WO2009052035A1

WO2009052035A1 - Emission control for mobile fuel tanks

Info

Publication number: WO2009052035A1
Application number: PCT/US2008/079683
Authority: WO
Inventors: John Bowser; Donald Stookey; Sudipto Majumdar
Original assignee: Cms Technologies Holdings, Inc.
Priority date: 2007-10-17
Filing date: 2008-10-13
Publication date: 2009-04-23

Abstract

A method of reducing volatile organic compound (VOC) emissions from mobile fuel storage tanks uses a compact membrane separator highly selective and permeable to non-hydrocarbon compounds relative to the VOC's to remove the non-hydrocarbon compounds from vapor in the ullage of the fuel tank which lowers the pressure of the ullage and reduces the incidence of venting to atmosphere VOC-containing vapor. A system of reducing the VOC emissions includes a fuel storage tank and membrane separator with an active membrane having nitrogen to propane selectivity of about 8 or higher and a nitrogen permeance of at least about 50 GPU. The system is adapted for use on mobile vehicles such as automotive vehicles of all types and portable equipment having onboard liquid VOC-driven internal combustion engines.

Description

EMISSION CONTROL FOR MOBILE FUEL TANKS

FIELD OF THE INVENTION

This invention relates to reducing excessive emissions of volatile organic compounds from mobile fuel tanks. More specifically it relates to separating with a selectively permeable membrane environmentally benign components from a vapor mixture with organic compounds present in the ullage of an automotive vehicle fuel tank to control the ullage pressure buildup and thereby reduce venting of the organic compounds to the atmosphere.

BACKGROUND OF THE INVENTION Automotive vehicles are omnipresent in great number throughout the world. The overwhelming majority of such vehicles rely on internal combustion engines in whole or in part as the ultimate source of power to motivate the vehicle and to operate its power- consuming accessory equipment. The fuel for the engines is typically a liquid mixture of hydrocarbon compounds, sometimes characterized as volatile organic compounds or "VOC". The most common automotive fuel is gasoline. Fuel consumed by the vehicles is stored in one or more onboard storage tanks.

Storing fuels, dispensing the fuels and refilling storage tanks with the fuels provide potential for emitting hydrocarbon vapor to the atmosphere with cumulatively significant adverse environmental impact. Much technology has and continues to be developed to reduce and prevent escape of fugitive hydrocarbon emissions primarily from large and stationary fuel storage and distribution facilities. Comparatively little attention has been paid to reducing or preventing emissions of hydrocarbons from onboard fuel tanks of common automotive vehicles.

The potential for emitting hydrocarbon vapors when refilling onboard vehicle fuel tanks is recognized and control technology in that situation usually involves equipment that is part of and located at the fuel dispensing station. In the aggregate there is also significant potential of emissions of hydrocarbon vapors from automotive fuel tanks at other times, that is, when the vehicle is in operation or even when parked with the engine off. At such times pressure can build up in the tank above a maximum design limit which necessitates the venting of vapor to atmosphere to relieve the excess pressure. One possible cause for pressure increasing is diurnal atmospheric temperature changes. As temperature rises during the day the tank of a parked vehicle can warm up causing expansion of the vapor within the fixed volume and increase of the pressure of the tank.

Another possible cause is the volatilization from the liquid fuel of gaseous components such as lower molecular weight hydrocarbons. As fuel is consumed, some air enters the ullage as a consequence of normal operating circumstances. For example, ambient air can get drawn into the ullage by lowered ullage pressure from time to time due to temperature changes, barometric or altitude changes and consumption of the liquid. Air in the ullage causes hydrocarbon liquid to vaporize especially during times that the engine is not consuming fuel. As this vaporization continues, pressure of the ullage increases. When the engine runs, liquid fuel is consumed and accordingly more volume is made available in the tank for vapor to occupy which reduces ullage pressure. At those times the pressure increase effect of volatilizing hydrocarbons is at least partially offset. However, no offset occurs when the engine is not in operation. For currently typical hybrid engine vehicles in which the power source is partially internal combustion engine and partially electrical motor, the combustion engine is in operation a smaller fraction of the time than for a non-hybrid vehicle. Hence the pressure reducing effect caused by consuming liquid fuel is less and the pressure build up leading to potential hydrocarbon emissions from the tank of a hybrid vehicle is increased.

