GB2044901A - Combustion method for removal of impurities from a gas - Google Patents

Abstract

A pollutant-containing gas (3) is passed in heat exchange contact with two packed beds (21a, 21b) of heat exchange material between which it is heated in a combustion zone (20), the gas being preheated by the bed upstream the combustion zone and giving up heat to the bed downstream. Flow of gas is periodically changed after the exhaust gas (4) reaches a temperature at least 20 DEG C higher than that of the gas to be treated. A portion of exhaust gas is recycled as a purge gas to the bed which has been serving to preheat the gas to be treated before that bed is again used to absorb heat from gas flowing from the combustion zone, the purge gas sweeping untreated gas remaining in the bed to the combustion zone for the combustion treatment. The system may have three heat exchange beds for system continuity without a break for purging. <IMAGE>

Description

SPECIFICATION Combustion method for removal of impurities from a gas This invention relates to a method for treating a gas for the removal of impurities therefrom by combustion.

One widely adopted method for the treatment of a gas containing impurities such as CO, HCN, organic solvents, amines and sulfides by combustion, involves passing a stream of the gas in heat exchange contact with two beds of heat exchange material between which it is heated by a burner for combustion of the impurities. In this method, the gas is preheated by contact with the bed upstream of the burner, and then cooled by giving up heat to the bed downstream of the burner, the direction of the gas stream being periodically changed whereby the beds alternatively serve to preheat gas and to absorb heat from gas which has been heated by the burner.

As a heat exchange material for the packed beds, refractory bricks have been conventionally used.

However, a packed bed formed by the bricks typically has a per unit volume heat transfer surface area as small as 10 - 25 m2/ m3 and a voidage as low as 0.3, resulting in a large-sized furnace structure as a whole and high production and operation costs. In an attempt to remove these drawbacks, it has been proposed to use ceramic or metallic packing elements for forming packed beds with a heat transfer surface area of 100 400 m2 / m3 and a voidage of 0.7 - 0.74, for example, Racshig rings and saddles such as are generally used in gas-liquid contacting packed towers. However, such packing materials with large specific surface area and voidage impose adverse affects on the efficiency of the treatment of the harmful substances.

It is, therefore, the object of the present invention to provide a method by which the efficiency of combustic treatment of a pollutant-containing gas in a regenerative furnace can be improved.

Thus, the present invention provides a method for treating a gas for the removal of impurities therefrom by combustion in an incineration system including a combustion zone for the combustion of the impurities and at least two packed beds of heat exchange material, each of which communicates for gas flow with the or each other packed bed through the combustion zone, wherein the gas to be treated is passed successively through and in heat exchange contact with both packed beds, where there are only two said packed beds, or through and in heat exchange contact with two of the packed beds where there are more than two said packed beds, and between the two packed beds through which the gas to be treated passes the impurities contained therein are combusted in said combustion zone, the packed bed upstream of said combustion zone being cooled by the flow of gas to be treated and the packed bed downstream of said combustion zone being heated by the gas flowing from said combustion zone; and wherein periodically the flow of gas through the incineration system is changed so that the heated bed in the previous cycle of combustion is used to preheat the gas to be treated and the or a cooled bed is used to absorb heat from the flow of gas from the combustion zone, each cycle of combustion continuing until the exhaust gas discharged from the combustion system reaches a temperature at least 20"C higher than that of the gas to be treated, and prior to using the or a cooled bed for absorbing heat from the gas flowing from the combustion zone, said cooled bed is purged with a portion of exhaust gas having a temperture at least 20"C higher than that of said gas to be treated for removal therefrom to said combustion zone of untreated gas remaining therein.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figures 1 to 3 are diagrammatic illustrations showing the positions of valves and gas flow directions in different cycles of a method of this invention using a gas treating system with two packed beds; Figure 4 is a graph showing the temperature distribution of the packed layer and the temperature distributions of contaminated gas and exhaust gas flows in the packed bed; Figure 5 is a graphic representation of the relationship between purge gas feed time and efficiency of removal of harmful material; Figure 6 is a cross-sectional horizontal view taken on line I - I of Figure 7, diagrammatically showing a furnace with three packed beds for carrying out an alternative embodiment of the method of this invention;; Figure 7 is a cross-sectional elevational view taken on line II - II of Figure 6; and Figure 8 is a time chart illustrating the timing for switching flow control valves.

