WO1998056491A1

WO1998056491A1 - A combined concentrator-oxidation system for voc emission control

Info

Publication number: WO1998056491A1
Application number: PCT/US1998/009104
Authority: WO
Inventors: James M. Chen; Pascaline H. Nguyen
Original assignee: Engelhard Corporation
Priority date: 1997-06-13
Filing date: 1998-05-04
Publication date: 1998-12-17
Also published as: AU7471998A

Abstract

A combined volatile organic compound (VOC) concentrator and oxidizer is provided in an integral rotary system. The system adsorbs VOC from incoming process streams and desorbs and catalytically oxidizes the VOC in-situ.

Description

A COMBINED CONCENTRATOR-OXIDATION SYSTEM FOR VOC EMISSION CONTROL

BACKGROUND OF THE INVENTION

1. Field of the Invention This invention relates to pollution abatement, particularly to systems, methods, and apparatus useful for volatile organic compound (VOC) emission control.

2. Related Art

Among VOC emission control systems, the technology that embodies a VOC concentrator followed by a oxidation system is a energy efficient system to treat streams that have the combination of high gas flow and low VOC concentration. The VOC concentrator is typically a rotor or a rotating wheel that contains an adsorbent material. This type of concentrator is typically divided into an adsorptive and desorptive sections. In the process, the VOC- laden stream passes through the adsorption section, in which VOC compounds are removed from gas phase onto the adsorbent, before being vented. After rotating from the adsorptive to the desorptive section, the adsorbent is heated by an incoming hot stream that strips the adsorbed VOC and reactivates the adsorbent for adsorption. The VOC compounds are now concentrated in the desorbed stream which is typically about 1/10 of the process flow. This concentrated stream is then fed to a catalytic or thermal oxidation system to oxidize the VOC compounds to C0₂ and water. Because of the smaller size stream, the oxidation system is substantially smaller for handling the concentrated stream. Also, because higher VOC concentration gives off higher exothermic heat from the oxidation reaction, the energy consumption cost for such a system is substantially lower than those that are not concentrated.

U.S. Patent No. 5,567,229 discloses a rotary adsorption unit in combination with a regenerative thermal oxidizer ("RTO") for removing impurities from a gas flow. The system uses a closed desorption loop wherein the desorption air is cleaned in the RTO and returned to the rotary adsorption unit. However, this design is somewhat complicated as two separate process units are required (i.e., the rotary adsorber and RTO) . Interconnecting process flow lines and controls are also required further complicating the system.

The present invention offers an advance over previous VOC control systems by providing a combined VOC concentration-oxidation system.

SUMMARY OF THE INVENTION

This invention is an advance from the concentrator/ oxidation systems of the prior art . It combines both concentration and oxidation of VOC in one integral system. In this invention, the adsorbent material in the concentrator is catalyzed. In the adsorption zone where the temperature is low, typically in the range of 15 to 50°C (60 to 120°F), the catalytic adsorbent performs only the adsorption function; namely, it retains the incoming VOC compounds by adsorbing on the material to achieve high VOC removal efficiency. Upon rotating to the high temperature reactivation zone where the adsorbent is exposed to an incoming regeneration stream of 175°C

(350°F) or higher temperatures, the VOC adsorbates then react in situ with 0₂ on the adsorbent catalyst surface to form C0₂ and water. The oxidation reaction not only reactivates the adsorption capacity, but releases heat from the oxidation reaction. While the regeneration stream may require a separate source of heat to raise its temperature to the 175 °C during startup, after startup sufficient heat is released and is desirably recovered by a heat exchanger to maintain the inlet temperature of the reactivation zone in a self-sustaining mode; i.e., no additional heat is required to be added to the system. To facilitate 99+% destruction efficiency, it is desirable to bring back the flue gas from oxidation to the process gas for feeding into the adsorption zone. In doing so, the residual VOC not oxidized is re-adsorbed by the concentrator.

Compared to the current commercial systems that have separate concentrator and oxidizer designs, this invention is substantially simpler to operate. It also eliminates the steps of interprocessing for desorption, mass transfer resistance and re-adsorption that are inherent in the current commercial design. Further, advantages include steady pressure operation of the system, i.e., there is no "pressure pulse" in this invention as are experienced when separate concentration and oxidation units are used. This feature enables enhanced efficiencies through avoidance of energy wasteful pressure losses. Another feature of this invention is that catalysis is accomplished in an environment which requires less oxygen. These results in more compact designs than if outside air is used to accomplish oxidation of the desorbed VOC. Yet another advantage of this invention's combined system is that seal losses are minimized as compared to designs that have separate concentration and oxidation units. The losses from use of a two unit design detracts from achieving high destruction efficiencies as noted below. Also, by recycling the regeneration stream back to the feed stream, it enables one to achieve 99+% overall VOC destruction efficiency. This is an advantage over known commercial two unit systems because these systems only are capable of obtaining destruction efficiencies of no more than 98%.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic of a preferred embodiment of this invention showing a combined concentrator/oxidizer for VOC abatement .

