JP7513374B2 - Method for operating pretreatment facility of cryogenic air separation unit - Google Patents

Description

The present invention relates to a method for operating a pretreatment facility of a cryogenic air separation unit, and more specifically, to a method for operating a pretreatment facility of a cryogenic air separation unit that purifies the feed air by adsorbing and removing impurities, including moisture and carbon dioxide, from the feed air supplied to the cryogenic air separation unit using a temperature swing adsorption method.

In the front stage of the cryogenic section of the air separation plant, a pretreatment facility using an adsorption device is installed that switches between multiple adsorption towers in sequence between the adsorption process and the regeneration process, and the adsorbent filled in the adsorption tower adsorbs and removes impurities such as moisture, carbon dioxide, hydrocarbons, and nitrogen oxides from the feed air to purify the feed air. As a method of operating the adsorption device in this pretreatment facility, the temperature of the feed air flowing into the adsorption tower in the adsorption process is measured, and the switching time between the adsorption process and the regeneration process is changed according to the measured feed air temperature (see, for example, Patent Document 1). In addition, the switching time is also changed based on the accumulated amount of carbon dioxide in the feed air introduced into the adsorption tower (see, for example, Patent Document 2).

JP 2006-258302 A JP 2014-113594 A

However, in conventional methods, the switching time is changed based on conditions such as the temperature of the feed air and the accumulated amount of carbon dioxide during one adsorption step. For example, if the switching time is lengthened based on the previous condition and a long regeneration step is being performed, and an external factor causes an increase in the temperature of the feed air or the amount of carbon dioxide to increase, in order to reliably remove moisture and carbon dioxide in the adsorption tower during the adsorption step, it is necessary to shorten the switching time and the adsorption step time, which may result in insufficient time for the regeneration step. In addition, it may be difficult to respond if the amount of feed air changes significantly and suddenly due to fluctuations in the amount of product gas used.

The present invention aims to provide a method for operating a pretreatment facility of an air cryogenic separation unit that can optimize the operation of the pretreatment facility even if the feed air conditions change.

In order to achieve the above object, the present invention provides a method for operating a pretreatment facility of a cryogenic air separation unit, which, as a first configuration, uses a purifier in which a plurality of adsorption towers filled with an adsorbent are sequentially switched between an adsorption step and a regeneration step by a temperature swing adsorption method to remove at least moisture and carbon dioxide from feed air and purify the feed air to be introduced into a cryogenic air separation unit, the method comprising: dividing the adsorption step and the regeneration step into a plurality of operation steps based on a fixed time interval; and calculating a moving average value for each step from the flow rate, temperature and pressure of the feed air at the inlet of the purifier measured at preset intervals during the previous operation step, and when the moving average value is increased compared to the value adopted in the previous step, the time for the next step is shortened and the flow rate of the regeneration gas in the adsorption tower is increased based on the rate of increase relative to the previous step, and conversely, when the moving average value is decreased compared to the value adopted in the previous step, the time for the next step is extended and the flow rate of the regeneration gas in the adsorption tower is decreased based on the rate of decrease. Thus, among the lengths of the operation steps estimated from each , the shortest length of the operation step is set as the length of the next operation step, and, among the flow rates of the regeneration gas estimated from each , the highest flow rate of the regeneration gas is set as the flow rate of the regeneration gas for the next operation step, so that the product of the flow rate of the regeneration gas and the length of the operation step is constant .

In addition, the second configuration of the present invention is characterized in that the adsorption step and the regeneration step are divided into a plurality of operation steps based on a certain time interval, and the length of one operation step and the flow rate of the regeneration gas are calculated as moving average values for each step from the flow rate, temperature, pressure and carbon dioxide concentration of the raw air at the inlet of the purifier measured at preset intervals during the previous operation step, and when the flow rate is higher than the value used in the previous step, the time of the next step is shortened and the flow rate of the regeneration gas in the adsorption tower is increased based on the rate of increase relative to the previous step, and conversely, when the flow rate is lower than the value used in the previous step, the time of the next step is extended and the flow rate of the regeneration gas in the adsorption tower is decreased based on the rate of decrease. Among the lengths of the operation steps estimated from each , the shortest length of the operation step is set as the length of the next operation step, and among the flow rates of the regeneration gas estimated from each , the largest flow rate of the regeneration gas is set as the flow rate of the regeneration gas in the next operation step, so that the product of the flow rate of the regeneration gas and the length of the operation step is constant .

