WO2000050154A1

WO2000050154A1 - Humidity swing adsorption process and apparatus

Info

Publication number: WO2000050154A1
Application number: PCT/US2000/004081
Authority: WO
Inventors: Barry K. Speronello; Appadurai Thangaraj
Original assignee: Engelhard Corp
Current assignee: BASF Catalysts LLC
Priority date: 1999-02-22
Filing date: 2000-02-17
Publication date: 2000-08-31
Anticipated expiration: 2001-08-22
Also published as: AU3494100A

Abstract

A gaseous mixture has at least one component separated therefrom by a method comprising the steps of: (a) contacting a feed gas stream containing at least two components with an adsorbent material to adsorb at least one component from the feed gas stream to produce a depleted gas stream diminished in the adsorbed component and a loaded adsorbent material; and (b) contacting a desorption gas stream with the loaded adsorbent material to desorb the adsorbed component therein and produce an enriched gas stream containing a desorbed component and a regenerated adsorbent material depleted in the adsorbed component wherein the regenerated adsorbent material is suitable for reuse in step (a) and wherein the relative humidity of the desorption gas stream is higher than the relative humidity of the feed gas stream. The gaseous components are effectively separated in an efficient and economical manner. The separation process is readily adapted to accommodate numerous process streams and widely differing applications. Thus, it is useful in large industrial processing plant as well as in small scale applications. An apparatus for separating gaseous components is also disclosed.

Description

HUMIDITY SWING ADSORPTION PROCESS AND APPARATUS

BACKGROUND OF THE INVENTION

Field of the Invention:

The present invention relates to a method for separating gaseous mixtures; and more particularly, to the use of humidity swing adsorption processes to separate such mixtures.

Description of the Prior Art:

Rising energy costs have led to use of adsorption technology to separate gaseous mixtures into their several components, thereby avoiding more costly distillation processes. In adsorption processes, gas to be separated is fed through a bed of adsorbent material to extract the adsorbed component and produce a stream that is rich in the non- adsorbed component. These procedures are used, for example, to extract nitrogen from air, remove volatile organic compounds ("VOC's") from process streams, and clean and purify industrial feed air.

Typical adsorption processes are based on thermal or pressure swing, purge gas stripping and displacement desorption. All of these however, have numerous drawbacks, making them suitable only for specific separation processes.

In pressure swing adsorption ("PSA") processes, of which U.S. Patent Nos. 4,925,461; 5,176,722; 5,507,857 and 5,707,425 are representative and which are incorporated herein by reference, an adsorbent is used to selectively adsorb one constituent from a gas mixture at higher pressure to produce a gas stream that is depleted of the adsorbed constituent. When the adsorbent is sufficiently loaded in the adsorbed phase, the pressure of the gas in contact with the adsorbent is reduced to a lower pressure which causes the adsorbed gas constituent to substantially desorb from the adsorbent to produce a gas stream enriched in the adsorbed gas constituent, and a regenerated adsorbent material. Typically, a minimum of two adsorbent beds is used to maintain a steady flow of product gas so that at any time at least one bed is producing product gas while the other is undergoing regeneration. Vacuum swing adsorption ("VS A") processes such as disclosed in U S Patent No 5,015,271, and pressure/vacuum swing adsorption ("P/VSA") processes such as disclosed in U S Patent No 5,702,504, both patents of which are expressly incorporated herein by reference, are subsets of PSA which utilize a lower pressure of less than 1 atmosphere for regeneration of the adsorbent bed Equipment used in the PSA, NSA, and P/NSA process is expensive, requiring a multitude of pressure or vacuum vessels, compressors or vacuum pumps, valves to control the flow of the different gas streams to and from the adsorbent beds, as well as a large amount of adsorbent material Substantial amounts of energy are needed to pressurize and repressurize or evacuate the adsorbent beds, causing large energy costs to be incurred by the system

In thermal swing adsorption ("TSA") processes as represented by U S Patent Νos 5,846,295 and 5,570,582, the entireties of which are incorporated herein by reference, an adsorbent selectively adsorbs one constituent of a gas mixture at a lower temperature to produce a gas stream that is depleted of the adsorbed constituent Once the adsorbent is sufficiently loaded with the adsorbed constituent, the temperature of the adsorbed phase is raised to desorb the adsorbed gas and produce a gas stream that is enriched in the adsorbed gas

Like PSA, TSA is usually conducted with two or more fixed beds of adsorbent material, one of which is producing product gas while the other is undergoing regeneration The fixed bed arrangement is expensive, requiring multiple adsorbent vessels capable of withstanding the relatively high regeneration temperatures Numerous valves are needed to control the various gas flows to and from the adsorbent vessels, and expensive heating equipment is required to generate the high temperatures required for regeneration of the adsorbent beds Alternatively, the TSA process uses a rotating monolithic bed which cycles the adsorbent between the lower temperature adsorption zone and the higher temperature desorption zone The rotating bed arrangement is expensive as it requires a sophisticated structure to support the moving bed of adsorbent as it expands and shrinks due to thermal cycling TSA also requires relatively high amounts of energy to heat the adsorbent material to the regeneration temperature, with the additional consequence that the heated regeneration purge stream dilutes the desorbed material The process is further limited by the requirement that the temperature of the gas in the adsorption zone be kept relatively low Any increase in temperature that brings the adsorbent close to the temperature of the desorption zone will decrease the ability of the adsorbent to adsorb and desorb useful amounts of adsorbate during temperature cycling Consequently, a hot gas mixture may require an additional cooling step prior to introduction into the adsorption zone of the TSA process

