KR930003239B1 - Turn-dowm control system for membrane seperating system - Google Patents

Abstract

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Description

Turndown Control Method for Membrane Separation System

The figure shows a schematic of the membrane system.

* Explanation of symbols for main parts of the drawings

2: compressor 4: permeable membrane device

6: product flow meter 7: transmitter

9 controller (process computer) 12 capacity controller

FIELD OF THE INVENTION The present invention relates to permeable membrane systems, and more particularly to the use of permeable membranes under various requirements.

Permeable membrane processes and systems are known in the art and have been used or discussed for a wide range of gas and liquid separations. In such an operation, the feed stream comes into contact with the membrane surface and more permeable components in the feed stream are recovered as permeate streams. Less permeable components are recovered in the membrane system as a non-permeable stream, or as a residual stream.

The inherent simplicity of such a fluid separation operation provides the motivation to extend the use of the membrane in a virtually economical manner in the art. Of course, the selectivity and permeability properties of the membrane system must be compatible with the overall production requirements of a given application. In addition, the efficiency of the membrane system must be constantly improved to increase the convenience of the membrane system used for convenience under operating conditions encountered in the art.

A critical factor in the design and overall efficiency of the membrane system is the total membrane surface area required for a given separation, and for example the desired permeability and selectivity or separation element required to achieve the quality and quantity of the desired product, such as the separation element. Partial pressure difference. The design of a practical membrane system requires optimization of the combination between the membrane surface area and the partial pressure difference described above. Thus, the greater the partial pressure difference or drive force through the membrane, the smaller the membrane surface area required for a given separation. This requires more economical pumping equipment and higher pumping costs, but the membrane equipment costs can be relatively low. On the other hand, with less driving force, more membrane surface area is required and the relative cost and handling of various aspects of the overall system will change accordingly.

Membrane systems are typically designed and optimized for sufficient capacity, fixed constant flow conditions, i.e. design conditions, which are not fully utilized in practice. Under design and other operating conditions, different combinations of optimum operating conditions will take precedence over the membrane area versus partial pressure difference. For fluid separation applications it is typically desirable for the membrane system not to operate under fixed flow conditions. Requirements in the membrane system will vary with the quantity and / or quality of the product. For example, product requirements for nitrogen gas in an air separation membrane system can vary considerably within 24 hours depending on the nitrogen flow rate and / or required purity.

Membrane systems typically operate in one of three forms while demand ceases, or under so-called turndown conditions. In one method, there is no turndown to control product requirement reduction. In this case, the feed flow and the partial pressure difference remain constant. As a result, the product quality will increase beyond the design level, while the required power remains at the full design level. This method is therefore disadvantageous in that power reduction cannot be realized and the product is obtained at a desired quality level or higher.

In another method, the membrane face used for separation is varied. Under reduced requirements, part of the membrane area is shut down. This reduces the amount of feed stream processed to meet the reduced requirements. The disadvantage of this method lies in the inefficiency associated with the lack of available membrane surface area. This factor prevents the combination between membrane surface area and partial pressure difference from optimizing for turndown conditions beyond design.

In a third method for reduced requirements, surge tanks are used to handle conditions that may change and operate at design conditions when the membrane system is in operation. When the surge tank is full, the membrane system is temporarily stopped to relieve the load, that is, the feed pump is idle, or to save energy. This method also has the disadvantage of not using the entire installed membrane area, because this membrane area is not available during all periods of plant operation or temporary shutdown. Such start / stop operation also has the disadvantage of increasing the wear of the associated device.

Therefore, there is a genuine need for the art to utilize the separation capacity of membrane areas installed under turndown conditions sufficiently and efficiently. Such a method will always enable the membrane system to be installed with optimal efficiency and will provide more reliable operation than the start / stop mode described above.

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

It is yet another object of the present invention to provide a method that allows efficient use of sufficient separation capacity of a membrane system installed under reduced product requirements.

With these and other objects in mind, the present invention is described in more detail below, and novel features of the present invention are set forth in detail in the appended claims.

