Abstract:
An apparatus for the treatment of a waste gas that is capable of recovering the heat generated by the catalytic oxidation of a waste gas to the largest possible extent and allowing the temperature of the treated gas in a first waste gas pre-heater to be adjusted to an arbitrary level incapable of condensing moisture and/or corrosive gas is provided. The apparatus for the treatment of the waste gas includes a molecular oxygen-containing gas supplying device, second to n&#39;th waste gas pre-heaters, a heat-recovering device, and

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to an apparatus for the treatment of waste gas containing a combustible compound and a method for the treatment of the waste gas by the use of the apparatus.  
           [0003]    2. Description of the Related Art  
           [0004]    The waste gas discharged from the process for the production of chemical substances possibly contains combustible compounds such as saturated hydrocarbons like methane, ethane or propane, unsaturated hydrocarbons like ethylene, or propylene, carbon monoxide, organic acids like phthalic acid, maleic acid, or (meth)acrylic acid, aldehydes like formaldehyde, esters, or alcohols in small amounts either singly or in the form of a mixture. These combustible compounds will emit offensive odor even in a small amount. Their release into the atmosphere without treatment is hardly agreeable.  
           [0005]    As one such measure, JP-A-57-58012 proposes an apparatus for the treatment of the waste gas. FIG. 1 is a diagram illustrating the apparatus for the treatment of the waste gas, which is disclosed in JP-A-57-58012. In FIG. 1, the apparatus is operated by heating the waste gas supplied through a first heat exchanger  2  and a second heat exchanger  3  disposed in series between a device for supplying the waste gas and a catalytic oxidation reactor  1 , supplying part of the hot treated gas emanating from the reactor  1  to the second heat exchanger  3  as the heat source, supplying the treated gas emanating from a heat-recovering device  4  to the first heat exchanger  2  as the heat source, supplying part or the whole of the treated gas used in the second heat exchanger  3  to the reactor  1 , and supplying the remainder of the treated gas together with the waste gas emanating from the reactor  1  to the heat-recovering device  4 . By allowing the treated gas used in the first heat exchanger  2  to be discharged out of the system, it is possible to maintain the appropriate reaction in temperature with the reactor  1  without addition of unnecessary air. Further by recovering the generated heat utilizing the heat-recovering device  4 , it is possible to decrease the heat loss to the outside of the system. That is, since the heat-recovering device  4  absorbs the variation of the heat generated by the reaction due to the conversion of the composition of combustible substances in the waste gas, the temperature of the treated gas emanating from the first heat exchanger is maintained at a substantially fixed level. When the amount of oxygen in the waste gas is not sufficient for the treatment of the combustible component, air may be added in advance into the waste gas to treat it.  
           [0006]    In consequence of variations in the operating conditions during the production process, the composition of the waste gas discharged from the process is often changed and this change possibly induces a change in the dew point of the treated gas and causes condensation in the first heat exchanger  2 . The waste gas may include a small amount of corrosive substances such as sulfur, which causes corrosion in the heat exchanger due to the condensation. Further, since the dust of the aerated catalyst is scattered from the reactor  1  in the long run, the dust adheres to the portions covered with the condensate possibly to lower the heat exchange ability.  
           [0007]    For the purpose of avoiding this situation, it is necessary to elevate the temperature of the treated gas being supplied to the first heat exchanger  2 , namely the temperature of the treated gas at the outlet of the heat-recovering device  4 . When a waste heat boiler is used for the heat-recovering device  4 , for example, the method of decreasing the amount of heat recovered in the boiler and elevating the temperatures of the treated gas at the inlet and the outlet of the first heat exchanger by increasing the steam pressure generated in the boiler, namely elevating the evaporation temperature is conceivable. Since the steam generated in the boiler is generally connected to the steam system in the plant, it is commendable for the pressure of the steam supplied to the steam system to be maintained at a fixed level. For this reason, it is extremely difficult to adjust the temperature of the treated gas at an arbitrary level by varying the steam pressure generated in the waste heat boiler in accordance with the operation condition of the apparatus for the treatment of the waste gas. As a result, the waste gas subjected to the treatment is limited in a narrow range and the apparatus possibly fails to apply to the treatment of the waste gas discharged from the production process.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is, therefore, to provide an apparatus for the treatment of the waste gas that has a wide adaptability for operation in correspondence to variations in the composition of the waste gas discharged mainly from the production process of organic compounds, namely that is capable of adjusting the temperature of the treated gas in a first waste gas pre-heater to an arbitrary temperature not lower than the dew point thereof, capable of being stably operated without the supply of the molecular oxygen-containing gas in an unduly large amount, and capable of recovering the heat generated by the catalytic oxidation of the waste gas to the largest possible extent.  
           [0009]    We, as a result of pursuing a diligent study concerning the problems mentioned above, have found that by supplying a molecular oxygen-containing gas to the inlet of a catalytic oxidation reactor, utilizing part or the whole of the treated gas emanating from the reactor as the heat source for a second waste gas pre-heater and/or a heat-recovering device, and further supplying part or the whole of the treated gas emanating from the second pre-heater as the heat source for the first pre-heater, it is possible to adjust the temperature of the treated gas in the first pre-heater at an arbitrary temperature not lower than the dew point of the gas. This invention has been perfected as a result.  
