GB2277795A

GB2277795A - Method and apparatus for consuming volatiles or solids entrained in a process plant fluid

Info

Publication number: GB2277795A
Application number: GB9408155A
Authority: GB
Inventors: William Bradshaw; Douglas Handley
Original assignee: BRADSHAW HANDLEY
Current assignee: BRADSHAW HANDLEY
Priority date: 1993-04-24
Filing date: 1994-04-25
Publication date: 1994-11-09
Also published as: GB9408155D0; GB9308537D0

Abstract

Volatiles are exhausted in a fluid from an oven. The fluid is passed through the secondary circuit of a heat exchanger and then the volatiles are incinerated. The gaseous product from the incinerator is passed through the primary circuit of the heat exchanger before being used as an energy source for the oven.

Description

METHOD AND APPARATUS FOR CONSUMING VOLATILES OR SOLIDS ENTRAINED IN A PROCESS PLANT FLUID There are many industrial processes in which polluting volatiles or solids are present in fluid leaving the process plant, and where it is environmentally desirable (and in some instances legally prescribed) that the pollutant shall be incinerated. A typical example is the coating of fabrics or paper with decorative film as occurs in the manufacture of wall coverings. The materials which are applied to the fabric or paper are usually carried in a solvent. The solvent may be water, but in some processes it contains pollutants, for example hydrocarbons such as toluene; diamethylformamide; methyl ethyl ketone or White Spirit.Regulations in force in some countries limit the levels at which such materials may be discharged into the atmosphere and current practice is to incinerate these products, leaving the process plant, to reduce/oxidise the pollutant to an acceptable level.

The principle drawback of incineration is the cost of the necessary fuel. Taking the example of the fabric or paper coating process, the vapours leaving the process plant might typically have a temperature in the order of 1500C, but the combustion temperature required for incineration may be approximately 7500C. The solvents present in the vapour stream have a calorific value in themselves, which will contribute to the attainment of the 7500C temperature required for combustion, but that still requires the burning of fuel to add the difference between the combustion temperature and the sum of the input temperature (1500C) and the temperature attributable to the released calorific value of the solvent. A natural gas burner may be employed to make up this difference.

It will be appreciated that the application of materials to fabric or paper is simply an illustration of a process to which the present invention can be applied. In other processes, the process fluid (which may be either gaseous or liquid, or even solid particles entrained in a gas) may have a different combustion temperature, and both the temperature of the fluid leaving the process plant and the calorific value of the fluid may be different, but in most cases, there is an appreciable fuel demand to produce operation of the incinerator at an effective level.

The primary object of the present invention is to improve the thermal efficiency of a process for consuming volatiles or solids entrained in a fluid leaving a process plant by an incineration process. In some instances, it will be possible to achieve substantially auto-thermal conditions dependent on the calorific value of the volatiles or solids being consumed. In most cases, however, it is to be expected that some additional energy input will be required and this may be achieved by providing the incinerator with a gas burner connected to a source of methane, for example. In any event, some form of burner will probably be required for start-up purposes, even when the process becomes auto-thermal after start-up.

According to a first aspect of the invention, a method of operating a process plant which employs a volatile or solid entrained in a fluid in contact with material being processed in a process chamber or region comprises: exhausting the fluid from the region; passing the fluid through the secondary circuit of a heat exchanger and raising the temperature of the fluid in the heat exchanger; burning off the volatile or solid in an incinerator, so as to utilise the potential energy of the volatile or solid in the combustion process; passing the gaseous produce from the incinerator through the primary circuit of the heat exchanger, so as to use some of the heat energy of the gaseous product to raise the temperature of the fluid in the secondary circuit of the heat exchanger, and using the gaseous product from the primary circuit of the heat exchanger as an energy source for the process region. Preferably, the entraining fluid is a gas containing a volatile pollutant.

According to a preferred feature of this aspect of the invention, atmospheric air is mixed with the gaseous product from the primary circuit of the heat exchanger before it is admitted to the process region. It is further preferred that the flow of atmospheric air to be mixed with the gaseous product from the primary circuit of the heat exchanger is controlled in response to sensed temperature in the process region to maintain a predetermined temperature range in the process region by using the atmospheric air to cool the gaseous product.

In one method in accordance with this aspect of the invention, the process region is divided into two or more temperature zones, the gaseous product from the primary circuit of the heat exchanger being divided between the temperature zones, atmospheric air is mixed with the gaseous product flowing into at least one of the temperature zones of the process region and the atmospheric air flow to the said temperature zone is controlled in response to sensed temperature in that temperature zone by using the atmospheric air to cool that part of the gaseous product flowing to that temperature zone.

According to another preferred feature of this aspect of the invention, part of the gaseous product from the incinerator bypasses the heat exchanger and is mixed with the gas from the primary circuit of the heat exchanger before being used as an energy source for the process region.

According to another preferred feature of this aspect of the invention, part of the entraining gas from the process region bypasses the heat exchanger and the incinerator and is mixed with the gaseous product from the primary circuit of the heat exchanger before being returned into the process region to increase the concentration of the volatile in the process region.

According to yet another preferred feature of this aspect of the invention, the rate of flow of the entraining gas to the secondary circuit of the heat exchanger is regulated by flow rate controlled valve means, any excess flow of gas being diverted back into the process region.

It is further preferred that atmospheric air is admitted into the combustion region of the incinerator, to limit the temperature of the combustion gases. The rate of flow of air into the combustion region may be regulated in response to detected temperature in the combustion region.

According to a still further feature of this aspect of the invention, some product gas leaving the incinerator is mixed with atmospheric air, to control its temperature and then mixed with the product gas leaving the primary circuit of the heat exchanger.

The operating temperature of the heat exchanger may be controlled by a water spray.

