Thermal swing adsorption process with purification

A process for regenerating a temperature swing adsorption unit comprising:sending a heated purge gas stream through an adsorption bed to remove impurities from said adsorption bed and producing a contaminated stream; sending said contaminated stream to a separator to produce a liquid stream and a vapor stream; returning said vapor stream as at least a portion of said heated purge stream until said vapor stream comprises above a predetermined level of impurities; and purging a portion of said vapor stream until the heated purge stream has a level of impurities below a second predetermined level.

BACKGROUND OF THE INVENTION

The present invention relates to a temperature swing adsorption process for the removal of an impurities, such as an acid gas, from a gas mixture, such as a natural gas stream. More specifically, the present invention relates to a temperature swing adsorption process that enables high purity levels at reduced capital costs and costs of operation including minimizing the loss of hydrocarbons to downstream processes and conservation of hydrogen.

A liquid temperature adsorption process which is integrated with a refinery may take advantage of the pressure swing adsorption (PSA) hydrogen which is available for use as the regeneration gas, especially as it tends to be low in impurities. However, the liquid hydrocarbon which is present in the macropores of the adsorbent will tend to be carried in the waste PSA hydrogen into downstream processes causing operational issues. Therefore, an issue is to use as much of the PSA hydrogen as possible and minimize the amount of liquid hydrocarbon carried over into the downstream processes.

Gas separation is important in many industries and can typically be accomplished by flowing a mixture of gases over an adsorbent that preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. One of the more important types of gas separation technology is swing adsorption, such as pressure swing adsorption (PSA) or temperature swing adsorption (TSA). PSA processes rely on the fact that when gases are under pressure they tend to be adsorbed within the pore structure of the microporous adsorbent materials or within the free volume of a polymeric material. When the pressure of the adsorbent is decreased, the adsorbed gas is released, or desorbed. By cyclically swinging the pressure of adsorbent beds between high pressures to adsorb and lower pressures to desorb, PSA processes can be used to separate gases in a mixture when used with an adsorbent that is selective for one or more of the components in a gas mixture that are to be removed.

TSA processes also rely on the fact that at cold temperatures gases tend to be adsorbed within the pore structure of the microporous adsorbent materials or within the free volume of a polymeric material. When the temperature of the adsorbent is increased, the adsorbed gas is released, or desorbed. By cyclically swinging the temperature of adsorbent beds between low temperatures to adsorb and higher temperatures to desorb, TSA processes can be used to separate gases in a mixture when used with an adsorbent that is selective for one or more of the components in a gas mixture that are to be removed.

DETAILED DESCRIPTION OF THE INVENTION

When operating a thermal swing adsorption unit, the adsorption bed generally goes through six distinct phases:

Offline Preparation: In a liquid TSA process the bed is first drained of the process liquid. In a gas TSA process the bed may be purged or pressure equalized.

Heating of the bed: During this stage, hot purge gas is passed into the bed and the temperature of the adsorbent rises. Typically, in treatment of natural gas, the entry temperature of the purge gas may be about 600° F. (315° C.) while the outlet temperature is over 100° F. less than the inlet temperature of the hot purge gas. During this stage a large fraction of the contaminants is removed from the adsorbent.

Thermal soak: Hot purge gas at about 600° F. (315° C.) continues to be fed to the bed. The exit temperature of the purge gas is less than 100° F. cooler than the inlet temperature of the hot purge gas. During this stage, the remaining contaminants are removed from the bed.

Cooling: Cool purge gas is fed to the bed (typically 80-100° F. (26 to 38° C.)), at a temperature close to ambient, and the adsorbent bed is cooled. During this stage the temperature of the purge gas leaving the bed falls from about 600° F. (315° C.) to less than about 200° F. (93° C.).

Online Preparation: In a liquid TSA arrangement, the bed is now filled with the process liquid in preparation for being brought online and in a gas TSA arrangement, the bed may be purged or pressure equalized.

Operation: Contaminated process fluid is fed to the bed and the clean process fluid is removed. The contaminants are adsorbed.

