JPH0268112A - Load control method for membrane separation system - Google Patents

Abstract

PURPOSE: To efficiently use the total separation capacity of an installed membrane system even under the condition of a decreasing demand for a product by utilizing a turndown control method by which a supply flow rate and partial driving force across a membrane are reduced when the demand decreases. CONSTITUTION: a) The supply flow rate and the partial driving force across the membrane are maintained so that an obtd. penetrating material and non- penetrating material are kept at the design quantity and purity level and that the installed membrane surface area of the membrane system is completely used. Next, b) the product flow is recovered from the membrane system in a desired adjusted quantity and/or quality during the period of demand decreasing by reducing the supply flow rate and the driving force across the membrane while maintaining the complete use of the installed membrane surface area of the membrane system during the period of demand decreasing for the quantity of the product flow and/or purity of either of the penetrating material or the non-penetrating material. During these stages a), b), the quantity of the supply liq. and the driving force across the membrane are adjusted.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to permeable membrane systems and, more particularly, to methods for effectively using such systems under variable demand conditions.

Permeable membrane processes and systems are well known in the art and are currently being used or being considered for a wide variety of gas and liquid separations. In such operations, a feed stream is brought into contact with the surface of the membrane and the readily permeable components of the feed stream are recovered as a permeate stream. Components that do not readily permeate are removed from the membrane system as a non-permeate, or retentate stream.

The inherent simplicity of such fluid separation operations has led to
The industry is expanding the use of membranes in practical industrial operations. Of course, it is necessary that the selectivity and permeability properties of the membrane system be compatible with the overall manufacturing requirements of a given application, further enhancing the practical ability to use the membrane system under the operating conditions encountered in the industry. It is also necessary for the efficiency of membrane systems to be continually improved.

Important factors in membrane system design and overall efficiency are:
The total membrane surface area required for a given separation and the partial pressure difference across the membrane required to obtain the desired product quantity and quality, i.e. permeability and selectivity or separation factor. be. The design of practical membrane systems requires optimization of the balance between membrane surface area and partial pressure difference across the membrane. That is, the greater the driving force, or partial pressure difference, across the membrane, the less membrane surface area is required for the desired separation. Although this requires the use of more expensive pumping equipment and - more expensive pump operating costs, it makes it possible to keep the membrane equipment costs relatively low. On the other hand, the lower the driving force used, the larger the membrane area required, and the relative costs of various aspects of the system and overall operation will change accordingly.

Although conventional membrane systems are typically designed and optimized for full volume steady flow conditions or design conditions, they are not always utilized to 100% of their capacity in practice. Under operating conditions other than design conditions, other combinations of optimal operating conditions may be advantageous in terms of membrane area versus partial pressure difference. Fluid separation applications that desire membrane systems typically are not operated under steady flow conditions. Demand from membrane systems often varies in terms of product quantity and/or quality. For example, the demand for nitrogen gas as a product from an air separation membrane system depends on the required nitrogen flow rate and/or
Alternatively, purity may vary significantly over a 24 hour period.

During load adjustment conditions that deviate (lower) from design conditions, so-called turndown conditions, the membrane system is typically operated in one of three modes.

In one approach, no corresponding turndown is effected to address the decrease in product demand. In this case, the feed flow rate and the partial pressure difference remain constant. As a result, product quality increases above the design level, while power requirements remain at the design level. This approach is therefore disadvantageous in that no power savings are realized and the product is obtained at a higher quality level than required.

In yet another approach, the membrane area used for separation is varied. Under conditions of reduced demand, a portion of the membrane area is taken out of service. This reduces the amount of supply flow processed to meet the reduced demand. A disadvantage of this approach is the inefficiency associated with not using some of the available membrane surface area. This approach certainly does not optimize the balance between membrane surface area and partial pressure difference during off-design load reduction conditions.

