WO2016160949A1

WO2016160949A1 - Hydrostatic control of a pressure differential between gas and liquid streams

Info

Publication number: WO2016160949A1
Application number: PCT/US2016/024954
Authority: WO
Inventors: Steven G. CALDERONE
Original assignee: Synata Bio, Inc.
Priority date: 2015-03-31
Filing date: 2016-03-30
Publication date: 2016-10-06

Abstract

A control method simply and reliably regulates the relative pressure across boundary surfaces that comprise opposite faces of a single element or are the outer surfaces of compound structure and at least partially separates the flow of a gas and a liquid thereacross by sending at least some of the gas that contacted boundary surface to a column of liquid (38) and adjusting the height of the liquid therein to provide a consistent back pressure on the gas flow from its boundary surface. The control method also permits regulation of back pressure on the liquid by maintaining a column of the liquid that flows from its boundary surface at a relatively constant elevation and controlling the flow of liquid out of such retained column of the liquid flow stream.

Description

HYDROSTATIC CONTROL OF A PRESSURE DI FFERENTIAL BETWEEN GAS AN D LIQUID

STREAMS BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to pressure differential control of relative pressures between gas and liquid streams.

Description of the Prior Art

Many processes have the need for a very precise control of relative pressures between one or more gas phase streams and one or more liquid phase streams. Many reasons arise for needing a reliable control of relative pressure between gas and liquid phase streams, these include overcoming pressure drop requirements, achieving optimum conversion conditions, adapting a process to a given supply pressure of one or more of its process streams.

This invention applies to a subset of such processes where gas phase and liquid phase streams are partitioned, i.e. separated from each other by a partition or barrier. As non-limiting examples, such partitions or barriers may comprise: permeable, impermeable or semi-permeable barriers such as membranes; or a heat exchange surface. Thus, the barrier or partition (hereinafter referred to as a partition for simplicity of description, and not for purposes of limitation) typically permits controlled communication of at least some portion or property of the gas and/or liquid stream with the other. Such partitions may for example communicate heat, when in the form of a heat exchange surface, provide a controlled transfer or cross-exchange of the fluids, or in the case of membrane partitions, the selective communication of different components of the liquid across the partitions.

Typically the separated streams are under pressure. Process requirements such as flow, reaction conditions or desired transport of fluid or their components across the partition will often dictate the use of different pressures for each separate fluid. Therefore, processes that require the sequestration of different interacting streams commonly attempt to maintain the separated streams at different pressures. In these cases the partition must withstand a differential pressure between the gas and liquid stream.

Often process that sequester fluids achieve better results as thickness of the partition decreases. In fact, a partition with minimal thickness serves best in many processes.

However, minimal thickness partitions between the process streams provide little surplus strength for withstanding differential pressure. The desirability to use a minimal thickness partition combined with the need to maintain or allow for pressure differential between the two streams creates a particular need for pressure control systems that can prevent significant deviations from target pressure differentials.

An especially beneficial application for this type of system is in bioreactors that use membranes as partitions. There are many forms of bio-reactors commonly used in the production of biofuels or biochemicals. These reactors use living organisms to convert inorganic or organic materials, particularly organic materials in the form of solids to the desired liquid products. Examples of biofuels and chemicals include ethanol, acetic acid, acetate, propanol, propionate, butanol and butyrate.

One of the main drivers for biofuels is their derivation from renewable sources by fermentation and bioprocess technology. Conventionally, biofuels have been made from readily fermentable carbohydrates such as sugars and starches. For example ethanol can be derived from many high carbohydrate sources. Among them are sugar cane, sugar beet, corn, sorghum, hemp seed, potatoes, sweet potatoes, cassava, sunflower, fruits, molasses, and various grains such as wheat and barley. Sugarcane, corn and other high carbohydrate containing plant products in addition to starches and sugars predominantly contain three separate classes of components as building blocks: cellulose (C6 sugar polymers), hemicellulose (various C5 and C6 sugar polymers), and lignin (aromatic and ether linked heteropolymers).

The cellulosic and lignin components of plants are refractory and not readily fermentable to alcohols and other chemicals. The very heterogeneous nature of lingo-cellulosic materials enables them to provide the mechanical support structure of plants and trees makes them inherently recalcitrant to conventional fermentations. Breaking down these recalcitrant materials to provide fermentable sugars for bioconversion to ethanol typically requires pretreatment steps together with chemical/enzymatic hydrolysis. Furthermore, conventional yeasts are unable to ferment the C5 sugars to ethanol and lignin components are completely unfermentable by such organisms. Often lignin accounts for 25 to 30% of the mass content and 35 to 45% of the chemical energy content of lignocellulosic biomass. For all of these reasons, processes based on a pre- treatment/hydrolysis/fermentation path for conversion of lignocellulosic biomass to ethanol, for example, are inherently difficult and often uneconomical multi-step and multi-conversion processes.

Adapting a fermentation process to use the cellulosic and lignin components of biomass can greatly increase the efficiency of biofuel production and has many advantages over carbohydrate based fermentations. High cellulose and lignin sources of biomass include switch grasses, miscanthus, bagasse, stover, kenaf, hemp fiber, straw, cotton, forest products, and many types of cellulose waste. Use of these types of sources avoid competition with food stuffs in the production of liquid fuels or chemicals and are among the most abundantly available raw material on earth for the production of bio-fuels.

Some recent work in biofuels production focuses on the conversion of lignin and cellulose material. This alternative technology path captures the energy value of the cellulose and lignin by converting it to syngas (also known as synthesis gas, primarily a mix of CO, H₂ and C0₂ with other components such as CH₄, N₂, NH₃, H₂S and other trace gases) and then ferment this gas with anaerobic microorganisms to produce biofuels such as ethanol, N-butanol or chemicals such as acetic acid, butyric acid and the like. This path can be inherently more efficient than the pretreatment /hydrolysis/fermentation path because the gasification step can convert all of the components to syngas with good efficiency (e.g., greater than 75%), and some strains of anaerobic microorganisms can convert syngas to ethanol, N-butanol or other chemicals with high (e.g., greater than 90% of theoretical) efficiency. Moreover, syngas can be made from many other carbonaceous feedstocks such as natural gas, reformed gas, peat, petroleum coke, coal, solid waste and land fill gas, making this a more universal technology path.