Additional adsorption processes include purge gas stripping and displacement adsorption In purge gas stripping, an adsorbent bed is regenerated at constant pressure and temperature by purging with a non-adsorbing inert gas This process is only useful for weakly held adsorbed species, owing to the large, and hence, expensive, amount of purge gas otherwise required Displacement desorption is similar to purge gas stripping, but instead of utilizing an inert purge, the adsorbed species are displaced by a stream containing a competitively adsorbed species In such processes however, product recovery becomes complex and expensive due to product contamination by the displacing agent Additionally, the choice of the adsorbent material is critical One subset of displacement desorption uses water as a displacing agent, but in all cases the adsorbent requires a subsequent high temperature thermal treatment to desorb the water from the adsorbent or, alternatively, regenerate the adsorbent See, for example, U S Patent Nos 4,966,611 and 4,319,893, the entire disclosures of which are expressly incorporated herein by reference, and Seballo et al , Low-Temperature Displacement Desorption of Substances Adsorbed on NaX Zeolite, Journal of Applied Chemistry of the USSR, Nol 43, No 11, pp 2439-43 (Nov 1970), Seballo et al , Study of Adsorptwnal Separation of Hydrocarbons in Moving Zeolite Bedswith the Aid of Low-Temperature Displacement Desorption, Journal of Applied Chemistry of the USSR, Nol 44, No 1, pp 44-48 (Jan 1971), and Astakhov et al , Investigation of Displacement Desorption of Carbon Disulfide from Active Carbons, Journal of Applied Chemistry of the USSR, Vol 48, No 9, pp 1975-78 (Jan 1975) Due to this high temperature thermal treatment, the cost of such processes is high

There remains a need in the art for a low cost process which can effectively separate the constituents of gaseous mixtures It would be useful if such a process was not limited to a single gaseous component, but rather, could be readily adapted to numerous process streams SUMMARY OF THE INVENTION

The present invention provides a method for separating gaseous mixtures comprising (a) contacting a feed gas stream containing at least two components with an adsorbent material to adsorb at least one component from the feed gas stream to produce a depleted gas stream diminished in the adsorbed component and a loaded adsorbent material, and (b) contacting a desorption gas stream with the loaded adsorbent material to desorb the adsorbed component therein and produce an enriched gas stream containing a desorbed component and a regenerated adsorbent material depleted of the adsorbed component wherein the regenerated adsorbent material is suitable for reuse in step (a) and wherein the relative humidity of the desorption gas stream is higher than the relative humidity of the feed gas stream Advantageously, the adsorbent material does not have to be thermally regenerated before reuse and in that way, significantly differs from the prior art

Another aspect of the present invention is that the temperature of the feed gas and desorption gas streams need not be controlled so long as the relative humidity of the desorption gas stream is higher than the feed gas stream Thus, the temperature of the feed gas stream may be greater than the temperature of the desorption gas stream so long as the relative humidity of the desorption gas stream is higher than that of the feed gas stream In another aspect, the method of the invention can separate gaseous mixtures in which the component to be adsorbed is less polar than at least one of the remaining constituents of the gaseous mixture For example, in the separation of gaseous hydrocarbon-water containing mixtures, less polar hydrocarbons will be adsorbed onto the adsorbent material instead of the more polar water molecules This is particularly surprising in light of generally accepted knowledge that polar molecules are more strongly attracted to adsorbent materials than their less polar counterparts

The method described and claimed herein can effectively separate gaseous components without additional processing requirements or costly equipment It is far less expensive to operate than the prior art separation processes Additionally, the method of the present invention may be readily adapted to numerous process steams, making it ideally suited for a wide variety of applications Advantageously, a single separation scheme may be used in a large industrial processing plant in which a number of different streams require separation The method of the present invention is also useful in small scale applications such as solvent vapor recovery from dry cleaning establishments

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages will become apparent when reference is had to the following detailed description and the accompanying drawings in which

FIG. 1 is a schematic representation depicting a fixed two bed apparatus in which each bed is alternately contacted by the lower humidity feed gas stream and the higher humidity desorption gas stream in accordance with the present invention,

FIG.2 is a diagrammatic perspective view depicting an adsorbent-coated rotating honeycomb wheel which cycles between the lower humidity feed gas stream and the higher humidity desorption gas stream in accordance with the present invention, FIG. 3 is a graph depicting the carbon content of the adsorbent material as a function of relative humidity, the graph illustrates that volatile organic compounds adsorb onto adsorbent material at low humidity conditions and desorb from the adsorbent material at high humidity conditions,

FIG. 4 is a graph depicting the carbon content of the adsorbent material as a function of temperature, the graph illustrates that adsorption of acetaldehyde and formaldehyde at low relative humidity is relatively temperature insensitive, and

FIG. 5 is a graph depicting outlet hydrocarbon concentration from a bed of adsorbent material as a function of time during the adsorption and desorption steps of the present invention

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is an improved method for separating gaseous mixtures comprising an adsorption step at a lower relative humidity and a desorption step at a higher relative humidity in which the higher humidity condition causes desorption of the adsorbed phase from the adsorbent material and in that way regenerates the adsorbent

More specifically, the method of the present invention comprises (a) contacting a feed gas stream containing at least two components with an adsorbent material to adsorb at least one component from the gas stream to produce a depleted gas stream diminished in the adsorbed component and a loaded adsorbent material, and (b) contacting a desorption gas stream with the loaded adsorbent material to desorb the adsorbed component therein and produce an enriched gas stream containing a desorbed component and a regenerated adsorbent material depleted in the adsorbed component wherein the regenerated adsorbent material is suitable for reuse in step (a) and wherein the relative humidity of the desorption gas stream is higher than the relative humidity of the feed gas stream The invention can be more fully understood from the following description taken in connection with the appended drawings Turning now to FIG 1 of the drawings, there is shown a schematic view of a fixed two bed adsorption system 10 comprised of adsorption beds A and B, each of which is packed with adsorbent material 12α and 12/3, respectively The system is equipped with piping and valving to permit adsorption beds A and B to be operated in parallel or out of phase, such that one adsorbent bed is in the adsorption phase while the other adsorbent bed is in the desorption/regeneration phase In this manner, the system may be operated in a single, or more preferably, as a continuous cycle Although the system can be used to separate many types of gaseous mixtures, it will be described with reference to the removal of hydrocarbon components from an air stream

