WASTE GAS EMISSION CONTROL SYSTEM

Abstract

A process for the production of formaldehyde is disclosed. The process comprises 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.

Claims

1. A process for the production of formaldehyde, the process comprising: a. feeding a feed stream comprising methanol to a reactor; b. converting the methanol to formaldehyde in the reactor using a mixed oxide catalyst to produce a process stream comprising formaldehyde; c. separating formaldehyde from the process stream to create a product stream comprising formaldehyde and a waste gas stream; d. 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 e. 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.

2. A process according to claim 1, wherein the steam condenser and the catalyst bed are contained within a single vessel.

3. A process according to claim 1, wherein 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.

4. A process according to claim 1, wherein the process further comprises: f. Feeding the combusted waste gas stream to a steam generator wherein the combusted waste gas stream is cooled and steam is produced.

5. A process according to claim 4, wherein 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.

6. A process according to claim 4, wherein, 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.

7. A process according to claim 4, wherein the process further comprises: g. Feeding steam from the steam generator to the steam condenser to raise the temperature of the waste gas stream in step d.

8. A process according to claim 7, wherein the steam condenser, the catalyst bed and the steam generator are contained within a single vessel.

9. 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.

10. An emissions control system according to claim 9, wherein the emissions control system comprises a vessel containing both the catalyst bed and the steam condenser.

11. An emissions control system according to claim 9 wherein 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.

12. An emissions control system according to claim 11, wherein the emissions control system comprises a vessel containing the steam condenser, the catalyst bed and the steam generator.

13. An emissions control system according to claim 11, wherein the emissions control system further comprises a turbocharger having an expander side inlet in fluid communication with the catalyst bed and an expander 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 an expander side of the turbocharger.

14. An emissions control system according to claim 11, wherein 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.

15. 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, wherein the emissions control system is for use in a process according to claim 1.

16. Use of 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 to treat the waste gas stream in a process according to claim 1.

Description

DESCRIPTION OF THE DRAWINGS

[0033] The invention will be further described by way of example only with reference to the following figures, of which:

[0034] FIG. 1 is a diagram of a prior art Formox process for the production of formaldehyde;

[0035] FIG. 2 is a diagram of a process for the production of formaldehyde according to an embodiment of the present invention;

[0036] FIG. 3 is an emissions control system according to an embodiment of the invention;

[0037] FIG. 4 is an emissions control system according to another embodiment of the invention;

[0038] FIG. 5 is an emissions control system according to another embodiment of the invention; and

[0039] FIG. 6 is an emissions control system according to another embodiment of the invention.

DETAILED DESCRIPTION

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.