However, this technology path requires that the syngas components CO and H2 be efficiently and economically dissolved in the aqueous medium and transferred to anaerobic microorganisms that convert them to the desired products. And very large quantities of these gases are required. For example, the theoretical equations for CO or H2 to ethanol are:

6CO+3H₂0— >C₂H₅0H+4C0₂

6H₂+2C0₂— >C₂H₅OH+3H₂0

Thus 6 moles of relatively insoluble gases such as CO or H₂ have to transfer to an aqueous medium for each mole of ethanol. Other products such as acetic acid and N-butanol have similar large stoichiometric requirements for the gases.

Furthermore, the anaerobic microorganisms that bring about these bioconversions generate very little metabolic energy from these bioconversions. Consequently they grow very slowly and often continue the conversions during the non-growth phase of their life cycle to gain metabolic energy for their maintenance. To get high yields and production rates, the cell concentrations in the bioreactor need to be high and this requires some form of cell recycle or retention. Cell retention by formation of biofilms is a very good and often inexpensive way to increase the density of microorganisms in bioreactors. This requires a solid matrix with large surface area for the microorganisms to colonize and form a biofilm that contains the metabolizing microorganisms in a matrix of biopolymers that the microorganisms generate.

U.S. Ser. No. 11/781,717 filed Jul. 23, 2007, U.S. Ser. No.11/833,864 filed Aug. 3, 2007 and U.S. Ser. No. 11/972,454 filed Jan. 10, 2008 disclose a membrane based bioreactor wherein anaerobic bacteria that have the ability to convert syngas to ethanol or other liquids, having formed biofilms on the outer surface of hydrophobic membranes with the syngas fed to the bacterial biofilm through the inner surface of the membrane. Such a bioreactor system has been able to directly convert the primary components of synthesis gas, CO and H₂/C0₂, to ethanol and other liquid products such as N-butanol, acetic acid and butyric acid. In these systems the gas flows through a porous region of a hydrophobic membrane and then reaches a biofilm which is hydrophilic. One drawback of this arrangement is that if water reaches and deposits/condenses on the hydrophobic porous region, it will severely decrease the gas transfer rate.

U.S. Ser. No. 12/036,007 filed Feb. 22, 2008 discloses an asymmetric membrane based bioreactor wherein anaerobic bacteria have the ability to convert syngas to ethanol or other liquids, having formed biopores to retain biofilms therein on the outer, spongy, surface of the membrane to feed the syngas directly to the bacterial biofilm and not through the membrane. The lumen of the membrane transports fermentation liquid or broth past the membrane for feeding of nutrients to the biofilm and reflux of liquid products from the biofilm back through the membrane and into the fermentation liquid.

Asymmetric membranes are known for use in a variety of membrane separations processes such as ultra and nano-filtration. Asymmetric membranes are typically hydrophilic and have a relatively tight semipermeable "skin" layer on one side supported on a porous "spongy" polymer layer. U.S. Pat. Nos. 4,442,206 and 4,440,853 show the use of the "spongy" polymer layer in an asymmetric membrane to immobilize microorganisms for certain biological processes that use soluble carbon sources. However, the asymmetric membrane typically have little ability to avoid rupture under pressure. Furthermore when in their typical hollow fiber form, the lumen are subject to collapse when the external fiber pressure significantly exceeds the lumen pressure. Collapse or rupture can result in loss of production capacity and/or excessive downtime for replacement membrane replacement.

It is the goal of this invention to provide a method for controlling the relative pressure on opposite sides of a partition that restricts communication between a gas stream and a liquid stream wherein the method reduces transient changes in the desired pressure of each stream through the use of a hydrostatic column of liquid. An especially useful application is the ability to regulate differential pressure between the partitioned gas and liquid streams within a specified range to increase efficiency of the production of biofuel using wherein a membrane serves as the partition.

Brief Summary of the Invention

This invention can simply and reliably regulate the pressure of any gas stream. A very advantageous application uses it to control the relative pressure across boundary surfaces that comprise opposite faces of a single element or are the outer surfaces of compound structure and at least partially separates the flow of a gas and a liquid thereacross by sending at least some of the gas that contacted a boundary surface to a column of liquid and adjusting the height of the liquid therein to provide a consistent back pressure on the gas flow from its boundary surface. Back pressure on the liquid stream may also be regulated in a somewhat analogous manner by maintaining a column of the liquid that flows from its boundary surface at a relatively constant elevation and controlling the flow of liquid out of such retained column of the liquid flow stream.

Accordingly the invention in one form provides a method for the control of a pressure differential between a gas stream primarily comprising a gas phase fluid and a liquid stream primarily comprising a liquid phase fluid wherein the gas stream is partitioned from the liquid stream at boundary surfaces and fluid communication between the gas stream and liquid stream through the boundary surfaces is restricted. The method passes the liquid stream into contact with a first boundary surface and recovers, at a liquid boundary pressure, a recovered liquid stream comprising liquid that is in communication with the first boundary surface and passes the gas stream into contact with a second boundary surface and recovers, at a gas boundary pressure, a recovered gas stream comprising gas that is in communication the second boundary surface; at least a portion of the recovered gas stream passes into a column of liquid at a gas entry elevation. The process controls the volume of liquid in the column in a manner that provides an interface comprising the top surface of liquid in the column and above which upper gas is retained; keeps the interface above the gas entry elevation; and recovers column gas from the column of liquid. The process thereby regulates a pressure differential that corresponds to the relative difference in pressure between the gas boundary pressure and the liquid boundary pressure by maintaining the interface at an about constant elevation or within a range elevations such that the pressure of water above the gas entry elevation at least in part controls the gas boundary pressure. In the most common cases the gas boundary pressure will exceed the liquid boundary pressure by no more than approximately 200 kPa (29 psi) and preferably by no more than approximately 70 kPa (10 psi) and more preferably by no more than approximately 35 kPa ( 5 psi.) The gas boundary pressure itself is usually in a range of from 3.5 kPa (0.5 psi) to 140 kPa (20 psi.)