A feed gas stream comprising air and minor amounts of at least one hydrocarbon component is introduced into the system of FIG 1 through inlet line 13 which, on its downstream end, is joined to inlet gas manifold 14 Manifold 14 is connected to adsorbent bed inlet lines 15a and 15b, which are, in turn, connected to the inlet ends of adsorbent beds A and B, respectively The gas outlet ends of adsorbent beds A and B are connected to adsorbent bed outlet lines 16 and 16b, respectively High humidity gas inlet line 17 is connected on its downstream end to inlet gas manifold 18 Manifold 18 is connected to adsorbent bed inlet lines 19a and 19b, which are, in turn, connected to the inlet ends of adsorbent beds A and B, respectively Flow between manifold 14 and inlet line 15α and manifold 14 and inlet line 15b is controlled by valves 20a and 20b, respectively, which are located in inlet lines 15α and 15 b, respectively Flow between manifold 18 and inlet line 19a and manifold 18 and inlet line 19b is controlled by valves 2\a and 21b, respectively, which are located in inlet lines 19α and 19b, respectively

In the first half-cycle of the process, adsorbent bed A is in the adsorption phase and adsorbent bed B is in the desorption/regeneration phase In the first half-cycle, valves 20α and 21b are open and all other valves (21α and 20b) are closed

Preferably, the process is conducted at atmospheric pressure, although pressures of from about 0 01 atmospheres to about 100 atmospheres are within the scope of the present invention

When the feed gas stream is introduced into the system via line 13, it flows through manifold 14 and enters adsorbent bed A via inlet line 15a This feed gas stream should have a relative humidity that is from 0 to about 80%, preferably from 0% to about 50%, and most preferably from 0 to about 30% By relative humidity, it is meant the ratio of actual water vapor pressure of the gas stream to the saturation water vapor pressure, expressed as a percentage Typically, the feed gas stream has a temperature range of from room temperature to about 250 °C where hydrocarbon components are to be separated That at least one component from the feed gas can be readily adsorbed onto the adsorbent material \2a at a high temperature is particularly surprising in light of the generally accepted knowledge in the art that high temperature causes desorption of hydrocarbons from an adsorbent material Optionally, the feed gas stream can be dried or heated slightly to reduce its relative humidity For example, the feed gas stream may be heated to a temperature of from about 35 °C to about 250°C Such heating of the feed gas stream to reduce its relative humidity in accordance with the present invention, should be distinguished from the fact that the chemical composition of the feed gas entering the adsorbent bed A is temperature independent Alternatively, the chemical composition of the feed gas stream may be modified to remove certain contaminants For example, a dryer, filter or scrubber (not shown) may be employed to remove gases such as water vapor, carbon dioxide or sulfur dioxide, or solid particles

As the feed gas passes through adsorbent bed A in the early stage of the first half- cycle, the at least one hydrocarbon component in the feed gas is adsorbed by adsorbent material 12α to form a loaded adsorbent material Thus, a loaded adsorbent material can contain at least a minor amount of adsorbable components A loaded adsorbent material can contain from about 0 1 weight % to about 80 weight % of the adsorbable component, and typically from about 2 weight % to about 50 weight % of the adsorbable component The adsorbent material can, but need not adsorb, 100% of the at least one hydrocarbon component from the feed gas Accordingly, the adsorbent material can adsorb and separate even trivial amounts of the at least one hydrocarbon component from the feed gas Usually the adsorbent material will adsorb from about 25 weight % to about 100 weight %, and more typically, 50 weight % to about 99 weight % of the at least one hydrocarbon component from the feed gas Meanwhile, the depleted gas stream exiting adsorbent bed A via outlet line 16a is depleted of at least a minor amount of the at least one hydrocarbon component that was contained in the feed gas entering adsorbent bed A Thus, this gas stream can contain any lesser amount of the at least one hydrocarbon component than the gas stream entering the adsorbent material In general, the depleted feed gas stream contains from about 0 mole % to about 75 mole % and more typically, from about 0 1 mole % to about 50 mole %, of the at least one hydrocarbon component

Typically, feed gas continues to be pumped through adsorbent bed A until the pores of adsorbent material \2a are sufficiently loaded with the at least one hydrocarbon component such that the hydrocarbon concentration of the depleted gas stream leaving through outlet line 16a is increased from its initial value

During this first half-cycle, adsorbent bed B undergoes desorption/ regeneration This is accomplished by passing a desorption gas stream having a relative humidity which is higher than the relative humidity of the feed gas stream via high humidity gas inlet line 17 through manifold 18 and inlet line 21b into adsorption bed B High levels of relative humidity may be simply and inexpensively produced by fiinneling the gas stream through a water saturated monolithic structure (not shown) prior to entry into manifold 18 Such devices are often referred to as evaporator pads and are conventionally used to cool and humidify interior environmental air in dry geographic regions These devices, which are generally known as " Swamp Coolers, " are attractive because they are inexpensive, contain few moving parts and do not require additional energy to operate The relative humidity of the desorption gas stream entering adsorption bed B should be as high as possible to maximize displacement of the at least one hydrocarbon component from the adsorbent material 12b Preferably, the relative humidity of the desorption gas stream is from greater than about 0% to about 100%, more preferably, from about 30% to about 80%, most preferably from about 50% to about 80% and, optionally, from greater than 80% to about 100% Importantly, the relative humidity of the desorption gas stream need only be greater than the relative humidity of the feed gas Thus, in the instance where the feed gas has a relative humidity approaching 0%, the desorption gas stream can be slightly greater than 0% Accordingly, the difference in the relative humidity of the feed gas stream and the desorption gas stream may range from about 0 1% to about 100%, and typically, from about 30% to about 100%

