GB2277369A - Method and apparatus for consuming volatiles or solids entrained in a process plant fluid - Google Patents

Abstract

Volatiles are produced at a given rate in an oven 301. Fluid containing the volatiles is taken off through the conduit 302. The conduit 303 recycles a proportion of the fluid containing the volatiles to maintain the flow velocity of fluid over the product giving off the volatiles. The remaining fluid is taken away through the conduit 304 for incineration of the volatiles. Under steady state conditions the percentage of volatiles in the fluid in the conduit 304 is greater than the percentage of volatiles in the fluid leaving the oven thereby reducing the heat input that is required to raise the volatiles to the incineration temperature. <IMAGE>

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 750"C. The solvents present in the vapour stream have a calorific value in themselves, which will contribute to the attainment of the 750"C 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 (150"C) 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 consuming volatiles or solids entrained in a fluid leaving a process plant comprises: passing the entraining fluid through the tubes of a shell and tube heat exchanger to raise the temperature of the entrained volatiles or solids; incinerating the fluid after it emerges from the tubes of the heat exchanger and directing the gaseous products of combustion through the shell of the heat exchanger in a direction generally parallel with the tubes to provide the primary fluid of the heat exchanger, whereby some or all of the temperature difference between the temperature of the entraining fluid leaving the production process and the combustion temperature of that fluid is derived from the gaseous products of combustion of the incineration.

Preferably, the gaseous products of combustion flow substantially unimpeded through the shell of the heat exchanger. It is further preferred that the entraining fluid is pumped through the tubes of the heat exchanger.

According to a preferred feature of this aspect of the invention, the gaseous flow in the shell of the heat exchanger is subject to induced Eddys. Alternatively or additionally the gaseous flow in the shell may be caused to flow in an undulating manner. It is further preferred that the gaseous flow in the shell has a Reynolds Number approximately equal to the Reynolds Number of the fluid flow in the tubes.

It will be appreciated that the method of this aspect of the invention differs from prior proposals in that the entraining fluid passes through the tubes of the heat exchanger and in that the gaseous products of combustion pass through the shell side of the heat exchanger.

However, with known apparatus in which the entraining fluid is preheated before incineration, in an attempt to improve the heat transfer efficiency of the heat exchanger, the gases on the shell side are caused to flow across the tubes (i.e. transversally of tubes). This is achieved either by providing the inlet and outlet openings to the shell on opposite sides of the tube stack or by the use of baffles on the tubes or some combination of these methods.

The area of the tubes required for heat exchange and the heat flow per unit area are a function of the Nusselt Number. Although the cross-flow configuration gives an improvement in the heat transfer coefficient as against straight counter-current flow, this improvement is achieved at the cost of an increase in the pressure drop in the fluid in the shell, and to compensate for this, energy has to be expended driving a fan for inducing the fluid flow in the shell. Thus there is a design conflict between improving the heat transfer efficiency and maintaining a minimum pressure drop in the fluid passing through the shell.

Preferably, the process is controlled so that the temperature of the fluid leaving the heat exchanger is lower than the combustion temperature of the volatiles or solids, but the summation of the temperature of the fluid leaving the heat exchanger and the temperature potential of the volatiles or solids is greater than the combustion temperature of these volatiles or solids.

An object of a second aspect of the invention is to create conditions in the heat exchanger whereby the heat transfer coefficient is at an acceptable high level, whilst keeping the pressure drop in the shell fluid down to a figure near to that associated with direct counter-current flow.

According to a second aspect of the invention, a tube and shell heat exchanger is provided, wherein one or more of the tubes is provided with a shell-flow regulation device comprising two or more arms, extending radially from the outside of the said tube, at least some of these arms extending across substantially the full space between the tube and an immediately adjacent tube, but adjacent arms projecting from the same tube being spaced apart, so that only a portion, less than 50% of the total annular space defined by the periphery of the said tube and an imaginary circle touching the peripheries of the tubes immediately adjacent to the said tube, is obstructed by the regulation device and the thickness of the regulation device is no greater than 20% of the diameter of the bore of the said tube.

