Carbon Materials for Advanced Technologies Part 10 pps

295 Table 4 shows the composition of a typical British natural gas, including the components as relative pressures, and their potential for adsorption on a coal based pellet (SSC 207EA 4mm). Table 4. Typical Composition of British Natural Gas expressed as relative pressure and their potential for adsorption on a coal based carbon. Bacton Terminal Gas Concentration Relative Potential Component vol.% Pressure Uptake g/g Carbon dioxide 0.25 Nitrogen Hydrocarbons Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Benzene Odorants Diethyl sulfide Methyl ethyl sulfide Ethyl mercaptan Tert. butyl mercaptan 3.17 92.81 2.84 0.58 0.20 0.067 0.032 0.017 0.007 0.001 0.022 7.00E-04 6.00E-05 6.00E-05 1.20E-04 7.32OE-04 2.350E-04 9.480E-04 1.370E-03 2.24OE-03 3.970E-03 4.800E-03 2.220E-03 2.96OE-03 1.160E-04 4.260E-06 1.180E-06 8.500E-06 0.019 0.068 0.127 0.176 0.232 0.259 0.262 0.127 0.143 2.920E-02 8 600E-04 0.079 Comparison of the volumeholume composition data with the relative pressure data shows that although C2-C4 hydrocarbons are present to the greatest volume percent, their actual pressures are an order of magnitude lower than the C5 plus hydrocarbons. Hence, the C5 plus hydrocarbons would be adsorbed in preference to the C2-C4 hydrocarbons and would displace them over a number of cycles. It is apparent therefore that the C5 plus hydrocarbons must be considered the primary target gases for pre-adsorption in guard bed systems The added odorants in natural gas require specific consideration. Table 4 shows that odorants are present at low partial pressures. Hence, adsorption of these odorants within a guard bed is likely to be small, especially when associated with the competitive adsorption of the hydrocarbon gases. It is probable therefore that some odorants e.g. ethyl mercaptan, will in fact pass through the guard bed and be present within the storage tank. Adsorption of odorants within the storage tank will be small because of their low partial pressures, and competitive adsorption of C4 and C5 hydrocarbons. Therefore, it seems unlikely that the odorant gases would accumulate within the storage vessel and thus would have an insignificant effect upon the storage performance. Indeed, their presence within the storage tank may be advantageous ensuring that the natural gas is odorized throughout the ANG storage system. 5.3 Guard Bed Adsorbent Characteristics It is difficult to make generalizations regarding the desirable characteristics of active carbons for guard bed applications without consideration of specific guard bed designs, e.g., fixed or mobile, and method of operation, i.e., heated or non-heated. However, consideration of the target gases and their likely adsorptioddesorption behavior, allows some generic classification to at least be intimated. The basic function of the guard bed is to adsorb C5 plus other hydrocarbons, preventing their accumulation within the main adsorbent storage bed. The relatively low partial pressures or relative pressures (relative to the pure substance vapor pressure) of these trace components suggests the need for an adsorbent of high adsorption capacity, i.e. containing a high proportion of micropores. However, probably more important than the adsorptive properties are the desorptive properties of the adsorbent. Facile desorption is required to prevent retention of the C5 plus gases on the guard bed, shortening its operating life and increasing the need for bed replacement. The importance of adsorbent desorptive properties are already widely appreciated in Evaporative Loss Control Devices (ELCDs), where the saturation uptake of butane under dynamic conditions, and weight desorbed in 200- 300 bed volumes of air passing through the adsorbent, are used to define the optimum adsorbent characteristics [73]. For ELCDs, it is generally accepted that adsorbents exhibiting a high proportion of pores at the upper end of the @cropore range and the lower end of the mesopore range exhibit the desired adsorptioddesorption behavior. Such carbonaceous adsorbents tend to be typically (but not exclusively) those from coal or wood based precursors. Since the guard bed is a specialized ELCD, adsorbents already optimized for these applications should be well suited to the guard bed application. However, the porous structure of the adsorbent and its adsorptioddesorption properties are not the only features of importance in 297 defining the requirements of a guard bed adsorbent, the heat capacity and thermal conductivity of the adsorbent must additionally be considered. The heats of adsorption and desorption need to be dissipated and subsequently returned if good cyclic efficiencies are to be gained. Indeed, the thermal effects of adsorption are critical factors for mobile guard beds, where heat load for desorption may place an additional electrical load on the vehicle systems. Adsorbents should possess high heat capacity and thermal conductivity values, properties which favor high density carbons. To some extent, desirable adsorptive and thermal properties