Furthermore, in a third aspect of the present invention, the adsorption step and the regeneration step are divided into a plurality of operation steps based on a fixed time interval, and a moving average value for the length of one operation step and the flow rate of the regeneration gas in each step is calculated from the flow rate, temperature, pressure, carbon dioxide concentration, hydrocarbon concentration, and nitrous oxide concentration of the raw air at the inlet of the purifier measured at preset intervals during the previous operation step, respectively. When the flow rate is higher than the value used in the previous step, the time of the next step is shortened and the flow rate of the regeneration gas in the adsorption tower is increased based on the rate of increase relative to the previous step, and conversely, when the flow rate is lower than the value used in the previous step, the time of the next step is extended and the flow rate of the regeneration gas in the adsorption tower is decreased based on the rate of decrease. In this way, among the lengths of the operation steps estimated from each , the shortest length of the operation step is set as the length of the next operation step, and among the flow rates of the regeneration gas estimated from each , the largest flow rate of the regeneration gas is set as the flow rate of the regeneration gas in the next operation step, so that the product of the flow rate of the regeneration gas and the length of the operation step is constant .

In addition, in a fourth aspect of the present invention, the adsorption step and the regeneration step are divided into a plurality of operation steps based on a certain time interval, and a moving average value for each operation step is calculated from at least one of the flow rate of the raw air at the inlet of the purifier, the temperature, the pressure, the carbon dioxide concentration, the hydrocarbon and the nitrous oxide measured at a preset interval during the previous operation step, and when the flow rate is higher than the value used in the previous step, the time for the next step is shortened and the flow rate of the regeneration gas in the adsorption tower is increased based on the increase rate relative to the previous step, and conversely, when the flow rate is lower than the value used in the previous step, the time for the next step is extended and the flow rate of the regeneration gas in the adsorption tower is decreased based on the decrease rate. In this manner, among the lengths of the operation steps estimated from each , the shortest length of the operation step is set as the length of the next operation step, and among the flow rates of the regeneration gas estimated from each , the largest flow rate of the regeneration gas is set as the flow rate of the regeneration gas in the next operation step, so that the product of the flow rate of the regeneration gas and the length of the operation step is constant .

According to the method for operating the pretreatment equipment of the cryogenic air separation unit of the present invention, the operating state of the purifier can be accurately controlled according to the conditions of the feed air introduced into the purifier, so that the purification of the feed air can be performed reliably and the operating costs can be reduced.

1 is a system diagram showing an example of a cryogenic air separation unit equipped with a pretreatment facility to which the method of the present invention can be applied. FIG. 2 is an explanatory diagram showing an example of a basic driving cycle. FIG. 2 is an explanatory diagram for explaining the operation control procedure of the method of the present invention. FIG. 13 is a diagram showing an example of a change in purifier load.

Figures 1 to 4 are used to explain the method of operating the pretreatment equipment of the cryogenic air separation unit in the present invention, and Figure 1 is a system diagram showing an example of a cryogenic air separation unit equipped with a pretreatment equipment to which the method of the present invention can be applied. The cryogenic air separation unit shown in Figure 1 is equipped with a pretreatment equipment 11 for compressing and purifying the feed air, and a cryogenic section 12 for cooling the feed air to separate it into liquid form.

In the pretreatment equipment 11, raw air from which dust has been removed by the air filter 13 is compressed to a predetermined pressure by the compressor 14, and the heat of compression is removed by the pre-cooling equipment 15. After that, impurities in the raw air, such as moisture, carbon dioxide, hydrocarbons, and nitrous oxide, are removed by the purifier 16. The purifier 16 uses an adsorption device that employs a temperature swing adsorption method in which multiple adsorption towers 17 filled with an adsorbent capable of removing the impurities are sequentially switched between an adsorption process and a regeneration process to continuously purify the raw air. This adsorption device is provided with multiple switching valves on one end and the other end of the adsorption towers 17, and each adsorption tower is switched between an adsorption process and a regeneration process by switching these switching valves open and close in a predetermined order.

The adsorption device is also provided with a regeneration gas supply unit 18 for switching between supplying heated gas and cooled gas to the adsorption tower undergoing the regeneration process. During the heating stage of the regeneration process, the low-temperature exhaust gas drawn out from the deep-cooling section is heated to a predetermined temperature by the heater 18a and introduced into the adsorption tower undergoing the heating stage, and during the cooling stage after the heating stage, the valves are switched open and closed so that the low-temperature exhaust gas is introduced through the bypass path 18b at a low temperature into the adsorption tower undergoing the cooling stage.

