AUTOMATIC CONTROL FOR GAS FRACTIONATION APPARATUS

Abstract: A Pressure Swing Adsorption (PSA) appatus 100 uses controller 102 to make possible automatic and continuous modification of the air input flow in order to ensure that the produced nitrogen contains only a preselected range of an impurity which consists essentially of oxygen and to keep the output flow of nitrogen relatively high. The apparatus 100 has a valve 18a to control the flow of air through chambers 38 and 40 containing adsorbent. The controller 102 includes a converter 110 for operating valve actuator 18al. Flow detector means 106 senses the rate of flow of air through the chamber and imputiry detector 46 senses the level of oxygen impurity in the product nitrogen. A first comparator means 114 compares the output of the impurity detector 46 with a reference signal V ref 3 and generates a signal proportional to the difference therebetween. This signal is transmitted to a second comparator means 118 which compares it with the output from the flow detector means 104 to produce an output that is coupled to the input of the converter means 110. The converter means 110 is thereby cause to generate a signal that adjusts the position of the valve 18a.

Description: This invention relates to fractionation of air particularly by selective adsorption and is particularly concerned with controlling a Pressure Swing Adsorption (PSA) apparatus which fractionates air to recover a high purity component(s) thereof. A variety of control systems exist to control various parts of different processes for the separation of air by preferential adsorption. U.S. Patent No. 4,472,177 is directed to a vacuum swing system for the recovery from air of a high purity nitrogen and an oxygen-enriched gas fraction. The system monitors the percentage of an impurity gas in the product gas as it comes out of an adsorption column and modifies the time duration of the steps of adsorption and regeneration (rinse) while keeping air input flow rate substantially constant. One problem with this type of control is that the equipment which senses the impurity gas in the product gas must have a relatively fast response time and is therefore relatively expensive. A buffer tank is usually coupled between the outputs of the chambers and a valve supplied by a customer to control the amounts of nitrogen received from the system. A second impurity measuring device is typically coupled to an inlet of a customer′s valve to measure an impurity in the product gas. In addition, varying the time periods of the various steps of the process can cause disruptions in the steady state flow rate of the end gas product. A Pressure Swing Adsorption (PSA) system is marketed which comprises two adsorption chambers. Each adsorption chamber contains a carbon adsorbent which preferentially adsorbs oxygen from incoming air. First and second pressure regulators (mechanically adjustable pressure control valves) are in series with an inlet line to the chambers and an outlet line from the chambers, respectively. A holding tank coupled to the outlet line of the adsorption chambers collects nitrogen gas which contains oxygen as an impurity. An oxygen detector is coupled to an outlet line of the holding tank. The system is designed to automatically shut down if the oxygen content of the outlet gas exceeds a preselected limit. The amount of oxygen is manually read from the analyser, and the pressure set point of the first regulator is mechanically adjusted to control the pressure and therefore flow rate of air into the system to control the percentage of oxygen in the recovered nitrogen. After this mechanical adjustment, the system is allowed to run for an hour to an hour and a half to allow it to come to a new steady state operating point. If the percentage of oxygen impurity is determined to be within preselected limits, then no further adjustments are needed. If not, then another adjustment of the set pressure of the first regulator is made and the system is again run until it comes to a new steady state point. This process is repeated until the oxygen impurity level is within a preselected range. If the input flow rate of air is decreased too much, then the concentration of oxygen will decrease to a value below the preselected limit with output flow rate of the desired nitrogen also decreasing. The first regulator must then be readjusted to increase the input flow of air so as to increase the output flow of nitrogen while keeping the oxygen impurity percentage at a preselected level. These manual adjustments can be very time consuming and are typically required on a regular basis. The extent of adsorption of oxygen by the carbon structures is a function of the ambient temperature of the incoming air and the adsorption chambers. Thus, the oxygen impurity level generated in the above described system is a function of the ambient temperature of the incoming air and the adsorption chambers. For example, an apparatus can be designed to produce 4000 standard cubic feet (113 standard cubic metres) per hour (SCFH) with 1% impurity at an ambient temperature of 70° C. This apparatus may produce an output with 1.3% impurity at 4000 SCFH (113 sm³) at an ambient temperature of 90° C. The system must therefore be designed to provide the required impurity level under worst case conditions which means at normal ambient conditions the system is over designed in terms of capacity. The need for repeated time consuming adjustments by maintenance personnel, coupled with the requirement of an over-designed system, results in a product in which the economies of production and operation can clearly use improvement. It is desirable to have a PSA or like apparatus in which the inlet air flow rate is automatically and continuously adjusted so as to maintain the oxygen impurity level at a preselected value while permitting the nitrogen flow rate to be as high as possible at existing temperature conditions while essentially eliminating the need for manual readjustment of the apparatus. The present invention is directed at a control apparatus for use with a gas fractionation apparatus. A first portion of the control apparatus continually monitors the level of an impurity component in a recovered gas ("the product gas") being produced and automatically and continually adjusts gas flow through the fractionation apparatus so as to keep the impurity component in the product gas within preselected levels while maintaining the product gas flow rate as high as reasonably possible. According to the present invention, there is provided gas fractionation apparatus having a valve with an actuator which controls the size of an opening therein, which valve is in communication with at least one chamber containing adsorbent which is adapted preferentially to adsorb one or more components of gas entering therein, and also having a control apparatus comprising: convertor means, which has an input and an output with the output being coupled to the actuator of the valve, for controlling the size of the opening in the valve; flow detector means in communication with the downstream side of the valve for generating at an output thereof a signal that is proportional to the rate of gas flowing out of the valve; an impurity detector coupled to an outlet of the chamber, said impurity detector being adapted to generate at an output thereof a signal that is proportional to the amount of an impurity component of gas in a product component of the gas exiting the chamber; a first comparator means, which has a first input which is adapted to be coupled to a first reference signal indicative