Overall it is sought to provide a method and apparatus capable of efficiently confining the combustible components of the fuel in the fuel tanks of internal combustion engine driven automotive vehicles and portable apparatus, during periods of vehicle inactivity as well as active operation, such that deleterious environmental contamination by such components is reduced and productive power value of the combustible fuel is improved. To these ends it is desirable to have a method of selectively removing the low-boiling non-hydrocarbon contaminants, such as nitrogen, from the ullage of vehicle fuel tanks while retaining the volatile organic components in the tank, and thereby reduce the frequency of relieving overpressurization of the tank with associated volatile organic compound emissions. It is also needed to have a lightweight and compact method to remove such non-hydrocarbon contaminants suitable for use on all types of automotive vehicles such as tracks, buses, passenger cars, off-road vehicles, motor homes, motorcycles and boats. An apparatus and method for carrying out such nojd-hydrocarbon removal that are simple to operate and have a minimum amount of moving parts and no consumable materials is of great value. It is further desired to have an apparatus to accomplish the non-hydrocarbon removal method which is inert to exposure to the hydrocarbon compounds present in automotive fuels so that the apparatus is durable and maintains a high level of performance for extended periods of time. There is great need for an apparatus to remove hydrocarbons from onboard vehicle fuel tank ullage that requires minimal power to function. SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method of reducing volatile organic compound (VOC) emissions from mobile fuel storage tanks using a compact membrane separator highly selective and permeable to non-hydrocarbon compounds relative to the VOCs to remove the non-hydrocarbon compounds from vapor in the ullage of the fuel tank. Removal of non-hydrocarbons lowers the pressure of the ullage and reduces the incidence of venting to the atmosphere mainly VOC-containing vapor. A preferred system of reducing the VOC emissions includes a fuel storage tank and membrane separator with an active membrane having nitrogen to propane selectivity of 8 or higher and a nitrogen permeance of at least about 50 GPU. The system is adapted for use on automotive vehicles of all types and portable apparatus having onboard liquid VOC-driven internal combustion engines. The invention also provides a storage tank nozzle having a selectively permeable membrane separator within the nozzle.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic flow diagram of an embodiment according to this invention of a system for removing non-hydrocarbon components from the vapor of the ullage of a vehicle fuel tank using an external selectively permeable membrane device.

Fig. 2 is a schematic flow diagram of another embodiment according to this invention of a system for removing non-hydrocarbon components from the vapor of the ullage of a vehicle fuel tank using an external selectively permeable membrane device. Fig. 3 is a schematic flow diagram of another embodiment according to this invention of a system for removing non-hydrocarbon components from the vapor of the ullage of a vehicle fuel tank using an external selectively permeable membrane device.

Fig. 4 is a schematic flow diagram of another embodiment according to this invention of a system for removing non-hydrocarbon components from the vapor of the ullage of a vehicle fuel tank using an external selectively permeable membrane device.

Fig. 5 is a schematic flow diagram of an embodiment according to this invention of a system for removing non-hydrocarbon components from the vapor of the ullage of a vehicle fuel tank using a selectively permeable membrane device internal to the fuel tank.

Fig. 6 is an elevation section view of an embodiment according to this invention of an apparatus for removing non-hydrocarbon components from the vapor of the ullage of a vehicle fuel tank in which a separator comprising plural hollow fiber membrane units is postioned within a nozzle or recess in a wall of the tank. Fig. 7 is an elevation section view of another embodiment according to this invention of an apparatus for removing non-hydrocarbon components from the vapor of the ullage of a vehicle fuel tank in which a separator comprising a coiled tubular membrane unit is postioned within a nozzle or recess in a wall of the tank.

Fig. 8 is a partially exploded, side elevation view taken in section at line 8-8 in Fig. 9 of another embodiment according to this invention of an apparatus for removing non-hydrocarbon components from the vapor of the ullage of a vehicle fuel tank in which a separator comprises flat sheet membrane elements spirally wound about a mandrel and is postioned within a nozzle or recess in a wall of the tank.

Fig. 9 is a plan view of a non-sectioned apparatus of the type of Fig. 8 taken at line 9-9.

Fig. 10 is a section view of another embodiment according to this invention of an apparatus for removing non-hydrocarbon components from the vapor of the ullage of a vehicle fuel tank in which the apparatus is postioned within a recess of the wall of the tank. DETAILED DESCRIPTION OF THE INVENTION

With reference to Fig. 1 it is seen that one embodiment of the invention is adapted to remove non-hydrocarbon components of a fuel vapor mixture of an automotive fuel tank 2 using a selectively permeable membrane 6 positioned within a membrane separator 5. The separator is located external to the tank. The tank contains an inventory of liquid fuel 4 and has an ullage 3, i.e., part of the tank volume above the liquid level which is occupied by vapor. Under normal circumstances the vapor is a predominantly organic composition of hydrocarbon components in equilibrium with the corresponding hydrocarbon components present in the liquid phase 4. The tank as well as the other elements of the non-hydrocarbon vapor removing apparatus are parts of an automotive vehicle, that is, they are mobile with the vehicle. Conventional features of fuel tanks such as liquid fuel refill nozzles, vents, discharges for fuel flow to the internal combustion engine are not shown in the figures.