Referring to Figures 1 to 3, the incinerator there schematically illustrated includes a furnace 10 generally of a cylindrical shape the interior space of which is divided into two or more contiguous, laterally spaced chambers by partition wall member 12. Each chamber is provided with a packed bed 21a or 21b of heat exchange material supported on a gas permeable plate (not shown). The partition wall member 12 has a common space 20 at its upper end, through which the packed beds 21 a and 21 b are in gas communication with each other.

The packed beds in the present invention preferably are formed of ceramic or metallic elements having a nominal diameter of about 3/4 to 3 inchs. The per unit volume heat transfer surface area and voidage of the packed bed depend upon the particular kind of the elements employed but are suitably in the range of 350 60 m2 / m3 and 0.6 - 0.95, respectively.

The common space 20 above the packed beds 21 a and 21 b serves as a combustion zone and is provided with a burner 11 to which an auxiliary fuel and air are supplied through lines 1 and 2, respectively. Formed below the respective beds 21 a and 21 b are lower chambers 5a and 5b for uniform admission and exhaustion of gases to and from the packed beds. The lower chambers 5a and 5b communicate with one end of untreated gas admission lines 3a and 3b, the other ends of which are connected to an untreated gas feed line 3 through untreated gas inlet valves 7a and 7b, respectively.Further, the lower chambers 5a and 5b communicate with one end of combustion gas discharge pipes 4a and 4b, respectively, the other ends of which are connected to a combustion gas discharge line 4 through exhaust combustion gas outlet valves 8a and 8b and a blower 6. The lower chambers 5a and 5b further communicate with one end of recirculating gas admission lines 1 3a and 1 3b, respectively, the other ends of which are connected to an exhaust gas recirculating line 13 through recirculating gas feed valves 9a and 9b, respectively. The exhaust gas recirculating line 13 is connected to a combustion gas discharge line 4.In Figures 1 to 3, the valves indicated by black symbols are in closed positions and the valves indicated by white symbols are in open positions.

The operation of the incinerator with the above-described arrangement is as follows. In the first cycle, the valves 8a, 9a, 7b and 9b are closed while the valves 7a and 8b are opened, as shown in Figure 1, to feed the gas stream to be treated to the furnace 10 via the line 3 and valve 7a by the suction of the blower 6. The gas which has been admitted into the lower chamber 5a is passed through the packed bed 21 a and thereby preheated before entering the combustion chamber 20. The burner 11 in the zone 20 is supplied with air and, if necessary, an auxiliary fuel such as a fuel oil to heat the preheated gas to a temperature sufficient to cause combustion of the harmful substances contained therein.The hot combustion gas which is generated in the combustion zone 20 is passed through the packed bed 21b and lower chamber 5b and sent to the combustion gas discharge line 4 through the valve 8b to exhaust the combustion gas into the air, with or without further treatment. While continuing above-described operation over a certain time period, the temperature of the packed bed 21a is lowered since its heat is absorbed by the gas passing therethrough. Qn the other hand, the temperature of the packed bed 21 b is elevated by contact with the hot combustion gas.

Therefore, the temperature of the exhaust combustion gas increases with the temperature rise of the packed bed 21 b. Upon the exhaust gas reaching a predetermined minimum temperature, a control unit (not shown) automatically produces a valve switching signal to switch the valve 7a into closed position and the valve 9a into open position, as shown in Figure 2, thereby recirculating part of the exhaust combustion gas flowing through the line 4 into the furnace, instead of the untreated gas, to serve as a purge gas for the combustion system. The exhaust gas admitted into the furnace is passed through the lower chamber 5a, packed bed 21 a, combustion zone 20 and packed bed 21 b. By this recirculation of the exhaust gas, the untreated gas which remains in the spaces of the lower chamber 5a and packed bed 21 a is purged into the combustion zone 20 and burned there.As a result of the recirculation of the hot exhaust gas, the temperature of the packed bed 21 a is elevated to a level higher than its temperature in the previous cycle during which untreated gas was passed therethrough, causing the adsorptive harmful substances which remain in the packed bed 21 a to be desorbed therefrom and carried away by the recirculated exhaust gas for pyrolytic decomposition in the combustion zone 20.