Thus, this invention particularly addresses the situation where the flow rate of gas in contact with the gas contact surface and/or the flow rate of liquid in contact with the liquid contact varies over time and keeps the pressure differential constant, or in a constant range, in response to such changes by adjusting and maintaining the elevation of the interface as necessary. In essence the volume of liquid in the column is controlled in response to a pressure differential input and/or a determined value of the pressure differential by adding or removing of liquid from the column of liquid based on pressure differential monitoring and/or input.

The boundary surface constitutes the face of an element, preferably one of planar or tubular shape that inhibits the flow of fluid thereacross in some manner. In a broad sense the boundary surfaces block, restrict or impede the flow of fluids, fluid components or distinct flow regimes. The element forming the boundary surface may completely inhibit fluid flow, i.e. prevent fluid flow, when in a form such as a solid plate. Alternately an element that permits only a portion of the stream to flow across it may provide the boundary surface. Elements of this type will usually comprise a fine screen or filter that allows only an aliquot portion of the stream to pass through it or a membrane that can also selectively permit only the passage of desired stream components.

For purposes of this description the streams are described as partitioned from each other and the invention may use a partition as part of the structure that provides this partitioning. Preferably the invention uses a partition that at least assists with the partitioning of the gas stream and the liquid stream. The boundary surfaces may comprise an integral part of the partition that provides the first and second boundary surfaces on its opposite sides. In another form the partition may comprise part of a compound structure retaining one or more separate elements that provide the boundary surfaces. Thus, the partition may include distinct and/or separate boundary surfaces. For example, one type of boundary surface may comprise a membrane that receives support from a partition in the form of a perforated plate that poses no functional flow impediment and may serve as a spacer to position two membranes in a spaced apart relationship.

The invention can apply broadly to the control of a pressure differential, which in some cases may refer more appropriately to a pressure drop where there is substantial flow across a boundary surface. The pressure differential occurs across any combination or arrangement of two boundary surfaces having a fluid in contact therewith and each boundary surface has an orientation away from each other. The boundary surfaces are often integral with, or connected, to a partition in a partition arrangement. Thus, and not intended in the way of limitation, where the boundary surfaces are integral with the partition, such partitions include those previously described, e.g. heat exchange tubes; permeable or impermeable conduits defining an inner flow channel and an outer flow channel; semi-permeable hollow fiber membranes; permeable or impermeable parallel plate elements defining alternate flow channels of liquid flow and gas flow; alternating flow channels defined by the annuli of concentric tubes; or flow channels created between multiple spiral wound elements.

It is possible to use this invention with two liquid phase fluids that are partitioned by creating a flowing gas phase cushion. The most common use of this invention applies to controlling pressure between fluids where at least one of the fluids is present in a substantial gas phase.

The invention can control the differential pressure in a variety of ways. Usually the invention keeps the pressure differential within a range of differential pressures by maintaining the interface between a minimum and maximum height above the gas entry elevation. The pressure may be kept below a maximum value by setting a maximum elevation above the gas inlet for the interface. In most cases the differential pressure is no greater 35 kPa (5 psi.) Of course, the control system can operate to maintain a minimum differential pressure which in some cases may be equal to zero. In other arrangements the invention can tie the pressure of the liquid phase stream to the gas phase stream. This is readily accomplished by sending some or all of the liquid phase stream to an additional vessel, usually in the form of a disengagement or separator vessel, along with gas from the column of liquid. The additional vessel equalizes the liquid and gas pressures at the point that they enter it and the pressure in the vessel can set a base pressure such that the hydrostatic head above the gas entry elevation serves to adjust the differential. In this manner the invention simplifies regulation of the pressure differential since its adjustment only requires adding or removing a control liquid from the column of liquid.

In another application the invention is a process for the biological conversion of a gas substrate using microorganisms that converts the gas into useful liquids and uses membranes for controlling communication between the gas substrate and a fermentation liquid using hydrostatic control of the relative pressures of the gas stream and liquid stream across the membranes. The process passes the fermentation liquid into contact with one side of each membrane that serves as a liquid contacting surface; recovers, at a liquid surface pressure, a recovered liquid comprising fermentation media and a conversion compound; passes the gas substrate into contact with the opposite side of the membrane, relative to liquid contacting surface, that serves as a gas contacting surface and recovers, at a gas surface pressure, a recovered gas stream comprising residual components of the gas substrate and conversion gas components; and passes at least a portion of the recovered gas stream, at a gas entry elevation, into a regulating vessel that retains a volume of a control liquid. The volume of liquid in the regulating vessel is controlled in a manner that provides an interface between the top of the control liquid and a volume of column gas retained in the vessel above the interface such that interface stays above the gas entry elevation. Column gas is recovered from the regulating vessel. In this manner the process provides regulation of a pressure differential that corresponds to the relative difference in pressure between the liquid surface pressure and gas surface pressure by maintaining the interface within a range elevation such that all elevations are above the gas entry elevation. There is no limitation on the type of control liquid that the invention may use. Liquids with low viscosity provide the most use. Water or aqueous streams make highly suitable control liquids.

The use of a stream that is normally present in the process or system to which the invention is applied have added advantages. In particular where the invention controls a fermentation process, the control liquid advantageously comprises the fermentation broth originating from the fermentation zone.