Improved turndown control is achieved by reducing feed stream flow and partial pressure drive under reduced requirements while fully utilizing the membrane surface area installed under all operating conditions. The desired quality and amount of product can be efficiently obtained by increased membrane plant reliability by avoiding increased device load under turndown conditions and increased wear associated with start / stop type operation under varying requirements.

The invention will now be described in more detail with reference to the accompanying drawings, which are schematic diagrams of membrane systems suitable for the practice of generally preferred embodiments of the turndown control method of the invention.

The object of the invention is achieved by using a turndown control method in which the feed flow rate and the partial pressure drive across the membrane are reduced when the requirements are reduced. The partial pressure driving force is a function of the partial pressure of the feed and permeate stream components. Such low energy manipulations are carried out not only under design conditions, but also during full utilization of the membrane surface area installed under the overall reduced required operating conditions. As a result, with a constant and fixed operation of the device under reduced load in turndown conditions, more efficient operation can be achieved and increased wear of the start / stop operation can be avoided.

The turndown method of the present invention is initiated by a necessary signal indicating the required yield and purity reduction design conditions. This signal can be a product flow rate flow measurement that indicates below the design flow, or a purity input such as indicating that below the design conditions are required. Membrane systems are generally designed to work with a process computer programmed to lower the operating pressure ratio across the membrane for new optimization conditions based on the full utilization of the mebrain surface area available for the system. The desired pressure ratio reduction is achieved by sending a turndown signal to a pumping device, ie a compressor, a vacuum pump, a liquid pump or the like.

Referring to the drawings, a generally preferred system for carrying out the turndown control method of the present invention is described, where the inlet line 1 is used to deliver feed gas to the compressor 2 and the feed gas compressed in the compressor And through (3) to the permeable membrane device (4). The desired product stream, which is permeate or non-permeable gas, is discharged through the product flow line 5. Although not shown, other streams removed from the membrane device 4 are discarded or used for any desired purpose. The standard product flow meter 6 is located in line 5, which has a conventional product flow signal transmitter associated with it, which sends the input signal indicated by the number 8 to the process computer / controller 9 Applies to send. Thus, the product flow meter 6 is used to monitor the product flow, and the transmitter 7 is used to send a process signal 9 which may vary, i.e. a signal proportional to the product flow. Also used as input to the computer described above is the purity set point indicated by number 10, which may vary depending on the product purity requirements of the operation.

The process computer 9 is turned by a suitable mechanical or electrical connection, indicated by the number 13, by adjustment of a recirculation valve, intake valve, variable speed motor, etc., suitable for pressure and flow conditions suitable for the indicated requirements. To the capacity control device 12 used in the down compressor 2, it is programmed to transmit an output signal indicated by the number 11. As the design conditions change, the method of the present invention can be easily reprogrammed so that the process computer can be adapted to the new design conditions of different or modified process applications as a result of differences in process applications, improved membrane performance, and the like. It is very flexible in that it can.

In alternative processes, product purity levels can be used as process variables to initiate membrane system turndown control. In this case, product purity rather than product flow can be monitored and a signal proportional to product purity will be passed to the process computer / controller 9. In this case, the computer is programmed to adjust the compressor capacity and pressure to maintain the desired purity.

Those skilled in the art will appreciate that the desired product purity may be a variable pressure from downward manipulation that allows product gas or liquid to be produced in the controlled membrane system. As product flow decreases, product purity will begin to increase, and the process computer will detect this increase and reduce the compressor load. However, it should be noted that using product purity as a basic measure may in some cases lead to instability of certain elements due to the inherent delay time involved in the collection and analysis of product purity. In contrast, this makes it easy to achieve constant monitoring of product flow changes.

The invention is further described with reference to an exemplary embodiment, which is a permeable membrane used for air separation to produce a nitrogen generating gas. It can be seen that with the use of a typical membrane system capable of separating oxygen as an air component through which the membrane material can more easily permeate, the desired nitrogen enrichment product stream is recovered impermeable, ie as residual gas. In this example, the oxygen enriched permeate gas constitutes a waste stream. The feed air is compressed to a pressure above atmospheric pressure and the product nitrogen impermeable stream is withdrawn from the membrane system at a pressure slightly below the feed pressure. Permeate gas is typically recovered at atmospheric pressure. Membrane systems are constructed and used on the basis of availability such as design conditions, that is, the desired sufficient volumetric operation, pressure level and desired partial pressure drive force, power ratio generally optimized based on the desired product flow and purity.

