WO1995030470A1 - Process for treating volatile organic compounds in a gas stream - Google Patents

Abstract

A process for treating volatile organic compounds known as VOCs, carried in a gaseous process medium from an industrial process plant (11) includes the steps of concentrating the VOCs in gaseous medium by an adsorption/desorption process and supply the thus concentrated VOC to a combustion engine (14) as a fuel for the engine. Two or more adsorption desorption units (22, 23) are provided whereby adsorption of VOCs in one of said units is decoupled from desorption of VOCs for combustion from another of said units. In this way the rates of adsorption and desorption can be controlled independently. Similarly desorption need not necessarily take place at the same time as adsorption and thus need not necessarily take place at the same time as process plant operation.

Description

Process for treating volati le organic compounds 1n a gas stream.

The invention relates to gas treatment processes and associated apparatus and particularly to the control of emission of volatile organic compounds (VOCs) from industrial processes.

Typical processes from which VOCs are emitted include painting, printing, textile coating, coating of metal, adhesive coating, coating synthetic films (eg for magnetic tapes and packaging) and various pharmaceutical and chemical production processes.

Control of VOC emission is becoming increasingly important, due in part to environmental protection legislation and in part to other environmental pressures. Collection of multi-component VOCs for re-use is generally impractical, partly because they become contaminated or mixed with other VOCs and may require subsequent separation. Simple combustion is the easiest way to dispose of VOCs but generally this requires an additional fuel to bring the concentration of VOCs up to a level where they become combustible. This simple combustion process is both inefficient and costly.

It has also been proposed, for example in EP-A-0566304 to use VOCs from an industrial process as a fuel in an internal combustion engine. Typically the engine also requires some additional fuel. The mechanical output from the engine can then be used directly or via electrical conversion to power other parts of the process plant. Heat in the engine exhaust and from its cooling system may also be used as process heat in the industrial process. An operation of this kind can result in net energy savings as well as controlling emission of VOCs to the atmosphere. It has also been proposed to increase the concentration of VOCs in air flowing through the industrial process prior to supply to an engine. However, difficulties can still arise in matching efficient operation of the engine to effective operation of the industrial process, particularly where the process is intermittent or produces varying amounts of VOCs with time.

An object of the invention is to provide an improved system whereby these problems may be reduced or overcome.

According to one aspect of the invention a process for treating VOCs carried in a gaseous process medium from an industrial process plant includes the steps of: concentrating the VOCs in gaseous medium by an adsorption/desorption process, and supplying the thus concentrated VOC to a combustion engine as a fuel for the engine, characterised in that at least two adsorption/desorption units are provided whereby adsorption of VOCs in one of said units is decoupled from desorption of VOCs for combustion from another of said units. In this way the rates of adsorption and desorption can be controlled independently. Similarly desorption need not necessarily take place at the same time as adsorption and thus need not necessarily take place at the same time as process plant operation.

Preferably the engine is an internal combustion engine but it could be a Stirling cycle engine or other external combustion engine. The most suitable form of internal combustion engine is generally a spark ignition reciprocatory engine but a gas turbine or compression ignition engine could be used.

Preferably the power from the engine is used to provide at least part of the mechanical power for the industrial process and/or for driving the gaseous medium through the treatment process.

Preferably heat from the engine is used as process heat in the industrial process plant. This may be done by providing a heat exchanger between on the one hand the engine exhaust or engine coolant and on the other hand the gaseous process medium upstream of the plant.

Adsorption may be effected at a pressure above atmospheric to improve its effectiveness.

The invention also extends to apparatus for carrying out a process as defined above.

An embodiment of the invention will be described with reference to the accompanying drawings in which:

Figure 1 is a flow diagram illustrating a process in accordance with the invention in a simple manner; and

Figure 2 illustrates a modification of part of Figure 1 showing a development of the invention.

A VOC emission control installation illustrated in Figure 1 is connected to process plant 1 1 . Flow paths are illustrated in full lines and control paths and sensors are illustrated in dash lines. The process plant is such that a gaseous process medium, normally air, is received at inlet 12 and the same air leaves at outlet 13 laden with VOC. For example, the plant may be such that VOC is employed as a solvent in a coating process and the solvent is removed from the coating by evaporation. Further details of the process are not described because it could be any process which produces VOC in significant quantities such that direct discharge of the VOC into the atmosphere is unacceptable.

