JPH0558766B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates to permeable membrane systems and, more particularly, to the effective use of such systems under variable demand conditions.

BACKGROUND OF THE INVENTION 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 encouraged the industry to expand the use of membranes in practical industrial operations. Of course, it is necessary that the properties of the membrane system with respect to selectivity and permeability be compatible with the overall manufacturing requirements of a given application. Continuous improvements in the efficiency of membrane systems are also needed to further improve the performance of using membrane systems under the operating conditions encountered in the industry.

The key factors in the design and overall efficiency of a membrane system are the total membrane surface area required for a given separation and the amount and quality of product desired, i.e. permeability and selectivity or separation factor. and the required partial pressure difference across the membrane. 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 differential partial pressure 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 higher pump operating costs, it allows membrane equipment costs to be kept 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.

BACKGROUND OF THE INVENTION Membrane systems are typically designed and optimized for full capacity steady flow conditions or design conditions, although in practice they are not always utilized to 100% of their capacity. 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 can vary significantly over a 24 hour period with respect to the required nitrogen flow rate and/or purity.

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. the result,
Product quality increases above the design level while power requirements remain at full design level. This system 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 the third method for the demand reduction condition,
The membrane system is operated at operating design conditions;
Surge tanks are then 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 method also
This has the disadvantage of underutilizing the installed membrane area because it is not fully or not fully utilized during periods of reduced demand and idling or downtime of the equipment. Moreover, such frequent start/stop operations also present the significant disadvantage of increased wear on the associated equipment parts.

Problems to be Solved by the Invention Therefore, in this field, load adjustment (turndown)
There is a strong desire to establish a method that fully and efficiently utilizes the separation capacity of the installed membrane area under certain 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.

OBJECTS OF THE INVENTION It is an object of the 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.

SUMMARY OF THE INVENTION The inventors have developed improved turndown control by reducing feed flow rates and partial pressure driving force at reduced demand conditions and by fully utilizing the installed membrane surface area under all operating conditions. succeeded in realizing it. In this way, the desired quantity and quality of product can be achieved at start-up/start-up and under fluctuating demand conditions and reduced equipment loads under turndown conditions.
Membrane equipment reliability is improved and efficiently achieved by avoiding increased equipment component wear associated with standstill operations.

Thus, the present invention provides a fluid feed stream having readily permeable and less easily permeable components under design operating conditions that can selectively permeate 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 low 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 the partial pressure driving force across the membrane system 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; (b) During periods of reduced demand on the quantity and/or purity of the product stream, either permeate or non-permeate, the feed flow rate and flow rate across the membrane can be reduced while maintaining full utilization of the installed membrane surface area of the membrane system. (c) reducing the driving force so that the product stream is recovered from the membrane system in a desired controlled quantity and/or quality during the period of demand reduction; the design conditions of step (a) for a design demand condition in which the stream is obtained at a design amount and purity level and a step (a) for a lower demand condition in which the product stream is recovered at a lower amount and/or purity level than the b) adjusting the feed flow rate and driving force across the membrane between the reduced feed flow rate and driving force conditions during the process, such that the efficiency of the membrane separation process is adjusted to meet varying demands. A regulatory control method for a membrane permeation process is provided, characterized in that it is enhanced by full utilization of the membrane surface area available under conditions.

DESCRIPTION OF THE EMBODIMENTS The objects of the present invention are achieved by using a turndown control method to reduce the feed flow rate and partial pressure driving force across the membrane upon demand reduction. 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 load during turndown conditions may 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 computers 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. 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 in line 3 to permeable membrane unit 4. 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) cut off 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 indicated 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 10, which can be changed 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 . The capacity control device 12 is numbered 13
The compressor 2 is reduced (turndown) by adjusting the appropriate recycling valve, suction valve, variable speed motor, etc. to the appropriate pressure and flow conditions for the specified demand conditions by mechanical or electrical connection means represented by used to. The method of the present invention 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 can be 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 sluggishness begins to rise and the process computer notices this rise and reduces the compressor load. However, the use of product purity as the primary metric is limited as a result of the inherent time delays involved in product collection and purity analysis.
It should be noted that in some cases this may introduce some instability. 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 is capable of separating 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 designed based on design conditions, i.e., generally the desired product flow rate and purity, the pressure level and the desired partial pressure driving force required, the desired total capacity optimized taking into account utility costs, e.g. electricity costs. Constructed and used on an operational basis.