After continuing the purge cycle for a predetermined time period, the respective valves are switched to the positions shown in Figure 3. More particularly, the valves 9a and 8b are closed while the valves 8a and 7b are opened, this time allowing the gas to be treated to flow in the reverse direction i.e. through the lower chamber 5b, packed bed 21 b, combustion chamber 20, packed bed 21 a and the line 4, in that order. The gas is preheated when passed through the packed bed 21b by the heat which has been accumulated in the packed bed 21b during the previous cycle.The contaminated gas is treated in this manner for a predetermined time period, and, when the temperature of the exhaust gas from the bed 21 a reaches the above-mentioned predetermined minimum, the valves are switched again (closing the valve 7b and opening the valve 9b in Figure 3), to recirculate to the furnace part of the exhaust gas flowing through the line 4, in a direction reverse to the recirculating operation of Figure 2. The exhaust gas is thus passed through the combustion system which is constituted by the packed layers 21 a and 21 b and the combustion zone 20. After purging the furnace in this manner for a predetermined time period, the valves are switched to the initial positions shown in Figure 1, and the above-described cycles of operations are repeated in the manner described.

Figure 4 shows the temperature distributions T-a and T-b in the packed beds 21 a and 21 b during the operation of Figure 1, along with the temperature distributions T-Ga and T-Gb in the gas flows Ga and Gb in the packed beds 21 a and 21 b, respectively. In this figure, the vertical axis represents the height of the packed beds 21 a and 21 b from the respective lower chambers, which is zero at the point X (where each packed bed is exposed to the lower chamber) and maximum at the point Y (where each packed bed is exposed to the combustion zone 20). The horizontal axis represents the temperature of the above mentioned gas flows Ga and Gb and the packed beds 21 a and 21 b, the temperature becoming higher in the arrowed direction.In Figure 4 plotting the temperature distributoins of the gas flows Ga and Gb and of the packed beds 21 a and 21 bat a time point immediately before switching the valves, the curve T-Ga shows the lowest temperature distribution in the untreated gas flow Ga, the curve T-Gb shows the highest temperature distribution of the combustion gas flow, the curve T-a shows the lowest temperature distribution in the packed bed 21 a, and the curve T-b shows the highest temperature distribution in the packed bed 21 b. The corresponding temperature distributions in the operation of Figure 3 are obtained by simply substituting the reference characters a and b with each other in Figure 4.

As a result of the alternate flows of the untreated gas and the combustion gas, the packed beds 21 a and 21 b undergo temperature changes. The greatest temperature changes occurs immediately after switching the gas flow. For example, the temperature distribution of the packed bed 21 a changes from T-a to the vicinity of curve T-Gb in Figure 4. This is because the packed bed 21 a with the lowest temperature distribution T-a is contacted with the hot gas with the highest temperture distribution T-Gb as a result of the switching operation.

Figure 5 shows the relation between the efficiency of treatment (removal rate of impurities) and the purge gas passing time, as obtained from an operation using as the gas to be treated, air of room temperature containing 0.1 - 0.2 % of lower hydrocarbons of low adsorptivity (propane gas) and, as a purge gas, air of room temperature. In Figure 5, the curve 1 is a plot where the ratio of the flow velocity r of the purge gas to that of the untreated gas is 0.1. Similarly, the curve 2 is a plot for r = 0.5; the curve 3 a plot for r = 0.8; and the curve 4 a plot for r = 1.0.As seen from the graph, the order to attain an efficiency of treatment above 99.9 % in an operation involving harmful substances of low adsorptivity like lower hydrocarbons, it suffices to satisfy the conditions that the flow velocity of the purge gas should be at least 0.8 times greater than that of the untreated gas and the purge gas passing time should be iongerthan 7 seconds. However, where the harmful substances include mercaptanes, amines or other organic based, aldehydes, organic acids, phenols, alcohols or other substances containing a hetero-atom (e.g. O, N, S etc.) or an ordorous substance like an aromatic hydrocarbon with a relatively high boiling point (e.g., naphthalene), it is more difficult to attain a high efficiency of treatment.It has been found that the above-mentioned conditions established for a gas which contains lower hydrocarbons alone can attain an efficiency of treatment of at most 96 - 98 % in these more difficult cases. In order to overcome this difficulty, the method of the present invention utilizes, as a purge gas, part of the combustion gas exhausted from the combustion system and having a temperature at least 20"C higher, preferably at least 500C higher than the temperature of the untreated gas. This is advantageous since the exhaust combustion gas of a temperature higher than that of the untreated gas is readily available from the combustion system itself.In view of the fact that the temperature of the untreated gas is generally in the range of from room temperature to 1 300C, the temperature of the recirculating exhaust gas is determined normally in the range of about 50 - 250"C, preferably in the range of 70 - 200"C in relation with the temperature of the untreated gas.