In a third approach to reduced demand conditions, the membrane system is operated at operating design conditions and a surge tank is used to handle fluctuating demand requirements. When the surge tank is full, the membrane system is either unloaded (ie, the feed pump is idle) or shut down to save energy. This approach also has the disadvantage of underutilizing the membrane area, since the installed membrane area is not fully or fully utilized during periods of reduced demand and equipment idle or downtime.

Moreover, such frequent start/stop operations also present the significant disadvantage of increased wear on the associated equipment parts.

Therefore, there is a strong need in the art for a method to fully and efficiently utilize the separation capacity of the installed membrane area under load control (turndown) conditions. If such a method were developed, it would be possible for the installed membrane system to operate at optimum efficiency throughout and would provide much more reliable operation than the start/stop operation schemes described above.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved method for operating a permeable membrane system under turndown conditions.

Another object of the present invention is to provide a method that allows the full separation capacity of an installed membrane system to be used efficiently even under conditions of reduced product demand.

The inventors achieved improved turndown control by reducing the supply flow rate and partial pressure driving force under reduced demand conditions and by fully utilizing the installed membrane surface area under all operating conditions. succeeded in doing so. In this way, the desired quantity and quality of product can be obtained under reduced equipment loads under turndown conditions and under fluctuating demand conditions.
Membrane equipment reliability is improved and efficiently achieved by avoiding increased equipment component wear associated with start/stop type operations.

Thus, the present invention provides a method for selectively permeating easily permeable components at a desired feed flow rate and pressure for a fluid feed stream having readily permeable and less easily permeable components under design operating conditions. A desired amount of the less easily permeable component is removed as non-permeate at substantially the design feed flow pressure level while at the same time a desired amount of the easily permeable component is passed into contact with the feed side of a permeable membrane system having a predetermined installed membrane surface area capable of being removed. In a membrane permeation process in which the components are withdrawn as permeate at a lower pressure level and the partial pressure of the feed stream components across the membrane provides the driving force for the membrane separation, (a) the feed stream flow rate and the membrane maintaining a partial pressure driving force across the membrane such that the resulting permeate and non-permeate are at the designed amount and purity level, yet the installed membrane surface area of the membrane system is fully utilized; (b) ) during periods of reduced demands on the quantity and/or purity of product streams, either permeate or non-permeate, to reduce the feed flow rate and driving force across the membrane while maintaining full utilization of the installed membrane surface area of the membrane system. reducing and thus recovering the product stream from the membrane system in a desired controlled quantity and/or quality during the period of demand reduction;
(c) the product stream, either permeate or non-permeate;
The design conditions in step (a) for a design demand condition obtained at a design amount and purity level and in step (b) for a lower demand condition where the product stream is recovered at a lower amount and/or purity level than the design level. adjusting the feed flow rate and driving force across the membrane between the reduced feed flow rate and driving force conditions, thereby increasing the efficiency of the membrane separation process by reducing the available membrane capacity under fluctuating demand conditions. A method for regulating and controlling a membrane permeation method is provided, characterized in that it is improved by full utilization of surface area.

The objects of the present invention are achieved by using a turndown control method that reduces the supply flow rate and the partial pressure driving force across the membrane in the event of a decrease in demand.
The partial pressure driving force is a function of the partial pressures of the components of the feed and permeate streams. Such energy-reduced operations are performed not only under design conditions, but also under reduced demand operating conditions while fully utilizing the installed membrane surface area. The resulting steady-state operation of equipment under reduced loads during turndown conditions can provide more efficient operation and avoid increased wear on equipment parts associated with start-up/shutdown operations. .

The method is initiated by a demand signal indicating that an off-design condition is required. This signal can be, for example, a product flow rate measurement indicating a below-design flow rate or a purity input that also indicates that a below-design condition is required. Membrane systems have traditionally been designed to incorporate process combinators programmed to reduce the operating pressure ratio across the membrane to a new optimum based on full utilization of the membrane surface area available in the system. There is. The desired pressure ratio reduction is accomplished by sending turndown signals to pump system equipment, such as compressors, vacuum pumps, liquid pumps, etc.

