GB1591241A - Method for controlling reversing heat exchanger in air separation plant - Google Patents

Description

(54) METHOD FOR CONTROLLING REVERSING HEAT EXCHANGER IN AIR SEPARATION PLANT We, KOBE STEEL LTD., a corporation organised under the laws of Japan, of 3-18, l-chome, Wakinohama-cho, Fukiai-ku, Kobe-city, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to an improvement in a reversing heat exchanger system for an air separation plant. More particularly, the invention relates to a method of coping with defects occurring in temperature-detecting means and temperature control valves in the above-mentioned heat exchanger system.

In general, a reversing heat exchanger used in an air separation plant comprises three channels as shown in Fig. 1 of the accompanying drawings: namely, a channel A for feed air, a return gas channel B for impure nitrogen, impure oxygen or the like, and a separated gas channel C for product nitrogen, product oxygen or the like. In this heat exchanger, the channels A and B are periodically interchanged with each other by a changeover device At a certain point, feed air flows in the channel A and the return gas flows in the channel B, and heat exchange is effected through heat transfer surfaces of these channels. Separated gases flow in the channel C irrespective of the changeover of the channels A and B and heat exchange is effected between channels A and C and between channels B and C.

When feed air flows in either of the channels A and B, the feed air is cooled and impurities in the feed air are precipitated on the heat transfer surface, and after the changeover of the channels A and B, the precipitated impurities are removed by the return gas flowing in the channel in which the feed air has been flowing. Thus, the channels A and B are changed over with each other so as to prevent accumulation of impurities in the reversing heat exchanger in normal operation, and this prevention of the accumulation of impurities can be accomplished by utilizing the difference between the vapour pressure of impurities in the feed air and that of impurities in the return gas.

In a large-scale air separation plant, a plurality of heat exchanger cores, sometimes several score, are combined in parallel with one another. Accordingly, if an imbalance of the flow rates occurs in individual heat exchangers, the imbalance is increased by the influence of the changeover operation.

The imbalance in the flow rates has hitherto been controlled by observation with the naked eye of indications on temperature detectors mounted in the intermediate portion and cold ends of the heat exchanger, and controlling the flow rate by corresponding automatic temperature control valves or the like. Thus, in the conventional techinque, the operation is conducted while maintaining the temperatures of the intermediate and cold end portions of all the reversing heat exchangers within a certain allowable temperature range.In case of a relatively small air separation plant where the number of heat exchanger cores is small, the above control operation can be performed manually, but in a large-scale air separation plant where scores of heat exchanger cores are used, interferences are produced in the control operations in the respective heat exchangers and the dead time and time constant of the process is increased and control becomes difficult.

The invention described in U.K. Patent Number 1548942 is aimed at solving the above mentioned problems. According to that invention there is provided a method of automatically maintaining the temperature of a plurality of cores of a reversing heat exchanger connected in parallel relationship substantially equal, wherein each core has a channel in which a feed stream flows and at least one further channel for at least one transfer medium stream, the said feed stream and the said heat transfer medium stream being interchanged between the channels at a given interval, the method comprising the steps of: detecting the temperature of each core at at least one point thereof at a plurality of successive moments in time, periodically calculating a mean temperature of each core from a plurality of the preceding detected values, calculating a mean temperature of all the heat exchanger cores considered together from the calculated mean temperatures, calculating the difference between the mean temperature of each core and the mean temperature of all the cores considered together, and the variation in the said difference with respect to time and, adjusting flow control valves of the heat exchanger in dependence on the said difference and variation in difference with respect to time to substantially equalise the respective temperatures of the cores.

When the temperature control is performed in all of the reversing heat exchan guess according to this temperature control method, noise is generated in temperature detectors of the heat exchangers or difficulties or defects are caused in measurement instruments or temperature control valves.

Thus, continuation of the control operation is often impossible. For example, when a defect occurs in the equipment of a particular reversing heat exchanger and it is desired to repair this equipment, a precise temperature signal of this heat exchanger may not be obtained owing to the above mentioned noise difficulties or defects. Therefore, in such case, in temperature control method of Patent 1548942 the control operation must be temporarily stopped and after the normal state is restored in the equipment, the control operation can be started again. The object of the present invention is to moderate this disadvantage so that the control operation can be performed continuously.