The optimum conditions for purging the furnace are determined according to the size and shape of the furnace, the kinds and compositions of impurities in the gas to be treated, the temperature of the recirculating exhaust gas, the required efficiency of treatment, and the cost of the equipment. For example, in a case using two packed beds, the feed of the untreated gas to the combustion system has to be stopped temporarily while the purge gas is passed through the furnace. Therefore, it is desirable to conduct the purging in as short a time period as possible and with sufficient effect.These requirements are generally met by feeding the purge gas to the furnace at a flow velocity (a flow rate per unit time, t/sec) 0.5 - 1.0 time as much as, normally 0.7 - 0.9 times as much as the flow velocity (else) of the untreated gas and for a time period of 2 - 15 seconds, normally 7 - 12 seconds, in a case where the untreated gas feed time is about 60 seconds. The total amount of the purge gas feed is generally about 1.5 to 4.5 times and the sum of the volume of void spaces in the packed bed on the upstream side plus the volume of the inner space of the ensuing lower chamber (i.e. the volume of the entire residual untreated gas within the furnace).

By effecting the purging under the above-mentioned conditions, the residual unreacted gas within the combustion system, that is to say, within the spaces of the chamber 5a and packed bed 21a, is purged into the combustion zone 20 by the purge gas i.e. the reciruclated exhaust gas, and at the same time adsorptive impurities in the packed bed 21 a are released therefrom the carried away into the combustion zone 20. The impurities are desorbed from the packed bed 21 a since the purge gas has a temperature higher than the temperature T-Ga of the untreated gas flow as seen in Figure 4 and the temperature of the packed bed is raised to a temperature range suitable for the desorption of the harmful substances from the low temperature level T-a immediately before the recirculation of the purge gas.

The present invention has so far been described in connection with a combustion system having one combustion chamber between two packed beds as shown in Figures 1 to 3, for simplicity of explanation.

However, the invention is also applicable to a combustion system having three or more packed beds in combination with one combustion chamber, as will be described hereinafter with reference to Figure 6.

Referring to Figure 6, there is shown a regenerative combustion furnace 10 for the pyrolytic treatment of gaseous harmful substances, having a combustion system with three packed beds 21 a, 21 b and 21 c which are separated from each other by partition walls 12 and which are communicable with each other through the common combustion zone 20 formed thereabove. An auxiliary burner 11 is provided within the combustion zone 20, the burner 11 receiving auxiliary fuel and air through lines 1 and 2, respectively. Lower chambers 5a, 5b and Sc (the latter not shown) are provided under the respective packed beds 21a, 21 band 21 c for uniform admission and exhaustion of gases to and from the respective packed beds.The lower chambers 5a, 5b and Sc communicate with one end of T-ducts 1 spa, 1 sub and 1 sic (the latter not shown), respectively, which have the other ends thereof connected to a manifold 3m through untreated gas inlet valves 7a, 7b and 7c (the latter not shown), respectively. The manifold 3m is connected to the untreated gas feed line 3. The remaining ends of the T-ducts are connected to a manifold 4m through exhaust combustion gas outlet valves 8a, 8b and 8c (the latter not shown), respectively. The manifold 4m is in turn connected to the exhaust combustion gas discharge line 4. Further connected to the middle portions of the T-ducts 1 spa, 1 sub and 1 sic are recirculating exhaust gas inlet valves 9a, 9b and 9c, respectively, which are connected to a reciruculating exhaust gas admission manifold 1 3m. The manifold 1 3m communicates through a blower 6 with an exhaust gas recirculating line 13 which communicates with the exhaust combustion gas discharge line 4. In Figure 6, the valves indicated by black symbols are in the respective closed positions while the valves indicated by white symbols are in the respective open positions.

