DE112004000175T5 - Cyclic membrane separation process - Google Patents

Abstract

A cyclic gas separation process for separating preferred permeable gas constituents from less preferred permeable gas constituents of a feed gas mixture of such constituents, the process comprising the steps of:
(a) providing a membrane module having a selectively gas-permeable membrane for the preferred permeable gas constituents and for the less preferred permeable gas constituents,
(b) simultaneously (i) feeding the feed gas mixture into the module to contact the feed gas mixture with a first side of the membrane; (ii) ejecting a permeate gas mixture enriched with the preferred permeable gas constituents from the module Fluid communication with a second side of the membrane and (iii) withdrawing a retentate gas mixture enriched with the less preferred permeable gas constituents from the module in fluid communication with the first side of the membrane,
(c) stopping the supply of the feed gas mixture to the first side, stopping the ejection of the permeate gas mixture, and stopping the withdrawal of the retentate gas mixture,
(d) charging the module with a diluent gas,
(e) stopping the feed ...

Description

Territory of invention

These The invention relates to a cyclic method in which a selectively gas permeable membrane is used to separate a component from a gas mixture. In particular, it relates to a membrane separation process for recovery volatile useful for organic compounds is that expelled from storage tanks, with a membrane is used, which comprises a selectively gas-permeable membrane polymer. The method involves repeated cycling between flow and none Flow of gas through the membrane.

background the invention

Liquid, volatile organic Compounds ("VOC") are in tanks stored and submitted from these. A very common example is found in the field of fuel delivery for internal combustion engines, such as z. As gasoline for refueling of motor vehicle and aircraft engines. The storage tanks usually have a big Capacity, take major deliveries of fuel from a supply source and give several times smaller quantities, z. B. when filling individual automobile tanks at petrol stations. The gas space over the liquid in the tank is sometimes referred to as "Ullage" of the tank in the ullage of fuel storage tanks a high VOC concentration in front.

Before the air pollution by VOC emissions an environmental protection topic emissions controls on storage tanks were mainly on it directed to avoid fire and explosion hazards. Few controls aimed, fleeting Emissions, such. B. with the delivery of fuel from large storage tanks and VOC emissions associated with storing the fuel in the tanks, curb.

In younger In the past, there has been a growing awareness of the need to volatile emissions, that result from the storage and delivery of VOC. As a result, were getting higher developed vapor recovery systems for VOC used. For example, emissions of VOC vapor into the environment when refueling motor vehicles and other types of refueling too Fuel suppliers and distributors have started at petrol stations Vapor recovery systems to install. These systems usually have a suction device on, the VOC vapors and air present during fuel transfer to the fuel nozzle are in the ullage of a large storage tank back sucks. The refluxing gas mixture flows in the tank empty space, which is generated when the discharged liquid exit.

traditionally, Storage tanks had only P / V valves (pressure-vacuum valves) that covered the tank in the area a slight over- or negative pressure, d. H. at a water pressure of several inches. The refluxing gas mixture from disbursements as well as other factors to the fact that in Ullage pressure built up over time. Of course, if the tank pressure exceeded the upper limit for the P / V valve, excess, VOC-containing gas expelled into the environment.

Certain advanced systems to control the emission of volatile VOCs are designed with a slight negative pressure in the ullage of the bulk storage tank work. The tank is located in a vacuum opposite the surrounding atmosphere. These systems offer the advantage that any occurring Leaks are more likely to enter outside air into the vapor recovery systems as lead to the escape of steam into the atmosphere. In addition to the aforementioned gas construction carries air leakage to increase the pressure in the tank. The liquid fuel evaporates in the inflowing Fresh air, and the mass of vaporized fuel plus the mass The air inside the solid Ullagevolumens increase the pressure. This can be a negative pressure only be maintained if from time to time gas in the Environment expelled becomes. However, it is necessary to completely or partially withdraw the VOC from the Remove exhaust gas. Otherwise, the VOC will be in the gas spouted negate the purpose of the pollution control system.

Various methods have been proposed for extracting VOC emissions from bulk storage tanks operating at reduced pressure. One method that gains commercial acceptance utilizes a selectively gas permeable membrane to separate the VOC component from the useful air component of the ullage mixture. The non-VOC component, which consists predominantly of nitrogen and oxygen is preferably permeable through the membrane and is expelled substantially free from the VOC fraction into the atmosphere. VOCs are less permeable, do not pass much through the membrane, and are returned to the storage tank.

The Membrane separation-vapor recovery system is for cyclical working and discontinuous emission into the atmosphere intended. Emissions occur only when the tank pressure is a preselected High pressure limit is exceeded. At other times, the flow through the membrane is interrupted. For example, falls the tank pressure due to the discharge of predominantly non-VOC components containing gas in the surrounding atmosphere below the high pressure limit. At a preselected Low pressure limit, the output is interrupted. At these times the steam stagnates in the separation membrane module and in the gas transfer lines, which are immediately upstream and downstream of the module.

