WO1998057049A1

WO1998057049A1 - Pollutant reduction catalyst in thermal oxidizer

Info

Publication number: WO1998057049A1
Application number: PCT/US1998/012253
Authority: WO
Inventors: Joseph D. Chiles; Melanius D'souza
Original assignee: Smith Engineering Company
Priority date: 1997-06-13
Filing date: 1998-06-12
Publication date: 1998-12-17
Also published as: AU8069198A

Abstract

A conversion device including at least two reaction chambers (15, 17A) which are in fluid communication with each other, for treating a polluted gas stream containing organic compounds. Thermal oxidation occurs in one reaction chamber (15). The other reaction chamber (17A) includes heat exchange materials (20) with a catalyst (30) maintained above the catalyst activation temperature, allowing catalytic conversion of the residual or thermally-generated pollutants to occur. The conversion device thus provides high destruction efficiencies in an efficient manner.

Description

TITLE OF THE INVENTION: POLLUTANT REDUCTION

CATALYST IN THERMAL OXIDIZER

Background Of The Invention

The present invention generally relates to the use of a conversion device for treating polluted process streams containing volatile organic compounds (VOCs). More

specifically, the invention relates to the use of a conversion device, such as a thermal oxidizer, for treating pollutants using both thermal and catalytic oxidation techniques.

Thermal oxidizers are known for heating process streams containing VOCs, such as hydrocarbon vapors in air, to temperatures above the oxidation temperature of the VOCs. Oxidation of organic compounds is achieved by elevating the temperature of

the gas above the ignition temperature of the organic components in the presence of oxygen using a heat source such as natural gas burners or electric heaters. Following oxidation, the organic constituents resolve into carbon dioxide and water vapor, which can be released to the atmosphere.

It is also known to provide a catalyst material in the exhaust stack of a thermal

oxidizer. The catalyst does not directly participate in the conversion reaction but

permits it to occur at lower temperatures, thus acting as a secondary oxidizer when high

conversion levels are required, or as a "polisher" to facilitate the reduction of CO

emissions. The use of a catalyst in this manner requires the exhaust stream

temperature of the thermal oxidizer to be maintained consistently above the activation

temperature of the catalyst. If the exhaust temperature falls below this temperature, it

must then be raised. In the past, this has been accomplished either by supplementing the VOC load to the oxidizer (e.g., by adding fuel to the inlet), derating the heat

exchanger, or by providing an additional heat source (e.g., electric coil, burner, etc.).

Each of these options reduces the overall energy efficiency of the system.

Regenerative thermal oxidizers (RTOs) which oxidize pollutants catalytically are

also known, as disclosed in U.S. Patent Nos. 5,516,499, 5,364,259 and 5,262,131.

The inherent disadvantage of these systems is the cost of the catalyst and their

typically lower destruction efficiencies. U.S. Patent No. 3,870,474 discloses an RTO for

oxidizing pollutants, and teaches that catalysts used with cleanable regenerators or for

the purpose of removing oxides of nitrogen can be provided as a part of the regenerator

packing material (see col. 6, lines 3-13; col. 8, lines 66-68 through col. 9, lines 1-2).

In order to reduce CO emissions in a process stream, it is also known to run the

retention chamber of a thermal oxidizer at elevated temperatures (e.g., 1550°F -

1700°F). While this is effective in reducing CO emissions, it is obviously less efficient

and consumes more fuel and/or energy.

Systems are also known which have the alternating capability of operating either

as a thermal combustor or as a catalytic oxidizer, so that when one operates the other

is quiescent, as disclosed in U.S. Patent No. 4,983,364. However, it would be

advantageous to provide a system for converting pollutants in a process gas that

benefits from simultaneous thermal and catalytic conversion processing, and which

does not add to the operating cost of the unit. Accordingly, an object of the present invention is to provide a conversion device

such as a thermal oxidizer which can convert a polluted gas stream containing VOCs

into a clean gas, releasable to the atmosphere, in a more efficient manner and without

violating applicable governmental emission standards.

Summary Of The Invention

These and other objects are achieved by the present invention, which preserves

the advantages of known reaction vessels and techniques for converting VOC-laden

process streams. In addition, it provides new advantages not available with such

systems, and overcomes disadvantages associated with such systems.

