DE69911905T2 - Performance savings for gas separation in part-load operation - Google Patents

Description

The invention relates to operation cryogenic or low-temperature decomposition at a flow rate under the optimal ("design") flow rate disassembly (i.e., "shutdown" or partial load operation or "turndown"). Such a method is known from GB-A-2126700. It finds special ones, but not exclusive application in the separation of air to a nitrogen product to disposal to deliver.

Gas mixtures from a "light" component and with a "heavy" component a higher boiling point as the light ingredient can separated by cryogenic or cryogenic decomposition to make a product from the gaseous, light component available to deliver. The gas mixture is compressed and a distillation column system supplied in which the gas mixture is rectified to extract the product from the light component available to deliver. The distillation column system has one or more Distillation columns as is well known in the art is. When two or more columns are arranged in a row, work at different pressures, being the upstream located column (s) works at a higher pressure than the downstream located column (s).

A process for cryogenic decomposition, which provides a product made of a light component, is designed to work with optimal efficiency, if the product from the light component at a predetermined Desired flow rate to disposal is provided. In many conditions or circumstances, however, the need changes for the Product, and the design or target flow rate is not always needed. Accordingly, there are occasions when the decomposition process takes place under the target capacity is operated (the so-called "turndown" or shutdown). The extent of the shutdown but is typically by the centrifugal processor or centrifugal compressor been limited to the usual used to the gaseous Compress mixture feed to the distillation column system. The extent in which the flow rate can be reduced by the compressor, by pumping or the sudden increase that damages the compressor and fluctuations in the discharge or Output pressure causes. The shutdown to 70% of the target flow the compressor can be variable with the use of guide vanes Angle and with an accompanying, slight reduction in output pressure due to the change of resistance can be obtained by the distillation column system. If the distillation column system with a stronger one Shutdown must be done, compressed gas, in flow direction seen behind the compressor, drained or recycled so that no energy savings with stronger Shutdown or partial load operation can be achieved.

It has been suggested energy savings when shutting down more than 30% (i.e. less than 70% the design or target flow) through the use of multiple compressors with partial flow received, for example two compressors with 60% flow. This leads when shutting down to significant energy savings, however at the expense of both a significant increase in the cost of capital as well even a slight increase the energy requirements for target flow.

US-A-5,501,078 discloses one Integrated gas turbine and air separation unit ("ASU" for air separation Unit) where the feed air to the ASU is through the gas turbine is compressed and the ASU works to oxygen at a first To produce purity and a first pressure when the gas turbine Desired flow works, but oxygen with lower purity and lower Generate pressure when the gas turbine is operating at the pressure that at partial load. The oxygen product from the ASU is compressed and fed to a fuel gasification plant that feeds the gas turbine. A control arrangement responsive to the shutdown state of the gas turbine causes the ASU in its reduced purity / reduced mode Pressure works. The reduction in the purity of the oxygen product is limited the reduction in the pressure of the oxygen product, which is from the reduced pressure of the feed air when shutting down of the turbine, compared to maintaining the Purity of the oxygen product, so that on the compressor for the oxygen product a lower compression ratio needed becomes.

In the example given embodiment according to US-A-5,501,078 the ASU is a conventional double column system, which compresses the nitrogen product from the column at lower pressure and is supplied to the combustion chamber of the gas turbine. Under target conditions the ASU with the compressed feed air of the gas turbine fed at 160 psi (1.1 MPa) and carries oxygen (95 purity) and nitrogen from the column lower pressure at 48.1 psia (332 kPa) or 47.1 psia (325 kPa) too. When shutting down or at partial load, the pressure of the Feed air reduced to 130 psia (0.9 MPa). Without reduction the oxygen purity the oxygen and nitrogen feed pressures from the lower column Pressure at around Reduced 22% to 37.4 psia (258 kPa) and 36.7 psia (253 kPa). However, if the oxygen purity is reduced to 90%, these target supply pressures increase by approximately 16% 40.4 psia (279 kPa) or 39.7 psia (274 kPa) reduced.

GB-A-2126700 (corresponding to US-A-4,566,887) discloses a process and plant for low temperature fractionation of air to variable amounts of oxygen from a constant To generate air supply. The characteristic feature of the process is that compressed air is over that is expanded, which is required by the fractionation, for additional cooling or cold to disposal to ask, making a higher Proportion of nitrogen as a liquid deducted and for use in a subsequent period with increased nitrogen requirements can be saved. It is found that a pressure indicator controller ("PIC" for Pressure Indicator Controller) will exert a throttling effect on the system pressure to maintain when the need for nitrogen gas over the capacity the system rises. If there is a decrease in demand, there is an initial one Pressure increases and the PIC will open fully. If the need continues falls can further increase the Pressure through a suitable arrangement, for example a suction control valve on the air compressor, to be used to feed the air flow reduce. This reduction in flow is then determined by the FIC, which is designed to open a valve, after which the pressure drops, causing the suction control valve opens again and the flow again manufactures. There is no disclosure of steady state operation with a suction throttled centrifugal compressor.