Figure 2 shows an example of a basic operating cycle. In this example, the switching time between the adsorption process and the regeneration process is set to 240 minutes to form one cycle. The regeneration process is divided into a 10-minute depressurization stage in which the pressure inside the tower, which has been pressurized by introducing compressed air, is lowered to atmospheric pressure; a 90-minute heating stage in which heated regeneration gas heated by heater 18a is circulated inside the tower to heat and regenerate the adsorbent; a 116-minute cooling stage in which low-temperature regeneration gas is circulated inside the tower to cool the adsorbent to a temperature suitable for the adsorption process; and a 24-minute pressurization stage in which a portion of the purified feed air is introduced into the tower to raise the pressure inside the tower to the adsorption process pressure.

In the method of the present invention, the one cycle is divided into a number of operating steps (hereinafter referred to as steps), and each step is controlled based on the information obtained. For example, if the basic time for one step is 60 seconds, taking into account the responsiveness when the flow rate is changed, one cycle is divided into 240 steps for the adsorption process, and 10 steps for the depressurization step, 90 steps for the heating step, and 116 steps for the cooling step in the regeneration process.

In addition, the information obtained at each step includes the flow rate, temperature, and pressure of the raw air at the inlet of the purifier 16, as well as impurity components that can be adsorbed and removed by the adsorbent, such as carbon dioxide concentration, hydrocarbon concentration, and nitrous oxide concentration. The amount of moisture in the raw air can be easily calculated based on the flow rate, temperature, and pressure of the raw air, assuming that the raw air extracted from the pre-cooling equipment 15 contains a saturated amount of moisture. This information can be easily obtained by placing a flow meter, thermometer, pressure gauge, and various analyzers corresponding to each piece of information at the inlet of the purifier 16.

Furthermore, the value of each piece of information is calculated as a moving average of measurements taken at preset intervals, for example, 1 second intervals, 5 second intervals, etc. For example, when measurements are taken at 5 second intervals and one step is 60 seconds, the moving average value obtained by dividing the sum of 12 (60/5) measurements taken in 60 seconds by 12 is adopted as the value to be used for control. Then, the moving average is calculated including the last measurement value in each step, and if the value adopted from this is higher than the value adopted in the previous step, the time of the next step is shortened based on the increase rate relative to the previous step and the regeneration gas in the adsorption tower is increased, and conversely, if it is lower than the value adopted in the previous step, the time of the next step is extended based on the decrease rate and the regeneration gas is decreased. Also, even when one moving average value increases and the other moving average value decreases, the conditions for the next step can be calculated by performing similar calculation processing.

For example, as shown in FIG. 3, the step time and regeneration gas flow rate for the nth step (step n) in one cycle are determined based on a moving average including the final measurement result for the previous step, i.e., step n-1, and the step time and regeneration gas flow rate for the next step, step n+1, are determined by a moving average including the final measurement result for step n.

Here, as shown in FIG. 4, when operating under the design conditions where the feed air flow rate at the time of step switching is 10,000 Nm ³ /h, the time for one step is 60 seconds, and the regeneration gas flow rate is 1,600 Nm ³ /h, if the feed air flow rate gradually decreases after step switching and the purifier load decreases, the feed air flow rate measured at 5-second intervals gradually changes, and the moving average of 12 times including the flow rate measured just before the end of step n-1 becomes 9,873 Nm ³ /h. If it is determined that the flow rate has decreased from 10,000 Nm ³ /h, that is, the purifier load has decreased with the decrease in the amount of feed air, the regeneration gas flow rate is decreased from 1,600 Nm ³ /h to 1,580 Nm ³ /h (1,600 * (9,873/10,000)), and the time of the next step n is extended from 60 seconds to 61 seconds ((60/9,873) * 10,000).

Similarly, for step n+1, the regeneration gas flow rate and the time for step n+1 are set based on the moving average of the flow rate measured immediately before the end of step n. For example, when the moving average of the flow rate becomes 9550 ^Nm3 /h, the regeneration gas flow rate is reduced from 1580 ^Nm3 /h to 1528 ^Nm3 /h (1580*(9550/9873)), and the time for the next step n+1 is extended from 61 seconds to 63 seconds ((61/9550)*9873). On the other hand, when the flow rate of the feed air gradually increases in the latter half and the purifier load increases, the value obtained by the moving average becomes larger than 1, so that the regeneration gas flow rate in the next step increases and the step time is shortened.

