Gas separation: processes – Solid sorption – With plural indirect heat transfer steps on solid sorbent or...
Reexamination Certificate
2001-07-05
2002-10-01
Simmons, David A. (Department: 1724)
Gas separation: processes
Solid sorption
With plural indirect heat transfer steps on solid sorbent or...
C095S141000, C095S148000
Reexamination Certificate
active
06458186
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for regenerating electrically conducting adsorbents laden with organic substances which are heated by passing electrical current therethrough.
2. Description of the Related Art
Methods for removing organic gaseous substances of exhaust gas streams by adsorption on adsorbents, in particular, activated carbon, have been used industrially for many years, for example, in devices for recovering solvents. In most cases, the activated carbon is in the form of solid beds of granular material. Less frequently, fiber-shaped activated carbon in the form of mats, for example, nonwoven, woven or knit fabrics, is used, wherein the fabrics are wound to annular adsorbers or clamped into a frame construction. The entire process of exhaust gas cleaning by adsorption is carried out in general in three main steps: the loading step, the regeneration step, and the cooling step. During the loading step, the exhaust gas to be cleaned is passed through the adsorbent material and the organic substances are adsorbed during this process in the inner pore structures of the adsorbent material. During the course of loading, the loading front migrates in the flow direction through the adsorber. When the adsorbent material at the end of the adsorber, viewed in the flow direction, is loaded such that the clean gas concentration that exits the adsorber has reached a preset limit value, the loading process of this adsorber is terminated and the gas flow is switched to another freshly regenerated adsorber. For economic reasons, it is generally desirable to regenerate the laden adsorbent. However, many cases are known in which a regeneration is not possible or it is not economical. This is particularly the case when the concentration of the organic substances is very small, as is the case for intake air of buildings or the circulating air of air conditioning systems of buildings. In these cases, the laden adsorbent is disposed of and is replaced with new adsorbent material.
Since the adsorption is a reversible process, the adsorbed organic substances can be removed from the adsorbent material in a regeneration step by changing the equilibrium conditions. This can be achieved, for example, in that the adsorbent material is heated and the adsorbed organic substances are then removed from the adsorbent by means of a flushing gas stream. This method is, for example, employed in activated carbon adsorption devices which are regenerated with steam (water vapor) or inert gas. In both cases, the flushing gas serves at the same time as a heating medium for the adsorbent. Heating of the adsorbent and desorption of the organic substances are thus coupled in an un-separable way. Because of this indirect heating of the laden adsorbent with the flushing gas very large volume streams of the flushing gas are required. In the case of regeneration with steam, the steam, after exiting from the adsorber, must be condensed together with the desorbed organic substances that have been removed from the adsorbent. The resulting condensate must be regenerated in complicated processes, for example, by rectification such that water and organic substances are separated from one another. The inert gas regeneration in which the inert gas is generally nitrogen, is even more complex than the steam regeneration because the nitrogen is circulated through the adsorber and the nitrogen must then be cooled to very low temperatures in a condensation step in order for the desorbed organic substances to be condensed and for the residual loading of the adsorbent at the end of the regeneration step to be so low that the desired clean gas concentration can be maintained in the subsequent loading step.
Since the heating of the adsorbent by the inert gas as a result of its minimal heat capacity occurs only very slowly and since large flow velocity, which must be between 0.1 and 0.5 m/s, are required, enrichment or concentration factors (=maximum regenerating gas concentrations/crude gas concentration) of only approximately 40 can be achieved with this method . An already significant improvement can be achieved when the adsorbent is not heated indirectly by the flushing gas but directly by passing electrical current therethrough. In this case, as a result of decoupling of the processes heating and flushing, the flushing gas amount can be significantly reduced, and significantly higher enrichment factors can be achieved.
From German patent application 195 13 376 A1 a device for recovering organic solvents is known which is comprised of an annular solid bed filled with granular activated carbon and electrically heatable, which, for adsorption of the solvent, is first loaded with the exhaust gas stream and is subsequently loaded with the regenerating gas for desorption or regeneration. In this device, the outer and inner mantle of the annular solid bed is comprised of a metal grate which forms the electrodes. With this method, enrichment factors of 120 can be achieved and the required flushing gas volume stream can thus be reduced to 5-10% of the crude gas volume streams. With the very small specific gas amounts relative to the inflow surface area of the carbon bed against which the gas streams flow, the required uniform loading of the carbon with the flushing gas, which is required for a good regeneration, can be achieved only when it is carried out centrally in the interior of the adsorber, for example, by a distribution tube with numerous fine bores. A disadvantage in regard to the service life for loading and the quality of the clean gas concentrations in this method is that the regenerating gas flow itself is heated by the activated carbon and that, for a regeneration from the interior to the exterior, the carbon has a significantly reduced regeneration temperature in the innermost carbon layer as compared to the outer layers. As a result of this, the innermost carbon layer is regenerated only insufficiently, and therefore it is not possible to achieve very low clean gas concentrations during the loading step of, for example, a few &mgr;g/m
3
as required, for example, for the intake air for clean rooms.
From German patent document 41 04 513 C2 an adsorber is known which is also heated directly with electrical current and which is characterized in that the adsorbent material is in the form of fibrous activated carbon which is, for example, in the form of mats. One embodiment of such an adsorber is schematically illustrated in FIG.
1
. The filter frame contains 5 to 15 layers of ACF (activated carbon fiber) fabric of 250 g/m
2
per layer. At the top of the frame an electrical power supply of 220 V and 15 kW is provided and the bottom of the frame is connected to ground, while the vertical lateral frame parts are insulating. Crude gas (wide arrow) passes from the left to the right through the filter at 3,500 m
3
/h with a load of 100 mg/m
3
at the inlet side and exits at the outlet side with a load of <1 mg/m
3
at 3,500 m
3
/h. The regenerating gas (narrow arrow) flows from the right to the left through the filter at 50 m
3
/h and exits to the left at 200° C., 50 m
3
/h with a maximum load of 150,000 mg/m
3
.
Several of these frames can be switched together air flow-technically and electrically for the treatment of larger exhaust gas amounts in numerous sensible arrangements. Because of the very quick electrical heating of the fabric which can be achieved in less than a minute, very high enrichment factors of up to 1,500 can be achieved which are advantageous when, for example, solvents are to be recovered.
A disadvantage is that a large expenditure must be provided in order to distribute the regenerating gas uniformly across the filter surface. For example, the regenerating gas velocity relative to the filter surface in the above example of a 1 m×3 m filter is only 0.01 m/s. For this flow velocity a pressure loss of only 10 Pa results across, for example, five layers of activated carbon fiber (ACF) fabric so that, at the time when the regenerating gas i
Chmiel Horst
Möhner Cornelia
Schippert Egbert
Huckett Gadrun E.
Lawrence Frank M.
M+W Zander Facility Engineering GmbH
Simmons David A.
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