are somewhat contradictory. Adsorbents possessing large inherent pore volumes will exhibit low thermal conductivity. Additionally, granular beds exhibit poor heat transfer characteristics. Thermal conductivity values in the range of 0.86 W/m.K have been calculated for single grains of SSC 208C, which reduced to 0.17 W/m.K for a bed of 208C used in ammonia adsorption studies [74]. However, good adsorptive and thermal properties can be combined in densified or immobilized adsorbents, provided the incorporation of binder phase is not deleterious to adsorptive capacity. A thermal conductivity of 0.33 W/m.K was reported for an immobilized 208C adsorbent used in ammonia studies [74]. Therefore, the desired guard bed adsorbent is one which combines high adsorptive capacity with low retentivity and which also has good thermal conductivity, a particularly difficult target to achieve. 5.4 Guard Bed Design Two guard bed design concepts need to be considered, a large fmed unit present at the fuel source or filling point, or a small mobile pre-adsorption unit incorporated into the vehicle mounted ANG storage system. The fixed system has the obvious advantage of scale, making possible the use of conventional regeneration technologies e.g. hot gas or steam, with proper gas handling facilities for the enriched desorbed phase. However, the fixed system has the primary disadvantage that it produces a substantially deodorized gas stream to downstream pipework and the vehicle refueling point. This fact, in addition to the large fixed capital costs associated with the installation of such facilities at every filling station, has tended to rule out their use in favor of small vehicle mounted guard bed units in most ANG storage concepts. The smaller mobile unit suffers the disadvantage of scale, and would be less efficient in complete removal of undesirable gas species. However, it would offer the advantage of allowing some odorized gas throughout the storage system. Heat management in mobile guard beds also must be considered. Being relatively small units, with a high external surface to mass ratio, heats of adsorption can be relatively quickly dissipated. The heat of desorption needed to effectively purge mobile guard beds must come from an external source. Ths could be made available 298 via a thermal feed back loop from the vehicle cooling or exhaust systems. Such systems would be complex and possibly too heavy for practical application. However, the guard beds could be heated by internally mounted electrical cartridge heaters, powered from the vehicle electrical system. Such an approach has been shown to be successful [69,70]. Data on the long term performance of guard bed systems has not been widely reported because of its proprietary nature. Work reported to Future Fuels Inc. [75], confirmed the observations above, i.e., that guard beds were effective in the removal of C5 plus hydrocarbons. The C2 - C5 hydrocarbons were shown to pass to the ANG storage vessel where they desorbed again on depressurization. C2 - C5 hydrocarbons were desorbed from the guard bed on flow through and the guard bed was as effective in desorbing these hydrocarbons when cold as it was when heated. However, the fate of the adsorbed C5 plus hydrocarbons was not discussed in this work and it is likely that a guard bed would require heating to desorb these species. 6 Summary. In excess of one million vehicles worldwide presently use natural gas as their fuel. Predominantly, it is stored as CNG at about 20 MPa. An alternative whch may be safer and more advantageous to use is an ANG storage system operating at considerably lower fill pressures. However, a successful adsorbent storage system for NGVs requires much more than a good adsorbent, but, without a high performance adsorbent, ANG can not become a commercial reality. With limited space available on-board a vehicle, storage performance must be based on the energy which can be stored within a given volume. The minimum acceptable level is considered to be 150 VN, 6.2 1b.icubic foot, equivalent to about one gallon of gasoline. To achieve this level of Performance the adsorbent has to adsorb about 120 mg gas per ml of adsorbent, where the adsorbent volume must be the practical packed volume. To date, porous carbons have yielded the best performance, but the micropore volume and pore size must be carefully controlled to make such an uptake possible. The second essential element of an ANG system is the storage vessel itself. The high pressures (20+ MPa) used for CNG storage demand the use of a cylindrical vessel. The external envelope of large cylinders cannot easily be placed efficiently within a small vehicle structure. The lower AVG pressure (<5 MPa) provides for more versatility in vessel design compared to CNG. The AGLARG tank design helps solve the problem of efficient space utilization. Possibly, future vessels of a similar type can be integrated into the vehicle structure. 