The membrane divides the separator into two chambers, namely a feed-retentate chamber and a permeate chamber on opposite sides of the membrane. Vapor from the ullage enters the feed-retentate chamber via transfer line 7. In the membrane vapor from the ullage contacts one side of the membrane 6 under conditions conducive to selective permeation of the non-hydrocarbon components such that a permeate rich in the non- hydrocarbon components is formed in the permeate chamber and can exit from the separator via transfer line 9. The permeate composition has a lower concentration of volatile organic compounds ("VOC") than the feed-retentate composition. Usually the concentration of VOC in the permeate is low enough that the permeate composition is considered environmentally benign and therefore the permeate transfer line 9 can discharge directly to the atmosphere. Optionally, a secondary polishing filter such as an activated carbon particle bed in a canister, for example, can be included in permeate transfer line 9 between separator 5 and the atmosphere. Such filter would scavenge all or some of the possible minor amount of VOC in the permeate. It is contemplated that in an embodiment of this invention the permeate can discharge to the carbon bed canister that is commonly incorporated in the fuel vapor recovery systems of many modern automobiles. If the permeate is vented to the conventional carbon bed canister, modification may be called for such that the discharge from the membrane separator is not blocked by the canister. Normally, a secondary polishing filter should not be needed. ζ The retentate from the feed-retentate chamber leaves the separator via transfer line 8 and ultimately returns to the ullage of tank 2. Motivation is provided to circulate the vapor from the ullage to the feed-retentate chamber and from that chamber back to the tank. A conveyor means for moving the vapor through the system can be used. Such conveyor means can be a blower, fan, compressor, turbine, vacuum pump or other similarly well known type of gas moving apparatus. Usually the conveyor means is driven by electrical power as an additional accessory of the automotive vehicle. For example it can derive power from a battery. Alternatively the power source can be a solar source. In the embodiment illustrated in Fig. 1 , a blower 1 is positioned in transfer line 8 such that it draws suction on the feed-retentate chamber of the separator and discharges its exhaust to the ullage via transfer line 10. Thus, blower 1 operates to blow vapor from the feed-retentate chamber to the ullage which displaces additional vapor from the ullage into the feed-retentate chamber via line 7. Preferably transfer line 10 enters into the ullage at a distance from the outlet at which line 7 connects to the ullage. Such distance between the inlet and outlets promotes mixing of the returned retentate vapor with the ullage such that the recirculation does not merely recycle retentate through the membrane. If the positions at which transfer lines 7 and 10 attach to the tank are mutually proximate, the preferred separation distance can be achieved by extending either one of the transfer lines with a tube inside the tank such that the mouth of line 7 is appropriately far from the discharge of line 10.

For the invention to operate with optimum effectiveness, the feed-retentate chamber pressure should be higher than the pressure of the permeate chamber. As applied to Fig. 1, the pressure of transfer line 9 is substantially atmospheric. Consequently, blower 1 should not draw the feed-retentate pressure below atmospheric pressure. Discharge of blower 1 can be an elevated pressure provided that it does not exceed the design limitation for pressure of the fuel tank. Raising blower 1 discharge pressure raises the ullage pressure which makes this embodiment less preferred.

A different embodiment is shown in Fig. 2. In the drawings, like parts have the same reference numbers. In Fig. 2 a blower 11 is present in transfer line 7 and blower 1 is absent, i.e., blower 11 is the only conveyor means present. In this case, blower 11 draws vapor from the ullage and forces it into the feed-retentate chamber. The discharge pressure of the blower should be high enough to motivate permeation of the feed through the membrane to the substantially atmospheric pressure of transfer line 9. The feed- retentate pressure would also be higher than the ullage and thus would enable return of the feed-retentate flow to the fuel tank through line 8. This embodiment provides the advantage that pressure of the ullage within the tank can be below atmospheric pressure provided that the blower 11 can raise the pressure to above atmospheric pressure in the feed-retentate chamber.

Another embodiment is depicted in Fig. 3. Here a vacuum pump 12 is inserted in transfer line 9. Accordingly, the vacuum pump creates a suction pressure in the permeate chamber which promotes permeation of the feed-retentate vapor through the membrane. In addition to vacuum pump 12, two blowers 1, 11 are shown to be present in lines 8 and 7, respectively in this embodiment. These conveyor means circulate the vapor between the fuel tank and the feed-retentate chamber. Although conveyor means in all three positions as shown in Fig. 3 can be used, two recirculating blowers can be redundant, excessive, costly and hence less preferred. However, either one of the two conveyor means in lines 7 and 8 combined either with or without conveyor means 12 in line 9 can be utilized in combination to operate the non-hydrocarbon component removal method.

It is contemplated that a two conveyor means combination of the vacuum pump 12 with either one of blowers 1 or 11 is preferred for the embodiment of this invention which employs an external membrane separator. In such configuration, the vacuum pump creates a vacuum pressure in the permeate chamber for maximum pressure differential and driving force across the membrane while a blower 1 or 11 effectuates a high volumetric recirculation flow of ullage gas between the tank and the membrane separator. Fig. 4 illustrates a particularly preferred embodiment in which blower 11 and vacuum pump 12 are deployed.

In an another aspect of the invention the membrane separator can be mounted in a nozzle or a recess of the wall of the storage tank and is in close communication with the vapors within the ullage of the tank. Particularly preferable is the membrane separator configuration in which the selectively gas permeable membrane is directly exposed to the vapor mixture inside the ullage. In such membrane arrangements the nonpermeating species may concentrate on or near the membrane surface. However, advantageously these concentrating species readily convect from proximity to the membrane back into the ullage, and they can condense and drain back to the tank liquid inventory.