According to the present invention there is provided a method of controlling the temperature in a reversing heat exchanger system for an air separation plant which comprises a plurality of heat exchanger cores connected in parallel relationship, wherein each core has a channel in which feed air flows and a channel in which return gas flows, the feed air flow and the return gas flow being periodically interchanged between the channels, the method comprising: detecting the temperature of a specific channel of each core at at least one point thereof at a plurality of successive moments in time, the said specific channel being between the feed air channel and the return gas channel, storing the temperatures detected during the most recent changeover period, periodically calculating a mean temperature of each core from the stored values, calculating a further mean temperature of all the heat exchanger cores considered together from the calculated mean temperatures, comparing the mean temperature of all the cores considered together, adjusting the flow rate in each of the channels in dependence on the said comparison, wherein each heat exchanger core is provided with means for switching it to a maintenance-appointed state, the core being switched to that state automatically or manually when a defect occurs therein, whereafter the control operation is performed using the mean temperature of all the reversing heat exchangers except the maintenance-appointed heat exchanger as the said further mean temperature, and when the defect in the maintenance-appointed heat exchanger core is remedied the heat-exchanger core is switched out of the maintenance appointed state automatically or manually and the control operation is then performed using the mean temperature of all the normal heat exchanger cores, inclusive of the that one which had been in a maintenance-appointed state, as the said further mean temperature.

When defects occur successively in a considerable number of reversing heat exchangers and these heat exchangers are switched into a maintenance-appointed state, the overall heat exchanger capacity of the entire heat exchanger system is sometimes insufficient for the air separation process and continuous operation becomes impossible. The number of heat exchanger cores which can be switched into a maintenance-appointed state is therefore preferably limited.This can be done by checking the number of maintenance-appointed cores and if this number exceeds the predetermined number, this is automatically indicated to an operator.

The present invention will now be described by reference to embodiments. An automatic detector for detecting a disorder in a measured process value such as a measured temperature value is provided, and a normal measured value range of operating values, such as temperature values, measured when the reversing heat exchanger is operated in the normal state is given to a control computer of the disorder detector.

A check is continuously carried out by the control computer as to whether or not the measured process value of the reversing heat exchanger is within the normal range, and if the measured value exceeds the lower or upper limit of the normal range, for example when a defect such as the generation of noise, a line breakage or a disorder is caused in a temperature detector or other measurement instrument occurs, this heat exchanger is switched into a maintenanceappointed state in a response to a disorderindicating signal from the automatic disorder detector. The temperature control operation is continued, with an average temperature value of all the heat exchangers except the maintenance-appointed heat exchanger being used as the intended control value.

When repair of the maintenace-appointed heat exchanger is completed and when the measured operating value, such as the measured temperature value, is checked and compared by the control computer of the disorder detector and found to be within the normal range, the heat exchanger is switched out of the maintenance appointed state automatically in response to a signal from the automatic disorder detector, and the temperature control operation is continued using an average temperature value of all the heat exchangers, inclusive of the maintenance appointed-released heat exchanger, as the intended control value.

A similar automatic disorder detector is provided to detect occurrence of a defect in the temperature control valve, and a normal control deviation range of the temperature control valve observed when the reversing heat exchanger is normally operated is given to a control computer of this automatic disorder detector. The control deviation of the control valve is always checked and compared by the control computer during operation, and when the checked value exceeds the upper or lower limit of the normal control deviation range, it is judged that the temperature control valve is out of order and the corresponding heat exchanger is switched into a maintenance-appointed state by a disorder-indicating signal from the automatic disorder detecter.The temperature control operation is continued using an average temperature of all the heat exchangers except the maintenance-appointed heat exchanger as the intended control value.

When repair of the maintenance-appointed temperature control valve of the heat exchanger is completed and when the control deviation of the temperature control valve is checked and compared by the control computer of the automatic disorder detector and is found to be within the normal control deviation range, the maintenance appointed is automatically released, and the temperature control operation is continued using an average temperature of all the heat exchangers, inclusive of the maintenance appointment-released heat exchanger, as the intended control value.

If a defect in the reversing heat exchanger is not detected by the automatic disorder detecter and the judgement of an operator is required, or if a cause of a defect is detected only by an operator, for example during routine inspection, maintenance appointment and maintenance release on the reversing type heat exchanger are manually effected by the operator. For this purpose, a changeover switch is disposed on each reversing heat exchanger so that when the changeover switch is operated, the heat exchanger is switched to a maintenanceappointed state. In this case, the temperature control operation is continued, with an average temperature of all the heat exchangers except the maintenance appointed heat exchanger being used as the intended control value.When repair of the maintenance-appointed heat exchanger is completed and the repaired heat exchanger is checked, the position of the changeover switch is reversed to release the maintenance appointment, and the temperature control operation is continued using an average temperature of all the heat exchangers, inclusive of the released heat exchanger, as the intended control value.

As will be apparent from the above description, when a defect occurs in a temperature detector, measuring equipment or temperature control valve of the reversing heat exchanger, that heat exchanger is automatically switched to a maintenanceappointed state and it is excluded from use in temperature control. However, the number of cores of heat exchangers which are in a maintenance-appointed state and which are separated from the heat exchange system should be limited if the plant is to operate efficiently. As shown in Fig. 2 of the accompanying drawings, the total number of cores of heat exchangers which can be in a maintenance-appointed state varies depending on the output of product oxygen and product nitrogen by the air separation plant.

