Sub-atmospheric pressure gas scrubbers

In order to minimize the amount of contaminants that pass through a pump, or that are allowed to accumulate therein, a sub-atmospheric pressure gas scrubber suitable for use upstream of a vacuum pump is provided. The gas scrubber comprising a vessel and means for forming a spray of cleaning solvent within the vessel, the vessel comprising an inlet and an outlet for the gas to be cleaned at sub atmospheric pressure, the inlet and outlet being arranged to cause the gas to be cleaned to pass through the spray of cleaning solvent, the means for forming a spray comprising a cleaning solvent inlet connected to at least one spray nozzle and a cleaning solvent outlet through which used solvent can be pumped from the vessel using a pump, wherein the pump is arranged to recirculate the used solvent back to the cleaning solvent inlet.

This application is a national stage entry under 35 U.S.C. §371 of International Application No. PCT/GB2012/051584, filed Jul. 6, 2012, which claims the benefit of G.B. Application 1112715.6, filed Jul. 22, 2011. The entire contents of International Application No. PCT/GB2012/051584 and G.B. Application 1112715.6 are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to sub atmospheric pressure gas scrubbers. In particular, the present disclosure relates to a gas scrubber that is suitable for use up stream of a vacuum pump.

BACKGROUND

Many industrial processes require vacuums in order to work. For example, in the manufacturing of polymers, foodstuffs, moldings and the like a vacuum is often required to initiate or sustain a chemical reaction, drive off excess moisture, to de-gas materials or to assist in the forming and shaping of manufactured products. In many cases, vacuums are used in conjunction with wet chemical processes, which leads to partial evaporation of the chemicals involved, which in turn means that gases drawn from the processes can contain contaminants.

An industrial pumping system will often comprise a number of pumps connected in series or in parallel to provide increasingly higher vacuum pressures from one pumping stage to the next. A dry vacuum pump, that is one that is designed to pump gases without using a sealing liquid in the vacuum pump swept volume, generally comprises one or more rotors arranged to cooperate with a stator: movement of the rotors causing the gas to be drawn into the pump and pushed through it towards an exhaust port of the pump. In order that the pumped gas does not backflow, and to improve the pumping efficiency of the pump, the gaps between the rotors and stators are minimized—usually to within a few hundred microns.

A problem therefore arises when the gas being pumped contains contaminants, which can build up on the surfaces of the rotors and stators thereby closing the gap between them. When a certain thickness of contaminants has built-up on the surface of the rotors and stators there is a chance that the pump will jam preventing further operation or restarting without remedial maintenance.

In most situations, vacuum pumps run hot, that is the act of compressing the gas being pumped causes heating of the gas, which heat is transferred to the rotors and stators of the pumps. In such a situation the majority of the contaminants within the gas being pumped will not condense on the rotors and stators and will simply pass through the pump. Moreover, because the pump components are hot, in use, contaminants that are viscous liquids will generally remain in a low-viscosity state thereby enabling the pump to continue running. However, when the pump is switched off, the contaminants may have a tendency to thicken or harden upon cooling, thereby preventing the pump from being restarted, even though, when hot, it was able to run.

In the alternative situation, that is where the pump components exceed certain temperatures, there is a chance that contaminants within the pumped gas will react to form hard compounds that may build up on the rotor and stator surfaces. Hard compounds on the rotor and stator surfaces can cause excessive or premature wear of the pump components, thereby degrading the efficiency of the pump or even leading to catastrophic failure.

SUMMARY

It is an object of this disclosure to provide a solution to one or more of the above problems, to provide an improved or alternative gas scrubber for use in conjunction with a vacuum system, or for protecting dry vacuum pumps, ejector pumps and interstage condensers, any of which can also become blocked by process gas contaminants.

According to a first aspect of the disclosure there is provided a gas scrubber suitable for use upstream of a dry vacuum pump, the gas scrubber comprising a vessel and means for forming a spray of cleaning solvent within the vessel, the vessel comprising an inlet and an outlet for the gas to be cleaned, the inlet and outlet being arranged to cause the gas to be cleaned to pass through the spray of cleaning solvent, the means for forming a spray comprising a cleaning solvent inlet connected to at least one spray nozzle and a cleaning solvent outlet through which used solvent can be pumped from the vessel using a pump, wherein the pump is arranged to recirculate the used solvent back to the cleaning solvent inlet.

