WO2002018032A2

WO2002018032A2 - A method of treating an effluent gas stream, and apparatus for use in such method

Info

Publication number: WO2002018032A2
Application number: PCT/IB2001/001573
Authority: WO
Inventors: Barthlo Von_Moltitz Harmse
Original assignee: Harmse Barthlo Von Moltitz
Priority date: 2000-08-31
Filing date: 2001-08-30
Publication date: 2002-03-07
Also published as: WO2002018032A3; AU2001284325A1

Abstract

A method of treating an effluent gas stream comprises the steps of bringing the effluent gas stream into contact with a process liquid in a pre-contactor (14), to provide a mixture of the effluent gas and the process liquid, and causing the mixture to pass through a liquid-ring compressor (16) whose seal liquid is formed by the process liquid. The process liquid, through its contact with the effluent gas in the pre-contactor and the liquid-ring compressor, serves to treat the effluent gas. Downstream of the liquid-ring compressor the process liquid is again separated from the mixture, filtered, cooled, and recirculated by returning it to the pre-contactor. The method is particularly suitable for treating gases (including vapours, fumes, hazardous air pollutants (HAPs), volatile organic compounds (VOCs), and so on) emitted from the process industries and product loading systems.

Description

A METHOD OF TREATING AN EFFLUENT GAS STREAM. AND APPARATUS FOR USE IN SUCH METHOD.

This invention relates to a method of treating an effluent gas stream, and to apparatus for use in such method. More particularly, it relates to a method of, and apparatus for, treating an effluent gas stream for purposes of scrubbing, vapour recovery, odour removal, or the like.

The term "gas" in this specification is to be interpreted broadly, so as to include also vapours, fumes, and the like. Gases, which may include vapours, fumes, hazardous air pollutants (HAPs) and volatile organic compounds (VOCs), emitted from the process industries and product loading systems may result in the production and release of harmful or unwanted gases, which may be detrimental to personnel or to the environment, or be the source of offensive odours. Several problems are experienced with presently available extraction, scrubbing and filtration systems used to treat these gas streams, and there is therefore a need for an efficient treatment system.

According to the invention there is provided a method of treating an effluent gas stream, which comprises the step of causing the effluent gas stream to pass through a liquid-ring compressor whose seal liquid is formed by a process liquid whose contact with the effluent gas serves to treat the effluent gas.

The method may comprise first bringing the effluent gas stream into contact with the process liquid in contacting means, to provide a mixture of the effluent gas and the process liquid, and then causing the mixture to pass through the liquid-ring compressor.

After the mixture has passed through the liquid-ring compressor, the process liquid in the mixture (together with any substances removed from the effluent gas stream through its contact with the process liquid) may again be separated from the mixture.

At least part of the process liquid which is separated from the mixture downstream of the liquid-ring compressor may be recirculated by returning it to the contacting means.

The process liquid may be cooled before being returned to the contacting means. The term "cooling" is to be interpreted as including chilling.

The process liquid may be filtered to remove any particulates that may be entrained therein, before being returned to the contacting means.

That part of the mixture that remains after separation of the process liquid therefrom may be contacted with an activated carbon/resin in an activated carbon/resin bed. The liquid-ring compressor may provide the suction that is required to extract the effluent gas stream from its source, and to draw it through the contacting means.

A relatively dense fraction of the process liquid (together with any substances removed from the effluent gas stream through its contact with the process liquid) may be separated from a relatively less dense fraction thereof by centrifugal action in the liquid-ring compressor, and extracted via one or more perforations (78) in the barrel (70) of the liquid-ring compressor.

Further according to the invention there is provided apparatus for treating an effluent gas stream, which comprises: contacting means for bringing the effluent gas into contact with a process liquid, to provide a mixture of the effluent gas and the process liquid; and a liquid-ring compressor downstream of the contacting means, the liquid-ring compressor being arranged to use the process liquid as its seal liquid; whereby the process liquid in use, through its contact with the effluent gas, serves to treat the effluent gas.

The apparatus may include a gas/liquid separator downstream of the liquid-ring compressor, for again separating the process liquid in the mixture (together with any substances removed from the effluent gas stream through its contact with the process liquid) from the gas in the mixture.

Apart from providing the motive force to extract the gas stream from its source and draw it through the contacting means, the liquid-ring compressor, through its seal liquid, provides additional contact between the effluent gas and the process liquid, and provides for elevated pressure in a downstream separator.