According to a second aspect of this invention, apparatus for use with a process plant to at least partially neutralise a pollutant entrained in a process fluid emitted from a process region comprises: a heat exchanger comprising primary and secondary circuits; an incinerator, and means for exhausting the fluid from the region and passing it successively through the secondary circuit of the heat exchanger, the incinerator, the primary circuit of the heat exchanger and to the process region.

Preferably there are means for admitting atmospheric air to the process fluid before it is admitted to the process region.

According to another preferred feature of this aspect of the invention, control means are provided regulating the flow of atmospheric air for mixing with the process fluid from the primary circuit of the heat exchanger in response to detected temperature in the process region.

According to another preferred feature of this aspect of the invention, the process region is divided into two or more temperature zones and conduits are provided dividing the fluid from the primary circuit of the heat exchanger between the temperature zones of the process region, at least one of those conduits being provided with means for admitting atmospheric air and temperature control apparatus adapted to control the flow of atmospheric air into this conduit in response to detected temperature in the temperature zone corresponding to that conduit.

According to yet another preferred feature of this aspect of the invention, a bypass conduit leads from the incinerator direct to the return conduit to the process region, so that some of the gaseous product from the incinerator can bypass the primary circuit of the heat exchanger.

In one form of the invention, a concentration bypass is provided from a conduit leading from the process region to the secondary circuit of the heat exchanger, the concentration bypass returning directly to the process region.

According to yet another feature of this aspect of the invention, a flow rate control means is provided for controlling the flow of fluid from the process region to the secondary circuit of the heat exchanger in response to detected flow rate in the secondary circuit and means are provided for returning excess fluid directly to the process region.

Means may be provided for admitting atmospheric air into the combustion region of the incinerator. In that case, there may also be means for regulating the flow of air into the combustion region in response to detected temperature in that region.

According to yet another feature of this aspect of the invention, there is provided a mixing region and conduit means connecting the combustion region of the incinerator with the mixing region and second conduit means leading from the mixing region to the process region.

Spray means may be provided for directing cooling fluid onto the heat exchanger.

The present invention includes any combination of features or limitations herein referred to.

The invention will be more clearly understood from the following description of various embodiments of the invention, which are given here by way of examples only, and with reference to the accompanying drawings, in which: - Figure 1 is a diagram illustrating the basic principle of the invention, Figure 2 is a diagram illustrating one form of the invention, Figure 3 is a diagram similar to Figure 2, but showing an arrangement employing an existing incinerator, Figure 4 is a diagram showing an arrangement for supplying clean gases from the process to a plurality of units of a process plant, Figure 5 is a diagram similar to Figure 2, but showing a more complex arrangement, including zonal temperature control of a process plant, Figure 6 is a diagram similar to Figure 5, but showing the use of an existing incinerator, Figure 7 is a diagram similar to Figure 2, but showing an arrangement for volatile concentration, Figure 8 is a diagrammatic arrangement of a clean gas cooler, Figure 9 is a diagram similar to Figure 2, but showing a more sophisticated combustion system, and Figure 10 is a diagram showing a modification including two oven units each containing separate ovens.

Referring to Figure 1, there is represented at 20 a drying oven, which in this example is used in a process for the application of a decorative film to paper in the manufacture of wallpaper. In this example, the material which forms the decorative film is applied in a solvent such as toluene. The oven is heated by a thermal oil heating battery with oil at a temperature in the range 24 OOC to 300 C. Heating the oven causes the volatile solvent to be given off as vapour into the gas in the oven and the process can be controlled so that a known level of volatiles is discharged with the waste gases from the oven. Fresh air is supplied to the drying oven 20 at 22 to provide a calculated amount of oxygen for a burning off process, and in a specific example, this air is at 200C and is supplied at a rate of 3,000 Kg per hour (Kg/h).It has a heat energy content of 60,000 kilojoules per hour (Kj/h). There is a gaseous discharge from the oven at 24, in the form of waste oven gas which now contains toluene pollutant picked up from the product in the oven. This gas is emitted at 18,000 Kg/h but now has an energy potential of 2,983,673 Kj/h and is at 1620C. The energy potential is provided by the temperature of the oven gas and the calorific value of the entrained toluene. Figure 1 also shows that there is a heat loss to the product and to the environment, illustrated diagrammatically at 26, which is at the rate of 1,084,927 Kj/h.

The problem which is addressed by the invention is that of preventing discharge of the toxic oven gas containing toluene into the atmosphere. It is to be understood, however, that this is purely exemplary. The invention is capable of application to any industrial process in which there is an emission of a fluid (usually a gas, but conceivably a liquid) in which there is entrained volatile, or even solid, particulate matter, which must not be released into the atmosphere without at least a measure of combustion/oxidation. In most instances, the noxious matter will be a volatile hydrocarbon such as toluene; diamethylformamide; methyl ethyl ketone or White Spirit and the entraining gas will be air, but there are many other possibilities.

The oven is maintained at a negative pressure to ensure that there is no leakage of oven gas into the atmosphere.

Essentially, the process comprises incinerating the oven gases in an incinerator 28, so as to consume (oxidise) the pollutant volatiles, or at least reduce them to an acceptable level in the gas, before discharging them to atmosphere at 30. However, Figure 1 also shows that some of the "clean" gas from the incinerator 28 can be returned to the oven 20, that is to say there is some recycling of the gas originally introduced at 22 as clean air.

Specifically the clean gas can be fed across a secondary thermal oil heat exchanger (not shown) which is then used to supply process heat to the oven. In some instances this may be sufficient to provide all the process heat so that there is no fuel consumption, all the process energy being derived from the combustible volatiles. Any excess energy can be used to provide process steam.