The invention is generally directed to a conversion device for reducing the

organic compound content of a gas stream. The conversion device includes a

combustion chamber for thermally oxidizing the organic compounds within the gas

stream, and at least one other reaction chamber in fluid communication with the

combustion chamber. The reaction chamber houses heat exchange materials which

provide a plurality of flow paths for the gas stream. Appropriate duct work or other

means are employed for circulating the gas stream between the combustion chamber

and the at least one other reaction chamber. Substantially all of the oxidation reaction

of the polluted gas stream occurs within the combustion chamber. A catalyst, such as a

precious metal catalyst like platinum or palladium, is positioned at a level within the heat

exchange materials, and within the flow paths of the gas stream, so that the catalyst remains above its activation temperature for a time period sufficient to catalytically react

residual hydrocarbons in the gas stream following thermal oxidation, as well as to

catalytically react any carbon monoxide generated during thermal oxidation. Preferably,

a substantial portion of the residual hydrocarbons and the generated carbon monoxide

is reacted. Preferably, the finally treated gas stream includes carbon monoxide in trace

amounts such as less than 50 ppm. While the combustion chamber reaches

temperatures in excess in 1300°F, such as 1500°F or greater, the catalyst band is

maintained at a temperature below 800°F, and typically below 600°F.

In one preferred embodiment, the conversion device is a regenerative thermal

oxidizer. In another, it is a recuperative thermal oxidizer. The catalyst may be coated

directly on the heat exchange materials of a regenerator chamber of an RTO, for

example, so that the catalyst-coated heat exchange materials form a relatively narrow

band within the heat exchange materials. The heat exchange materials may include a

random-packed media or a structured media. For an RTO, the catalyst/bed volume

ratio will be less than 10%, and preferably less than 6%.

A method for reducing the organic compound content of a gas stream using a

conversion device also forms part of the present invention. The conversion device

includes a combustion chamber and at least one reaction vessel containing heat

exchange materials. The conversion device may be an RTO with multiple regenerator

chambers in communication with a combustion chamber, or it may include a recuperative thermal oxidizer. The at least one reaction vessel is in fluid

communication with the combustion chamber. A gas stream is directed into the

combustion chamber, which thermally oxidizes organic compounds within the gas

stream. The gas stream is then directed into the at least one reaction vessel

containing the heat exchange materials. The heat exchange materials are at least

partially coated with a catalyst, and the gas stream is catalytically reacted to remove

hydrocarbons initially present within the gas stream or generated during thermal

oxidation within the combustion chamber. Preferably, the catalyst is substantially

uniformly dispersed throughout a cross-section of the heat exchange materials and in

the flow path of the gas stream. The catalyst is located at a level within the heat

exchange materials of the at least one reaction vessel which will ensure that the

catalyst remains above its activation temperature for at least a time period sufficient to

catalytically react a substantial portion of the hydrocarbons remaining in the gas stream

following thermal conversion or generated during thermal conversion. The generated

hydrocarbons which are targeted for removal include carbon monoxide.

Brief Description Of The Drawings

The novel features which are characteristic of the present invention are set forth

in the appended claims. The invention itself, however, together with further objects and

attendant advantages, will be best understood by reference to the following description

taken in connection with the accompanying drawings in which: FIGURE 1 is a perspective view of a regenerative thermal oxidizer of the present

invention employing random-packed heat exchange media;

FIGURE 2 is a partial cross-sectional view of the right-side regenerator chamber

of the regenerative thermal oxidizer shown in FIGURE 1 ; and

FIGURES 3 and 4 are schematic views of two different embodiments of a

recuperative thermal oxidizer according to the present invention.

Detailed Description Of The Preferred Embodiments

Referring first to FIGURES 1 and 2, in a preferred embodiment of the present

invention, the thermal oxidizer shown, designated generally as 10, is a "reverse flow"

regenerative thermal oxidizer (RTO). RTOs typically include separate incinerator

chambers or "regenerators" which house layers of heat exchange materials that store

the heat remaining in the reacted gas after incineration. This heat may then be used to

increase the temperature of the feed gas and thereby reduce external fuel

requirements. By alternating the flow of cool gas to be cleaned through a hot heat

exchanger, and moving the hot cleaned gas from the combustion chamber and through

a "cooled" heat exchanger, the RTO continuously operates to efficiently oxidize

pollutant-laden gases, as is well known in the art. Examples of RTOs are disclosed in

U.S. Patent Nos. 5,026,277 and 5,352,115, incorporated herein by reference.