Preferably, each arm of each shell-flow regulator tapers inwardly as seen from one end of the tube as it approaches the said tube from which it radiates. It is further preferred that each arm of each shell-flow regulator has concave side edges as seen from one end of the tube.

According to a further preferred feature, there is a bank of tubes and the majority of the tubes, other than the tubes around the edges of the bank, are provided with shell-flow regulators. It is still further preferred that all the tubes, other than the tubes around the edges of the bank, are provided with shell-flow regulators.

According to another preferred feature of this aspect of the invention, there are more than one shell-flow regulator on the said tube, spaced apart longitudinally along the tube.

According to yet another preferred feature of this aspect of the invention, the design of the shell-flow regulator(s) is such as to produce a mass-flow in the shell adjacent to the said tube of approximately the same Reynolds Number as the flow inside the said tube.

Preferably, at least some of the arms of each shell-flow regulator engage with immediately adjacent tubes and provide spacers locating the said tube and the immediately adjacent tubes with respect to each other.

According to another aspect of the present invention a method of consuming volatiles or solids entrained in a fluid from a process region comprises recycling a first proportion of the fluid to the process region and incinerating at least some of the volatiles or solids in a second proportion of the fluid leaving the process region with the percentage of volatiles or solids in a second proportion of the fluid leaving the process region with the percentage of volatiles or solids in the second proportion of fluid being greater than the percentage of the volatiles or solids in the fluid coming from the process region.

The first and second proportions of fluid coming from the process region together before the second proportion leaves for incineration of at least some of the volatiles or solids and the first proportion leaves for recycling.

The first and second proportions of fluid may comprise substantially the complete proportion of fluid coming from the process region.

The first proportion of fluid may assist in the generation of fluid containing volatiles or solids in the process region during its recycling.

According to another aspect of the present invention a process plant includes a process region arranged, in use, to generate volatiles or solids in a fluid, means for taking a first and second proportion of fluid from the process region, recycling means arranged to return the first proportion of fluid to the process region and incineration means arranged to incinerate at least some of the volatiles or solids in the second proportion of fluid.

According to a further aspect of the present invention a method of operating a tube and shell heat exchanger comprises causing the fluid flowing through the shell to be deflected from a direction in line with the longitudinal axes of the tubes by no more than 450 to the longitudinal axes.

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

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 an illustration of a heat exchanger, being part of the apparatus shown in Figure 2, Figure 4 is a horizontal cross-section through the heat exchanger shown in Figure 3, Figure 5 is a diagram similar to Figure 2, but showing an arrangement employing an existing incinerator, Figure 6 is a diagram showing an arrangement for supplying clean gases from the process to a plurality of units of a process plant, Figure 7 is a diagram similar to Figure 2, but showing a more complex arrangement, including zonal temperature control of a process plant, Figure 8 is a diagram similar to Figure 7, but showing the use of an existing incinerator, Figure 9 is a diagram similar to Figure 2, but showing an arrangement for volatile concentration, Figure 10 is a diagram showing an arrangement for concentrating the volatiles, Figure 11 is a diagrammatic arrangement of a clean gas cooler, and Figure 12 is a diagram similar to Figure 2 but showing a more sophisticated combustion system.

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 2400C 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 20"C 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 Kg/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 and to ensure that oxygen lends are maintained.

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 or other heat supply means.

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 162"C, 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 162"C 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 solution to this problem is the provision of a heat exchanger arrangement having design characteristics, which are illustrated diagrammatically in Figures 2 to 4.