Safety regulations and insurance requirements for process plant of the kind under consideration require that the mixture strength of VOC in air is kept well below the level at which combustion can occur, typically below 50% of the lower explosive limit. Practical operating requirements in a process plant may require concentrations which are even well below this 50% level.

The VOC emission control installation incorporates a reciprocatory spark- ignition internal- combustion engine 14 in which VOCs are employed as fuel. The engine drives an electrical generator 15, the output from which may be employed to provide mechanical power for the emission control installation and/or the process plant 1 1 and/or export of electrical power from the installation. Heat produced in the engine from its coolant system

16 and exhaust 17 is also used in the installation as will be described subsequently.

Fuel such as natural gas from a separate source 18 is supplied at the inlet to the engine to increase the concentration of combustibles in the inlet air to enable the engine to run. It is desirable to keep the requirement for additional fuel to a minimum bearing in mind that the main purpose of the installation is to dispose of the VOCs in a convenient manner.

A VOC concentration unit 21 is provided for the purpose of increasing the concentration of VOCs to the maximum permissible level. Typically the objective is to increase the VOC concentration to 50% of the lower explosive limit or to some other maximum permissible level. For example, if the concentration unit can be integrated sufficiently closely with the engine so that its output is seen as equivalent to the combustible mixture generated at an engine inlet, higher levels of concentration may be permissible and would be desirable.

The VOC concentration unit incorporates two adsorption/desorption beds 22 and 23 of activated carbon or zeolite or other suitable material. Depending on the nature of the VOCs, it may be desirable to employ compound beds of more than one material to adsorb different constituent VOCs or graded beds to improve adsorption. The material is such that when VOCs in air are passed through the bed in one set of conditions, the VOCs are adsorbed by the bed resulting in clean air, or at least air with very little VOCs, leaving the bed. In another set of conditions, VOCs previously adsorbed into the bed can be desorbed in much higher concentration into air or into a weak air/VOC mixture to result in a more concentrated VOC mixture.

For adsorption of VOCs from air leaving the process, outlet 13 from the process plant is connected via a cooling heat exchanger 19, an extractor fan 24 and flow directing valves 27 and 28 to an inlet of each bed 22 and 23. A corresponding outlet from each bed in one direction is connected through a respective flow directing valve 31 or 32 to a clean air outlet 29.

For desorption of VOCs into air for combustion in the engine, heated air from a source to be described below is supplied to the other inlets of each bed 22 and 23 via flow directing valves 25 and 26. The opposite outlet from each bed is connected through a respective flow directing valve 33 or 34 through a cooler 20 to the inlet of the engine 14.

In the simplest form of operation, the arrangement is such that valves 25, 27, 31 and 33 are all closed when valves 26, 28, 32 and 34 are all open as shown. All of these valves have their open/closed condition changed simultaneously so that in another operating state, valves 25, 27, 31 and 33 are open when valves 26, 28, 32 and 34 are closed.

In the position of the valves as shown, air carrying VOCs from the outlet 13 of process plant 1 1 flows through bed 22 from valve 28 to valve 32, most or all of the VOCs being adsorbed into the bed 22. Simultaneously, bed 23 which has previously adsorbed VOCs in a similar way, is subjected to a much lower flow rate of air through valves 26 and 34 to the engine inlet. During this operation, the bed 23 desorbs VOCs for combustion in the engine. A remote engine inlet throttle 30 controls the desorption flow rate through bed 22 or 23. The throttle may be adjusted to provide a desired substantially constant flow rate or may be controlled to provide constant VOC concentration as explained in more details below.

With a more typical arrangement a cooling period is required after desorption in preparation for the next adsorption phase. In such a case desorption is arranged to occur over a shorter period than adsorption and a cooling phase follows desorption. For the cooling phase, ambient air from inlet 8 is supplied through valve 9 or 10, bed 22 or 23 and valve 32 or 31 respectively while other valves associated with the beds remain closed.

The adsorption/desorption bed is such that adsorption occurs effectively with relatively high flow rate at relatively low temperature through the bed and desorption occurs effectively with a relatively low flow rate at relatively high temperature. Instead of or in addition to the temperature variation, adsorption may take place at relatively high pressure and desorption at relatively low pressure. Clearly the conditions have to be selected to achieve the required result for the particular VOCs and installation. To achieve the required temperatures, air for desorption to be supplied through valve 25 or 26 is heated in a heat exchanger 43, 44, referred to below, and air/VOC mixture leaving the process plant 1 1 is cooled in a cooler 19. The heating and cooling may be achieved at least in part by heat exchange with other parts of the installation.