of the amount of impurity that is to be tolerated in the product component of gas received from the chamber and which has a second input coupled to the output of the impurity detector, the first comparator means being able to generate at an output thereof a signal which is proportional to the difference between the reference signal and the output signal of the impurity detector; and a second comparator means which has a first input coupled to the output of the flow detector means, a second input coupled to the output of the first comparator means and has an output coupled to the input of the convertor means, the second comparator means being able to compare signals received from the first comparator means and the flow detector means and to generate at the output thereof a signal which causes the convertor means to generate a signal causing the actuator of the valve to adjust the size of the opening in the valve so as to maintain the purity of gas exiting the chamber at a preselected level. In a preferred embodiment, the fractionation apparatus is a Pressure Swing Adsorption (PSA) apparatus with the input gas being air, the product gas being nitrogen, and the impurity gas being oxygen. The flow detecting means is a pressure monitor which generates an electrical current as an output signal. The convertor means responds to an electrical current signal at the input thereof and generates a force proportion thereto at the output thereof. This force causes the valve to open or close by an amount which is proportional to the force and the direction of same. The first and second comparators comprise discrete electronic or pneumatic components. In a preferred embodiment a microprocessor includes software routines to enable the comparator functions to be performed. The apparatus according to the present invention is adapted to adjust automatically and continuously the input flow rate of air to the PSA system to such an extent such that the nitrogen produced is typically 97 or more percent pure. The control circuitry automatically increases the input air flow rate so as to increase the output nitrogen flow rate while still keeping the impurity oxygen level thereof at no more than a preselected level if the percentage of an impurity becomes lower than is required. Conversely, if the percentage of input becomes higher than a preselected level, the control circuitry automatically decreases the input air flow rate so as to decrease the output nitrogen flow rate and maintain the percentage of impurity at a preselected level. In the preferred embodiment, a Pressure Swing Adsorption (PSA) system separates air into nitrogen and oxygen. The nitrogen is the product gas and oxygen which is mixed with the nitrogen is considered an impurity. The PSA system uses two chambers and a cycle of operation which includes: (a) a pressurising portion in which one of the chambers fills with air which remains in the chamber for a long enough time for a carbon structure or adsorbent in the chamber to a adsorb a significant amount of the oxygen from the air and until a preselected pressure is built up in the chamber, (b) a production portion in which nitrogen with some oxygen exits the chamber and is collected in a buffer tank, and (c) an equalisation portion in which the two chambers are coupled together and the pressure in both equalises. During the time one chamber is pressurised and subsequently produces nitrogen, the other chamber is allowed to vent oxygen-rich waste gas through an exhaust outlet so as to regenerate and be ready again to adsorb more oxygen when it is pressurised. The oxygen-rich waste gas typically includes oxygen, nitrogen, water vapour, carbon dioxide and other trace elements. After the equalisation portion of the cycle, the second chamber is pressurised and the first chamber is allowed to vent the oxygen-rich waste gas. During the production portion of the cycle the control system automatically and continually adjusts the position of the first valve. This adjusts the rate of air flow into the chambers so as to keep the level of oxygen in the produced nitrogen at a preselected level. The control apparatus preferably further comprises: pressure sensing means, pressurisation time measuring means, and a third comparator means; the pressure sensing means being coupled to the chamber for sensing pressure within the chamber; the pressurisation time measuring means, being coupled by a first input to the pressure sensing means, being adapted to be coupled by a second input thereof to a preselected pressure level, and being able to generate an output signal proportional to the amount of time measured for the chamber to pressurise from a pressure reading determined by the pressure sensing means to a preselected level; and the third comparator means being coupled by a first input thereof to an output of the pressurisation time measuring means, being also coupled by an output thereof to the control of the convertor means, and being adapted to be coupled by a second input thereof to a source of a reference signal which is proportional to a preselected time period in which it is desired that the chamber reach a preselected pressure level, the third comparator means being able to compare the output signal of the pressurisation time measuring means with the reference signal coupled to the second input thereof and to generate at the output thereof a signal which causes the convertor means to generate a signal at the output thereof causing the actuator of the valve to adjust the size of the opening in the valve and thus the rate of flow of gas into the chamber such that the chamber is pressurised to a preselected level during a preselected time period. Thus, during the pressurisation portion of the cycle of operation, the control apparatus senses pressure in the chamber being pressurised and automatically adjusts the opening of the valve at the beginning of the next pressurisation portion of the cycle so as to control the flow of air into the chamber to facilitate pressurisation occurring within a preselected time period providing adequate residence time in the chamber which allows for sufficient oxygen adsorption before product gas is released. The present invention will now be described by way of example with reference to the accompanying drawings. FIG. 1 shows one illustrative embodiment of a commercially available PSA apparatus; FIG. 2 shows an illustrative embodiment of PSA apparatus in accordance with the present invention; FIG. 3 shows another illustrative embodiment of PSA apparatus in accordance with the present invention; and FIGs. 4A and 4B collectively show a flow chart illustrating the operation of part of the embodiment of FIG. 3 in accordance with the present invention. FIG. 4 is a diagram showing the proper alignment of the flow charts of FIGs. 4A and 4B. Referring now to FIG. 1, there is shown the essential components of a commercially available Pressure Swing Adsorption (PSA) apparatus 10. Apparatus 10 comprises an air compressor 12 having an inlet 13, an air receiver tank 14, a water and oil filter 16, pressure regulators 18 and 20, an orifice plate 22, valves 24, 26, 28, 30, 32, 34a, 34b and 36, first 38 and second 40 adsorption chambers each containing carbon structures (not shown) adapted preferentially to adsorb oxygen from air in the chambers, an exhaust vent 42, nitrogen buffer tank 44, and an impurity (oxygen) detector (oxygen sensor and transmitter) 46. Typically, an end user of system 10 couples a valve 48 to an outlet of tank 44 to be able to control the amount of nitrogen gas received from system 10 at an outlet 50 of valve 