Referring still to FIGURES 1 and 2, RTO 10 of the present invention includes a

retention or combustion chamber 15 with a burner 18, inlet ducts 22, 24 (including dampers 23) leading to regenerative heat exchanger chambers 17A, 17B, and outlet

ducts 26 (one shown). In the "reverse flow" RTO shown in FIGURE 1 , a polluted

process gas stream, represented by an arrow in FIGURE 1 , is first directed into inlet

duct 22, past open damper 23 and into regenerator chamber 17A. As the gas stream

passes through regenerator chamber 17A on its way to combustion chamber 15, it

passes through catalyst band 30, whose purpose is explained below. After most of the

hydrocarbons within the gas stream are thermally oxidized within combustion chamber

15 using burner 18, often at temperatures in excess of 1500°F, the gas stream is then

directed (as shown by the arrows) through "cooling" regenerative heat exchanger 17B,

and again passes through catalyst band 30. The "clean" gas stream then passes out

through outlet duct 26 to stack.

RTO 10 includes heat exchange media 20 which may be random-packed (a

"saddle bed") as shown in FIGURES 1 and 2, or structured, as taught in U.S. Patent

Nos. 5,026,277 and 5,352,115. According to the present invention, within the heat

exchange materials there is placed a layer or band 30 of catalyst material. This catalyst

may be in the form of a relatively thin layer of catalyst material deposited on a substrate

or as an extrudate (e.g., in the form of pellets or beads). This layer should be installed

in the heat exchange material so that the catalyst is exposed to the air flow within the

media bed and is uniformly dispersed throughout the cross-section of the media bed.

The catalyst coating can be applied to the substrate using processes performed by catalyst vendors such as, for example, Prototech Company, 32 Freemont Street,

Needham, Massachusetts (a division of United Catalysts), or Johnson Matthey, 460 E.

Swedesford Road, Wayne, Pennsylvania.

Catalyst layer 30 is provided at a location within the heat exchange materials that

corresponds to a temperature at or above the activation temperature of the catalyst so

that there is no need to add additional energy when the catalyst is used as a secondary

oxidizer. This temperature is typically about 400-600 °F, and is generally below 800 °F.

When activated, the catalyst will induce hydrocarbon conversion into water vapor and

carbon dioxide.

While in the preferred embodiment the catalyst is coated directly on the heat

exchange materials of the media beds within an RTO, it will be appreciated that other

embodiments are within the scope of the present invention. Thus, the catalyst may be

installed in forms other than a relatively narrow strip or band of coated random-packed

materials. Examples include coating the catalyst on other substrate materials such as

balls, grating(s), structured heat exchange materials such as metallic or ceramic

monoliths, or other structures located within the media bed. The amount and form of

the catalyst coating will depend upon the system requirements, the type of media bed

(i.e., random-packed or structured), the type of catalyst, etc. Whatever the form, the

catalyst should be substantially uniformly dispersed throughout the cross-section of the media bed, and properly exposed to the air flow through the bed, to provide appropriate

catalytic conversion.

RTO media bed 20 contains heat exchanger materials at a temperature ranging

from about 70°F at the inlet to a typical maximum temperature of 1500°F. The

appropriate location of catalyst band 30 will depend upon the particular media bed

temperature profile, as well as the activation temperature of the chosen catalyst.

Referring still to FIGURE 1 , when the products of combustion exit retention chamber

15, compounds that have not been converted thermally will pass through catalyst band

30 and convert catalytically. In most systems the combustion chamber of the thermal

oxidizer will handle the vast majority of the oxidation load, and the catalyst band will

only be required to oxidize a relatively small percentage (e.g., less than 5%-10%) of the

VOCs which remain, or which are generated through thermal oxidation. It is believed

that a catalyst band as narrow as 4 inches can be employed with a saddle media bed,

for example. For typical RTOs having an 80% efficiency and a 6-foot bed, or a 95%

efficiency and 8-9-foot bed, the relative percentage of catalyst band width to media

height is between about 3%-6%. More specifically, analyzing the two cases of an RTO

with a four-inch thick catalyst layer in a: (1) 9-foot tall bed designed for a 100 ft/min

cross-sectional area; and (2) 6-foot tall bed designed for 400 ft/min cross-sectional

area, this equates to a catalyst volume/media bed volume ratio (termed here the

"catalyst/bed volume ratio") having a range of 3%-6%. It is believed that virtually all thermal oxidizers using heat exchange materials, including RTOs, will have a catalyst/

^' bed volume ratio of less than 10%, and typically less than 6%.