A major problem with the incinerating process is that of the energy required to raise the oven gas to the temperature at which the volatiles can be consumed. In the specific instance quoted above, the oven gases leave the oven at 1620C, but the required combustion temperature in the incinerator is 750"C. There is in fact calorific value in the volatiles themselves which will supply some of the temperature rise from that of the gases leaving the oven to the combustion temperature, but there will always be a shortfall which can only be made up by heat energy, shown in Figure 1 as a methane gas supply 32.It is the cost of this added energy which makes incineration of the waste gases unattractive, although sometimes legal regulations require incineration and therefore the cost of the additional energy becomes a significant proportion of the costs of the process.

In Figure 1, there is shown a heat exchanger 34 which takes the form of a tube and shell heat exchanger, arranged so that the oven gases are passed through the tube side between exiting the oven 20 and entering the incinerator 20, and the gaseous products of combustion in the incinerator (flue gas) are taken in the opposite direction through the shell side of the heat exchanger on their way to atmosphere or to the oven 20. A fan 36 is provided to draw the flue gas from the incinerator through the heat exchanger. In the heat exchanger, heat is transferred from the flue gas to the oven gas, thus raising the temperature of the oven gas and thereby reducing the added energy input required at the incinerator 28. Again, in the specific example, oven gases entering the heat exchanger at 1620C may be raised to say 480 OC before entering the incinerator. When the temperature equivalent of the calorific value of the volatiles is added, comparatively little energy need be added at 32 and in some instances, the added energy requirement can be reduced to zero, i.e. the incineration process becomes auto-thermal. It has already been proposed to provide a heat exchanger between the oven (or other process plant) and an incinerator, but the problem has been to achieve a viable heat transfer without such a large pressure drop in the gas flow that the power requirements of the fan 36 become excessive. (It will be appreciated that there is little point in saving added energy requirement at 32, if this can only be achieved by another - possibly greater - energy requirement at the fan 36.) The present invention, however, provides a system for consuming the volatiles, and using the products of combustion in such a manner as greatly to increase the thermal efficiency of the system.

In Figure 2, there is illustrated a heat exchanger 40, comprising two banks 48 and 50 of tubes 42, and the tubes 42 of each bank are arranged in a shell. As the construction of each bank 48 and 50 is largely conventional, it is unnecessary to describe it in detail.

However, it is to be noted that the tubes are straight and parallel with each other. The heat exchanger is preferably constructed as described and claimed in the specification of our co-pending Patent Application No. GB 93 08534.8, but for present purposes, the precise construction of the heat exchanger is immaterial.

However, it is important to note that the tubes of the two banks 48 and 50 are connected at the top end of the heat exchanger, and at the bottom end the tubes 48 open into an inlet plenum 70, into which the waste gases from the oven 20 are led. Hence, the waste gases pass upwardly through the tubes 48, and downwardly through the tubes 50, which are open at the bottom end, into the incineration zone 72 of the incinerator 28. This constitutes the secondary circuit of the heat exchanger. There is an opening from the incineration zone of the incinerator 28 into the shell of the heat exchanger 40 near to the lower end of the bank 50 of tubes, and there is a passage connecting the upper end of the shell surrounding the tubes 50 with a similar passage surrounding the tubes 48, and an outlet 55 from a position near to the lower end of that second passage.

Hence, it is possible for gases from the incinerator 28 to flow upwardly through the shell around the tubes 50, and then downwardly through the shell around the tubes 48, before passing out through the exit 55, which is connected by conduit means not shown to the oven 20. This constitutes the primary circuit of the heat exchanger.

Figure 2 shows that the oven gas is taken from the oven into the inlet ends of the tubes in the stack 48 and the gas from the lower (outlet) end of the tubes in the stack 50 at 70, are directed into the incinerator 28 at 72. The oven gases in a specific instance, which contain toluene or other solvent, are at 500C where they enter the heat exchanger at 70 but in passing through the heat exchanger, they are raised to a temperature of 6500C. The calorific value of the solvent in the oven gas is more than adequate to supply the energy required to bring the gas escaping into the incinerator up to the 75 OOC required for combustion.

A gas burner 74 is provided in the incinerator and is connected to a supply of methane. This gas burner is used to start up the incinerator and would be used continuously if the heat added to the oven gas in the heat exchanger 40, plus the released calorific value of the toluene or other solvent, is not adequate to bring the temperature of the gas in the incinerator up to 7500C - the ignition temperature. Consequently, one part of the energy required to operate the system is that of the methane added at 74.

The gas leaving the heat exchanger at 55 flows through the fan 36 from whence it may be directed via a vent 30 to atmosphere or some or all of it may be re-directed into the oven 20 via a pipe 84 (Figure 1). It is the fan 36 which provides the suction to pull the gases from the oven 20 through the secondary (tube) side of the heat exchanger 40 and then through the incinerator 28. Therefore, the power requirements of the fan are the only energy requirement of the incineration process, other than that of the gas burner 74. (In the case where the calorific value of the volatile(s) enables the system to be autothermal, the power requirements of the fan 36 are the total energy requirements of the incineration system.) Obviously, therefore, it is desirable to reduce the pressure drop in the heat exchanger 40 due to friction in the tubes and shell to as low a figure as possible.

Figure 3 illustrates an arrangement which is the same as that shown in Figure 2, excepting that the heat exchanger 40 is not combined with an incinerator, but is connected by flow and return pipes 100 and 102 to an existing incinerator (not shown). The flow pipe 100 carries the gases which have passed through the tubes 48 and 50 of the heat exchanger at an elevated temperature to the incinerator. The process has to be controlled, to ensure that the temperature of the gases leaving the heat exchanger is low enough to prevent explosion, but sufficiently high to sustain combustion in the incinerator. The incinerator will be provided with a burner, but this need only be used for start-up purposes, so that the fuel consumption of the incinerator is negligible.Another way of stating the temperature requirement of the gases entering the incinerator is that the volatile energy content (calorific value) of the vapours from the heat exchanger must be sufficient to raise the temperature of the gas entering the incinerator above ignition temperature.