The control liquid is derivable from a variety of liquids including a fermentation broth originating from a fermentation zone that biologically converts a gas substrate. The gas substrate in this aspect of the invention comprises at least one of hydrogen together with C02 and CO. In this aspect microorganisms produce a conversion compound comprising an oxygenate and the gas stream emanates at least in part from the fermentation zone. Typically the conversion compound(s) will comprise at least one of ethanol, acetic acid, propanol and n-butanol.

The membranes serve as part of a bioreactor and preferably comprise hollow fiber membranes. In many cases the fermentation liquid passes through the lumens of the hollow fibers, the inside of which provide the liquid contacting surface. In such arrangement the outside of the fiber provides the gas contact surface that also retains the microorganisms thereon or within close proximity thereto.

Another application of the invention takes the form of a system for regulating the pressure differential between a gas stream, primarily comprising a gas phase, and a liquid stream primarily comprising a liquid phase stream. The system includes a partition arrangement comprising at least one partition for separating the gas stream from the liquid stream and that defines at least one gas flow channel arrangement comprising at least one gas flow channel adapted to receive the gas stream and at least one liquid flow channel adapted to receive the liquid stream. A regulating vessel is adapted to retain a vertical column of liquid therein and adapted to receive at least a portion of the gas stream from the gas channel arrangement through a gas inlet located at a gas inlet elevation and defined at least in part by the regulating vessel. The system also includes at least one liquid communication port defined at least in part by the regulating vessel and adapted to allow addition of control liquid into the regulating vessel and/or removal of control liquid from the regulating vessel and at least one gas outlet defined at least in part by the regulating vessel for allowing gas to flow out of the regulating vessel. An additional fluid communication port is defined by the regulating vessel if necessary for the addition or withdrawal of control liquid. The system uses at least one pressure sensor adapted to obtain at least one of: i) a liquid pressure value indicative of the pressure of the liquid stream in the liquid flow channel and ii) a gas pressure value indicative of the pressure of the gas stream in the gas flow channel in conjunction with a liquid level control arrangement capable of adjusting the volume of the control liquid in the regulating vessel by adding or removing control liquid through the at least one liquid communication port to i.) establish an interface between a gas phase and a liquid phase in the regulating column, ii.) maintain the interface above the gas inlet elevation, iii.) control the differential pressure in response to an input setting indicating a range of desired variation of the differential pressure, iv) maintain the differential pressure in the range of desired variation.

Brief Description of the Drawings

Figure 1 shows a cross section of a partition arrangement having two separate boundary elements.

Figure 2 shows an enlarged portion of the cross section of Figure 1.

Figure 3 presents a schematic representation of one possible arrangement for practicing this invention by a method of control for maintaining a pressure differential between gas and liquid streams passing through a membrane separator within a vessel, also referred to as a shell.

Detailed Description of the Invention

This invention has broad application to any method, process or system with a need to control the pressure of at least one fluid stream that is used therein. The most useful applications may be those involving the partitioning of multiple streams as previously described wherein at least one of the process streams is in a primarily gaseous phase. The further description of this invention will be in the context of controlling a pressure differential between a primarily gas phase stream separated from a primarily liquid phase stream across by a partition arrangement, but is not intended to preclude the application of the invention in the control of other streams with any manner of boundary surfaces that may benefit from its use.

Definitions

The term "boundary surface" refers to a surface formed by a solid material about which one of gas or liquid will flow and that receives support or positioning by virtue having a connection to or with a partition. A boundary surface may restrict flow of gas or liquid in the manner typically accomplished by impermeable, semi-permeable or permeable materials and may be provided by an element that is separable from the partition or may provide relatively unrestricted flow of gas or liquid to a partition element.

The term "partition" refers to a barrier structure that by itself or in combination with one or more boundary surfaces separates the gas from the liquid and may in some cases serve to restrict flow of gas or liquid in the manner typically accomplished by impermeable, semi-permeable or permeable materials and will typically have the construction of a relatively thin flat or rolled sheet that may block communication between the gas and liquid thereacross, or permit limited communication of the gas and/or into or through it. Either side of the partition may comprise a boundary surface as described herein. The partition may comprise single or multiple elements and may provide a single boundary surface or multiple boundary surfaces and, in a preferred form, will comprise a single unitary element that provides the desired control of gas and/or liquid and provides both boundary surfaces on its opposite sides. A partition assists with the partitioning of the gas or liquid stream where it supports or positions at least one of the boundary surfaces or restricts the flow of gas or liquid across the boundary surfaces. The preferred form of the partition will also define the boundary surfaces, non-limiting examples of which are described as heat exchange tubes; permeable or impermeable conduits defining an inner flow channel and an outer flow channel; semipermeable hollow fiber membranes; permeable or impermeable parallel plate elements defining alternate flow channels of liquid flow and gas flow; alternating flow channels defined by annuli of concentric tubes; or flow channels created between multiple spiral wound elements.

The term "common boundary" refers to a combination of any partitions and elements that provide boundary surfaces and include all elements of any partitions that are connected to or with any boundary surfaces.

The term "oxygenate," also referred to as an "oxygenated organic compound" means one or more organic compounds containing two or more, and preferably two to six carbon atoms together with hydrogen and oxygen atom and preferably selected from the group of aliphatic carboxylic acids and salts, alkanols and alkoxide salts, and aldehydes. An Oxygenate is often part of a mixture of organic compounds produced by microorganisms in a fermentation process, particularly a fermentation process that uses syngas as the gas substrate.