For any given pressure ratio across the membrane, the gas or other fluid separation operation is defined by two key variables called area and compressor factors. The area factor is the amount of membrane surface area required to produce 1 ncfh (regular cubic feet per hour) of produced nitrogen gas in an exemplary embodiment. The compressor factor is the amount of feed gas required to produce 1 ncfh of the above product. Once the product requirements of a given application are established, the optimal or desired pressure ratio across the membrane, and the corresponding area and compressor factors can be determined.

Optimization of these process variables should be understood to include the optimization of the combination between the high pressure ratio and the required power associated with the high compressor cost at the larger product flows that can be produced at such a high pressure ratio. In air separation applications, larger product flows are associated with higher feed pressures as the permeation pressure is generally and conveniently fixed at atmospheric pressure.

As mentioned above, the product flow is generally increased at high feed pressures for constant product purity, constant membrane area manipulation, and vice versa. Likewise, for constant purity operation, the area self generally decreases as the supply pressure increases. In addition, for constant product purity, constant membrane area operation, the feed stream, ie the compressor stream, generally increases at high feed pressures and vice versa. Such a relationship is taken into account in determining the design conditions, i.e. the complete requirements, which preferably optimize the combinations indicated to achieve the design conditions optimized for the particular membrane system and the product requirements of a given application.

In an exemplary air separation operation, the membrane plant is operated at a feed gas pressure of 200 psig and optimized to sufficient requirements to produce 98% nitrogen 10,000 ncfh as a non-permeable gas. If the product requirement is reduced to 7,000 ncfh, the overall utilization of the available membrane area can be maintained in the practice of the present invention by reducing the feed gas pressure to 145 psig, where the product purity is essentially at the design purity level. maintain. In addition to the reduced feed gas pressure, the feed air flow requirements for producing 7,000 ncfh of nitrogen product are also reduced. At a lower feed gas pressure of 145 psig, less gas is permeated through the membrane and less feed gas is required to achieve the desired turndown production. In the turndown example mentioned above, the desired product is reduced from 10,000 ncfh to 7,000 ncfh and the feed air pressure is reduced from 200 psig to 145 psig, and the compressor flow, the feed flow to the membrane system, appears to be 72% of the complete design flow. Thus, during production of the product at a design purity level, the conditions for maintaining sufficient use of the membrane area available at turndown are combined with the indicated turndown from the design feed stream flow, eg feed gas pressure. It includes the indicated reduction in driving force across the membrane by the reduction of.

The amount of energy stockpile at turndown will depend on the type of compression equipment used in the practice of the present invention. It should be appreciated that various capacity control devices may be used in the art depending on the type of compression device used. Variable speed motors, internal recirculation valves and intake valve unloaders are examples of available devices that can be used in the turndown compression equipment according to the present invention.

Compression equipment, ie reciprocating compressors with suction valve unloaders, oil filled screw compressors with internal valves for returning to the suction site, or other such commercially available equipment, may provide a signal for turndown. Receiving, its feed stream flow and pressure ratio across the membrane is reduced, which reduces power draw. In an automated embodiment of the present invention, the process computer is a turndown, such as a valve unloader, variable speed motor, etc., to optimize the flow rate and pressure ratio across the membrane that can be used for a given desired flow or purity signal. It can be programmed to control the mechanism and automatically set the compression equipment.

In the above-mentioned exemplary embodiment where a screw-type compression system is used with an internal recirculation valve, the power draw for turndown from 10,000 to 7,000 ncfh nitrogen is 77% of the design value. At a reduced feed gas pressure of 145 psig, the power draw is 85% of the design value. Since the total power draw is reduced by both factors, the feed gas volume and the decrease in feed gas pressure to reduce the driving force across the membrane, the total power draw is 85% of the design 77%, or 65% of the power draw under the design conditions. .