Advantageously, the temperature of the desorption gas stream need not be controlled as long as the relative humidity is higher than that of the feed gas stream Thus, the adsorption and desorption/regeneration steps may occur under different temperature conditions as, for example, where the temperature of the feed gas stream is higher than the temperature of the desorption gas stream Typical temperature ranges of the desorption gas stream is from about 0°C to about 250 °C and typically from about 0°C to about 100°C In this manner, the present invention is completely opposite than the TSA processes of the prior art The desorption gas stream functions to desorb at least a minor amount of the at least one adsorbed hydrocarbon component from the adsorbent material As the at least one hydrocarbon component is desorbed from adsorbent material 12b, it exits adsorbent bed B via outlet line 16b to form a gas stream enriched in the hydrocarbon component, hereinafter the enriched gas stream Typically, this enriched gas stream will contain from about 0 01 mole % to about 100 mole % of the at least one hydrocarbon component, and more preferably, from about 0 1 mole % to about 50 mole %

The higher relative humidity of the desorption gas stream causes at least a minor amount of the at least one adsorbed hydrocarbon component to desorb from adsorbent material 12b, thus forming a regenerated adsorbent material In this way, adsorbent material 12b is not dried at a high temperature, and as such, specifically differs from high temperature drying of water saturated adsorbent material which is required by, for example, A A Seballo, et al , Low-Temperature Displacement Desorption of Substances Adsorbed on NaX Zeolite, Journal of Applied Chemistry of the USSR, Vol 43, No 1 1, pp 2439-43 (Nov 1970) By high temperature drying is meant heating the regenerated adsorbent material to a temperature exceeding 250°C to prepare the adsorbent material for the next adsorption cycle

The regenerated adsorbent bed may be void of all of the at least one hydrocarbon component or, alternatively, it may have only slightly less of the at least one hydrocarbon component than the loaded adsorbed material discussed previously with reference to adsorbent bed A, or it may contain any range of hydrocarbon concentration therein Typically, the regenerated adsorbent bed will contain from about 0 weight % to about 79 weight % and more typically, from about 0 1 weight % to about 49 weight % of at least one hydrocarbon component

Accordingly, the humidity swing process described herein does not require high temperatures to effectively desorb/regenerate the adsorbent material In some instances, however, it may be advantageous to heat the desorption gas stream, using an optional heater or heat exchanger (not shown), entering adsorbent bed B so as to maximize the concentration of water vapor in the desorption gas, as for example, where it is desired to pass more highly concentrated water vapor through the adsorbent material to obtain a higher purity gas stream exiting adsorbent bed B via outlet line 16b In this instance, extra water would be added by an evaporator pad, mist or the like Thus, where the desorption gas stream entering adsorbent bed B comprises substantially 100% water vapor at 100° C and 1 atmosphere pressure (i e , it is saturated steam), it can displace the adsorbed component from the loaded adsorbent material to obtain an enriched gas stream in outlet line 16b that is substantially pure in the adsorbed component For a given high relative humidity, warm desorption gas contains a higher concentration of water vapor than its cooler counterpart and is thus capable of displacing more adsorbed component from the loaded adsorbed per unit volume of desorption gas than a cooler, high relative humidity desorption gas Use of hotter, wetter gas to displace the adsorbed component from the adsorbent material should be distinguished from separation processes, such as thermal swing, which utilize superheated steam at temperatures of at least 250 °C In those processes, high temperature, not high relative humidity, causes desorption of the adsorbed component from the adsorbent material Instead, the high temperature, high relative humidity gas used to concentrate water vapor in the humidity swing process has a temperature less than about 250°C, preferably, from about 35°C to about 100°C, and more preferably from about 65 °C to about 100°C Temperatures of about 100°C and relative humidity approaching 100% effectively concentrate the water in the gas stream, thereby accelerating displacement of adsorbed components without requiring that the process operate at super-atmospheric pressure, and accordingly, are preferred

When desorption/regeneration of the adsorbent material 12b is finished, usually determined when the gas stream exiting outlet line 16b has the same composition as the gas stream entering adsorbent bed B via inlet line 19b, the first half-cycle is terminated and the second half-cycle is begun At this point, valves 21 and 20b are opened and all other valves are closed The second half of the cycle is identical to the first half of the cycle except that the phases conducted in adsorbent beds A and B are reversed, such that in the second half-cycle, adsorbent bed B is in the adsorption phase and adsorbent bed A is in the desorption/regeneration phase, as described above

It may be desirable to provide for a purge step between the desorption and adsorption steps During a purge step, the adsorbent material may be contacted with relatively dry gas at a temperature not exceeding about 250 °C to at least partially desorb some of the adsorbed water vapor from the adsorbent material and better prepare the adsorbent material for the next adsorption cycle

FIG 2 illustrates a preferred embodiment of the present invention 30 wherein the adsorbent material is contained in a rotating monolithic bed which alternately contacts the lower and higher humidity gas streams This type of configuration is particularly suitable for use in open-cycle air conditioning systems, such as described in U S Patent No 4,594,860, the entirety of which is incorporated herein by reference The outer enclosure of the device shown in FIG 2 has been omitted so the internal working parts may be shown more clearly

Drive belt 45, driven by drive motor 44, causes the adsorbent-coated wheel assembly 35 mounted inside of the mounting enclosure 36 to rotate continuously An optional heater or heat exchange unit 32 warms feed gas stream 33 prior to contact with the adsorbent-coated wheel assembly 35 The internal portion of apparatus 30 is divided via partitions 34 and 39 so as to minimize intermixing of the feed and desorption gas streams and to separate apparatus 30 into its respective adsorption and desorption/regeneration zones via sliding seal 43 Fan 46 pushes feed gas stream 33 through adsorbent-coated wheel assembly 35 where at least a minor amount of the at least one hydrocarbon component contained in feed gas stream 33 is adsorbed by the adsorbent material to form a loaded adsorbent material It is recognized that a plurality of hydrocarbons may be present in feed gas stream 33, and at least a portion of at least one or more may be adsorbed The remainder of the stream passes through adsorbent-coated wheel assembly 35 to form depleted gas steam 38 Sliding seal 42, located between the perimeter of rotating adsorbent-coated wheel 35 and mounting enclosure 36, insures that feed gas stream 33 passes through wheel 35 Fan 47 pushes a countercurrent flow of desorption gas stream 41 through adsorbent-coated wheel assembly 35 via an optional humidifier 40 In an alternate embodiment, the flow of desorption gas stream 41 may be cocurrent As it was described with reference to FIG 1 , the feed gas may optionally pass through a heater to reduce its relative humidity In the desorption phase, desorption gas stream 41 passes through adsorbent-coated wheel assembly 35, where it displaces at least a minor amount of the hydrocarbon component from the adsorbent material to form a regenerated adsorbent material and enriched gas stream 37 which is subsequently pushed out of the system via fan 47 Adsorbent-coated wheel 35 rotates inside of mounting assembly 36 to continuously move the loaded adsorbent material from the adsorption zone into the regeneration zone of apparatus 30 for regeneration Meanwhile, regenerated adsorbent material is continuously moved from the regeneration zone to the adsorption zone of apparatus 30 to continue the adsorption cycle