It will now be understood that the present invention provides the benefits of

catalytic conversion without sacrificing thermal conversion efficiency. At least two

separate reaction zones are provided by the invention: a thermal oxidation reaction

takes place in the combustion chamber, while a catalytic reaction takes place within the

catalyst band of the heat exchanger layers of the reaction chambers. As a process

stream enters the inlet bed of a regenerator chamber, for example, if the temperature of

the inlet bed is at or above the activation temperature of the catalyst, some catalytic

conversion will occur. The vast majority of the conversion reaction of the hydrocarbons

within the process stream to carbon dioxide and water vapor will then occur through

thermal oxidation, in the combustion chamber. Finally, as the process stream exits the

exhaust regenerator chamber, further catalytic conversion will occur within the catalytic

band.

The present invention may be advantageously used with systems requiring low

CO emissions, such as less than 50 parts/million (as currently required in some states).

In this case, combustion chamber 15 can be run at a lower temperature (e.g., 1400°F)

to convert the VOCs in the process stream. Any unconverted CO, or CO produced

during the combustion process, will be converted when it passes through catalyst band

30 in the exhaust heat exchanger. Preferably, the system is designed so that a substantial portion of the residual or thermally generated pollutants (VOCs and CO) is

removed. As used here, the term "substantial portion" shall mean about 80% or more

of such pollutants, although it will be understood that, typically, a much greater

percentage of residual or generated pollutants will be oxidized catalytically.

It is believed that use of the present invention will substantially reduce CO

emissions without any increase in operating temperatures or fuel consumption, as

compared to prior art systems. Systems of the present invention will also provide

increased VOC destruction efficiency without raising operating temperatures, or

requiring either supplemental heat or an increased retention chamber volume and/or

residence time.

The present invention also protects the user of a conventional thermal oxidizer

from CO emission violations. Thus, should the burner system produce excessive CO

emissions, those emissions will be converted as they pass through the catalyst band of

the exhaust bed. Unlike previous designs, this is done without the use of additional

hardware, such as a separate catalyst plenum, additional burners, associated duct

work, etc.

Those of ordinary skill in the art will appreciate that various catalysts, such as

base metal or precious metal catalysts, can be used with the present invention. In a

preferred embodiment, a precious metal catalyst, such as a platinum or palladium

catalyst, may be used. The invention is envisioned as adaptable for potential use with virtually any type

of reaction chamber housing heat exchange materials for converting hydrocarbons in a

polluted gas stream. As examples, ceramic or metallic monolithic/structured (e.g.,

honeycomb) or random-packed/saddle heat exchange media may be used.

The present invention is also envisioned for use with a recuperative oxidizer.

Referring to FIGURES 3-4, multi-pass heat exchanger systems, generally designated

as 40, are shown. A process stream "A" to be treated flows in the direction of the

arrows. Heat exchanger systems 40 each include a burner 45, a shell-and-tube heat

exchanger 50, a catalyst module 30, and an exhaust stack 60. Catalyst module 30 is

located inside the heat exchanger and within the process gas flow path at a point where

the temperature is above the catalyst activation temperature. In these applications,

rather than using catalyst-coated random-packed media, it would be desirable to install

a wall of a ceramic grid or monolith that is coated with a catalyst.

As used in the claims, the term "fluid" (as in "fluid communication") refers to the

ability of a gas or gaseous vapor to move between locations.

It will be understood that the invention may be embodied in other specific forms

without departing from its spirit or central characteristics. An RTO is a particularly

preferred embodiment of the present invention since a catalyst can be coated directly

on the same materials which are used as the heat exchange media. However, those of

ordinary skill will appreciate that the present invention can be used with other types of oxidizers, such as recuperative oxidizers supplied with a saddle or structured media

having a coated catalyst as described above. The present examples and

embodiments, therefore, are to be considered in all respects as illustrative and not

restrictive, and the invention as recited in the claims is not to be limited to the details

given here.