Syngas refers to a gas made from a variety of carbonaceous feedstocks. These include sources of hydrocarbons such as natural gas, biogas, biomass, especially woody biomass, gas generated by reforming hydrocarbon-containing materials, peat, petroleum coke, coal, waste material such as debris from construction and demolition, municipal solid waste, and landfill gas. Syngas is typically produced by a gasifier or reformer. Any of the aforementioned biomass sources are suitable for producing syngas. The syngas produced thereby will typically contain from 10 to 60 mole% CO, from 10 to 25 mole% C0₂ and from 10 to 60 mole% H₂. The syngas may also contain N₂ and CH₄ as well as trace components such as H₂S and COS, NH₃ and HCN. Gases from other sources, such as those generated during petroleum and petrochemical processing, may comprise or undergo conversion or separation to comprise either or both of CO, or C02 with Hydrogen as the majority of the components in the gas and although some of these gases may differ from the typical composition of a syngas, the term syngas for ease of reading refer to these gases as well.

Substrate - refers to a gaseous stream used in a biological process conversion as a feed gas for conversion by microorganisms into a desired liquid that is a product of the conversion. Typically such may be obtained directly from gasification or from petroleum and petrochemical processing or may be obtained by blending two or more streams. The gas substrate may be treated to remove or alter the composition including, but not limited to, removing components by chemical or physical sorption, membrane separation, and selective reaction. Components may be added to the gas substrate such as nitrogen or adjuvant gases such as ammonia and hydrogen sulfide.

Essentially equal pressure means a pressure that varies by no more than ordinary head losses through piping, valves, meters and similar equipment, but excludes any variation in pressure resulting from the use of pressure control devices.

A gas stream primarily comprising a gas phase fluid means that at least 50% of the gas stream components are in the gas phase.

A liquid stream comprising primarily a liquid phase fluid means that the liquid stream has no more than 5% of its volume in a gas phase.

Pressure tying refers to a direct or indirect equilibration of the pressures of the different fluids that are subject to pressure control in accordance with this invention where in a direct tying at least a portion of the fluid streams enter a common chamber downstream of the fluid stream partitioning and in an indirect tying at least a portion of the two streams exert pressure against opposite sides of a highly flexible separation element such as a bladder or a corrugated expansion element arranged to provide a desired degree of pressure equalization between each of the fluid streams.

Arrangement and Control of the Gas Stream

Typically the addition and withdrawal elevations for controlling the volume of liquid in the column are below the gas entry elevation and recovery of gas takes place above the highest anticipated elevation of the interface, i.e. the top of the column of liquid. In most cases a gas recovered from the column of liquid flows out of a nozzle located at the top of a regulating vessel that retains the column of liquid. Having the interface above the gas entry is necessary to effectively practice the invention. Of course the fluid withdrawal elevation must be below the interface to have a practical means of withdrawing liquid. However, it is possible to have alternate liquid withdrawal points that are at times above the elevation of the interface as its elevation moves up and down in the control of the pressure differential. It is possible, but not particularly practical, to withdraw gas from the column of liquid at a location below the interface since this will typically create a need to separate liquid from the any gas that is recovered from the column.

The hydrostatic head of the control liquid must supply a consistent pressure to the gas stream that enters the column of liquid. Coalescing of the gas into large bubbles or, even worse, slugs of gas poses the main disrupting influence on the consistency of the back pressure from the control liquid at a constant elevation of control liquid and gas stream flow. Discharging the gas into the control liquid through a distributor that disperses the gas into fine bubbles can help to eliminate or minimize variations in the back pressure of the column of liquid. Selecting a suitable diameter (on the basis of gas flow, control liquid make-up, gas composition, etc.) of the regulating vessel will also minimizes the coalescing of gas bubbles in the control liquid, wherein larger vessel diameter promote greater bubble stability. Use of a surfactant will promote the creation and maintenance of the gas into a fine bubble dispersion as it passes through the control liquid. The addition of a surfactant may offer significant advantages to the practice of the invention when used alone or in conjunction with a reduction in the size of the regulating vessel. Thus, maintaining a predetermined concentration of surfactant in the control liquid may reduce the necessary diameter of the regulating vessel. Many processes, such as the biological processes as described herein that convert a gas into products, use or produce compounds with inherent surfactant properties, e.g. ethanol. Therefore, many of the possible applications of this invention can readily employ the use of a surfactant.

Reducing the flow of the amount of process gas that flows through the control liquid can also reduce the size of the regulating vessel or allow a reduction or elimination in the use of surfactant. A process flow arrangement can divide the gas stream from its boundary surface into a stream that passes to the regulating vessel and a remaining stream that passes through a fixed or variable flow restrictor. In this type of flow arrangement only a portion of the gas enters the regulating vessel for control and maintenance of a consistent back pressure while the flow restrictor maintains a stable pressure drop, on the basis of the properties of the gas stream passing through it. In this manner the flow restrictor and control liquid together establish a stable pressure of the gas stream that contacts the boundary surface.

Pressure tying the streams that are subject to the pressure control of this invention to equilibrate the pressure between the fluid streams downstream of the partition and the column of liquid can facilitate regulation of the pressure at the partition. Pressure equilibration can take many forms. Perhaps the simplest is passing at least a portion of the fluid streams to a common chamber or vessel that mixes the fluid streams therein. Typically such a vessel will take the form of a gas separation drum or receiver that into which the gas stream and a liquid stream flow and in which the gas stream is disengaged from the liquid stream for separate recovery of both streams. Sending only a portion of any or all of the fluid streams to a common vessel can also effect pressure equilibration while reducing the volume of fluid streams that enter the same vessel. Where intermixing of the regulated fluids is unsuitable the pressure tying may occur in any manner that will keep the fluid sequestered. One such arrangement can use a flexible pressure separation element with each regulated fluid contacting an opposite side of the element. In one detailed form the pressure separation element will have enough elasticity and a suitable containment arrangement such that the pressure between the regulated fluid streams, or a portion of the streams, can reach equilibrium through adjustment of the relative volume of the fluids on opposite sides of the pressure separation element.