The Although separation membrane leaves selectively oxygen and nitrogen, but has VOC compounds not completely back. Consequently contains the gas that passes through the membrane and into the environment subtracting some VOC vapor, albeit less than withdrawn would, if the membrane was not used. It was determined, that a very highly concentrated stream of VOC vapor at the beginning a venting cycle, d. H. immediately after the pressure increase in the tank through a river initiates the membrane and at the end of a stagnation period, the bleed begins to emerge from the membrane module. Decreased after a while the VOC concentration in the permeate / exhaust gas is expected to be the same lasting value. A significant amount of VOC vapor will reach up to the time at which the gas vent section of the cycle is interrupted, released into the atmosphere. This leads to, that the time-averaged amount of VOC compounds attached to the Air is discharged, is still unacceptably high.

It is desirable Total emissions of VOC compounds below value reduce that from conventional Fuel tank vapor recovery systems stir on the basis of a separating membrane.

detailed description

From the schematic flow diagram of FIG 1 It can be seen that a conventional system for dispensing liquid fuel is a large capacity fuel tank 1 which has a stock of liquid fuel 2 contains. The volume of the tank above the liquid level 3 is as Ullage 4 known. The liquid fuel is typically a highly volatile organic compound ("VOC"), and therefore the oil is taken up by a gas composition having a high concentration of VOC vapor in a typical fuel dispensing operation designed to refuel automotive tanks with gasoline. The liquid gasoline is transferred via a transfer line 5 leading to a pumping station 6 leads, withdrawn from the storage tank. The gasoline comes from a fuel dispensing pump 7 over a hose 8th through a neck 9 in the fuel nozzle 11 and in the vehicle tank in the automobile 10 issued.

Modern, conventional fuel delivery systems also typically include a steam trap 12 on. Typically, the steam trap is part of the fuel filler neck. The trapping device 12 is configured to transfer volatile VOC vapor that exits when the fuel flows into the fuel nozzle into the vapor transfer line 13 to draw. This vapor is generated by a small amount of the volatile liquid fuel which evaporates as it flows into the tank and VOC vapor present in the motor vehicle tank which is displaced by the incoming liquid fuel. The capture device may also draw some air that passes through gaps in the seal between the capture device and the fuel nozzle.

The Ullage of the large storage tank and the steam transfer line 13 are preferably maintained at negative pressure, so that any leaks draw vapor into the tank. This prevents pollution that could be caused by leakage of VOC vapor if the pressure were positive to the atmospheric pressure. Some dispensing system designs use a vacuum pump in the vapor transfer line 13 (Not shown). This complements the vacuum impulse provided by the low pressure in the storage tank to trap fugitive emissions at the stub. Many independently working Dispensing stations can be connected to the bulk storage tank, although only one is shown. For systems with multiple storage tanks, a common steam transfer line is often used.

The large storage tank is equipped with a vapor recovery system 20 equipped, among other elements 28 . 15 . 23 . 24 and associated transfer lines. The recovery system 20 works mostly so that it in the Ullage 4 generates a negative pressure. Gas in the ullage is by means of a gas conveyor 28 subtracted and through the membrane module 15 bubbled. Then the gas passes through the vent line 25 ejected into the atmosphere. A second gas conveyor 23 additionally helps to remove the vented gas.

The operation of the vapor recovery system 20 takes place in repeated cycles, each having two consecutive sections. Typically, system operation cycles between portions needed to maintain the pressure in the ullage at a vacuum between a lower vacuum limit and an upper vacuum limit. When dispensing fuel at the station 6 steam continues to flow through the pipe 13 into the Ullage 4 and the pressure in the ullage may increase or decrease depending on the volume ratios of discharged fuel to gas being returned to the ullage, but usually it increases. Inward leakage causes the pressure inside the tank to gradually increase. The pressure in the storage tank may also increase as a result of refilling the tank itself as a result of temperature changes or evaporation of the liquid over time. As soon as the pressure has risen to the upper limit of vacuum, the first section, and the gas conveyors, begin 28 and 23 start to run. As this first operating section proceeds, gas is expelled from the vent by the continued operation of the gas delivery devices and the pressure in the tank drops. When the pressure falls below the lower pressure limit, the second section begins and the gas delivery devices shut down. This causes the movement of the gases through the vapor recovery system 20 to stop. The pressure builds up and the cycle repeats.

More specifically, the vapor recovery system includes a vapor extraction conduit 14 which is in a membrane module 15 leads, which is a selective gas-permeable separation membrane 16 having. The membrane divides the interior of the module into a feed-retentate chamber 17 and a permeate chamber 18 which are respectively in contact with opposite sides of the membrane. The membrane material is able to pass certain components of the gaseous mixture removed from the ullage and reject other components. Typically, oxygen, nitrogen and other low concentration, low molecular weight gaseous components present in air, e.g. Argon, ozone, carbon dioxide and the like, through the membrane. VOC vapor molecules pass very slowly compared to the other constituents passing through. Consequently, the feed-retentate chamber is enriched with VOC passing through the reflux line 19 in the large storage tank 1 to be led back. An air conveyor 28 , such as As a vacuum pump, a fan, a fan or similar mechanism forces the VOC-enriched gas through the return line 19 ,

As a result of selective permeation, the gas in the permeate chamber 18 a lower concentration of VOC than the gas in the ullage of the tank. However, as is usually the case, it may contain a small amount of VOC. Usually, the purified air is reduced with reduced VOC contamination through the line 22 using the air conveyor 23 withdrawn from the permeate chamber. This air can then be expelled into the atmosphere. A check valve 24 or a similar conventional flow control device may be employed to prevent ambient air from flowing back into the storage tank through the vapor recovery system and increasing the pressure in the tank. The main task of the vapor recovery system is to emit as little VOC as practicable into the environment.