           [0010]    Further, the object of this invention, therefore, consists in eliminating the problems mentioned above and providing a method for the treatment of the waste gas by the use of the apparatus for the treatment of waste gas that is capable of adjusting the temperature of the waste gas in the waste gas pre-heater to an arbitrary level.  
           [0011]    This invention relates to an apparatus for the treatment of waste gases, characterized by comprising a catalytic oxidation reactor for the treatment of a waste gas containing combustible compounds; a device for supplying the waste gas to the reactor; plural preheating devices for preheating the waste gas; and a heat-recovering device for recovering the heat of the treated gas emanating from the reactor; being provided with a molecular oxygen-containing gas supplying device for supplying a molecular oxygen-containing gas to at least one arbitrary point between the waste gas inlet of a first waste gas pre-heater and the inlet of the reactor (wherein the term “first waste gas pre-heater” as used herein means the pre-heater to which the waste gas is supplied first where the pre-heaters are disposed in series or either of the pre-heaters where the pre-heaters are disposed parallelly); being adapted to supply the treated gas emanating from the reactor as the heat source to at least one of the second to n&#39;th waste gas pre-heaters and the heat-recovering device (wherein the term “n&#39;th waste gas pre-heater” means the pre-heater to which the treated gas is supplied first where the pre-heaters are disposed in series or the pre-heater to which the treated gas is supplied first where the pre-heaters are disposed parallelly); being constructed to use the treated gas emanating from at least one of the second to n&#39;th waste gas pre-heaters as mixed with the waste gas supplied to the reactor and supply the treated gas emanating from the heat-recovering device to the first pre-heater; or being constructed to supply the treated gas emanating from any one of the second to n&#39;th waste gas pre-heaters and the treated gas emanating from the heat-recovering device to the first pre-heater as the heat source.  
           [0012]    Further, this invention relates to a method for the treatment of the waste gas by the use of the apparatus mentioned above.  
           [0013]    Though the number, n, of the pre-heaters is not particularly limited, the limit up to n=2 is preferred from the viewpoint of the convenience of operation.  
           [0014]    Further, this invention relates to an apparatus for the treatment of waste gases, characterized by comprising a catalytic oxidation reactor for the treatment of a waste gas containing combustible compounds; a device for supplying the waste gas to the reactor; plural preheating devices for preheating the waste gas; and a heat-recovering device for recovering the heat of the treated gas emanating from the reactor; being provided with a molecular oxygen-containing gas supplying device for supplying a molecular oxygen-containing gas to at least one arbitrary point between the waste gas inlet of a first waste gas pre-heater and the inlet of the reactor (wherein the term “first waste gas pre-heater” as used herein means the pre-heater to which the waste gas is supplied first where the pre-heaters are disposed in series or either of the pre-heaters where the pre-heaters are disposed parallelly); being adapted to supply part or the whole of the treated gas emanating from the reactor to at least one of the second to n&#39;th waste gas pre-heaters (wherein the term “n&#39;th waste gas pre-heater” as used herein means the pre-heater to which the treated gas is supplied first where the pre-heaters are disposed in series or the pre-heater to which the treated gas is supplied first where the pre-heaters are disposed parallelly); and being constructed to supply the treated gas emanating from the second to n&#39;th waste gas pre-heaters to the first pre-heater as the heat source and supply the treated gas emanating from the first pre-heater and the reactor (inclusive of the case wherein the amount of the treated gas to be supplied is zero)to the heat-recovering device as the heat source.  
           [0015]    Further, this invention relates to a method for the treatment of a waste gas by the use of the apparatus mentioned above.  
           [0016]    Though the number, n, of the pre-heaters is not particularly limited, the limit up to n=2 is preferred from the viewpoint of the convenience of operation.  
           [0017]    In accordance with the present invention, it is enabled to elevate the temperature of the treated gas at the outlet of the first pre-heater to a level higher than the dew point of the gas for the same amount of the molecular oxygen-containing gas as in the conventional apparatus and consequently eliminate the trouble related to condensation and permit the heat generated by the catalytic oxidation of the waste gas to be recovered to the largest possible extent.  
           [0018]    The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0019]    The accompanying drawing incorporated in and forming a part of the specification, illustrates several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings:  
         [0020]    [0020]FIG. 1 is a diagram illustrating the flow sheet of the apparatus for the treatment of a waste gas according to the conventional technique;  
         [0021]    [0021]FIG. 2 is a diagram illustrating one flow sheet of the apparatus of this invention for the treatment of a waste gas;  
         [0022]    [0022]FIG. 3 is a diagram illustrating another flow sheet of the apparatus of this invention for the treatment of a waste gas; and  
         [0023]    [0023]FIG. 4 is a diagram illustrating yet another flow sheet of the apparatus of this invention for the treatment of a waste gas. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    This invention will be described below based on the drawings attached hereto. As used hereinafter, the term “waste gas” refers to the combustible compound-containing gas to be treated and the term “treated gas” refers to the gas that has been treated in a catalytic oxidation reactor and subsequently discharged from the reactor.  