Referring now to the drawings illustrating a preferred system example for implementing the control method of the present invention, an inlet line 1 is used to deliver feed gas to a compressor 2, from which a pressurized supply Gas is sent to the permeable membrane unit 4 in the conduit 3. The desired product or product stream (either permeate gas or non-permeate gas) is discharged through product flow line 5. The other stream (not shown) removed from the membrane unit is either discarded or used for some desired purpose. A standard flow meter 6 is located within line 5 and has a conventional flow meter associated therewith and adapted to send an input signal designated by numeral 8 to a process computer/controller 9. It is equipped with a generation flow signal transmitter 7. That is, the flow meter 6 is used to sense the product flow rate, while the signal transmitter 7 is used to send a process variation signal, ie a signal proportional to the product flow rate, to the process computer. A purity set point, represented by number 1o, which is varied depending on the product purity requirements of the operation, is also used as an input to the process computer.

Process computer 9 is programmed to send an output signal represented by number 11 to capacity controller 12 . Capacity control device 12, by mechanical or electrical connection means represented by numeral 13, controls compression by adjusting appropriate recycle valves, suction valves, variable speed motors, etc. to appropriate pressure and flow conditions for specified demand conditions. Used to turn down aircraft 2. The method of the present invention includes
It is highly flexible in that as design conditions change as a result of different process applications, improvements in membrane performance, etc., the process computer can be easily and quickly reprogrammed to meet the new design conditions.

In another example of processing, product purity level is used as a process variable to initiate membrane system regulation control. In this case, product purity rather than product flow rate is sensed and a signal proportional to product purity is sent to process computer/controller 9. in this case,
The computer is programmed to adjust compressor capacity and pressure to maintain the desired purity.

Those skilled in the art will appreciate that the desired product purity can often be a variable input from downstream operations to the product gas or liquid being produced in a controlled membrane system. As the product flow rate is reduced, the product purity begins to increase and the process computer takes note of this increase and reduces the compressor load. However, it should be noted that the use of product purity as the primary metric may in some cases introduce an element of instability as a result of the inherent time delays involved in product collection and purity analysis. Should. In contrast, it is easily possible to achieve instantaneous detection of product flow rate changes.

The invention will be described in more detail with reference to a specific example in which a permeable membrane is used for air separation to produce nitrogen product gas.

It will be appreciated that using typical membrane systems in which the membrane material separates oxygen as a readily permeable component of air, the desired nitrogen-enriched product stream is recovered as a non-permeate or residual gas. The oxygen-enriched permeate gas then constitutes the waste stream. The feed air is compressed to a pressure above atmospheric and the product nitrogen non-permeate stream is withdrawn from the membrane system at a pressure slightly below the feed pressure. Permeate gas is typically withdrawn at atmospheric pressure. Membrane systems are subject to design conditions,
That is, it is generally configured and used on the basis of the desired product flow rate and purity, the pressure level and the desired partial pressure driving force required, the desired full capacity operation being optimized taking into account utility costs such as, for example, electricity costs. be done.

For a given pressure ratio across a membrane, gas or other fluid separation operations are characterized by two key factors: the area factor and the compressor factor. The area factor is
In the illustrated embodiment, it is the amount of membrane surface area required to produce 1 ncfh of product nitrogen gas (ft"7 hours under standard conditions). The compressor factor is
is the amount of feed gas required to produce h. Once the product requirements for a given application are established, the optimal or desired pressure ratio across the membrane and corresponding area and compressor factors can be determined.

Optimization of these process variables optimizes the balance between the higher compressor costs and associated power requirements at higher pressure ratios and the increased product flow produced at such higher pressure ratios. It will be understood that this is related to In air separation applications, the permeate pressure is generally and conveniently fixed at atmospheric pressure, so increasing product flow is associated with increasing feed pressure.