The method and apparatus according to this invention differ from the conventional VOC vapor recovery technology primarily in that for at least a portion of the second portion of the cyclic process, an amount of diluent gas enters the membrane module 15 is introduced. As the diluent enters, the valves in the vapor recovery system are adjusted to direct the flowing diluent in a manner that will be discussed in more detail below. The diluent gas may be any gaseous composition that is free of the components that are rejected by the membrane, ie, free of VOC. The diluent gas should also not react with VOC under the conditions found in the vapor recovery system. Examples of suitable diluent gas compositions include air, carbon dioxide, hydrogen, helium, nitrogen, and mixtures thereof. Preferably, the diluent gas is air.

As an advantageous consequence of the introduction of diluent gas into the membrane module during the second portion of each cycle, the amount of VOC expelled from the vapor recovery system to the environment per cycle is reduced. The exact reason for the reduction in VOC emissions, which has been found to occur when air is deliberately introduced into the module during the second section of each cycle, is not yet clear. Without wishing to be bound by theory, it is believed that this reduction is based on two phenomena. First, during operation of a conventional vapor recovery system, as shown in FIG 1 is shown at the end of each first section VOC in the module. The gas concentration in the feed chamber is the same as in the ullage of the storage tank. The second portion of each cycle typically lasts about 30 minutes and is significantly longer than the first portion. During this period, the VOC concentration on both sides of the membrane equalizes. Thus, a comparatively large amount of VOC migrates into the permeate chamber. At the beginning of the first section of the next cycle, the amount of VOC present in the permeate chamber flows forwards to the vent transfer line and finally to the surrounding environment. In the new method, however, a significant portion of the VOC present in the module at the end of the first section is displaced by the diluent gas to the storage tank before the first section of the next cycle begins. Therefore, at the beginning of each first section, fewer VOCs are available to flow through the module and exit through the vent line.

Secondly the diluent gas tends to purge the free volume of the selectively gas permeable membrane. in the In comparison, the conventional method causes the action the high concentration of VOC during the second section on the membrane that the VOCs present in the module occupy a high proportion of the free volume of the membrane composition. To name In this state, the term "plastification" (membrane) is used. The plasticized membrane is not in optimal condition, to selectively remove the non-VOC ingredients while the first section of each cycle. Especially would be too expect a VOC plasticized membrane to be a higher amount at VOC, as one that is not plasticized. After the new procedure flows the diluent gas past or through the membrane. At least that's what draws remove some of the VOC from the free volume and bring the membrane thereby in a better condition to the gas mixture components while of the first section of the next Cycle through selectively.

The new procedures and the new system are thus preferably at least about 5% of the VOC, otherwise would be present at the start of the first section. More preferably, the withdrawn Amount at least about 10%, and more preferably at least about 25%. VOC emissions are lower than those that would occur if the diluent gas during the second section would not have been added to the module. The VOC emissions are preferably reduced by more than 10%.

The effectiveness of the new vapor recovery system can be demonstrated by 2 be explained. The curve A represents the typical performance to be expected during a cycle for a fuel storage tank provided with a conventional vapor recovery system as shown in FIG 1 shown is working. It is a graphical representation of the concentration in the vent line 25 of the VOC content in volume percent of the gas exiting a hypothetical system. The accumulation of gas during fuel dispensing operations in the storage tank 1 flows back, and the leakage inside increases the pressure inside the storage tank to a pressure above the high vacuum limit. This triggers the operation of the vapor recovery system. The operation of the first section begins with the commissioning of the gas conveying equipment 28 and 23 (Point A1). The detection of an increase in VOC concentration by a sensor in the vent line 25 occurs a short time later, usually about several seconds later (point A2). As the operation of the first section of the recovery system continues, ullage gas selectively passes through the membrane and displaces the gas in the permeate chamber, which initially has a high VOC concentration, for venting. This lowers the pressure in the storage tank and also causes the VOC concentration in the vent line to rise sharply (point A3). The membrane causes rejection of VOC, and thus the concentration of VOC in the emitted gas peaks and begins to decrease (point A4). Thereafter, the VOC concentration gradually decreases (point A5) and begins to reach a constant value. The time elapsed between points A1 and A6 is relatively short, usually ranging from about 30 seconds to about 30 minutes. When a sufficient amount of gas has been withdrawn from the system to lower the pressure in the storage tank below the lower vacuum limit (item A6), an automatic control system will cause the gas delivery equipment to be shut off 28 and 23 stop operating when the second section starts. The second section usually takes a long time compared to the first section. It is not uncommon for the duration to be at least about 15 minutes, and it can range from about 30 minutes to about 1 to 3 hours or even longer, which depends on the size of the storage tank and the Ullagegasvolumens and the leakage rate inward. Fractions in the abscissa and the curves in 2 show the longer duration. Because the sensor in the vent line 25 is normally spaced from the module, and since the flow is stopped during the second section, the VOC concentration remains at a value VOC ₀ . This value is well above zero, as the VOCs sometimes pass through the membrane during the first stage and are present in the leaked gas. The first section of the next cycle begins at point A1 '.