A representative embodiment is illustrated schematically in Fig. 5. The tank 22 contains hydrocarbon liquid 24 and has an ullage 23 of predominantly hydrocarbon vapor above the liquid level. Inside the tank 22 and specifically deployed in the ullage 23 is a membrane separator 25. This separator has a case 29 and a membrane 26 configured such that one side of the membrane is directly exposed to the vapor of the ullage. The enclosed space within the separator between the membrane and the case defines the permeate chamber. A transfer line 30 exits through the tank wall via a port 28 and connects permeate vapor to the suction of conveyor means 21. The permeate composition is discharged via transfer line 31. Preferably in this configuration conveyor means 21 is a vacuum pump. This embodiment functions by directly selectively permeating vapor of the ullage through the membrane under the driving force of pressure differential created primarily by the vacuum pump. Under ideal circumstances and design, it is contemplated that the pressure build up in the ullage can drive permeation without drawing a vacuum on the permeate, for example without a vacuum pump. That is, non-hydrocarbon components can be removed by allowing the pressure build up in the ullage to vent to atmosphere through the membrane separator. Optionally, a check valve can be included in line 30 or 31 to prevent back flow of ambient atmosphere into the ullage.

The invention embodied in Fig. 5 presents a number of advantages. Firstly, it is simple and does not use more than a single vapor conveyor means. This potentially reduces the incremental weight and power demand of the system. Also it saves space that would otherwise be occupied by an external piece of equipment by placing the membrane separator inside the fuel tank. Furthermore, there is little concern that liquid fuel entrains in the feed. Indeed it is expected that vehicle motion is likely to cause the liquid fuel to splash onto the surface of the exposed membrane from time to time. The liquid that contacts the membrane will drain back into the liquid inventory.

Several other representative embodiments of this aspect are illustrated in additional drawing figures. One preferred embodiment is understood with reference to Fig. 6 showing an elevation section view of the tank wall taken through the nozzle 602. Tank wall 601 is seen to have an outwardly protruding nozzle 602. Position of the nozzle 602 on the wall 601 is not critical provided that the interior volume 607 of the nozzle is in vapor communication with the ullage 605 of the tank. Preferably the nozzle is on a top horizontal or near horizontal wall of the tank as shown. The nozzle includes a cap 603 which seals the end of the nozzle and provides an outlet port 609 through which permeate vapor discharges as will be explained.

A membrane separator is positioned inside the nozzle 602. The membrane separator includes a tube sheet 606 and a plurality of hollow fiber membranes 608. One end of each hollow fiber is potted in the tube sheet. The fibers extend from the tube sheet in substantially parallel orientation toward the ullage of the tank. In the illustrated embodiment the fibers are suspended by the tube sheet 606 and hang downward in the nozzle where they are exposed to the vapor in the ullage of tank. The fibers comprise a gas permeable membrane component that is selectively permeable preferably for non- hydrocarbon compounds such as nitrogen and oxygen, relative to volatile organic compounds. The tube sheet comprises a conventional hollow fiber membrane potting composition, such as an epoxy material, as is well known in the art. The tube sheet composition, as well as other elements of this device, is resistant to chemical reaction with, and dissolution by, the organic compounds stored in the tank.

The tube sheet separates the interior volume 607 of the nozzle from a permeate chamber 614. The lumina of all of the fibers 608 at the tubesheet 606 are open to permit flow of vapor from inside each fiber into the permeate chamber. The opposite ends of the fibers 615 are closed. A conduit 611 of an appropriate material, for example, rubber, plastic or metal tubing, is mounted on port 609 such that vapor within the permeate chamber 614 can be removed from the nozzle as represented schematically by arrow 612.

The method of mounting the membrane separator in the nozzle is not critical and any conventional technique can be employed. Access to the membrane separator is desirable for replacement or repair from time to time. Consequently a removable cap 603 is preferred. Understandably the nozzle fittings should be vapor tight to ullage vapor in the tank except as consistent with the purposeful operation of the membrane. Sealing and fastening elements such as o-rings, gaskets, screw threads, clamps and the like are not shown in the drawing.

Another embodiment of the membrane separator in a tank nozzle or recess is shown in Fig. 7 which is also an elevation section view of the tank wall through the nozzle. In this embodiment the membrane separator is in the form of a coiled tube 702. The tube is hollow and has a selectively permeable component. Preferably the tube comprises a porous substrate and a thin nonporous layer of selectively permeable composition on either the inside or outside surface of the substrate. To more clearly show the coiled nature of the membrane separator, the portion 704 of the coil that would not normally be evident in a section diagram is shown by dashed lines. The free end 706 of the coiled membrane separator is sealed to prevent flow of ullage gas into the lumen of the tube. At the discharge end 710 of the coil, the membrane unit is mated by a potting composition 709, or equivalent known fastening technique, with the nozzle cap 703 through a single conduit connection 705. Although a single coiled tube membrane separator is illustrated, variations with multiple coils, for example with plural tubes mounted in concentric coils, are contemplated.

