JPH0515487B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a dialysis membrane gas separation system (device).
More particularly, the present invention relates to preventing condensation in such systems.

BACKGROUND OF THE INVENTION Dialysis membranes capable of selectively permeating one component of a gas mixture are considered in the art to be a convenient and potentially highly advantageous means of achieving desired gas separations. In order to realize this potential in actual commercial operation, the membrane system must be maintained so that environmental factors associated with its use do not result in excessive maintenance or unacceptably shortened membrane life. ,
It must be possible to achieve and maintain the desired degree of process efficiency.

One such factor relates to condensation of components of the feed gas on the surface of the membrane. Such condensation can lead to reduced permeation rates, corrosion, increased maintenance, and reduced membrane life. Additionally, there are some cases in which condensation in the membrane system can lead to contamination of the desired product stream.
Thus, membrane systems typically require more membrane surface area for a given gas separation operation due to such condensation. As a result, both capital and maintenance costs are increased over what would be incurred with a membrane system without condensation problems.

Therefore, efforts have been made in the art to minimize or eliminate condensation in membrane systems. One approach that has been used for this purpose is to superheat the feedstock to the membrane system and individually insulate the membrane modules contained within the membrane system to maintain the superheated condition internally. Superheating is typically provided from an external source such as steam or an electric heater. Another approach involves pre-drying the feed stream with an adsorbent or refrigerant dryer to a dew point temperature below the membrane operating temperature.

Although such an approach serves to minimize or eliminate condensation, it is recognized that the associated capital and operating costs are relatively high. Preheaters thus typically require an external energy source, and insulation for individual membrane assemblies can be relatively expensive and make accessing the membranes cumbersome for maintenance. Dryer systems similarly tend to be expensive, both in operating costs and capital expenditures.

Although solutions to the problem of condensation in membrane systems have thus been developed, further improvements in the field, namely condensation, can be achieved with lower initial capital costs, less There remains a need for such developments that would allow operating and maintenance costs to be minimized or eliminated. Such improvements in the art contribute to the technical and economic viability of using dialysis membrane systems in a wide range of commercially important gas separation operations.

It is therefore an object of the invention to provide an improved membrane separation system and method that eliminates the problem of condensation.

Another object of the invention is to provide a membrane separation system and method that accommodates improved means for eliminating or minimizing condensation of feed gas components on membrane surfaces.

With these and other objects in mind, the invention hereinafter is described in detail, and the novel features of the invention are pointed out with particularity in the claims.

SUMMARY OF THE INVENTION An insulated and processed enclosure is used to provide and/or introduce superheat to a feed gas passed through a membrane system contained within the enclosure, the superheat being applied to the surface of the membrane material. plays a role in preventing condensation. There is no need to heat individual membrane modules or to preheat or predry the feed gas before passing it through the membrane system.
Preferably, heat recovered from the feed gas compression operation is used as the superheat.

DETAILED DESCRIPTION OF THE INVENTION It is an object of the invention to provide a membrane system in a vessel separate from itself, insulating and heating the vessel to maintain a desired superheat for the feed gas passing through the membrane system within the vessel. Achieve it. The individual membrane modules making up the membrane system thus do not require individual insulation.
In practicing the invention, capital and operating costs for condensation control are reduced. In addition, a high degree of flexibility is achieved in regulating operating temperatures and achieving process optimization.

The invention relates to any desired membrane structure capable of selectively permeating more easily permeable components of a feed gas mixture containing more easily permeable components and less easily permeable components. I hope it will be applied and understood. That is, a composite type membrane, an asymmetric type membrane, or any other type of membrane structure can be employed. Composite membranes consist of a thin separation layer or coating of a suitable dialysis membrane material stacked on a porous support, with the separation layer determining the separation properties of the composite structure. Asymmetric membranes, on the other hand, have a thin dense semi-permeable skin that essentially determines the membrane's separation properties and a less dense porous non-selective support region that serves to prevent the thin skin region from collapsing under pressure. Constructed of a single dialysis membrane material with Such membrane structures can be made in a variety of shapes, such as spiral wraps, hollow fibers, flat sheets, etc.

When used in actual commercial operations, such membrane structures are commonly used in membrane assemblies, which are typically placed within a container to form membrane modules that constitute the main elements of the overall membrane system. Membrane systems as used in connection with the invention herein consist of a membrane module or multiple membrane modules arranged for either parallel or series operation. As mentioned above, the membrane system is surrounded by a separate insulated container that is heated to maintain superheat, and the individual modules within the system do not require individual insulation as in prior art implementations. Membrane modules can be made in the form of spirally wound cartridges, hollow fiber bundles, pleated flat sheet membrane assemblies, and other such assemblies common in the membrane industry. The membrane module is configured to have a feed side and a counter permeate outlet side. In the conventional module,
The enclosure is configured to contact the feed stream mixture to the membrane feed side. Conduit means are provided for removing the non-permeate portion of the feed stream and separately for removing the permeate gas that has passed through the membrane.