48. Air is pulled in through inlet 13 of air compressor 12 and nitrogen exits system 10 through outlet 50. Air pulled into inlet 13 of air compressor 12 is fed into tank 14. Compressor 12 is typically an oil flooded screw-type compressor. Air from tank 14 passes through filter 16 where any oil and water droplets are removed. The air then passes through regulator 18. Regulator 18 is a valve with a mechanical pressure regulator which senses the pressure at an outlet thereof and keeps the valve open when the outlet (down stream) pressure is at or below that which the valve is mechanically set to control. The pressure at which regulator 18 controls (known as the set pressure) can be adjusted mechanically by turning a portion thereof to adjust compression on an internal spring (not shown) thereof. The air now passes through an opening (not shown) in orifice plate 22 which limits the maximum flow rate of air which enters chamber 38 or 40 so as to prevent damage to the carbon structures. Valves 24 and 26 are on-off type valves with same being operated with one being open and the other being closed. Valves 32 and 36 are unidirectional type on-off valves (check valves) which allow a flow of gas from chambers 38 and 40, respectively, and through regulator 20, but prevent the flow of gas in the reverse direction. Typically, product gas (nitrogen) flows from chamber 38 or 40 and therefore valve 32 or valve 36 is open and the other is closed. Air can flow into chamber 38 but cannot flow into chamber 40 with valves 26 and 32 open and valves 24 and 36 closed. Air can flow into chamber 40 but cannot flow into chamber 38 with valve 26 closed and valve 24 open. Automated controls (not illustrated) are typically used to open and close all the valves except for regulators 18 and 20 and valves 32, 36 and 48. Apparatus 10 operates on a cycle which includes a pressurization portion, a production portion, and an equalization portion. Only one of the chambers 38 and 40 receives air at a time. Assume that valves 26, 30 and 32 are open and that valves 24, 28, 34a, and 34b and 36 are closed. Air from compressor 12 fills chamber 38 and causes the pressure of chamber 38 to rise until the pressure in chamber 38 is at the level at which regulator 20 opens. This is known as the pressurization portion of the cycle. When the pressure in chamber 38 reaches the set pressure of regulator 20, regulator 20 opens and releases product nitrogen. This begins the production portion of the cycle. Air continues to enter chamber 38 during the production portion of the cycle with continued adsorption of oxygen. Product nitrogen exits chamber 38 and passes through regulator 20 and collects in tank 44. During the pressurization and production portions of the cycle for chamber 38, chamber 40 is allowed to regenerate by releasing oxygen-rich waste gas through valve 30 and out through exhaust vent 42 into the atmosphere. The oxygen-rich waste gas typically includes oxygen, nitrogen, water vapor, carbon dioxide and other trace elements. At this time, valve 28 is closed and therefore no air or component thereof can exit chamber 38 via valve 26 and exhaust vent 42. At the end of the production portion of the cycle for chamber 38, valves 24, 26, 28, 30, 32 and 36 are closed and valves 34a and 34b are opened. This allows pressure in chambers 38 and 40 to equalize. Typically, valve 34a is located near the top of the chambers and valve 34b is located near the bottom of the chambers. This portion of the cycle of operation is denoted as the equalization portion. Now valves 26, 30, 32 and 34 are closed and valves 24, 28, and 36 are opened. This allows air to flow into chamber 40 and causes the pressure of chamber 40 to rise until the pressure in chamber 40 is at the level at which regulator 20 opens. This is the pressurization portion of the cycle for chamber 40. Nitrogen within chamber 40 then passes through regulator 20 and is collected in tank 44. This is the production portion of the cycle for chamber 40. During the pressurization and production portion of the cycle of operation of chamber 40, oxygen-enriched waste gas exits chamber 38 through valve 28 and vents out through exhaust vent 42 into the atmosphere. This regenerates the carbon structure (or adsorbent) of chamber 38. Valves 24, 26, 28, 30, 32 and 36 are then closed and valves 34a and 34b are opened. This equalizes the pressures in chambers 38 and 40. A new cycle now begins with chamber 38 being again pressurized with air. In one application, apparatus 10 is typically operated on a two minute cycle. During the first approximate 25 seconds air is fed into chamber 38 through open valve 26 and oxygen is adsorbed in the carbon structure of chamber 38 as the pressure therein rises until it reaches the set point pressure of regulator 20. When this occurs, regulator 20 opens and nitrogen flows out of chamber 38, through valve 32, regulator 20 and into tank 44 for the next 31 seconds. Valves 34a and 34b are then opened up for 4 seconds to equalize the pressure in chambers 38 and 40 while all other valves are closed. Valves 26, 30, 32, 34a and 34b are closed and valves 24, 28 and 36 are opened for the next 56 seconds. Oxygen in the carbon structure (or adsorbent) of chamber 38 vents through vent pipe 42 into the atmosphere and air flows into chamber 40 for the next 56 seconds. Oxygen is now adsorbed in chamber 40 and nitrogen flows out of chamber 40 through open valve 36 and regulator 20 and then into tank 44. Valve 34 is then again opened for 4 seconds to equalize pressure between chambers 38 and 40 while valves 24, 26, 28, 30, 32 and 36 are closed. The cycle then starts again with valves 26, 30 and 32 opening and valves 24, 28, 34 and 36 being closing. In the commercially available PSA apparatus, regulator 20 is typically set to maintain an essentially constant pressure in chambers 38 and 40 during the production part of the cycle when nitrogen is being produced. The set pressure of regulator 18 is manually adjusted by turning a portion thereof to create a differential pressure between an outlet of the regulator 18 and an inlet of the regulator 20 so as to establish and control the flow rate through the system. The set pressure of the regulator 18 is adjusted so as to cause the regulator 18 to further restrict the flow of air into chamber 38 or 40 if it is determined by reading a gauge (not shown) of oxygen detector 46 that the percentage of oxygen in tank 44 is greater than a preselected limit. This allows chambers 38 and 40 to adsorb a higher percentage of oxygen from the incoming air which lowers the amount of oxygen which reaches tank 44. Conversely, the set pressure of 18 is adjusted so as to increase the flow of air into chamber 38 or 40 if it is determined that the percentage of oxygen in tank 44 is below a preselected limit. This results in the chambers adsorbing a lower percentage of oxygen from the incoming air, which increases the amount of oxygen reaching tank 44. It typically takes from one to one and a half hours after the set pressure of the regulator 18 is adjusted for the effect thereof to be fully felt. Further time consuming adjustments are necessary if the first adjustment does not bring about the desired result. The temperature of the incoming air and the temperature of the chambers affects the amount of oxygen adsorbed in the carbon structures or adsorbent of chambers 38 and 40. Thus, the percent of oxygen which is contained in the nitrogen product also varies with these parameters. It has been found that the commercially available system 10 requires frequent adjustments in the field to compensate for temperature