The adsorbent-coated wheel 35 is constructed of a plurality of adjoining parallel channels which are generally hexagonal in shape, such as described in U S Patent No 5,733,451, the entire disclosure of which is expressly incorporated herein by reference Preferably, the material used to construct the adjoining parallel channels comprise a non- metallic, high- strength, temperature-resistant, low thermal conductivity material such as Nomex^® aramid in paper form, commercially available as 0 55 Hexagonal Core honeycomb from Engelhard/Hex Core, L L P , Iselin, N J The hexagonal honeycomb is coated with the adsorbent material by methods which are well known and readily available to the skilled artisan This adsorbent-coated material interacts with the feed or desorption gas streams to achieve adsorption or desorption as necessary

Although the apparatus and method of the present invention has been described with reference to the separation of hydrocarbons components from gaseous air streams, the method described and claimed herein may be readily adapted to any number of process streams Thus, a "gaseous mixture" which may be separated in accordance with the method taught herein refers to any stream which contains at least two components in the gaseous phase and may comprise two, three, four and more components Accordingly, the method of the present invention can separate hydrocarbon laden mixtures containing olefmic and paraffinic components or fermentation mixtures, it may be used to purify chemical mixtures, or it may be used to recover carbon dioxide, sulfur dioxide or a mixture of different gases from a multi component gas stream In particular, the method described herein is effective for separating styrene from flue gas and VOC's such as acetylaldehyde and/or formaldehyde from indoor air Due to the many different gaseous mixtures which may be separated, it is contemplated that the feed gas stream may contain any combination of components to be separated Thus, the molecular size of the component or components to be adsorbed may be the same or different from the other components in the mixture Or, the component to be adsorbed may or may not be polar That is to say, there is no requirement that the component to be adsorbed have a greater polarity than the remainder of the gaseous mixture In fact, the method of the present invention is particularly advantageous for separating gaseous mixtures in which the component to be adsorbed is less polar than the remainder of the gaseous mixture Therefore, in the instance where the feed gas comprises a gaseous hydrocarbon laden water mixture, the hydrocarbon component rather than the water component will be adsorbed onto the adsorbent material Likewise, the desorption gas stream need not be more polar than hydrocarbon component in order to effectively desorb/regenerate the adsorbent material This result is particularly surprising in light of generally accepted principles in the adsorption art wherein the electrostatic charge inherent within most adsorbent materials attracts more polar, rather than less, polar molecules Numerous adsorbent materials may be used in the method of the present invention The adsorbent material may be any material in which the adsorption capacity is higher at a low relative humidity than at a high relative humidity Adsorption capacity refers to the amount of a given adsorbable component which can be retained on an absorbent material and is derived by the formula mass of adsorbed component/mass of adsorbent x 100% Additionally, the adsorption capacity of the adsorbent material should not be affected by the temperature of the feed gas Typical adsorbent materials include activated carbons, activated clays, activated alumina, inorganic gels such as silica gel, silica-alumina gels, silica-magnesia gels, titanium-silica gels, and crystalline naturally occurring or synthetic zeolites such as types L, Beta, X, Y, chabazite, clinoptihlite, and the like, as well as any other molecular sieve material which can extract gaseous components from process streams

Selectivity of the adsorbent material for a particular adsorbable component is generally governed by the volume and distribution of the pore size in the adsorbent material Gaseous molecules having a kinetic diameter less than, or equal to, the pore size of the adsorbent material can enter the intracrystalline void space in the adsorbent material while molecules having a diameter larger than the pore size of the adsorbent are excluded from and not retained by the pores of the adsorbent material The adsorbent material thus can sieve gaseous molecules according to their molecular size The kinetic diameters of various molecules are provided in D W Breck, Zeolitic Molecular Sieves, John Wiley and Sons (1974), p 636 The adsorbent material may also separate molecules according to their different rates of diffusion in the pores of the adsorbent material The specificity shown by an adsorbent material towards an adsorbed component may also be influenced by the electrostatic charge of the adsorbent material In some instances, therefore, it may be preferable to use an adsorbent material which possesses at least some electrostatic charge Accordingly, selection of an appropriate adsorbent material will depend on the particularities of the component to be adsorbed from the feed gas stream

For example, activated carbons, activated alumina and silica gel have pores which are non-uniformly sized Consequently, many different gaseous molecules can enter the pores of these compounds By way of contrast, most zeolitic materials have uniformly sized pores of about 3 Angstroms to about 10 Angstroms, making these materials well suited to adsorb only those gaseous molecules which are smaller than the pore size of the particular zeolitic material Accordingly, these materials are particularly useful in the practice of this invention In particular, a large-pored crystalline titanium silicate molecular sieve zeolite ("ETS-10") as claimed in U S Patent No 4,853,202, the entire disclosure of which is incorporated herein by reference, in its substantially hydrogen ion exchanged form is particularly preferred Methods for ion-exchanging molecular sieve materials such as zeolites to obtain, for example, a hydrogen ion exchanged form, are well known and readily available to the skilled artisan