Preferably a flame arrester is provided at the inlet to the incinerator to prevent the gases burning in the flow pipe 100. The return pipe 102 carries the flue gases from the incinerator back to the shell part of the heat exchanger 40 (primary circuit).

Turning now to Figure 4, there is illustrated diagrammatically a process plant oven 110 which is divided into a series of different temperature zones 112; 114 and 116. It is to be understood that the term "oven" is used herein in a broad sense and is intended to include a variety of process plant in which heat is employed. It is an important feature of the oven 110, that the stages of the process carried out in it require different operating temperatures. By way of example, in a paper printing process, the required temperatures in the first two zones may be: Zone 112: 2000C Zone 114: 178"C.

A hot gas supply pipe 118 leads from the outlet 55 of a heat exchanger such as that shown in Figure 2 or Figure 3, and there are branch pipes 120 and 122 leading from the pipe 118 into, respectively, the zones 112 and 114.

Again, in the specific example, the temperature of the clean gas leaving the heat exchanger 40 is at approximately 280"C. It is important that this temperature is equal to, or preferably significantly higher than, the desired operating temperature in the zone 112 of the oven.

Now if the hot gas from the heat exchanger were directed into the zones 112 and 114, without regulation, the temperature in these zones would be the same and this would not give the required differential temperature operating conditions.

A temperature regulation system for the oven zone 112 comprises a thermocouple or other temperature sensor 124, sensing the temperature in zone 112; a temperature control device 126 and an atmospheric air inlet 128. The control device 126 essentially comprises a flow regulation valve acting on the air inlet 128, the output from the control device 126 being led into the branch pipe 120 immediately in advance of the position where the pipe 120 enters the zone 112. The control device receives an input signal from the thermocouple 124 and in response to that signal, adjusts the valve in the control device 126 to control the flow of (relatively cool) atmospheric air into the gas stream in the branch pipe 120 and the control is such that the resulting flow of mixed hot gas and air into the zone 112 is at a temperature to produce the required operating temperature in the zone 112.

There is a similar temperature regulation system for the oven zone 114, comprising: a thermocouple 130; a temperature control device 132 and an atmospheric air inlet 134. The setting of the control device is such that it admits atmospheric air so as to reduce the temperature of the gases flowing into the zone 114 to give the desired operating temperature of 178"C. It will be appreciated that similar temperature regulation systems could be provided on other zones of the oven, and that, if necessary, a gas vent to atmosphere can be provided to discharge any hot gas surplus to that required to produce the various temperature zones in the oven.Since the hot gas from the heat exchanger is itself gas from which the volatiles have been purged in the incinerator, and the input at the air inlet 128 is ordinary atmospheric air, no pollution problems should arise from any such venting.

If, on the other hand, the total hot gas available from the heat exchanger is insufficient to provide all the heat required for the oven, then additional gas burners may be provided in the oven, or the gas burner in the incinerator may be operated to raise the temperature of the gas flowing through the secondary circuit of the heat exchanger, and hence into the pipe 118. However, the use of the cleaned gases from the heat exchanger in the oven greatly improves the overall thermal efficiency of the process plant, whether or not total auto-thermal conditions are achieved.

Although in Figure 4 there is shown a process oven having three zones 112, 114 and 116, it is to be understood that the temperature regulation system could be employed on a process oven with any number of temperature zones, including a basic form in which there is only a single temperature zone.

In Figure 5, there is shown a more developed example of the application of the invention to a continuous process plant, such as the paper coating plant using toluene in the process, to which reference has been made previously.

The process chamber comprises a long oven indicated diagrammatically at 140, a shell and tube-type heat exchanger 142 of the same kind as that described with reference to Figure 2, and an incinerator 144, which is connected to the heat exchanger and which is provided with a gas burner 146.

An oven flue 148 leads exhaust gas, containing the volatile toluene, from the oven 148 to the secondary circuit (tubes) of the heat exchanger 142. In a specific instance, the exhaust gas leading the oven is at 500C, but the temperature of the exhaust gas is raised in the heat exchanger, so that it enters the incinerator at 650"C. The incinerator is operated with or without use of its gas burner, but in the specific instance where the volatile is toluene and the exhaust gas enters the incinerator at 6500C, it should be possible, once the system has been started, to operate without consumption of fuel gas, i.e.

the system in the heat exchanger and incinerator is autothermal.

Some of the incinerator gaseous product of combustion essentially clean air - is taken through the primary circuit (shell) of the heat exchanger as previously described. From the shell, the gaseous product of combustion passes through a conduit 150 into a mixing chamber 152. The gas flowing into the mixing chamber 152 through the conduit 150 is, in the specific instance, at 1500C, since most of its heat energy has been given up to the fluid in the primary circuit.

A bypass conduit 154 leads directly from the incinerator 144 into the mixing chamber 152, where it is mixed with the gas from the heat exchanger. The bypass flow is regulated either by the original design of the bypass or by valve means (not shown), so that the temperature of the mixed gas is 2800C. This is the temperature required to operate the oven 140, and from the mixing chamber 152, the hot clean air flows via a conduit 156 into the oven.

For some processes, all the gas from the mixing chamber 152 can be taken straight into the oven. This would apply if the process were such that there was only a single oven space and any temperature gradient required could be obtained simply by supplying the hot gas at one end and exhausting it at the other end. However, Figure 5 illustrates a process in which the oven 140 has a plurality of temperature zones, of which three are specifically indicated at 158, 160 and 162. The operating temperature required in each zone is: Zone 158: 2400C Zone 160: 2500C Zone 162: 1780C.