The apparatus may include means for recirculating at least part of the separated process liquid to the contacting means. The apparatus may include cooling means for cooling said at least part of the separated process liquid before it is returned to the contacting means.

The apparatus may include a filter for removing any particulates that may be entrained in said process liquid before it is returned to the contacting means.

The barrel of the liquid-ring compressor may be provided with one or more perforations through which a relatively dense fraction of the process liquid (together with any substances removed from the effluent gas stream through its contact with the process liquid), which becomes separated from a relatively less dense fraction thereof through centrifugal action in the liquid-ring compressor, can be extracted.

The invention extends to apparatus for treating an effluent gas stream which comprises a liquid-ring compressor which is arranged to make use of a process liquid as its seal liquid, whereby the process liquid in use, through its contact with the effluent gas in the liquid-ring compressor, serves to treat the effluent gas, the barrel of the liquid-ring compressor being provided with one or more perforations through which a relatively dense fraction of the process liquid (together with any substances removed from the effluent gas stream through its contact with the process liquid), which becomes separated from a relatively less dense fraction thereof through centrifugal action in the liquid-ring compressor, can be extracted.

The invention will now be described in more detail, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings: Figure 1 is a schematic diagram illustrating apparatus for treating an effluent gas stream, in accordance with a first embodiment of the invention;

Figure 2 is a schematic diagram illustrating similar apparatus, in accordance with a second embodiment of the invention; and

Figure 3 illustrates a liquid-ring compressor which has been modified specially for use in the apparatus of Figures 1 or 2. Referring first to Figure 1 , reference numeral 10 generally indicates apparatus for treating a gas stream coming from a process vent. The apparatus can be designed as a skid-mounted unit, which allows for a modular construction and field installation. The gas stream enters the apparatus via a line 12. The apparatus comprises contacting means in the form of a pre-contactor 14 whose inlet is connected to the line 12, and a liquid-ring compressor 16 whose suction port is connected to the outlet of the pre-contactor via a line 18.

The design of the pre-contactor 14 can vary, depending on the specific application, but essentially it comprises a vessel with packing therein to ensure that intimate contact is established between the gas stream and a process liquid.

The delivery port of the liquid-ring compressor 16 is connected to a gas/liquid separator or knock-out vessel 20 via a line 22. The separator vessel 20 has a vent line 24 which opens to atmosphere via a controlled valve 26, and a liquid outlet line 28 which leads via a filter 30 to the suction of a recirculation pump 32. The recirculation pump may be omitted in cases where no liquid spray is required in the separator vessel 20 and there is sufficient head available for recirculation flow to the pre-contactor 14. The delivery of the recirculation pump 32 leads via a line 34 to process liquid inlets 36 of the pre-contactor 14. The line 34 passes through a heat exchanger 38. The heat exchanger 38 has a cooling medium circulating therethrough, via a line 40.

Operation of the valve 26 may be controlled in response to the pressure in the line 18, as indicated by pressure sensor 41 .1 and control line 41 .2 which links the pressure sensor to the valve 26.

In operation recirculated process liquid is introduced via the inlets 36 into the pre-contactor 14, where it comes into contact and is thoroughly mixed with the gas stream entering the pre- contactor via the line 12.

The recirculated process liquid may, if desired, be cooled before it is reintroduced into the gas stream. The degree of cooling depends on the specific process requirements. For example: (a) The recirculated process liquid may be cooled to enhance the scrubbing efficiency and to overcome the heat of compression, friction, and/or heat of absorption in the process, (b) If the apparatus is predominantly used for vapour recovery of VOCs, then the process liquid may be cooled to a temperature which is low enough for the VOCs to be condensed to such an extent that the VOC content of the remaining gas stream is below the most stringent of the mandated emission limits. The process liquid also serves as the seal liquid of the liquid-ring compressor.

The centrifugal action of the liquid-ring compressor can be used to separate water and/or solid particles that may enter the system, from the process liquid (if water is immiscible with the process liquid). The water/particles will be forced radially outwardly to the barrel of the liquid-ring compressor and can be extracted or recovered via perforations in the barrel. This will be described in more detail hereinafter, with reference to Figure 3.

The resulting two-phase mixture of gas and process liquid flows via the line 18 to the inlet port of the liquid-ring compressor 16, where the mixture is contacted with more process liquid (i.e. the seal liquid of the pump).