         [0025]    [0025]FIG. 2 is a diagram illustrating one example of the flow sheet of the apparatus of this invention for the treatment of a waste gas. In FIG. 2, two waste gas pre-heaters are disposed between a waste gas-supplying device  205  and a catalytic oxidation reactor  201  and disposed in series along the flow of the waste gas. A waste gas  212  containing combustible compounds is supplied by the waste gas-supplying device  205  to a first waste gas pre-heater  202  and a second waste gas pre-heater  203 , heated to an appropriate temperature with the treated gas emanating from the reactor  201  and a heat-recovering device  204 , and thereafter supplied to the reactor  201 .  
         [0026]    This waste gas  212  generally contains the waste gas discharged from the production process such a chemical substance as phthalic acid, maleic acid, ethylene oxide, (meth)acrylicacid,ora (meth) acrylicester. Thewastegases from the various processes arise single or in the form of a mixture. The waste gas-supplying device  205  is not particularly limited but only required to be capable of supplying the waste gas  212 . Examples of the waste gas-supplying device may include rotary machines such as blower and fan. Besides, supplying method utilizing the column operating pressure itself in case of the waste gas is supplied from absorbing or stripping columns operated under pressurized condition, or the waste gas supplying device installed at the outlet of the treated gas from the apparatus for the waste gas treatment to evacuate the apparatus and eventually suck the waste gas can be cited as means for supplying the waste gas.  
         [0027]    The first pre-heater  202  refers to a waste gas pre-heater that effects the temperature elevation of the waste gas by using the treated gas emanating from the heat-recovering device  204  and/or the second pre-heater  203  as the heat source. The second pre-heater  203  refers to a waste gas pre-heater that effects the temperature elevation of the waste gas by using the treated gas emanating from the reactor  201  as the heat source. These waste gas pre-heaters are only required to be installed one each at least. The numbers thereof are not particularly limited. These waste gas pre-heaters are not restricted on account of the model and only required to be capable of elevating the temperature of the waste gas by exchange of heat. Any of the heretofore known heat exchanges can be used as the waste gas pre-heaters. The plate type heat exchangers can be used particularly advantageously among other types.  
         [0028]    A molecular oxygen-containing gas supplying device  207  is connected to the waste gas line at an arbitrary point between the waste gas inlet of the first pre-heater  202  and the inlet of the reactor  201 . Specifically, the supplying device  207  is connected to the waste gas inlet of the first pre-heater  202 , between the waste gas outlet of the first pre-heater  202  and the waste gas inlet of the second pre-heater  203 , and/or between the waste gas outlet of the second pre-heater  203  and the inlet of the reactor  201 . Preferably, this connection is made between the waste gas outlet of the first pre-heater  202  and the inlet of the reactor  201 , especially to the inlet of the reactor  201 . The term “molecular oxygen-containing gas  213 ” as used herein involves air, oxygen gas, oxygen-enriched air, and the molecular oxygen-containing treated gas discharged from the other facilities.  
         [0029]    The device for supplying the molecular oxygen-containing gas  213  will be considered now on the assumption that a blower without the function to control revolution, for example, is adopted. The flow volume of the molecular oxygen-containing gas  213  supplied to between the waste gas-supplying device  205  and the first pre-heater  202  (hereinafter occasionally referred to as “the amount of the molecular oxygen-containing gas for premix”) is adjusted in accordance with the oxygen gas concentration in the treated gas or the supplying amount of the bypass oxygen-containing gas (the oxygen-containing gas supplied to the inlet of the second pre-heater and/or the inlet of the reactor) is adjusted in accordance with the temperature of the treated gas at the outlet of the first pre-heater, respectively by the use of the control valve. In this case, since the amounts of such molecular oxygen-containing gases are controlled in response to the signals issued from the detectors disposed respectively, the discharging amount of the blower possibly fluctuates from the amount inherently necessary for the treatment of the waste gas. This fluctuation due to the deviation from the amount to be required for waste gas treatment may cause hunting of the system. As a result, it becomes difficult to operate the reactor stably. Further, since decreasing the amount of the discharge gas reduces the pressure loss in the reactor, the discharge pressure of the blower can be reduced. Since diminishing the flow volume with the control valve increases the pressure loss, the blower consumes unnecessary electric power and the condition will results in an uneconomical operation. For the purpose of performing the treatment of the waste gas itself stably, it is necessary to prevent the fluctuation of the amount of the gas from the blower. Measuring the oxygen gas concentration in the treated gas and adjusting the amount of discharge gas from the blower based on this concentration can eliminate this problem.  
         [0030]    The molecular oxygen-containing gas supplying device  207  is so constructed as to receive the signal from an oxygen concentration detector  209  disposed in the pipe on the treated gas outlet side of the first pre-heater  202 . To be specific, when the oxygen gas concentration in the treated gas discharged from the first pre-heater  202  is found by measurement to be unduly low, the molecular oxygen-containing gas is supplied with a conventionally known gas-supplying device such as a blower or a fan that is furnished with a driving part capable of being driven at various revolutions. As regards the method for measuring the oxygen gas concentration in the treated gas, since the treated gas system from the reactor  201  onward admits no gas from any other device and the oxygen concentration is not changed in the system, the oxygen concentration detector  209  can be installed at an arbitrary position in the treated gas system. Examples of the measuring means may include known oxygen gas measuring means of the magnetic type and the zirconia type. For a device  211  to adjust the amount of the discharged molecular oxygen-containing gas, inverter control device to control revolution or reduction ratio control device, in particular a method to control revolution of motor by inverter control device may be cited.  
