Patent Publication Number: US-2020276537-A1

Title: Waste gas emission control system

Description:
FIELD OF INVENTION 
     The present invention concerns an emissions control system for the catalytic combustion of components of a process waste gas stream. In particular, but not exclusively, the present invention concerns an emissions control system for use in a process for the production of formaldehyde, for example as formalin or UFC. The present invention also concerns a process for the production of formaldehyde, for example as formalin or UFC. 
     BACKGROUND 
     Formaldehyde can be produced by the catalytic oxidative dehydrogenation of methanol. Processes for carrying out such production are known, for example from WO9632189 or US2504402. The catalyst typically comprises a so-called “mixed oxide” catalyst comprising molybdenum and iron oxides. A well-known process for the production of formaldehyde is the Formox process offered by Johnson Matthey. The Formox process involves catalytic oxidative dehydrogenation of methanol over a mixed oxide catalyst. The Formox process is illustrated in  FIG. 1 . Methanol is mixed with air, vaporised and fed as a feed stream to a reactor, where it is converted into formaldehyde. The process stream leaving the reactor is passed to an absorber and the formaldehyde is removed from the process stream and exits in a product stream at the bottom of the absorber, typically as formalin or UFC. A part of the waste gas stream from the top of the absorber is fed to an emissions control unit (the rest, for example, being recycled), where hazardous components such as carbon monoxide, DME and methanol, are burned by catalytic combustion to produce a combusted waste gas stream that can be vented via a stack. In the present design, the combusted waste gas stream is used to pre-heat the waste gas stream entering the emissions control system to the required ignition temperature for the catalytic combustion. The present emissions control system offers significant advantages over processes without such a system, but it is desirable to seek to improve the system further so as to reduce capital costs and reduce pressure drops. That is particularly the case since emission control systems may be retro-fitted onto existing processes to improve their emissions standards. 
     The present invention seeks to provide an improved emissions control system and process for the production of formaldehyde. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a process for the production of formaldehyde, the process comprising: 
     feeding a feed stream comprising methanol to a reactor; 
     converting the methanol to formaldehyde in the reactor using a mixed oxide catalyst to produce a process stream comprising formaldehyde; 
     separating formaldehyde from the process stream to create a product stream, comprising formaldehyde, and a waste gas stream; 
     feeding at least part of the waste gas stream to a steam condenser to raise the temperature of the at least part of the waste gas stream to create a heated waste gas stream; and 
     feeding the heated waste gas stream to a catalytic combustion bed to catalytically combust components of the heated waste gas stream to create a combusted waste gas stream. 
     By feeding the waste gas stream to a steam condenser to raise the temperature of the waste gas stream, the temperature of the heated waste gas stream as it is fed to the catalyst bed can be controlled so as to maintain a constant temperature of the heated waste gas stream entering the catalyst bed. The control is more straightforward than prior art systems where the combusted waste gas stream is used to heat the incoming waste gas stream since the steam can be controlled independently. For example, in order to maintain a certain temperature into the catalyst bed, a minimum required amount of steam with a minimum required steam pressure may be used. For example, a steam pressure of 14.6 barg corresponds to a steam temperature of 200° C. and a steam pressure of 22.2 barg corresponds to a steam temperature of 220° C. In an example process, the catalyst bed contains a catalyst comprising PPd and PPt catalysts available from Johnson Matthey Formox and operates with 12 barg steam pressure. That pressure advantageously corresponds to a minimum export steam pressure from a typical plant. The steam temperature will typically need to exceed the inlet temperature of the catalyst bed by an approach temperature, to allow for effective heat exchange. In some embodiments therefore, the steam condenser is preferably fed with steam at a pressure of from 10 to 25 barg, preferably from 15 to 25 barg and most preferably from 17 to 20 barg. Such steam pressures may efficiently heat the waste gas stream. Steam condensate may be collected and reused. Efficient heat transfer in the steam condenser may also permit lower pressure drops as the waste gas passes through the steam condenser. Reducing pressure drops may be advantageous in providing a cost- and energy-efficient process as the energy required to compress the gases entering the process, and the cost of that energy, may be significant. Preferably the steam condenser and the catalyst bed are contained within a single vessel. Such an arrangement reduces the need for connecting pipework and may thus further reduce the pressure drop of the system. 