A wide range of practical gas separation operations can suffer from unwanted condensation problems and require improved processes and systems to maintain a superheated and stable temperature environment during optimal operation of gas separation membrane systems. The properties of this are demonstrated herein with respect to air separation to produce a nitrogen-rich product gas stream.
In such air separation operations, there is a loss in membrane permeability, ie, a loss in permeation rate, as the relative humidity of the air is increased. On the other hand, heating the feed air to reduce its relative humidity increases the rate of permeation of the more easily permeable oxygen component of the air. It will be appreciated that as the permeation rate is increased, less membrane surface area is required for a given gas separation, such as the separation of nitrogen and oxygen in the case of air separation. This reduces the effective membrane surface area requirement for a particular membrane system as the feed gas becomes more and more superheated as it is heated above the dew point temperature, thus reducing the capital costs associated with gas separation operations. can be reduced.

A number of other problems can be caused by the accumulation of other condensates on the membrane assembly. Corrosion may now become a major problem, requiring additional capital expenditures for construction materials that can withstand such corrosion and/or increased maintenance costs. Additionally, condensate build-up can eventually carry over into the product gas stream, thereby contaminating the product. In the case of air separation, water can condense within the membrane module and be carried over into the non-permeate nitrogen product stream. However, water vapor is extremely permeable. Thus, if superheated, water vapor will normally permeate through the membrane and exit the system along with other permeate waste, ie, the oxygen-enriched permeate gas stream in a typical air separation operation.

In addition to the effects of relative humidity and condensation on membrane performance, membrane operating temperature also has a significant effect on membrane performance. That is, the capacity of the membrane system per unit area of the membrane surface and the capacity per unit of gas supplied vary considerably depending on the operating temperature. As the temperature increases, the capacity per unit area increases, but the capacity per unit supply gas decreases. As a result, higher operating temperatures can produce more product gas for a given membrane surface area, but require proportionately more feed gas under such higher temperature conditions. Such an increase in feed gas requirements necessarily means that larger air compressors are required and power consumption is greater.
At lower operating temperatures, the opposite effect occurs, reducing compressor and power requirements, but requiring a greater amount of membrane surface area for a given gas separation operation.

With these temperature effects in mind, it is of course highly desirable to optimize the membrane system design with respect to surface area and feed gas requirements for a particular operating temperature. Once a membrane system is designed, it is important that the system can be maintained at the design temperature during normal commercial operation. It is also desirable to have the ability to vary the operating temperature of the system to better meet production and/or purity requirements during periods of reduced demand, ie, turndown from design conditions. The practice of the invention provides a method for adjusting temperature between the design temperature and the desired temperature at various turndown conditions, or between one turndown temperature and another turndown temperature, or between such a temperature and a temperature above the design temperature. Provides desirable flexibility in changing operating temperatures even between temperatures. As can be seen from the disclosure herein, the invention provides a gaseous separation dialysis membrane system that provides a constant, stable,
To provide a simple, inexpensive and efficient means of providing a flexible heating environment.

For purposes of the invention, an inexpensive container or building is mounted over the membrane system, which typically includes a number of membrane modules. Typical building construction insulation materials, such as 3″ (7.6cm) or 6″ (15cm) R-
11 or other typical fiberglass or other suitable insulating material. It will be appreciated that the container will be large enough to allow an operator to enter the container to repair the membrane system located therein. The container may be constructed of sheet metal or any other suitable material and lined with such insulation. Various suitable safety features, such as vents, fans, sneak ports, etc., may be included as part of the container construction. To facilitate use, the container will be provided with membrane system equipment located within and within the container, and suitable doors etc. will be provided to facilitate maintenance.