In this way, a richer VOC/air mixture than leaves the process plant can be applied to the inlet to the engine 14. Cooler 20 reduces the temperature of the desorption mixture to an acceptable level prior to the inlet to the engine. Fuel as required is added from fuel supply 18 to provide a combustible mixture.

Generally there is a requirement for inlet air in a process plant at a temperature elevated above ambient temperature. For example this may be required to facilitate evaporation of VOC solvent. In this example, air from inlet 41 is supplied by a fan 42 through a first heat exchanger element 43 where it is heated from the engine cooling system 16 and a second heat exchanger element 44 where it is further heated from the engine exhaust 17. This heated air is then supplied to air inlet 12 for the process plant. Further air from inlet 41 heated in heat exchanger 43, 44 is used as air for desorption supplied through valve 25 or 26.

A complete adsorption/desorption cycle may take of the order of 200 minutes, namely with an adsorption time of about 100 minutes and a desorption time of about 100 minutes possibly less a short cooling period of the order of 10 minutes. In such a case, the flow directing valves have their state changed at about 100 minute intervals.

The electrical power from generator 15 may be employed to power fans 24 and 42. Further power from the generator may be used for additional heating of inlet air if required. In some conditions at least, the electrical power generated exceeds the electrical power requirements of the emission control installation in which case the installation exports electrical power for other purposes. Typically the generator is connected electrically to a 3-phase electric mains supply which fixes generator speed and thus also fixes engine speed.

During start up, the main process in the process plant should not be carried out until normal operating temperature is reached. Thus, depending on the quantity of VOC if any adsorbed into one of the beds 22 or 23 during a previous operation, the whole of the engine fuel requirement may have to be met from fuel supply 18. Also external electric power may be required for fans 24 and 42 during start up. Once suitable conditions of temperature and air flow rate have been achieved in the process plant, the process may be commenced.

A control unit 51 monitors conditions in the system and controls its operation. An oxygen sensor 52 in the engine exhaust is employed to provide the control system with an oxygen ratio signal which is used as a measure of the air/fuel mixture ratio at the engine inlet. A VOC sensor 53 may be a commercially available hydrocarbon analyser or may be a simpler device which gives a measure of the VOC concentration in terms of the lower-explosive-limit. The VOC sensor is connected to the outlet from the beds 22, 23 and provides its output to the control unit 51 .

The control unit 51 provides a fuel control signal 54 to a fuel valve 55 which controls the supply of additional fuel from supply 18 to the engine. The additional fuel supply is dependent primarily on the signal from oxygen sensor 52 and is adjusted to maintain the exhaust oxygen level and thus the inlet air/fuel ratio (based on total fuel) at a required level.

The control unit 51 also provides an engine throttle signal 56 to the engine throttle 30. Operation of the throttle modulates both the flow rate and pressure during the desorption phase. The pressure reduction is brought about by suction from the engine, particularly when it is operated on a conventional spark ignition Otto cycle. The desorption rate tends to vary with time, particularly with a VOC mix of substances having different boiling points and also towards the end of desorption. The throttle signal is arranged to tend to close the throttle when the VOC sensor 53 detects a lowered VOC concentration. The associated reduced flow rate and lower pressure tend to increase the VOC concentration and also reduce the total engine charge so that less additional fuel is required to maintain a constant air/fuel ratio in the engine. Eventually the outlet VOC concentration can no longer be maintained at a required level, indicated by the throttle setting signal reducing to a set level, even with the low flow rate and reduced pressure associated with a small throttle opening. The control unit then issues a change over command to commence the cooling phase by closing valve 27 or 28 and opening valve 9 or 10. The cooling phase may be limited to a specific duration or may be terminated in response to a signal indicating that a satisfactorily low temperature has been reached.

Throughout the desorption phase, the engine speed is maintained constant in a conventional way but its power varies widely as the throttle closes. An arrangement of this kind produces power efficiently because a minimum of additional fuel is used as the VOC concentration is kept substantially constant at its maximum concentration.