For operating the furnace 10 of the above construction, the respective valves are switched according to the time chart of Figure 8. In Figure 8, the trapezoidal blocks represent the open time periods of the respective valves and the letters I, E and P indicate the kind of the gas which is flowing therethrough, namely, the gas to be treated, the combustion gas and the purge gas (recirculated exhaust gas), respectively.

Figure 6 shows the valve position in the time period t of Figure 8, in which the valves 7a, 8b and 9c are in open state while the valves 7b, 7c, 8a, 8c 9a and 9b are in closed state. Generally, the open periods for the inlet and outlet valves are about 60 seconds and the time necessary for switching the valves is 1 to 2 seconds. The time for the purge is about 55 seconds. The valves are switched according to a programmed sequence on the basis of the temperature at the downstream end of the respective one of the packed beds which is serving to absorb heat by contact with the exhaust gas flow.For example, a signal is produced by a detector including a thermometer when the temperature of the exhaust gas at the downstream end of the bed 21 b becomes a predetermined value and is fed in the direction of the arrowed broken line, thereby to close the purge valve 9c for the bed 21 c and at the same time to open the outlet valve 8c, thereafter closing the valve 8b, opening the valve 7b, closing the valve 7a and opening the valve 9a.

Atypical operation using the three packed beds proceeds as follows. Referring to Figure 6, an odorous inimical gas of, say, 80"C which is fed into the lower chamber 5a through the manifold 3m and the inlet valve 7a is preheated to about 730 while it is passed upwardly through the packed bed 21 a before entering the combustion zone 20. In the combustion zone 20, it is heated to 800 C by the combustion gas of the burner 11 and dwells therein for a sufficient time period to complete the combustion of the harmful substances.

The exhaust gas produced in the combustion zone 20 flows downwardly through the packed bed 21 b where it is cooled by giving up its heat to the packed bed 21 b. The exhaust gas then flows through the lower chamber 5b and the outlet valve 8b of the T-duct and into the manifold 4m to be released through the exhaust duct 4. Part of the exhaust gas is sent to the recirculating gas manifold 13m and to the lower chamber 5e throug h the purge gas valve 9c by the operation of the blower 6, the recirculating exhaust gas passing through the packed bed 21 c as a purge gas. In this instance, the flow rate of the recirculating exhaust gas is about 10 - 15 % of the input inimical gas. Therefore, the recirculation of the exhaust gas requires only pipings and valves of small scale.

Upon the exhaust gas temperature at the downstream end of the packed bed 21 b reaching a predetermined valve, e.g. 1 50 C, a signal is produced to change over the valves, whereupon the outlet valve 8c of the lower chamber Sc is opened and at the same time the purge gas valve 9c is closed. Then, the outlet valve 8b of the T-duct which communicates with the lower chamber 5b is closed, succeeded by opening of the inlet valve 7b of the just-mentioned duct. As soon as the inlet valve 7b is opened, the inlet valve 7a of the T-duct which communicates with the lower chamber 5a is closed and simultaneously the purge gas inlet valve 9a is opened.As a result of these valve switching operations, the input contaminated gas is in this cycle fed to the combustion zone 20 through the lower chamber 5b and the packed bed 21 b to undergo the same pyrolytic treatment as described hereinbefore. The heat of the resulting exhaust gas is collected during its passage through the packed bed 21 c. The exhaust gas is then discharged through the outlet valve 8c of the T-duct which communicates with the lower chamber 5c. Part of the exhaust gas is recirculated into the lower chamber 5a through the valve 9a and T-duct 1 spa and sent again into the combustion zone 20, purging the T-duct 15a, lower chamber 5a and bed 21a.

By repeating similar valve switching operations sequentially and periodicallyforthe respective packed beds, it becomes possible to remove the impurities from the contaminated gas continuously by combustion with a high removal rate as a whole without stopping the supply of the gas to the furnace.