The problem with prior art processes is that to achieve a high purity of the clean process fluid, it is necessary to ensure that the contaminants have been removed from the bed to a sufficient level during the regeneration phase (during steps 2-4 above with the heating of the bed, thermal soak, and subsequent cooling). To achieve a successful regeneration of the adsorbent bed, it has generally been recommended that high purity gas be used for the regeneration step (with only low levels of the contaminants which need to be removed).

However, many operations have neither a source of purge gas meeting the purity levels nor a secondary operation which can utilize this waste purge gas. Therefore, the operators of many TSA processes generally prefer to use a closed loop regeneration system (FIG. 2). In a closed loop system, the operation may be similar to the open loop regeneration cycle, even though the fresh purge gas is drawn from the waste purge gas. For many operations, this is a suitable approach. However, there are a large number of processes where the purity levels of the waste purge gas are insufficient to enable the required purity of the clean process fluid to be achieved.

There are many ways to enable the waste purge gas to meet the purity specifications. The most common modification is to run the chiller at colder temperatures. This reduces the vapor pressure of the contaminants and a larger amount of the contaminants can be removed in the separator. If temperatures below 80° F. (27° C.) are required, this will typically necessitate the inclusion of a chiller loop, adding capital cost (CAPEX) and operating costs (OPEX). A typical arrangement for this type of solution can be found in U.S. Pat. No. 8,936,669 where in order to achieve the purity levels necessary, the chiller needed to operate at temperatures around 50° F. (10° C.)

Another solution is to use a two-step process in order to achieve the necessary purity levels. A chiller/separator operating at or near ambient conditions is then coupled to a second device which further removes contaminants from the waste purge gas. These devices can be one of multiple options including chemisorbant guard beds, another physisorbant, lean oil wash, fractionation, and catalytic destruction. U.S. Pat. No. 4,971,606 is an example of a situation where a reactive component needs to be removed and through the use of a second adsorption step, the waste purge gas can achieve the necessary purity. It is clear that this approach can be used to remove many different contaminants and it has been utilized frequently to achieve the specifications needed to run a close loop TSA system where the waste purge gas purity cannot be achieved by a simple cooler/separator arrangement.

The problem that is created in these solutions is that the solution used to remove the impurities not only leads to a major increase in the capital cost, but also operating costs. Unfortunately, in trying to solve the main problem of meeting the purity specification for the purge gas they have added unnecessary cost increases.

FIG. 1shows a prior art open loop configuration to regenerate adsorbent bed8with purge gas2which is first heated by heater4with heated purge gas6entering adsorbent bed8. A gas stream10containing impurities is first cooled by cooler12and then is sent to separator14to produce a waste purge gas stream16and a liquid stream18.

FIG. 2shows a closed loop configuration in which a treated purge stream20is heated by heater22with heated purge stream24entering adsorbent bed26to remove contaminants that are being desorbed. A stream28containing removed contaminants is cooled by cooler30and then enters separator32to be separated into liquid stream34and vapor stream36that is compressed to become treated purge stream20.

An alternative system for regenerating an adsorbent bed is shown inFIG. 3which shows a semi-closed loop system. A feed40is shown as stream42passing through heater44to produce heated stream46that enters adsorbent bed48. A contaminated stream50containing the impurities removed from adsorbent bed50is cooled by cooler52and sent to separator54with impurities sent into liquid stream56and vapor stream58is compressed by compressor60with compressed stream61sent to line64and combined with feed40. A portion of compressed stream61is shown being removed into purge stream62.

Under certain operating conditions it is desired to have additional treating of the vapor stream that is being used to desorb the impurities from the adsorbent in the adsorbent bed.FIG. 4shows such a system with a purge gas stream80that is heated by heater82with heated gas stream84passing through adsorbent bed86. A contaminated gas stream88that contains the impurities removed from adsorbent bed86is cooled by cooler90and then separated in separator92into liquid stream94and vapor stream96. Vapor stream is compressed by compressor98with compressed stream100sent through purifier102to result in purge gas stream80.FIGS. 5, 6, 7 and 9show several embodiments of the invention with scrubbing elements used to remove impurities from the purge stream.

