KR800001335B1 - Temperature Control Method of Switchable Heat Exchanger of Air Separator - Google Patents

Abstract

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Description

Temperature Control Method of Switchable Heat Exchanger of Air Separator

1 is a schematic diagram of a flow path of a switched heat exchanger,

2 is a temperature distribution diagram of a countercurrent heat exchanger,

3 is a temperature distribution diagram of the switched heat exchanger,

4 is a temperature control system diagram of a switch type heat exchanger showing an example for carrying out the present invention;

5 is a temperature control method diagram in the case of a switch type exchanger composed of five blocks with one core as one block;

6 is a method of correcting the control operation point of the temperature control valve when the temperature control valve of the switchable heat exchanger in a block (block 1) is out of the controllable range.

7 is a method of modifying the control operating point of the temperature control valve when the temperature control valve of the switchable heat exchanger of a block is equally out of the controllable range,

8 is a method of modifying the control operating point of the temperature control valve when all the temperature control valves of the switchable heat exchanger are equally out of the controllable range.

9 is a control state diagram during normal operation;

10 is a control state diagram at the time of increasing operation,

11 is a control state diagram at start-up after a planned pause;

12 and 13 are modified state diagrams of the control operation points of the temperature control valve by the raw air block valve.

The present invention relates to a temperature control method of a switch heat exchanger used in an air separation device, and to manage the switch heat exchanger group most suitably by automatic temperature control, thereby to heat the entire system of the air separation process. The aim is to balance equilibrium and maintain normal long-term stable continuous operation of the process.

In general, a switch heat exchanger used in an air separation device includes, for example, a return gas flow B through which three flow paths, a raw air flow path A, an impure nitrogen, an impurity oxygen, etc., flow, as shown in FIG. It consists of C, which is a separated gas oil through which nitrogen, oxygen, etc. flow.

At this time, the flow paths of A and B are periodically switched alternately by switching of the switching device, at which point raw material air flows into A, and return gas flows into B, and heat exchange is performed through their heat transfer surfaces. Is done.

Regardless of the switching of the flow paths of A and B, C flows through the heat transfer surface in the same manner as the separation gas flows. When the raw material air flows in either of the flow paths A and B, the raw material air is cooled and impurities in the raw air are precipitated on the heat transfer surface, but the precipitated impurities have flowed before the switching by the switching of the flow paths A and B. Gas returned to the flow path is removed by flow.

This type of switch heat exchanger switches the flow paths A and B so that impurities do not accumulate during normal operation, and this prevents accumulation of impurities by utilizing the difference in vapor pressure of impurities in the source air and the return gas. have. In order to effectively remove this impurity, it is necessary to limit the temperature difference between the raw air and the return gas within a certain allowable value.

As the switching heat exchanger becomes a low temperature zone, the temperature difference tends to be large due to the difference in specific heat between the raw air and the returning gas, which deteriorates the condition for removing impurities. In other words, impurities in the raw air adhering to the flow path are not completely removed by the returning gas, resulting in a situation of blockage.

As a means to solve this problem, the temperature difference between the raw material air and the returning gas is controlled within the allowable value by providing a reheating circuit for backflowing the gas extracted from the rectification column to the low temperature side of the switching heat exchanger.

Usually, this type of heat exchanger has the largest cross section dimension of 1,200mm in the current manufacturing technology, and in the large air separation unit, this heat exchanger uses a combination of several cores from several cores in parallel, By using the combination in this way, when an unbalance of flow rate occurs in each of the switching heat exchangers, there is a tendency that the unbalance of the flow rate increases due to the effect of switching.

In order to control the unbalance of the flow rate in this way, conventionally, the thermometers T ₁ , T ₂ ... … Tn, T ' ₁ , T' ₂ ... Automatic valve A ₁ , A ₂ ... An, N ₁ , N ₂ ,... Nn, B ₁ , B ₂ . The flow rate was controlled by Bn or the like to operate the central temperature or the cold end temperature of all switch heat exchangers within a certain allowable temperature range (each symbol is referred to in FIG. 4).

