Gas separation: processes – Solid sorption
Reexamination Certificate
2001-11-02
2003-07-15
Smith, Duane (Department: 1724)
Gas separation: processes
Solid sorption
C095S900000, C423S717000, C502S400000, C502S439000, C502S514000
Reexamination Certificate
active
06592651
ABSTRACT:
FIELD OF THE INVENTION
The present invention provides for processes for activating monolith adsorbents to remove the binders present in the monolith to improve kinetics while maintaining structural integrity. The treated monoliths of this invention are suited for enhanced gas and vapor separation operations.
BACKGROUND OF THE INVENTION
Cyclic adsorption processes are frequently used to separate the components of a gas mixture. Typically, cyclic adsorption processes are conducted in one or more adsorbent vessels that are packed with a particulate adsorbent material which adsorbs at least one gaseous component of the gas mixture more strongly than it adsorbs at least one other component of the mixture. The adsorption process comprises repeatedly performing a series of steps, the specific steps of the sequence depending upon the particular cyclic adsorption process being carried out.
In any cyclic adsorption process, the adsorbent bed has a finite capacity to adsorb a given gaseous component and, therefore, the adsorbent requires periodic regeneration to restore its adsorption capacity. The procedure followed for regenerating the adsorbent varies according to the process. In VSA processes, the adsorbent is at least partially regenerated by creating vacuum in the adsorption vessel, thereby causing adsorbed component to be desorbed from the adsorbent, whereas in PSA processes, the adsorbent is regenerated at atmospheric pressure. In both VSA and PSA processes, the adsorption step is carried out at a pressure higher than the desorption or regeneration pressure.
A typical VSA process generally comprises of a series of four basic steps that includes (i) pressurization of the bed to the required pressure, (ii) production of the product gas at required purity, (iii) evacuation of the bed to a certain minimum pressure, and (iv) purging the bed with product gas under vacuum conditions. In addition, a pressure equalization or bed balance step may also be present. This step basically minimizes vent losses and helps in improving process efficiency. The PSA process is similar but differs in that the bed is depressurized to atmospheric pressure and then purged with product gas at atmospheric pressure.
As mentioned above, the regeneration process includes a purge step during which a regeneration gas stream that is depleted in the component to be desorbed is passed counter currently through the bed of adsorbent, thereby reducing the partial pressure of adsorbed component in the adsorption vessel which causes additional adsorbed component to be desorbed from the adsorbent. The non-adsorbed gas product may be used to purge the adsorbent beds since this gas is usually quite depleted in the adsorbed component of the feed gas mixture. It often requires a considerable quantity of purge gas to adequately regenerate the adsorbent. For example, it is not unusual to use half of the non-adsorbed product gas produced during the previous production step to restore the adsorbent to the desired extent. The purge gas requirement in both VSA and PSA processes are optimization parameters and depend on the specific design of the plant and within the purview of one having ordinary skill in the art of gas separation.
Many process improvements have been made to this simple cycle design in order to reduce power consumption, improve product recovery and purity, and increase product flow rate. These have included multi-bed processes, single-column rapid pressure swing adsorption and, more recently, piston-driven rapid pressure swing adsorption and radial flow rapid pressure swing adsorption. The trend toward shorter cycle times is driven by the desire to design more compact processes with lower capital costs and lower power requirements. The objective has been to develop an adsorbent configuration that demonstrates a low pressure drop, a fast cycle time and an ability to produce products such as oxygen.
Honeycomb structured monoliths, which are normally made by high temperature treatment of a mixture of binder, additives and catalyst or adsorbent materials are suitable for fast cycle sorption processes. These monoliths, either in the form of one single block or in the form of extrudates with multiple random channels, exhibit unique features of low pressure drop, good mechanical properties and freedom from attrition and fluidization problems of conventional catalysts and adsorbents. These types of monoliths have historically been employed as catalyst supports in automobile catalytic converters, catalytic combustion, electrochemical reactors and biochemical reactors. These monoliths, however, have very low loading of active catalyst or adsorbent and not all of the adsorbent or catalyst material is accessible to the gas molecules passing through them.
Monoliths, however, that are made from paper like sheets containing polymeric fibers as described in U.S. Pat. Nos. 5,660,048; 5,660,221; 5,685,897; and 5,580,369, exhibit very high loading of adsorbent material. Active adsorbent materials such as zeolites, carbon molecular sieve (CMS), alumina and other porous adsorbent materials can be embedded in the paper during the manufacturing process. In order to bind adsorbent particles with fibers and to have uniform distribution of adsorbent particles, many ingredients and additives may also be added into the slurry during the sheet manufacturing. Normally, the non-woven-fabric sheet (paper), which will be shaped into the monolith in later stages, comprises fibers such as polyaramids, one or more binders such as acrylic latex, a retention aid and active adsorbent materials. A retention aid is any material that enhances the retention of very small fibers and adsorbents.
The binder is added to the slurry to bind the adsorbent particles to the fibers. Through this process, adsorbent/catalyst particles tend also to be encapsulated by the polymeric binder material. The adsorbent containing monoliths or sheets need to be activated at high temperatures to desorb water or other sorbed species from their active sites. The temperature at which the activation is undertaken is dependent on the nature of the adsorbent material. As described in U.S. Pat. No. 5,580,369, this activation is typically performed at temperatures below the decomposition temperature of the binder. This degree of activation is sufficient, if the adsorbate molecules can diffuse through the binder layer to reach the adsorbent/catalyst. It has been demonstrated that such an activation below the temperature of binder decomposition works well in dehumidification applications, where the diffusivity or solubility of water through the binder is high and adsorbent does not need higher temperature activation. However, it has been found in the current invention that in certain types of applications such as the adsorption of N
2
from air, the binder layer typically provides a kinetic barrier to adsorbate molecules from reaching the adsorbent particles. When this happens, the resulting monolith has poor kinetic performance for gas adsorption and desorption, which results in poor PSA/VSA performance in gas separation processes.
The present inventors have discovered that using two binders in the sheet making process and removing one of them during monolith activation, will provide for a better performing adsorbent monolith.
SUMMARY OF THE INVENTION
The present invention provides for a method for removing binders from monolith adsorbents. This method utilizes a combination of two binders one of which decomposes at a lower temperature than the other in the handsheets used to prepare the monolith adsorbents. After decomposing the lower temperature binder, the higher temperature binder remains and adds structural integrity to the final formed monolith. The heat used to decompose the lower temperature binders also activates the adsorbent material contained in the monolith.
The present invention also provides for a method for separating a first gaseous component from a gas mixture by passing the gas mixture into an adsorption zone containing a monolith adsorbent that has been treated by he
Jain Ravi
Jale Sudhakar R.
Lawrence Frank M.
Neida Philip H. Von
Pace Salvatore P.
The BOC Group Inc.
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