FIG. 5shows a system to purify the vapor stream used in regenerating the adsorbent bed. A vapor stream120is heated by heater122to produce heated vapor stream124that passes through adsorbent bed126. A stream128containing impurities is cooled by cooler130and then in separator132is separated into liquid134and vapor stream136which is compressed by compressor138. Compressed stream140is passed through lean oil wash142with lean oil entering at146and waste exiting at144. Stream148then passes through a dehydration bed150to produce vapor stream120.

FIG. 6shows a regeneration system for two adsorbent beds. Hot vapor stream160is sent through adsorbent bed162with contaminated stream164cooled by cooler166and then passing through separator168to produce a liquid stream170and a vapor stream172that is compressed by compressor174with compressed vapor stream176being heated by heater178to form stream275and then passing through a dehydration bed180where water is removed from the bed to produce hot vapor stream160.

FIG. 7shows a lean oil wash being used to remove impurities from the vapor before it passes through the adsorbent bed. In this embodiment of the invention, a vapor stream200is heated by heater202with heated vapor stream passing through adsorbent bed206to remove impurities. Contaminated vapor208is cooled by cooler210and separator212results in liquid stream214and vapor stream216which is then compressed by compressor218. Compressed vapor stream220is sent to lean oil wash222to remove impurities with a purified stream224sent to dehydration unit226to produce the vapor stream200. Also shown are light naphtha232that is used in the lean oil wash222and that is passed through pump230to a feed line234for feed light naphtha, Fed light naphtha is subsequently treated in unit236used to produce product light naphtha238.

FIG. 8show several alternative methods to regenerate the adsorbent bed (such as the adsorbent bed226inFIG. 7). The adsorbent bed may be regenerated either in a TSA, PSA or combination of both (TPSA). Different guard bed regeneration are shown inFIGS. 8A, 8B and 8C.FIG. 8Ashows purifying a vapor stream224that passes through guard bed226to produce a purified vapor stream200. Also shown is the vapor stream208where the bed is in regenerating mode passing through a bed226with vapor stream210when the guard bed is being regenerated. InFIG. 8B, the guard bed is a temperature swing adsorption bed shown in purifying mode with stream224passing through TSA bed226to produce purified stream200. The TSA bed226is shown in regeneration mode with a hot fluid275and a fluid160containing contaminants shown.FIG. 8Cshows a PSA bed in purifying mode with vapor stream224to bed226and purified vapor stream200as well as a PSA bed in regeneration mode with a low pressure sweep gas238passing through PSA bed240and sweep gas242exiting containing contaminants removed from PSA bed226. It is noted that bed226may represent one or multiple beds in different stages of operation

FIG. 9show several alternative methods to scrub impurities from the vapor (such as the purifier102inFIG. 4). The scrubber may be a guard bed or a fractionator.FIG. 9Ashows a heavies fractionation unit to treat a feed liquid to produce clean vapor252. A feed vapor100is first condensed in cooler252to form liquid stream258and then passes through fractionation unit250with liquid251exiting, passing through reboiler254, waste liquid256exiting and a portion253returning to fractionation unit250. A purge gas stream80is produced as a vapor product

FIG. 9Bshows a light fractionation unit in which a feed vapor100enters light fractionation unit260with clean liquid270produced. A vapor stream262is cooled by heat exchanger264with a portion268returning to fractionation unit260and a purge gas stream80exiting.

When condensing a stream, the mixture will typically follow a condensation curve as shown inFIG. 10. The hot purge gas enters the cooler at TAand a loading of the contaminants of PA. At the end of the cooler the temperature has been reduced to TCand as a result the contaminants are now present at PC. Therefore, the amount of contaminants which have been removed is PA−PC. In order to achieve the necessary bed cleanliness it has been defined that a purge gas purity of PSis required, and this has driven companies to either use chillers to drive down the condensation to TS, or look at ways in which the loading PC−PScan be removed after the cooler/separator. The erroneous assumption has been that during the regeneration process, a loading of PSin the purge gas is always required. It is not required and at the early stages or regeneration, it is prudent to not have it at PS.