Detailed Description of Figures 1 and 2

Figures 1 and 2 illustrate one example of a compound partition arrangement 100 that comprises a partition and separate elements defining the boundary and that is suitable for the practice of this invention. The compound partition is bounded on both sides by separate boundary layer elements. The figures show partitions arrangements 102 the define gas flow channels 110 and liquid flow channels 101 that define gas boundary surfaces 103 and liquid boundary surfaces 104. Gas streams pass through gas channels 110 and contact gas boundary surfaces 103. Liquid streams pass through channels 109 are contacted by a gas streams and contact liquid boundary surfaces 104.

Figure 2 shows the partition arrangements 102 in further detail. A perforated plate 107 bent into the form of corrugated spacer provides a central partition permits fluid flow thereacross with minimal restriction to fluid flow. Plate 107 provides alternating peaks 111 and 112. Each set of peaks 112 and 111 support a different membrane element to respectively support opposing membrane elements 106 and 108 that provide boundary surfaces 103 and 104 respectively.

In a particular application of partition arrangement 100 a portion of the liquid streams flowing down channels 109 passes across boundary surfaces 104 and into corrugation volume 114 at rate determined by the permeability of membrane 108. Liquid in corrugation volume 114 flows freely through plate 107 and into corrugation volume 113. Corrugation volume 113 receives gas from the gas stream that contacts boundary surface 103 and flows into corrugation volume at a rate controlled by the permeability of membrane 106.

Although not depicted by the Figures, it will be readily understood that other arrangements of the partitions can have both boundary surfaces situated to one of side of the partition. An example of such an arrangement is an at least two layer membrane composition where a highly porous support layer is bonded to a less permeable layer that and the highly porous layer provides no significant inhibition of the fluid flow to the membrane to which it is bonded.

Detailed Description of Preferred Embodiment

As is known in the art, it is difficult to control slight differential pressures across a membrane at high static pressure. In the particular arrangement of the invention shown by Figure 3, a shell 11 of a bioreactor 10 houses plurality of hollow fiber membranes 12 that provide a partition arrangement separating a gas substrate of syngas and a fermentation media. Figure 2 generally shows only those elements of the process arrangement that promote an understanding of the invention and those familiar with such process arrangements can readily provide those additional process elements such as additional equipment and piping necessary for the full practice of the invention. The process depicted by Fig. 3 is particularly well suited for use in producing biofuels, though this should not be construed as limiting but rather as exemplary, within which said method of control for maintaining a pressure differential between gas and liquid streams passing through a vessel in the form of shell that retains tubes in the form of hollow fiber membranes. The shell may withstand external or internal pressurized as desired for the particular application of the invention.

With respect to the fermentation media, most of it circulates through the process from a disengagement vessel 24 through bioreactor 10 and back into vessel 24 while a typically lesser volume passes out of the process. A line 22 carries the fermentation media from disengagement vessel 24 through a pump 26. A line 27 carries a portion of the fermentation media from line 22 into the bioreactor 10 and to a manifold 20 that distributes the media to individual lumens of the hollow fiber membranes 12. The fermentation media delivers nutrients across membranes 12 and into contact with microorganisms retained on the outer surface of the hollow fibers. The microorganisms produce conversion products from the substrate surrounding the fibers. The conversion products pass across the membrane into the lumens and become mixed with the outward flow of fermentation media from the hollow fibers. A manifold 30 collects liquid from the lumens of membranes 12 and a line 32 transports the recovered fermentation media back into disengagement vessel 24. A line 15 carries a portion of the fermentation media from line 22 through a control valve 19 and on to a separation zone (not shown), usually comprising a distillation section, for recovery of one or more conversion products from the fermentation media. The separation zone will usually recover fermentation media and return it to the circulating flow of fermentation media at a suitable location (not shown.) Some portion of the syngas substrate may come into contact with the fermentation liquid and become entrained therein, however only minor amounts of gas will become entrained and the fermentation liquid will at all times constitute a primarily liquid phase stream.

The process of Fig. 3 also retains additional liquid as control liquid in a vertically extended vessel, i.e. the regulating vessel, that provides a control column 38. A line 17 carries a liquid addition stream through a flow controller/control valve assembly 19 and delivers the liquid into column 38 where it mixes with a volume 21 control liquid in column 38. A line 23 withdraws control liquid from column 38, carries the withdrawn liquid through a control valve 33 and passes the withdrawn liquid into disengagement vessel 24.

The liquid addition stream typically comprises sterile water. The addition of sterile water helps minimize fouling of the control liquid within column 38. Alternatively fermentation media could circulate through the control column as the control liquid, however the fermentation media contains a wide variety of compounds and even residue such that it use could pose operational problems for the control column. Liquid added to column 38 can also provide make-up liquid such as sterile water to the fermentation media. On the gas flow side, the syngas enters the process as the conversion substrate and exits the process from disengagement vessel 24. A line 16 carries the gas substrate through a flow controller/control valve assembly 18 and into a shell volume 29 enclosed by shell 11 of bioreactor 10 wherein the substrate gas contacts the outside of membranes 12. A line 34 receives effluent gas from volume 24 and passes it into column 38 through a gas inlet 36 located at a gas inlet elevation 37. A line 40 withdraws a stream of column gas from column 38 above a gas liquid interface 39 positioned at an upper liquid level 31. Line 40 delivers column gas to disengagement vessel 24. A line 42 withdraws effluent gas from disengagement vessel 24 and passes it through a flow controller/control valve assembly 44. A certain amount of the fermentation media that passes across the membrane become entrained with the gas as a liquid, however the substrate at all times remains in a primarily gas phase.