Centrifugal forces acting on the process liquid in the liquid-ring compressor 16, form the liquid in a ring against the inside cylindrical surface of the compressor barrel. The compressor compresses the gas through the action of an eccentric impeller which causes gas pockets that move circumferentially around the casing as the impeller rotates, to decrease in volume. This action also facilitates effective heat and mass transfer between the gas and the process liquid, and results in near isothermal compression. The process liquid is chosen to be specific to the compounds that are to be removed or recovered from the gas stream. It can be any of the following, depending on what the apparatus is to be used for: (a) A lean scrubbing oil for the scrubbing/absorption of fumes and volatile organic compounds (VOCs). (b) A caustic medium for scrubbing/neutralisation of acid gases and/or odours.

(c) An acid medium for the scrubbing/neutralisation of alkali fumes and/or odours. (d) A strong oxidation medium such as ozone, hypochlorides, permanganates, etc., for odour removal and/or to oxidise organic compounds.

(e) If the apparatus is configured for vapour recovery, the condensed vapour recovered from the gas stream can be used as the process liquid.

(f) Any suitable solvent, absorbent, or liquid not already mentioned above, that will react, absorb, condense, precipitate components, or selectively absorb or neutralise components in the gas stream.

Unlike the conventional practice in the operation of a liquid-ring compressor, the continuously recirculated seal liquid (that in this instance also acts as a scrubbing and/or vapour condensing medium) is not introduced into the barrel section of the liquid-ring compressor but instead enters the compressor via the suction port. The suction port may be enlarged for this purpose.

In other applications the recirculated process liquid may be cooled to overcome the heat of compression, friction, absorption, or, where required if the gas stream that enters the apparatus is at an elevated temperature, to reduce the temperature of the gas stream.

The delivery of the liquid-ring compressor 1 6, which has some of the process liquid entrained therein, flows via the line 22 to the separator vessel 20, where the process liquid is separated from the gas in which it is entrained. The gas from which the process liquid has been separated is vented to atmosphere via the vent line 24, the gas first passing through a demister/coalescer section 42 in the vessel, to prevent any process liquid from being entrained in the gas that is vented to atmosphere.

The process liquid that is separated out in the separator vessel 20 is re-circulated by means of the recirculation pump 32, the pump 32 pumping the process liquid from the separator vessel back to the inlets 36 of the pre-contactor 14 via the lines 28 and 34.

The filter 30 in the line 28 serves to remove any solid impurities that may be present in the process liquid. Referring now to Figure 2, reference numeral 10.1 generally indicates apparatus which is in part similar to the apparatus 10 illustrated in Figure 1 , the same reference numerals being used to indicate the same or similar parts.

The apparatus 10.1 differs from the apparatus 10 in that, in addition to the liquid outlet line 28, the separator vessel 20 is provided with a water drain 50, a packed bed 52 at a level above that at which the line 22 enters the vessel, and spray nozzles 54 above the packed bed. The spray nozzles 54 are connected via a line 56 to the line 34 at a point which is downstream of the heat exchanger 38. The apparatus 10.1 has a further heat exchanger 58, the line 56 passing through this heat exchanger. The line 40 for cooling medium which passes through the heat exchanger 38 also passes through the heat exchanger 58. Thus, the process liquid that flows from the line 34 to the spray nozzles 54 is additionally cooled in the heat exchanger 58, before being contacted with the residual gas in the packed bed 52, to further reduce the level of contamination in the residual gas.

The discharge line 34 of the recirculation pump 32 is provided with an outlet line 60, which allows for the removal of accumulated process liquid from the system. Flow through the outlet line can be modulated by means of a valve 62, to control the level in the separator vessel 20. This control may be automatic, as indicated by level control sensor 64 on the vessel 20 and control line 66 linking the sensor 64 to the valve 62. A connection (not shown) is also provided on the suction side of the recirculation pump 32, so that process liquid may be added to the system if necessary. In the case of vapour recovery, the condensed vapour recovered from the gas stream is pumped back to the process (or into a tank) via the line 60 and valve 62 under level control from the separator vessel 20. The process liquid is exhausted when, in the case of lean oil, it is fully saturated or, in the case of a caustic/oxidising/acid medium, it is depleted or neutralised. In certain applications, where the density and viscosity of the process liquid changes as it becomes exhausted, the condition of the process liquid can be measured indirectly by measuring the power consumption of the liquid-ring compressor. This is possible because power consumption of the liquid-ring compressor 16 varies linearly with the viscosity and density of the seal liquid (i.e. the process liquid). Thus, for example, an indication of when the process liquid has reached the end of its service life and requires replacement can be obtained by measuring the current that is drawn by the motor driving the pump.