The quantity of gas that is adsorbed by the adsorbent material depends on the pressure, temperature, and the nature of the gas and the adsorbent material Any gas which is capable of adsorbing on a surface may be separated by the method described and claimed herein Thus, noble gases and other weakly adsorbed species are not particularly suitable in the practice of the present invention

The method taught herein is also useful to adsorb more than one component at a time For instance, a gaseous mixture containing several components adsorbed by a single adsorbent material may be removed from the feed gas at a single time Alternatively, several components may be adsorbed by sequentially contacting the feed gas with a series of different adsorbent materials, each of which is selective for a different gaseous molecule (i e , the adsorption step of the process may be repeated one or more times prior to beginning the desorption step of the process)

Each of the feed and desorption gas streams may contact the adsorbent one or more times For example, it might be desirable to produce a stream containing only a single component from a feed gas containing a number of different gaseous species In this instance, the feed gas stream might contact several different adsorbent materials, each of which is selective for adsorbing a different species contained in the feed gas such that only a single desired constituent remains unadsorbed Essentially, the number of times the process may be repeated is almost limitless

The gas stream exiting the adsorbent material may be recovered for use in another process stream, or alternatively, it may be recycled in the method described herein as the desorption gas stream such as, for example, where a single component is stripped from an air stream In this instance, some or all of the resultant stripped air stream may be humidified and used as the desorption gas stream Thus, the gas stream exiting the adsorbent material may be a purified product gas, a stream of waste gas material in which the adsorbed component is recovered, a multi-component gaseous mixture diminished in a single adsorbed component, or the like The method described and claimed herein may be applied to any number of process streams in which at least one component is to be separated Thus, the method described herein may be adapted to small scale process streams such as recovery of solvent vapors from dry cleaning establishments, or to large industrial process streams such as flue gas clean-up, chemical purification, separation of azeotropic mixtures, and the like

Advantageously, the method of the present invention can separate even very minute quantities (vppm) of components from a gaseous stream without the use of high temperature or pressure apparatuses Thus, the method described herein is more efficient and economical than any of the separation systems known in the art The following examples are presented to provide a more complete understanding of the invention The specific techniques, conditions, materials, proportions and reported data set forth to illustrate the principles and practice of the invention are exemplary and should not be construed as limiting the scope of the invention

Example 1

This Example illustrates the separation of acetaldehyde from a gaseous water stream by humidity swing adsorption under static conditions

10 grams of commercial grade ETS-10, a large pored crystalline titanium molecular sieve, described inU S PatentNo 4,853,202, in the substantially hydrogen ion exchanged form ("HETS-10") available from the Engelhard Corporation, Edison, N J , was exposed to acetaldehyde (" AcA") vapor in sealed containers until sufficient AcA was adsorbed on the HETS-10 to yield 2 7% carbon (as measured with a LECO carbon analyzer) One gram portions of this AcA laden adsorbent material were then exposed to either 30%, 80%, or 100% relative humidity ("RH") conditions at room temperature in closed polyethylene containers Thereafter, the carbon content of the samples was again measured with a LECO carbon analyzer These results are illustrated in FIG 3 Additionally, one gram samples of the AcA laden adsorbent were placed in aluminum foil pans in a preheated oven at either 140°F or 190°F and between about 1% and about 4% RH for 30 minutes After this heat treatment, the carbon content of the samples was measured with a LECO carbon analyzer These results are illustrated in FIG 4

FIG 3 demonstrates that at room temperature and a RH of 30%, the adsorbent material can adsorb sufficient AcA to correspond to about 2 5 weight percent carbon Upon exposure to 80% RH and room temperature conditions, approximately 50% (1 2 weight percent) of the carbon will desorb from the adsorbent material At 100% RH and room temperature conditions, about 80% (2 0 weight percent) of the AcA will desorb from the adsorbent material This provides a humidity swing adsorption capacity of between about 1 5% and 2 5%, depending upon the desorption RH FIG 4 demonstrates that the adsorption capacity of the adsorbent material for AcA at low RH is relatively insensitive to temperature up to about 190°F

Example 2 This Example illustrates the separation of VOC's other than AcA from a gaseous water stream by humidity swing adsorption under static conditions

10 grams of HETS-10 as described in Example 1 was exposed to formaldehyde ("FA") vapor in sealed containers until 3 7% carbon, as measured by a LECO carbon analyzer, was adsorbed on the adsorbent material One gram portions of this FA laden adsorbent material were exposed to either 30%, 80%, or 100% RH conditions at room temperature in closed polyethylene containers for 3 hours The carbon content of these samples was thereafter measured and the results illustrated in FIG 3 One gram samples of the FA laden adsorbent material were additionally placed in aluminum foil pans in a preheated oven at either 140°F or 190°F and between about 1% and about 4% RH for 30 minutes After this heat treatment, the carbon content of the samples was measured with a LECO carbon analyzer These results are illustrated in FIG 4 FIG 3 demonstrates that at room temperature and RH below 80%, it is possible to adsorb sufficient FA to correspond to about 3 2 weight percent carbon About 25% (0 8 weight percent) of the carbon will desorb when the adsorbent material is exposed to 100% RH at room temperature This provides a humidity swing adsorption capacity of about 1 % FIG 4 demonstrates that the adsorption capacity of the adsorbent material for FA at low RH is relatively insensitive to a temperature up to about 140°F

Example 3 This Example details the preparation of an adsorbent material for use in the humidity swing method A core material having a plurality of centrally disposed longitudinal channels and a generally hexagonal shape, as described in U S Patent No 5,733,451, the disclosure of which is expressly incorporated herein by reference, was fashioned out of Nomex^® aramid sheet, a non-metallic, high-strength, temperature- resistant, low thermal conductivity material, available as 0 55 Hexagonal Core honeycomb from Engelhard/Hex Core, LLP, Iselin, N J This honeycomb core material had a wall thickness of about 0 0015 inches and a channel diameter of about 0 055 inches From this honeycomb were cut small pieces having a square cross section (i e , perpendicular to the direction of the channels) of about 0 6 inches on a side and having a length (parallel to the direction of the channels) of about 1 5 inches