variations which are undesirable. Referring now to FIG. 2, there is shown a Pressure Swing Adsorption (PSA) apparatus 100 which comprises an automatic control apparatus or controller shown within the dashed line rectangular figure 102 (denoted as control apparatus 102) which is coupled to a system which is essentially identical to system 10 of FIG. 1 except that regulators 18 and 20 have been replaced with valves 18a and 20a. Valves 18a and 20a have actuators 18al and 20al, respectively, which control the size of openings (not shown) in valves 18a and 20a, respectively. Control apparatus 102 comprises first, second and third pressure sensor and transmitters 104, 106 and 108 (also denoted as detecting means), first and second convertors (also denoted as convertor means) 110 and 112, and first, second and third comparator-controllers (also denoted as comparator means) 114, 116 and 118. Pressure sensor and transmitter 104 may be denoted as flow detecting means or as pressure detector means. An inlet of pressure sensor and transmitter 104 is coupled to a conduit which couples an outlet of valve 18a to an inlet of orifice plate 22. Pressure sensor and transmitter 104 senses (measures) the pressure in the conduit between valve 18a and orifice 22 (denoted as inlet air pressure). At an output thereof it generates a current signal which is proportional to the measured pressure. The output of pressure-sensor and transmitter 104 is coupled to an input of comparator-controller means 118. An output of comparator-controller means 118 is coupled to an input of convertor 110. An output of convertor 110 is coupled to an actuator of valve 18a. Convertor 110 is typically a current-to-pressure convertor which in response to a current signal received on the input thereof generates a force at the output thereof. A force applied to actuator 18al of valve 18a causes an opening (not shown) in valve 18a to enlarge or contract proportionally to the magnitude and direction of the force exerted thereon. Comparator 118 has a set point input s.p.1 which is coupled to an output of comparator-controller 114. As will become clear later herein, a signal received from comparator-controller 114 at input s.p.1 of 118 effectively adjusts the flow rate of air into chamber 38 or 40 by adjusting the size of the opening of valve 18a. The signal level at s.p.1 is the same as is generated at the output of comparator 114 and is representative of the required inlet air pressure. Comparator 118 compares this signal and the signal generated by pressure sensor and transmitter 104 (the measured pressure) and generates a signal at the output thereof which is proportional to the difference between the two input signals. Convertor means 110 causes valve 18a to contract the size of the opening therein so as to cause the pressure at the output thereof to drop to the s.p.1 pressure level if the measured pressure is greater than the s.p.1 pressure. This causes the air flow through valve 18a and into chamber 38 or 40 to decrease. The size of the opening in valve 18a enlarges so as to allow the pressure at the outlet of valve 18a to increase up to the s.p.1 level if the measured pressure is less than the s.p.1 pressure. This causes the air flow through valve 18a and into chamber 38 or 40 to increase. When the measured pressure is at the s.p.1 level the size of the opening through valve 18a is maintained. The pressure at the outlet of valve 18a is known to be proportional to the air flow into chamber 38 or 40. Accordingly, the adjusting of the pressure also adjusts the flow rate of air into one or the other of chambers 38 and 40. Over a useful range the flow rate of air into chamber 38 and 40 is proportional to the amount of impurity in the nitrogen product collected in tank 44. As will become clear later herein, if the amount of oxygen in the produced nitrogen is above a preselected level, then comparator-controller 114 generates an output signal thereof which causes the size of the opening in valve 18a to decrease so as to reduce the air flow rate into chamber 38 or 40. This in turn allows for greater adsorption of oxygen from the air that enters chamber 38 or 40. Accordingly, the nitrogen product which reaches tank 44 has a smaller amount of oxygen contained therein. Conversely, if the amount of oxygen in the product nitrogen is below the preselected level, then comparator-controller 114 generates an output signal which causes the size of the opening in valve 18a to increase so as to increase the air flow rate into chambers 38 and 40. The mathematical function performed by comparator-controller 118 is of the following form: M = G (r-c) + K, (Eq.1) where M is the output, G = the proportional gain, r = the second input signal to input port s.p.1 of comparator-controller 118 (the set pressure), c = the first input signal to comparator-controller 118 (the process variable being pressure), and K is the output level of comparator-controller 118 when r-c = O (i.e., there is no difference (error) between r and c). In a preferred embodiment comparator-controllers 114, 116 and 118 are all implemented by an Eagle Signal Model No. EPTAK 240 microprocessor based programmable controller using software provided therewith which includes Proportional-Integral and Derivative (P-I-D) control algorithms. An inlet of pressure sensor and transmitter 106 is coupled to a conduit which couples chambers 38 and 40 to an inlet of valve 20a. Pressure sensor and transmitter 106 senses (measures) the pressure in the conduit between chamber 38 or 40 and an inlet of valve 20a and generates at an output thereof a current signal which is proportional to the measured pressure. The output of the pressure sensor and transmitter 106 is coupled to a first input of comparator-controller means 116. An output of comparator means 116 is coupled to an input of convertor 112. An output of convertor 112 is coupled to actuator 20al of valve 20a. Convertor 112 is essentially identical to convertor 110 and causes an opening (not shown) in valve 20a to enlarge or contract proportionally to the magnitude and direction of the force exerted on actuator 20al. A second input s.p.2 of comparator means 116 is coupled to a fixed reference signal Vref2. Vref2 corresponds to a preselected pressure at which it is desired to operate chambers 38 and 40 during the production portion of the cycle of operation thereof. Comparator 116 compares the signal levels at the two inputs thereof and generates an output signal having a level proportional to the differences between the pressure measured at the input of valve 20a and the applied Vref2 at input s.p.2. Convertor means 112 causes the opening in valve 20a to enlarge so as to decrease the pressure if the measured pressure is greater than Vref2. Valve 20a contracts the opening therein so as to allow the pressure to increase if the measured pressure is less than Vref2. The combination of the pressure sensor and transmitter 106, and the convertors 110 and 112 acts to control the size of the opening through valve 20a so as to keep the pressure conditions at an essentially constant preselected level in chambers 38 and 40 during the production portion of the cycle of operation. The changes in pressure at the outlet of valve 18a caused by adjustments to the size of the opening therein are essentially dropped across orifice plate 22 such that the pressure 8720/MW in chamber 38 or 40 during the production portion of the cycle of operation is essentially constant and determined by Vref2 applied to 116. System 100 is designed such that there is a positive pressure drop across chambers 38 and 40 during