The gas scrubber is most suited to use in sub-atmospheric pressure conditions, which, in many practical situations, will be from about 1-50 mbarA. One of the main restrictions on the working pressure of the scrubber is the vapor pressure of the solvent. In other words, the working pressure of the scrubber must be higher than the vapor pressure of the cleaning solvent to prevent the cleaning solvent from boiling or evaporating. However, in situations where a lower operating pressure is required, a cleaning solvent having a correspondingly lower vapor pressure will need to be selected.

The pump can be a re-circulating pump, for example centrifugal recirculating pump, although other pump types may be used.

The process gas inlet and outlet ports are preferably offset, inclined or angled with respect to one another to cause the process gas to change direction as it passes through the vessel.

The process gas and solvent sprays are preferably arranged to contraflow.

A heat exchanger is preferably interposed between the provided downstream cleaning solvent outlet and the cleaning solvent inlet to regulate the temperature of the cleaning solvent. The heat exchanger may be of any suitable type, although a water-cooled shell and tube type of heat exchanger is preferred. The cooling water preferably passes on the shell side of the heat exchanger with the cleaning solvent passing through the tubes.

A temperature sensor may be provided for sensing the temperature of the cleaning solvent. The temperature sensor, where provided, is preferably connected to means, for example a mechanical valve actuated by gas pressure caused by a temperature differential or a motorized valve, for controlling the flow of cooling water through the heat exchanger, and hence regulating the temperature of the cleaning solvent.

Means is preferably provided for priming the pump and or providing the pump with a minimum head pressure. To achieve this, the pump is preferably located at a level below the vessel. Additionally or alternatively, a desired minimum volume of cleaning fluid may be maintained within the vessel. Additionally or alternatively, a desired maximum volume of cleaning fluid may be maintained within the vessel. The vessel may therefore be provided with an either or both of an upper level sensor and a lower level sensor for sensing whether the volume of cleaning solvent within the vessel is between the desired maximum and minimum. The upper and lower level sensors, where provided, are preferably connected to control means for draining and topping-up the volume of cleaning solvent within the vessel, respectively.

The lower level sensor, where provided, may be connected to a motor controller of the pump to cause the pump to slow down or stop in the event that the volume (or level) of cleaning solvent within the vessel falls below the desired minimum. In addition, the lower level sensor, where provided, may be connected to a servo-operated valve of a top up line connected at one end to a supply of fresh solvent and at its other end to an outlet located within the vessel above the upper solvent level line.

The upper level sensor, where provided, may be connected to the motor controller of the pump, that is to speed-up the pump, thereby draining the vessel, when the volume of cleaning solvent within the vessel exceeds the desired maximum. The upper level sensor may also be connected to a drain valve.

A solvent quality sensor may be provided for analysing the condition of the solvent. The solvent quality sensor, where provided, can be any sensor that senses any property of solvent that varies with quality, for example a specific gravity sensor, resistivity sensor, a capacitance sensor, a viscosity sensor, an in-line flow sensor, and the like. Additionally or alternatively, the power consumption of the pump, which is related to the solvent's viscosity, could be monitored as an indicator of the solvent quality or purity.

One or more in-line filters are preferably provided, for example upstream of the pump or upstream of the heat exchanger. Isolating valves may be provided either side of the filters, where provided, to enable them to be changed or cleaned should they become clogged.

The gas scrubber is preferably mounted on a skid.

According to a second aspect of the disclosure there is provided a gas scrubber suitable for use upstream of a vacuum pump, the gas scrubber comprising a vessel and means for forming a spray of cleaning solvent within the vessel, the vessel comprising an inlet and an outlet for the gas to be cleaned, the inlet and outlet being arranged to cause the gas to be cleaned to pass through the spray of cleaning solvent, the means for forming a spray comprising a plurality of spray nozzles each spray nozzle being arranged to form a spray pattern of cleaning solvent within the vessel, wherein the spray patterns are arranged to overlap radially, axially or radially and axially.

The gas scrubber is preferably locatable upstream of a dry vacuum pump.

Preferably, the gas to be cleaned passes, by at least a desired minimum distance, through at least one spray pattern of cleaning solvent.

Preferably, there is a main spray nozzle, which may be mounted substantially coaxially with the vessel. The additional spray nozzles are may be offset radially, axially or radially and axially with respect to the main spray nozzle. Most preferably, there is one main spray nozzle and three additional spray nozzles.