Figures 8 and 9 together illustrate yet another preferred embodiment of the invention in which the membrane separator includes a spiral wound sheet membrane deployed within a nozzle or recess of the tank wall 801. Fig. 8 shows a side elevation section view of the nozzle 802 partially exploded with the top 803 and discharge conduit 811 separated from the nozzle. Fig. 9 is a plan view of the nozzle 802 and membrane separator 800 taken at level 9-9 of Fig. 8. Fig. 8 is thus the section taken along the centerline 8-8 of the membrane unit in Fig. 9. The membrane separator includes a central hollow tube 806 and at least one flexible, flat sheet membrane envelopes 808. The envelopes are formed of two congruent sheets, for example, 812 and 814, at least one of which has a selectively permeable component. The sheets are sealed together at their mutual perimeters to form the envelope structure. The envelope has an opening that is in vapor communication with the lumen 805 of the tube 806 such that vapor permeating through the sheet into the envelope can subsequently flow into the lumen. Preferably, there are a plurality of substantially rectangular envelopes sealed on three edges of the sheets. The tube then preferably has a plurality of corresponding longitudinal slits through the cylinder of the tube such that the envelopes are affixed to the tube with each slit aligned and mated to the fourth, unsealed edge of an envelope.

Preferably both sheets of each envelope have a selectively permeable component. Any conventional type of membrane structure can be used such as monolithic membrane, composite membrane and asymmetric membrane. Preferably the membrane structure is a composite membrane with a porous substrate layer coextensively incorporating a layer of nonporous selectively permeable composition. When only one of the congruent sheets of an envelope is selectively permeable, the other surfaces of the envelope should be non-permeable to prevent indiscriminant ullage vapor migration into the envelopes and the lumen.

The envelopes of permeable sheets extend radially outward from the axis of the tube as seen in Fig. 9. The envelopes are wound in a spiral arrangement to further increase the surface area for permeation of any particular membrane separator. The central tube 806 thus also serves as a mandrel for winding of the envelopes. Gaps between adjacent envelopes are exaggerated in the illustration. It is contemplated that the envelopes will usually be more tightly wound around the mandrel to provide a large surface area for permeation within a small cross section area. The gaps are preferably packed with inert spacer material 815 to prevent adjacent envelopes from contacting each other and thereby blocking access to the selectively permeable surfaces of the sheets. The spacer material should have an openwork structure effective to permit vapor flow throughout the inter-envelope gaps. For example, the spacer material can be open cell foam, open mesh screen, granules, column packing and the like. The bottom of the membrane separator can optionally include a plate 818 to support the envelopes and to maintain the spacer material in position. The spacer material is not necessarily affixed to the envelopes or the tube 806 and, in that event, especially for loose spacer material particles, would likely fall out of the gaps without a bottom plate 818. The bottom plate 818 should have perforations 819 or other openings to permit ullage gas to communicate with the permeable sheets of the envelopes and to allow droplets of condensed vapor or entrained liquid from the tank to drain downward into the liquid tank inventory.

Another preferred embodiment of a membrane separator within a nozzle or recess 950 of the tank wall 952 can be understood with reference to Fig. 10. Here the nozzle is defined by walls 956 protruding outward from the tank and covered by a cap 957 to encompass cavity 958. The cap includes a port 951 adapted to mate with a conduit 955 provided to discharge exhaust vapor permeated through the membrane separator 960.

The membrane separator is similar in style to the GKSS type designed by GKSS- Forschungszentrum Geesthacht GmbH (Geesthacht, Germany) in that it includes a horizontal central tube 961 and an array of plural, thin flat disks 962 of membrane elements extending radially outward from the central tube. The disks substantially surround the tube 961 with portions 963 located above the tube and other portions 964 located below. The tube is mounted in seats 967 and 968 of the nozzle walls 956. Any form of conventional technique can be utilized for mounting the tube horizontally in fixed position in such way that the tube with the whole separator 960 can be removed for inspection, repair or replacement after lifting cap 957. Fig. 10 illustrates a representative technique in which one end 955 inserts within a retaining depression of the nozzle wall at seat 967. Opposite tube end 971 rests in a similar depression of seat 968. Seat 968 has an upper portion of its body removed to permit lifting tube end 971 from the nozzle for service.

Each disk has two platters 977 and 978 that are sealed at their outer edges 979 thus defining an internal core 975. The surface 974 of the disk platters include a gas permeable material that enables non-hydrocarbon components contacting the disk to selectively permeate to the core. A plurality of perforations 981 are positioned on tube 961 such that the internal cores 975 of the disks 962 have vapor communication with bore 982 of the central horizontal tube. A discharge tube 983 connects in vapor communication with the bore 982 so that permeated vapor from the cores of the disks can flow from the bore, through the discharge tube and out of the port 951 for removal. Sealant 984 provides a vapor tight seal between discharge tube 983 and cap 957.

The membrane separator of the embodiment seen in Fig. 10 functions by exposing vapor in cavity 958 from the ullage of the tank to the surface of the disk platters. A low pressure is applied through conduit 955 which creates a pressure gradient across the selectively permeable membrane due to vapor communication between discharge tube 983, central tube bore 982, perforations 981 and disk cores 975. Narrow gaps 987 allows close packing of the disks along the longitudinal axis of tube 961 and provides very effective high surface area per unit volume of the cavity. The shape of the disks, shape of the nozzle or both can be modified, e.g, not all disks have same height, to maximize the permeable membrane surface area in the nozzle. Another contemplated variation is to employ a cylindrical nozzle that has a horizontal longitudinal axis, that is, parallel to the tank wall 952 and to have the central tube discharge through the nozzle wall at the size of the nozzle in a direction parallel to the tank wall. An important common feature of the embodiments according to the GKSS type membrane separator is that the disk membrane surfaces are oriented vertically and have vertical gaps 987 between disks. This conformation allows liquid splashing up from the tank inventory and liquid condensed from vapor at the disk surfaces to drain downward into the tank.