the pressurization and production portions of the cycle of operation so as to establish continuous flow through the chambers. The mathematical function performed by comparator-controller 116 is in the following form: M = G(c-r) + K, (Eq. 2) where M, G, c, r and K are as defined in the equation previously given for the operation of comparator-controller 118. An output of oxygen detector 46 is coupled to a first input of comparator-controller 114. A second input s.p.3 of comparator-controller 114 is coupled to a reference signal Vref3 which is representative of the maximum level of oxygen to be allowed in the product. An output of comparator-controller 114 is coupled to input s.p.1 of comparator-controller 118. Oxygen detector 46 provides an output current signal whose level is proportional to the percentage of oxygen in the nitrogen received from tank 44. Vref3 is a signal which has a level proportional to the maximum desired level of oxygen which is to be tolerated in tank 44. Comparator-controller 114 compares the percentage of oxygen measured (the output signal of impurity detector 46) to Vref3, the maximum desired, and generates an output signal at the output thereof which is inversely proportional to the difference between the two. If the level of oxygen in tank 44 is above the Vref3, the output signal of comparator-controller 114 is lower than if the oxygen level is at or below that desired. This output signal becomes the input signal at input s.p.1 of comparator-controller 118. This causes comparator-controller 118 to reduce the output signal thereof which causes convertor 110 to adjust the force applied to actuator 18al of valve 18a to cause the opening in valve 18a to contract. This causes a reduction in the flow rate of air into chambers 38 and 40 during the production portion of the cycle of operation which effectively reduces the amount of oxygen which reaches tank 44. This keeps the nitrogen in tank 44 at the desired purity. Comparator-controller 114 causes the input signal to s.p.1 to increase so as to increase the flow rate of incoming air if the level of oxygen in tank 44 is significantly below the maximum desired level (Vref3). This also increases the flow rate of produced nitrogen while maintaining the oxygen content at a preselected level. The mathematical function performed by comparator- controller 114 is of the following form: M = G[(r-c) + 1/I (r-c)dt + Dd(r-c)/dt] +K, (Eq. 3) where M = the output of comparator-controller 114, G = the proportional gain, c = the second input signal to comparator-controller 114 (the process variable - the impurity percentage of oxygen), r = the third input signal to input s.p.3 of 114 (Vref3), I = an integration constant, D = the derivative or rate constant, and K is a constant which is the output if r-c = 0 (there is no difference (error) between r and c). A user of apparatus 100 may have a decreased need for nitrogen and therefore may significantly close valve 48. This significantly reduces the output flow of nitrogen and also significantly limits the flow of air into chambers 38 and 40. Chambers 38 and 40 are, therefore, able to adsorb a high percentage of the oxygen from the incoming air which reduces the content of oxygen in the product nitrogen. Oxygen sensor transmitter 46 therefore provides a decreased current signal to comparator-controller 114 which generates an increased output signal to comparator-controller 118 which in turn generates an increased output signal to convertor 110 that causes an increase in the opening of valve 18a. This would provide a means for allowing a greater flow of air into chambers 38 and 40 in an attempt to increase the oxygen content to the value of s.p.3. The air flow into chambers 38 and 40, however, is limited by the opening in valve 48 which is set in accordance with the user's nitrogen requirement. Comparator-controller 114 will continue to act to increase the size of the opening in valve 18a until it is fully open. If the user then requires more nitrogen and opens up valve 48, the chambers would become flooded with air and the percentage of impurity of oxygen would rise rapidly. Comparator-controller 114 could not respond rapidly enough and the system would automatically shut down. To prevent this from occurring, tank 44 is monitored for a build up in pressure which would occur if valve 48 was significantly closed. This pressure build-up is transmitted via pressure sensor and transmitter 108 to comparator-controller 114 which is adapted upon receiving such a signal to cause the s.p.3 input signal level to assume the level existing at the output of 46 such that a zero error is generated within comparator-controller 114. This freezes the output signal level of comparator-controller 114 in accordance with the mathematical control algorithm of Eq. 3. This effectively maintains s.p.1 of comparator-controller 118 at the value it was at prior to valve 48 being significantly closed. When valve 48 is significantly reopened, the inlet air flow resumes at the rate that existed prior to valve 48 being closed. Thus, the purity of the nitrogen flowing into tank 44 returns to the previous level. There may be situations in which the user has only a minimal reduction in the need for nitrogen and only slightly closes valve 48. The oxygen content in the product will decrease slightly, while the pressure in tank 44 increases causing pressure sensor and transmitter 108 to deactivate comparator-controller 114. During this reduced requirement period, ambient temperature changes may cause the impurity content of the nitrogen to increase to a level greater than the set point Vref3. In such a case, comparator-controller 114 is adapted to ignore the deactivation signal from pressure sensor and transmitter 108 and to continue responding to the signal from impurity detector 46 to bring the oxygen content to the preselected level. Comparator-controller 114 is designed to allow operation of system 100 in a manual or in an automatic mode of operation via a selector switch. Comparator-controller 114 is deactivated in a manner similar to that when nitrogen demand is reduced. In the manual mode of operation, an operator selects and applies a signal level to the s.p.1 input of comnparator controller 118. Comparator-controller 114 is designed to allow the applied level to input s.p.1 of comparator-controller 118 to remain and thus to control comparator-controller 118. Manual operation is useful during start up of apparatus 100 or during testing of system 100. During normal operation, apparatus 100 is operated in the automatic mode. Comparator-controller 114 is also designed to provide a bumpless transfer when apparatus 100 is switched from an automatic mode to a manual mode. When apparatus 100 is in an automatic mode of operation, the signal level into input s.p.1 of comparator-controller 118 is controlled by comparator-controller 114. When apparatus 100 is in the manual mode of operation, the signal level into input port s.p.1 of comparator-controller 118 is set manually and comparator-controller 114 is adapted so as to not override this manual setting. When the apparatus 100 is switched from manual to automatic, comparator-controller 114 causes the previously manually applied value to input port s.p.1 of comparator-controller 118 to be maintained for a short period of time so as to ensure a bumpless transfer from one mode of operation to another. Referring now to FIG. 3 there is shown a Pressure Swing Adsorption (PSA) apparatus 1000 which includes essentially all of the components of apparatus 100 of FIG. 2 and, in addition, comprises within dashed line rectangle 