As mentioned above, for constant product purity, constant membrane area operation, product flow rate generally increases as feed pressure increases, and vice versa. The same (for constant purity operation, the area factor generally decreases as the feed pressure increases; in addition, for constant product purity, constant membrane area operation, the feed flow rate or compressor flow rate generally decreases as the feed pressure increases. The higher the supply pressure, the greater the increase, and vice versa. These relationships are taken into account in determining the design conditions, or 100% demand conditions, to be optimized for the specific membrane system and manufacturing requirements of a given application. The above balance is preferably optimized to achieve the desired design conditions.

In an exemplary air separation operation, the membrane installed steel is 200 psLg
10 to produce 10,000 ncfh of 98% nitrogen as non-permeate gas.
Be optimized under 0% demand conditions.

If the product demand decreases to 7,000 ncfh, in the practice of this invention the feed gas pressure is reduced to 14
By reducing to 5 psig, full utilization of the available membrane area can be maintained while product purity is maintained substantially at the design purity level. In addition to the reduction in supply gas pressure,
The feed gas flow rate is also reduced to produce 7.000 ncfh of nitrogen product.

At a reduced feed gas pressure of 145 psig,
Only a reduced amount of gas passes through the membrane (increasing residual product) and therefore a reduction in feed gas is required to achieve the desired product reduction. The desired product is 10,00
In the product reduction example above where the feed gas pressure is reduced from 0 ncfh to 7.000 ncfh and the feed gas pressure is reduced from 200 psig to 145 psig, the compressed flow rate, i.e. the feed flow rate to the membrane system, is 72% of the 100% design flow rate. was discovered. (Thus, the conditions for maintaining full utilization of available membrane area during reduction while producing product at the design purity level are: Consists of a specified reduction in driving force across the membrane due to a reduction in pressure.

The amount of energy savings achieved in product reduction will depend on the type of compression equipment used to carry out the invention. A variety of capacity control devices are available in the art depending on the type of compression equipment used. A variable speed motor, an internal recycle valve, and a suction valve offloader are one example of a crane device used to throttle a compression equipment in accordance with the present invention.

Compression equipment, such as a reciprocating compressor with suction valve offloading device, an oil-immersed screw compressor for recycling back to the point of suction, or other commercially available such equipment, receives and receives the abatement signal. In an automated embodiment of the present invention, the process computer described above is configured to reduce the flow rate of the feed stream and the pressure ratio across the membrane, which results in reduced power usage. Load relief devices, variable speed motors, etc. can be programmed to automatically set to allow the optimum flow rate and pressure ratio across the membrane to be used for a given demand signal, i.e., flow rate or purity signal. It can be done.

In the above exemplary embodiment using a screw type compression equipment with an internal recycle valve, 10.000 n of nitrogen
cfh to 7. The power reduction effect for the reduction to OOOncfh is 77% of the design value. At a reduced supply gas pressure of 145 psig, the power reduction effect is 85% of the design value. The total power usage is reduced by both factors, i.e., the reduction in supply gas volume and pressure, such that the total power usage is 85% of 77% of the design value, or 65% of the power usage under design conditions. This creates a reduction in driving force across the road.

It should be noted that the control method of the present invention can be effectively used in embodiments where product purity control rather than product flow control is desired. −In the main product flow rate,
If a reduction in product purity is required, the feed pressure is reduced to reduce the partial pressure across the membrane while maintaining the product flow rate at the design condition level. In these specific examples,
The total available membrane surface area is again fully utilized during this conditioning. Those skilled in the art will appreciate that the process combinator described above can also be easily programmed to automatically adjust compression equipment operations to simultaneously provide product purity and product flow regulation control.

In this case, the pressure differential that creates the driving force across the membrane is varied in response to changes in both product purity and flow rate and the operating conditions and overall requirements associated with a given gas or other fluid separation operation. It will be optimized taking into account.