With reference now to FIG. 1 of the accompanying drawings, the invention will now be further described in relation to an insulated hermetic enclosure heated by an external heat source such as a steam, gas or electric heater. In this embodiment, the feed gas is sent in line 1 to a suitable compressor 2 for compression to the desired feed gas pressure. The compressed feed gas is then
It is passed in line 3 to a suitable conventional gas cooling zone 4 for cooling below the design operating temperature level of the membrane system. Upon cooling, the feed gas is typically supersaturated, ie, it is saturated with its condensable vapor components and also contains free droplets. Thus, at this point in the air separation process, the feed air stream typically contains compressed air below the design operating temperature along with water droplets. The feed gas is passed from the cooling zone 6 in line 6 to a conventional water separation zone 6 of the desired type for the removal of any free water or other droplets present in the feed gas stream. The separated liquid is withdrawn from the separation zone 6 via line 7, whereas the feed gas is withdrawn from the separation zone 6 via line 8. In this regard, the compressed and cooled feed gas stream is typically saturated with its condensable vapor component, such as a water vapor saturated feed air stream, at a particular operating pressure. The feed gas stream enters the inventive insulated sealed enclosure 9 in line 8. The insulating layer of the enclosure is indicated generally by the reference numeral 10, but doors, vents, fans, ports for detecting gas leaks, etc. are not shown in the drawings. The thermally insulated enclosure is heated in the example shown by heating means 11 which are arranged within the enclosure and which receive their heat by external means designated generally by the reference numeral 12.

Feed gas is conveyed in line 8 to a permeable membrane system 13 in a heated, insulated closed enclosure 9, and then to individual membrane modules (not shown) containing the system in desired rows and/or parallel flow paths. will be passed through. The more permeable components of the feed gas mixture pass through the permeable membrane material of the membrane module for discharge via line 14 on the permeate outlet side of the membrane system. The less easily permeable components of the feed gas mixture are withdrawn from the membrane module on its feed or retentate side for discharge from the membrane system 13 via line 15.

Those skilled in the art will appreciate that the feed gas should be heated within the insulated closed enclosure 9 to a superheat temperature, ie above the saturation temperature of the feed gas at the operating pressure of the membrane separation operation. For this purpose, the amount of heat added must be sufficient to superheat the feed gas and compensate for heat losses from the insulated enclosure. The degree of superheating of the feed gas should generally be at least 3°C, preferably at least 5°C, to ensure that undesirable condensation does not occur. Heating may be applied to the atmosphere within the insulated enclosure by heating means 11, or directly to the feed gas as explained below.

It has been found that practice of the present invention provides a number of significant advantages over the prior art, as described below. Thus, the capital cost of the entire system is
This can be reduced compared to prior art methods in which the feed gas is directly superheated and the individual membrane modules, e.g. bundles, are each insulated to maintain the desired superheat conditions. This is especially true when using multi-film modular systems. In addition,
Compared to prior art methods of insulating individual membrane modules, the method of the present invention, which does not insulate each module individually, improves module maintenance service accessibility.

It has also been found that using the heated insulated enclosure of the present invention, more stable temperature conditions can be maintained throughout the entire membrane system than when using prior art methods. Furthermore, in contrast to prior art methods of directly heating the feed gas and insulating individual modules, the operating temperature of the membrane system is made easier by varying the overall temperature within the closed enclosure. I also found out that it is adjustable. The method and apparatus of the present invention thus make it possible to obtain considerably greater flexibility in optimizing membrane system efficiency under variable operating conditions than was previously possible.

The benefits of the present invention are provided when the operation of a membrane system for gas separation may be adversely affected by condensation of components in the feed gas stream to be treated. Practice of the present invention can also provide benefits when temperature control of the membrane system is necessary to optimize overall process/membrane system operation. In a specific example as explained below,
The benefits of the present invention are further enhanced when the energy requirements of gas separation operations can be advantageously reduced by recovery of heat from the entire gas separation system.

Referring now to FIG. 2, the preferred embodiment shown hereinabove and illustrated in FIG.
It will be seen that an insulated hermetic enclosure as shown in the illustrated embodiment is used. However, instead of using external heating means to achieve and maintain the desired superheat conditions within an insulated closed enclosure, heat is recovered from the entire gas separation system itself, and this recovered heat is preferably It is used to achieve superheat conditions within the membrane system by directly heating the feed gas within an insulated closed enclosure. Thus, line 2
The feed gas from 1 is sent to an oil flooded screw compressor 22 from which the compressed feed gas is sent in line 23 to a conventional oil separator 24. The thus treated feed gas stream is then sent in line 25 to a postcooler 26 and in line 27 to a conventional liquid (eg water) separator 28. Condensed water or other liquid flows from the separator into line 2
9. The feed gas stream then passes through line 30 and enters the insulated sealed enclosure of the present invention. The thermal insulation material with which the enclosure is insulated is designated generally by the reference numeral 32.