2 Figure 4 also shows the VOC concentration versus time versus time curve V of a similar VOC fuel dispensing unit operating the vapor recovery system of this invention. The events in the cycle take place exactly at the same instants just described. This means that the first section takes place in the period between points B1 and B6. The period of the second section lies between point B6 and point B1 '. However, part of the ambient air is introduced into the membrane module during the second part of the operation. Due to the supply of air, the maximum concentration (point B4) is lower than the maximum concentration in the conventional method. In addition, the VOC concentration of gas in the vent line eventually decreases to near zero as soon as the next first section (point B1 ') begins because the module is purged with diluent gas that is free of VOC. The rate of reduction of the VOC concentration in the second section (ie, between points B6 and B1 ') depends on the geometry of the particular system and the location of the sensor relative to the position at which the diluent gas is introduced. Overall, the cumulative area under curve B1 during the first section (ie, between points B1 and B6), which represents the total amount of VOC released into the environment per cycle, is less than is usually the case.

Now, various embodiments of the new method and system will be described with reference to FIG 3 described. Like parts in different figures of the drawings have common reference numerals.

According to one aspect of the invention, the dilution gas is supplied to the membrane module at a position that is in fluid communication with the permeate chamber of the module. Thus, in one embodiment of the present invention, the vapor recovery system is modified to be an automatic diverter valve 36 in the diluent gas supply line 35 having. During operation, the valve becomes 36 opened for a period during the second section of the cycle. As a result, a supply of diluent gas, preferably ambient air, can enter the permeate chamber. Under the driving compressive force due to the vacuum state, which then in the ullage 4 is present, the air flows back through the membrane and into the feed-retentate chamber. The fresh air thus dilutes the VOC in the permeate chamber and purges the plasticizing VOC from the free volume of membrane material, as expected. At a point in time defined by a predetermined period of time, a detected VOC concentration in the system, or a pressure in the system, backflow into the ullage is halted. This prevents a further increase in the pressure in the storage tank. The arrest of reflux can be achieved by closing the valves 32 and 33 or alternatively by closing the valve 36 be accomplished. Before the first section of the next vapor recovery cycle begins, the valves become 32 and 33 in the open and the valve 36 returned to the closed state.

The position of the valve 32 is not critical. It can be located anywhere in the feed gas transfer line 14 . 21 between the Ullage 4 and the feed-retentate chamber. The location of the valve near the inlet to the feed-retentate chamber is preferred because it reduces the volume of dead space encountered by the diluent gas as it flows to the ullage. Likewise, the valve 33 be arranged arbitrarily in the retentate line between the storage tank and the membrane module, but it is preferably close to the feed-retentate chamber. Similarly, the dilution air supply line 35 downstream of the gas conveyor 23 to be ordered. Of course, this requires that the gas conveyor 23 and any other intervening means in the permeate effluent line from the air inlet point to the permeate chamber of the module do not appreciably impede gas reflux. It is recalled that the dilution air is supplied during the second portion of the vapor recovery operation, during which the gas delivery devices 28 and 23 are turned off.

There are also other variants of the above-mentioned embodiment into consideration. For example, the line 35 lead directly into the permeate chamber. When air passes through the conveyor 23 can flow back when the device is stopped, the function of the valve 36 also through the valve 37 or a bidirectional valve, which is the check valve 24 replaced, replaced. For example, the check valve 24 be replaced by a pressure-vacuum ("P / V") valve that introduces dilution air into the permeate chamber during the second section until the pressure gradient in the entire P / V valve is below a preselected minimum value falls.

In another aspect, the diluent gas is supplied to the membrane module at a position in fluid communication with the feed-retentate chamber of the module. For example, dilution air would be through the pipe 35a fed and through the valve 36a controlled. In operation, the check valve 32 closed in due course while air passes through the valve 36a flows. Since the ullage is in negative pressure relative to the incoming diluent gas, the gas flows through the feed-retentate chamber and over the line 19 back to the Ullage.

For better results, ie lower VOC emissions, the diluent gas should be the feed-retentate area of the membrane 16 sweep. Care should be taken to the position of the line 35a for incoming diluent gas to be configured to ensure that a substantial portion of the gas does not bypass contact with the membrane surface. This can often be achieved by the supply line 35a and the reflux line 19 At opposite ends of the feed-retentate chamber can be arranged.

Other contemplated variants are those in which the diluent gas supply line is located at a different position in the module supply line. For example, the line 35a in the transfer line 14 upstream of the gas conveyor 28 be arranged. With the line 35a in this position should the valve 38 be closed during the second section, and the gas conveyor 28 Optionally, it can be used to force air through the feed-retentate chamber. If the check valve 24 is not designed to prevent the escape of a possible outflow, it is recommended for this purpose an optional divisional valve 37 in the permeate discharge line.

In another possible variant, the line 35a in the pipe 19 between the valve 33 and the feed-retentate chamber.

As mentioned a main task of the new vapor recovery system is to to allow reduced environmental emissions of VOC vapor while the vapor system is in fluid communication with the ullage of the storage tank at a pressure below the ambient air pressure is operated. Lower emissions occur when a diluent gas during at least a portion of the second portion of each vapor recovery operating cycle is added in the membrane module. Certain control protocols will be considered to achieve this.