In this way, by changing the length of the current step and the flow rate of the regeneration gas according to the change in the moving average calculated in the previous step, the purifier 16 can be operated under optimal conditions according to the load, and the amount of impurities adsorbed by the adsorbent in each adsorption process can be controlled to a substantially constant amount, ensuring the removal of impurities in the raw air, and regeneration of the adsorbent can also be reliably performed with a substantially constant amount of heated regeneration gas. In particular, since the values for changing the step length and the flow rate of the regeneration gas are calculated using a moving average, variations and errors in the measured values can be absorbed compared to when the measured values are used directly for control, and, for example, the valve for changing the flow rate of the regeneration gas does not operate frequently.

Under normal atmospheric conditions, the concentration of each impurity component in the feed air hardly changes, and the pressure and temperature of the feed air also hardly change, so sufficient control can be achieved simply by measuring the change in the flow rate of the feed air that accompanies fluctuations in the amount of product gas used that is separated in the cryogenic section 12. This is particularly effective when there are large fluctuations in demand for product gas, such as in air liquefaction separation units for steelworks.

In addition, when there are many factories in the vicinity and there is a risk of the carbon dioxide concentration increasing or decreasing, by adding the carbon dioxide concentration in the raw air as control information, it is possible to reliably remove the carbon dioxide with the adsorbent even when the carbon dioxide concentration increases, and also to reliably regenerate the adsorbent that has adsorbed the carbon dioxide. Furthermore, when there is a chemical complex or steelworks in the vicinity, or when the air separation unit is located on the premises of a chemical complex or steelworks, it is possible to operate more reliably by adding the hydrocarbons and nitrous oxide that can be removed with the adsorbent as control information. In addition, since the switching interval of the adsorption tower is longer in normal operation compared to the design conditions assuming maximum load, for example, the switching operation can be reduced from six cycles per day to five cycles or less, thereby reducing the energy required to regenerate the adsorption tower.

In the actual operation of an air liquefaction separation unit, even if the feed air flow rate is reduced due to a reduction in the required product flow rate, and as a result it becomes necessary to reduce the regeneration gas flow rate in the next step and lengthen the step time, the carbon dioxide concentration in the feed air at the inlet of the purifier 16 temporarily increases, and this increase in carbon dioxide concentration may require an increase in the regeneration gas flow rate in the next step and a shortening of the step time.

In other words, conditions such as the flow rate, temperature, pressure, carbon dioxide concentration, hydrocarbon concentration, or nitrous oxide concentration of the raw air at the inlet of the purifier are basically independent elements that affect the step length and the flow rate of the regeneration gas. In the present invention, the shortest step length determined from each element is adopted, and the highest regeneration gas flow rate is adopted, so that the optimal step length and regeneration gas flow rate can be set according to the raw air conditions. Therefore, the adsorption process and regeneration process can be optimized according to each condition, so that the removal of various impurities and the regeneration of the adsorbent can be reliably performed.

In addition, in the present invention, any one of the various conditions such as the flow rate, temperature, and pressure of the raw air at the inlet of the purifier, the carbon dioxide concentration, the hydrocarbon concentration, and the nitrous oxide concentration in the raw air may be used. However, in order to ensure proper operation of the purifier in more cases, it is preferable to use a method of calculating a moving average per unit time based on the flow rate, temperature, and pressure of the raw air at the inlet of the purifier, a method of calculating a moving average per unit time based on the flow rate, temperature, and pressure of the raw air at the inlet of the purifier and the carbon dioxide concentration in the raw air at the inlet of the purifier, or a method of calculating a moving average per unit time based on the flow rate, temperature, and pressure of the raw air at the inlet of the purifier and the carbon dioxide concentration, hydrocarbon concentration, or nitrous oxide concentration in the raw air at the inlet of the purifier.

Furthermore, in the present invention, it is possible to associate the length of a specific step with the flow rate of the regeneration gas. In other words, if the length of the next operation step determined from the previous step is 10% longer, the flow rate of the regeneration gas can simply be reduced by 10%. In other words, the product of the length of the operation step and the flow rate of the regeneration gas can be made constant in each step. By adopting such a control method, it is only necessary to calculate the step length from each element in each control method, and the flow rate of the regeneration gas can be easily determined.

The time for one step can be set arbitrarily, and the measurement interval can also be set arbitrarily. In special cases, such as when starting up a cryogenic air separation unit, it can be set so that control is not performed until the operating conditions are stabilized.