The spray nozzles are preferably located within an upper portion of the vessel at a level below that of the process gas outlet port.

The nozzles are preferably adapted to produce a substantially solid conical, downwardly directed spray of cleaning solvent. The cone of spray form each nozzle preferably emanates from the nozzle and travels axially and a radially to impinge upon the inner sidewalls of the vessel a point below the level of the nozzle, but above the level of the inlet port for the process gas.

The main nozzle is preferably adapted to form a full circular spray pattern that covers the whole inside diameter of the vessel.

The nozzles may be co-planar, but have differing spray patterns to produce overlapping sprays of cleaning solvent. The additional nozzles may be located radially outwardly of the main nozzle, but adapted to provide substantially conical sprays of cleaning solvent that impinge upon the inner sidewalls of the vessel at a point higher up the vessel than the cone of the main nozzle.

The nozzles may be inclined with respect to a lateral plane of the vessel to produce angled sprays of cleaning solvent, for example a helical flow of cleaning solvent spray.

The nozzles, where provided, preferably have an opening of, or about 6.4 mm. The nozzles are preferably self-cleaning, for example, using the pressure of the recirculating pump recirculating the solvent at a high pressure to keep the nozzle orifice open and may have removable vanes that can also be adjustable.

The vessel is preferably provided with an inwardly-projecting annular flange on its inner sidewall. The location and dimensions of the annular flange, where provided, is preferably adjustable or interchangeable.

DETAILED DESCRIPTION

It is therefore desirable to minimize the amount of contaminants that pass through a pump, or that are allowed to accumulate therein. In many cases, the adverse effects of contaminants in the pumped gas can be controlled satisfactorily by providing the pump with a thermal jacket to heat or cool the pump such that the contaminants building up on the pump are prevented from solidifying or hardening or reacting to form hard compounds. Most dry pumps require a cooling system although a heating jacket could be provided to keep the pump warm when it is not running. This is inconvenient and expensive to install and may use a lot of energy, (albeit less expensive than the cost of reinstating a seized pump). A heating jacket could, for example, comprise a hot water circuit acting as coolant when the pump is not running or even when it is running. However, thermal jackets require considerable amount of energy to operate and may not always be reliable.

A need therefore arises for an improved method of protecting vacuum pumps from contamination.

It is known to provide a gas scrubber upstream of a pump and a known gas scrubber is shown inFIG. 1, and described in detail below. Such a known scrubber comprises a vessel into which a gas cleaning solvent is pumped to form a spray or mist that contraflows with the process gas to be cleaned. The solvent interacts with the contaminants in the process gas to remove them, and the solvent and contaminants are sluiced to waste. Such a known type of gas scrubber consumes a large amount of solvent and is thus expensive to operate. Moreover, the relatively large volume of contaminated solvent produced can be difficult or costly to process. In addition, a known type of gas scrubber can provide non-uniform gas scrubbing as process gas passing through different parts of the vessel is subjected to different amounts of interaction with the cleaning solvent.

Finally, in the known gas scrubber, the used solvent effectively drains to an open dump tank and to ensure that the process gas vacuum is maintained, a barometric leg must usually be employed. In modest or high vacuum systems, the barometric leg must be relatively large so as to not break the process vacuum, which greatly increases the overall height of apparatus needed and places other constructional constraints on the design of the scrubber. This can make indoor installations of gas scrubber impractical or uneconomic.

Another type of known gas scrubber utilises a packed column or vessel with demister pads to filter the process gas. However, filter-type scrubbers can have high pressure drops, which are undesirable in a vacuum application, and the filters can become blocked with contaminants.

InFIG. 1, a known in-line gas scrubber10suitable for use in a vacuum pumping system shown. The gas scrubber10comprises a hollow vessel12having an inlet port14through which process gas16enters scrubber10and outlet port18leading to the first stage of a vacuum pumping system (not shown).

Inlet port14is located at a position that meets the requirements of enabling the spray to develop, the provision of a disengagement space, optimal vapor velocity and a desired hold-up volume. The outlet port18is located towards the top of the vessel12. It will be noted that the inlet port14is aligned substantially horizontally whereas the outlet port18is inclined to the horizontal. The purpose of this is to cause the process gas16to have to change direction a few times as it passes through the vessel thereby increasing the amount of time it spends inside the vessel12before being extracted by the vacuum pumping system.