     The product stream comprising formaldehyde is preferably formalin or UFC. The mixed oxide catalyst preferably comprises molybdenum and iron oxides. The conversion of the methanol to formaldehyde in the reactor and the separation of the product stream comprising formaldehyde may, for example, be carried out according to the Formox process. 
     Preferably the waste gas stream enters in the region of the bottom of the steam condenser and flows up through the steam condenser and the heated waste gas stream flows up through the catalyst bed. That may have several advantages. For example, the catalyst bed in an emissions control system is typically supported on a catalyst net. Flowing the heated waste gas stream up through the catalyst bed means that the net is at the cooler, inlet end of the catalyst bed. That is advantageous as the net does not have to withstand as high temperatures and the risk of the net failing is reduced. It may thus be more straightforward to provide a net having the required mechanical strength. A further net may be required at the top of the catalyst bed to reduce movement of the catalyst due to the upward heated waste gas flow, however that net is not subjected to the same forces as the net at the bottom of the catalyst bed and thus does not need to be as strong. It can therefore be more easily designed to handle the high temperatures at the exit of the catalyst bed. Furthermore, flowing the waste gas stream in a single direction through the steam condenser and then onward through the catalyst bed may advantageously reduce pressure drops. Flowing the waste gas stream upwards may be particularly advantageous as condensate (for example, water) condensing in the steam condenser can then be allowed to flow downwards under gravity in a counter-current manner so that the hottest temperature is at the top of steam condenser where the heated waste gas stream passes to the catalyst bed. Preferably therefore steam enters the steam condenser in the region of the top of the steam condenser and flows down through the steam condenser, condensing to form a condensate and the condensate exits the steam condenser in the region of the bottom of the steam condenser. 
     Preferably the steam condenser is a shell and tube steam condenser and the waste gas stream flows through the tube side of the steam condenser and steam condenses in the shell side of the steam condenser. Such an arrangement may optimise pressure drop and heat transfer efficiency. 
     Preferably the process further comprises: 
     Feeding the combusted waste gas stream to a steam generator wherein the combusted waste gas stream is cooled and steam is produced. 
     Thus, the process may use the heat from the combustion to generate steam that can be used elsewhere in the process or elsewhere in the plant. For example, the steam could be provided to a plant steam net. While providing the steam to a plant steam net, and using steam from elsewhere in the plant in the steam condenser may be an efficient option if different pressures of steam are used in the steam condenser and generated in the steam generator, in a particularly preferred embodiment the steam produced in the steam generator is used in the steam condenser. Thus, the process preferably further comprises: 
     Feeding the steam from the steam generator to the steam condenser to raise the temperature of the waste gas stream in step d. 
     Thus the heat is recovered from the combusted waste gas stream and used to heat the waste gas stream before it is fed to the catalyst bed, but that recovery and heating is done indirectly using steam. The steam is produced using the heat from the combusted waste gas stream and is then used to transfer that heat to the incoming waste gas stream. An advantage of such a system is that the pressure drops on the steam side of the process do not affect the overall pressure drops of the formaldehyde production process. Thus, the effectiveness of the heat transfer on the steam side can be optimised without needing to account for the effect of any pressure drop on the overall formaldehyde process. The heat transfer may in any case be more efficient in a condenser than in a gas-gas heat exchanger as might be used if heating the waste gas stream with the combusted waste gas stream directly. Moreover, the heat integration can be more efficiently balanced as additional steam can be added if more heat is needed, or some steam can be removed and used elsewhere if there is an excess of heat. The system may also have advantages on start up since steam from another source can be used initially to heat the waste gas stream. That may remove the need to incur the cost of an electrical heater to heat the emissions control system on start-up. 
     Preferably the steam generator is a shell and tube steam generator and the combusted waste gas stream flows through the tube side of the steam generator and steam is generated in the shell side of the steam generator. Advantageously, that may reduce the pressure drop of the combusted waste gas stream. In some embodiments, the steam generator may comprise a steam super-heater and in some embodiments the steam generator may be a steam super-heater. 