         [0031]    Specifically, by being furnished with the detector  209  to measure the oxygen gas concentration in the treated gas, the molecular oxygen-containing gas supplying device  207  provided with a driving part capable of being driven at various revolutions, and the device  211  to adjust the discharge amount of the supplying device, it is possible to adjust the amount of the molecular oxygen-containing gas and supply the oxygen-containing gas required for the catalytic oxidation of the waste gas just enough.  
         [0032]    A controller  210  to adjust the amount of the bypass molecular oxygen-containing gas is so constructed as to receive the signal from a treated gas temperature measuring means  208  that is disposed in the pipe on the treated gas out let side of the first pre-heater  202 . When the temperature of the treated gas from the first pre-heater  202  is found by measurement to be lower than the prescribed level, the controller  210  is actuated to increase the amount of the bypass molecular oxygen-containing gas. Incidentally, the temperature of the treated gas emanating from the first pre-heater is controlled by utilizing the known detector such as a resistance thermometer using platinum, nickel, copper, or sheath or a thermocouple thermometer using copper-constantan, iron-constantan, chromel-alumel, or platinumrhodium-platinum. The molecular oxygen-containing gas is supplied to the point between the first pre-heater  202  and the second pre-heater  203  and/or between the second pre-heater  203  and the reactor  201 . When the temperature of the treated gas emanating from the first pre-heater  202  is lower than the dew point of the gas, for example, the amount of the bypass molecular oxygen-containing gas can be increased under the condition where the amount of oxygen-containing gas discharged from supplying device  207  is kept at a fixed level. In an extreme case, the discharge gas from the molecular oxygen-containing gas supplying device  207  is supplied in its whole amount as the bypass molecular oxygen-containing gas. Conversely, when such temperature is unduly high from the dew point of the gas, it becomes necessary to lower the temperature from the economic point of view. As occasion demands, the discharge gas from the molecular oxygen-containing gas supplying device  207  is supplied in its whole amount as the molecular oxygen-containing gas for premix. Example of the controller  210  to adjust the amount of the bypass molecular oxygen-containing gas may include a control valve, which has been known to the art.  
         [0033]    Specifically, by being provided with the controller  208  to measure the temperature of the treated gas emanating from the first pre-heater and the controller  210  to adjust the amount of the molecular oxygen-containing gas supplied to an arbitrary point between the waste gas outlet of the first pre-heater and the inlet of the reactor, it is possible to divide the molecular oxygen-containing gas being supplied and adjust the temperature of the treated gas emanating from the waste gas pre-heater at an arbitrary level.  
         [0034]    By adjusting the discharge amount of the blower by the revolution control and adjusting the amount of the bypass molecular oxygen-containing gas by the control valve, therefore, it is possible to operate the reactor stably with the temperature of a treated gas  214  arbitrarily adjusted meanwhile.  
         [0035]    In the reactor  201 , combustible compounds contained in the waste gas are disposed, namely treated by oxidation, by the use of an oxidation catalyst. The degree of perfect oxidation of the combustible organic compounds in the waste gas is confirmed with an analyzer such as a gas chromatography, as occasion demands. The oxidation catalyst used herein may be selected from among the oxidation catalysts known to the art. Examples of the oxidation catalyst may include molded pieces of a refractory inorganic oxide such as activated alumina, activated carbon, titania, or zeolite having an active component such as a noble metal like platinum or palladium or a base metal like copper, cobalt, nickel, or iron dispersed therein or superposed thereon or honeycomb structures of cordierite coated with the active component dispersed in the refractory inorganic oxide. Since the saturated hydrocarbons are not easily burned while the unsaturated hydrocarbons are burned relatively easily, it is commendable to fix the inlet temperature of the reactor at a level of not lower than 250° C. and the outlet temperature of the reactor at a level in the range of 650-700° C. and further the temperature elevation in the catalyst layer at a level of not higher than 430° C.  
         [0036]    The heat-recovering device  204  is disposed between the reactor  201  and the first pre-heater  202 . The second pre-heater  203  is disposed between the reactor  201  and a circulation blower  206 . The outlet of the heat-recovering device  204  is connected with a pipe to the inlet of the circulation blower  206  as well as to the first pre-heater  202 . The outlet of the circulation blower  206  is connected to the reactor  201  via a control valve  215  and to the inlet of the first pre-heater  202  via a control valve  216  respectively. Part or the whole of the treated gas emanating from the reactor  201  is utilized as the heat source for the second pre-heater  203  and the heat-recovering device  204 .  
         [0037]    The hot treated gas emanating from the reactor  201  is supplied to the heat-recovering device  204 , which is a known waste heat boiler and the second pre-heater  203 . The treated gas that has been supplied to the second pre-heater  203  is used to elevate temperature of the waste gas therein and then supplied to the inlet of the first pre-heater  202  via the circulation blower  206  and the control valve  216 . By this procedure, it is possible to preheat the waste gas and, at the same time, elevate the temperature of the treated gas emanating from the first pre-heater  202 . Thus, the apparatus is so constructed as to utilize the treated gases emanating from the heat-recovering device  204  and the second pre-heater  203  as the heat source for the first pre-heater  202 .  