According to such a protocol, the diluent gas is supplied to the module at times when the pressure at a preselected position in the system is within a predetermined pressure range. Again, it is recognized that the start of the second section of the vapor recovery cycle is characterized by a low pressure in the steam treatment system. Feeding the module with diluent gas causes the system pressure to increase. This control protocol allows the diluent gas to flow into the module until the system pressure rises to a predetermined upper pressure limit. This protocol can be implemented in several ways. For example, an electronic pressure sensor may be used to initiate control of the diluent gas inlet. In another representative example, the control may be mechanical, e.g. B. by using a P / V valve in position 24 ( 4 ), As previously mentioned.

Another operating protocol requires the diluent gas feed for a duration effective to maintain a particular concentration at a position in the module or a connected piping system. In this case, a sensor for a VOC concentration analyzer can be brought into fluid communication with the feed-retentate chamber or the permeate chamber. The analyzer should be able to provide real-time VOC concentration analysis and generate a signal for input to an automatic control system. The control system is adapted to the valve 36 or 36a in response to the input signal. Such analyzers and control systems are well known to those skilled in the art. An example of such a control system is the C series programmable logic controllers (PLCs) available from Omron Electronics LLC, One East Commerce Drive, Schaumburg IL, 60173. An example of a suitable analyzer is Model 317 WP of the non-dispersive infrared hydrocarbon sensor manufactured by Nova Analytical Systems, LTD., 270 Sherman Ave N., Hamilton, ON, CA, L8L 6 N5. The vapor in the module or piping system has a relatively high initial VOC concentration at the beginning of the second section. Upon introduction of the diluent gas, the VOC concentration decreases. This protocol provides that the diluent gas flows into the module until the VOC concentration at the sensor position falls below a predetermined lower concentration limit.

at In another operation log, the diluent gas feed is via one Period of a predetermined period of time. In this case, the gas dilution feed valve open at a time after the start of the second section the entry of diluent gas to allow in the module. The valve remains only for one in advance elected Duration open. After expiration of the time limit, the diluent gas supply valve closed. The diluent gas delivery period preferably starts simultaneously with the onset of the second Section. The flow rate of the diluent gas is another parameter that can be adjusted to reduce to optimize VOC emissions. For example, during the predetermined duration of the gas feed the flow rate at a fixed Value to be kept. Emission results may be one or more consecutive following cycles are observed. Either the dilution gas flow rate, the diluent gas delivery time or a combination of speed and duration in different Cycles changed to determine which settings provide optimal emission performance bring. In another embodiment considered can the flow rate of the diluent gas throttled according to a predetermined program. there can open when Dilution gas valve the flow rate elevated, reduced or otherwise for best results are set.

It It is understood that any combination of more than one of previously mentioned Control protocols can be implemented. One with the teaching of this Revelation armored One of ordinary skill in the art will be able to adjust the control variables so affect him without excessive experimentation the lowest VOC emissions can reach.

The new procedures and the new system work with a module that a selectively gas permeable Membrane comprises. Any membrane composition that has good selectivity for constituents from the air VOC can be used. Usually, the membrane a polymer composition. VOCs are known to be solvents for many Polymers. Therefore, the membrane composition versus VOC be inert.

The membrane should preferably contain a thin layer of a selectively permeable, nonporous polymer having a large free volume. The non-porous layer can be applied to a porous substrate, such. As a microporous hollow fiber, be applied. Representative polymers include polytrimethylsilylpropyn, poly-fluoroallyl vinyl ether), copolymers of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole and tetrafluoroethylene (TFE), as well as certain amorphous copolymers of perfluoro-2,2-dimethyl-1,3- dioxol ("PDD"). PDD copolymers are particularly preferred because they have a unique combination of superior permeability and selectivity for various gas mixtures, particularly preferred are PDD copolymers with fluoromonomers such as TFE, vinylidene fluoride, perfluoromethyl vinyl ether, hexafluoropropylene , Chlorotrifluoroethylene, and mixtures thereof. Gas separation membranes comprising PDD are disclosed in U.S.P. US 5,051,114 (Nemser et al.), The entire disclosure of which is hereby incorporated by reference.

The structure of the membrane module is not critical. Flat, folded, spiral wound, ribbon tubular membranes and hollow fiber membranes can be used. Hollow fiber membranes are preferred. Hollow fiber membranes can be combined in large numbers in a so-called hollow fiber membrane module. The structure and method for hollow fiber membrane modules are well known to those skilled in the art, see for example US 3,339,341 (Maxwell et al.) And US 5,985,002 (Grantham), the disclosures of which are hereby incorporated by reference.

The The above disclosure is broadly based on embodiments of this invention directed, in which the polymer component of the selectively gas-permeable membrane in a so-called glassy polymer state. It is well known in the field of physical chemistry of polymers that amorphous polymers and amorphous domains of crystalline polymers phase transitions second Subject to order, which are defined by a glass transition temperature ("Tg"). At temperatures well below Tg, these polymers harden, stiff and glassy, though not necessarily brittle. The Polymers are leathery and in a temperature range near Tg at temperatures well above Tg rubbery.

The performance of the selectively gas permeable polymer membranes is affected by whether the polymer is glassy or rubbery. For example, the selectivity between the atmospheric gas species commonly present in ambient air, ie, oxygen, nitrogen, argon, carbon dioxide, and the like, and vapor species of VOC is such that glassy selective polymers preferentially pass atmospheric gas species over the VOC species. Conversely, rubbery selective polymers are be preferably for VOC and less preferably permeable to atmospheric gases. The vapor recovery systems used in the 1 and 3 are designed to expel the permeate compositions into the atmosphere. Therefore, these systems use glassy polymer membranes that separate ullage gas into a permeate that is enriched in air and depleted of VOC vapor. The VOC vapor-enriched retentate is returned to the storage tank.