     Preferably the steam condenser, the catalyst bed and the steam generator are contained within a single vessel. That may advantageously remove pressure drops that would be associated with connections between separate vessels. It may also provide a single unit that can be retrofitted to an existing plant. Containing the steam condenser, the catalyst bed and the steam generator within a single vessel may also be advantageous mechanically as it may eliminate the need for high-temperature piping and flanges that would otherwise be needed, particularly between the catalyst bed and the steam generator. The combusted waste gas stream leaving the catalyst bed in prior art systems may reach temperatures of around 550° C. and any piping and flanges between the catalyst bed and steam generator may therefore need to handle such temperatures. The temperature of the combusted waste gas stream leaving the steam generator may be around 230° C.-245° C. if, for example, 22.2 barg 220° C. steam is generated in the steam generator. Thus, if the catalyst bed and the steam generator are contained within the same vessel, the pipework and flanges of that vessel can be designed for temperatures of around 230° C.-245° C., instead of 550° C., which may result in significant savings. Moreover, when there is no need for piping and flanges between the catalyst bed and the steam generator in the present invention, the process temperature at the exit of the catalyst bed can advantageously be increased, for example to at least 580° C., preferably to at least 590° C. and more preferably to at least 600° C., at reasonable cost. Such an increase may improve the emissions control of the process. Preferably the steam generator produces steam having a pressure of from 10 to 25 barg, more preferably 15 to 25 barg. The steam generator may produce steam having a pressure of from 17 to 20 barg. 
     Preferably the steam condenser is a shell and tube steam condenser, wherein the waste gas stream flows through the tube side of the steam condenser and steam condenses in the shell side of the steam condenser; and the steam generator is a shell and tube steam generator, wherein the combusted waste gas stream flows through the tube side of the steam generator and steam is generated in the shell side of the steam generator. Keeping the waste gas stream and the combusted waste gas stream (that is, the process waste gas streams) on the tube side of the steam condenser and steam generator can have significant advantages for scaling up the emissions control system. In such a system, pressure drop can be maintained when scaling the system up by scaling the numbers of tubes with the capacity demand. That is desirably advantageous over prior art systems where the combusted waste gas stream is on the shell side and the waste gas stream is on the tube side and scale up is more complex. 
     Preferably, before being fed to the steam generator, the combusted waste gas stream is fed through an expander part of a turbocharger to drive a compressor part of the turbocharger in order to pressurise an air stream fed to the process to form part of the feed stream. Using at least some of the energy from the combusted waste gas stream in a turbocharger used to pressurise an air stream fed to the process to form part of the feed stream, and thus to pressurise the feed stream, is advantageously an efficient way to recover as much energy as possible from the combusted waste gas stream. Feeding the combusted waste gas stream to the turbocharger before feeding the combusted waste gas stream to the steam generator may be advantageous in making the best use of the high temperature combusted waste gas stream leaving the catalyst bed. 
     According to a second aspect of the invention there is provided an emissions control system for the catalytic combustion of components of a process waste gas stream, the emissions control system comprising: a catalyst bed comprising a catalyst for the catalytic combustion of the components of the process waste gas stream; and a steam condenser having a tube side in fluid communication with a process waste gas stream inlet and the catalyst bed, and a shell side in fluid communication with a steam inlet and a condensate outlet, such that, in operation, a process waste gas stream entering the process waste gas stream inlet is heated in the steam condenser before passing to the catalyst bed. 
     Preferably the emissions control system comprises a vessel containing both the catalyst bed and the steam condenser. Both the catalyst bed and the steam condenser being in the same vessel may advantageously result in a less expensive apparatus and an apparatus with an advantageously low pressure drop across the emission control system. 
     Preferably the process waste gas stream inlet is in the region of the bottom of the vessel; the tube side of the steam condenser comprises tubes, preferably vertical tubes, having an inlet end lower than an outlet end; the steam inlet is in the region of the top of the shell side of the steam condenser; the condensate outlet is in the region of the bottom of the steam condenser; and the catalyst bed is arranged above the steam condenser, such that, in operation, the process waste gas stream entering the process waste gas stream inlet flows up through the tube side of the steam condenser and up through the catalyst bed, and steam entering the steam inlet flows down through the shell side and condenses to form a condensate, wherein the condensate flows down through the shell side and out through the condensate outlet. Such an apparatus may be particularly efficient to operate and control, for example by controlling the level of condensate in the shell side. 