Located within an upper portion of the vessel12is one or more downwardly directed nozzles22adapted to spray a mist of solvent into the interior of the vessel12. Solvent is fed into the nozzle22via a supply line24from a tank (not shown) of solvent. The delivery of the solvent to the nozzle22is controlled using an in-line valve26on the supply line24. Thus, solvent enters the vessel12via the supply line24and a spray downwardly to form a mist within the vessel12by the nozzle22. Meanwhile, process gas16entering the vessel through inlet port14moves upwardly, that is contraflows, through the solvent mist enabling contaminants within the process gas16to be removed from it by dissolution into the droplets of solvent. The contaminated solvent falls to the bottom of the vessel12under the influence of gravity and is removed and via a drain line28.

The drain line28optionally has an in-line valve30to control the egress of contaminated solvent from the vessel12. If the flow rate of solvent into the vessel12is higher than the flow rate of solvent out of the vessel12, then a quantity of solvent will pool32in the bottom of the vessel12. To avoid the vessel12from over-filling, a solvent level sensor34is provided in the bottom of the vessel12for sensing the level36of the pool of solvent32. If the level36rises above the level of the solvent level sensor34control lines38and40operate the inlet valve26and the outlet valve32drain the vessel and maintain the level36below that of the level sensor34.

To maintain the low pressure of the process gas16, a barometric leg is required between the outlet line38and the dump tank44into which the contaminated solvent drains. The barometric leg is necessarily quite tall-often around 10 m in length, thereby making system difficult to install indoors. Additionally, it will be noted that a continuous supply of solvent is required and a corresponding amount of contaminated solvent needs to be disposed of. Accordingly, the known gas scrubber10occupies a large volume, consumes a lot of solvent and produces large volumes of contaminated solvent, which need to be disposed of in an environmentally friendly manner.

FIG. 2shows a gas scrubber100in accordance with the disclosure, which is similar in number of respects to the known gas scrubber shown inFIG. 1. As such, identical features inFIG. 2are identified using identical reference signs to corresponding features inFIG. 1for ease of understanding.

InFIG. 2, a glycol or butanediol (BDO) gas scrubber system100is installed upstream of a vacuum pump (not shown) connected to a polymer manufacturing plant, in this case, a PBT process. The scrubber can be used for in other applications, for example in a fatty acid deodorization and its use in a PBT process is only by way of example. The scrubber100is designed to remove oligomers, which are present in the process gas16being pumped, which are created during PET (Polyethylene terephthalate) or PBT (Polybutylene terephthalate) polymerization processes. The scrubber system100is also designed to catch droplets of process chemicals, which assists in protecting the vacuum pumps downstream of the scrubber100. The scrubber can also act as a spray condenser, which might reduce the capacity of a vacuum pump in certain circumstances.

The scrubber100consists of a hollow vessel12mounted on a skid (for which seeFIG. 3, in particular) alongside its auxiliary equipment and control systems. The scrubber100removes oligomers using glycol, butanediol (BDO) or other suitable solvent sprays and is particularly suitable for use as a vacuum gas scrubber for use with a dry vacuum pump system. Suitable solvents generally exhibit a vapor pressure that is lower than the system pressure at that point in the vacuum system (that is, at the pump's inlet).

The scrubber has a primary spray nozzle22fitted in the center of the main body of the vessel12and has three, additional, secondary spray nozzles220fitted at 1200 instances on a pitch circle diameter of 170 mm around the primary nozzle in the same horizontal plane.

The scrubber system100consists of a re-circulating pump102capable of delivering solvent (glycol or BDO) at required pressure and temperature. The material of construction for impellers is polypropylene with stainless steel material for various parts. A solvent cooler104is installed in the re-circulation circuit to remove the heat of condensation and excess heat of the process vapor16.

InFIG. 2, process gas16passes through a cleaning vessel12via an inlet port14and an outlet port16has previously described. The process gas16travel upwardly through the interior of the vessel12following a serpentine path, contra-flowing against the downwardly directed sprays or mists of solvent released by a number of spray nozzles22located towards the top of the vessel12. Notably, it will be seen that there are a number of spray nozzles22located in different radial positions within the vessel, the function of which configurations shall be explained in greater detail below.