In such a case, if the number of cores of the switchable heat exchanger is small with a relatively small air separation device, it can be controlled to some extent by human operation. However, in the recent large air separation devices, the number of cores is more than the collecting core. In addition, the control operation of each other interferes with each other, and the operation time becomes more difficult because the waste time and time constant of the process are long.

SUMMARY OF THE INVENTION The present invention aims to solve these problems of the prior art, which is characterized by the fact that the automatic switching device always switches to the switching heat exchanger every time the switching timing of the automatic switching device interlocked with each switching heat exchanger. Measure and update the temperature of each switchable heat exchanger for each cycle, and calculate the average value of the data, that is, the average temperature of each switchable heat exchanger, and use this as the control amount. It is used to calculate the total average temperature and to control the automatic valve of each switchable heat exchanger so that the target value is converged to the target value.

That is, in the temperature control method of the switchable heat exchanger group for an air separator having at least two cores having a raw material air flow path, a return gas flow path and a separation gas flow path, and a heat exchanger for switching between the raw air flow path and the return gas flow path. For each heat exchanger, the temperature of a specific flow path in each flow path is detected at the time of switching each time the raw air flow path and the return gas are switched, and the memory temperature is always updated and updated at the same time for the latest switching cycle. Calculate the average temperature of each temperature calculated as the control amount, and calculate the average temperature again from each calculated average temperature to set this as the control target value, and compare the control target value with the control amount to adjust the gas flow rate in each flow path. A method of temperature control of a switched heat exchanger for an air separator.

In general, the temperature distribution of a countercurrent heat exchanger is as shown in FIG. 2, but the temperature distribution changes as shown in FIG. 3 by installing a reheating circuit like a switch type heat exchanger. The temperature difference with the gas is maximum.

The temperature difference t ₅ of the central part of the heat exchanger is larger than that of t ₃ , t ₄ , and if there is confusion in the amount of reheating gas, the central temperature difference t ₅ is more sensitive than t ₃ , t _4. It is preferable to perform temperature control by measuring the central temperature. In addition, the air separation apparatus needs to make the cold cooling required for the device completely parallel to the cold cooling generated in the expansion turbine.

In addition, it is necessary to prevent the thermal balance of other systems from being disturbed by controlling the temperature of the switching heat exchanger. In addition, if the heat balance of the entire apparatus is maintained, the heat balance of the switchable heat exchanger group is inevitably stable, so that even if an unbalance occurs in the temperature of the switchable heat exchanger group, the total exchange heat of the switchable heat exchanger group is constant. In addition, it is necessary to control the temperature of the switching heat exchanger in a stable state of the process.

In order to solve these problems, the temperature of the heat exchanger must be controlled so as not to break down the thermal equilibrium by analyzing the characteristics such as the flow temperature of the switchable heat exchanger. It is possible to distribute the heat within a range that does not change the total heat loss of the heat exchanger group because it is a control amount and the average temperature is calculated again from the above average temperatures, and the control target value is used to control the temperature of the switched heat exchanger. It is suitable.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated concretely by drawing.

As shown in FIG. 4, the switchable heat exchanger has 1 to n cores, which are commanded by a switching device (not shown), and switch valves V ₁ , V ₂ ... … By Vn, that is, switching of the flow paths A and B is performed in sequence for each block unit, and heat exchange between the raw air and the gas returned is performed. One block is composed of a 1-3 core switchable heat exchanger. May know the temperature of the heat exchanger switching expression to facilitate description, even the central temperature _{(T 1, T 2 ...... Tn} ) in the ₍₁ naengdan temperature T', _T'2 ... T'n instead central temperature), and the like.

또는 2절환 주기분 등이라도 좋다)으로 하여 각 절환식 열교환기의 중부온도(T₁,T₂……Tn)는 각 열교환기의 유로 A, B가 절환되는 시점에서 자동제어장치에 의하여 절환시마다 측정기억이 된다.1 change cycle for temperature

Or two switching cycles, etc.), the central temperature (T ₁ , T ₂ ... Tn) of each switchable heat exchanger is changed at the time of switching by the automatic control device at the time when the flow paths A, B of each heat exchanger are switched. It becomes measurement memory.