     Preferably the emissions control system further comprises a steam generator having a tube side in fluid communication with the catalyst bed and a process waste gas stream outlet, and a shell side in fluid communication with a boiler feed water inlet and a steam outlet, such that, in operation, the process waste gas stream leaving the catalyst bed is cooled in the steam generator, converting boiler feed water entering through the boiler feed water inlet into steam exiting through the steam outlet, before exiting the process waste gas stream outlet. The steam outlet may be connected to a plant steam net to export steam to the plant. Preferably the steam outlet is in fluid communication with the steam inlet of the steam condenser, such that, in operation, steam generated in the steam generator is passed to the steam condenser to heat the process waste gas stream entering the process waste gas stream inlet. By providing a steam generator linked to the steam condenser the apparatus can advantageously be used to transfer heat from a combusted process waste gas stream exiting the catalyst bed to an incoming process waste gas stream that is to be fed to the catalyst bed. An advantage of carrying out that heat transfer via a steam generator and a steam condenser is that the pressure drop on the process side of the emissions control system can be kept low whilst still retaining efficient heat transfer through the design of the steam side of the emissions control system. Moreover, extra steam can be added, or steam can be removed, as necessary to balance the required heat transfer. For that reason, it may be that the steam outlet of the steam generator is also in fluid communication with a connector for connecting to a steam network, for example a plant steam network. 
     Preferably the emissions control system comprises a vessel containing the steam condenser, the catalyst bed and the steam generator. Combining all three stages in a single vessel advantageously reduces the cost of the equipment and keeps pressure drops below. Such a combination may also remove the need for high temperature, for example 600° C., flange connections between vessels. The temperature between the catalyst bed and the steam generator may be in the region of 600° C., but if those components are in the same vessel, the only connection needed is that downstream of the steam generator, where the temperature may for example be in the region of 230° C.-245° C. 
     Preferably the steam generator is located above the catalyst bed. In that way, the process waste gas stream flows up through all parts of the emissions control system one after the other, thus avoiding bends or other significant direction changes that may increase pressure drop. 
     Preferably the emissions control system further comprises a turbocharger having a turbine side inlet in fluid communication with the catalyst bed and a turbine side outlet in fluid communication with the tube side of the steam generator such that, in operation, the process waste gas stream leaving the catalyst bed is passed to the tube side of the steam generator via a turbine side of the turbocharger. Thus, the energy in the process waste stream can be used to drive the turbine in the turbocharger to recover some of the energy in the process waste stream. The turbocharger may, for example, be configured to pressurise a stream, for example an air stream, fed to the process, thus reducing the new energy required for pressurising the feed stream. 
     Preferably the emissions control system is for use in a process according to the first aspect of the invention. Desirably, the emissions control system is suitable for retrofitting to existing processes or plants for the production of formaldehyde. Fitting an emissions control system according to the invention may help a plant or process achieve better environmental performance without adversely affecting the pressure drop of the process as a whole. 
     Preferably the emissions control system is used to treat the waste gas stream in the process. 
     It will be appreciated that features described in relation to one aspect of the invention may be equally applicable to other aspects of the invention. For example, features described in relation to a process of the invention for the production of formaldehyde may be equally applicable to an emissions control system of the invention and vice versa. It will also be appreciated that optional features may not apply, and may be excluded from, certain aspects of the invention. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be further described by way of example only with reference to the following figures, of which: 
         FIG. 1  is a diagram of a prior art Formox process for the production of formaldehyde; 
         FIG. 2  is a diagram of a process for the production of formaldehyde according to an embodiment of the present invention; 
         FIG. 3  is an emissions control system according to an embodiment of the invention; 
         FIG. 4  is an emissions control system according to another embodiment of the invention; 
         FIG. 5  is an emissions control system according to another embodiment of the invention; and 