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the broadest embodiment of the invention, an apparatus and method is provided for concentrating and catalytically oxidizing VOC in-situ in an integral rotating system. VOC are adsorbed from a VOC-laden stream in a first section ("adsorption section") of the rotary system. A second section ("regeneration section") of the system desorbs and catalytically oxidizes in-situ the adsorbed VOC with a regeneration stream. The first step of VOC adsorption and the second step of desorption and in-situ catalytic oxidation is accomplished in a rotating manner such that the VOC-laden stream enters the desorbed regeneration section continuously as the system rotates .

In another embodiment, a cooling stream is introduced into a third section of the system ("cooling section") . The cooled section is positioned between the adsorption and regeneration sections such that the desorbed and oxidized regeneration section will rotate into the cooling section in order to lower the temperature of the cooling section before this section is rotated into the adsorption section. In further embodiments, the regeneration stream is provided from a portion of the desorbed process stream.

Yet another embodiment provides for the cooling stream to be taken from a portion of the incoming process stream and used to cool the third section of the rotating system. Afterwards, this cooling stream is used as the regeneration stream.

In yet further preferred embodiments, the regeneration stream exiting the regeneration section of the system, exchanges heat with the cooling stream exiting the cooling section of the rotating system such that heat is transferred to the cooling stream. The regeneration stream may either be vented or combined with the incoming VOC-laden stream for removal of any residual VOC.

Figure 1 depicts a preferred embodiment of the present invention. Rotating wheel 10 revolves around axis A-A and comprises an adsorption section 12, cooling section 14, and a catalytic oxidation section 16. A VOC- laden stream 1 is divided into a cooling stream 2 and an adsorptive stream 3. Adsorptive stream 3 enters the adsorptive section 12, wherein the VOC are adsorbed onto a suitable adsorptive material, to provide a clean vent stream 5. Cooling stream 2 enters cooling section 14 and exits as stream 4 after having cooled the portion of the wheel rotating out from catalytic oxidation section 16. Section 16 is substantially warmer than adsorption section 12, due to the exothermic reactions occurring in catalytic oxidation section 16. It is advantageous to cool rotating wheel 10 after rotating out of catalytic oxidation section 16 and before rotating into adsorption section 12 because the process of adsorption is more effective at lower temperatures.

While the present embodiment has been depicted with a rotating wheel, it should be appreciated by one skilled-in-the-art that distribution of the VOC-laden stream 1, cooling stream 2, and adsorptive stream 3 may be accomplished through use of a rotary distributor/ valve with the rotary wheel 10 not rotating at all i.e., kept in a fixed position.

Stream 4 then enters heat exchanger 20, wherein steam 4 is heated to stream 6 by stream 7 which returns from catalytic oxidation section 16 having catalyzed the VOC and being of a higher temperature than stream 6. Stream 7 is cooled in heat exchanger 20 and exits as stream 8. Stream 8 may then be combined with vent stream 5, in whole or in part, or may be retained as stream 9 to be combined with adsorptive stream 3.

Desirably the apparatus of this invention is operated in a self-sustaining mode without the need of external heat input. To accomplish this, the flow of stream 6 (stream to the catalytic oxidation zone) is set according to the VOC loading entering the system in stream 1. When the VOC loading is low, say 50 ppm, the flow of stream 6 is adjusted to provide approximately a 750 ppm VOC concentration in the catalytic oxidation zone (corresponding to a 15-to-l concentration ratio or 15-to- 1 flow split ratio of stream 1 to stream 6) . This will permit a temperature rise from the oxidation reaction such that the temperature of stream 7 (stream exiting the catalytic oxidation zone) is raised by approximately 100 °C over the temperature of incoming stream 6. With this temperature rise, a recuperative heat exchanger of 50 to 70% thermal efficiency will raise the regeneration gas temperature (stream 4 to stream 6) in the range of 175° -250 °C. When the VOC loading is higher in stream 1, say 100 ppm, the flow of stream 6 is increased as compared to a lower VOC loading to maintain the desired 750 ppm VOC concentration in the catalytic oxidation zone (corresponding to a 7.5-to-l concentration ratio or 7.5- to-1 flow split ratio) . This will lower the exotherm in the catalytic oxidation zone to maintain the 100 °C temperature rise between streams 6 and 7. As illustrated, one skilled in the art will know the flow rate of stream 6 can be conveniently controlled say at the stream 2 location to achieve the desired system design criteria thereby achieving the highest possible VOC destruction efficiencies.