Suction throttling is known as Means to increase the part-load efficiency of centrifugal compressors. she reduces the suction pressure on the compressor and makes it possible the compressor to work at lower mass flow without steep climbs or critical angles occur for the guide wings. The effect from suction throttling to centrifugal compressors is, for example, from J. R. Gaston in "Centrifugal Compressor Operation & Control ", Part II," Compressor Operation "(Adv. Instrum. 31 (1976) Proc. 31st Annual ISA Conf. & Exhb., Houston, Texas, October 10-14 1976); F. B. Horowitz in “Graphical Interpretation of Centrifugal Compressor Controls "(Proc. Inst. Mech. Closely. IMechE Conf. (1989) 37-48); and by R. P. Williams in “An examination of the methods used to vary the output of centifugal compressors with particular reference to part-load efficiency "(Ind. Process Control Conference (1994) Proc. Workshop (1979) 120-124).

1 and 2 show the effect of suction throttling on the shutdown or partial load operation of a centrifugal compressor. In 1 the effect on the shutdown from the target or design compressor flow (100%, point A) to 70% (point B) is shown by using changes in the angle of the guide vanes. The output pressure drops slightly due to the lower pressure drops in the air separation plant. A further shutdown cannot be achieved due to the approach to the sudden rise or the pumping of the compressor.

2 Figure 4 shows the reduction in mass flow from 70% to 50% during shutdown resulting from changes in the angle of the guide vanes when the suction throttling has reduced the outlet pressure to 70% of the target pressure. Since both the suction and discharge pressure are reduced, the compressor curves shift in comparison with 1 significant. The lower pressures decrease the density of the air, thereby increasing the current volumetric flow compared to the same flow rate at target conditions (ie normal operating pressures), thereby keeping the efficiency of the compressor stage at target values. However, the prior art requires that the same pressure conditions in the distillation column system be maintained at shutdown as at the desired flow, so that essentially the same outlet pressure from the compressor is required to supply the feed. Furthermore, it is conventionally required that the values for the product recovery (as a molar percentage of the feed) are to be kept at target conditions under target conditions for a conventional double-product ASU. Different criteria apply when feed air to an ASU is compressed by an integrated gas turbine, because the pressure of the feed air depends on the shutdown of the gas turbine.

It has now been found that a low-temperature gas separation to product at target or design pressure to disposal to be provided, selectively with target flow or with partial load or after shutdown can be operated, especially in strong Shutdown (i.e. a partial load of less than 30%; less as 70% target flow), namely by suction throttling of the compressor for the supply of the feed at Shutdown to print for the feeder to reduce the feed to the distillation column system and the distillation column system at a sufficiently reduced one Operate product acquisition to maintain product pressure receive. Previously, operation required a severe shutdown the use of two or more compressors for the routing of the feed with accompanying increases the cost of capital and the energy requirements for the target flow. The Use of suction throttling in accordance with the present invention is associated with only low capital costs, increases energy requirements with target flow not and has shutdown power requirements that are equivalent on the use of two or more compressors for feeding the Are infeed.

The present invention provides a method for cryogenic separation of a gaseous mixture having a "light" component and a "heavy" component having a higher boiling point than the light component, whereby the gaseous mixture is fed into a Amount selected from at least two flow rates in a centrifugal compressor or centrifugal compressor is compressed; the compressed feed is fed to a distillation column system having a "high" pressure ("HP" for high pressure) column and a "low" pressure ("LP" for low pressure) column operating at a lower pressure than the HP column ; the compressed feed is rectified in the HP column to form a gaseous overhead of the HP column and a liquid bottoms of the HP column; at least a portion of the liquid bottoms of the HP column is fed to the LP column and rectified therein to provide a gaseous product of light components at a substantially constant pressure regardless of these flow rates and a waste stream that is rich in the heavy component ; at least a portion of the gaseous overhead of the HP column is condensed against the liquid bottoms of the LP column to provide boiling or reboil for the LP column and to condense the overhead of the HP column; the respective proportions of the condensed top product of the HP column provide reflux or reboil for the HP column and the LP column, characterized in that the lower, but not the higher, of the two flow rates, the feed to the compressor is throttled and the Recovery (relative to the feed) of the product from the light component (compared to that at the higher flow rate) can be reduced to the pressure of the compressed feed to the HP column essentially without reducing the pressure of the product from the lighter component of the Reduce LP column (compared to that at the higher flow rate).

Usually becomes the upper flow rate the target or design rate for be the compressor while the lower flow rate 50 to 70% of the target flow rate will be because of conventional modifications such as a change the angle of the guide vanes, for shutdown can cause up to 30%.