1002 (denoted as control apparatus or controller 1002) a pressurization time measuring device (means) 1004 and a comparator-controller 1006. Components 1004 and 1006 permit apparatus 1000 to control the air flow into chamber 38 or 40 during the pressurization portion of the cycle of operation so as to control the time needed to cause either of chambers 38 or 40 to reach a pressure of Vref2. It has been found that it is desirable to control the time in which chamber 38 or 40 is pressurized to a preselected pressure to insure that there is sufficient residence time in the chamber to allow adequate oxygen adsorption before product gas is released. Pressurization time measuring device 1004 is coupled by a first input thereof to an output of pressure sensor-transmitter 106, is coupled by a second input to Vref2 and is coupled by an output thereof to a first input of comparator-controller 1006. A second input s.p.4 of comparator-controller 1006 is coupled to a reference signal Vref4 which has a level proportional to the preselected desired time for pressurizing chamber 38 or 40. Pressurization time measuring device 1004 measures the time it takes for the sensed pressure at the inlet of valve 20a to reach Vref2. An output signal is generated at the output of pressurisation time measuring device 1004 which has a level proportional to the time measured for the pressure in chamber 38 or 40 to reach the level of Vref2. Comparator-controller 1006 compares the output signal from pressurisation time measuring device 1004 with Vref4 and generates an output signal at the output thereof which is proportional to the difference between the two. This output signal is a current signal which controls convertor 110. Convertor 110 adjusts the size of the opening through valve 18a at the beginning of pressurization such that, during the pressurization portion of the next cycle of operation, the rate of flow of air into chamber 38 or 40 is adjusted so as to pressurize the chamber within the desired time period Vref4. Comparator-controller 118 is adapted to be decoupled from convertor 110 during the pressurization and equalization portions of the cycle of operation of apparatus 1000. Comparator-controller 1006 is adapted to be decoupled from convertor 110 during the production portion of the cycle of operation of apparatus 1000. The mathematical function performed by comparator- controller 1004 is of the following form: M = G (r-c) + K, (Eq. 4) where M is the output, G = the proportional gain, r = the first input signal level to pressurisation time measuring device 1004 (the process variable- time), c = the second input signal level to the s.p.4 input of pressurisation time measuring device 1004 (the Vref4 level), and K is the output level of pressurisation time measuring device 1004 when r-c = 0 (i.e., there is no difference (error) between r and c). In the preferred embodiment, pressurisation time measuring device 1004 and comparator-controller 1006 are implement by an Eagle Signal Model No. EPTAK 240 microprocessor using software provided therewith. The memory of the microprocessor contains instructions in coded form to provide the sequence of control signals described herein. Referring now to the flow diagram of FIG. 4, there is shown a basic operation of apparatus 1000 of FIG. 3. In block 200 a decision is made as to whether a start button associated with apparatus 1000 has been engaged (pushed in) to start apparatus 1000. If the start button has been engaged, then all input and output signal levels of impurity detector 46, pressure sensor and transmitters 104, 106, and 108, convertors 110 and 112, comparator-controllers 114, 116 and 118, pressurisation time measuring device 1004 and comparator-controller 1006 are scanned and recorded as indicated in block 202. As denoted in block 204, a determination is then made if the impurity level of oxygen in the nitrogen exiting tank 44 is less than a preselected shut down value of apparatus 1000. If the impurity level is not below the shut down value, then the output (decision) of block 204 is NO. In block 206 a decision is made as to whether a bypass switch has been activated so as to ignore the greater than shut down level of the oxygen. If the system has not been in operation for a period of time, then there may be substantial amount of oxygen in tank 44. An operator knowing this will generally elect to push in the bypass button (i.e. a YES decision) and allow the system to run until a steady state condition is achieved. If the by pass button is not activated (i.e. a NO decision), then apparatus 1000 stops operating as is indicated by block 208. If the answer to the determination in block 204 is YES, then block 210 determines if an automatic preselected time delay has been completed such that apparatus 1000 has reached a steady state operation. If the answer is NO, then block 214 sets apparatus 1000 into the manual mode of operation. Block 215 then sets the value of a signal applied to input s.p.3 of comparator-controller 114 to the value of the signal existing at the output of impurity detector 46 which was recorded in block 202. If the answer of the determination of block 210 is YES, then block 212 determines whether or not an automatic/manual switch has been switched to automatic. If the answer is NO, then block 214 sets apparatus 1000 to the manual mode of operation. If the answer is YES, then block 216 determines if the measured pressured in buffer tank 44 is below a high preselected pressure limit. If the answer is YES, then block 220 sets apparatus 1000 to an automatic mode of operation and then block 222 sets the signal level applied to s.p.3 to a preselected level (a desired value). If the answer is NO, then block 218 determines if the impurity level of oxygen in the nitrogen exiting tank 44 is greater than a preselected limit (s.p.3). If the answer is NO, then block 214 sets system 1000 to the manual mode of operation. If the answer is YES, then block 220 sets apparatus 1000 to the automatic code of operation. If apparatus 1000 has been set to automatic, then the value set by block 222 is entered into block 224 and the value of the signal at s.p.3 generated by block 224 is the same as the value set by block 222. If apparatus 1000 has been set to manual, then the value of the signal at s.p.3 set by block 215 is entered into block 224 and the value of the signal at s.p.3 generated by block 224 is the value set by block 215. Block 226 receives an input from block 228 which provides the value of the oxygen impurity level in tank 44 and receives the s.p.3 signal level from block 224. Block 226 generates an output signal level which is proportional to the difference between the two input signals thereto and effectively performs the mathematical function of Eq. 3. An output signal from block 226 is coupled to blocks 230 and 234. Block 230 determines if apparatus 1000 is on manual control. If the answer is YES, then block 232 subtracts the output level received from a level existing at input s.p.1 of a block 242 and couples the calculated value to blocks 233 and 234. If the apparatus is on manual control, then the value of the signals from blocks 232 and 233 is identical. If the apparatus is on automatic control, then the last value of the output signal of block 232 is stored in block 233 and is sent by block 233 to block 234. If the decision of block 230 is NO, then the value which existed at block 232 just before the automatic mode was selected is coupled to block 234. Block 234 adds the input levels received from blocks 226 and 232 or 233 and couples the calculated output signal thereof to block 