Upon entering the insulated hermetic enclosure 31 (which it will be appreciated is adapted to control and/or minimize their heat loss), the lines 3
0 feed gas is sent to a combining overzone 33 where any residual oil droplets present in the feed gas are separated therefrom and withdrawn via line 34. From zone 33, the feed gas is passed in line 35 to a heat exchange zone 36, such as a shell-and-tube heat exchanger, where the feed gas is heated by hot oil from feed gas compressor 22. The gas exit temperature is conveniently regulated by controlling the amount of oil passed through heat exchange zone 36. Therefore, the desired degree of heating can be easily adjusted to accommodate various dew point conditions or other factors associated with a given gas separation application. As mentioned above, the feed gas is in any case superheated to a temperature above the saturation temperature of the feed gas at the desired operating pressure.

After heating in zone 36, the feed gas passes through line 37
to the membrane modules of the permeable membrane system 38 within the insulated closed enclosure 31, the flow being in a continuous or parallel flow pattern with respect to the individual modules contained within the system. As can be seen from the above, modules such as hollow fiber bundles are not individually insulated. The less easily permeable components of the feed gas mixture are withdrawn from the membrane system 38 as impermeable gas via line 39 on the feed side of the system essentially at the feed gas pressure level. The more easily permeable components of the feed gas are withdrawn separately as permeate gas via line 40 on the permeate side of the system at a lower pressure.

It will be appreciated that the oil separated from the compressed feed gas in the oil separator 24 can be conditioned and sent to the flooded screw compressor 22 via line 41. In this preferred embodiment, the heat of compression associated with the use of compressor 22 is recovered and used in place of, or in addition to, an external heat source as shown with respect to the embodiment of FIG. For this reason, although no external heat source is shown in FIG. 2, it will be appreciated that such an external heat source may also be used with the beneficial use of the recovered heat of compression within the system.

To extract the heat of compression from the compressor 22, heated oil is passed from the compressor in line 42 to an oil cooler 43.
from where the cooled oil is returned to the compressor via line 44. For heat utilization purposes of the present invention, a portion of the oil in line 42 is transferred to heat exchanger 36.
The oil can then be diverted via line 45 to bypass the oil cooler 43 for delivery into and out of the insulated enclosure 31. Cooled oil exiting the heat exchanger is routed via line 46 to join cooled oil in line 44 for recirculation to compressor 22. The desired control of operating temperature in this embodiment is facilitated by the use of a suitable control valve located in bypass line 48 to control the amount of heated oil desired to be delivered to heat exchanger 36. can be achieved. In this case, the remaining oil is sent via line 48 to join the cooled oil that is being recycled via line 46 from the heat exchanger. control valve 4
7 is instrument 4 in line 37 leading to membrane system 38
It will be appreciated that operation can be performed in response to any suitable temperature measuring means such as 9. It should be noted that reference numeral 50 represents the usual communication means between temperature meter 49 and control valve 47.

It will be appreciated by those skilled in the art that numerous changes and modifications may be made in the details of the invention without departing from the scope of the invention as set forth in the claims. In the air separation applications described above, it is common to use permeable membrane materials that are capable of permeating oxygen as a highly permeable component in the feed gas stream. Thus, nitrogen is the less easily permeable component in the feed air stream, and the nitrogen-rich product stream can be recovered as a retentate stream if desired, in which case the permeable gas is combined with the feed air stream. It consists of a residual oxygen-nitrogen stream that is comparatively enriched in oxygen. In other applications of the invention, it is possible to use permeable membrane materials with opposite permeation properties, such that, for example, in air separation applications, the permeable membrane can be used as the more easily permeable component in the feed air stream. Permeates nitrogen rather than oxygen. Those skilled in the art will appreciate that the improved membrane separation systems and methods of the present invention can be used in any desired gas separation operation and/or in which condensation of feed gas components is a problem that should desirably be overcome. Generally applied to operations where it is necessary or desirable to achieve a constant stable temperature environment in a superheated environment with desired temperature control capabilities in excess of that which can be obtained using a thermally insulated membrane module. It will be understood that it is possible. Examples of such gas separation applications are hydrogen purification from off-gases that also contain methane, ethane, and other hydrocarbons, and hydrogen recovery from ammonia purge gas and separation of carbon dioxide and methane.

As previously noted, the permeable membrane comprising the membrane system disposed within the insulated closed enclosure of the present invention may be of any desired form, although hollow fiber membranes are generally preferred.
The membrane material used in a particular gas separation application may be any suitable material that is capable of selectively permeating more easily permeable components in a gas or liquid mixture containing less easily permeable components. Good things will be understood. Cellulose derivatives such as cellulose acetate, cellulose acetate butyrate, etc., polyamides and polyimides including aryl polyamides and aryl polyimides, polysulfones, polystyrene, etc. are representative examples of such materials. Those skilled in the art will appreciate that numerous other permeable membrane materials are known in the art and are suitable for use in a variety of separation applications.
As previously described, membranes such as those used in the practice of the present invention may be in composite membrane configurations, asymmetric configurations, or for the particular gas separations sought to be performed using the systems and methods of the present invention. It may take any such form that is useful and effective.