It is contemplated that the new method of reducing VOC emissions may be applied to a vapor recovery system using a rubbery polymer. Broadly speaking, such a system differs from a glassy polymer membrane system in that the retentate gas composition of the former is discharged into the atmosphere and the permeate gas composition is returned to the storage tank's ullage. A gas recovery system of this type is in the US 5,571,310 by Nanaji, the disclosure of which is hereby incorporated by reference in its entirety.

4 FIG. 12 shows a schematic flow diagram for the novel vapor recovery system of this invention that incorporates a gas selective membrane. FIG 16r made of rubbery polymer. During the first part of the cyclical operation Ullagegas flows from the Ullage 4 through transfer lines 14 and 21 in the feed-retentate chamber 17 of the module 15 , The feed gas conveyor 28 pressurizes the feedstock for separation through the membrane 16r to facilitate and the useful retentate through the exhaust gas transport line to the vent valve 24 to force. VOC components preferably pass through the membrane into the permeate chamber 18 , and this VOC-enriched composition is transferred via the transfer line 19 returned to Ullage. A vacuum pump 42 supports the aspiration of the permeate through the membrane.

As described above, the second operation section starts at an appropriate time. The vacuum pump 42 and the gas conveyor 28 are stopped, and the valve 33 will be closed. In the second operating section, the valve 36 opened to a diluent gas, preferably air, from the line 35 in the feed-retentate chamber 17 involved. The administration 35 can be designed so that the diluent gas directly or indirectly via the transfer line 25 as shown in the chamber 17 is introduced. The valves 32 and 38 as well as other system elements in the lines 14 and 21 are designed to provide a return of purified gas from the feed-retentate chamber back to the ullage 4 permit. Alternatively, an optional secondary return line 46 intended. If one of the system elements, such as. B. the device 28 , prevents backflow, therefore, the valve can 44 be opened and purified gas from the feed-retentate chamber 17 through the secondary return line 46 into the Ullage 4 flow.

The diluent gas should be introduced into the feed-retentate chamber such that the chamber 17 is cleaned according to VOC types before the first section of the next cycle begins. Preferably, the diluent gas should flow through the chamber to maximize the cleaning action. Therefore, an introduction of diluent gas over the line 35a upstream of the module, e.g. B. in the transfer line 21 , less preferred.

The feed-retentate chamber 17 should not by introducing the diluent gas into the permeate 18 of the module. Without wishing to be bound by theory, it is believed that VOC species preferentially migrate through the rubbery polymer selective membrane by passing through a polymer in which VOCs are highly absorbed. Should diluent gas flow back from the permeate chamber through the rubbery polymer membrane to clean the feed-retentate chamber, it is believed that the membrane polymer would also be cleared of VOC. This would make the membrane less effective for permeation of VOC immediately after the onset of the first portion of the next cycle.

Theoretically, one could choose a membrane of a particular amorphous polymer and opt for operation at temperatures above Tg where the polymer is rubbery or below Tg where the polymer is glassy. Then the appropriate configuration, ie either the 3 or the off 4 , chosen for the vapor recovery system. In practice, however, it can be assumed that one chooses a polymer which has an optimum combination of performance characteristics, mechanical and physical properties. Thereby, selectivity and permeability to the substances to be separated as well as the ability to process the polymer into a stable membrane of desired shape and to operate the membrane at a temperature compatible with the vapor recovery process should all be considered. All of these factors together determine whether the polymer is glassy or rubbery under separation conditions, and what flow structure should be used.

Returning to the new vapor recovery system in which the selectively gas permeable membrane comprises a glassy polymer, an additional embodiment of the invention, which will be referred to herein as a "vacuum process", will now be discussed 5 understandable.

A major difference between the vacuum method of reducing VOC emissions and the embodiments described above is that no diluent gas is supplied to the membrane during the second portion of the cycle. Instead, the permeate chamber of the membrane module is subjected to an increased suction, ie, a lower absolute pressure, to draw off VOC components. In general, the method is operated in the manner described above, but during the second cycle, the valves 32 and 53 closed and a suction is applied to the permeate or feed / retentate chamber. The suction may be provided by an additional vacuum pump (not shown), the suction port of which is in fluid communication with the membrane module. In a preferred embodiment of the vacuum process disclosed in U.S. Pat 5 is shown, the second gas conveyor 23 operated so that it subtracts the contents of the membrane module. This not only removes the contents of the permeate chamber, but also withdraws gas from the feed-retentate chamber through the membrane. In one mode of operation, the vapor is released from the module through the vent on the check valve 24 by opening the valve 54 escape to the atmosphere. In a more preferred mode of operation is a vacuum reflux transfer line 50 with a compartment valve 52 intended. With the valve open 33 (and closed valve 54 ) That can from the second conveyor 23 escaping gas into the ullage 4 flow back. Before the beginning of the first section of the next cycle, the valve becomes 52 closed.