Rotating wheel 10 can be made of a variety of designs and constructed from a variety of materials. In a preferred embodiment, wheel 10 is generally circular in design and is formed of a heat resistant material having a plurality of axial gas flow passages.

The term heat resistant material, as used herein, is intended to encompass materials that will not melt or deform when exposed process temperatures necessary to destroy the VOC. Thus, such materials should be able to retain their structural integrity up to temperatures of about 400 °C.

Rotating wheel 10 has a substantially constant cross-sectional area throughout its length. As apparent to one skilled in the art, the size of rotating wheel 10 will depend upon the rated capacity of the unit. Also, size of the flow passages may be varied to achieve a desired pressure drop and/or effective gas contact surface area. In one embodiment, rotating wheel 10 comprises a plurality of blocks of a heat resistant material . Each block includes a plurality of spaced small gas flow passages, and the blocks are arranged to form a generally circular cross section with the gas flow passages extending generally parallel to the flow axis. In this embodiment, the blocks may be generally rectangular, each having a plurality of small gas flow passages.

In another embodiment, rotating wheel 10 comprises a plurality of tubes formed of a heat resistant material. Each tube includes an axial bore, and the tubes are arranged to form a circular cross-section with the axial bores extending parallel to the flow axis of the heat exchange passages. Most preferably rotating wheel 10 is a monolithic or unitary structure having a plurality of spaced passages extending parallel to the flow axis of the heat exchanger and with each passage preferably having a constant cross-sectional area. This type of arrangement is commonly referred in the art as "honeycombed" . The passages are comprised of thin walls which form a multiplicity of open-ended cells extending between its ends. Examples of useful cell densities are about 172 cells/cm² (1100 cells/in²) , about 94 cells/cm² (600 cells/in²) , about 62 cells/cm² (400 cells/in²) , about 47 cells/cm² (300 cells/in²) , about 31 cells/cm² (200 cells/in²), about 15 cells/cm² (100 cells/in²), about 2.5 cells/cm² (16 cells/in²), or about 1.5 cells/cm² (9 cells/in²) . Wall thicknesses range typically from about 0.01 to about 1.3 mm (about 0.4 to about 50 mils). Other combinations of cell densities and wall thicknesses can be used.

Suitable heat resistant materials include but are not limited to ceramic-based, glass-based, and metallic- based refractory materials.

Examples of refractory ceramic and glass-based materials and/or combinations of these are: cordierite, mullite, clay, talc, zircon, zirconia, zirconates, zirconia-spinel, magnesium aluminosilicates, spinel, alumina, silica, silicates, borides, alumino-silicates, such as, porcelains, lithium aluminosilicates, alumina silica, feldspar, titania, fused silica, nitrides, borides, carbides, such as, silicon carbide, silicon nitride, or mixtures thereof.

In a preferred embodiment, the ceramic or glass- based materials are in the form of porous-fibrous materials. Such materials are low heat-capacity materials . The term "low-heat capacity" for purposes of this invention is intended to describe materials having heat capacities in the range of 2 to 10 Btu/Ft³ • °F, preferably less than 6 Btu/Ft^{3 ■} °F. The advantages of a low-heat capacity material is that it can be quickly heated or cooled which facilitates the desorption and adsorption functions of the concentrator part of this invention.

Examples of refractory metals are stainless steel, aluminum, and alloys of aluminum. The preferred heat resistant materials are the refractory metals and the low heat capacity ceramic and glass-based materials.

Suitable adsorptive materials are any materials capable of adsorbing VOC. Such materials include but are not limited to activated carbons, zeolites, molecular sieve materials, silica gel, etc.

Preferred adsorbents are dealuminized zeolites such as β-zeolite, y-zeolite, ZSM-5 type zeolite, and silicalite. The absorptive materials can conveniently be applied to the heat resistant material of the rotating wheel 10 with a binder also referred to as a washcoat binder. Washcoat binders typical for use in the formulation of slurries include but are not restricted to the following: sols of alumina, silica, ceria and zirconia; inorganic and organic salts and hydrolysis products thereof of aluminum, silicon, cerium and zirconium such as nitrates, nitrites, halides, sulfates and acetates; hydroxides of aluminum, silicon, cerium, zirconium, and mixtures of all of the above components. Also useful as binders are organic silicates which are hydrolyzable to silica include tetraethyl orthosilicates .

