Adsorption systems economics

Many gas streams can be deodorized by using solid adsorption systems to remove the odor before the stream is released to the atmosphere. Such procedures are often both effective and economical. 

Engineering Considerations To effect the good engineering design of an activated carbon adsorption system, it is first necessary to obtain information on the following the actual cubic feet per minute (ACFM) of air to be processed by the adsorber, the temperature of gas stream, the material(s) to be absorbed, the concentration of the material to be adsorbed, and if the intended application is air pollution control such as odor control - then the odor threshold of the material to be adsorbed. In addition, data is needed on the presence of other constituents in the gas stream, and whether or not solvent recovery is economical. 

Solvent recovery systems would also necessitate the specification of condenser duties, distillation tower sizes, holding tanks, piping, and valves. It is important to note that the engineering design of an adsorption system should be based on pilot data for the particular system. Information can usually be obtained directly from the adsorbent manufacturer. The overall size of the unit is determined primarily by economic considerations, balancing the operating costs against the capital costs. 

Nitrogen usually is produced by cryogenic air separation, liquefaction of atmospheric air, and removal of the nitrogen by distillation, as described earlier. Noncryogenic adsorption technology, including PSA and VSA systems, economically provides nitrogen for requirements between 4,000 and 30,000 standard cubic feet per hour (SCFH).9... 

The adsorption system is distinguished by an extremely economical operation with a good ratio of energy consumption to toluene recovery. Solvent recovery rate is about 99.5 %. 

In the Molex process the Sorbex system is used for the separation of linear and branched hydrocarbons, using as adsorbent 5A molecular sieve. This is the same adsorbent as is used in the cyclic batch processes such as Ensorb and Isosiv. In contrast to the situation with the Cg aromatics, the separation factor is very large and it seems unlikely that for such an easy separation the increased cost of a Sorbex unit is justified by the reduction in adsorbent inventory and/or desorbent circulation rate. Detailed cost comparisons do not appear to have been published, but it seems likely that any economic advantage which the Molex process may have arises more from the energy savings associated with liquid phase operation than from the intrinsic advantages of a countercurrent adsorption system. 

Evaluation of Book-Kept Economics of Adsorption Systems... 

Adsorbent drying systems are typicaHy operated in a regenerative mode with an adsorption half-cycle to remove water from the process stream and a desorption half-cycle to remove water from the adsorbent and to prepare it for another adsorption half-cycle (8,30,31). UsuaHy, two beds are employed to aHow for continuous processing. In most cases, some residual water remains on the adsorbent after the desorption half-cycle because complete removal is not economically practical. The difference between the amount of water removed during the adsorption and desorption half-cycle is termed the differential loading, which is the working capacity available for dehydration. 

Although the continuous-countercurrent type of operation has found limited application in the removal of gaseous pollutants from process streams (Tor example, the removal of carbon dioxide and sulfur compounds such as hydrogen sulfide and carbonyl sulfide), by far the most common type of operation presently in use is the fixed-bed adsorber. The relatively high cost of continuously transporting solid particles as required in steady-state operations makes fixed-bed adsorption an attractive, economical alternative. If intermittent or batch operation is practical, a simple one-bed system, cycling alternately between the adsorption and regeneration phases, 1 suffice. 

The use of a combination dryer should be investigated when a very low dew point is necessary. Placing a refrigeration system ahead of an adsorption dryer will allow the more economical refrigeration unit to remove most of the vapor and reduce the load on the desiccant. 

Commission. The work in this project will start in April 2003 and will continue for three years. One of the objectives is the development of a gas fired adsorption heat pump coupled to an underground storage system. That means the size of the sorption storage system is reduced significantly, a large number of cycles is possible and the actual seasonal storage is now realized by the UTES system. The economics of this concept are expected to be much better. 

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