Claims

We Claim:

1. A conversion device for reducing the organic compound content of a gas

stream, comprising: a combustion chamber for thermally oxidizing the organic compounds

within the gas stream;

at least one other reaction chamber in fluid communication with the

combustion chamber, the at least one other reaction chamber housing heat exchange

materials providing a plurality of flow paths for the gas stream;

means for circulating the gas stream between the combustion chamber

and the at least one other reaction chamber; and

a catalyst positioned at a level within the heat exchange materials, and

within the flow paths of the gas stream, so that the catalyst remains above its activation

temperature for a time period sufficient to catalytically react residual hydrocarbons in

the gas stream following thermal oxidation, as well as any carbon monoxide generated

during thermal oxidation.

2. The conversion device of Claim 1 , wherein a substantial portion of the

residual hydrocarbons and the generated carbon monoxide is reacted.

3. The conversion device of Claim 1 , wherein the finally treated gas stream

includes carbon monoxide in an amount less than 50 ppm.

4. The conversion device of Claim 1 , wherein the combustion chamber

reaches temperatures in excess in 1300┬░F, and the catalyst band is maintained at a

temperature below 800┬░F.

5. The conversion device of Claim 1 , wherein the conversion device

comprises a regenerative thermal oxidizer.

6. The conversion device of Claim 1 , wherein substantially all of the

oxidation reaction of the polluted gas stream occurs within the combustion chamber.

7. The conversion device of Claim 1 , wherein the catalyst is coated directly

on the heat exchange materials.

8. The conversion device of Claim 7, wherein the catalyst-coated heat

exchange materials form a relatively narrow band within the heat exchange materials.

9. The conversion device of Claim 5, wherein the catalyst/bed volume ratio is less than 10%.

10. The conversion device of Claim 5, wherein the catalyst/bed volume ratio

is less than 6%.

11. The conversion device of Claim 1 , wherein the conversion device

comprises a recuperative thermal oxidizer, the at least one reaction chamber includes a

heat exchanger, and a catalyst is positioned within the gas flow path inside the heat

exchanger.

12. The conversion device of Claim 1 , wherein the catalyst comprises

platinum.

13. The conversion device of Claim 1 , wherein the catalyst comprises

palladium.

14. The conversion device of Claim 5, wherein the heat exchange materials

comprise a random-packed media, and the catalyst is coated on the random-packed

media.

15. The conversion device of Claim 5, wherein the heat exchange materials

comprise a structured media, and the catalyst is coated on the structured media.

16. A method for reducing the organic compound content of a gas stream

using a conversion device with a combustion chamber and at least one reaction vessel

containing heat exchange materials, the at least one reaction vessel being in fluid

communication with the combustion chamber, comprising the steps of:

a. directing the gas stream into the combustion chamber and

thermally oxidizing organic compounds within the gas stream;

b. directing the gas stream into the at least one reaction vessel

containing the heat exchange materials, wherein the heat exchange materials are at

least partially coated with a catalyst; and

c. catalytically reacting the gas stream within the at least one reaction

vessel to remove residual hydrocarbons initially present within the gas stream or

hydrocarbons generated during thermal oxidation within the combustion chamber.

17. The conversion device of Claim 16, wherein the catalyst is substantially

the flow path of the gas stream.

18. The method of Claim 16, wherein the catalyst is located at a level within

the heat exchange materials of the at least one reaction vessel which will ensure that

the catalyst remains above its activation temperature for at least a time period sufficient

to catalytically react a substantial portion of the hydrocarbons remaining in the gas

stream following thermal conversion or generated during thermal conversion.

19. A method for reducing the organic compound content of a gas stream

using a regenerative thermal oxidizer having a combustion chamber and at least first

and second regenerator chambers, comprising the steps of:

a. providing the first and second regenerator chambers with heat

exchange materials having catalytic properties;

b. directing the gas stream through the first regenerator chamber and

into the combustion chamber;

c. thermally oxidizing the organic compounds within the gas stream

using the combustion chamber; and

d. directing the gas stream through the second regenerator chamber,

wherein catalytic oxidation is allowed to occur within the heat exchange materials of the

second regenerator chamber.

20. The method of Claim 19, wherein the catalyst is located at a level within