         [0038]    Further, the treated gas supplied via the second pre-heater  203  and part of the treated gas supplied via the heat-recovering device  204 , when necessary, are supplied to the reactor  201  via the circulation blower  206  and the control valve  215 . By this procedure, it is possible to increase the amount of the gas entering the reactor  201  and permit effective recovery of the heat generated therein. Thus, the treated gas emanating from the second pre-heater  203  and part of the treated gas emanating from the heat-recovering device  204 , when necessary, can be supplied to the reactor  201 .  
         [0039]    Further, the hot treated gas emanating from the reactor  201 , if necessary, may be passed solely through the heat-recovering device  204 .  
         [0040]    The treated gas that has elevated the temperature of the waste gas in the first pre-heater  202  and has been consequently brought to a lowered temperature is supplied to the treated gas-discharging device (not illustrated) and released into the atmosphere. Further, since the oxygen in the treated gas has been consumed in the oxidation of harmful components and, therefore, is in a very scarce state, it is permissible to promote effective utilization of the treated gas as an inert gas, as a seal gas for storage tanks, or as a diluent gas for the oxidation reaction in some other facilities.  
         [0041]    The treatment of the waste gas is carried out stably by adjusting the flow volume of the treated gas with the control valve, in accordance with the increase or decrease of the molecular oxygen-containing gas and the change of the inlet and outlet temperatures of the reactor. When the heat generated by the waste gas is small, for example, it is possible to fix the temperature of the treated gas of the first pre-heater at a level not lower than the dew point of the gas by using the whole amount of the molecular oxygen-containing gas as the bypass oxygen-containing gas and supplying the treated gas from the heat-recovering device and the second pre-heater to the first pre-heater as the heat source. When the heat generated by the waste gas is large, it is possible to fix the temperature of the treated gas from the first pre-heater at a level not lower than the dew point of the gas by using the molecular oxygen-containing gas as the molecular oxygen-containing gas for premix and as the bypass oxygen-containing gas and supplying the treated gas from the heat-recovering device to the first pre-heater as the heat source (in this case, the treated gas of the second pre-heater and, as occasion demands, part of the treated gas from the heat-recovering device are introduced into the inlet of the reactor). As a result, the flow volume balance and the heat balance of gases in the apparatus for the treatment can be changed more widely than when the molecular oxygen-containing gas is supplied in advance and disposed in the conventional apparatus and the temperature of the treated gas outlet of the first pre-heater can be adjusted to an arbitrary level not lower than the dew point of the gas. Thus, the supplying device does not need to be operated with an unduly high discharge pressure. This fact proves highly advantageous from the viewpoint of saving energy.  
         [0042]    Incidentally, the equipments, devices, and means that form the apparatus are suitably connected with pipes.  
         [0043]    The apparatus described above has two waste gas pre-heaters disposed in series. In the case of the apparatus using three such waste gas pre-heaters, the method of disposing the three waste gas pre-heaters between the first pre-heater  202  and the second pre-heater  203  or the method of disposing them between the second pre-heater  203  and the reactor  201  (prior to the supply of the molecular oxygen-containing gas) may be conceived. In this case, the flows of the waste gas and the treated gas are substantially the same as illustrated in FIG. 2. These waste gas pre-heaters are reckoned as the first, second, and third waste gas pre-heaters as viewed from the upstream side of the waste gas. The treated gas emanating from the reactor  201  may be supplied to the other waste gas pre-heaters. When the number of the waste gas pre-heaters is further increased, these waste gas pre-heaters can be disposed in the same manner as when the number of the waste gas pre-heaters is three.  
         [0044]    [0044]FIG. 3 is a diagram illustrating another flow sheet of the apparatus of this invention for the treatment of the waste gas. The equipments and the devices that are used in the apparatus illustrated in FIG. 3 have the same functions as those illustrated in FIG. 2. They are denoted by reference numerals that result from adding 100 to the corresponding reference numerals used in FIG. 2. A large difference between the flow sheets of FIGS. 3 and 2 resides in the point that the apparatus in FIG. 3 has waste gas pre-heaters disposed parallelly, whereas the apparatus in FIG. 2 has waste gas pre-heaters disposed in series.  
         [0045]    In FIG. 3, two waste gas pre-heaters  302  and  303  are disposed parallelly between a waste gas-supplying device  305  and a catalytic oxidation reactor  301 . Since the waste gas pre-heaters in the present apparatus are substantially the same as those in the apparatus of FIG. 2 excepting they are disposed parallelly, the waste gas and the treated gas flow in the same manner as those in the apparatus of FIG. 2.  
         [0046]    The apparatus described above has two waste gas pre-heaters disposed parallelly. In the case of the apparatus using three waste gas pre-heaters, the method of disposing such waste gas pre-heaters between the first pre-heater  302  (prior to the circulation blower) and the second pre-heater  303  or the method of disposing them between the second pre-heater  303  and the reactor  301  may be conceived. In this case, the flows of the waste gas and the treated gas are substantially the same as those illustrated in FIG. 3. The waste gas pre-heaters are reckoned as the first, second, and third waste gas pre-heaters as viewed from the downstream side of the treated gas. The treated gas emanating from the reactor  301  may be supplied to the other waste gas pre-heaters. When the number of the waste gas pre-heaters is further increased, these waste gas pre-heaters can be disposed in the same manner as when the number of the waste gas pre-heaters is three.  