The main difference between the gas scrubber100of the disclosure and that 10 of the prior art is that the solvent is recirculated and to do this additional controls and circuits are required. Solvent enters the vessel12via an inlet line24and the nozzles22in a manner similar to that described previously. However, rather than simply draining the contaminated solvent32to a dump tank, it is recirculated using a centrifugal recirculation pump102. In other words, solvent enters the vessel12via the inlet line24and is drained from the vessel12via a drain line28. The pump102is interposed between the drain line28and the inlet line24, such that the solvent can be recirculated through the vessel12.

The process gas16contains contaminants, which are removed by the mist solvent by dissolution or condensation processes. The latent heat of condensation of the contaminants (and the hot gas from a booster pump, should the apparatus be located between booster and dry vacuum pumps) causes the solvent32to be heated, which heat must be removed if the solvent is to be maintained at a desired operating temperature. In certain circumstances, it may be necessary or desirable to provide additional heaters, for example, trace heating around the pipes and vessel, to heat up the solvent if it becomes too cool, especially if the solvent has a relatively high viscosity at low temperature.

To achieve this, a heat exchanger104is provided downstream of the pump102and upstream of the inlet line24. The heat exchanger104ensures that the coolant is not contaminated and that the velocity of the fluid through the tubes is high to avoid blockages. The heat exchanger104comprises surface heat exchanger of the shell and tube type adapted for glycol or BDO cooling and being capable of removing heat at a rate of ˜6 kW using a cooling water flow rate of ˜2 m3/h. The heat exchanger has a design pressure of 22 bar and a design temperature of 1200 C. In this example, the cooling water passes on the shell side, whereas the glycol passes through the tubes, the tube material being Cupro-Nickel (90/10 grade).

The heat exchanger104is generally of a conventional design and comprises a coolant (for example, cold water, kerosene, ethylene glycol, and the like) inlet106and a coolant outlet108. The coolant is fed from a tank (not shown) using a pump (not shown) into the heat exchanger104by the coolant inlet106. The coolant passes through internal pipes (not shown) within the heat exchanger104to cool the solvent. Coolant leaves the heat exchanger104via the outlet108and fed back into the tank (not shown). Provided the tank (not shown) is sufficiently large, it is not always necessary to provide a chiller to cool the coolant. The coolant inlet106is provided with a flow control valve110, which is connected to a temperature sensing head112, which monitors the temperature of the solvent in the inlet line24. If the temperature of the solvent in the inlet line24exceeds a predetermined temperature, the temperature control head112opens the inlet valve (not shown) increasing the cooling capacity of the heat exchanger, and vice versa. A further valve114is provided on the coolant outlet108to limit the flow of coolant through the heat exchanger104and to enable the heat exchanger104to be isolated, if desired, from the tank (not shown). As such, the temperature of the solvent can be regulated automatically using the heat exchanger104and its associated controls110,112,114.

The recirculating pump102is capable of delivering up to maximum, and in the exemplary embodiment shown, 22 m3/hr of liquid. The maximum head can be achieved up to 22 m (at reduced flow). The pump102is designed to deliver a maximum of about 9.2 m3/hr of glycol at 1.5 barg discharge head.

To achieve this, the recirculating pump102must be primed at all times and is provided with a minimum head pressure. This is achieved by locating the recirculating pump102below the level of the vessel12and by maintaining the level36of the solvent32within the vessel12between an upper level116and a lower level118. To achieve this, the vessel12is provided with an upper level sensor120in the lower level sensor122whose positions within the vessel correspond substantially to the upper116and lower118levels, respectively.

The recirculating pump102is driven by an electric motor124whose speed and direction is controlled using an integral motor controller. A lower level sensing control circuit comprises a control line126that connects the lower level sensor122to the motor controller124. The lower level sensing control circuit is configured to cause the recirculating pump102to slowdown or stop in the event that the level36of solvent32within the vessel12falls below the lower level118. In addition, the lower level sensing control circuit comprises a valve control line130connected to a servo-operated valve132of a top up line134. The top-of-the-line134is connected at one end136to a supply of fresh solvent (not shown) and at its other end to an outlet138located within the vessel12above the upper solvent level line116. Should the level36of solvent32within the vessel fall below the lower level118, this will be detected by the lower level sensor112, which causes the top valve132to open thereby enabling level36of solvent32within the vessel12to be topped-up.