It is preferred that the centrifugal compressor Has variable-angle guide vanes, and the selectable ones flow rates for the Feed include a lot between the higher and the lower rate, where the angles of the vanes at this intermediate flow rate of which at the higher flow rate differ, the compressor, however, the compressed feed to the HP column at the same pressure as the higher flow rate supplies. The intermediate flow rate is usually larger than 70% of the higher flow rate be while the lower flow rate between 50 and 70% of the higher flow rate will lie.

The invention has special, however not exclusive Application in the separation of air to a gaseous nitrogen product to disposal to deliver. It has special application to a nitrogen generator process, where the distillation column system has a "high" pressure ("HP" for High Pressure) column and a "low" pressure ("LP" for Low Pressure) column that operates at a lower pressure when the HP column works; the compressed air feed is fed to the HP column and rectified therein to a gaseous product of the HP column and a fluid To form bottom product of the HP column; at least part of the liquid bottoms the HP column is fed to the LP column and therein to the gaseous nitrogen product and rectified an oxygen-rich waste stream; at least one Part of the gaseous Top product of the HP column is condensed against the liquid bottom product of the LP column, to boil or reboil to the LP column and condense the top product of the HP column; and respective proportions of the condensed top product of the HP column represent reflux to the HP column and the LP column. The HP and LP columns can the only rectification columns in the distillation system and / or all of the gaseous Top product of the HP column is suitably against the liquid bottom product the LP column condenses to reboil for the LP Column available to provide and the top product of the HP column completely condense.

The following is a description based on only one embodiment with reference to the figures of the accompanying drawings currently preferred embodiment the invention. In the drawings:

1 shows the effect of reducing the mass flow through an ASU air feed compressor from the target or design compressor flow (100%, point A) to 70% of the target flow (point B) by using changes in the angle of the guide vanes the rapid surge of the compressor and the discharge pressure of the compressor;

2 shows the effect of reducing the mass flow through the same compressor as 1 from 70% of the target flow (point C) to 50% of the target flow (point D) by using changes in the angle of the guide vanes to the rapid surge ("surge") of the compressor and the discharge pressure of the compressor, but with suction throttling has reduced the output pressure of the compressor to 70% of the target or design pressure;

3 Fig. 4 is a schematic illustration of a conventional nitrogen generator with a double column system that contains the compressor for the feed air; and

4 is a schematic representation of the nitrogen generator with double column system according to 2 which has been modified to carry out the method according to the present invention.

First up 3 Referred; feed air 100 is in a multi-stage centrifugal compressor or centrifugal compressor 1 compressed with intermediate stage cooling devices and an aftercooler (not shown). The water condensed from the air is removed in a decomposition pot or from the coolers, and the compressed feed air becomes an upstream purification system 2 fed where the remaining water, carbon dioxide and other contaminants are removed. The clean, dry air 101 is close to their dew point in a main heat exchanger 3 against a nitrogen product stream 106 and an oxygen-rich exhaust gas stream 108 cooled.

The chilled air 102 becomes a high pressure column 4 supplied, where it is rectified to a gaseous, nitrogen-rich top product and a liquid bottoms product from crude oxygen. The nitrogen top product is in a reboiler / condenser 5 condenses in the bottom of a low pressure column 6 located. Part of the condensed nitrogen overhead product becomes the high pressure column 4 returned while the rest of the part 104 in its pressure on a valve 7 reduced and the upper end of the low pressure column 6 is fed. The liquid soil product 103 raw oxygen becomes a valve in its pressure 8th reduced and an intermediate point in the low pressure column 6 fed.

The feeds 103 and 104 to the low pressure column 6 are rectified in the column to give a gaseous nitrogen overhead stream 105 and an oxygen-rich exhaust gas stream 107 to provide. After heating in the main heat exchanger 3 represents the nitrogen flow 105 the stream 106 of the nitrogen product. The exhaust gas flow 107 is in the main heat exchanger 3 partially heated, in an expansion device 9 expands to recover cold and in the main heat exchanger 3 warmed up before being a waste stream 108 is eliminated. This waste stream 108 can be released to the atmosphere and / or as a regeneration gas in the upstream purification system 2 be used.

If less than the design or target flow of the nitrogen product 106 is required, the mass flow of air 100 to the main compressor with no significant reduction in the feed or discharge pressure of the main compressor 1 reduced. The reduction in mass flow of supply air to the main compressor is limited to avoid a rapid rise and critical angles of the vanes in the compressor. The operating pressures of the high pressure column 4 and the low pressure column 6 remain unchanged compared to the target flow, and accordingly the nitrogen pressure and nitrogen recovery remain essentially constant.

