Power savings for gas separation at turndown

Abstract: Cryogenic gas separation to provide product (105,106) at design pressure is selectively operated at design flow or at turndown, especially at greater than 30% turndown (less than 70% design flow), by suction throttling (10) the supply feed compressor (1) at turndown to reduce the supply feed (102) pressure to the distillation column system (3,4,5,6) and operating the system at sufficiently reduced product (105,106) recovery to maintain the product pressure. Preferably, the compressor (1) has variable angle guide vanes permitting up to 30% turndown before suction throttling (10) the compressor (1) to allow greater turndown without compressor surge. The arrangement has particular application to dual column nitrogen generators.

Description: The invention relates to operation of cryogenic separation at a flow rate below the optimum ("design") flow rate of the separation (i.e. at "turndown"). Such a process is known from GB-A-2126700. It has particular, but not exclusive, application to the separation of air to provide a nitrogen product. Gaseous mixtures comprising a "light" component and a "heavy" component having a higher boiling point than the light component can be separated by cryogenic separation to provide a gaseous light component product. The gaseous mixture is compressed and fed to a distillation column system in which the gaseous mixture is rectified to provide the light component product. The distillation column system comprises one or more distillation columns as is well known in the art. When there are two or more columns arranged in series, they operate at different pressures with the upstream column(s) operating at a higher pressure than the downstream column(s). A cryogenic separation process providing a light component product is designed to operate at optimum efficiency when providing the light component product at a predetermined design flow rate. However, in many circumstances, the demand for the product varies and the design flow rate is not always required. Accordingly, there are occasions upon which the separation process is operated at below design capacity (so-called "turndown"). However, the extent of turndown has typically been limited by the centrifugal compressor conventionally used to compress the gaseous mixture feed to the distillation column system. The extent to which the flow rate through the compressor can be reduced is limited by surge which damages the compressor and causes fluctuations in the discharge pressure. Turndown to 70% of design flow through the compressor can be obtained with the use of variable angle guide vanes and with an accompanying slight decrease in discharge pressure due to the change in resistance through the distillation column system. If the distillation column system is required to operate with a greater turndown, compressed gas is vented downstream of the compressor or recycled and hence no power savings are achieved at higher turndown. It has been proposed to obtain power savings at turndowns of greater than 30% (i.e. to less than 70% design flow) by the use of multiple partial flow compressors, for example two 60% flow compressors. This results in significant power savings at turndown but at the expense of both a significant increase in capital cost and a slight increase in power requirements at design flow. US-A-5,501,078 discloses an integrated gas turbine and air separation unit ("ASU") in which the feed air to the ASU is compressed by the gas turbine and the ASU operates to produce oxygen at a first purity and pressure when the gas turbine operates at design flow but to produce oxygen at lower purity and pressure when the gas turbine operates at a turndown pressure. The oxygen product from the ASU is compressed and fed to a fuel gasifier feeding the gas turbine. Control means responsive to a turndown state of the gas turbine causes the ASU to operate in its reduced purity/reduced pressure mode. The reduction in purity of the oxygen product limits the reduction in oxygen product pressure resultant from reduced feed air pressure on turbine turndown compared with maintaining oxygen product purity and hence a lower compression ratio is required across the oxygen product compressor. In the exemplified embodiment of US-A-5,501,078, the ASU is a conventional dual column system in which nitrogen product from the lower pressure column is compressed and fed to the combustion chamber of the gas turbine. Under design conditions, the ASU is fed with gas turbine compressed feed air at 160 psi (1.1 MPa) and supplies oxygen (95% purity) and nitrogen from the lower pressure column at 48.1 psia (332 kPa) and 47.1 psia (325 kPa) respectively. At turndown, the feed air pressure is reduced to 130 psia (0.9 MPa). Without reduction in oxygen purity, the oxygen and nitrogen supply pressures from the lower pressure column are reduced by about 22% to 37.4 psia (258 kPa) and 36.7 psia (253 kPa) respectively. However, when the oxygen purity is reduced to 90% these design supply pressures are reduced by about 16% to 40.4 psia (279 kPa) and 39.7 psia (274 kPa) respectively. GB-A-2126700 (corresponding to US-A-4,566,887) discloses process and apparatus for the cryogenic fractionation of air to produce variable quantities of nitrogen from a constant supply of air. The characterising feature of the process is that compressed air in excess of that required by the fractionation is expanded to provide additional refrigeration enabling a higher proportion of the nitrogen to be withdrawn as liquid and stored for use in a subsequent period of increased nitrogen demand. It