         FIG. 6  is an emissions control system according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In a prior art Formox process  1  for producing formaldehyde in  FIG. 1  a fresh air stream  5  is passed through a pressurisation blower  4  and then mixed with a recirculation stream  22  to form a mixed stream  23  before being fed via a recirculation blower  3  to a vaporiser  10 . In the vaporiser  10 , the mixed stream  23  is mixed with a methanol stream  2  and vaporised using heat from a process stream  24  leaving a reactor  9 . The resulting feed stream  25  is fed to the reactor  9  which, in this embodiment, is an isothermal reactor cooled by vaporisation of a heat transfer fluid  32 . The heat transfer fluid  32  passes to a condenser  8 , where it is condensed and steam  6  generated from boiler feed water  7 , before returning to the reactor  9 . In the reactor  9 , the methanol in the feed stream  25  reacts on an iron/molybdenum oxide catalyst to produce formaldehyde, which exits the reactor  9  in a process stream  24  comprising the formaldehyde and unreacted parts of the feed stream  25 . The process stream  24  passes through the vaporiser  10 , where heat in the process stream  24  is used to vaporise the feed stream  25 , and is fed to an absorber  11 . In the absorber  11 , process water  12  and optionally urea  13  flows down and strips the formaldehyde from the process stream  24  flowing up the absorber  11 . The water  12 , and optionally urea  13 , together with the formaldehyde exits the bottom of the absorber as a product stream  21 . That product stream  21  is typically 55% formalin, if just process water  12  is used, or UFC if urea  13  is used. The remainder of the process stream  24  exits the top of the absorber as a waste gas stream  26 . That waste gas stream  26  is partially recycled as the recirculation stream  22  and the remainder is sent to an emissions control system  16 . In the emissions control system  16 , the waste gas stream  26  is first heated in a pre-heater  14  using energy from the combusted waste gas stream  27  leaving the emissions control system  16  and then combusted in a catalyst bed  15  having a catalyst comprising PPd and PPt to form the combusted waste gas stream  27 . The combusted waste gas stream  27  leaving the catalyst bed  15  has a temperature of around 500° C. to 550° C. and is fed to a steam generator  20 , where the combusted waste gas stream  27  is cooled and boiler feed water  19  is turned into steam  18 , and then fed back to the pre-heater  14  of the emissions control system  16  to heat the incoming waste gas stream  26 . The combusted waste gas stream  27  leaving the pre-heater  16  is sent to a stack  17 . 
     In  FIG. 2  a process according to the invention is presented. A fresh air stream  55  is passed through a pressurisation blower  54  and then mixed with a recirculation stream  72  to form a mixed stream  73  before being fed via a recirculation blower  53  to a vaporiser  60 . In the vaporiser  60 , the mixed stream  73  is mixed with a methanol stream  52  and vaporised using heat from a process stream  74  leaving a reactor  59 . The resulting feed stream  75  is fed to the reactor  59  which, in this embodiment, is an isothermal reactor cooled by vaporisation of a heat transfer fluid  82 . The heat transfer fluid  82  passes to a condenser  58 , where it is condensed and steam  56  generated from boiler feed water  57 , before returning to the reactor  59 . In the reactor  59 , the methanol in the feed stream  75  reacts on an iron/molybdenum oxide catalyst to produce formaldehyde, which exits the reactor  59  in a process stream  74  comprising the formaldehyde and unreacted parts of the feed stream  75 . The process stream  74  passes through the vaporiser  60 , where heat in the process stream  74  is used to vaporise the feed stream  75 , and is fed to an absorber  61 . In the absorber  61 , process water  62  and optionally urea  63  flows down and strips the formaldehyde from the process stream  74  flowing up the absorber  61 . The water  62 , and optionally urea  63 , together with the formaldehyde exits the bottom of the absorber as a product stream  71 . That product stream  71  is typically 55% formalin, if just process water  62  is used, or UFC if urea  63  is used. The remainder of the process stream  74  exits the top of the absorber as a waste gas stream  76 . That waste gas stream  76  is partially recycled as the recirculation stream  72  and the remainder is sent to an emissions control system  66 . In the emissions control system  66 , the waste gas stream  76  is first heated in a steam condenser  79 . The waste gas stream  76  flows in to the bottom of the steam condenser  79  and up through the condenser  79 . Steam  68  entering the steam condenser  79  condenses on the tubes and flows down and out of the steam condenser  79  as condensate  80 . The condensate  80  is collected and re-used. The heated waste gas stream thus created flows from the steam condenser  79  to the catalyst bed  65  having a catalyst comprising PPd and PPt. In the catalyst bed  65  components of the heated waste gas stream such as carbon monoxide, DME and methanol are combusted to form a combusted waste gas stream, which enters a steam generator  70 . In the steam generator  70  the combusted waste gas stream is cooled and boiler feed water  69  is turned into steam  68 . The steam  68  may be 12 barg steam, which coincides with the minimum export steam pressure from a standard plant. The