In effecting the essence of the invention a differential pressure controller 46 senses, via sensing tubes 48 and 50 respectively, the difference between the pressure of the liquid in line 32 and the pressure of the gas in line 34. The differential pressure sensed by controller 46 represents, with head loss effects taken into account, the difference in pressure of the gas and the liquid across the membranes 12. This differential pressure may be kept essentially equal or for more typical membrane arrangements may seek to keep the gas pressure approximately 35 kPa above the liquid pressure. Typically the control will keep the differential pressure in a desired range of pressure about a value of 5 kPa and more typically will set a minimum positive pressure for the gas above the liquid pressure and a maximum pressure of 5 kPa for the gas above the liquid.

When sensing variations in the differential pressure or resetting the desired differential pressure, pressure controller 46 signals level controller 58 via a signal transmission 56. Level controller 58 senses pressure at the bottom of column 38 and removes control liquid from the column via control valve 33 to lower the upper liquid level 31 or can add more control liquid to the column via liquid addition stream 17 via a signal sent to flow controller/ control valve assembly 19. In this manner the control elements 46, 58, 33, and 19 maintain a head of liquid above the gas inlet that can range from the gas inlet elevation 37 to the established height of the upper liquid level 31.

The gas stream passed to gas inlet 36 must overcome the back pressure from the head of liquid as it enters column 38. Once in the column the gas bubbles up through the sub-volume of control liquid that extends above the gas inlet 36 and then passes across interface 39 for removal from the control column. The column of control liquid above gas inlet 36 provides a back pressure with minimal un-programmed fluctuation so that the control system can simply and efficiently maintain a consistent pressure differential.

Application of the process will include additional equipment for instrumentation and flow control. Pressure indicators 52 and 54 represent such additional equipment for monitoring the pressure of the liquid and gas streams, respectively, to allow control of absolute pressure of each stream in addition to control of the pressure differential between the gas and the liquid. Flow controller/pressure controller assemblies 18 and 44 respectively control the overall flow of substrate gas into the bioreactor and the flow of gas out of the process. The combination of level control 62 and control valve 19 maintain, via a transmission signal 64, the desired level of the liquid in the disengagement vessel.

Those familiar with methods and processes for pressure control will recognize other applications for this invention and the following claims apply to all such applications that fall within their scope.

Claims

1. A method for control of a pressure differential between a gas stream primarily comprising a gas phase fluid and liquid stream comprising primarily a liquid phase fluid wherein the gas stream is partitioned from the liquid stream at a common boundary and fluid communication between the gas stream and liquid stream between opposing surfaces of the common boundary is restricted, said method comprising:

a) passing the liquid stream into contact with a first boundary surface and recovering, at a liquid boundary pressure, a recovered liquid stream comprising liquid in communication with the first boundary surface;

b) passing the gas stream into contact with a second boundary surface and recovering, at a gas boundary pressure, a recovered gas stream comprising gas in communication the second boundary surface;

c) passing at least a portion of the recovered gas stream into a column of liquid at a gas entry elevation;

d) controlling the volume of liquid in the column in a manner that provides an interface comprising the top surface of liquid in the column and above which upper gas retained;

e) keeping the interface above the gas entry elevation;

f) recovering column gas from the column of liquid; and,

g) regulating a pressure differential that corresponds to the relative difference in pressure between the gas boundary pressure and the liquid boundary pressure by maintaining the interface at an about constant elevation or within a range elevations such that the pressure of water above the gas entry elevation at least in part controls the gas boundary pressure.

2. The method of claim 1 wherein a partition is proximate at least one boundary surface and at least assists with the partitioning of the gas stream and the liquid stream.

3. The method of any of the preceding claims wherein the partition comprises a unitary element that provides the first and second boundary surfaces on its opposite sides.

4. The method of any of the preceding claims wherein the partition comprises: heat exchange tubes; permeable or impermeable conduits defining an inner flow channel and an outer flow channel; semipermeable hollow fiber membranes; permeable or impermeable parallel plate elements defining alternate flow channels of liquid flow and gas flow; alternating flow channels defined by the annuli of concentric tubes; or flow channels created between multiple spiral wound elements.

5. The method of any of the preceding claims wherein the partition comprises a hollow fiber membrane with the liquid stream passing through the lumen of the hollow fiber and the gas stream contacting the outside surface of the hollow fiber.

6. The method of any of the preceding claims wherein the pressure differential is kept within a range of differential pressures by maintaining the interface between a minimum and maximum height above the gas entry elevation

7. The method of any of the preceding claims wherein the pressure differential is kept below a maximum value set by keeping the interface from rising past a predetermined elevation set above the first gas entry elevation.

8. The method of any of the preceding claims wherein the pressure differential equals zero.

9. The method of any of the preceding claims wherein the pressure of the liquid stream and the gas stream are equilibrated downstream of the partition and the column of liquid by pressure tying the gas stream and the liquid stream.

10. The method of any of the preceding claims wherein the liquid in the column of liquid comprises a control liquid and the elevation of the interface is positioned by the addition or removal of control liquid from the column of liquid.

11. The method of any of the preceding claims wherein the control liquid is removed from the column of liquid at a liquid outlet elevation located below the gas inlet elevation and the control liquid is added to the column at a liquid inlet elevation located below the liquid outlet elevation and the column gas is recovered from the column of liquid at an elevation above the gas entry elevation.

12. The method of any of the preceding claims wherein the control liquid comprises a fermentation broth from a fermentation zone and the gas stream is an effluent gas from the fermentation zone that comprises an oxygenate and at least one of C02, hydrogen, and CO.

13. The method of any of the preceding claims wherein the gas boundary pressure exceeds the liquid boundary pressure by no more than approximately 200 kPa (29 psi) and preferably by no more than approximately 35 kPa ( 5 psi).

14. The method of any of the preceding claims wherein at least one of the flow rate of gas in contact with the gas contacting or the flow rate of liquid in contact with the liquid contact surface varies over time and the elevation of the interface is adjusted in response to variations in at least one of the gas stream flow rate and the liquid stream flow rate.