The data on the graph of Fig. 2 are fed to a computer of a detector for detecting the number -of maintenance-appointed cores.

When the detected number exceeds the allowable limit, an alarm is automatically generated and an operator takes appropriate action is response to this alarm.

In performing the temperature control in a plurality of parallel reversing heat exchangers for an air separation plant with an average temperature of the individual heat exchangers being used as the intended control value, in accordance with the present invention, when a defect occurs in any of the heat exchangers the defective heat exchanger is switched to a maintenanceappointed state and this heat exchanger is excluded from use in the temperature control. Advantageously, the total number of heat exchanger cores which are in a maintenance-appointed state is always checked and when the checked number exceeds a predetermined number, an alarm is automatically generated.The invention and this advantageous embodiment enables the following advantages to be achieved: (1) Even if a defect occurs in a temperature detector, instrument or temperature control valve of any of heat exchangers, the temperature control operation can generally be continued fully automatically without stopping other heat exchangers which are operating normally.

(2) A heat exchanger in which defect is detected can be immediately subjected to the temperature control.

(3) Since the total number of maintenance-appointed reversing heat exchangers is always checked automatically, then, unless the number of maintenanceappointed heat exchangers exceeds an allowable limit continuous operation of the air separation process can be maintained.

WHAT WE CLAIM IS:- 1. A method of controlling the temperature in a reversing heat exchanger system for an air separation plant which comprises a plurality of heat exchanger cores connected in parallel relationship, wherein each core has a channel in which feed air flows and a channel in which return gas flows, the feed air flow and the return gas flow being periodically interchanged between the channels, the method comprising: detecting the temperature of a specific channel of each core at at least one point thereof at a plurality of successive moments in time, the said specific channel being between the feed air channel and the return gas channel, storing the temperature detected during the most recent changeover period, periodically calculating a mean temperature of each core from the stored values, calculating a further mean temperature of all the heat exchanger cores considered together from the calculated mean temperatures, comparing the mean temperature of all the cores considered together, adjusting the flow rate in each of the channels in dependence on the said comparison, wherein each heat exchanger core is provided with means tor switching it to a maintenance-appointed state, the core being switched to that state automatically or manually when a defect occurs therein, whereafter the control oper ation is performed using the mean tempera ture of all the reversing heat exchangers except the maintenance-appointed heat exchanger as the said further mean tempera ture, and when the defect in the maintenance-appointed heat exchanger core is remedied the heat-exchanger core is switched out of the maintenance appointed state automatically or manually and the con trol operation is then performed using the mean temperature of all the normal heat exchanger cores, inclusive of the that one which had been in a maintenance-appointed state, as the said further mean temperature.

2. A method according to Claim 1, wherein when at any time the total number of heat exchanger cores in a maintenance appointed state exceeds a predetermined number this is automatically indicated.

3. A method of controlling the tempera ture in a reversing heat exchanger system, substantially as herein described with refer ence to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. (3) Since the total number of maintenance-appointed reversing heat exchangers is always checked automatically, then, unless the number of maintenanceappointed heat exchangers exceeds an allowable limit continuous operation of the air separation process can be maintained. WHAT WE CLAIM IS:-

1. A method of controlling the temperature in a reversing heat exchanger system for an air separation plant which comprises a plurality of heat exchanger cores connected in parallel relationship, wherein each core has a channel in which feed air flows and a channel in which return gas flows, the feed air flow and the return gas flow being periodically interchanged between the channels, the method comprising: detecting the temperature of a specific channel of each core at at least one point thereof at a plurality of successive moments in time, the said specific channel being between the feed air channel and the return gas channel, storing the temperature detected during the most recent changeover period, periodically calculating a mean temperature of each core from the stored values, calculating a further mean temperature of all the heat exchanger cores considered together from the calculated mean temperatures, comparing the mean temperature of all the cores considered together, adjusting the flow rate in each of the channels in dependence on the said comparison, wherein each heat exchanger core is provided with means tor switching it to a maintenance-appointed state, the core being switched to that state automatically or manually when a defect occurs therein, whereafter the control oper ation is performed using the mean tempera ture of all the reversing heat exchangers except the maintenance-appointed heat exchanger as the said further mean tempera ture, and when the defect in the maintenance-appointed heat exchanger core is remedied the heat-exchanger core is switched out of the maintenance appointed state automatically or manually and the con trol operation is then performed using the mean temperature of all the normal heat exchanger cores, inclusive of the that one which had been in a maintenance-appointed state, as the said further mean temperature.

2. A method according to Claim 1, wherein when at any time the total number of heat exchanger cores in a maintenance appointed state exceeds a predetermined number this is automatically indicated.

3. A method of controlling the tempera ture in a reversing heat exchanger system, substantially as herein described with refer ence to the accompanying drawings.