299 Good heat dissipation on adsorption (fueling) and good heat input during desorption (fuel use) are desirable features for maximizing capacity and use of an ANG system. The flat aspect and internal webs of the AGLARG tank design provides better heat transfer when compared to a cylindrical vessel, and greatly improve the overall performance of the ANG system. Natural gas composition varies greatly. Although principally methane, it often contains components such as higher alkanes which are irreversibly adsorbed at ambient temperature, and gradually reduce the adsorbent uptake of methane, lowering the overall storage capacity. Currently, it is unlikely that natural gas will be "cleaned up" prior to delivery to a NGV. Consequently a vehicle's ANG storage system will have to be protected from the deleterious components in natural gas. The use of guard beds, which in themselves are adsorbent systems where the adsorbent has to be carefully selected for rapid preferential adsorption of the higher alkanes, pentanes and above, has been shown to be effective in maintaining the storage capacity of the ANG tank. Thus the "guard bed" is an essential component of a satisfactory ANG storage system, Finally, although a good adsorbent is key to the success of ANG, it must be integrated into a well designed system which must compensate for the weaknesses inherent in the adsorption process, deleterious poisoning and heat effects. 7 References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Stephenson, J., A Position Paper on Natural Gas Vehicles 1993, International Association of NGVs, (1993) Darrow, K.G., Light Duty Vehicle Full Fuel Cycle Emissions Analysis Topical Report, Gas Research Institute Report GRI-93/0472 (1 994) European Natural Gas Vehicle Association Bulletin, April 1996 Hagen, M., "Clathrate Inclusion Compounds", Reinhold, New York, 1962 Dignum, M.J., Report 33259, Ontario Ministry of Transportation, 1982, 1201 Wilson Ave, Downsview, Ontario, Canada M3M 1 J8 Notaro, F., "Enhancement of Automotive Compressed Natural Gas Fuel Storage Via Adsorbents" New York State Energy Research and Development Authority Report 85-1 1, 1985 Komodromos, C., Pearson, S. & Grint, A,, "The Potential of Adsorbed Natural Gas for Advanced On-board Storage in Natural Gas Fueled Vehicles", International Gas Research Conference, Florida, 1992. Fricker, N. and Parkyns, N.D., "Adsorbed Natural Gas for Road Vehicle Applications", 3rd Biennial International Conference and Exhibition of Natural Gas Vehicles, International Association for Natural Gas Vehicles, Gothenburg, Sweden, 1992 Komodromos, C., Fricker, N. & Slater, G., "Development of Novel Tanks for Low-Pressure Adsorbed Natural Gas Storage in Vehicles", International Association for Natural Gas Vehicles, Toronto, 1994 Bennett, P.G. & Tilley, G., Methamotion Conference, London, 1995 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. 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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. 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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. [...]... 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... 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,... 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... 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... still adsorbed and thus the reduction in mass concentration is small Thus 310 the variation of pressure with adsorbate temperature approximates to that of an isostere as shown in Fig 4 -1n- -10 C 40°C + 3 20°C Fig 4 Clapeyron diagram for a simple solar refrigerator The situation changes when the system pressure becomes high enough for refrigerant to condense in the condenser and reject the resulting latent... Name 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 768 389 605 382 288 235 2258 508 1102 842 746... heat rejection temperature 50°C Maximum carbon temperature 150°C K ? O 0 5 IO 15 b Fig 10 Variation of heat pump COP with carbon properties 5 Improving Cost Effectiveness The cost effectiveness of an adsorption cycle machine depends both on the COP, which will affect the operating costs and also on its size, which will influence the capital cost The COP in a particular application will be both a function... hydrocarbons such as butane have been evaluated in detail by Critoph [3,4] but are significantly worse in performance than methanol or ammonia In 1996, 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,... Although the bed has very poor conductivity, the carbon (or other adsorptive) grains have a very high surface area which can be used for convective heat transfer In the case of heating a carbon - ammonia bed, the ammonia can be heated external to the bed in a conventional heat exchanger and then pumped through the bed, where it rapidly gives up its heat to the carbon and, in the process, desorbs a little . the C2-C4 hydrocarbons and would displace them over a number of cycles. It is apparent therefore that the C5 plus hydrocarbons must be considered the primary target gases for pre-adsorption. MacDonald, J.A.F., Carbon 2.9 109 9 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. 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