15. The method of any of the preceding claims wherein removal of liquid from the column is controlled based on determining the value of the pressure differential and adding or removing of liquid from the column of liquid based on the determined value of the pressure differential.

16. The method of any of the preceding claims wherein at least a portion of the recovered liquid stream and at least a portion of the column gas are passed into a recovery vessel.

17. The method of any of the preceding claims wherein a plurality of hollow fiber membranes comprise a membrane bioreactor, microorganisms adhere to the outside of each hollow fiber; a shell retains the hollow fiber membranes; the gas stream passes through the shell and contacts the hollow fiber membranes for the production of an oxygenate; the liquid stream comprises a fermentation broth; the hollow fiber membranes communicate the fermentation broth with the microorganisms and the oxygenate with the liquid stream; the liquid in the column comprises at least a portion of the fermentation broth; and the column gas is recovered at an elevation above the interface.

18. The method of any of the preceding claims wherein the differential pressure is no greater 35 kPa (5 psi.)

19. The method of any of the preceding claims wherein the gas boundary pressure is in a range of from 3.5 kPa (0.5 psi) to 140 kPa (20 psi.)

20. A process for the biological conversion of a gas substrate using microorganism that converts the gas into useful liquids using membranes for controlling communication between the gas substrate and a fermentation liquid with control of the relative pressures of the gas stream and liquid stream across the membranes, the process comprising:

a) passing the fermentation liquid into contact with one side of each membrane, with such side serving as a liquid contacting surface and recovering, at a liquid surface pressure, a recovered liquid comprising fermentation media and a conversion compound;

b) passing the gas substrate into contact with the opposite side of the membrane, relative to liquid contacting surface, that serves as a gas contacting surface and recovering, at a gas surface pressure, a recovered gas stream comprising residual components of the gas substrate and conversion gas components;

c) passing at least a portion of the recovered gas stream, at a gas entry elevation, into a regulating vessel that retains a volume of column liquid in the regulating vessel;

d) controlling the volume of liquid in the regulating vessel in a manner that provides an interface between the top of the control liquid and retains a volume of column gas retained in the regulating vessel above the interface;

e) keeping the interface above the gas entry elevation;

f) recovering column gas from the regulating vessel; and,

g) regulating a pressure differential that corresponds to the relative difference in pressure between the liquid surface pressure and gas surface pressure by maintaining the interface within a range elevations such that all elevations are above the gas entry elevation.

21. The process of any of claim 20 wherein the gas substrate comprises at least one of hydrogen together with C02 and CO and the conversion compound comprises oxygenates.

22. The process of claim 20 or 21 wherein the membranes comprises a hollow fiber membrane, fermentation liquid passes through the lumens of the hollow fibers, the gas substrate contacts the outside surface of the hollow fibers, and microorganisms on or within close proximity of the outer surface of the fiber.

23. The process of claim 20, 21 or 22 wherein at least a portion of the recovered liquid stream and at least a portion of the column gas are passed into a recovery vessel; at least a portion of the column gas is passed into the recovery vessel; fermentation liquid is recovered from the recovery vessel; at least a portion of the fermentation liquid from the recovery vessel passes into contact with liquid contacting surface in accordance with step (a) of claim 20; a conversion stream comprising conversion product is recovered from the recovery vessel; and a conversion product is recovered from at least a portion of the conversion stream.

24. The process of claim 20, 21, 22 or 23 wherein a liquid comprising water is added to the regulating column to at least in part adjust the elevation of the interface.

25. The process of claim 20, 21, 22, 23 or 24 wherein the gas surface pressure exceeds the liquid surface pressure by no more than approximately 70 kPa (10 psi) and preferably by no more than approximately 35 kPa (5 psi.)

26. The process of claim 20, 21, 22, 23, 24 or 25 wherein the fermentation product (20) comprises at least one of ethanol, acetic acid, propanol and n-butanol.

27. The method of claim 20, 21, 22, 23, 24, 25 or 26 wherein the membrane comprises an asymmetric membrane having at least one of i.) a semi-permeable skin comprising regenerated cellulose cast onto a porous polymer comprising a microporous polyethylene substrate or a ii.) a semi-permeable skin and porous polymer both comprising polysulfone or polyethersulfone.

28. A system for regulating the pressure differential of a gas stream on one side of a partition and a liquid stream on an opposite side of a partition, the system comprising:

a) a partition arrangement comprising at least one partition for separating a gas stream primarily comprising a gaseous phase fluid from a liquid stream comprising a primarily liquid phase fluid, said partition arrangement defining at least one gas flow channel arrangement comprising at least one gas flow channel adapted to receive the gas phase stream and at least one liquid flow channel arrangement defining at least one liquid flow channel adapted to receive the liquid stream;

b) a regulating vessel adapted to retain a vertical column of liquid therein and adapted to receive at least a portion of the gas stream from the gas channel arrangement through a gas inlet located at a gas inlet elevation and defined at least in part by the regulating vessel;

c) at least one liquid communication port defined at least in part by the regulating vessel and adapted to allow at least one of adding control liquid into the regulating vessel and removing control liquid from the regulating vessel;

d) at least one gas outlet defined at least in part by the regulating vessel capable of allowing gas to flow out of the regulating vessel;

e) at least one pressure sensor adapted to obtain at least one of: i) a liquid pressure value indicative of the pressure of the liquid stream in the liquid flow channel and ii) a gas pressure value indicative of the pressure of the gas stream in the gas flow channel;

f) a liquid level control arrangement capable of adjusting the volume of the control liquid in the regulating vessel by adding or removing control liquid through the at least one liquid communication port to i.) establish an interface between a gas phase and a liquid phase in the regulating column, ii.) maintain the interface above the gas inlet elevation, iii.) control the differential pressure in response to an input setting indicating a range of desired variation of the differential pressure, iv) maintain the differential pressure in the range of desired variation.