is stated that, when demand for nitrogen gas increases above the capacity of the plant, a Pressure Indicator Controller ("PIC") will throttle to maintain the system pressure. When there is a reduction in demand, there is an initial rise in pressure and the PIC will open fully. If demand continues to fall, the further increase in pressure can be employed by any suitable means, e.g. a suction control valve on the air compressor, to decrease the feed air flow. This reduction in flow is then detected by the FIC which is arranged to open a valve whereupon the pressure falls thereby causing the suction control valve to reopen and restore the flow. There is no disclosure of steady state operation with a suction throttled centrifugal compressor. Suction throttling is well known as a means of increasing part-load efficiency of centrifugal compressors. It decreases the suction pressure on the compressor and enables the compressor to operate with lower mass flow without encountering surge or severe guide vane angles. The effect of suction throttling of centrifugal compressors is discussed by, for example, 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. Eng. IMechE Conf. (1989) 37-48); and by R.P. Williams in "An examination of the methods used to vary the output of centrifugal compressors with particular reference to part-load efficiency" (Ind. Process Control Conference (1994), Proc. Workshop (1979) 120-124). Figures 1 and 2 show the effect of suction throttling on the turndown of a centrifugal compressor. In Figure 1, the effect on turndown from design compressor flow (100%, point A) to 70% (point B) through the use of guide vane angle changes is shown. The discharge pressure drops slightly due to lower pressure drops in the air separation plant. Further turndown cannot be achieved because of the approach to compressor surge. Figure 2 shows the reduction in mass flow at turndown from 70 to 50% resulting from guide vane angle changes when suction throttling has reduced the discharge pressure to 70% of design pressure. Since both the suction and discharge pressures are reduced, the compressor curves shift significantly compared with Figure 1. The lower pressures decrease the density of the air and thereby cause an increase in actual volumetric flow compared with the same flow rate at design (i.e. normal operation pressures) thereby maintaining the compressor stage efficiencies at design levels. However, the prior art requires that the same pressure conditions should be maintained in the distillation column system at turndown as at design flow and hence essentially the same discharge pressure is required from the supply feed compressor. Further, it is conventionally required that the product recovery levels (as molar percentage of feed) at design should be maintained at turndown for a conventional dual product ASU. Different criteria apply when feed air to an ASU is compressed by an integrated gas turbine because the feed air pressure depends on turndown of the gas turbine. It has now been found that cryogenic gas separation to provide product at design pressure can be selectively operated at design flow or at turndown, especially at high turndown (i.e. greater than 30% turndown; less than 70% design flow), by suction throttling the supply feed compressor at turndown to reduce the supply feed pressure to the distillation column system and operating the distillation column system at sufficiently reduced product recovery to maintain the product pressure. Previously, operation at high turndown required the use of two or more supply feed compressors with accompanying increases in capital cost and power requirement at design flow. Use of suction throttling in accordance with the present invention incurs little capital cost, does not increase power requirement at design flow, and has equivalent power requirement at turndown to the use of two or more supply feed compressors. The present invention provides a process for the cryogenic separation of a gaseous mixture, comprising a "light" component and a "heavy" component having a higher boiling point than the light component wherein a feed of said gaseous mixture in an amount selected from at least two flow rates is compressed in a centrifugal compressor; the compressed feed is fed to a distillation column system comprising a "high" pressure ("HP") column and a "low" pressure ("LP") column operating at a lower pressure than the HP column; the compressed feed is rectified in the HP column to form a HP column gaseous overhead and a HP column liquid bottoms; at least a portion of the HP column liquid bottoms is fed to the LP column and is rectified therein to provide a gaseous light component product at essentially a constant pressure independent of said flow rates, and a heavy component-rich waste stream; at least a portion of the HP column gaseous overhead is condensed against LP column liquid bottoms to provide reboil to the LP column and condense the HP column overhead; and respective portions of the condensed HP column overhead provide reflux to the HP column and LP column, characterized in that at the lower, but not at the higher, of said two flow rates the feed to said compressor is suction throttled and the recovery (relative to the feed) of said light component product is reduced (compared with at said higher flow rate) to reduce (compared with at said higher flow rate) the pressure of the compressed feed to the HP column essentially without reducing the pressure of the light component product from the LP column. Usually, the upper flow rate will be the design rate for the compressor and the lower flow rate