The conduit 156 leads into an inlet manifold 164, extending alongside the three temperature zones, and each of these zones is fitted with a regulation device 166 of the kind described with reference to Figure 6, and comprising a thermocouple detecting the temperature in the oven zone, an air inlet and a regulator valve. Hence, there are three separate atmospheric air inlets. The operation of the regulation devices needs no further description. It will be noted that in this specific example, the temperature rises in the oven towards the exhaust 148.

In many instances, use of the system shown in Figure 5, employing the bypass 154 will enable the heat required for the process in the oven to be obtained entirely from the gases from the incinerator, so that the total process becomes auto-thermal, the heat energy being derived entirely from the volatile(s) in the process fluid.

There are no fans illustrated in Figure 7 for pumping the exhaust gas and clean air round the system. It is to be understood, however, that one or more fans may be provided for this purpose, and this applies also to the system shown in Figure 8. It has been found advantageous to employ two fans, one a blower between the oven and the secondary side of the heat exchanger and the other a suction fan between the primary side of the heat exchanger and the oven, the required pressure drop being divided between the two fans. However, the choice of one or two fans is essentially a question of economics.

The process plant illustrated in Figure 6 is identical with that shown in Figure 5, excepting that in this arrangement, the incinerator 170 is separate from the heat exchanger 142. Therefore, like components have been given the same reference numerals as in Figure 5.

In this specific example, although the temperature of the gas leaving the primary circuit of the heat exchanger 142 in the conduit 150 is 1500C, sufficient hot gas is drawn through the bypass 154 to produce a gas mixture at 325"C in the mixer 152. Therefore, the highest temperature zone in the oven can have an operating temperature just below the 3250C and in the specific instance illustrated, the temperature in Zone 158 is 3000C, in Zone 160 it is 205 C and in Zone 162 it is 178or.

Figure 7 shows another configuration in which the process plant comprises two drying ovens 170 and 172, which are employed to dry paper or textile fabric to which has been applied a coating or colouring. The process employs White Spirit or similar hydrocarbon solvent, and the ovens are heated by gas at approximately 50"C. As a result, the gas exhaust from the ovens contains White Spirit in suspension. In the following description, specific values are quoted, but it is to be understood that these are given to assist in an understanding of the invention, and that they are only exemplary.

Exhaust gas from the oven 170 is drawn off at an exit 174 by a power-operated fan 176. A damper 178 is provided in the exit 174 to provide in effect a non-return valve in the exit. A similar exit 180 from the second oven 172 leads into the fan 176 and has its own damper 182.

From the fan 176 a conduit 184 leads to the input end of a tube and shell heat exchanger 186. The latter is illustrated as a single pass heat exchanger, but this purely diagrammatic; in practice, it may be a double pass heat exchanger, as shown in Figures 3, 5 and 6. The heat exchanger feeds into an incinerator 188, designed to incinerate the volatile White Spirit in the gas flowing through the secondary circuit of the heat exchanger. The operation of the heat exchanger and incinerator is the same as described with reference to, for example, Figure 3.Specifically, the exhaust gases from the ovens 170 and 172 are pulled through the fan 176 and enter the tubes of the heat exchanger at 47.5"C. These gases leave the heat exchanger and enter the incinerator at a temperature of 568"C. Within the incinerator, the gases are burnt and the volatile White Spirit provides the heat energy (calorific value) for combustion. (If necessary, additional energy could be supplied by a burner 190.) The products of combustion gas, essentially clean air, leaves the incinerator at 7500C and passes through the primary circuit, shell, of the heat exchanger 186, leaving at 2450C at the exit 192.

The exit 192 leads into a second power-operated fan 194, which forces the product gas along a conduit 196, which also includes a damper 198 providing another non-return valve. The conduit 198 branches into a conduit 200 with a damper 202 and a gas vent 204. Therefore, product gas which cannot pass the damper 202 because of back pressure in the conduit 200 will vent to atmosphere at 204, but this should not pose an environmental problem, because the toxic products will have been consumed in the incinerator.

The conduit 200 itself leads into an inlet conduit 206, which leads into the first oven 170. The conduit 206 may also lead into the second oven, or alternatively, the first and second ovens may be arranged in series so that some of the gases can flow from the first oven into the second oven. A fresh air inlet 208 leads into the conduit 206 and this is regulated to give a predetermined fixed volume per unit of time (flow rate) of atmospheric air in the ovens. (The air supply to the two ovens may be through a single entry or through separate entries.) A bypass conduit 210 branches off the conduit 184 downstream of the first fan 176 and leads into the conduit 200 downstream of the damper 202. The bypass conduit has its own damper 212.Hence, gas flowing along the bypass conduit does not flow through the heat exchanger or through the incinerator and therefore contains unburned White Spirit in suspension. It is to be noted that this gas cannot flow out at the vent 204 because of the nonreturn effect of the damper 202. This bypass arrangement provides a means whereby some of the exhaust gases from the ovens 170 and 172 can be forced directly back into those ovens without passing through the heat exchanger and incinerator.

In a specific instance, from each of the ovens 170 and 172, there is drawn off 11,095 Kg/h of gas (essentially air), this gas containing in suspension 47.5 Kg/h of White Spirit and the gas being at a temperature of 470C. From the fan 176, there flows from each of the ovens 170 and 172, through the conduit 184 into the secondary circuit of the heat exchanger 186, 2,774 Kg/h of gas containing 11.88 Kg/h of White Spirit, again at 47.50C. However, from each of the ovens 170 and 172, the fan 176 causes to flow through the bypass 210 some 8,321 Kg/h of air containing 35.6 Kg/h of White Spirit in suspension. The product of incineration gas (essentially clean air) leaving the heat exchanger in the conduit 192 is at 2450C, and the fan 194 causes the whole of the 2,774 Kg/h of incinerated gas to flow through the conduit 196.

