may be used (Smisak and Cernf[ 11). The mass concentration x can be related to the volume of adsorbed phase V by an assumed density of adsorbed phase r : The value of r can be estimated as that of saturated liquid at the same temperature or related to supercritical properties at temperatures above critical. Critoph [2] found that for the practical purposes of modelling ammonia - carbon adsorption cycles, using experimentally determined porosity data, that the complexity of estimating both r andp' at sub and supercritical levels was not justified. The measured porosity data could be fitted to a much simpler version of the equation with no loss of accuracy, as follows: x=pv where: x, is the limiting concentration, k isaconstant. Combining this with the relationship between saturation pressures and temperatures: lnp,, = a - - C where a and c are constants, Tat 308 K is a constant. T,,, is the saturation temperature of the adsorbate at the system pressure (Kelvin). Fig. 2. Clapeyron diagram showing saturated refrigerant and isosteres Lines of constant concentration (isosteres) are straight when the natural logarithm of pressure is plotted against the inverse of the absolute temperature. It is conventional to plot against -1/T so that temperature still increases when moving from left to right. Since adsorbents hold less adsorbate when hot the low concentration isostere is on the right of the high concentration isostere. The line labelled ‘pure refrigerant’ shows the variation of the refrigerant’s saturation pressure and temperature (i.e. the variation of its boiling/condensing temperature with its pressure). It takes energy (heat of desorption) to drive refrigerant from the pores and similarly, when gas is adsorbed into the pores heat is generated. This is analogous to the latent heat required or generated in boiling or condensation but is greater in size. The heat of desorption per mass of refiigerant is actually proportional to the slope of the isosteres. 309 2.2 The simple solar refi-igerator Now it is possible to understand the simple solar refrigerator illustrated in Fig. 3 below: Solar collector E II I :vapor 'ator Cold box Fig. 3. Schematic solar refrigerator Fig. 3 shows an idealised solar collector (generator) containing adsorbent which is connected to a condenser that rejects heat to the environment and an insulated box containing a liquid receiver and a flooded evaporator. Fig. 4 shows the p-T-x (pressure - temperature - concentration or Clapeyron diagram) for the adsorbent- adsorbate pair with typical temperatures. The cycle begins in the morning with the generator (solar collector) at ambient temperature and the evaporator (but not the receiver) full of cold liquid refrigerant from the previous cycle. The adsorbent contains the maximum quantity of refrigerant at this time. As the sun heats the collector, the adsorbent temperature rises and some refrigerant is desorbed. Since it is desorbed into a system of fixed volume the pressure in the system rises. The gas does not condense because the saturation temperature corresponding to the system pressure is below ambient temperature. As more heat is transferred to the adsorbent, more gas is desorbed and the pressure rises further. Since the volume of the gas in the system is not large, the mass of gas desorbed is small compared to that still adsorbed and thus the reduction in mass concentration is small. Thus [...]... 41 31 160 Latent heat L (kJ/kg) 1368 768 3 89 605 382 288 235 2258 508 1102 842 746 93 5 415 766 836 1 89 Liquid density p &g/m3) 681 815 1455 883 1214 1563 14 09 958 1780 791 7 89 833 688 1447 782 703 31 19 Latent heat per unit vol pl- xi05 (kJ/m3) 93 2 626 566 534 463 450 331 2163 90 5 872 665 62 1 643 600 599 588 588 1 Other chemicals not shown are superior to W2, but it is included as the best of the conventional... Ammonia Formaldehyde Vinyl Fluoride Sulphur Dioxide R32 Chlorine R22‘ Water Sulphur Trioxide Methanol Ethanol Ethylamine Hydrogen Cyanide Nitrogen Dioxide’ Acetonitrile Methylamine Bromine3 Boiling Point (“C) -34 - 19 -38 - 10 -52 -34 -4 1 100 45 65 79 57 26 21 81 -7 59 Mol Wt 17 30 64 46 52 71 86 18 80 32 46 43 27 46 41 31 160 Latent heat L (kJ/kg) 1368 768 3 89 605 382 288 235 2258 508 1102 842 746 93 5... hydrocarbons such as butane have been evaluated in detail by Critoph [3,4] but are significantly worse in performance than methanol or ammonia In 199 6, these two refrigerants are the only ones used in the major laboratories working on carbon adsorption cycles Having chosen a suitable refkigerant, the best adsorbent must be found Zeolites, silica gels and chemical adsorbents have been used as well as carbons,... as an environmentally friendly refi-igerant for other applications The best sub-atmospheric refrigerant is water Unfortunately it is not strongly adsorbed by carbons, but refrigerators and heat pumps based on water - zeolite pairs have been built and tested in research laboratories Methanol is adsorbed well by carbons and a solar refrigerator based on a carbon - methanol pair was marketed by Brissoneau... concentrate on the carbon adsorbents An indication as to the range of COP’S that can be expected and the influence of the type of cas-bon used can be obtained by modelling the performance of carbons with a range of adsorption parameters For this purpose it is preferable to use the Dubinin-Raduschkevich 32 1 equation, in which the exponent n ofthe D-A equation is made equal to 2 for theoretical reasons... which covers most carbons (and zeolites) is: 0.15 I xo I 0.75 11K517 Figs 9 and 10 below show the COP’S of rehigerators and heat pumps respectively in the basic cycle described in Section 2 0.6 I 0.5 0.4 0.3 Evaporating temperature -10°C Condensing and adsorption heat rejection temperature 30°C 0.2 Maximum carbon temperature 120°C 0.1 K 0 0 5 1 0 Fig 9 Variation of refrigeration COP with carbon properties... granular carbon used by the author has an xo of 0.25 with ammonia and other readily available carbons have an xo of 0.3 The current limit lies with a carbon such as Andersen AX-21 with an xo of 0.55 This material gives COP’S that are 10-20% higher than conventional carbons, but the possible improvement is not so large as to radically improve the cost effectiveness To achieve much higher performance,... granular adsorbent beds The preferred refrigerants for use with active carbons are methanol and ammonia Methanol - carbon systems have been studied in depth by Meunier’s team at LIMSI (Laboratoire d’Informatique pour la Mtchanique et les Sciences de 1’Ingtnieur) Guilleminot, Meunier and Paklesa [I21 modelled the two dimensional heat transfer in the methanol - carbon generator of a solar refrigerator The... predicted COP’S are similar A cooling COP of 0 .9 (based on heat input to the cycle) is predicted for one design with modest regeneration efficiency, evaporating at 5°C and condensing at 4OOC 334 5.3 Improving heat transfer Conventional beds of granular carbon have low thermal conductivity, typically 0.1 W/mK This presents a problem, both in terms of the performance and cost of systems Low power machines... scale 332 system for air conditioning A practical schematic is shown in Fig 17 The two ‘active’ beds are packed with activated carbon and the two ‘inert’ beds are packed with non-reactive particles such as steel balls The characteristic sue of the carbon particles and steel balls is in the range 1-3 mm The rest of the system contains ammonia refrigerant in either liquid or gaseous form Fig 17 shows . Strong, S.L., Carbon 93 Abstracts p 490 , Buffalo, 199 3 Kaneko, K. and Murata, K., Adsorption 199 7,3, 197 Chaffee, A. and Pandolfo, A., Carbon 90 Abstracts p246, Paris 199 0 also presentation. Utilisation Research Forum, London, 199 0 Verheyen, V., Jagtoyen, M. and Derbyshire, F., Carbon 93 Abstracts p 474 199 3 AGLARG Report to US Dept. of Energy, Contract 466 590 , 199 7 Private Communication,. Science and Technology (1 99 3) MacDonald, J.A.F. and Quinn, D.F., Carbon 34 11 03 ( 199 6) Barton, S.S Evans, M.J.B., and MacDonald, J.A.F., Carbon 2 .9 1 099 Alcaniz-Monge, J., dela