236. Block 236 determines if apparatus 1000 is in the automatic mode of operation. If the answer is YES, then a signal is sent to block 246. If the answer is NO, then a signal is sent to block 238. Blocks 230, 232, 233 and 234 cooperate to insure that there is a bumpless transfer when apparatus 1000 is switched from a manual mode of operation to an automatic mode of operation. This means that the signal level existing at the output of block 234 at the beginning of an automatic mode of operation is the same as existed at the end of a manual mode of operation. This, in turn, keeps the input level to input s.p.1 of block 242 at the same level during the start of automatic operation as it was during the end of manual operation. Block 238, when activated by a signal from block 236, sends out a signal to block 240 which is the last output signal of block 234 before apparatus 1000 left the automatic mode of operation. Block 244 decides if an operator has entered a value for s.p.1. If the answer is YES, then the entered value is coupled to block 240. Block 240 couples a signal received at either of its two inputs to an s.p.1 input of block 242. If input signals appear on both of the inputs to block 240, block 240 selects the input from block 244 and transmits it to the s.p.1 input of block 242. If block 246 receives a signal whose level is above a preselected level (a YES decision), then it transmits the received level. If block 246 receives a signal whose level is below a preselected level, then it does not transmit that level (a NO decision) and a default level is transmitted by block 245 to the s.p.1 input of block 242. This prevents gas flow through the chamber from being completely stopped. Block 246 transmits the received signal level from block 236 to block 242 if the received signal level from 236 is above the preselected level. Only one of output signals of blocks 240 and 246 is transmitted to block 242 at a given time. If apparatus 1000 is in the automatic mode of operation, then the signal to the s.p.1 input of block 242 comes from block 246. If apparatus 1000 is in the manual mode of operation, then the signal to the s.p.1 input of block 242 comes from block 240. Block 242 receives an input signal representing the inlet pressure of chamber 38 or 40 which is measured and transmitted by pressure sensor transmitter 104 from block 248. An output from block 242 is coupled to inputs of blocks 250 and 256. Block 242 effectively performs the mathematical function of equation Eq. 1. The output of block 204 is also coupled to block 274 which decides if the equalization portion of the cycle of operation of apparatus 1000 is complete. If the answer is YES, then a signal is sent to block 268 from block 274. Block 268 receives the measured value of the pressure in chamber 38 or 40 at one input from block 270 and receives a preselected applied reference signal pressure level Vref2 from block 272 at a second input thereof. Block 268, which performs the function of device 1004 of FIG. 3, measures the time it takes for the pressure in chamber 38 or 40 to go from that value existing at the end of the equalization portion of the cycle of operation until the end of the pressurization part of the cycle of operation when valve 20a opens. Block 264 receives at one input thereof the time measurement output signal of block 268 and receives at a second input s.p.4 thereof a preselected desired time Vref4 for pressurization of chamber 38 or 40 from block 266. The basic mathematical function of comparator-controller 1006 of FIG. 3, as shown in equation Eq. 4, is performed by block 264. Block 262 receives an output signal from block 264 and transmits that signal to block 258. If the signal received by block 262 is within preselected limits (a YES decision), then the output of block 262 is coupled directly to a first input of block 258. If the signal received by block 262 is not within preselected limits (a NO decision), then the output of block 262 is coupled to block 257 which in turn sends a default signal level to block 258. Block 258 determines if the pressurization portion of the cycle of operation is complete. If the answer is NO, then an output signal of block 258 is transmitted to blocks 254 and 256 (convertor 110). If the answer is YES, then the output of block 258 is coupled to block 255. Block 256 subtracts the output of block 258 from the output of block 242 and transmits the result to block 255. Block 255 stores the current value from the output of block 256 until the end of the pressurization portion of the cycle of operation. At the end of the pressurization portion of the cycle, block 258 allows the value of block 255, which is the last value of block 256 before pressurization was completed, to be added to the value of 242 in block 250 and then the resultant is transmitted from block 250 to block 254. At this time, block 258 inhibits the signal from 258 to 254. Blocks 250, 255 and 256 cooperate such that there is a bumpless transfer of control of block 254 from the end of the pressurization portion of the cycle of operation to the beginning of the production portion of the cycle of operation. The YES output of block 204 is also coupled to inputs of blocks 276 and 278. Block 276 represents a plurality of blocks which result in all the valves, except for valves 18a, 20a and 48, being opened and closed at the needed appropriate times during a cycle of operation of system 1000. Block 278 represents blocks which would illustrate the function of pressure sensor and transmitter 106, convertor 112, and comparator-controller 116 which was already explained as part of the above discussion of FIG. 2. Block 278 essentially performs the mathematical function of equation Eq. 2. It is to be understood that the embodiments described herein are merely illustrative of the general principles of the invention. Various modifications are possible within the general principles of the invention. For example, the comparator-controllers could be formed from hardware components or could be software and an other computer than the one denoted. Further, valve 20a and the pressure sensor and comparator-controller associated therewith could be replaced by a mechanical regulator like valve 20 of FIG. 1. Still further, the sensing and current generating components could be modified to provide output voltage signals instead of output current signals. Still further, the control systems which control the air flow during pressurization and production could be used separately. Furthermore, a plurality of chambers, which are greater in number than two, could be used. Still further, the oxygen vented into the atmosphere could be collected as a product. Furthermore, systems 100 and 1000 can be modified so as to control the input air flow into the chambers by varying an opening in a valve which replaces valve 48 instead of varying the opening in valve 18a. In this case, a regulator like regulator 18 of FIG. 1 or the combination of a valve, convertor means, pressure sensing means and a comparator-controller with fixed set input signal level could be substituted for valve 18a. Valve 20a and the control components associated therewith could be eliminated and valve 18a, pressure sensor 104, convertor means 110 and convertor-controller 118 could be substituted for valve 48, with pressure sensor and transmitter 104 being coupled to the inlet side of 18a instead of to the outlet side (as is shown in FIGs. 2 and 3). Still further, apparatuses 100 and 1000 can be modified such that the amount of impurity to be tolerated is within a preselected range rather than just a single level.