When non-condensing VOC species are present in the mixture, concentration polarization of the VOC vapor can occur in the zone near the membrane due to deep and narrow geometry of the nozzle or recess and the convection flux of the ullage vapor being drawn toward the membrane by the conveyor means. To reduce formation of such a barrier zone it is helpful to promote diffusion of the concentrated VOCs near the membrane back into the ullage vapor mixture. A recommended technique is to assure that the cross section area of the mouth of the nozzle or recess in which the membrane resides is large enough to prevent retention of the rejected VOC species in the membrane zone. Representative specific techniques that can be used include making the nozzle or recess shallow or outwardly flared (i.e., with a bell shaped elevation profile), for example.

The physical form of separator and membrane can be any selectively permeable membrane separator configuration well known in the art with the exceptions as will be set forth in greater detail, below. That is, the separator can be for example a flat membrane cell, a pleated membrane separator, a ribbon tube membrane separator, a tubular membrane separator, a spiral membrane separator, a hollow fiber membrane separator and the like. Hollow fiber membrane separators are often referred to as modules. Such modules typically include a plurality of membranes in form of hollow fibers bundled substantially parallel to an axis of an elongated case enclosing the fiber bundle. The fibers are fixed at their ends by tube sheets such that the volume of the case inside the fibers and beyond the tube sheets, sometimes called the "tube side" and the volume of the case outside the fibers between the tube sheets, sometimes called the "shell side", become chambers separated by the membrane. Hollow fiber membrane separators are well known in the art and are available commercially from many manufacturers including Compact Membrane Systems, Inc., (Newport, Delaware).

It has been disclosed here that the membrane separator can be deployed within a nozzle or recess of the tank wall. By "recess" is meant an outwardly extending 12 protrusion formed by the wall. The recess should have a port such as 609 (Fig. 6) to permit connection with a discharge conduit. It is also contemplated that in yet another embodiment, the membrane separator can be implemented in an "in-line" configuration. That is, the tank has a nozzle to which a hose or pipe, (hereinafter, "hose") is coupled. The membrane separator is disposed coaxially inside the hose such that the hose wall provides the function of the surrounding nozzle wall, (e.g., element 602).

If the membrane element is based on a flat conformation, such as a sheet, a spiral wound sheet or a pleated sheet, the active selectively permeable component can be monolithic and constitute substantially the whole element. Preferably the active component is a nonporous layer positioned on a preferably porous, and more preferably microporous substrate. The substrate provides structural support for the active layer and permits the active layer to be as thin as practicable to provide higher permeance while withstanding the pressure differential. When the membrane is a hollow fiber conformation preferably the active layer also is a thin nonporous layer on a surface of a microporous hollow fiber substrate. The active layer can be on the inner surface, the outer surface or both inner and outer surfaces of the hollow fiber.

The mobile storage tanks to which this invention is primarily intended to be applied are those deployed on all forms of automotive vehicles such as trucks, buses, passenger cars, off-road vehicles, motor homes and recreational vehicles, motorcycles, snowmobiles, tractors, lawnmowers and boats. It is preferably intended for use with vehicles driven by internal combustion engines that consume fuels which comprise volatile organic compounds. Application of this invention to cargo tank trailers, such as gasoline or other hydrocarbon fuel delivery trailers, is also contemplated. The main anticipated fuel is gasoline. It is also contemplated that this invention can deployed on portable internal combustion engine-powered apparatus such as portable compressors, generators and power washers.

Minimization of space and weight are primary design considerations for the vehicles and equipment with which this invention is expected to be practiced. Consequently, it is desired to have the apparatus be as small and as light weight as practical while providing an acceptably high rate of non-hydrocarbon component removal. To achieve such effective performance an extremely compact membrane separator is desired. The fuels toward which this invention is directed can be characterized as being liquid mixtures of volatile organic compounds (VOC). The fuels can comprise usually minor amounts of gaseous non-hydrocarbon components. Representative examples of liquid combustible components are hydrocarbon compounds such as gasoline, kerosene, other organic compounds such as methanol, ethanol, butanol, and mixtures of hydrocarbons and other organics, such as so-called "gasohol". The term "gaseous non- hydrocarbon component" means a typically inorganic chemical compound that is normally in the gaseous state at atmospheric temperature and pressure and which is not present in the fuel for its combustion value, such as nitrogen and oxygen although it can participate in combustion (e.g., oxygen). The fuels can also contain other components deliberately included normally in minor fractions, such as additives, (e.g., methy-t-butyl ether "MTBE"), lubricants, stabilizers and the like, or inadvertently or unavoidably included such as water.