The heat exchanger is shown at 40 and comprises two banks 48 and 50 of tubes 42; the tubes 42 of each bank are arranged in a shell 44. As the construction of each bank 48 and 50 is largely conventional (insofar as they relate to vertical tubes in a shell), it is unnecessary to describe it in detail. However, it is to be noted that the tubes 42 are straight and parallel with each other and, as appears in Figure 2, are disposed vertically and the two banks are arranged side by side. A spacer 52 made in a material which is not subject to the same thermal expansion characteristics as the tubes, is provided between the two banks 48 and 52, but the spacer does not extend to the bottom or to the top of the banks, so that there is an outlet area 54 between the banks at the top and an inlet area 56 between the banks at the bottom.

There are openings through the parts of the shell walls which face into the areas 54 and 56; the whole assembly of the two tube banks and the spacer 52 is located within a case 60 filled with thermal insulation material and there is an inlet opening through the case 60 into the inlet area 56 and an outlet opening through the case 60 into the outlet area 54. In addition, there is an inlet manifold (not shown) leading into the bottom ends of the tubes 42 in the bank 48; a transfer manifold (not shown) at the top, connecting the top end of each tube 42 in the bank 48 to the top end of a corresponding tube 42 in the bank 50 and an outlet manifold (not shown) connecting the bottom ends of all the tubes in the bank 50 to an outlet.

The heat exchanger therefore has two separate flow paths for primary and secondary fluids (gases). The primary gas flows into the inlet area 56 and then diverges into the bottom ends of the two shells 44 (see the arrow 62 in Figure 4). The design of the openings through the wall 44 which allow this flow of gases is such as to avoid turbulence at this point. The gas then flows upwardly through the shells 44 and around the tubes 42 in those shells and then emerges in the outlet area 54 before leaving the heat exchanger at an exit indicated at 55.

The secondary fluid (gas) flows into the bottom ends of the tubes 42 in the stack 48, up the insides of those tubes, across the top to the tubes in the stack 50, down those tubes and out at the manifold at the bottom of the stack 50. Thus, the primary and secondary gases are in concurrent flow in the stack 48 and in counter-current flow in the stack 50.

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 50"C 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 7500C 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 750"C - the combustion temperature. Consequently, one part of the energy required to operate the system is that of the methane added at 74.

In order to improve the thermal transfer efficiency of the heat exchanger, the tubes 42 are provided with shell-flow regulators, such as those illustrated at 76 and 78 in Figure 4. A regulator 76 is made of relatively thin heatresistant metal (e.g. chromium steel or nickel steel) or ceramics and in a specific example, where the tubes 42 have a nominal bore of 25 mm, the thickness of the regulator is approximately 4 mm. The regulator comprises an inner ring 80, which is a tight fit on the tube 42, and three radial arms 82 extending from the ring. Some of the arms 82 extend across the space between the tube 42 to which the regulator is attached and an immediately adjacent tube; the arm widens towards its distal end where, in some cases, it rests on and is secured to one of the other tubes. The side edges of the arms 82 are concave as seen from one end.

In Figure 4, regulators 76 are shown only on three of the tubes 42. In practice, they are provided on all the tubes except those around the edges of the stack. It will be appreciated, however, that they have to be offset longitudinally of the tubes and, in fact, there may be several such regulators at longitudinally spaced positions along the length of each tube. As a result, the entire shell area between the tubes 42 and within the shell 44 is occupied by many of the regulators, the arms 82 of which bridge most of the interstices between the tubes 42. The regulators may act as spacers to maintain the tubes at the correct distance from each other.

As the "clean" gas leaving the incinerator flows through the two stacks 48 and 50, it encounters the regulator arms. At each such encounter, the gas stream has to divide to flow around the edges of the regulator arms, but the divided streams re-combine on the downstream side of each arm. The overall effect is for the mass flow through the shell to be generally linear and to be generally in the direction of the tubes. Each part of the fluid that is deflected may have components of velocity in line with the extent of the tubes and at component at right angles to the extent of the tubes. However, the extent in the in line direction will be equal to or greater than the component at right angles. Thus each part of the flow will not be in a direction of more than 450 from the longitudinal extent of the tubes.Alternatively the regulator arms may create a multitude of local diversions setting up Eddys in the gas stream. These Eddys have comparatively little pressure reducing effect on the gas stream but on the other hand, it has been found that the overall effect is to produce a greatly enhanced heat transfer to the oven gas flowing in the tubes.