steam  68  raised in the steam generator  70  is fed to the steam condenser  79  to raise the temperature of the incoming waste gas stream  76 . The steam  68  can also be fed to, or supplemented from, the plant steam network  78 . The combusted waste gas stream  77  exiting the steam generator  70  is sent to a stack  67 . The stack  67  temperature depends on the pressure of the steam  68 . For example, with a temperature approach (that is, the temperature difference between the combusted waste gas stream  77  and the steam) of 25° C. a stack  67  temperature of 225° C. corresponds to a steam  68  pressure of 14.6 barg and a stack  67  temperature of 245° C. corresponds to a steam  68  pressure of 22.2 barg. The steam condenser  79 , catalyst bed  65  and steam generator  70  are all contained within a single vessel. The flanges and piping at the vessel exit need to be suitable to handle the stack  67  temperature, which is significantly lower than the 500° C.-550° C. that the connections between the emissions control system  16  and the steam generator  20  in the prior art process  1  of  FIG. 1  need to handle. Advantageously, this may even permit higher process temperatures, for example 600° C., to be used at the exit of the catalyst bed  65 , since, unlike the prior art, there is no need for pipework and flanges at the exit of the catalyst bed  65  when the steam condenser  79 , catalyst bed  65  and steam generator  70  are all contained within a single vessel. 
     During start-up steam from elsewhere in the plant steam network  78 , can be fed to the steam condenser  79 , thus removing the need for a separate electrical heater for the emissions control system  66 . 
     In  FIG. 3  an emissions control system  101  is provided for the catalytic combustion of components of a process waste gas stream  105 . The emissions control system  101  comprises a catalyst bed  111  comprising a catalyst for the catalytic combustion of the components of the process waste gas stream  105 . The catalyst typically comprises PPd and PPt, for example as supplied by Johnson Matthey Formox. A steam condenser  103  has a tube side in fluid communication with a process waste gas stream inlet, where the process waste gas stream  105  is fed to the emissions control system  101 , and with the catalyst bed  111 . The steam condenser  103  has a shell side in fluid communication with a steam inlet fed from a steam stream  112  and a condensate outlet  108 . Downstream of the catalyst bed  111  the emissions control system  101  further comprises a steam generator  102  having a tube side in fluid communication with the catalyst bed  111  and a process waste gas stream outlet  104 , and a shell side in fluid communication with a boiler feed water inlet  118  and a steam outlet  107 . The steam outlet  107  is in fluid communication with the steam inlet stream  112  of the steam condenser  103 . A steam stream  106  connects with the steam outlet  107  and the steam inlet stream  112  so that excess steam can be removed or make-up steam added as required at any particular time. 
     The steam condenser  103 , catalyst bed  111  and steam generator  102  are in a single vessel. The outlet temperature of the vessel is around 225° C.-245° C., which is significantly cooler than the 500° C.-550° C. temperature of the combusted waste gas stream leaving the catalyst bed  111 . By feeding that stream straight from the catalyst bed  111  to the steam generator  102  in the same vessel, the need for high temperature piping and connections is removed. The removal of piping and connections in the high temperature region downstream of the catalyst bed  111  may allow higher process temperature, for example 600° C., to be used at that point in the process. 
     The steam condenser  103  is at the bottom of the vessel, with the catalyst bed  111  above it and the steam generator  102  above that. In operation, the process waste gas stream leaving the catalyst bed  111  is cooled in the steam generator  102  before exiting the process waste gas stream outlet  104  and steam generated in the steam generator  102  is passed to the steam condenser  103  to heat the process waste gas stream  105  entering the process waste gas stream inlet. Chill gas  109 , which might for example be air at ambient temperature, or steam  110  for heating can also be fed to the emissions control system  101  to further control the temperature if required. The process waste gas stream  105  flows upwards through the emissions control system  101 , with steam stream  112  fed to the top of the steam condenser  103  and condensate removed from the condensate outlet  108  at the bottom of the steam condenser  103 . Steam condensing on the outside of the tubes of the steam condenser  103  will thus flow downwards under gravity toward the condensate outlet  108 . The process waste gas stream  105  enters the bottom of the emissions control system  101  and flows in a relatively straight path up through the emissions control system  101 , thus avoiding unnecessary pressure drops. Compression costs may be significant in formaldehyde production and any pressure drops, even in the emissions control system  101 , must be accounted for in the initial compression of the feed gases. Avoiding unnecessary pressure drops may therefore be important for producing a cost-efficient process. 