The relative proportions of absorbent and binder can range from about 1 to 20 percent by weight and preferably from about 5 to about 15 weight percent. A preferred composite comprises about 90 weight percent β-zeolite and about 10 weight percent of a silica sol and the silica sol. Preferably, the silica sol has substantially no alumin . The adsorbent material may be deposited onto a solid monolithic carrier by methods known in the art. It is usually most convenient to apply the adsorbent as a thin film or coating deposited on an inert carrier material which provides the structural support for the adsorbent . The inert carrier material can be any refractory material such as ceramic or metallic materials.

The amount of adsorbent applied can be varied based on factors including the specific type and amount of VOC adsorbed, the specific adsorbent and binder combination and concentrations, and the like. Typically, an adsorbent composition is in an aqueous slurry form having 5 to 50, preferably 10 to 40 weight percent solids, for use to coat a monolith. The resultant monolith preferably is coated with the adsorbent composition and preferably has from 0.3 to 3.0 g/in³ and preferably 0.5 to 2.5 g/in³ of coating based on the amount of adsorbent. Suitable catalytic materials are those which are capable of oxidizing VOC. Examples of these catalysts include metals and metal oxides known to promote catalytic oxidation of carbonaceous compounds, such as the oxides of vanadium, chromium, manganese, iron, nickel, cobalt, copper, and the platinum group metals. In a preferred embodiment, the catalytic material comprises platinum, palladium, rhodium and mixtures thereof.

Typically, the catalytic material is deposited upon a support material. The combination is referred to as catalyzed support material. Preferably, the support material is in powder or particle form. Suitable support materials include but are not limited to at least one metal oxide compound selected from the group consisting of Si0₂, Ti0₂, Zr0₂, MgO, CaO, a₂0₃, Y₂0₃, tin oxide, gamma alumina, delta alumina, theta alumina, transitional forms of alumina, silica-alumina, and zeolites. Preferred is A1₂0₃.

The amount of catalyst needed, as with the amount of adsorbent, depends on factors such as specific type and amount of VOC to be destroyed. The resultant amount of catalyst may typically range from 0.2 to 2.5 g/in³, preferably from 0.5 to 2.0 g/in³ based on the amount of catalyst .

The catalyst may be applied to rotating wheel 10 by conventional methods such as dipping the wheel in a washcoat slurry comprising a catalyst material, and drying and calcining the wheel by conventional methods. For example, a slurry can be prepared by means known in the art such as combining the appropriate amounts of the supported catalyst in powder form, with water. The resultant slurry is typically ball-milled for about 8 to 18 hours to form a usable slurry. Other types of mills such as impact mills can be used to reduce the milling time to about 1-4 hours. The slurry is then applied as a thin film or coating onto the monolithic carrier by means well known in the art. Optionally, an adhesion aid such as alumina, silica, zirconium silicate, aluminum silicates or zirconium acetate can be added in the form of an aqueous slurry or solution. A common method involves dipping the monolithic carrier into said slurry, blowing out the excess slurry, drying and calcining in air at a temperature of about 450°C to about 600°C for about 1 to about 4 hours. This procedure can be repeated until the desired amount of catalyst of this invention is deposited on said monolithic honeycomb substrate. It is desirable that the supported catalyst be present on the monolithic carrier in an amount in the range of about 1-4 grams of supported catalyst per in³ of carrier volume and preferably from about 1.5 - 3 grams/in³. An alternative method of preparation is to disperse the catalytic metal or metals and such other optional components on a monolithic substrate carrier which previously has been coated with only uncatalyzed support material by the above procedure . The compounds of catalytic metal which can be used and the methods of dispersion are the same as described above. After one or more of these compounds have been dispersed onto the support material coated substrate, the coated substrate is dried and calcined at a temperature of about 400°C to about 600°C for a time of about 1 to 6 hours. If other components are desired, they may be impregnated simultaneously or individually in any order.

The catalyst may be applied to rotating wheel 10 simultaneously with the adsorbent as a mixture, or separately as a first layer on the wheel or as a second layer over the adsorbent.

In a preferred embodiment, rotating wheel 10 comprises a low heat capacity material, a dealuminized zeolite, and a catalyst of platinum on an Al₂0₃ carrier. In a particularly preferred embodiment, the low heat capacity material of the rotating wheel 10 is stainless steel, the adsorbent is a dealuminized zeolite and is deposited as a first layer on the wheel, and the catalyst is platinum supported on an Al₂0₃ carrier which is deposited as an overlayer on the first adsorbent layer. In this embodiment, the metallic wheel, because of its low heat capacity, is capable of being quickly heated and cooled. This property is advantageous, as the adsorbent, which is the first layer on the wheel, can be more favorably controlled by temperature to perform its adsorption and desorption functions. The catalyst on the other hand, becomes activated when exposed to the high temperatures of the regeneration stream. Thus, having the catalyst as an upper layer permits the catalyst to be exposed to the hot regeneration gas thus providing temperatures conducive to catalytic oxidation of VOC.