The system is designed so that in normal operating conditions, there is no flow past the damper 202, and as a result, the whole of the 2,774 Kg/h of incinerated gas from the fan 194 is vented at 204. Consequently, the 8,321 Kg/h of gas leaving the fan 176 and flowing through the bypass 210 and containing 35.6 Kg/h of White Spirit flows back into the oven 170. 2,774 Kg/h of fresh air flows in at 208, mixing with the air and White Spirit flowing in the conduit 200, before entering the oven 170.

Under certain operating conditions, the damper 202 will open to allow some of the clean hot gas from the conduit 196 to flow through the conduit 202, and to mix with the gas containing White Spirit flowing in the conduit 202.

This will be compensated for automatically by a reduction in the fresh air input at 208.

Now, since the flow rate of atmospheric air into the oven is maintained at a constant, and fresh White Spirit will be entering the ovens on the product at a known rate, the return of some White Spirit via the bypass 210 into the.

oven has the effect of concentrating the proportion of volatiles in the oven gases leaving at 174 and being pumped into the heat exchanger 186. This is important, particularly if there is a low level of volatiles present in the oven gases without the concentration effect. Use of the bypass system means that, without increasing the quantity of volatiles on the product, it is possible to increase the percentage of volatiles in the gas flowing to the heat exchanger to a level which will support combustion in the incinerator.

The system shown in Figure 9, therefore, provides a means of controlling the volatile concentration in the gases going to the heat exchanger, and it is therefore possible to design the system, by control of the dampers for example, to attain a preferred increase in temperature of the gases in the tube side of the heat exchanger.

All the specific embodiments so far described cause the waste gases from the process plant to be heated in the heat exchanger to a temperature at which combustion can be sustained in the incinerator with a minimum of added fuel gas. In some instances, however, the conditions may be such that the temperature attained in the tubes of the heat exchanger would be too high, i.e. above the lower explosion level of the gases. To avoid this, a water cooling system can be employed, as shown in Figure 8.

The heat exchanger, which may be single or double backed, is indicated at 220. Waste gases from the process plant enter at 222 and flow out through the flame barrier 226 to an outlet 274 into the incinerator. The cleaned air (product of combustion) from the incinerator enters the shell of the heat exchanger at 228 and flows out to return to the process plant or to a low temperature discharge (vent) at 230. In a plenum 232 where the waste gases flow into the tubes of the heat exchanger, there is a water spray device 234, having its nozzles directed down onto the stack or stacks of tubes. This device 234 is supplied with mains water normally at about 130C to 160C. A temperature sensing device (not shown) senses the temperature of the gas leaving the primary circuit of the heat exchanger and is arranged to control the water supply to the spray device 234 through a valve (not shown).The control is so arranged that the water supply is turned on when the temperature of the waste gases exceeds a predetermined threshold value, and this causes the water to fall on to the tubes, cooling the gas in those tubes.

The threshold value is set such that the temperature of the gases in the tubes cannot attain the lower explosion level.

The arrangement shown in Figure 8 can also be used for removing incrustation on the inside of the tubes. The heat exchanger is arranged so that it is possible to cause hot gases from the incinerator to flow up the insides of the tubes for short periods to remove carbonaceous deposits. If required the water from the sprays 234 can be directed into the tubes to clean out waste solid matter.

A more sophisticated system for consuming volatiles is shown in Figure 9. The process plant is shown diagrammatically as an oven 250. It will be appreciated that the process chamber represented by the oven 250 may be a simple single chamber or a more complex arrangement with a plurality of temperature zones, as described for example with reference to Figure 4. Also, heat may be supplied to the oven 250 from any heat source, such as an oil or gas burner, and the gas generated in the oven 250 may contain any combustible volatile such as toluene, White Spirit or other hydrocarbon.

The system includes the heat exchanger 252 combined with an incinerator 254, having a gas burner 256. Waste gas from the oven 250 and containing the volatile is taken via a conduit 258 into the secondary circuit (tubes) of the heat exchanger; the gas heated in the heat exchanger passes into the incinerator 254, where the volatile(s) is burnt off (using the burner 256 as required); the gas from the incinerator (essentially clean air) flows through the primary circuit (shell) of the heat exchanger 250 and exits via a return conduit 260. The return conduit 260 leads back into the process oven 250, although there is a vent to atmosphere at 262, and there is a damper system in the conduit 260 which ensures that the oven 250 takes a sufficiency of gas from the conduit 260, but when the back pressure in the oven exceeds a threshold, gas can exit at the vent 262.There is no environmental problem with the venting of gas at 262, because this gas has had the volatile(s) burnt off in the incinerator. Thus, the main loop through the system passes through the conduit 258; heat exchanger tubes; incinerator; heat exchanger shell; conduit 260 back to the oven 250. The flow of gas through the system is maintained by a power-driven fan 264 in the conduit 258, and by a power-driven extractor fan 294 in the return conduit 260.

If the lower explosion level of the waste gases is low, then the efficiency of the system is improved by regulating the flow rate into the heat exchanger, to ensure that only a predetermined volume of gas can be presented to the heat exchanger during any increment of time. For this purpose, a flow indication control unit 266 is provided, operating on the conduit 258 between the fan 264 and the heat exchanger. The unit 266 includes a flow rate detector and a diaphragm-operated valve in the conduit. The detector is arranged to control the valve, so as to restrict the flow of gas into the tubes of the heat exchanger to a preset, safe level.A bypass conduit 268 leads from the upstream side of the flow indication control unit 266 back into the oven 250 and this conduit is provided with a non-return damper, which is normally closed, but which permits flow of gas from the conduit 258 into the oven when the control unit 266 is restricts flow in the conduit 258.