It has been discovered that highly effective and compact membrane separators for removing non-hydrocarbon vapor components from mixtures with VOC preferably should have an active selectively permeable membrane of a composition which has nitrogen/propane selectivity of at least about 8, more preferably at least about 9 and most preferably at least about 10. Selectivity of a membrane composition can be predicted from the ratio of the permeability of the pure components. Thus the selectivity of a candidate membrane composition for this invention can be determined by measuring the permeability of pure nitrogen through the membrane, measuring the permeability of pure propane through the membrane and dividing the former by the latter.

Another important parameter of the membrane suitable for use in this invention is transmembrane permeance. In accord with this invention, the nitrogen permeance of the selectively permeable layer of the membrane should be greater than about 50 gas permeation units ("GPU"). A GPU is equal to 1 x 10^"6 cm³ (STP) / [cm² . sec . cm Hg]. Permeability, as well as selectivity, are to large degree functions of the membrane composition. Additionally, permeance is a function of thickness of the membrane. To obtain transmembrane permeance in the preferred range for operating this invention the nonporous selectively permeable layer of the membrane should be thin, preferably less than 10 μm thick and more preferably less than 6 μm thick. Still another important characteristic of the whole of the apparatus including the membrane component is stability and resistance to corrosion, distortion, plasticization or other interference from exposure to volatile organic compounds in the liquid or vapor state. The need for this attribute is particularly evident with respect to the embodiment of Figs. 5-9 in which the membrane is likely to frequently contact the liquid fuel directly.

Certain compositions have been discovered to provide all of the desirable attributes for the apparatus and are preferred for use in the implementation of this invention. Among the preferred compositions are copolymers perfluoro-2,2- dimethyl- 1,3-dioxole ("PDD") and tetrafluoroethylene ("TFE"). Membranes and membrane separators having a nonporous selectively gas permeable layer of PDD/TFE copolymer and which are suitable for use in this invention are available from Compact Membrane Systems, Inc., Newport, Delaware. Other compositions for the selectively gas permeable layer suitable for use in this invention include copolymer of TFE and 2,2,4-trifluoro-5- trifluoromethoxy-l,3-dioxole (Hyflon®, Solvay Solexis, Thorofare, N. J.), and polyperfluoro (alkenyl vinyl ether) (Cytop®, Asahi Glass, Japan).

In operation, gasoline or similar liquid fuel comprising VOC is stored in the fuel tank. Vapor in the ullage comprises vapor state VOC in equilibrium with the liquid VOC in the tank. The vapor also typically includes concentrations of non-hydrocarbon components such as nitrogen and oxygen that have entered the fuel storage system with the fuel, during refilling the tank with fuel or by in-leakage from miscellaneous sources. At a certain condition, such as when the pressure in the ullage exceeds a preselected maximum pressure, vapor from the ullage is directed to flow into the feed-retentate chamber of the membrane separator. For example flow can be induced by operating blower 11 in line 7 (Fig. 4). The vapor in the feed-retentate chamber comes in contact with the selectively permeable membrane 6. Blower 11 elevates the pressure of the feed- retentate chamber vapor above that of the permeate chamber to create a pressure gradient across the membrane that promotes vapor permeation of the feed-retentate chamber components. A vacuum pump 12 (Fig. 4) can also reduce the pressure in the permeate chamber to increase the pressure gradient and thereby enhance the driving force for permeation. The selective permeation through the membrane causes the non- hydrocarbon components of the vapor fed to the feed-retentate chamber to preferentially permeate the membrane leaving a composition leaner in the non-hydrocarbons in the feed-retentate chamber. The thus non-hydrocarbon depleted retentate composition can be returned to the ullage via transfer line 8 motivated by blower 9. The predominantly non-hydrocarbon composition of the permeate is vented ultimately to the atmosphere. By discharging the permeate stream, ullage pressure is thus reduced to desired values. For a membrane separator internal to the tank, as in Fig. 5, vacuum source 21 can be activated to provide a pressure gradient across the membrane 26 effective to cause selective permeation of the ullage gas in proximity to the membrane. Components rejected by membrane may concentrate near the feed side of the membrane and can convect into the bulk of the ullage gas. Most simply, the permeate containing largely non-hydrocarbon components can be discharged directly to atmosphere. Additionally, the membrane permeate discharge can be conducted to the engine air intake, for example at the intake manifold, where the combustible components, i.e., residual hydrocarbons and oxygen, can be burned. Also in this optional configuration it is contemplated that engine vacuum can serve to help drive permeation of the feed through the membrane to the permeate side.

Example

An apparatus was set up basically as illustrated in Fig. 4. The tank was a two- gallon capacity stainless steel pressure vessel. The membrane separator was a cylindrical, hollow fiber membrane module having 1840 fibers of 0.305 mm inner diameter, 0.483 mm outer diameter and 20.32 cm active length. Active surface area for permeation was 6968 cm². The fiber substrate was polysulfone coated with an approximately 1.2 μm thickness selectively permeable layer of 65 mol% PDD/35 mol% TFE copolymer membrane composition.