After the cooling phase, the bed is in a condition for a new adsorption phase and the change over from adsorption in the other bed is instigated by a command from the control unit 51 to the relevant valves. Adsorption and desorption conditions should be so balanced that the changeover occurs shortly before the adsorbing bed reaches the limit of adsorption where a significant proportion of VOCs pass through it to outlet 29.

An alternative requirement to constant VOC concentration into the engine could be for constant power from the engine. In such a case, the throttle could be maintained substantially fully open during desorption. More additional fuel would then be used to maintain the air/fuel mixture at a suitable level.

Figure 2 illustrates a modification which takes advantage of the fact that adsorption/desorption beds operate more effectively at pressures well above atmospheric, for example at about 8 Bar. High pressure improves the capacity of the bed and the rate of adsorption. Desorption can take place at lower pressures and is more effective when there is a -large pressure differential between adsorption and desorption. Figure 2 shows a modified adsorption/desorption unit which may be substituted for that of Figure 1. Where appropriate, the same reference numerals are used in Figure 2 as in Figure 1 but with the suffix B. A two stage compressor made up of compressors 45 and 46 is provided to raise the inlet pressure to the beds in the adsorption phase to about 8 Bar. The main flow from the outlet of the adsorption/desorption units after desorption is expanded in a turbine 47 or other expander producing mechanical energy which provides part of the power requirement for the compressors. The remainder of the power requirement can be met in normal operating conditions from the generator 15 of Figure 1 or a geared mechanical connection to the output of engine 14. An inter-cooler 48 is provided between the two stages 45 and 46 of the compressor to improve the efficiency of the second stage.

In other respects, the arrangement of Figure 2 operates in the same way as for Figure 1. Smaller adsorption/desorption beds may be employed because of their increased efficiency associated with the higher pressure operation during the adsorption phase.

As an alternative to providing only two adsorption/ desorption beds, a greater number of beds may be provided to simplify the effect of transition from adsorption to desorption and cooling on the plant as a whole and to provide for desorption from one bed at all times, if required, even when one bed is adsorbing and another is being cooled after desorption. Similarly, where the process plant produces VOCs at varying rates, the effective provision of a storage medium for VOCs in the beds 22 and 23 permits the operation of other parts of the installation to be smoothed out.

Although the working gaseous process medium in the process plant and for desorption has been described as air, an alternative medium may be used for either or both of these. For example, filtered engine exhaust gas could be used in place of air or mixed with air for the desorption state. The reduction in oxygen would result in a medium which could carry more VOC without risk of explosion. In such a case, additional air or oxygen from another source may be needed to support combustion in the engine.

Claims

1 . A process for treating volatile organic compounds (VOCs) carried in a gaseous process medium from an industrial process plant including the steps of concentrating the VOC in gaseous medium by an adsorption/ desorption process, and supplying the thus concentrated VOC to a combustion engine as a fuel for the engine, characterised in that at least two adsorption/desorption units are provided whereby adsorption of VOCs in one of said units is decoupled from desorption of VOCs for combustion from another of said units.

2. A process according to claim 1 wherein the engine is an internal combustion engine.

3. A process according to Claim 2 wherein the engine is a spark ignition Otto cycle engine and an inlet throttle for the engine is arranged upstream of the adsorption/desorption units.

4. A process according to any one of the preceding claims including a control unit arranged to operate the process at substantially constant VOC concentration.

5. A process according to any one of Claims 1 to 3 arranged to operate the process at substantially constant power output from the engine.

6. A process according to any one of the preceding claims wherein power from the engine is used to provide at least part of the mechanical power for the industrial process and/or for driving the gaseous medium through the treatment process.

7. A process as claimed in any one of the preceding claims wherein heat from the engine is used as process heat in the industrial process plant.

8. A process according to any one of the preceding claims wherein adsorption is effected at a pressure above atmospheric pressure.

9. A process according to Claim 8 wherein the process medium is expanded in an expander which provides part of the mechanical power for compressing the process medium.

10. A process for treating VOCs as claimed in any one of the preceding claims wherein the gaseous process medium for desorption is or includes exhaust gas from the engine.

1 1. A process as claimed in claim 10 wherein additional oxygen is provided to the engine for the combustion process.

12. A process for treating VOCs carried in a gaseous process medium substantially as described with reference to the accompanying drawings.