A further temperature control arrangement shown in Figure 9 comprises an atmospheric air inlet and power-driven fan 270 and a temperature indication control unit 272. The latter unit comprises a temperature sensor (e.g. a thermocouple) located in the incinerator and a diaphragmcontrolled valve, arranged to control flow of air from the fan 270 into the incinerator. The unit 272 is preset, so that the valve is opened in response to increase in temperature in the incinerator so as to admit a sufficient flow of air into the incineration chamber to prevent excessive temperatures arising in that chamber. The unit 272 may be set to admit air whenever the temperature in the incinerator rises above, say, 750"C.

Gas from the incinerator can flow via a conduit 274 into a mixing chamber 276. Gases can leave the mixing chamber 276 via a conduit 278, which leads into the return conduit 260 and which is provided with a vent 280 and with a damper system set so that gas is only vented to atmosphere at 280 when the back pressure in the oven prevents further gas flow into the oven. It will be appreciated that the gases in the incinerator are expanding and attempting to flow out of the incinerator.However, a further air supply is available at 282 and there is a power-driven fan 290 for driving this air supply into the mixing chamber 276, but this is regulated by a temperature indication unit 292, comprising a temperature sensor detecting the temperature in the conduit 278 (i.e. the temperature of the gas leaving the mixing chamber) and a diaphragmcontrol led valve operating to control the flow from the fan 290 into the mixing chamber. As air is blown into the mixing chamber 276 under pressure, there is a venturi effect, which tends to draw gas into the mixing chamber from the incinerator. This assists in the mixing process.

Hence, it is possible to regulate the temperature of any gas flow from the mixing chamber into the return conduit 260. In the return conduit 260, in addition to the extractor fan 294 previously mentioned, there is a flow indication control unit 296 which is similar to the unit 266 previously described. The system includes necessary dampers and non-return valves, which have not been illustrated owing to the diagrammatic nature of the drawings.

When the apparatus shown in Figure 9 is in use, the effect of withdrawing hot gas from the incinerator and replacing it with cold air from the supply 270 is to cool down the shell side of the heat exchanger 252. This reduces the actual heat transfer quite drastically, as is necessary for instance if the temperature of the gas in the tubes in the heat exchanger is approaching the oxidation temperature of the tube metal. Obviously, this has the effect of reducing the contribution of the incinerated gases to the temperature of the exhaust gases, where the latter enter the incinerator, and in theory, this contribution could be reduced to zero, if necessary.

However, the heat from the incinerator would still be available to the oven via the tube 278.

In all the specific examples described with reference to the drawings, the fluid leaving the process plant has been gaseous, carrying volatile pollutants in vapour or gaseous phases. It is to be understood, however, that the process fluid could in some instances be a liquid, from which the pollutant could be removed by incineration, or indeed a gas (including air) in which particulate solid pollutant material, which can be reduced/oxidised by an incineration procedure, is entrained. Obviously, in the case of a liquid, it is necessary that the pollutant shall be sufficiently volatile to be given up under heat treatment.

In Figure 10 two oven units 300 and 301 each include separate ovens 302 and 303.

The hot gas from the heat exchanger 304 is fed through a conduit 305 via a fan 306. The conduit 306 leads to conduits 307 and 308 which each have conduits 309 leading to the individual ovens 302 or 303. A venturi opening 310 is included on each hot gas supply conduit 309 to the ovens through which air is pulled into the stream and then fed to the ovens. If desired the openings 310 may include a valve which can be regulated to determine the amount of air that is drawn into the conduit 309.

A temperature sensor 311 is provided downstream of the heat exchanger just before the fan. If desired the sensor 311 could be provided on the outlet of the incinerator.

If one or more ovens shuts down and is no longer producing volatiles this can be sensed and the oven can be isolated.

The temperature sensor detects the drop in temperature and can vary the flow rate induced by the fan and hence the energy input from the fan to ensure that the remaining ovens remain operating at an efficient level. The temperature sensor can also allow for adjustment in variation in the rate that solvents are being produced by particular ovens.

The ovens are each at a slightly negative pressure to prevent volatiles from escaping.

The plant shown in Figure 10 can be modified to include any of the features shown in the other figures and visa versa.

Claims

CLAINS

1. A method of operating a process plant which employs a volatile or solid entrained in a fluid in contact with the material being processed in a process region, which comprises, exhausting the fluid from the region; passing the fluid through the secondary circuit of a heat exchanger and raising the temperature of the fluid in the heat exchanger; burning off at least some of the volatile or solid in an incinerator, so as to utilise the potential energy of the volatile or solid in the combustion process; passing the gaseous product from the incinerator through the primary circuit of the heat exchanger, so as to use some of the heat energy of the gaseous product to raise the temperature of the fluid in the secondary circuit of the heat exchanger and using the gaseous product from the primary circuit of the heat exchanger as an energy source for the process region.

2. A method of operating a power process plant as claimed in claim 1, in which the entraining fluid is a gas containing a volatile pollutant.

3. A method of operating a process plant as claimed in claim 2, in which atmospheric air is mixed with the gaseous product from the primary circuit of the heat exchanger before it is admitted to the process region.

4. A method of operating a process plant as claimed in Claim 2 in which atmospheric air is mixed with the gaseous product from the primary circuit of the heat exchanger by a venturi effect.