In the automatic control device, the central temperature measured and stored at the switching timing for the latest one switching cycle from the present time is stored and updated. For the sake of clarity, a switch type heat exchanger composed of 5 blocks with 1 core as 1 block will be described as shown in FIG. 5, and the switching is performed in block units (1 → 2 → 3 → 4 → 5 → 1 → 2). In each switching heat exchanger, the central temperature of each switching heat exchanger is measured and memorized, and in the switching timing of A, the switching timing for the latest one switching cycle is always stored so that the central temperature detected by B is discarded and the central temperature detected by A is newly stored. The central temperature memorized and measured at the time of memory is updated.

Using the measured data of each latest one switching cycle of each switching heat exchanger obtained as described above, the average value of the stored data, that is, the average central temperature of each switching heat exchanger, is calculated as the control amount and all the switching again. The average median temperature is calculated using the average median temperature in the type heat exchanger, and this is set as the control target value, and then the automatic valve of each switchable heat exchanger is controlled to converge the control amount to the target value. Usually, the deviation of the control amount with respect to the target value is obtained and the manipulated value from the current state of the automatic valve is calculated from the deviation and the tendency of the deviation.

In calculating the manipulated variable, set the dead zone and the dead zone for the deviation and the tendency, and check the minimum manipulated value which does not give an operation signal when the maximum operation that can be issued at the same time or less and the final manipulated value is obtained. Calculate The control operation is performed on the automatic valve based on the operation amount obtained in the calculation, and temperature control is performed. However, in order to stabilize the effect, the control operation is paused, and then control is resumed. Usually, the pause time of the control operation is controlled by the number of switching signals of the automatic switching device, and is set to be almost equal to the overall time constant of the switching heat exchanger.

Control operation is separated gas control valve O ₁ , O ₂ , O ₃ . … On or return gas control valves N ₁ , N ₂ , N ₃ . … Nn or raw air control valve A ₁ , A ₂ , A ₃ . … An Raw air block valve B ₁ , B ₂ ,. … Although not shown in Bn or FIG. 4, the valve is performed by any of the reheat circuit valves of the reheat circuit provided on the low temperature side of the switch type heat exchanger.

Next, by the temperature control method as described above, the effect is apparent in the actual operating data (Figs. 9 to 11) obtained.

9 shows the control state in the normal operation, and it can be seen that all the switching heat exchangers maintain the heat balance well in the normal operation by the automatic control.

FIG. 10 shows the control state in the increase operation, and it can be seen that the following operation is satisfactory even if the heat balance state of the switch type heat exchanger is changed by the increase operation.

FIG. 11 shows a control state at start-up after a planned pause, and it can be seen that the process at start-up is stable and the heat exchanger operates well even with a change in the switching cycle to maintain the heat balance.

When the temperature control valve performing temperature control operation in the temperature control of the switching heat exchanger is out of the control operation possible range, the temperature control valve is controlled by combining one or two or more types of automatic valves other than the temperature control valve. Can be modified to the operable range.

To clarify the method of modifying the control operation point of the temperature control valve described above, in Figure 4, the switchable heat exchanger is composed of three blocks and one block of three cores to separate the temperature of the switchable heat exchanger into a central temperature and a temperature control valve. Gas control valves O ₁ , O ₂ , O ₃ . … On or return gas control valves N ₁ , N ₂ , N ₃ . … The case where either of Nn is adopted is demonstrated concretely. Moreover, when the temperature and temperature control valve of a switchable heat exchanger are other than the above, the correction method of the control operation point of a temperature control valve is the same.

(A) The temperature control valve of the switchable heat exchanger in a block (I block) is out of the controllable range (see Fig. 6).

By detecting the valve opening degree of the temperature control valve of the switchable heat exchanger in the I block and using this as a control amount, calculating the average value of the valve opening degree previously detected, that is, the average valve opening degree in the I block, and making this as the control target value. The raw air control valves A ₃ I-2, A ₃ I-1, and A ₃ I of each switchable heat exchanger in the I block are operated to converge the control amount to this control target value.

Normally, the deviation of the control amount with respect to the control target value is obtained, and the deviation is banded and deadband according to the deviation range, and from the current state of the raw air control valves A ₃ I-2, A ₃ I-1, A ₃ I Calculate the manipulated variable. When operating the raw air control valve A ₃ I-2, A ₃ I-1, A ₃ I, the central temperature of each switchable heat exchanger in the I block is controlled if the temperature control valve is within the controllable range. The control valve is to return within the controllable range.