Similarly, an upper level sensing circuit comprises a control line128connected to the motor controller124and the upper level sensor120to control the speed of the recirculating pump102. In this case, if the level36of solvent32within the vessel12exceeds the upper level116the control line128can signal the motor controller124to speed up the pump12thereby draining the vessel12. Alternatively, the pump102can be configured not to speed-up but rather to have a regulated flow that is set to minimize nozzle blockage ensure a desired amount of cooling through the heat exchanger. In addition, should the level36of solvent32within the vessel12remain above the upper limit of116, the upper level sensor120is also connected via a further control line142the valve132of the top-up line34to reduce or shut-off the flow of fresh solvent into the vessel12. In addition, the upper level sensor120is connected via a third control line142to a servo-actuated valve144of the drain line28. Located downstream of the servo-actuated valve144is adjustable flow control valve146, which limits the rate of flow of contaminated solvent into the dump tank44. The purpose of the adjustable flow control valve146is to limit or prevent the vessel12from emptying too quickly and setting is likely to be pre-set during commissioning.

Using such a system, the solvent can be recirculated rather than simply used once and discarded and the level36of solvent32within the vessel can be maintained at a level that ensures that the recirculating pump12is continuously and correctly primed.

It will be appreciated that as contaminants are removed from the process gas16, the solvent32will become increasingly contaminated over time. The viscosity of a contaminated solvent32is generally higher than that of a fresh solvent and so it will be necessary, from time to time, to purge the system. In this regard, a solvent quality sensor148is provided with a lower portion of the vessel12and is connected to a computer150that analyses the condition of the solvent in real-time. The solvent quality sensor can be any sensor that senses any property of solvent that varies with quality, for example a specific gravity sensor, resistivity sensor, a capacity sensor, viscosity sensor, pump power or current sensor, or the like. In the illustrated example, an in-line flow sensor152is provided that monitors the flow rate of solvent within the feed line24. In the event that the viscosity of the solvent increases, the flow rate of the solvent would be expected to produce correspondingly, and a control line154is provided in such a situation to cause the motor124of the recirculating pump102to work harder. The control line154can be connected to the computer150to provide a real-time indication of the viscosity of the solvent within the system.

In the event that the computer150detects that the quality of the solvent within the system has fallen below a predetermined threshold value, it may signal a full or partial purge of the solvent32.

Purging is achieved by opening the server-actuated valve144on the drain line28two partially empty the vessel12. As the solvent32within the vessel12drains off, its level will eventually fall below the lower level line118, which will automatically trigger the top-up valve132to open thereby replenishing the solvent32with fresh solvent from the fresh solvent supply136. It will be appreciated that because the solvent32is only trained when large amounts of contaminants are being extracted from the process gas or during a purge, the volume of solvent required to operate is considerably lower than a continuously-fed system. One of the main advantages of this are reduced use of solvents and a reduced output of waste solvent. Moreover, because waste solvent is outputted batch-wise, management of the waste solvent is better facilitated.

The frequency of purging can be reduced by providing one or more in-line filters in the recirculating solvent circuit. In the illustrated example, in-line filters156are provided upstream of the recirculating pump12and upstream of the heat exchanger104. Isolating valves158are provided either side of the filters156to enable them to be changed or cleaned should they become clogged. Bypass lines (not shown) may optionally be provided to enable the scrubber100to continue to operate without filters156present, although it will be appreciated that in some circumstances, no significant harm will be done in the short time the scrubber is not operating.

The filter or filters156(as shown in greater detail in cross-section inFIG. 2A) have a filter inlet size of preferably but not exclusively 40 mm and are manufactures of 315 grade stainless steel. The perforated screen size is of the filter is preferably 400 microns giving a rated pressure drop of between 0.05 and 0.2 bar. The filters156are in-line type filters whereby solvent enters through the end of a removably retained basket-type filter scrim, and exits through the sidewalls thereof to the outlet.

InFIG. 3, an embodiment of a gas scrubber100according to disclosure is shown and comprises a vessel12having the inlet port14and outlet port18previously described through which process gas16enters and leaves the vessel12, respectively. In the illustrated embodiment, the cleaning solvent is a mixture of glycol and butanediol (BDO) and enters the vessel12by inlet pipe24. The temperature of the glycol in the inlet pipe24is monitored using a temperature gauge112and the flow rate of the solvent is monitored using a flow gauge152. Temperature gauge112is connected to a servo-actuated control valve110via a pilot line200, the servo-actuated control valve110be used to control the flow of cooling water into a heat exchanger104. The temperature of the solvent within the pipe24can therefore be regulated automatically by adjusting the supply of cooling water into the heat exchanger104in response to sensed changes in solvent temperature.