5. A method of operating a process plant as claimed in claim 3, in which the flow of atmospheric air to be mixed with the gaseous product from the primary circuit of the heat exchanger is controlled in response to sensed temperature in the process region to maintain a predetermined temperature range in the process region by using the atmospheric air to cool the gaseous product.

6. A method of operating a process plant as claimed in any preceding claim in which the temperature downstream of the input to the incinerator and upstream of the process region is monitored in order to provide a control of the effective heat being supplied to the process region.

7. A method of operating a process plant as claimed in claim 6 including a drive means arranged to assist in conveying the gaseous product from the incinerator to the process region, the drive means being variable whereby the assistance provided by the drive means is varied in accordance with the temperature sensed.

8. A method of operating a process plant as claimed in any of claims 4 to 7, in which the process region is divided into two or more temperature zones, the gaseous product from the primary circuit of the heat exchanger is divided between the temperature zones, atmospheric air is mixed with the gaseous product flowing into at least one of the temperature zones of the process region and the atmospheric air flow to the said temperature zone is controlled in response to sensed temperature in that temperature zone by using the atmospheric air to cool that part of the gaseous product flowing to that temperature zone.

9. A method of operating a process plant as claimed in claim 8 in which, if one or more of the temperature zones is no longer operative it is isolated from the gaseous product.

10. A method of operating a process plant as claimed in any one of claims 2 to 9, in which part of the gaseous product from the incinerator bypasses the heat exchanger and is mixed with the gas from the primary circuit of the heat exchanger before being used as an energy source for the process region.

11. A method of operating a process plant as claimed in any one of claims 2 to 10, in which part of the entraining gas from the process region bypasses the heat exchanger and the incinerator and is mixed with the gaseous product from the primary circuit of the heat exchanger before being returned into the process region to increase the concentration of the volatile in the process region.

12. A method of operating a process plant as claimed in any one of claims 2 to 11, in which the rate of flow of the entraining gas to the secondary circuit of the heat exchanger is regulated by flow rate controlled valve means, any excess flow of gas being diverted back into the process region gases.

13. A method of operating a process plant as claimed in any one of claims 2 to 12, in which atmospheric air is admitted into the combustion region of the incinerator, to limit the temperature of the combustion gases.

14. A method of operating a process plant as claimed in claim 13, in which the rate of flow of air into the combustion region is regulated in response to detected temperature in the combustion region.

15. A method of operating a process plant as claimed in any one of claims 2 to 14, in which some product gas leaving the incinerator is mixed with atmospheric air to control its temperature and then mixed with the product gas leaving the primary circuit of the heat exchanger.

16. A method of operating a process plant as claimed in any preceding claim comprising maintaining the process region at a pressure below ambient pressure.

17. A method of operating a process plant in which the only burner that is included is in the incinerator.

18. A method of operating a process plant as claimed in any one of claims 1 to 17, in which the operating temperature of the heat exchanger is controlled by a water spray.

19. Apparatus for use with a process plant to neutralise a pollutant entrained in a process fluid emitted from a process region, comprising: a heat exchanger comprising primary and secondary circuits; an incinerator, and means for exhausting the fluid from the region and passing it successively through the secondary circuit of the heat exchanger, the incinerator, the primary circuit of the heat exchanger and to the process region.

20. Apparatus as claimed in claim 19, comprising means for admitting atmospheric air to the process fluid before it is admitted to the process region.

21. Apparatus as claimed in claim 19 or claim 20, further comprising control means regulating the flow of atmospheric air for mixing with the process fluid from the primary circuit of the heat exchanger in response to detected temperature in the process region.

22. Apparatus as claimed in any one of claims 19 to 21, in which the process region is divided into two or more temperature zones and conduits are provided dividing the fluid from the primary circuit of the heat exchanger between the temperature zones of the process region, at least one of those conduits being provided with means for admitting atmospheric air and control apparatus adapted to control the flow of atmospheric air into this conduit in response to detected temperature in the temperature zone corresponding to that conduit.

23. Apparatus as claimed in any one of claims 19 to 22, including a bypass conduit from the incinerator direct to the return conduit to the process region, so that some of the gaseous product from the incinerator can bypass the primary circuit of the heat exchanger.

24. Apparatus as claimed in any one of claims 19 to 23, including a concentration bypass from a conduit leading from the process region to the secondary circuit of the heat exchanger, the concentration bypass returning directly to the process region.

25. Apparatus as claimed in any one of claims 19 to 24, including a flow rate control means for controlling the flow of fluid from the process region to the secondary circuit of the heat exchanger in response to detected flow rate in the primary circuit and means for returning excess fluid directly to the process region.

26. Apparatus as claimed in any one of claims 19 to 25, including means for admitting atmospheric air into the combustion region of the incinerator.

27. Apparatus as claimed in claim 26, further comprising means for regulating the flow of air into the combustion region in response to detected temperature in that region.

28. Apparatus as claimed in any one of claims 14 to 27, further comprising a mixing region and conduit means connecting the combustion region of the incinerator with the mixing region and second conduit means leading from the mixing region to the process chamber.

29. Apparatus as claimed in any one of claims 14 to 28, including spray means for directing cooling fluid onto the heat exchanger.

30. A method of operating a process plant which employs a volatile or solid entrained in a fluid in contact with material being processed in a process region substantially as herein described with reference to Figures 1 and 2 or any one of Figures 3 to 7 and 9, or any one of Figures 2 to 7 and 9 as modified by Figure 8 of the accompanying drawings.

31. Apparatus for use with a process plant to neutralise a pollutant entrained in a process fluid emitted from a process region constructed and arranged substantially as herein described with reference to Figures 1 and 2 or any one of Figures 3 to 7 and 9, or any one of Figures 2 to 7 and 9 as modified by Figure 8 of the accompanying drawings.