The loading PSin the purge gas has been specified so that upon completion of the regeneration step, the bed has sufficiently been clean so that it is able to achieve the purity of the clean process fluid. While the adsorbent is hot, little if any contaminants remain in the pores of the adsorbent. It is only during the cooling phase (step 4 above) that the contaminants which are present in the purge. While the adsorbent cools it develops a greater affinity for the contaminants and will remove them from the purge gas. If the contaminant levels are too high in the bed, then when the contaminated process fluid is passed through it, the purity specification will not be achieved.

However, during the heating process, this is not a problem. As PA>>PCthe hot purge gas will remove the contaminants from the bed. The idea therefore is only at the end of thermal soak stage, to reduce the loading to PSand do so in a way that does not impact CAPEX and OPEX. InFIG. 3, the regen system operates as a regular TSA and the loading in the hot regen gas is PC. Towards the end of the thermal soak step, the waste purge gas exiting the compressor (or cooler) is passed through a scrubber which brings it down to a loading of PS. Because it is a closed loop, the transition from PCto PSdoes not need to be instantaneous or once-through and over a series of many passes through the scrubber, a loading of PSa can be achieved. As long at the loading in the purge gas has been reduced to PSat the beginning (or even during the early stages of the cooling step) the bed will be able to achieve the clean process fluid specifications.

This scrubber can take many forms. They can utilize adsorption, fractionation or many other approaches to reduce the loading of the contaminants in the regen stream. A benefit over the equipment used in U.S. Pat. No. 4,971,606 is that because the transition from PCto PSdoes not need to be achieved in a single pass, the scrubbers can be optimized to reduce CAPEX. Further, more than one scrubber can be used and a process developed which does not necessitate the generation of a second waste stream (as with U.S. Pat. No. 4,971,606)

InFIG. 5, two different contaminants need to be removed. The first is can be removed using a lean oil wash and the second (herein water) is removed in a second TSA bed (DeHy). During the purge purification step, the waste purge gas is passed sequentially through the two units and over about 10 cycles, the loading of contaminants in the purge gas is reduced to PS. InFIG. 6, the DeHy bed is regenerated during the heat of the bed or thermal soak stages of the regeneration cycle, and the water which was adsorbed onto the bed will leave the system in the separator liquid. InFIG. 7, the contaminants which were removed in the lean oil wash column, are sent to the feed to an on-stream bed and then removed in the separator liquid at during that bed's regeneration step. The result of this integration is that a closed loop regen system can be employed using an ambient temperature cooler but achieve loading levels only achievable if substantially lower temperatures were employed.

There are two further benefits of this approach over U.S. Pat. No. 4,971,606. In a long-cycle TSA, the amount contaminants removed during the scrubber step are substantially less. In an example, where the separator removes 10 times the amount of contaminant than the scrubber [(PA−PC)=10×(PC−PS)] and the amount of time it takes to run the scrubber in this arrangement in only 1/10thof the time of the heating of the bed and the thermal soak stages (THS) then the amount of contaminants removed is substantially less.

Scrubber on stream continuously:
Mass removed by the cooler/separator=(PA−PC)×THS
Mass removed by the scrubber=(PC−PS)×THS=0.1×(PA−PC)×THS
Scrubber on only once no more liquid flows from the separator.
Mass removed by the scrubber=½×(PC−PS)×0.1×THS=0.005×(PA−PC)×THS

Therefore, the only 1/20 if the contaminants are removed by the scrubber in comparison to the approach taken by U.S. Pat. No. 4,971,606. The reason for this is that if the scrubber was run continuously, the bed would be continuously replacing the loading removed by the scrubber (PC−PS). However, as has been discussed the lower loading to the bed during this stage provides very little benefit as the gas is at a high temperature and the adsorbent has very little affinity for the contaminants when hot. Once the bed has reached the end of the thermal soak cycle, the separator no longer removes any liquid. Therefore, bringing the scrubber on at this stage would result in the loading of the feed to the scrubber dropping from PCto PSover a period of time and approximately half the mass of contaminate would need to be removed than if it remained constant. This reduction opens up the potential to use a chemisorbant for the removal of the contaminant during the purification step or smaller, cheaper equipment.