By efficiently and conveniently overcoming condensation problems encountered in practical industrial operations, the present invention provides a highly desirable advance in membrane technology related to gas separation operations. The present invention also provides a highly desirable means to achieve a constant and stable temperature environment which further enhances the efficiency of membrane systems and methods for gas separation, thus making it possible for permeable membranes to The need for practical and convenient means of achieving separation can be more effectively met.

[Brief explanation of the drawing]

FIG. 1 is a schematic flow diagram of one embodiment of the temperature control system of the present invention, and FIG. 2 is a schematic flow diagram of a preferred embodiment of the present invention that provides beneficial heat recovery and temperature control.

Claims (1)

Claims: 1. (a) containing at least one membrane module capable of selectively permeating more easily permeable components of the feed gas stream over less easily permeable components; , a means for passing a feed gas stream to the feed side of the membrane module at a desired feed gas pressure and components that cannot permeate as easily as essentially non-permeable gases and more readily permeate at the level of the feed gas pressure; (b) means for extracting the obtained components as a permeate gas at lower pressure; the membrane modules are individually uninsulated to retain heat; (c) said membrane apparatus and said heat supply; (c) said membrane apparatus and said heat supply; surrounding the means to suppress and/or minimize heat losses such that any heat losses are no greater than the heat supplied by the heat supply means and superheat conditions are maintained with respect to the feed gas flow through the membrane device. including an insulated vessel to effectively prevent condensation of components of the feed gas stream within the membrane device and/or to maintain stable and uniform regulation of the temperature within the membrane device and to resist temperature changes within the device. An improved gas separation device having advantageous adaptability and achieving more efficient optimal operation of the membrane device. 2. The apparatus of claim 1, wherein the heat supply means includes means for supplying heat external to the gas separation operation to the insulating container. 3. Apparatus according to claim 2, including means for compressing said feed gas stream to a desired pressure level for passage through said heat supply means and a membrane device located within said insulated vessel. 4. means for cooling said compressed feed gas and means for removing droplets therefrom, so that said feed gas is compressed at a desired pressure level when passed through said heat supply means and a membrane device located within said insulated container. 4. The device according to claim 3, wherein the device is saturated. 5. Apparatus according to claim 1, including compressor means for compressing said feed gas stream to a desired pressure level for passage through said heat supply means and a membrane device located within said insulated vessel. 6. The apparatus of claim 5, wherein said heat supply means includes heat exchange means for supplying heat to said feed gas stream within said insulated container. 7. Claim 6, wherein said heat exchange means are adapted to supply heat directly to said feed gas stream.
Apparatus described in section. 8. Apparatus according to claim 7, including means for recovering the heat of compression generated in compressing the feed gas stream and passing it to heat exchange means either within or outside the insulated container. . 9. A claim in which said compressor means comprises oil-flat compressor means and conduit means for passing oil heated by said compressor means to said heat exchange means either within or outside said insulated container. The device according to item 8. 10 (a) passing the feed gas stream through an insulated vessel adapted to suppress and/or minimize heat loss; and (b) superheating the feed gas stream within the insulated vessel to reduce the feed gas pressure. the insulating container suppresses and/or minimizes heat losses such that any loss of heat is no greater than the heat supplied into the insulating container;
(c) passing the thus superheated feed gas stream through a dialysis membrane system located within the insulated vessel, the dialysis membrane system containing at least one membrane module capable of selectively permeating more easily permeable components of the feed gas stream over less easily permeable components, the membrane module for retaining heat therein; (d) components that cannot be permeated as easily are withdrawn from the membrane system as non-permeable gas and from the insulated vessel essentially at the feed gas pressure; separately withdrawing readily permeable components as a permeate gas from the membrane system and from the insulated vessel at a lower pressure, thereby effectively preventing condensation of components of the feed gas stream within the membrane system and/or An improved gas separation method that maintains a stable and uniform regulation of the temperature within the system, is advantageously adaptable to temperature changes within the system, and achieves more efficient optimal operation of the gas separation process. 11 The heat that superheats the feed gas stream in an insulated container is
11. A method as claimed in claim 10, characterized in that the supply is provided by heat exchange means located outside or inside the insulated container. 12. Compressing the feed gas using oil-flat compressor means and passing heated oil from the compressor through said heat exchange means to provide the heat necessary to superheat the feed gas. The method according to paragraph 11.