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 the amount of membrane surface area required to produce 1 ncfh of product nitrogen gas (ft ³ /hour under standard conditions) in the illustrated embodiment. The compressor factor is the amount of feed gas required to produce 1 ncfh of product. 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 that can be 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 an increase in product flow rate is associated with an increase in 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. Similarly, for constant purity operations, the area factor generally decreases as feed pressure increases. In addition,
For constant product purity, constant membrane area operation, the feed flow rate, or compressor flow rate, generally increases with higher feed pressures and vice versa. These relationships are taken into account in determining the design conditions, i.e., 100% demand conditions, and the above balance is preferably made to achieve a design condition that is optimized for the manufacturing requirements of a particular membrane system and a given application. Optimize.

In the exemplary air separation operation, the membrane equipment:
It is operated at a feed gas pressure of 200 psig and is optimized at 100% demand conditions to produce 10,000 ncfh of 98% nitrogen as the non-permeate gas. If the product demand is reduced to 7000 ncfh, in the practice of the present invention, full utilization of the available membrane area can be maintained by reducing the feed gas pressure to 145 psig, yet the product purity is substantially less than designed. Maintained purity levels. In addition to reducing the feed gas pressure, the feed gas flow rate is also reduced to produce 7000 ncfh of nitrogen product. At a reduced feed gas pressure of 145 psig, only a reduced amount of gas permeates through the membrane (increasing residual product) and therefore a reduction in feed gas is required to achieve the desired product reduction. be done. Desired product is 10000ncfh
to 7000ncfh and supply gas pressure
In the above product reduction example where the product reduction was reduced from 200 psig to 145 psig, the compressor flow rate or feed flow rate to the membrane system was found to be 72% of the 100% design flow rate. Thus, the conditions for maintaining full utilization of available membrane area during reduction while producing product at the design purity level are, for example, the feed gas pressure in combination with a specified reduction from the design feed flow rate. consists of a specified reduction in the driving force across the membrane due to a reduction in .

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. Variable speed motors, internal recycle valves, and suction valve offload devices are examples of devices that may be used to throttle compression equipment in accordance with the present invention.

reciprocating compressor with suction valve load reduction device;
The flow rate and pressure across the membrane of the feed stream when the compression equipment, typically an oil-immersed screw compressor or other commercially available such equipment, receives a reduction signal for recycling back to the point of suction. The ratio is reduced, which results in lower power usage. In an automated embodiment of the invention, the process computer described above is configured to operate the compression equipment and throttling mechanism, load reduction device, variable speed motor, etc. at the optimal flow rate and across the membrane for a given demand signal, i.e., flow rate or purity signal. can be programmed to automatically set the pressure ratio to allow the pressure ratio to be used.

In the above illustrative embodiment where a screw-type compression equipment is used with an internal recycle valve, the nitrogen
The power reduction effect for reducing from 10000ncfh to 7000ncfh 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. Total power usage is reduced by both factors: supply gas volume and pressure reduction;
This produces a reduction in driving force across the membrane such that the total power usage is 85% of 77% of the design value, or 65% of the power usage under design conditions.

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. At a constant 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 such embodiments, the total available membrane surface area is still fully utilized during this conditioning. Those skilled in the art will appreciate that the process computer described above can also be easily programmed to automatically adjust compression equipment operations to provide simultaneous product purity and product flow regulation control.
In this case, the pressure differential that produces the driving force across the membrane is varied in response to changes in both product purity and flow rate and the overall operating conditions and conditions associated with a given gas or other fluid separation operation. It will be optimized taking into account the specific requirements.