It will be appreciated that the efficiency of turndown control depends on the compression equipment design used in a given membrane separation operation as well as its ability to efficiently control the feed flow rate to the membrane. It should also be noted that it depends on the product pressure requirements of the application. In the air separation for nitrogen recovery described above, if a minimum product pressure of 1 OOpslg is required, then the limit of reduction would be 45% of the maximum product flow rate of the compression equipment. but,
In most cases, this factor should not affect the overall reduction efficiency since the optimal design membrane operating pressure is typically significantly higher than the required product pressure. Therefore,
For most applications, implementation of the present invention will be the easiest to implement and most efficient control method. Those skilled in the art will desire that the method of the present invention be optimized to the extent commensurate with the importance of the reduction operation in the overall process being controlled.

The selection of compressor equipment and design operating pressure are important parameters in determining the efficiency and scope of abatement in practical industrial operations.

The present invention is not simply concerned with turndowns to one particular flow rate and driving force condition set level that is lower than the design conditions, but rather between the design conditions and various lower demand conditions. It should be understood that adjusting these conditions or recovering less than the designed amount and/or purity of the product may also involve adjusting between different lower demand conditions, if desired. Therefore, the adjustment of the present invention encompasses all of these.

Furthermore, the design conditions represent the total capacity of a given membrane system in terms of the desired product flow rate and purity, but are designed to produce an amount of product in excess of the design amount at the expense of some product purity and product recovery. It should also be specified that it is generally possible to operate a membrane system beyond its design capacity to produce product above the design purity at some sacrifice of purity.

It is to be understood that operation beyond such design conditions constitutes substantially design conditions with respect to the present invention.

From this discussion, turndown conditions or reduction adjustments are applicable if the product flow rate or product purity requirements, or both, are substantially lower than the design product flow rate and purity for a given membrane system. be understood.

It is also within the scope of the invention to practice the methods described herein in a wide variety of fluid separation operations.

The air separation for nitrogen production mentioned here is one example where reducing conditions are encountered. Such operations include any in which a permeable membrane system is effectively used to separate readily permeable and less easily permeable components from a fluid mixture. A typical off-gas of this nature contains, for example, about 45 mole percent hydrogen, 25 mole percent methane, and lesser amounts of other hydrocarbons. Membrane systems can be used to purify the hydrogen to a desired purity of, for example, about 90%. Hydrogen recovery from ammonia purge gas and carbon dioxide and methane separation are examples of other industrial fluid separation operations to which the present invention may be applied under suitable conditions.

Although air separation applications have been described with convenient discharge of the permeate oxygen stream at atmospheric pressure, it should be noted that other preferred pressure conditions are also involved with other embodiments of the invention. In some cases, it may be desirable to take advantage of the feed pressure available under reduced conditions and achieve a reduction in the driving force across the membrane by increasing the permeate pressure without increasing the feed gas pressure. . The product purity and/or recovery achieved will, of course, vary somewhat depending on the overall requirements of a given membrane separation operation.

The invention can be practiced using any equalizing membrane system. The membrane material used can be any material that is selectively permeable to one or more permeable components from a gas or other fluid mixture, e.g. cellulose derivatives such as cellulose acetate, cellulose acetate butyrate, etc.; aryl polyamides; and polyamides and polyimides including aryl polyimides;
Polysulfone; polystyrene, etc. It is also within the scope of this invention to use any desired form of permeable membrane.

Thus, the permeable membrane can be of composite form, comprising a separating layer that determines the selectivity and permeability properties of the membrane, which is placed on a porous support. Asymmetric membranes may also be used in which the relatively dense surface area determines the selectivity and permeability properties of the membrane and the more porous areas provide support. Other forms of membranes, such as dense pus, are also useful for certain applications.

For purposes of the present invention, the permeable membrane may be a flat sheet, hollow fiber, spiral wound, or any other desired form. Hollow fiber membranes are preferred. Hollow fibers or other desired forms of membrane material are generally assembled into membrane modules consisting of hollow fiber bundles, folded flat sheet membrane assemblies or spirally wound cartridges, with feed inlet sides and permeate outlet sides, and conduits. Means are provided for separate withdrawal of unequal portions of the feed stream and for withdrawal of the permeate portion thereof.