Two different adsorbent materials were coated onto the honeycomb structures described above The first of these adsorbent materials was commercial grade HETS- 10, as described with reference to Example 1 herein The HETS-10 was obtained from an Engelhard Corporation manufacturing facility as a 40% solid aqueous slurry containing 2% by weight of latex binder solids (Nacrylic brand 4260, available from National Starch Company, Bridgewater, N J ) An equivalent slurry may be prepared using the method given in the paragraph below, but substituting HETS-10 powder for ammonium Y powder

The other adsorbent material was ammonium Y powder ("NH₄Y") obtained from an Engelhard Corporation manufacturing plant in Savannah, GA , substantially equivalent to Molecular Sieve Catalyst support-ammonium Y zeolite powder (Cat No 33,4413-3), Aldrich Chemical Co , Milwaukee, WI An aqueous slurry was prepared from the ammonium Y powder by adding 158 grams of the powder to 222 grams of deionized water and mixing with a spatula until the slurry was uniform To this was added 6 4 grams of Nacrylic brand 4260 latex emulsion (50% by weight latex solids), available from National Starch Company, Bridgewater, N J , and the slurry was stirred again with a spatula to produce a uniform slurry suitable for coating onto the hexagonal honeycomb material The honeycomb samples were coated with the different adsorbents as follows

1 At least a 2 inch depth of coating slurry containing the adsorbent powder was placed in a container and stirred until a uniform consistency was obtained

2 A honeycomb piece was weighed, oriented with its channels in the vertical direction and immersed into the coating slurry 3 The immersed honeycomb piece was held under the surface of the coating slurry and gently agitated up and down until no additional bubbles come to the surface of the coating slurry

4 The honeycomb piece was raised to remove it from the slurry with the honeycomb channels still oriented in the vertical direction, and excess coating slurry was allowed to drain from the honeycomb channels

5 Additional excess coating slurry was drained and removed from the channels of the wet, coated honeycomb piece by blowing with compressed air

6 The wet, coated honeycomb piece, which was free of excess coating slurry, was then dried by blowing through the channels with warm air from a laboratory heat gun until no further weight loss was detected

7 If a sufficient weight of adsorbent coating was achieved, the honeycomb piece was dried for 2 hours at about 100°C and weighed If a sufficient weight of adsorbent coating was not achieved after step (6), then steps (1) through (6) were repeated on the partially coated honeycomb piece until the target coating weight was reached

8 After final drying, the adsorbent loading (gm/in³) was calculated for each honeycomb piece using the equation below

_j _ Coated Honeycomb Weight - Uncoated Honeycomb Weight ^A Honeycomb Volume The adsorbent types and loadings for the honeycombs prepared as described above are set forth in Table 1.

Table 1

Example 4

This Example illustrates the separation of VOC's by a dynamic humidity swing adsorption system. An adsorbent coated honeycomb piece, prepared as described in Example 3, was wrapped with ceramic fiber blanket and inserted into a 2.5 centimeter diameter adsorber tube made of fused silica. The ceramic fiber material was gently compressed between the honeycomb piece and the inner wall of the adsorber tube to provide a sufficiently gas tight seal so that substantially all of the gas flow was through the channels of the honeycomb pieces. Air at 30°C and either <1% or 50% RH was passed through the adsorber tube containing the adsorbent coated honeycomb piece at a rate sufficient to produce a space velocity of 22,500 hr^"1 as determined by the equation: (Volume Air @ STP)/(Volume of Coated Honeycomb Piece/Hr). This space velocity was maintained for 30 minutes. At that time to begin the adsorption step of the process, VOC vapor was added to each of the <1% and 50% RH air streams at a concentration of nominally 50 vppm (parts per million by volume) (methane equivalents). The actual VOC concentration entering and leaving the adsorber tube was measured using a flame ionization detector hydrocarbon analyzer (Rosemont Analytical Model 400 A). This continued until the VOC concentrations entering and exiting the adsorber tube were equal (i.e., the adsorbent was loaded with VOC). At that time, the VOC concentration of the gas entering the adsorber tube was reduced to zero and the RH was increased to 100% until the VOC concentration leaving the adsorber tube fell to zero. The temperature during the desorption phase was 30 °C. These procedures were repeated four times to authenticate the results. FIG 5 depicts the outlet hydrocarbon concentration from the adsorber tube as a function of time using a HETS-10 coated honeycomb piece and acetaldehyde ("AcA") as the VOC The RH of the air prior to and during the adsorption step was 50% Time zero (0) was defined as the time when the VOC was first introduced into the inlet of the adsorber tube Data from the adsorption step of the humidity swing adsorption cycle is shown as unfilled squares, and it is scaled to the Y axis scale on the left side of the graph (0-50 vppm) Data from the desorption step of the cycle is shown as filled squares, and it is scaled to the Y axis scale on the right side of the graph 0-1000 vppm) FIG 5 illustrates that substantially all of the AcA was removed from the air stream by the adsorbent material during the first 2 hours of adsorption The VOC exiting the adsorbent material was less than 10% (less than 4 4 vppm) for the first 2 5 hours of the adsorption cycle The adsorbent material became fully loaded after 5 hours of the adsorption cycle That is to say, after 5 hours of adsorption at the lower RH condition, the VOC stream exiting the adsorbent material had a similar concentration as the VOC stream entering the adsorbent material (50 vppm)

By way of contrast, desorption of the adsorbed AcA from the adsorbent material was rapid The AcA concentration in the adsorber outlet increased immediately as the RH of the inlet air was increased from 50% to 100% Within 5 minutes after the RH changeover, the outlet AcA concentration had peaked at nominally 800 vppm and then began to decrease rapidly After 25 minutes, the outlet AcA concentration had returned to substantially zero, indicating the all of the AcA had been removed from the adsorbent