     In operation, the incoming process waste gas stream  105  is thus heated by the condensing steam in the steam condenser  103  before being combusted in the catalyst bed  111 . The hot combusted waste gas stream leaving the catalyst bed  111  is cooled in the steam generator  102 , generating steam  107  that is in turn used to run the steam condenser  103 . The heat transfer efficiency on the steam side of the steam generator  102  and steam condenser  103  can be optimised without affecting the pressure drop of the process side, unlike in prior art systems where heat is transferred directly between the outgoing combusted waste gas stream and the incoming process waste gas stream. When the steam generated in the steam generator  102  is not sufficient to pre-heat the incoming process waste gas stream  105 , for example during start up, the steam condenser  103  can be fed with steam from another part of the plant via steam stream  106 . That removes the need for a dedicated heater for start-up of the emissions control system  101 , thus saving on capital costs. 
     In  FIG. 4 , an emissions control system  201  is fed with a process waste gas stream  205 . At the upstream end of the emissions control system  201 , which is at the bottom of the vessel in which the emissions control system is contained in  FIG. 4 , there is a steam condenser  203 . The tube side of the steam condenser  203  is in fluid communication with the process waste gas stream  205  and the catalyst bed  211 . The process waste gas stream  205  flows up through the steam condenser  203  and through the catalyst bed  211  where hazardous components of the stream are combusted to form a combusted waste gas stream. 
     Downstream of the catalyst bed  211  there is a steam superheater  217 . Downstream of the steam superheater  217  is a steam generator  202  and an economiser  223 . The shell side of the economiser  223  is fed with boiler feed water  218  and has an outlet stream  216  which connects to a shell side inlet of the steam generator  202 . The shell side of the steam generator  202  has an outlet steam stream  207 , which connects with a steam stream  206  by which steam can either be removed or added as necessary. After the connection, the steam stream splits to a stream  214  that feeds to the steam superheater  217  to create superheated export steam  215  and to a steam stream  212  that is fed to the steam condenser  203 . The combusted waste gas stream leaving the catalyst bed  211  passes through the shell side of the steam superheater  217 , through the tube side of the steam generator  202  and then through the tube side of the economiser  223  before exiting through the combusted gas stream outlet  204 , which is typically fed to a stack. 
     As with the embodiment in  FIG. 3 , the process waste gas stream  205  is heated in the steam condenser  203  before being combusted in the catalyst bed  211  to combust hazardous components and create a combusted waste gas stream. The combusted waste gas stream is then cooled in the steam superheater  217 , steam generator  202  and economiser  223 . The economiser  223  may be replaced with a low-pressure steam generator. The economiser  223  or low-pressure steam generator improve the heat recovery efficiency by making use of the low temperature heat remaining in the combusted waste gas stream after it has passed through the steam generator  202 . Boiler feed water  218  fed to the shell side of the economiser  223  is heated by the cooling of the combusted waste gas stream and fed to the shell side of the steam generator  202  where it is turned into steam. The steam is fed to the steam superheater  217  to create superheated steam  215  for export to other parts of the plant or to the steam condenser  203  to pre-heat the incoming process waste gas stream  205 . Again, as with the embodiment in  FIG. 3 , the emissions control system  201  can be started using steam from elsewhere in the plant via steam stream  206 , thus removing the need for a dedicated start-up heater. Moreover, the heat transfer efficiency on the steam side of the emissions control system  201  can be optimised without affecting the pressure drop of the process side. 
     Again, the emissions control system  201  is contained in a single vessel. That may be advantageous as it reduces the need for inter-vessel connections, and particularly high-temperature inter-vessel connections. That may reduce capital costs and also pressure drops, which may in turn reduce operating costs. Because the steam condenser  203  is at the bottom of the vessel and the process waste gas stream flows up from the steam condenser  203  through the catalyst bed  211 , the support net on which the catalyst bed rests is at the cooler end of the catalyst bed  211 . That may be advantageous since a support net of sufficient strength may be more readily provided when it does not have to withstand the high temperatures at the exit of the catalyst bed  211 . A secondary net may be provided above the catalyst bed  211  to prevent catalyst being carried away in the combusted waste gas stream, but that net does not need to support the full weight of the catalyst bed  211 . 