In case where the present invention is applied to a furnace which is provided with three or more packed beds, at least one of the cooled packed beds is used for the purge operation by the recirculated exhaust gas so that there is no necessity for stopping the feed of the gas to be treated at the time of switching the respective flow control valves. This is advantageous as compared with the operation using a furnace with two packed beds. Generally, the purging is effected during the gas treating operation by recirculating the exhause gas to a packed bed to be purged at a flow rate of 5 - 25 %, preferably 10 - 15 % of the input contaminated gas.Of course, the purging may be effected by flowing a large amount of purge gas for a short time period as in the case of a furnace with two packed beds, although this is less advantageous in view of the necessity for the valves and blower to be of larger sizes and the increased cost of installation.

According to the method of the present invention, the efficiency of the gas treatment in a regenerative combustion furnace is improved by relatively simple valve switching operations. The method of the invention can thus contribute to the prevention of environmental pollution and is a wide application since it can handle various inimical substances including odorous material, air pollutants and the like.

The following example will further illustrate the present invention.

Example An odorous gas of 50"C (containing phenol, formaldehyde and organic solvents) discharged from a printed circuit board manufacting process was treated according to the process of the present invention using the incinerator system as shown in Figures 1 to 3. The gas was continuously fed to the furnace at a rate of 100 N m3/hour under the following conditions: single gas treating period of about 60 seconds; combustion zone temperature of 1100; maximum exhaust gas temperature of 140"C. For purging, ordinary clean air, heated air, and the exhaust gas from the furnace were used. The results are shown in the table below.

TABLE 1 Flow rate of Purge gas Purging Efficiency Feed Gas Time of Kind Amount N m3/h Temp. Treatment ( C) (sec.) (%)* 100 N m3/H Clean 16 25 12 95.5 Air 100Nm3/H " 16 70 12 99.0 100 N m3/H Exhaust 16 140 12 > 99.9 Gas * Removal Rate

Claims (6)

1. A method for treating a gas for the removal of impurities therefrom by combustion in an incineration system including a combustion zone for the combustion of the impurities and at least two packed beds of heat exchange material, each of which communicates for gas flow the or each other packed bed through the combustion zone, wherein the gas to be treated is passed successively through an in heat exchange contact with both packed beds, where there are only two said packed beds, or through and in heat exchange contact with two of the packed beds where there are more than two said packed beds, and between the two packed beds through which the gas to be treated passes the impurities contained therein are combusted in said combustion zone, the packed bed upstream of said combustion zone being cooled by the flow of gas to be treated and the packed bed downstream of said combustion zone being heated by the gas flowing from said combustion zone; and wherein periodically the flow of gas through the incineration system is changed so that the heated bed in the previous cycle of combustion is used to preheat the gas to be treated and the or a cooled bed is usd to absorb heat from the flow of gas from the combustion zone, each cycle of combustion continuing until the exhaust gas discharged from the combustion system reaches a temperature at least 20"C higher than that of the gas to be treated, and prior to using the or a cooled bed for absorbing heat from the gas flowing from the combustion zone, said cooled bed is purged with a portion of exhaust gas having a temperature at least 20"C higher than that of said gas to be treated for removal therefrom to said combustion zone of untreated gas remaining therein.

2. A method according to Claim 1,wherein the incineration system has only two said packed beds, and wherein the flow of gas to be treated is discontinued whilst said purging of said cooled bed is carried out.

3. A method according to Claim 1, wherein the incineration system has at least three said packed beds, and wherein, at the conclusion of each combustion cycle, the gas to be treated is passed through a cooled bed which has been purged during a previous combustion cycle, thereby permitting continuous flow of the gas to be treated.

4. A method according to any preceding claim, wherein the exhaust gas used for purging has a temperature of at least 50-C higher than that of the gas to be treated.

5. A method for treating a gas for the removal of impurities therefrom, according to Claim 1 and substantially as hereinbefore described with reference to Figures 1 - 3 or Figures 6 and 7 of the accompanying drawings.

6. A method for treating a gas for the removal of impurities therefrom, according to Claim 1 and substantially as described in the Example herein.