The second benefit can be found during the heating step. During the heating of the bed step, hot purge gas enters the bed and cooler gas leaves. This is because the majority of the specific heat (CP) of the gas is transferred to the adsorbent. Therefore, in order to heat the bed as rapidly as possible it is key to ensure that no heat is wasted during this step. As a result, the beds are frequently insulated. However, in running the scrubber continuously, a parasitic heat load is introduced. As the gas passes down through the bed, it is cooled and for a large fraction of the time will exit the bed at or near to ambient conditions. During this stage, contaminants will condense onto the adsorbent as the cool purge stream will leave the bed as a two-phase fluid. The gas phase would be loaded at PC. If the hot purge gas was introduced into the bed at PCthen during this phase or the bed heating the specific heat of the feed gas would be fully consumed in the heating of the adsorbent as any heat which had been used to vaporize the contaminant would be recovered by the lower part of the bed when the contaminant is condensed.

However, if the hot purge gas was introduced into the bed at PSand left at PCthen the latent heat which was necessary to saturate the gas up to PCwould not have been available to heat the adsorbent. In calculations, it is estimated that with a long 6-hour cycle necessary to heat the bed, this full utilization of the heat could potentially shorten the heating time by almost 30 minutes.

The idea is therefore to run a closed loop regeneration cycle for a TSA and then only once the separator no longer discharges liquid, bring the scrubber on-stream in order to reduce the loading from PCto PS. In doing so, in comparison to a solution like found in U.S. Pat. No. 4,971,606, a lower cost scrubber could be utilized and a smaller fraction of the contaminant needs to be removed by the scrubber.

In another embodiment of the invention, the traditional solution would be to use the PSA hydrogen in an open cycle arrangement. In a Naphtha CS2 case, 5,350 kg/h of PSA hydrogen would be needed, and after the condenser, it would be saturated with naphtha, around 2.5 wt %. This would continue until there is insufficient naphtha in the macropores of the adsorbent to raise the dew-point of the PSA hydrogen to the point that the condenser is able to crease a liquid. It is calculated that for approximately 6 hr a flow-rate of 134 kg/h of naphtha, totaling 770 kg will be delivered into the downstream processes as a result of the saturation of the PSA hydrogen. If a closed loop cycle was used, then while the naphtha is not sent into any downstream processes, the contaminates such as CS2 which were in the micropores would remain at elevated levels and the adsorbent would not be sufficiently regenerated. A possible solution would be to bleed in 10% of PSA hydrogen into a closed loop cycle and then send only 10% into the downstream processes. This would reduce the flow to 13.4 kg/h and a total of 111 kg. Still a potential issue. The invention operates the LTSA regeneration in a closed cycle for the first part of the process and then once the dew-point limit is reached, a small bleed of PSA-hydrogen is introduced (2-6% of flow). This bleed is slowly increased such that the peak naphtha flow does not increase. Once the closed loop has almost been completely purged, the full flow of the PSA hydrogen is allowed to remove the last traces of naphtha and CS2. The resulting analysis suggests that a total of 9 kg of naphtha would be sent into downstream processes at a peak flow-rate of 7 kg/h. In addition, given a longer cycle time, the peak could be reduced to less than 5 kg/h.

The invention is shown in the following Figures.FIG. 1shows an open loop regeneration scheme in which a purge gas stream2is sent through heater4with heated purge gas stream6entering the top of adsorbent bed8to remove impurities and producing a contaminated gas stream10that is cooled by cooler12and then in separator14, divided into a liquid stream18and a waste purge gas stream16. Operating in open loop mode there is an increased need for purge gas when it is not recirculated as in closed loop mode shown inFIG. 2where a compressed purge gas stream20is heated by heater22with heated purge gas stream24sent through adsorbent bed26to remove impurities and contaminated stream28being cooled by cooler30and then separated into liquid stream34containing the majority of the impurities removed from adsorbent bed26and vapor stream36in separator32. Vapor stream36is compressed by compressor38to produce purge gas20.

A third mode of operation is shown inFIG. 3which shows a regeneration scheme in semi-closed mode. A feed40is combined into vapor stream42that is heated by heater42to heated gas stream46. Impurities are removed from adsorbent bed48with contaminated stream50being cooled in cooler52and then separated in separator54into a liquid stream56and a vapor stream58to be compressed by compressor60. A portion of compressed stream61is purged and the remainder64of the stream is combined with feed40.