Claims: Gas fractionation apparatus (100, 1000) having a valve (18a) with an actuator (18a1) which controls the size of an opening therein, which valve (18a) is in communication with at least one chamber (38,40) containing adsorbent which is adapted preferentially to adsorb one or more components of gas entering therein, and also having a control apparatus (102) comprising: convertor means (110) which has an input and an output with the output being coupled to the actuator (18a1) of the valve (18a), for controlling the size of the opening in the valve (18a); flow detector means (104) in communication with the downstream side of the valve (18a) for generating at an output thereof a signal that is proportional to the rate of gas flowing out of the valve (18a); an impurity detector (46) coupled to an outlet of the chamber (38,40), said impurity detector (46) being adapted to generate at an output thereof a signal that is proportional to the amount of an impurity component of gas in a product component of the gas exiting the chamber (38,40); a first comparator means (114) which has a first input which is adapted to be coupled to a first reference signal (V ref 3) indicative of the amount of impurity that is to be tolerated in the product component of gas received from the chamber (38,40) and which has a second input coupled to the output of the impurity detector (46), the first comparator means (114) being able to generate at an output thereof a signal which is proportional to the difference between the reference signal (V ref 3) and the output signal of the impurity detector (46); and a second comparator means (118) which has a first input coupled to the output of the flow detector means (104), a second input coupled to the output of the first comparator means (114) and has an output coupled to the input of the convertor means (110), the second comparator means (118) being able to compare signals received from the first comparator means (114) and the flow detector means (104) and to generate at the output thereof a signal which causes the convertor means (110) to generate a signal causing the actuator (18a1) of the valve (18a) to adjust the size of the opening in the valve (18a) so as to maintain the purity of gas exiting the chamber (38,40) at a preselected level. Gas fractionation apparatus (100,1000) according to claim 1, wherein the control apparatus (102) further comprises: pressure sensing means (106), pressurisation time measuring means (1004), and a third comparator means (1006); the pressure sensing means (106) being coupled to the chamber (38,40) for sensing pressure within the chamber (38,40); the pressurisation time measuring means (1004) being coupled by a first input to the pressure sensing means (106), being adapted to be coupled by a second input thereof to a preselected pressure level, and being able to generate an output signal proportional to the amount of time measured for the chamber (38,40) to pressurise from a pressure reading determined by the pressure sensing means (106) to a preselected level; and the third comparator means (1006) being coupled by a first input thereof to an output of the pressurisation time measuring means (1004), being also coupled by an output thereof to the control of the convertor means (110), and being adapted to be coupled by a second input thereof to a source of a reference signal (V ref 4) which is proportional to a preselected time period in which it is desired that the chamber (38,40) reach a preselected pressure level, the third comparator means (1006) being able to compare the output signal of the pressurisation time measuring means (1004) with the reference signal (V ref 4) coupled to the second input thereof and to generate at the output thereof a signal which causes the convertor means (110) to generate a signal at the output thereof causing the actuator (18a1) of the valve (18a) to adjust the size of the opening in the valve and thus the rate of flow of gas into the chamber (38,40) such that the chamber (38,40) is pressurised to a preselected level during a preselected time period. Gas fractionation apparatus (100,1000) according to claim 1 or claim 2, being adapted to separate air by pressure swing adsorption and to perform a cycle of operation which includes pressurisation and production portions; and including first decoupling means coupled between the second comparator means (118) and the input of the convertor means (110) for decoupling the output of the second comparator means (118) from the input of the convertor means (110) during a production portion of the cycle of operation. Gas fractionation apparatus (100,1000) according to claim 3, wherein there are two adsorption chambers (38,40) and the cycle of operation includes an equalisation portion to equalise the pressures between the chambers (38,40), and additionally including: second decoupling means coupled between the third comparator (1006) and the input of the convertor means (110) for de-coupling the output of the third comparator means (1006) from the input of the convertor means (110) during the equalisation and production portions of the cycle of operation. Gas fractionation apparatus (100,1000) according to claim 1, wherein the control apparatus (102) additionally includes a pressure sensor (106) which has such a position that it is able to sense the pressure in the chamber (38 or 40) and generate a signal representative of such pressure, a fourth comparator (116) which is able to compare said signal with a reference signal (V Ref 2) and which is able to generate an outlet that causes a converter (112) to generate an output signal effective to operate an actuator (20a1) of a valve (20a) so as to adjust the setting of the valve (20a) in such a way as to tend to eliminate any divergence of the actual adsorption pressure from that represented by the reference signal (V Ref 2). Gas fractionation apparatus (100,1000) according to claim 1 or claim 5, additionally including a buffer tank (44) connected so as to receive product gas from the chambers (38,40), wherein the control apparatus (102) additionally includes a pressure sensor (108) positioned so as to sense the pressure in the tank (44) and able to transmit to the first comparator (114) a signal representative of the pressure in the tank (44).

Inventors: GUNDERSON, JEFF
Applicants: THE BOC GROUP, INC.
Application year: 1987
Main classification: B01D53
IPC codes: B01D53