In any of the above-described operations of the vacuum process, a suction generator (vacuum pump or device 23 ) work continuously for the duration of the second section. Alternatively, the valve 37 closed and the suction generator is stopped before the end of the second section according to a predetermined control protocol. For example, the suction may be stopped after a preselected duration after the pressure in the membrane module has dropped to a preselected vacuum limit, or after the VOC concentration at a reference location in the membrane module has reached a preselected value. Preferably, the suction is applied so that the absolute pressure in the module decreases to less than about 0.5 atmospheres. It will be appreciated that the vacuum process advantageously captures the VOCs present in the membrane module at the end of the first portion of the cycle without adding a volume of LPG to the system.

EXAMPLES

These Invention will now be described by way of example with particularity embodiments the same, all parts, parts and percentages, unless otherwise indicated, are by volume. All weight and units of measurement, the original not obtained in SI units were converted to SI units.

Examples 1 to 5 and Comparative Examples 1 to 5

Operation of the vapor recovery system according to one Protocol with fixed duration.

Experiments were carried out on-site at a large filling station in operation using a steam processing system with the in 3 configuration shown performed. The service station had three underground gasoline storage tanks in fluid communication with Ullageräumen. Together, they collected 50,327 liters of liquid gas at a temperature of 15 ° C and a total volume of 58,901 liters. The VOC contents were measured with a Nova Analytical Systems 7204 SF hydrocarbon analyzer modified to operate according to the NDIR (non-dispersive infrared) principle.

In the apparatus described above, a series of vapor recovery cycles were performed. During the second portion of the test cycles, an ambient air supply was either not admitted into the membrane module, or introduced into the module on the permeate chamber side of the membrane, or introduced into the module on the feed-retentate chamber side of the module. Optionally, the air was admitted at the beginning of the second section. The settings of the system valves during the second section were as shown in Table I below: Table I

During the feed air supply was a gas conveyor 28 , which was a fan, was turned on to move air through the feed-retentate chamber at a rate of 820 liters per minute.

The VOC concentration (VOC%) of the gas leaving the exhaust chimney was measured during the immediately following first section. Three concentration values were determined for each cycle. These were (i) the initial VOC concentration at the beginning of a first section, ie (point A2, 2 ) at the beginning of the membrane separation, (ii) the maximum VOC concentration, ie (point A4, 2 ) the maximum deaerating concentration and (iii) the final VOC concentration (point A6, 2 ) when terminating the membrane separation. Based on these determinations, the difference between the maximum height and the value at the end of the cycle (PE) was calculated. The air supply conditions and the analysis results are given in Table II.

Table II

The Data show that with air addition to the module the highest concentrations significantly lower in the new procedure (4.7% to 7.3% compared to 9.1% to 9.5%). The VOC end values (those for the stable operating state are also lower than the controls in the invention (3.8% to 4.5% compared to 4.9% to 5.4%). The P-E values were consistently in the conventional method about 4 percent units while they did not exceed 2.8 percent units in the operating examples. In general led longer admission times for the Air to lower total VOC concentrations from during the cycle of emitted air. Although Example 2 showed the effectiveness invention, but the VOC emission reduction was already due the current duration of the air supply only temporarily. From the results Apart from example 2, the operating examples clearly show that both the VOC peak emissions as well as the sustained VOC emissions by the practice of this invention are improved.

Although specific forms of the invention for illustrative purposes in the drawings were chosen and the above description with specific terms for Purpose of the complete and comprehensive description of these forms of the invention for a A person of ordinary skill in the relevant field, it goes without saying that various additions and modifications, the essentially equivalent ones or better results and / or performance than under the The scope and spirit of the following claims shall be considered as falling.

Summary

A cyclic process to control volatile organic compound (VOC) environmental emissions from vapor recovery in liquid storage and dispensing operations maintains a vacuum in the storage tank's tuyere. In the first part of a two-part cyclic process, ullage vapor is passed through a vapor recovery system in which VOC with a selectively gas-permeable membrane from the vented gas are withdrawn, ejected. In the second part, the membrane is deactivated while the gas pressure in the ullage increases. According to one aspect of this invention, ambient air is supplied to the membrane separation unit during the second part of the cycle. In another aspect, a vacuum is drawn into the membrane separation unit. The supply of air or vacuum draws VOC from the membrane unit, thus reducing total VOC emissions.

Claims (10)