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 effectively control the feed flow rate to the membrane. It should also be noted that this control method is dependent on the product pressure requirements of a given application. In the air separation for nitrogen recovery described above, if a minimum product pressure of 100 psig is required, then the reduction limit would be 45% of the maximum product flow rate of the compression equipment. However, 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 the most efficient control method. Those skilled in the art will desire that the method of the invention be optimized to the extent commensurate with the importance of the reduction operation in the overall process being controlled. It is an important parameter in determining the efficiency and scope of reduction in compressor equipment and design operating pressure in practical industrial operations.

The present invention does not simply relate to 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 adjustment of these conditions at or between different lower demand conditions where product recovery of less than the designed amount and/or purity is desired may also be involved. Therefore, the adjustment of the present invention encompasses all of these.

Furthermore, while the design conditions represent the total capacity of a given membrane system in terms of the desired product flow rate and purity, it may also be necessary to It should be specified that it is generally possible to operate a membrane system beyond its design capacity to produce product of greater than design purity at some sacrifice in recovery.
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 can be effectively used to separate readily permeable and non-readily 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 minimal 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 is 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. It will be. The product purity and/or recovery achieved will, of course,
There will be some variation 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; polysulfones; polystyrene, etc. It is also within the scope of this invention to use any desired form of permeable membrane. Thus, the permeable membrane may be of composite form, comprising a separating layer that determines the selectivity and permeability properties of the membrane, which is disposed 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 porous areas provide support. Other forms of membranes, such as high density membranes, 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. 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 loaded according to the present invention and the installed membrane surface area is fully utilized under all operating conditions.

ADVANTAGES OF THE INVENTION 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 can be conveniently used to achieve the desired quantity and quality of product under reduced conditions and the reliability of the membrane system can be increased by steady operation of the equipment under reduced load conditions.

[Brief explanation of the drawing]

The drawing is a schematic illustration of a preferred embodiment membrane system suitable for carrying out the invention. 1: inlet line, 2: compressor, 4: permeable membrane unit, 5: product flow line, 6: flow meter, 7: transmitter, 9: process computer, 12: capacity controller.

Claims (1)

[Scope of Claims] 1. A fluid feed stream having readily permeable components and less easily permeable components under design operating conditions is provided with a predetermined flow rate capable of selectively permeating easily permeable components at a desired feed flow rate and pressure. in contact with the feed side of a permeable membrane system having a membrane surface area of In a membrane permeation process in which the permeate is withdrawn at a low 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 transverse partial pressure driving force such that the resulting permeate and non-permeate are at the designed amount and purity level, and the installed membrane surface area of the membrane system is fully utilized; (b) During periods of reduced demand on the quantity and/or purity of the product stream, either permeate or non-permeate, the feed flow rate and flow rate across the membrane can be reduced while maintaining full utilization of the installed membrane surface area of the membrane system. (c) producing either permeate or non-permeate; the design conditions of step (a) for a design demand condition in which the physical stream is recovered at a design amount and purity level; and a step (a) for a lower demand condition in which the product stream is recovered at a lower amount and/or purity level than the design level; (b) adjusting the feed flow rate and driving force across the membrane between the reduced feed flow rate and driving force conditions in the process, thereby varying the efficiency of the membrane separation process. A regulatory control method for a membrane permeation process, characterized in that it is enhanced by full utilization of the membrane surface area available under demand conditions. 2 The driving force across the membrane is reduced by reducing the supply flow pressure during periods of reduced demand.
(b) The method according to claim 1, which is reduced to medium. 3. 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. 4. The method of claim 1, wherein the demand reduction period comprises a demand period during which a product of less than the design purity is required to be supplied at the design quantity level. 5. 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.