will be 50 to 70% of the design flow rate because conventional modifications such as guide vane angle variation can provide for turndown of up to 30%. It is preferred that the centrifugal compressor has variable angle guide vanes and the selectable feed flow rates include an amount intermediate the higher and lower rates and at which intermediate flow rate the guide vane angles differ from those at the higher flow rate but the compressor supplies compressed feed to the HP column at the same pressure as at the higher flow rate. The intermediate flow rate usually will be greater than 70% of the higher flow rate and the lower flow rate will be between 50 and 70% of the higher flow rate. The invention has particular, but not exclusive, application to the separation of air to provide a gaseous nitrogen product. It has especial application to a nitrogen generator process in which the distillation column system comprises a "high" pressure ("HP") column and a "low" pressure ("LP") column operating at a lower pressure than the HP column; the compressed air feed is fed to the HP column and rectified therein to form a HP column gaseous overhead and a HP column liquid bottoms; at least a portion of the HP column liquid bottoms is fed to the LP column and is rectified therein into the gaseous nitrogen product and an oxygen-rich waste stream; at least a portion of the HP column gaseous overhead is condensed against LP column liquid bottoms to provide reboil to the LP column and condense the HP column overhead; and respective portions of the condensed HP column overhead provide reflux to the HP column and LP column. The HP and LP columns can be the only rectification columns in the distillation system and/or all of the HP column gaseous overhead suitably is condensed against LP column liquid bottoms to provide reboil to the LP column and fully condense the HP column overhead. The following is a description by way of example only and with reference to the Figures of the accompanying drawings of a presently preferred embodiment of the invention. In the drawings:- Figure 1 shows the effect on compressor surge and compressor discharge pressure of reducing mass flow through an ASU feed air compressor from design compressor flow (100%, point A) to 70% of design flow (point B) by using guide vane angle changes; Figure 2 shows the effect on compressor surge and compressor discharge pressure of reducing mass flow through the same compressor as Figure 1 from 70% of design flow (point C) to 50% of design flow (point D) by using guide vane angle changes but in which suction throttling has reduced the compressor discharge pressure to 70% of design pressure; Figure 3 is a schematic representation of a prior art dual column nitrogen generator incorporating the feed air compressor; and Figure 4 is a schematic representation of the dual column nitrogen generator of Figure 3 modified for carrying out the process in accordance with the present invention. Referring first to Figure 3, feed air 100 is compressed in a multistage centrifugal compressor 1 with interstage coolers and an after cooler (not shown). Water condensed from the air is removed in a separation pot or from the coolers and the compressed feed air is passed to a front end clean-up system 2 where remaining water, carbon dioxide and other contaminants are removed. The clean, dry air 101 is cooled to near its dew point in main heat exchanger 3 against a nitrogen product stream 106 and a waste oxygen-rich stream 108. The cooled air 102 is fed to a high pressure column 4 where it is rectified into a gaseous nitrogen-rich overhead and crude liquid oxygen bottoms. The nitrogen overhead is condensed in reboiler/condenser 5 located in the bottom of a low pressure column 6. A portion of the condensed nitrogen overhead is returned to the high pressure column 4 and the remainder 104 is reduced in pressure across a valve 7 and fed to the top of the low pressure column 6. The crude liquid oxygen bottoms 103 is reduced in pressure across a valve 8 and fed to an intermediate location in the low pressure column 6. The feed 103 and 104 to the low pressure column 6 is rectified in the column to provide a gaseous nitrogen overhead stream 105 and a waste oxygen-rich stream 107. After warming in the main heat exchanger 3, the nitrogen stream 105 provides the nitrogen product stream 106. The waste stream 107 is partially warmed in the main heat exchanger 3, expanded in expander 9 to recover refrigeration and warmed in main heat exchanger 3 before removal as waste stream 108. This waste stream 108 can be discharged to atmosphere and/or used as regeneration gas in the front end clean-up system 2. When less than the design flow of nitrogen product 106 is required, the mass flow of air 100 to the main compressor is reduced without any significant reduction in the supply or discharge pressure of the main compressor 1. The reduction in mass flow of the supply air to the main compressor is limited to avoid surge and severe guide vane angles in the compressor. The operational pressures of the high pressure column 4 and low pressure column 6 remain unchanged compared with design flow and accordingly the nitrogen pressure and recovery remain essentially constant. Thus, if the turndown is to 70% of design flow, the amount of nitrogen product is reduced to 70% of design flow but the nitrogen product pressure and recovery (as a proportion of feed air) is substantially unchanged. Turndown of the compressor to less than 70% design flow cannot be obtained because of the onset of surge in the compressor 1 at the maximum guide vane angle. If the distillation column