         [0047]    [0047]FIG. 4 is a diagram illustrating yet another flow sheet of the apparatus of this invention for the treatment of a waste gas. The equipments and the devices that are used in the apparatus illustrated in FIG. 4 have the same functions as those illustrated in FIG. 2. They are denoted by reference numerals that result from adding 200 to the corresponding reference numerals used in FIG. 2. In FIG. 4, two waste gas pre-heaters  402  and  403  are disposed in series between a waste gas-supplying device  405  and a catalytic oxidation reactor  401 . A bypass is furnished between the inlet and outlet of the first pre-heater  402  through a control valve  418 . A molecular oxygen-containing gas-supplying device  407  is connected to the inlet of the reactor  401 . The molecular oxygen-containing gas-supplying device  407  is naturally connected in various patterns such as, for example, to the waste gas inlet of the first pre-heater  402 , between the first pre-heater  402  and the reactor  401 , between the first pre-heater  402  and the waste gas inlet of the second pre-heater  403 , and/or between the waste gas outlet of the second pre-heater  402  and the inlet of the reactor  401 . Among other patterns mentioned above, the connection between the first pre-heater  402  and the inlet of the reactor  401  and particularly the connection to the inlet of the reactor  401  prove advantageous.  
         [0048]    A waste gas  412  is advanced via the waste gas-supplying device  405  and the two waste gas pre-heaters  402  and  403  and introduced into the reactor  401  as mixed with a molecular oxygen-containing gas  413  from the molecular oxygen-containing gas supplying devices  407 .  
         [0049]    A heat-recovering device  404  is connected to the reactor  401  via a control valve  419  and further connected to the reactor  401  via the waste gas pre-heaters  402  and  403  as furnished parallelly thereto. The treated gas emanating from the first pre-heater  402  and, if necessary, the treated gas emanating from the reactor  401  are utilized as the heat source for a heat-recovering device  404 .  
         [0050]    The hot treated gas emanating from the reactor  401  is introduced to the heat-recovering device  404  via the two waste gas pre-heaters  402  and  403  and brought parallelly thereto to the heat-recovering device  404  via a control valve  419 , then deprived of the heat, and finally released into the atmosphere.  
         [0051]    When the heat of the waste gas generated in the reactor  401  is small and the dew point of the treated gas is comparatively low, it is commendable to use the apparatus illustrated in FIG. 4. In this case, the temperatures of the various parts in the apparatus can be maintained at respectively proper levels by adjusting the flow volume of the waste gas that is bypassing the first pre-heater  402 . Further, since the heat-recovering device  404  is disposed at the most backward part of the treated gas flow path, it is only natural that the first pre-heater  402  has no possibility of entailing such trouble as dewing. Further, since no flow path for the treated gas bypasses the heat-recovering device  404  and the pressure of the steam generated in the heat-recovering device  404  can be lowered, the heat generated in the reactor  401  can be effectively recovered and the waste heat loss to the outside of the system can be decreased.  
         [0052]    Incidentally, the equipments, devices, and means that form the apparatus are suitably connected with pipes.  
         [0053]    The apparatus described above has two waste gas pre-heaters disposed in series. In the case of the apparatus using three waste gas pre-heaters, methods of disposing these waste gas pre-heaters between the first pre-heater  402  and the second pre-heater  403  and of disposing them between the first pre-heater  403  and the reactor  401  (prior to the supply of the molecular oxygen-containing gas) are conceivable. In this case, the flows of the waste gas and the treated gas are substantially the same as those illustrated in FIG. 4. These waste gas pre-heaters are reckoned as the first, second, and third waste gas pre-heaters as viewed from the upstream side of the waste gas. The treated gas emanating from the reactor  401  may be supplied to the other waste gas pre-heaters. Further, when the number of the waste gas pre-heaters is increased, such waste gas pre-heaters can be disposed in the same manner as when the number of waste gas pre-heaters is three.  
         [0054]    Thus, a suitable apparatus for the treatment of the waste gas can be selected, depending on the heat of the waste gas and the degree of change of the dew point of the treated gas.  
       EXAMPLES  
       [0055]    Now, this invention will be described specifically below with reference to examples. The description is given as divided into two cases; the one case having a large amount of heat generated in consequence of the catalytic oxidation of the waste gas and the other case having a small amount of such heat.  
         [0056]    (1) Treatment of waste gas that generates a small amount of heat  
       Comparative Example 1  
       [0057]    Conventional Technique of FIG. 1  
         [0058]    Part of the treated gas at the outlet of the reactor was supplied to the second pre-heater (the second heat exchanger) and spent therein to preheat the waste gas to be treated so as to maintain the temperature in the reactor at a proper level. The treated gas emanating from the second pre-heater was supplied together with the remaining treated gas to the heat-recovering device to allow recovery of heat entrained by the steam, further elevate the waste gas temperature in the first pre-heater (the first heat exchanger), and released into the atmosphere. Air was used as the molecular oxygen-containing gas for the treatment of the waste gas. It was supplied and mixed with the waste gas in advance.  