The turndown control method of the present invention can be effectively used in implementations where product purity turndown is required rather than product flow turndown. If reduced product purity is desired in a constant product flow, the feed pressure may be lowered to reduce the pressure differential across the membrane while the product flow is maintained at the design level. In such embodiments, the total membrane surface area available is again fully utilized during the turndown operation. Those skilled in the art will appreciate that the process computer mentioned above is easily programmed to automatically adjust the compression system operation to account for product purity and product flow turndown control simultaneously. In this application, the pressure differential that generates the driving force across the membrane will change in response to changes in both product purity and flow and will be optimized in view of the overall requirements and operating conditions associated with a given gas or other fluid separation operation.

It can be seen that the rate of turndown control depends on the compression system design used for a given membrane separation operation and its ability to effectively control the feed stream flow to the membrane. It can also be seen that the efficiency of the turndown control method of the present invention is sensitive to the product pressure requirements of a given application. In the above example of air separation for nitrogen recovery, if a minimum product pressure of 100 psig is required, the limit of turndown will be 45% of the maximum product flow of the compression system. In most cases, however, this factor does not affect the overall turndown efficiency, since typically the optimized design membrane operating pressure is significantly higher than the product pressure required. Therefore, to obtain many advantages in the application, the practice of the invention will be a convenient and most effective turndown control method. One of ordinary skill in the art will appreciate that the turndown method of the present invention will preferably be optimized to the point that is guaranteed by the importance of the turndown operation of the overall process being controlled. The choice of compressor system and design operating pressure are the key determinants of turndown efficiency and placement in practical commercial operation.

The present invention is not merely concerned with the turndown from the design conditions for one particular set of low feed flow and drive force conditions, but the adjustment of the above conditions between the above design conditions and various lower requirements, or design quantities and It should also be appreciated that a relationship between various low requirements is involved, where product recovery of less than purity is required. Although the design conditions indicate the total capacity of a given membrane system with respect to the desired product flow and purity, in order to produce products of the design purity described above with a slight loss of product purity while slightly more than the product design quantity or a slight loss of product recovery. It should also be noted that it is generally possible to operate a membrane system at a capacity larger than the design capacity. Operation above such design conditions essentially constitutes the design conditions relating to the turndown feature of the present invention. From the description herein, it can be seen that turndown conditions are effective if either or both of the product flow or product purity requirements are lower than the design product flow and purity levels for a given membrane system.

It is also within the scope of the present invention to implement the turndown method described and claimed herein with a wide range of fluid separation operations in which air separation for the above-mentioned nitrogen production embodiments with turndown conditions is an illustrative example thereof. Such manipulations include all cases where the permeable membrane system can be effectively used to separate more permeable components from fluid mixtures with less permeable components. As in the desulfurization apparatus, hydrogen containing exhaust gas is an example of another suitable feed stream for use in permeable membrane operation and for the turndown control method of the present invention. Typical exhaust gases of this nature will contain about 45 mole percent hydrogen, 25% methane, 25% ethane and some other hydrocarbons. The membrane system method can be used to purify the hydrogen described above to the desired purity level, for example 90%. Ammonia purge gas and hydrogen recovery from carbon dioxide and methane separation are illustrative examples of other commercial fluid separation operations in which the turndown process of the present invention can be applied as appropriate. Although air separation applications have been described above with regard to the easy outflow of oxygen streams permeating at atmospheric pressure, other suitable and preferred pressure conditions may be suitable for various other embodiments of the present invention. In some instances, it may be desirable to utilize the supply pressure available under turndown conditions, with a reduction in driving force across the membrane resulting from increasing the permeation pressure rather than by lowering the air gas pressure. Of course, the product purity and / or product recovery achieved can vary somewhat depending on the overall requirements of any given membrane separation operation.