As previously mentioned, the solvent enters the chamber12by inlet pipe24and drains therefrom fire drain pipe28. Solvent is recirculated through the system by a recirculating pump102driven by an electric motor124. The level of solvent within the vessel12is monitored using the low level sensor122and the upper level sensor120in the manner previously described. Furthermore, the level of solvent within the vessel12can be top-up using top-of-the-line134.

It will be noted that the gas scrubber of the illustrated embodiment is mounted on a skid202for ease of transportation and mounting. It will also be noted that the overall size of the gas scrubber100is compact owing to the lack of a barometric leg that would otherwise need to be present in the scrubber100did not recirculated the cleaning solvent.

FIG. 4shows an arrangement of nozzles22within the vessel12in greater detail. Identical features inFIG. 4are designated by identical reference signs to identical features described previously, for ease of understanding. InFIG. 4, there are four spray nozzles located within an upper portion of the chamber12below the level of the outlet port18. In particular, there is a central nozzle206that is located substantially on the centerline of the vessel12and which provides a substantially solid conical, downwardly directed spray205of cleaning solvent. The “cone” of spray emanates from the nozzle206and travels axially and a radially to impinge upon the inner sidewalls of the vessel12at a point208below the level of the nozzle206, but above the level of the inlet port14. As such, process gas16entering the vessel12via the inlet port14must pass through the cone205spray created by the central nozzle206.

It will be noted that process gas210travelling up through the vessel12at or near to the centerline of the vessel12will travel through the cone205over a much greater distance212than the distance214traveled by process gas216moving upwardly through the vessel12nearer to its sidewalls.

In order to efficiently remove contaminants from the process gas16, the process gas16must travel a certain minimum distance through the spray of cleaning solvent. In order to ensure that this happens, one can simply elongate the vessel12to increase the distance that the process gas16must travel through the spray before it exits through the outlet port18. However, in a skid-based embodiment of the disclosure, such as that shown inFIG. 3, it is desirable to make the vessel12as compact as possible. Therefore, to ensure that the process gas16always travels through the cleaning solvent spray for more than a desired minimum distance, supplementary nozzles220are provided. It will be noted that the supplementary nozzles220,222are located radially outwardly of the main nozzle206and are arranged to provide substantially conical sprays207of cleaning solvent that impinge upon the inner sidewalls of the vessel12at a point209slightly higher up the vessel12than the cone205of the central nozzle206. By providing a plurality of axially spaced and overlapping cones205,209of solvent spray, even process gas to16moving upwardly through the vessel12adjacent the inner sidewalls thereof must pass through a minimum distance of spray to224, as defined by the difference in height of the intersection points208,209of the central205and the outer cones209on the inner sidewall of the vessel12.

It will be seen fromFIG. 5, that there are four nozzles including a central nozzle206and three radially outwardly spaced nozzles220at 120° instances.

It is also important that the nozzles do not become blocked during operation and for that the free passage of the nozzle is sufficiently large (˜6.4 mm) to maintain a free flow of solvent. The nozzles are also designed to be self-cleaning, although they can also be manually cleaned or serviced by removing the upper portion of the vessel12.

In the present example, the nozzles are of the type supplied by Spraying Systems Co., model: 1¼ H-SS6 FullJet Nozzles having female NPT connections. The nozzles are manufactured of stainless steel and have removable polypropylene vanes and a maximum temperature rating of 600 C. The nozzles can each deliver 38 liters/minute at 1.5 bar operating pressure, the optimum pressure being between 0.5 bar and 1.5 bar.

Turning now toFIGS. 6 & 7, which shows a variation on the arrangement shown inFIG. 4, it will be noted that the minimum distance224can be increased by providing an inwardly-projecting annular flange240on the inner sidewall of the vessel12. The purpose of the annular flange240is to provide an aperture located inwardly from the sidewalls of the vessel12through which the process gas16must flow thereby increasing the distance in which the process gas travels through the solvent mist or spray as it moves upwardly through the vessel. A further effect of the annular flange240is to disturb the downward flow of the solvent spray creating eddy currents241downstream of the flange240thereby creating a secondary spray nozzle beneath the flange240using solvent that runs down this inner sidewalls of the vessel12.