         [0059]    As a result, no treated gas was recycled from the second pre-heater to the reactor or from the heat-recovering device (waste heat boiler) to the outlet of the second pre-heater. Though the temperature of the treated gas at the outlet of the first pre-heater (the first heat exchanger) was 109° C., the ability of the apparatus was gradually degraded because the first pre-heater formed dew on the treated gas (elevated temperature) side thereof and the scattered dust of a catalyst adhered to the heating surface. Since the sulfur component contained in a minute amount in the waste gas was condensed, the heating surface corroded and sustained a crack.  
       Example 1  
       [0060]    Part of the treated gas was supplied to the first pre-heater without being passed through the heat-recovering device  
         [0061]    As a measure to cope with the trouble mentioned above, the flow path from the second pre-heater to the inlet of the heat-recovering device was changed so as to reach the outlet of the recovering device to decrease the amount of heat to be recovered. As a result, the temperature of the treated gas at the outlet of the first pre-heater was elevated till 115° C. and the condensation trouble was canceled.  
       Comparative Example 2  
       [0062]    Increased Amount of Air for the Oxidation Reaction  
         [0063]    When the amount of air in the apparatus illustrated in FIG. 1 was increased, the temperature of the treated gas was elevated only sparingly and the trouble of condensation was not cancelled. Further, the oxygen concentration in the treated gas was heightened and the operation was forced to proceed inefficiently by requiring supply of air in an unduly large amount.  
       Example 2  
       [0064]    In FIG. 2, the molecular oxygen-containing gas was thrown into the waste gas prior to the reactor and the hot treated gas emanating from the reactor was supplied to the first pre-heater via the heat-recovering device, the second pre-heater, the circulation blower, and the control valve. With the same amount of air as used heretofore, the temperature at the outlet of the first pre-heater was heightened till 127° C. and the condensation trouble was eliminated.  
         [0065]    The results of Comparative Examples 1 and 2 and Examples 1 and 2 are shown in Table 1.  
                                                                   TABLE 1                                       During generation of low heat                Comparative       Comparative               Example 1   Example 1   Example 2   Example 2                        (A)   1200   ←   ←   ←       (B)       (B-1)   400   ←   ←   ←       (B-2)   40   40   92   0       (B-3)   0   0   0   40       (B-4)   40   40   92   40       (B-5)   440   440   492   400       (B-6)   440   440   492   440       (B-7)   250   242   332   255       (B-8)   190   198   160   185       (B-9)   440   198   492   185       (B-10)   0   ←   ←   ←       (B-11)   250   242   332   255       (B-12)   0   ←   ←   ←       (B-13)   250   242   332   255       (B-14)   440   440   492   440       (C)       (C-1)   60   ←   ←   ←       (C-2)   412   412   443   412       (C-3)   700   ←   ←   ←       (C-4)   109   115   110   127       (D)   1.0   1.0   3.1   1.0                  
 
         [0066]    wherein:  
         [0067]    (A): Amount of heat generated by the reactor (10 4  kj/h)  
         [0068]    (B): Flow volume (Nm 3 /min)  
         [0069]    (B-1): Amount of waste gas to be treated  
         [0070]    (B-2): Amount of air supplied as premixed  
         [0071]    (B-3): Amount of air supplied via the bypass  
         [0072]    (B-4): Total amount of air  
         [0073]    (B-5): Amount of gas at the inlet of the first pre-heater  
         [0074]    (B-6): Amount of gas at the inlet of the reactor  
         [0075]    (B-7): Amount of gas: Outlet of the reactor→the second pre-heater  
         [0076]    (B-8): Amount of gas: Outlet of the reactor→the waste heat boiler  
         [0077]    (B-9): Amount of waste gas at the inlet of the waste heat boiler  
         [0078]    (B-10): Amount of gas: Waste heat boiler→Circulation blower  
         [0079]    (B-11): Amount of gas at inlet of the circulation blower  
         [0080]    (B-12): Amount of gas: Circulation blower→Reactor  
         [0081]    (B-13): Amount of gas: Circulation blower→First waste gas pre-heater  
         [0082]    (B-14): Amount of waste gas at the inlet of the first pre-heater  
         [0083]    (C): Temperature conditions (° C.)  
         [0084]    (C-1): Temperature of waste gas at in let of the first pre-heater  
         [0085]    (C-2): Temperature of the inlet of the reactor  
         [0086]    (C-3): Temperature of the outlet of the reactor  
         [0087]    (C-4): Temperature of the treated gas at the outlet of the first pre-heater  
         [0088]    (D): O 2  concentration (% by volume) in treated gas  
         [0089]    It is noted from the comparison of the results of Comparative Example 1 and Examples 1 and 2 given in Table 1 that in spite of the fact that the amount of air used for the oxidation reaction was equal between the examples under comparison, the temperature of the treated gas at the outlet of the first pre-heater could be elevated by supplying the treated gas emanating from the second pre-heater directly to the first pre-heater in the case of Examples 1 and 2, and the oxygen concentration in the treated gas at the outlet of the first pre-heater was equal between the examples under comparison.  