FIG. 4shows a mode of operation with purification of the stream that is used in regeneration of the adsorbent bed. A gas stream80that has been purified in scrubber102is heated by heater82and then the heated gas stream84is sent through adsorbent bed86. A contaminated gas stream88that contains impurities from adsorbent bed86is cooled by cooler90and then sent to separator92with a liquid stream94and a vapor stream96being the products of separator92. Vapor stream96is then compressed by compressor98.

FIG. 5shows a purifying mode for the system andFIG. 6has actual regeneration of the adsorbent bed. InFIG. 5, a gas stream120is heated by heater122with heated gas stream124passing through adsorbent bed126with contaminated stream128going to cooler130to separator132. A liquid stream134and a vapor stream136exit separator132with vapor stream136passing through compressor138to compressed stream140and a lean oil wash146with waste LN144exiting and wash LN146entering the lean oil wash. A clean stream146is then sent to be dehydrated in vessel150.

FIG. 6shows an embodiment of the invention in regeneration mode. A gas stream160is sent to an adsorbent bed162to remove impurities from the adsorbent. A contaminated gas stream164then is cooled by cooler166and then in separator168a liquid stream170and a vapor stream172are produced with vapor stream172compressed by compressor174. Compressed stream174is heated by heater178and dehydrated by bed180to produce gas stream160.

Any of the above conduits, unit devices, scaffolding, surrounding environments, zones or similar may be equipped with one or more monitoring components including sensors, measurement devices, data capture devices or data transmission devices. Signals, process or status measurements, and data from monitoring components may be used to monitor conditions in, around, and on process equipment. Signals, measurements, and/or data generated or recorded by monitoring components may be collected, processed, and/or transmitted through one or more networks or connections that may be private or public, general or specific, direct or indirect, wired or wireless, encrypted or not encrypted, and/or combination(s) thereof; the specification is not intended to be limiting in this respect.

Signals, measurements, and/or data generated or recorded by monitoring components may be transmitted to one or more computing devices or systems. Computing devices or systems may include at least one processor and memory storing computer-readable instructions that, when executed by the at least one processor, cause the one or more computing devices to perform a process that may include one or more steps. For example, the one or more computing devices may be configured to receive, from one or more monitoring component, data related to at least one piece of equipment associated with the process. The one or more computing devices or systems may be configured to analyze the data. Based on analyzing the data, the one or more computing devices or systems may be configured to determine one or more recommended adjustments to one or more parameters of one or more processes described herein. The one or more computing devices or systems may be configured to transmit encrypted or unencrypted data that includes the one or more recommended adjustments to the one or more parameters of the one or more processes described herein.

SPECIFIC EMBODIMENTS

A first embodiment of the invention is a process for regenerating a temperature swing adsorption unit comprising sending a heated purge gas stream through an adsorption bed to remove impurities from the adsorption bed and producing a contaminated stream; sending the contaminated stream to a separator to produce a liquid stream and a vapor stream; returning the vapor stream as at least a portion of the heated purge stream until the vapor stream comprises above a predetermined level of impurities; and purging a portion of the vapor stream until the heated purge stream has a level of impurities below a second predetermined level. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising cooling the contaminated stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising determining a purity of the heated purge gas, comparing the purity to a predetermined value and purging at least a portion of the heated purge gas when the predetermined value is exceeded. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising scrubbing a vapor stream prior to heating the vapor stream to become the heated purge gas stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the scrubbing is done by a scrubber comprising a guard bed or a fractionator. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the guard bed is a pressure swing adsorption bed or a temperature swing adsorption bed. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the process uses an open loop, closed loop or semi-closed loop. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the scrubber comprises a guard bed and a lean oil wash unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising at least one of sensing at least one parameter of the process and generating a signal from the sensing; sensing at least one parameter of the process and generating data from the sensing; generating and transmitting a signal; generating and transmitting data. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the signal results in a scrubbing of the purge gas stream.