11...pretreatment equipment, 12...deep cooling section, 13...air filter, 14...compressor, 15...precooling equipment, 16...purifier, 17...adsorption tower, 18...regenerated gas supply section, 18a...heater, 18b...bypass path

Claims (4)

In a method for operating a pretreatment facility of a cryogenic air separation unit, in which a purifier is used by sequentially switching between an adsorption step and a regeneration step using a temperature swing adsorption method, and at least moisture and carbon dioxide are removed from the feed air to purify the feed air to be introduced into the cryogenic air separation unit, the adsorption step and the regeneration step are divided into a plurality of operating steps based on a fixed time interval, the length of one operating step and the flow rate of the regeneration gas are calculated by calculating moving average values in each step from the flow rate, temperature and pressure of the feed air at the inlet of the purifier measured at preset intervals during the previous operating step, and the moving average values are increased compared to the values used in the previous step. a regeneration gas flow rate of the adsorption tower in a stepwise manner so that the product of the regeneration gas flow rate and the length of the operation step is constant; the length of the next operation step is determined to be the shortest among the lengths of the operation steps estimated from each of the steps, and the regeneration gas flow rate of the next operation step is determined to be the highest among the flow rates of the regeneration gas estimated from each of the steps , so that the product of the regeneration gas flow rate and the length of the operation step is constant . In a method for operating a pretreatment facility of a cryogenic air separation plant, in which a purifier is used by sequentially switching between an adsorption step and a regeneration step using a temperature swing adsorption method, and at least moisture and carbon dioxide are removed from the feed air to purify the feed air to be introduced into the cryogenic air separation plant, the adsorption step and the regeneration step are divided into a plurality of operation steps based on a fixed time interval, and the length of one operation step and the flow rate of the regeneration gas are calculated by calculating moving average values in each step from the flow rate, temperature, pressure and carbon dioxide concentration of the feed air at the inlet of the purifier measured at preset intervals during the previous operation step, and the moving average values are set to values that are higher than those used in the previous step. and when the regeneration gas flow rate is lower than the value used in the previous step, the time for the next step is shortened and the flow rate of the regeneration gas in the adsorption tower is increased based on the rate of increase relative to the value used in the previous step, and conversely, when the regeneration gas flow rate is lower than the value used in the previous step, the time for the next step is extended and the flow rate of the regeneration gas in the adsorption tower is decreased based on the rate of decrease, so that the length of the next step is determined to be the shortest of the lengths of the operation steps estimated from each of the steps , and the regeneration gas flow rate for the next operation step is determined to be the highest of the regeneration gas flow rates estimated from each of the steps, and the product of the regeneration gas flow rate and the length of the operation step is constant . In a method for operating a pretreatment facility of a cryogenic air separation unit, in which a purifier is used by sequentially switching between an adsorption step and a regeneration step using a temperature swing adsorption method, and at least moisture and carbon dioxide are removed from the feed air to purify the feed air to be introduced into the cryogenic air separation unit, the adsorption step and the regeneration step are divided into a plurality of operation steps based on a fixed time interval, and the length of one operation step and the flow rate of the regeneration gas are calculated by calculating moving average values in each step from the flow rate, temperature, pressure, carbon dioxide concentration, hydrocarbon concentration and nitrous oxide concentration of the feed air at the inlet of the purifier measured at preset intervals during the previous operation step, and comparing the moving average values with the values used in the previous step. a regeneration gas flow rate of the adsorption tower in accordance with the rate of decrease in the regeneration gas flow rate of the adsorption tower in response to ... In a method for operating a pretreatment facility of a cryogenic air separation unit, in which a purifier is used by sequentially switching between an adsorption step and a regeneration step by a temperature swing adsorption method, and at least moisture and carbon dioxide are removed from the feed air to purify the feed air to be introduced into the cryogenic air separation unit, the adsorption step and the regeneration step are divided into a plurality of operation steps based on a fixed time interval, the length of one operation step and the flow rate of the regeneration gas are calculated by calculating a moving average value in each step from at least one of the flow rate, temperature, pressure, carbon dioxide concentration, hydrocarbon concentration and nitrous oxide concentration of the feed air at the inlet of the purifier measured at preset intervals during the previous operation step, and resetting the moving average value to the value adopted in the previous step. a regeneration gas flow rate of the adsorption tower in a stepwise manner so that the product of the regeneration gas flow rate and the length of the operation step is constant; and, when the regeneration gas flow rate is increasing compared to the value used in the previous step, the time for the next step is shortened and the flow rate of the regeneration gas in the adsorption tower is increased based on the rate of increase relative to the previous step; and, when the regeneration gas flow rate is decreasing compared to the value used in the previous step, the time for the next step is extended and the flow rate of the regeneration gas in the adsorption tower is decreased based on the rate of decrease ;

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