system is to be turned down by more than 30%, the compressor must be operated at 30% turndown and the excess compressed air vented or recycled. Figure 4 differs from Figure 3 only in so far as a butterfly, or other suction throttle, valve 10 is provided in the air supply 100 to the main compressor 1. When the throttle valve 10 is fully open, the process of Figure 4 operates in the same manner as the process of Figure 3 and can be operated at up to 30% turndown (70% design flow) by increasing the guide vane angle. However, at greater than 30% turndown, the throttle valve 10 is operated to throttle the feed to the compressor 1 and the rate of removal of nitrogen product in stream 106 is reduced by an amount proportionally greater than the reduction in air feed from the compressor 1. The relative reduction of the nitrogen removal rate results in an increase in the nitrogen content of the waste stream 107 and hence a reduction in pressure in the high pressure column 4, thereby permitting the compression ratio across the compressor 1 to remain unchanged. Although the process of the invention has lower nitrogen recovery than the conventional system of Figure 3 (in which the compressor discharge pressure is maintained), this reduction is compensated for by the ability of the system to operate at much greater turndown (e.g. up to 50%) than is possible with the system of Figure 3. In order to avoid surge in the system of Figure 3, turndown of the distillation column system to less than 70% of design flow would require venting or recycle of access air immediately downstream of the compressor. Alternatively, the compressor 1 could be replaced by two 60% compressors connected in parallel. The Table below compares the material balance, pressures, relative capital and relative power of the system of Figure 3 of 30% turndown (70% of base flow) with the system of Figure 4 at 50% turndown. It also provides a comparison with the prior art modification of the system of Figure 3 in which the main compressor is replaced by two 60% compressors. As can be seen from the Table, the present invention provides comparable performance at turndown with the prior art system using two compressors but with minimal capital expenditure increase and no power penalty when operating under design conditions. It will be appreciated that the invention is not restricted to the particular details described above but that numerous modifications and variations can be made without departing from the invention as defined in the following claims.

Claims: A process for the cryogenic separation of a gaseous mixture, comprising a "light" component and a "heavy" component having a higher boiling point than the light component wherein a feed of said gaseous mixture in an amount selected from at least two flow rates is compressed in a centrifugal compressor; the compressed feed is fed to the HP column of a distillation column system comprising a "high" pressure ("HP") column and a "low" pressure ("LP") column operating at a lower pressure than the HP column; the compressed feed is rectified in the HP column to form a HP column gaseous overhead and a HP column liquid bottoms; at least a portion of the HP column liquid bottoms is fed to the LP column and is rectified therein to provide a gaseous light component product at essentially a constant pressure independent of said flow rates and a heavy component-rich waste stream; at least a portion of the HP column gaseous overhead is condensed against LP column liquid bottoms to provide reboil to the LP column and condense the HP column overhead; and respective portions of the condensed HP column overhead provide reflux to the HP column and LP column, characterized in that at the lower, but not at the higher, of said two flow rates the feed to said compressor is suction throttled and the recovery (relative to the feed) of said light component product is reduced (compared with at said higher flow rate) to reduce (compared with at said higher flow rate) the pressure of the compressed feed to the HP column essentially without reducing the pressure of the light component product from the LP column. A process as claimed in Claim 1, wherein said lower flow rate is 50 to 70% of said higher flow rate. A process as claimed in Claim 1 or Claim 2, wherein the centrifugal compressor has variable angle guide vanes and said selectable feed flow rates include an amount intermediate said higher and lower rates and at which intermediate flow rate the guide vane angles differ from those at the higher flow rate but the compressor supplies compressed feed to the HP column at the same pressure as at the higher flow rate. A process as claimed in Claim 3, wherein said intermediate flow rate is greater than 70% of the higher flow rate and said lower flow rate is between 50 and 70% of said higher flow rate. A process as claimed in any one of the preceding claims, wherein the gaseous mixture is air and the gaseous light component product is a gaseous nitrogen product. A process as claimed in any one of the preceding Claims, wherein the only rectification columns in the distillation system are the HP and LP columns. A process as claimed in any one of the preceding Claims, wherein all of the HP column gaseous overhead is condensed against LP column liquid bottoms to provide reboil to the LP column and fully condense the HP column overhead.

Inventors: BOULTON LAURA ANN, PETROWSKI JOSEPH MICHAEL, WEYRICH GREGORY SCOTT, BOULTON, LAURA ANN, PETROWSKI, JOSEPH MICHAEL, WEYRICH, GREGORY SCOTT
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1999
Main classification: F25J3
IPC codes: F25J3