For maximum efficiency, the Reynolds Number of the gas stream in the shell should be the same as that of the gas stream in the tubes. This can be achieved (or almost achieved) by appropriate design of the number, diameter and spacing of the tubes. It is, however, necessary to vary the design to allow for the small reduction in gas stream velocity in the shell brought about by the presence of the regulators 76. It will be observed that the regulator arms only occupy a small part of the crosssectional shell area between the tubes 42. The design should be such that the area occupied by the regulator arms is never greater than 50% of the shell cross-section comprising an annulus defined by the exterior of the tube to which a regulator is attached, and an imaginary circle touching the immediately adjacent tubes, and preferably, it is not more than 25% of this annular area.

In Figure 4, there is also shown a regulator 78, which is similar to the regulator 76 except that it has six arms and consequently bridges the spaces between the tube 42 from which its arms radiate and six adjacent tubes. It will be appreciated that other configurations are possible and that the heat exchanger may be fitted with regulators, all of one configuration or alternatively with regulators of different configurations.

Besides regulating the gas flow through the shell, the regulation devices 76 and/or 78 serve to locate the tubes 42 with respect to each other. At the top of the heat exchanger, there are stiffener braces (not shown) and the tubes are suspended from these braces; however, the tubes are free at their lower ends, so that they can expand longitudinally. The devices 76 and/or 78 ensure the spacing of the tubes irrespective of any such lengthwise expansion.

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.The shell flow regulators achieve this whilst still increasing the heat transfer efficiency of the heat exchanger.

Figure 5 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 6, 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: 1780C.

A hot gas supply pipe 118 leads from the outlet 55 of a heat exchanger such as that shown in Figure 2 or Figure 5, 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 28 OOC. 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.Alternatively the control device could be provided on the hot side on the piper 120.

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 6 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 7, 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 Figures 2 to 4, 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 50"C, 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 7 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 7, 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 8 is identical with that shown in Figure 7, 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 7.

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 3250C 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 1780C.

Figure 9 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 500C. 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 5, 7 and 8. 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 5. 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 47.50C.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 47"C. From the fan 176, there flows from each of the ovens 170 and 172, through the conduit 184 into the primary 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.

As shown in Figure 10, product 300 enters the oven 301 and volatiles are given off at a rate of 5kg/hr. Fluid is taken off the oven at a rate of 1000m3/hr through the conduit 302. That fluid takes with is Skg/hr of solvent.

The conduit 303 then recycles 750m3/hr back into the oven and the recycled fluid is used to maintain the flow and velocity of flow over the product to ensure that Skg/hr of volatiles are still given off by the product.

Under steady state conditions the flow of fluid through the conduit 304 of 250m3/hr also takes with it 5kg/hr of volatiles. It will be appreciated that the conduit 304 leads to an incinerator and may be combined with any of the other embodiments described herein. Similarly the recycling provided by the conduit 303 may be used with any of the other embodiments either directly back to the same oven or to another oven that may, if desired, lead to the oven from which the fluid is being taken off.

The concentration of volatiles in the conduit 304 is 5kg each hour for 250m3 compared to the considerably lesser concentration of 5kg each hour for every 1000m3 coming out of the oven. The increase in the temperature of the air achieved by this concentration when the volatiles are incinerated can be seen from the following calculations.

Take the calorific value of the solvent is 35000kj/kg and the heat capacity as 1.04.

Then, for 1000m3 with 5kg of solvent the temperature on combustion would be: 5 x 35000 = = 1680C 1000 x 1.04 However, for 250m3/hr with 5kg of solvent the temperature on combustion would be: 5 x 3500 = - 6730C 250 x 1.04 Accordingly by concentrating the percentage of solvent considerably less heat has to be provided to reach the incineration temperature of 7500C.