The annular flange240stops liquid from draining straight to the bottom of the vessel12and helps create an annular zone around the primary nozzle spray blocking any gas16escaping without being entrained by the primary spray nozzle. The location of the flange240is optimized with reference to the primary nozzle spray characteristics. The secondary function of the drip ring is to create another spray by gravitational drop of collected liquid from the wall thus enhancing the entrainment of the liquid particle from the process.

InFIGS. 6 & 7, it will be noted that all of the secondary jets207have impinged on the wall of the vessel at a level209higher than that of the annular flange240. As such, the small gap243shown inFIG. 7between the secondary jets is irrelevant because by the time any gas reaches this point, the gas has already been entrained by the full circular spray covering the whole inside diameter of the vessel and also because the effective inside diameter for the secondary spray needed is the inside diameter of the annular flange240, which is smaller than the inner diameter of the vessel12.

The nozzles206,222designs have an aperture therein that is large enough to not be clogged by particulate contaminants that may become entrained in the cleaning solvent and are adjustable during commissioning of the scrubber100to enable the shape, that is the spread and length of the components of spray, to be adjusted to suit particular applications. Furthermore, the annular flange240is removable or replaceable during commissioning so that the scrubber100can be set up to operate using a range of solvent having different viscosities or flow properties.

FIGS. 8 and 9show alternative embodiments of the invention as shown inFIG. 2in which the recirculation leg comprises an automatic siphon drainage system. InFIG. 8, the vessel12is part-filled with a solvent32up to a desired fill level36and comprises an outlet pipe28through which excess solvent32can drain. The outlet pipe28comprises a siphon arrangement500, which serves to maintain the solvent fill level36between upper502and lower504levels. The outlet pipe28comprises a gooseneck having a first tube portion506up to the desired upper fill level502and a second tube portion508connected to the first tube portion506by a U-bend portion510. The second tube portion terminates at a level corresponding to the lower fill level504and is arranged to discharge solvent32into a second vessel512. A pilot line514interconnects the evacuated spaces within the first12and second vessels512such that the vacuum pressure above the solvent30in?? (32 in) both vessels is equalised, which arrangement enables the siphon arrangement to function correctly.

As the first vessel12is filled with solvent32, the level36will increase until it reaches the upper fill level502, whereupon excess solvent will begin to flow over the U-bend portion510and siphon off into the second vessel512. When the fill level36reaches the lower fill level504, there is no head of solvent above the termination level of the second tube portion508, which breaks the siphon and prevents further emptying of solvent32from the first vessel12. Excess solvent is thus collected and stored in the second vessel, from where it can be drawn back to the solvent inlet via a feed pipe516which extends below the fill level of solvent32in the second vessel512, using a pump102as previously described. It will be noted that the pump102is connected to both the solvent32in the main vessel12and the second vessel512via pipes ensuring that it remains primed and able to recirculate solvent back to the solvent inlet24and nozzle(s)222.

Fresh solvent can be introduced into the first vessel12using a system described previously (not shown) and contaminated solvent can be taken off from either vessel12,512, also in a manner previously described.

FIG. 9shows a variation of the arrangement shown inFIG. 8in as much as the pilot line514is provided with a valve520(FIG. 9missing the valve designation) that enables a pressure differential between the vacuum pressures within the empty spaces of the first12and second vessels512to be maintained. In addition, the siphon arrangement ofFIG. 9additionally comprises a purge gas injection port522in the U-bend portion510of the outlet pipe28to enable the siphon effect to be artificially broken by the introduction of a purge gas into the siphon500. The purge gas524is introduced via a purge gas pipe526, and is controlled using a flow control valve528that is actuated by a level sensor530mounted inside the main vessel12. As the fill level36within the main vessel12exceeds the upper fill level502, the level sensor530triggers the purge gas control valve528to open allowing purge gas to enter the U-bend portion510thereby breaking the siphon and preventing further drainage of solvent32from the main vessel.

The embodiments shown inFIGS. 8 and 9are functionally similar to the arrangement shown inFIG. 1except that the barometric leg, which can cause installation problems in restricted spaces, has been removed. In addition, the embodiments ofFIGS. 8 and 9benefit from the advantages of the embodiment ofFIG. 2, with the added benefit of a mechanical/physical fill level regulation, which reduces the number of sensors and control valves need for the device to operate effectively. Nevertheless, in all cases, the fill level36of solvent32within the main vessel12is maintained within pre-set upper and lower limits, whilst maintaining the vacuum above the solvent32in the main vessel12.