While the above description constitutes the preferred embodiments of the present invention, it is to be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims .

Claims

WHAT IS CLAIMED IS:

1. An apparatus for the concentration and oxidation of volatile organic compounds comprising:

(a) a wheel comprising a heat refractory material, an adsorptive material, and a catalytic material;

(b) means for separating said wheel into an adsorption zone and a catalytic oxidation zone; each of said zones having an inlet and an outlet said adsorption zone receives a VOC-laden stream and produces an exiting stream having a reduced level of VOC, said catalytic oxidation zone receives a regeneration stream sufficiently hot to desorb and catalyze the VOC in-situ thereby providing a regenerated adsorption zone when said regeneration stream exits; and

(c) means for rotating said wheel or said separating means such that the inlet and outlet of said adsorption zone and said oxidation zone are in fluid communication with said wheel defining said respective adsorption and oxidation zones as rotation progresses.

2. The apparatus of claim 1, wherein said separating means further defines a cooling zone having an inlet and an outlet, said cooling zone being positioned between said oxidation zone and said adsorption zone such that as rotation progresses the heat generated in said catalytic oxidation zone is abated in said cooling zone to provide that the relative temperature of said adsorption zone is lower than the temperature of said catalytic oxidation zone.

3. The apparatus of claim 2, wherein a portion of said inlet VOC-laden stream is directed to the inlet of said cooling zone and exits said zone at a temperature higher than the temperature at said inlet, said cooling zone exiting stream then enters the inlet of said catalytic oxidation zone to oxidize adsorbed VOC and then exits said oxidation zone.

4. The apparatus of claim 3, wherein said cooling zone exiting stream exchanges heat with said oxidation zone exiting stream.

5. The apparatus of either of claims 3 or 4 , wherein said exiting oxidation zone stream either directly or after additional heating exchange as claimed in claim 4, combines with said VOC-laden stream entering said adsorption zone.

6. The apparatus of claim 1, wherein the heat refractory material is selected from the group consisting of metal and ceramic materials, the adsorptive material is selected from the group consisting of dealuminized zeolites and activated carbons, and the catalytic material is selected from the group consisting of platinum group metals and oxides of vanadium chromium, manganese, iron, nickel, cobalt and copper.

7. The apparatus of claim 6, wherein the heat refractory material is stainless steel, the adsorptive material is a dealuminized zeolite, and the catalyst is platinum, palladium, rhodium or mixtures thereof.

8. The apparatus of claim 7, wherein the adsorptive material and the catalyst are applied as a mixture to the refractory material.

9. The apparatus of claim 8, wherein the adsorptive material is applied to the refractory material as a first layer and the catalyst is applied as a second layer.

10. The apparatus of claim 8, wherein the catalyst is applied to the refractory material as a first layer and the adsorptive material is applied as a second layer.

11. A method for the concentration and oxidation of volatile organic compounds in a rotating system comprising the steps of:

(a) adsorbing VOC from a process stream in a first section of the rotary system;

(b) desorbing and catalytically oxidizing in- situ the adsorbed VOC with a regeneration stream in a second section of the rotary system; and

(c) continuously rotating the system so that the process stream of step (a) will enter the desorbed second section of step (b) and the adsorbed VOC in the first section of step (a) will be desorbed and catalytically oxidized by the regeneration stream as rotation progresses.

12. The method of claim 11, further comprising the step of cooling a third section of the rotating system with a cooling stream, the third section being positioned between the first and second sections such that the desorbing and oxidizing second section will rotate into the third section in order to lower the temperature of the third section before rotating into the adsorbing first section.

13. The method of either claims 11 or 12, wherein the regeneration stream is provided from a portion of the process stream exiting the first section of the rotary system.

14. The method of claim 12, wherein the cooling stream is provided from a portion of the incoming process stream of step (a) such that the cooling stream is first used to cool the third section of the rotating system and afterwards is used as the regeneration stream to desorb and oxidize the VOC in the second section of the rotating system.

15. The method of claim 14, wherein the regeneration stream exiting the second section exchanges heat with the cooling stream exiting the third section of the rotating system such that heat is transferred to the cooling stream.

16. The method of claim 15, wherein the regeneration stream, after having transferred heat to the cooling stream, combines with the incoming process stream of step (a) .