When used for vapour recovery, the apparatus 10.1 illustrated in Figure 2 can be combined with a pressure swing adsorption (PSA) activated carbon/resin adsorption system (not shown). Thus, the gas that remains after the process liquid has been separated from the mixture entering the separator vessel 20 via the line 22, can be polished to meet much more stringent emission requirements by passing it through an activated carbon/resin system. When this system operates in the adsorption mode any trace uncondensed VOCs are adsorbed on the activated carbon/resin at a pressure slightly above atmospheric pressure and the clean air/gas then vented to atmosphere. Regeneration of the activated carbon/resin is accomplished by reducing the pressure in the PSA system. This can be achieved by using the same liquid-ring compressor 16, i.e. by operating the liquid-ring compressor off-line and in reverse so that suction is applied to the PSA system.

Referring now to Figure 3, the liquid-ring compressor 16 comprises a casing having a barrel 70, and an eccentric impeller 72 which is rotatable in the casing, in the direction indicated by the arrow. At 74 there is a suction port in the end walls of the casing, and at 76 a delivery port.

Unwanted contaminants other than hazardous substances that need to be scrubbed out can cause process problems in the system in certain applications. For instance, atmospheric water vapour will condense out and freeze up the system in a vapour recovery application. In other applications where there may be particulates in the gas stream these can also cause downstream problems such as blockages.

To deal with this in an effective manner, the construction of the liquid-ring compressor 16 is modified by providing the barrel 70 with perforations 78 and a wall 80 which forms a collecting chamber 82 on the outside of the barrel. The collecting chamber 82 is connected to a decanter pot 84 via a line 86. The decanter pot has a drain outlet 88 which has a controlled valve 90 therein. An interface controller 92 is provided to open the valve 90 when liquid/particles in the decanter pot have reached a predetermined level. A balancing line 94 connects the inside of the decanter pot to the suction of the liquid-ring compressor.

This construction enables the liquid-ring compressor to be used as a centrifugal separating device for separating water and/or contaminant particles from the process liquid. Reference 96 indicates the process liquid in the liquid ring compressor and reference numeral 98 the water and/or contaminant particles which, because of the higher density thereof in comparison with the process liquid, migrates radially outwardly from where it is extracted via the perforations 78. The apparatus herein described is reliable, simple in design, compact in size (i.e. requires little space), and operates with minimal noise or vibration. The apparatus is flexible and can be used for many environmental control applications by selecting the appropriate process liquid for each particular application. The apparatus makes advantageous use of the following inherent characteristics of a liquid-ring compressor:

(a) The thorough mixing of the gas stream with the seal liquid promotes mass and heat transfer.

(b) Its ability to handle liquids entrained in the gas stream.

(c) The compression that takes place in the pump is near isothermal, since the seal liquid largely absorbs the heat of compression, condensation, and friction. (d) Its ability to handle small particulates entrained in the gas stream.

(e) Low noise and vibration.

(f) Simplicity of design and high reliability.

(g) The centrifugal action of the liquid-ring compressor can be used to separate denser, unwanted contaminants from the process liquid. The invention has many applications. These include: (a) The treatment of any process stream where hazardous air pollutants (HAPs) are present that require cleaning before being vented to the atmosphere. (b) In vapour recovery applications where a volatile liquid is loaded or transferred into containers or tanks, e.g. of a ship, barge, or rail or road tanker.

(c) The treatment of any vapour stream emitted from a process and that needs to be cleaned up or from which the vapour is to be recovered. The method and apparatus herein described have several advantages over the extraction, scrubber, and filter systems that are currently in use. Some of the advantages are: (a) It does not require filters that can be blocked by deposits from condensable fumes and by the build-up of particulates. For example, it will not suffer from the problems currently experienced with deposits on the blades of extraction fans, causing vibration and other problems, or in piping systems causing blockages.

(b) It does not require adsorption beds or filters that need regular replacement. Depending on the application, the system can use inexpensive and readily available process liquids, having a long service life.

(c) It can combine scrubbing and fume extraction operations in a single compact unit, making it ideal for standardised skid-mounted construction. (d) The system can be more efficient than systems that are currently in use, and should require significantly less maintenance.

(e) The cost of the system over its service life can be less than is the case with currently available systems.