The data obtained for VOC adsorption on the honeycomb samples described in

Example 3, is set forth in Table 2 For each combination of adsorbent material and VOC,

Table 2 reports the duration of complete VOC removal, the duration of >90% VOC removal, the time needed to saturate the adsorbent, the time to achieve peak outlet VOC concentration during desorption, the peak VOC concentration during desorption, and the time to complete desorption Table 2

Table 2 demonstrates that the humidity swing adsorption process can operate with different adsorbent material/adsorbed gas combinations Other adsorbents, such as silica- alumina gel, titania-silica gel and zeolitic molecular sieves such as types L, Beta, X, chabazite, clinoptililite, etc are also suitable for the humidity swing process Likewise, the adsorbed gas need not be a VOC, but may be any other gas which is capable of being adsorbed Finally, while the desorption step was conducted at the same temperature as the adsorption step, the two steps may be conducted at different temperatures as long as the relative humidity of the desorption gas is higher than the adsorption gas

Having thus described the invention in rather full detail, it will be understood that such detail need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the present invention as defined by subjoined claims

Claims

What is claimed is

1 A method for separating gaseous mixtures comprising (a) contacting a feed gas stream containing at least two components with an adsorbent material to adsorb at least one component from the feed gas stream to produce a depleted gas stream diminished in the adsorbed component and a loaded adsorbent material, and

(b) contacting a desorption gas stream with the loaded adsorbent material to desorb the adsorbed component therein and produce an enriched gas stream containing a desorbed component and a regenerated adsorbent material depleted in the adsorbed component wherein the regenerated adsorbent material is suitable for reuse in step (a) and wherein the relative humidity of said desorption gas stream is higher than the relative humidity of the feed gas stream

2 A method as recited in claim 1 , further comprising contacting the regenerated adsorbent material with the feed gas stream

3 A method as recited in claim 1 , wherein the regenerated adsorbent material of step (b) is maintained at a temperature not exceeding about 250 °C before reuse in step (a)

4 A method as recited in claim 1 , wherein step (a) is repeated before commencing step (b)

5 A method as recited in claim 1, wherein the relative humidity of the feed gas stream is from 0% to about 80%

6 A method as recited in claim 1, wherein the relative humidity of the feed gas stream is from 0% to about 50% 7 A method as recited in claim 1, wherein the relative humidity of the feed gas stream is from 0% to about 30%

8 A method as recited in claim 1, wherein the relative humidity of the desorption gas stream is from greater than 0% to about 100%

9 A method as recited in claim 1, wherein the relative humidity of the desorption gas stream is from about 30% to about 80%

10 A method as recited in claim 1, wherein the relative humidity of the desorption gas stream is from about 50% to about 80%

11 A method as recited in claim 1, wherein the relative humidity of the desorption gas stream is from greater than about 80% to about 100%

12 A method as recited in claim 1, wherein the chemical composition of the feed gas stream and the desorption gas stream is the same

13 A method as recited by claim 1, wherein the chemical composition of the depleted gas stream and the desorption gas stream is the same

14 A method as recited in claim 1, wherein the temperature of the feed gas stream is about room temperature

15 A method as recited in claim 1, wherein the temperature of the feed gas stream is higher than the temperature of the desorption gas stream

16 A method as recited in claim 1, wherein the temperature of the desorption gas stream is less than about 100°C 17 A method as recited in claim 1, further comprising the step of heating the feed gas stream

18 A method as recited in claim 1, further comprising a purge step prior to reuse of the regenerated adsorbent in step (a)

19 A method as recited in claim 1, wherein the adsorbent material is selected from the group consisting of activated carbons, activated clays, inorganic gels, and zeolites

20 A method as recited in claim 1 , wherein the adsorbent material is a zeolite

21 A method as recited in claim 1, wherein the adsorbent material is ETS- 10

22 A method as recited in claim 1 , wherein the adsorbable component of the feed gas stream is less polar than water vapor

23 A method as recited in claim 1, wherein the desorption gas stream consists of water

24 A method as recited in claim 1 , wherein the adsorbent material is contained in at least two adsorbent beds

25 A method as recited in claim 1, wherein the adsorbent material is contained in a rotating monolithic bed

26 A method for separating hydrocarbons from gaseous mixtures comprising (a) contacting a gaseous mixture comprising at least one hydrocarbon component with an adsorbent material to adsorb the hydrocarbon component from the gaseous mixture to produce a gas stream depleted of the hydrocarbon component and a loaded adsorbent material, and (b) contacting a desorption gas stream with the loaded adsorbent material to desorb the hydrocarbon component therein and produce a gas stream containing a desorbed component and a regenerated adsorbent material depleted in the hydrocarbon component wherein the regenerated adsorbent material is suitable for reuse in step (a) and wherein the relative humidity of the desorption gas stream is higher than the relative humidity of the gaseous mixture comprising at least one hydrocarbon component

27 Use of the method as recited in claim 1 for the purification of chemical compounds

28 Use of the method as recited in claim 1 for separating azeotropic mixtures

29 Use of the method as recited in claim 1 for separation of styrene from flue gases

30 Use of the method as recited in claim 1 for recovery of solvent vapors in dry cleaning establishments

31 Use of the method as recited in claim 1 for separating volatile organic compounds in indoor air purification systems

32 An apparatus for separating gaseous mixtures comprising (a) means for passing a feed gas stream containing at least two components through an adsorbent material, the adsorbent material being capable of adsorbing at least one component from the feed gas stream to form a loaded adsorbent material and a depleted gas stream diminished in the adsorbed component;

(b) means for moving the depleted gas stream from the adsorbent material;

(c) means for passing a desorption gas stream through the loaded adsorbent material to desorb the adsorbed component therein and produce a regenerated adsorbent material and an enriched gas stream containing the desorbed component; and

(d) means for moving the enriched gas stream from the adsorbent material, wherein the regenerated adsorbent material is suitable for reuse in step (a) and wherein the relative humidity of the desorption gas stream is higher than the relative humidity of the feed gas stream.