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 10.

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 16"C. 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.

A more sophisticated system for consuming volatiles is shown in Figure 12. 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 6. 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, and as these are constructed as described for example with reference to Figures 2 to 4, no further description is needed. 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 12 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 diaphragm controlled 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 diaphragmcontrolled 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 12 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.

Claims (17)

1. A method of consuming volatiles or solids entrained in a fluid from a process region comprising recycling a first proportion of the fluid to the process region and incinerating at least some of the volatiles or solids in a second proportion of the fluid leaving the process region with the percentage of volatiles or solids in the second proportion of fluid being greater than the percentage of volatiles or solids in the fluid coming from the process region.

2. A method as claimed in Claim 1 comprising the first and second proportions of fluid coming from the process region together before the second proportion leaves for incineration of at least some of the volatiles or solids and the first proportion leaves for recycling.

3. A method as claimed in Claim 1 or 2 in which the first and second proportions of fluid comprise substantially the complete proportion of fluid coming from the process region.

4. A method as claimed in any preceding claim in which the first proportion of fluid assists in the generation of fluid containing volatiles or solids during its recycling.

5. A method as claimed in any preceding claim in which at least a part of the first proportion of fluid passes over material being processed in the process region.

6. A method of consuming volatiles or solids entrained in a fluid from a process plant substantially as herein described with reference to, and as shown in any of the accompanying drawings.

7. A process plant including a process region which, in use, generates volatiles or solids in a fluid; means for taking a first and a second proportion of fluid from the process region; recycling means arranged to return the first proportion of fluid to the process region and incineration means arranged to incinerate at least some of the volatiles or solids in the second proportion of fluid.

8. A process plant substantially as herein described with reference to, and as shown in any of the accompanying drawings.

9. A tube and shell heat exchanger wherein one or more of the tubes is provided with a shell-flow regulation device comprising at least one arm extending radially from the outside of the said tube, the or at least some of these arms extending across substantially the full space between the tube and an immediately adjacent tube, but any adjacent arms projecting from the same tube being spaced apart, so that only a portion, less than 50%, of the total annular space defined by the periphery of the said tube and an imaginary circle touching the peripheries of the tubes immediately adjacent to the said tube is obstructed by the regulation device and the thickness of the regulation device is no greater than 20% of the diameter of the bore of the said tube.

10. A tube and shell heat exchanger as claimed in Claim 9, in which the or each arm of each shell-flow regulator tapers inwardly as seen from one end of the tube as it approaches the said tube from which it radiates.

11. A tube and shell heat exchanger as claimed in Claim 9, in which the or each arm of each shell-flow regulator has concave side edges as seen from one end of the tube.

12. A tube and shell heat exchanger as claimed in any one of Claims 9 to 11, in which there is a bank of tubes and the majority of the tubes other than the tubes around the edges of the bank are provided with shell flow regulators.

13. A tube and shell heat exchanger as claimed in any one of Claims 9 to 12, in which there are more than one shell-flow regulator on the said tube, spaced apart longitudinally of the tube.

14. A tube and shell heat exchanger as claimed in any one of Claims 9 to 13, wherein the design of the shell flow regulator(s) is such as to produce a mass flow in the shell adjacent to the said tube of approximately the same Reynolds Number as the flow inside the said tube.

15. A tube and shell heat exchanger as claimed in any one of Claims 9 to 14, in which the or each arm of each shell-flow regulator is able to engage with the immediately adjacent tube and provides a space locating the said tube and the immediately adjacent tube with respect to each other.

16. A tube and shell heat exchanger constructed and arranged substantially as herein described with reference to Figures 1 to 4 or Figures 1 to 4 as modified by any one of Figures 5 to 11 of the accompanying drawings.

17. A method of operating a tube and shell heat exchanger comprising causing the fluid flowing through the shell to be deflected from a direction in line with the longitudinal axes of the tubes by no more than 450 to the longitudinal axes.