Adsorptive solvent recovery systems

In 1995, one of Europe s most modem printing shops in Dresden was equipped with an adsorptive solvent recovery system (Supersorbon process (Figure 22.1.18.)). 

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. 

Recoveiy of solvent vapor (dicfaloromethane, CH2CI2) fiom air by pressure swing adsorption (PSA) was studied, using two columns packed with high silica zeolite and resin as adsorbent Gravimetric measurements wete made for dichloromethane on the adsorbents. Computer calculations were carried out to simulate the experimental results using the Stop-Go method to show the calculated results coincide well with experimental results. The method is useful to predict the performance of a solvent recovery system operated by PSA... 

In solvent recovery systems, air velocity rates through the bed should be between 0.2 to 0.4 m/s. The length of the MTZ is directly proportional to the air velocity. Lower velocities (<0.2 m/s) would lead to better utilization of the adsorption capacity of the carbon, but there is a danger that the heat of adsorption not be carried away which would cause overheating and possibly ignition of the carbon bed. 

The most common adsorbant used is granular or powdered activated carbon. This material, which is available from almost all forms of organic carbon-containing matter, is a microcrystalline nongraphite form of carbon. The production of activated carbon can be achieved by use of rotary kilns, hearth furnaces, or furnaces of the vertical shaft or fluidised bed type, and each is suitable for the generation of different pore size and the source of carbon. The pore volume and size are influenced by both the carbon source and method of production. The adsorption properties are directly related to the pore volume, pore size distribution and the nature of the functional groups on the surface of the carbon. Activation is achieved chemically, by treatment by dehydration with zinc chloride or phosphoric acid, or by treatment with steam, hot carbon dioxide or a mixture of both. The activated carbon is available in three basic forms, powder, granules or as cylindrical or spherical pellets. For solvent recovery systems the carbon is usually obtained from either wood charcoal, petroleum residues or coconut shells and is often used in the form of pellets. 

The surface of carbon is essentially nonpolar although a slight polarity may arise from surface oxidation. As a result, carbon adsorbents tend to be hydrophobic and organophilic. They are therefore widely used for the adsorption of organics in decolorizing sugar, water purification, and. solvent recovery systems as well as for the adsorption of gasoline vapors in automobiles and as... 

In many solvent recovery systems, adsorption represents only one step in a complex series of chemical engineering operations. The design of a eomplete system for recovering methylene chloride and methanol from air emitted from a dryer in a resin processing plant has been described by Drew (1975). The overall solvent recovery system includes a water scrubber to remove resins and cool the air to 100°-110°F a standard 2-bed carbon adsorber unit designed for 95% solvent removal efficiency a condenser and decanter to handle the vapors that are stripped from the carbon by steam an extraction column in which water is used to remove the water soluble methanol from the methylene chloride phase a stripping column to remove dissolved methanol and methylene chloride from the waste water and a drying column to remove water from the recovered methylene chloride. These items of equipment and operations are representative of those required for complete solvent recovery systems however, each system must, of course, be tailored to the profierties of the specific solvent involved. 

The removal of aromatics and relatively heavy hydrocarbons from gas streams with fixed beds of activated carbon is essentially the same process as. solvent recovery, and similar adsorbents and equipment are used. The principal differences are that in hydrocarbon recovery the feed is typically a natural gas or other combustible gas stream rather than air, and adsorption is usually (but not always) conducted at elevated pressure. The basic design approach for hydrocarbon recovery systems follows the same general logic as that described for solvent recovery systems. A brief outline of the key design steps is given in the Calgon Carbon Corporation bulletin, Heavy Hydrocarbon Removal or Recovery from Gas Streams (1987). 

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. 

Determinations of the adsorption isotherms for a number of organic solvent-water systems in contact with hydrocarbonaceous stationary phases have shown that a layer of solvent molecules forms on the bonded-phase surface and that the extent of the layer increases with the concentration of the solvent in the mobile phase. For example, methanol shows a Langmuir-type isotherm when distributed between water and Partisil ODS (56). This effect can be exploited to enhance the resolution and the recoveries of hydrophobic peptides by the use of low concentrations, i.e., <5% v/v, of medium-chain alkyl alcohols such as tm-butanol or tert-pentanol or other polar, but nonionic solvents added to aquo-methanol or acetonitrile eluents. It also highlights the cautionary requirement that adequate equilibration of a reversed-phase system is mandatory if reproducible chromatography is to be obtained with surface-active components in the mobile phase. 

Figure 22.3 Schematic drawings of solvent vapor recovery systems (a) conventional thermal swing adsorption, (b) conventional pressure swing adsorption, and (c) rotary adsorber.

Adsorption in low-pressure gas systems such as vent-stream cleanup and solvent-recovery applications presents some special problems. Because of their low density and tbe resulting large velocities, such nenr-atmospheric-preasure streams have much higher pressure drops. This is compounded by the fact that the compression ratio (and thus the power) to evercome any AP is much greater than in high-preasure cases. For this reason, low-pressure adsorption is often carried out in vety shallow beds, sometimes in horizontal vessels,... 

Adsorption is an exothermic process. The adsorption enthalpy, decreases as the load of adsorbed molecules increases. In activated carbon adsorption systems for solvent recovery, the liberated adsorption enthalpy normally amounts to 1.5 times the evaporation enthalpy at the standard working capacities which can result in a 20 K or more temperature increase. In the process, exothermic adsorption mechanisms may coincide with endothermic desorption mechanisms. ... 

In solvent recovery plants, temperature-swing processes are most frequently used. The loaded adsorbent is direct heated by steam or hot inert gas, which at the same time serves as a transport medium to discharge the desorbed vapor and reduce the partial pressure of the gas-phase desorpt. As complete desorption of the adsorpt cannot be accomplished in a reasonable time in commercial-scale systems, there is always heel remaining which reduces the adsorbent working capacity. 

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