The turndown control of the present invention can be used for any desired permeable membrane system. Thus, the membrane material used may include, for example, cellulose derivatives such as cellulose acetate, cellulose acetate butyrate and the like; Polyamides and polyimides including aryl polyamides and aryl polyimides; Polysulfone; It may be any suitable material capable of selectively permeating more permeable components in a gas or other fluid mixture such as polystyrene. It should be appreciated that the use of any desired type of permeable membrane is also within the scope of the present invention. Thus, the permeable membrane will be in the form of a composite with a separation layer that determines the selectivity and permeability characteristics of the membrane located on the porous support layer. Asymmetric type membranes may also be used in which relatively dense surface portions determine the selectivity and permeability properties of the membrane and more pores provide support layers. Other types of membranes, such as dense membranes, are also useful for particular applications. The permeable membrane for the present invention may be in any desired shape, such as flat sheet, hollow fiber, spiral wound, or other desired shape, with hollow fiber membranes being generally preferred. Hollow fiber or other desired type of membrane material is generally a hollow fiber bundle, with a feed inlet and a permeate outlet side, with conduit means provided for separating off the feed stream impermeable portion and removing the permeate portion thereof. It is assembled with a membrane modulus comprising a jean flat sheet membrane assembly or a spiral wound container. All such systems are effectively turned down by the present invention, and the membrane face then installed is fully utilized under all operating conditions.

The present invention thus provides a very practical and effective means of turndown control. Not only can the installed capacity of the membrane system be fully utilized, but the use of start / stop mode operations can be avoided in response to changing requirements or turndown conditions. As a result, membrane systems can be commonly used to obtain the desired quantity and quality of product under turndown conditions, and the reliability of the membrane system is increased with reduced load, constant and stable operation of the equipment under turndown conditions.

Claims (12)

Permeability with an installed membrane surface area under which design fluid conditions can selectively permeate more easily permeable components in the aforementioned feed streams that contain less permeable components at the desired feed flow rate and pressure. Conveyed in contact with the feed side of the membrane system, wherein a predetermined amount of said less easily permeable component is recovered as a non-permeate at the design feed stream pressure level and a predetermined amount of more easily permeable component is lower as a permeate. In a permeable membrane process which recovers at a pressure level and the partial pressure of the feed stream components across the membrane provides a driving force for the permeable membrane separation, the turndown control method comprises: (a) the installed membrane surface area of the membrane system; To make full use of the feed stream flow rate and membrane Horizontally across the partial pressure driving force, the step of the obtained transmission and non-transparent water quantity and purity maintained to be the design quantity and purity levels; (b) During the period of reduced requirements for the amount and purity of the permeate or non-permeable product stream, the product stream is desired during the turndown period of the reduced requirements, while maintaining sufficient use of the installed membrane surface area of the system. Reducing the feed flow rate and said driving force across the membrane to be recovered in the membrane system in turn down quantity and quality; And (c) the design conditions of step (a) for the design requirement that the permeate or non-permeable product stream is obtained at the design quantity and purity level, and the lower requirement that the product stream is recovered in an amount and purity lower than the design level described above. Adjusting the feed flow rate across the membrane and the feed flow rate between the lower feed flow rate and the driving force condition during step (b) for the conditions, thereby fully utilizing the membrane surface area available under variability requirements. Turndown control method characterized in that the efficiency of the membrane process is increased by. The method of claim 1, wherein the driving force across the membrane is reduced during step (b) by reducing the feed stream pressure during the reduced requirement period. The method of claim 1 wherein the permeate stream is the desired product stream. The method of claim 1 wherein the non-permeate stream is the desired product stream. The method of claim 1, wherein said period of reduced requirements includes a period of time less than the amount of design of product to be supplied at a level of design purity. 2. The method of claim 1, wherein said period of reduced requirements includes a period of time less than the design purity of the product to be supplied at the design quantity level. 2. The turndown method of claim 1, wherein the driving force across the membrane is reduced during step (b) by increasing the permeate stream pressure during the period of reduced requirements. 4. The method of claim 3 wherein the feed stream comprises a hydrogen containing stream and the product permeate stream comprises purified hydrogen. 5. The method of claim 4 wherein the feed stream comprises air and the product impermeable stream comprises a nitrogen enriched stream. 9. A method according to claim 8, wherein the driving force across the membrane is reduced during step (b) by increasing the permeate pressure during the period of reduced requirements. 10. The method of claim 9, wherein the driving force across the membrane is reduced during step (b) by reducing the feed stream pressure during the reduced requirement period. The method of claim 1, wherein the permeable membrane system consists of a membrane modulus containing a hollow fiber membrane.

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