For each experiment, about 1 gallon of gasoline was charged to the vessel which was sealed and pressurized to the 18-19 psia range with nitrogen gas. The vessel contents were allowed to equilibrate about 16 hours. The gas mixture above the vessel liquid was recirculated by a blower through the shell side of membrane module fibers and returned to the vessel vapor space while the pressure in the tank was monitored. Permeate from the tube side was drawn simultaneously from the module by a vacuum pump and discharged to an exhaust vent. Within 3 minutes from starting, pressure in the vapor space of the tank dropped to about 14.7 psia (i.e., atmospheric pressure). The vapor in the tank was measured at the start and end of each trial using a NOVA Analytical Systems Inc. (Niagara Falls, NY) model 317WP nondispersive infrared analyzer indicating concentration of gasoline as percent propane. The experiment was repeated multiple times. Conditions and results are reported in Table I.

In all cases the concentration of volatile organic compounds increased indicating a successful removal of nitrogen by the membrane module and a lowering of the vessel pressure.

Table I

Ullage Ullage

ExperiInitial Feed Flow Permeate Permeate Gasoline Gasoline ment Tank Rate Pressure Flow Rate Vapor Vapor

Pressure at start at end

(psia) (scfh) (psia) (scfh) (wt % propane)

A 18.3 20.0 3.2 1.5 30.4 40.1

B 18.6 15.5 3.4 1.4 29.7 39.0

C 18.2 15.5 3.5 2.2 31.5 39.7

D 18.3 15.0 3.4 1.2 30.6 41.3

Although specific forms of the invention have been selected in the preceding disclosure for illustration in specific terms for the purpose of describing these forms of the invention fully and amply for one of average skill in the pertinent art, it should be understood that various substitutions and modifications which bring about substantially equivalent or superior results and/or performance are deemed to be within the scope and spirit of the following claims.

Claims

What is claimed is

1. A method of controlling volatile organic compound emissions from an ullage of a mobile storage tank on a mobile vehicle, the tank containing liquid comprising volatile organic compounds, the method comprising (a) providing a membrane separator onboard the mobile vehicle and comprising a selectively gas permeable membrane having a selectivity of nitrogen to propane, (b) bringing vapor from the ullage comprising a mixture of volatile organic compounds and non-hydrocarbon compounds in contact with one side of the selectively gas permeable membrane, (c) creating a pressure gradient across said membrane effective to selectively permeate said vapor through the membrane and thereby forming on the opposite side of the membrane a permeate enriched in the non-hydrocarbon compounds and forming on the one side of the membrane a retentate vapor depleted of non-hydrocarbon compounds, and (d) removing the permeate.

2. The method of claim 1 in which the selectivity of nitrogen to propane is at least about 8.

3. The method of claim 1 which further comprises the step of (e) returning the retentate vapor to the ullage.

4. The method of claim 1 which further comprises positioning the selectively gas permeable membrane inside the storage tank.

5. The method of claim 4 in which the selectively gas permeable membrane is directly exposed to the vapor within the ullage.

6. The method of claim 4 in which the pressure gradient is created by pressure buildup in the ullage without drawing vacuum on the permeate through a vacuum pump.

7. The method of claim 1 in which the selectively gas permeable membrane has a nitrogen permeance greater than about 50 GPU.

8. The method of claim 1 in which the mobile storage tank is a fuel tank of an automotive vehicle or a portable internal combustion engine-powered apparatus.

9. A system for removing non-hydrocarbon compounds from a mixture with volatile organic compounds in a mobile storage tank, the system comprising (a) a mobile vehicle having a mobile storage tank onboard the mobile vehicle, (b) a liquid comprising volatile organic compounds in the storage tank, (c) an ullage in the storage tank comprising a vapor mixture of volatile organic compounds and non-hydrocarbon compounds, and (d) a membrane separator onboard the mobile vehicle and comprising a selectively gas permeable membrane having a selectivity of nitrogen to propane, in which the membrane separator defines a feed-retentate chamber on one side of the membrane in fluid communication with vapor mixture, and a permeate chamber on the opposite side of the membrane.

10. The system of claim 9 which further comprises a conveyor means onboard the mobile vehicle for moving vapor through the system.

11. The system of claim 9 in which the conveyor means comprises a vacuum pump having a suction in fluid communication with the permeate chamber.

12. The system of claim 9 in which the selectivity of the selectively gas permeable membrane is at least about 8 and the membrane has nitrogen permeance of at least about 50 GPU.

13. The system of claim 9 in which the membrane separator is within the storage tank.

14. The system of claim 13 in which the membrane is directly exposed to the vapor within the ullage of the storage tank.

15. A nozzle for a storage tank for an inventory of liquid volatile organic compounds and for an ullage above the liquid in the tank, the nozzle comprising (1) a nozzle wall adapted to protrude outwardly from the tank, (2) a cap sealing the nozzle wall distant from the ullage, (3) a port through the nozzle wall or cap adapted to discharge vapor from the nozzle, (4) a membrane separator positioned within the nozzle and comprising a selectively gas permeable membrane defining (i) a feed-retentate chamber within the nozzle on one side of the membrane in fluid communication with the ullage and (ii) a permeate chamber within the nozzle on the opposite side of the membrane in fluid communication with the port, in which the membrane has a selectivity of nitrogen to propane of at least about 8 and a permeance of nitrogen of at least about 50 GPU.

16. The nozzle of claim 15 which the one side of the membrane is directly exposed to the ullage of the storage tank.