When the correction operation is performed, the correction operation is paused to wait for the stability of the effect, and then the correction operation is performed when the temperature control valve is out of the controllable range. The separation gas control valves O ₁ , O ₂ , O ₃ . … On or return gas control valves N ₁ , N ₂ , N ₃ . … Even when Nn is used, the correction method is the same as described above.

(B) The temperature control valve of the switching heat exchanger of any block is constantly outside the controllable range (see Figure 7).

For each block, the total block average opening is calculated using the average valve opening in each block from the valve opening of the temperature control valve of the switchable heat exchanger in the block. The raw material air block valve B of the switchable heat exchanger so that the average valve opening in each block of the temperature control valve of the switchable heat exchanger is the control amount, and the control block value is converged to the control target value with the average block opening degree of all the blocks as the control target value. ₁ , B _2, B ₃ ... Manipulate Bn. Normally, the deviation of the control amount to the control target value is obtained, and the dead band and dead band are set according to the deviation range, and the current raw air block valves B ₁ , B _2, B ₃ . The manipulated value in the state of Bn is calculated. Raw material air block valve B ₁ , B _2, B ₃ . By operating Bn, the central temperature of the switchable heat exchanger of each block is disturbed so that the temperature control valve is within the controllable range, so if the temperature control is continued, the temperature control valve returns to the controllable range. When the correction operation is performed, the correction operation is paused to wait for the stabilization of the effect, and then the correction operation is performed when the temperature control valve is out of the controllable range. In addition, as a temperature control valve, a separation gas control valve O ₁ , O ₂ , O ₃ . … On or return gas control valves N ₁ , N ₂ , N ₃ . … Even when Nn is used, the correction method is the same as described above.

(C) All temperature control valves of switchable heat exchangers are constantly outside the controllable range (see Figure 8).

By operating the temperature control valves of all switch heat exchangers uniformly, the temperature control valves are separated gas control valves O ₁ , O ₂ , O ₃ . … In case of ON, the separation gas source valve D is replaced by the gas control valves N ₁ , N ₂ , N ₃ . … In the case of Nn, by operating the return gas source valve C, the temperature control valve can be returned to within the controllable range without disturbing the flow rate balance and the heat balance of the entire switchable heat exchanger.

When the correction operation is performed, the correction operation is paused to wait for the stabilization of the effect, and then the correction operation is performed when the temperature control valve is out of the controllable range. As described above, it is well shown that the actual operation data (12th and 13th degrees) or the effect obtained based on the method of correcting the control operation point of the temperature control valve is very large.

12 and 13 show the state of modification of the control operation point of the temperature control valve by the raw air block valve, and the three-block temperature control valve is modified in the closed direction as the raw air block valve is operated. At the same time, it can be seen that the eight-block temperature control valve is modified in the open direction. If only the reheat nitrogen outlet temperature (the same characteristic as the central temperature) of 3 blocks and 8 blocks is distracted in the correcting direction of the control operation point of the temperature control valve and temperature control is continued, as shown in FIG. 13, the temperature control valve of the other block. It can be seen that the control operating points of only 3 blocks and 8 blocks can be modified without changing the control operating points of the blocks.

According to the present invention as described above, in the switchable heat exchanger group of the large-sized air separation device, the unbalance of the flow rate can be automatically controlled, and the mutual interaction in each heat exchange period can be made very small.

Claims (1)

In a well-known switch type heat exchanger group for an air separator having a source air flow path and a return gas flow path and a separation gas flow path, wherein at least two cores or more are provided together with a heat exchanger in which the raw air flow path and the return gas flow path are exchanged. At the time of switching between the source air flow path and the return gas flow path for each heat exchanger, the temperature of a specific flow path in each flow path is detected, and the average temperature of each temperature is always stored and updated in the latest temperature for any switching cycle. Is calculated as the control amount, and the average temperature of the calculated average temperatures is calculated as the control target value, and the gas flow rate of each flow path is adjusted by comparing the control target value with the control amount. Temperature control method of switchable heat exchanger for equipment.

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