         [0090]    (2) Treatment of waste gas that generates a large amount of heat  
       Comparative Example 3  
       [0091]    Conventional Technique  
         [0092]    Similarly to (1), part of the treated gas at the outlet of the reactor was supplied to the second pre-heater and spent therein to preheat the waste gas. In this case, since the heat generated in the reactor was large, the inside of the reactor was heated abnormally high temperature when the waste gas supplied from the outlet of the second pre-heater was supplied directly to the reactor. The abnormal elevation of the temperature inside the reactor was repressed by recycling the treated gas at the outlet of the second pre-heater to the inlet of the reactor. When the heat generated was larger, part of the gas at the outlet of the heat-recovering device was also recycled to the reactor. Incidentally, the air for the oxidation reaction was supplied and mixed with the waste gas in advance.  
         [0093]    As a result, no inflow of gas occurred from the circulation blower to the first pre-heater (waste heat boiler).  
         [0094]    In this case, though the temperature of the treated gas at the outlet of the first pre-heater was 128° C., the same trouble occurred as in the operation of Comparative Example 1.  
       Comparative Example 4  
       [0095]    Increased Amount of Air for the Oxidation Reaction  
         [0096]    In this case, since no inflow of gas occurred from the circulation blower to the first pre-heater (waste heat boiler), the temperature of the treated gas at the outlet of the first pre-heater could not be elevated by a simple change of the flow path as in the case of (1). In spite of the increase of the air for the oxidation reaction from 114 to 164 Nm 3 /min, the temperature was not elevated appreciably and the oxygen concentration in the treated gas was increased to 2.7 vol. % and the operation was forced to proceed inefficiently by requiring supply of air in an unduly large amount.  
       Example 3  
       [0097]    In FIG. 2, part of the air (74 Nm 3 /min) was supplied as bypassed to the inlet of the reactor from the viewpoint of such factors as the catalyst, heat resistance of the component devices, and the stable operation of the apparatus. The abnormal elevation of the temperature in the reactor was repressed by recycling the treated gas at the outlet of the second pre-heater to the inlet of the reactor. When the heat generated was still larger, part of the outlet gas of the heat-recovering device was also recycled to the reactor.  
         [0098]    With the same amount of the air as in Comparative Example 3, the temperature of the treated gas at the outlet of the first pre-heater was elevated till 137° C. and the trouble of condensation was eliminated.  
         [0099]    The results of Comparative Examples 3 and 4 and Example 3 are shown in Table 2.  
                                                               TABLE 2                                       During generation of high heat                Comparative   Comparative               Example 3   Example 4   Example 3                            (A)   3000   ←   ←           (B)           (B-1)   420   ←   ←           (B-2)   114   164   40           (B-3)   0   0   74           (B-4)   114   164   114           (B-5)   534   584   460           (B-6)   534   584   534           (B-7)   190   200   207           (B-8)   344   384   327           (B-9)   344   384   327           (B-10)   125   67   108           (B-11)   315   267   315           (B-12)   315   267   315           (B-13)   0   ←   ←           (B-14)   534   584   534           (C)           (C-1)   60   ←   ←           (C-2)   300   ←   ←           (C-3)   700   ←   ←           (C-4)   128   129   137           (D)   1.0   2.7   1.0                      
 
         [0100]    wherein:  
         [0101]    (A): Amount of heat generated by the reactor (10 4  kj/h)  
         [0102]    (B): Flow volume (Nm 3 /min)  
         [0103]    (B-1): Amount of gas to be treated  
         [0104]    (B-2): Amount of air supplied as premixed  
         [0105]    (B-3): Amount of air supplied via the bypass  
         [0106]    (B-4): Total amount of air  
         [0107]    (B-5): Amount of gas at the inlet of the first pre-heater  
         [0108]    (B-6): Amount of gas at the inlet of the reactor  
         [0109]    (B-7): Amount of gas: Outlet of the reactor→the second pre-heater  
         [0110]    (B-8): Amount of gas: Outlet of the reactor→the waste heat boiler  
         [0111]    (B-9): Amount of waste gas at the inlet of the waste heat boiler  
         [0112]    (B-10): Amount of gas: Waste heat boiler→Circulation blower  
         [0113]    (B-11): Amount of gas at inlet of the circulation blower  
         [0114]    (B-12): Amount of gas: Circulation blower→Reactor  
         [0115]    (B-13): Amount of gas: Circulation blower→First waste gas pre-heater  
         [0116]    (B-14): Amount of waste gas at the inlet of the first pre-heater  
         [0117]    (C): Temperature conditions (° C.)  
         [0118]    (C-1): Temperature of waste gas at inlet of the first pre-heater  
         [0119]    (C-2): Temperature of the inlet of the reactor  
         [0120]    (C-3): Temperature of the outlet of the reactor  
         [0121]    (C-4): Temperature of the treated gas at the outlet of the first pre-heater  
         [0122]    (D): O 2  concentration (% by volume) in treated gas  
         [0123]    It is noted from the comparison of the results of Comparative Example 3 and Example 3 given in Table 2 that in spite of the fact that the amount of air used for the oxidation reaction was equal between the examples under comparison, the temperature of the treated gas at the outlet of the first pre-heater could be elevated in the case of Example 3.  
         [0124]    The oxygen concentration in the treated gas at the outlet of the first pre-heater was equal between the examples under comparison.  
         [0125]    The entire disclosure of Japanese Patent Application Nos. 2000-167973 filed on Jun. 5, 2000 including specification, claims, drawings and summary are incorporated herein by reference in its entirety.