If the shutdown or the partial load drops to 70% of the target flow, the amount of nitrogen product is reduced to 70% of the target flow, but the pressure and the recovery of the nitrogen product (as a proportion of the feed air ) remain essentially unchanged. Shutting down the compressor to less than 70% of the desired flow may occur due to the onset of the rapid surge in the compressor 1 cannot be obtained at the maximum angle of the guide vanes. If the distillation column system is to be shut down by more than 30%, the compressor must be operated at 30% part load, and the excess compressed air is discharged to the outside or recycled.

4 differs from 3 only to the extent that a butterfly or wing cock 10 - or another suction throttle - in the air supply 100 to the main compressor 1 is provided. If the throttle valve 10 is fully open, the procedure reworks 4 in the same way as the procedure after 3 and can be operated up to a 30% shutdown (70% target flow) by increasing the angle of the vanes. If the shutdown is more than 30%, however, the throttle valve 10 operated to feed to the compressor 1 to throttle and the rate of removal of the nitrogen product in the stream 106 is reduced by an amount that is proportionately greater than the reduction in air feed from the compressor 1 is. The relative reduction in the rate of nitrogen withdrawal leads to an increase in the nitrogen content of the exhaust gas flow 107 and thus a reduction in the pressure in the high pressure column 4 , causing the compression ratio on the compressor 1 can remain unchanged.

Although the process according to the invention has a lower nitrogen recovery than the conventional system 3 (at which the compressor output pressure is maintained) this reduction is compensated for by the system's ability to operate at a much more severe shutdown (e.g. up to 50%) than the system after 3 is possible. For a rapid surge in the system after 3 to avoid, shutting down the distillation column system to less than 70% of the desired flow would require venting to the outside or venting or recycling the access air in the flow direction, directly behind the compressor. As an alternative, the compressor could 1 be replaced by two 60% compressors connected in parallel.

The following table compares the material balance, the pressures, the relative capital and the relative energy of the system 3 at 30% shutdown (70% of base flow) with the system 3 at 50% shutdown. It also compares with the traditional modification of the system 3 available, in which the main compressor is replaced by two 60 ° compressors.

As can be seen from the table, the present invention provides comparable performance when shutting down with the conventional system when using two compressors, but with a minimal increase in capital expenditure and without loss of energy when working under target conditions.

It is understandable that the invention is not limited to the specific details described above, but that numerous modifications and changes are made can, without departing from the invention as defined in the following claims is.

Claims (7)

Process for the cryogen or low-temperature separation or decomposition of a gaseous mixture with a "light" component and a "heavy" component which has a higher boiling point than the light component, whereby the gaseous mixture is fed in in an amount resulting from at least two flow rates are selected, is compressed in a centrifugal compressor or centrifugal compressor; the compressed feed is fed to the HP column of a distillation system with a "high" pressure ("HP" for high pressure) column and a "low" pressure ("LP" for low pressure) column operating at the lower pressure when the HP column works; the compressed feed is rectified in the HP column to form a gaseous overhead of the HP column and a liquid bottoms of the HP column; at least a portion of the liquid bottoms of the HP column is fed to the LP column and rectified therein to provide a gaseous product of light components at a substantially constant pressure regardless of these flow rates and a waste stream that is rich in the heavy component ; at least a portion of the gaseous overhead of the HP column is condensed against the liquid bottoms of the LP column to provide boiling or reboil for the LP column and to condense the overhead of the HP column; the respective proportions of the condensed top product of the HP column provide reflux for the HP column and the LP column, characterized in that the feed to the compressor is throttled at the lower, but not at the higher, of the two flow rates and the recovery (relative to the feed) of the product from the light component (compared to at the higher flow rate) to reduce the pressure of the compressed feed to the HP column substantially without reducing the pressure of the product from the lighter component from the LP column (compared to at the higher flow rate). The method of claim 1, wherein the lower flow rate 50 to 70% of the higher Flow rate is. The method of claim 1 or claim 2, wherein the centrifugal compressor guide wing or -variable angle blades, and being the selectable ones flow rates of the feed a quantity or an amount or a rate between the higher and the lower rate includes and at this intermediate flow rate, the angles of the guide vanes different from those at the higher flow rate differ, the compressor, however, the compressed feed the HP column at the same pressure as the higher flow rate supplies. The method of claim 3, wherein the intermediate flow rate larger than 70% of the higher flow rate is and the lower flow rate between 50 and 70% of the higher flow rate lies: Method according to one of the preceding claims, wherein the gaseous Mixture is air and the gaseous Product from the light component is a gaseous nitrogen product. Method according to one of the preceding claims, wherein the only rectification columns in the distillation system the HP and the LP column are. Method according to one of the preceding claims, wherein the whole gaseous Top product of the HP column against the liquid bottom product of the LP column is condensed to boil or reboil for the LP column place and the top product of the HP column completely condense.