Cyclic gas separation process for separating preferred permeable gas constituents of less preferred permeable gas constituents a feed gas mixture of such components, the method the Steps includes: (a) providing a membrane module that a selectively gas permeable Membrane for the more preferred permeable gas components and the less permeable ones Has gas components, (b) simultaneous (i) feeding the Feed gas mixture in the module to the feed gas mixture with a first Side of the membrane into contact, (ii) ejecting a Permeate gas mixture with the preferred permeable gas components enriched, from the module in fluid communication with a second Side of the membrane and (iii) withdrawing a retentate gas mixture, enriched with the less preferred permeable gas components is, from the module in fluid communication with the first side of the membrane, (C) Stopping the feed of the feed gas mixture to the first side, stopping of ejection the permeate gas mixture and stopping the withdrawal of the retentate gas mixture, (D) Charging the module with a diluent gas, (e) stop feeding the module with diluent gas, and (f) Repeat of steps (b) to (e). The method of claim 1, wherein the membrane is a Polymer with a glass transition temperature The preferred permeable gas constituents are volatile organic compounds Compounds are, the diluting gas free from volatile is organic compounds and the method is feeding the Feed gas mixture into the module at a temperature above the Glass transition temperature and the filling of the diluent gas in fluid communication with the first side of the membrane. The method of claim 1, wherein the membrane is a Polymer with a glass transition temperature has, the diluent gas is free from the less preferred permeable gas components and the method involves feeding the feed gas mixture into the module a temperature below the glass transition temperature. The method of claim 1, wherein the feed gas mixture volatile Includes organic compounds and the diluent gas air. A method of reducing atmospheric emissions of volatile organic compounds containing vapor from storage gas of a liquid organic volatile organic compound storage tank, the method comprising: (a) providing a vapor recovery system comprising: (i) a membrane module having a bilateral gas permeable membrane comprising a polymer having a glass transition temperature and a selectivity for the permeation of air against the permeation of volatile organic compounds, wherein one side of the membrane has a feed-retentate chamber on a first side of the membrane in fluid communication with the oil gas and the second side of the membrane Diaphragm defines a permeate chamber in fluid communication with the suction pump inlet, and (ii) a suction pump having an inlet in fluid communication with the feed-retentate chamber or permeate chamber and an ambient atmosphere outlet (b) simultaneously and continuous (i) Conveying ullage gas into the feed-retentate chamber of the module to contact the first side of the membrane with the ullage gas, (ii) separating the ullage gas to produce a low-organic gas, opposite to the ullage gas of volatile organic compounds is depleted, and to form a high organic content gas enriched in volatile organic compounds with respect to the ullage gas, (iii) discharging the low organic content gas from the module into the surrounding atmosphere, and (iv) recycling the gas over a first period of time, (c) stopping the production of the ullage gas, separating the ullage gas, discharging the low organic content gas and returning the high organic content gas over a high organic content from the module to the ullage gas in the storage tank second period, (d) charging the vapor recovery system with a volatile organic compound for stripping indun gene from the membrane module effective amount during step (c) and (e) repeating steps (b) to (d). The method of claim 5, further comprising separating of the ullage gas at a temperature below the glass transition temperature includes, so the volatile compared to organic compounds less preferably pass through the membrane, ejecting the Low organic content gas from the permeate chamber of the Module and return of the High organic content gas from the feed-retentate chamber of the module. The method of claim 6, further comprising air a job in the vapor recovery system is filled in fluid communication with the permeate chamber and causes Air passes through the membrane from the second side to the first side, flows into the feed-retentate chamber and enters the Ullagegas in the storage tank, whereby volatile organic Connections are removed from the module. Cyclic gas separation process for separating preferred permeable gas constituents of less preferred permeable gas constituents a feed gas mixture of such components, the method the Steps includes: (a) providing a membrane module that a selectively gas permeable Membrane for the more preferred permeable gas components and the less permeable ones Has gas components, (b) simultaneous (i) feeding the Feed gas mixture in the module to the feed gas mixture with a first Side of the membrane into contact, (ii) ejecting a Permeate gas mixture with the preferred permeable gas components enriched, from the module in fluid communication with a second Side of the membrane and (iii) withdrawing a retentate gas mixture, enriched with the less preferred permeable gas components is, from the module in fluid communication with the first side of the membrane, (C) Stopping the feed of the feed gas mixture to the first side, stopping of ejection the permeate gas mixture and stopping the withdrawal of the retentate gas mixture from the first page, (d) sucking a vacuum into the module, wherein the vacuum causes a portion of a residual gas to be added to the Time at which the feed, the ejection and the peeling stopped is present in the module, is subtracted from the module, (E) Stopping the suction of a vacuum in the module and (f) Repeat of steps (b) to (e). Method for reducing atmospheric Emissions of steam, the volatile contains organic compounds, from the Ullageraum a storage tank, for liquid, volatile organic compounds, wherein the method comprises: (a) providing a vapor recovery system, comprising: (i) a membrane module having a bilateral gas permeable membrane, the one polymer with a glass transition temperature and a selectivity for permeation from the air the permeation of volatile having organic compounds, wherein one side of the membrane a feed-retentate chamber on a first side of the membrane in Fluid communication with the ullage gas and the second side of the membrane a permeate chamber in fluid communication with the suction pump inlet and (ii) a suction pump having an inlet in fluid communication with the feed-retentate chamber or the permeate chamber and an outlet to the surrounding atmosphere, (B) simultaneous and continuous (i) pumping of ullage gas from the Ullageraum in the feed-retentate chamber of the module to the first Side of the membrane into contact with the feed gas, (ii) Separating the feed gas to a low organic content gas, the opposite the feed gas to volatile is depleted of organic compounds, and a gas with high organic Salary form opposite the feed gas with volatile Enriched organic compounds, (iii) expelling the Low organic content gases from the module into the surrounding The atmosphere, and (iv) returning the High organic content gas from the module into the Ullageraum of the storage tank, over a first period, (c) stopping the production of the feed gas, the expulsion of the low organic content gas and the return of the High organic content gases over a second period, (D) Aspirating a vacuum into the membrane module to an extent which the removal of a portion of a residual gas causes, in the module for Time of stopping the promotion of the feed gas is present during step (c) and (E) Repeating steps (b) to (d). The method of claim 9, further comprising recycling the residual gas to the storage room summarizes. 38. The method of claim 37, wherein the preselected concentration is about ... mg / m ³ .