All such membrane systems are effectively load balanced according to the present invention and the installed membrane surface area is fully utilized under all operating conditions.

The present invention provides a very practical and efficient off-design load regulation control means. Not only is the installed capacity of the membrane system fully utilized, but the use of start/stop type operations in response to demand fluctuations is avoided.

As a result, the membrane system is conveniently used to achieve the desired quantity and quality of product under reduced conditions and the reliability of the membrane system is increased by steady operation of the equipment under reduced load conditions.

FIG. 2 is a schematic representation of the lateral membrane system.

1: Inlet pipe 2: Compressor 4: Permeable membrane unit 5: Product flow line 6: Flow meter 7: Transmitter 9: Process computer 12: Capacity control device

[Brief explanation of the drawing]

The drawings are preferred illustrations of drawings suitable for carrying out the present invention (no changes in content).

Claims (1)

[Scope of Claims] 1) A fluid feed stream having readily permeable components and non-easily permeable components under design operating conditions is capable of selectively permeating the easily permeable components at a desired feed flow rate and pressure. is passed into contact with the feed side of a permeable membrane system having a predetermined installed membrane surface area, and a desired amount of the less easily permeable component is removed as non-permeate at substantially the design feed flow pressure level while simultaneously removing the desired amount of the easily permeable component. In a membrane permeation process in which the permeate is withdrawn at a lower pressure level and the partial pressure of the feed stream components across the membrane provides the driving force for the permeate membrane separation, (a) the feed stream flow rate and across the membrane (b) maintaining the partial pressure driving force of the permeate such that the resulting permeate and non-permeate are at the designed amount and purity level and that the installed membrane surface area of the membrane system is fully utilized; During periods of reduced demands on the quantity and/or purity of product streams, either permeate or non-permeate, the feed flow rate and driving force across the membrane can be reduced while maintaining full utilization of the installed membrane surface area of the membrane system. (c) recovering the product stream, either permeate or non-permeate, from the membrane system in a desired controlled quantity and/or quality during a period of demand reduction;
The design conditions in step (a) for a design demand condition obtained at a design amount and purity level and in step (b) for a lower demand condition where the product stream is recovered at a lower amount and/or purity level than the design level. adjusting the feed flow rate and driving force across the membrane between the reduced feed flow rate and driving force conditions, thereby increasing the efficiency of the membrane separation process by reducing the available membrane capacity under fluctuating demand conditions. A method of regulating and controlling a membrane permeation method, characterized in that it is improved by full utilization of surface area. 2) The method of claim 1, wherein the driving force across the membrane is reduced during step (b) by reducing the supply flow pressure during periods of reduced demand. 3) The method of claim 1, wherein the permeate stream is the desired product stream. 4) The method of claim 1, wherein the retentate stream is the desired product stream. 5) The method of claim 1, wherein the demand reduction period comprises a demand period during which less than the designed amount of product is required to be supplied at the designed purity level. 6) The method of claim 1, wherein the demand reduction period comprises a demand period during which the product of less than the design purity is required to be supplied at the design quantity level. 7) The method of claim 1, wherein the driving force across the membrane is reduced during step (b) by increasing the permeate flow pressure during the period of reduced demand. 8) The process of claim 3, wherein the feed stream comprises a hydrogen-containing stream and the product permeate stream comprises purified hydrogen. 9) The method of claim 4, wherein the feed stream comprises air and the product retentate stream comprises a nitrogen-enriched stream. 10) The method of claim 8, wherein the driving force across the membrane is reduced during step (b) by increasing the permeate membrane pressure during periods of reduced demand. 11) The method of claim 9, wherein the driving force across the membrane is reduced during step (b) by reducing the supply flow pressure during the period of reduced demand. 12) The method of claim 1, wherein the permeable membrane system comprises a membrane module housing the hollow fiber membrane.