(f) The liquid-ring compressor is virtually noiseless. (g) The system can tolerate particles that cannot always be handled by currently available systems.

(h) The centrifugal action of the liquid-ring compressor can be used to remove or separate entrained or condensed water and/or particulate matter from the process liquid.

Claims

CLAIMS:

1 . A method of treating an effluent gas stream, characterised in that it comprises the step of causing the effluent gas stream to pass through a liquid-ring compressor (1 6) whose seal liquid is formed by a process liquid whose contact with the effluent gas serves to treat the effluent gas.

2. A method as claimed in claim 1 , which comprises first bringing the effluent gas stream into contact with the process liquid in contacting means (14), to provide a mixture of the effluent gas and the process liquid, and then causing the mixture to pass through the liquid-ring compressor (1 6).

3. A method as claimed in claim 2, wherein, after the mixture has passed through the liquid-ring compressor (16), the process liquid in the mixture (together with any substances removed from the effluent gas stream through its contact with the process liquid) is again separated from the mixture.

4. A method as claimed in claim 3, wherein at least part of the process liquid which is separated from the mixture downstream of the liquid-ring compressor (1 6) is recirculated by returning it to the contacting means (14).

5. A method as claimed in claim 4, wherein said at least part of the process liquid is cooled (38) before being returned to the contacting means (14).

6. A method as claimed in claim 4 or claim 5, wherein the process liquid is filtered (30) to remove any particulates that may be entrained therein, before being returned to the contacting means (14).

7. A method as claimed in any one of claims 4 to 6, wherein that part of the mixture that remains after separation of the process liquid therefrom is contacted with an activated carbon/resin in an activated carbon/resin bed.

8. A method as claimed in any one of the preceding claims, wherein the liquid-ring compressor provides the suction that is required to extract the effluent gas stream from its source, and to draw it through the contacting means.

9. A method as claimed in any one of the preceding claims, wherein a relatively dense fraction (98) of the process liquid (together with any substances removed from the effluent gas stream through its contact with the process liquid) is separated from a relatively less dense fraction (96) thereof by centrifugal action in the liquid-ring compressor, and is extracted via one or more perforations (78) in the barrel (70) of the liquid-ring compressor.

10. Apparatus for treating an effluent gas stream, characterised in that it comprises: contacting means (14) for bringing the effluent gas into contact with a process liquid, to provide a mixture of the effluent gas and the process liquid; and a liquid-ring compressor (16) downstream of the contacting means, the liquid-ring compressor being arranged to use the process liquid as its seal liquid; whereby the process liquid in use, through its contact with the effluent gas, serves to treat the effluent gas.

1 1 . Apparatus as claimed in claim 10, which includes a gas/liquid separator (20) connected downstream of the liquid-ring compressor, for again separating the process liquid in the mixture (together with any substances removed from the effluent gas stream through its contact with the process liquid) from the gas in the mixture.

12. Apparatus as claimed in claim 10, which includes means (28, 32, 34, 36) for recirculating at least part of the separated process liquid to the contacting means (14).

1 3. Apparatus as claimed in claim 12, which includes cooling means (38) for cooling said at least part of the separated process liquid before it is returned to the contacting means (14).

14. Apparatus as claimed in claim 12 or claim 13, which includes a filter (30) for removing any particulates that may be entrained in said process liquid before it is returned to the contacting means (14).

15. Apparatus as claimed in any one of claims 10 to 14, wherein the barrel (70) of the liquid-ring compressor is provided with one or more perforations (78) through which a relatively dense fraction (98) of the process liquid (together with any substances removed from the effluent gas stream through its contact with the process liquid), which becomes separated from a relatively less dense fraction (96) thereof through centrifugal action in the liquid- ring compressor, can be extracted.

1 6. Apparatus for treating an effluent gas stream, characterised in that it comprises a liquid-ring compressor (16) which is arranged to make use of a process liquid as its seal liquid, whereby the process liquid in use, through its contact with the effluent gas in the liquid-ring compressor, serves to treat the effluent gas, the barrel (70) of the liquid-ring compressor being provided with one or more perforations (78) through which a relatively dense fraction (98) of the process liquid (together with any substances removed from the effluent gas stream through its contact with the process liquid), which becomes separated from a relatively less dense fraction (96) thereof through centrifugal action in the liquid- ring compressor, can be extracted.

1 7. A method of treating an effluent gas stream, substantially as herein described and illustrated.

18. Apparatus for treating an effluent gas stream, substantially as herein described and illustrated.