     In  FIG. 5  an emissions control system  301  comprises a steam condenser  303 , a catalyst bed  311  and a furnace-type steam super heater  319 . The furnace-type steam super heater  319  may be used to generate super-heated steam. Producing super-heated steam in this way may increase the stack temperature as it is not possible to recover low temperature heat in the furnace-type steam super heater  319 . However, it has the advantage of generating super-heated steam, which may be valuable elsewhere on the plant. A process waste gas stream  305  is pre-heated in the steam condenser  303  before passing to the catalyst bed  311  where the hazardous components are combusted to form a combusted waste gas stream. The combusted waste gas stream is fed to the furnace-type steam super heater  319  which generates super-heated steam while cooling the combusted waste gas stream. The cooled combusted waste gas stream exits the furnace-type steam super heater  319  via the outlet  304  and passes to a stack. Super-heated steam raised in the furnace-type steam super heater  319  can be fed to the shell side of the steam condenser  303  to be used in pre-heating the incoming process waste gas stream  305 . In this embodiment, the furnace-type steam super heater  319  is in a different vessel to the vessel containing the steam condenser  303  and the catalyst bed  311 . While there may be advantages, for example in terms of reduced connections and hence reduced pressure drops, by having everything in one vessel, there may be occasions when it is preferable to use more than one vessel, for example due to space constraints when upgrading an existing process. 
     In the emissions control system  401  of  FIG. 6  a catalyst bed  411  is located downstream of, and in this embodiment above, a steam condenser  403 . A process waste gas stream  405  flows up through the tube side of the steam condenser  403  and then up through the catalyst bed  411 . As explained above in relation to other embodiments, flowing the process waste gas stream  405  up through the catalyst bed  411  provides advantages in terms of the temperature conditions to which the support net for the catalyst bed  411  is exposed. The steam condenser  403  is fed with steam from a steam inlet stream  412  near the top of the shell side and condensate exits through a condensate outlet  408  near the bottom of the shell side. Thus, the steam condenses on the tubes and flows down under gravity to the condensate outlet  408 . In doing so it heats the process waste gas stream  405  before it is fed to the catalyst bed  411 . 
     The combusted waste gas stream leaving the catalyst bed  411  is fed to a turbocharger  420 . In the turbocharger  420  the pressure of the combusted waste gas stream is reduced and a feed stream to the process is pressurised. Typically, the combusted waste gas stream passes through the expander part of the turbocharger  420 , and a fresh air feed stream to the process passes through the compressor part of the turbocharger  420 . Compression of process gases may be a significant operating cost in a formaldehyde production process and recovering some of the energy in the combusted waste gas stream as compression of a feed stream may therefore be advantageous. 
     From the turbocharger  420  the combusted waste gas stream passes through the tube side of a steam generator  402 , which is fed with boiler feed water  421  on the shell side to raise steam  422 . The steam thus raised is fed to the steam inlet stream  412 , either with withdrawal or addition of further steam as necessary, and used to pre-heat the incoming process waste gas stream  405 . Thus, the energy in the combusted waste gas stream is used to pre-heat the incoming process waste gas stream  405 , but the heat is transferred indirectly using the steam generator  402  and steam condenser  403 . As discussed above, that has several advantages including the opportunity to reduce pressure drops for the process waste gas stream and to use substitute steam from another part of the plant during start-up, thus removing the need for a dedicated start-up heater for the emissions control system  401 . Including the turbocharger  420  permits the energy in the combusted waste gas stream to be used effectively by using it in the turbocharger  420  while the combusted waste gas stream is at its hottest and then using it to generate steam in the steam generator  402  after it has passed through the turbocharger  420 . 
     The emissions control systems  101 ,  201 ,  301 ,  401  of  FIGS. 3, 4, 5 and 6 , could be used, for example, in the process  51  of  FIG. 2 . 
     It will be appreciated that the embodiments set out above are examples of the invention and that the skilled person would appreciate that variations were possible within the scope of the invention. For example, the steam condenser and steam generator may be in the same or different vessels and the system could be arranged horizontally or with side-by-side vessels. The process waste gas stream may flow down or horizontally through some or all parts of the process.