Analysis and treatment of causes of excessive temperature difference at hot end of main heat exchanger

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Analysis and Treatment of Excessive Temperature Difference at Hot End of Main Heat Exchanger Qian Jiqing Abstract from Design Institute of Hangzhou Hangyang Co., Ltd.: The phenomenon of large temperature difference at the hot end of the main heat exchanger of a 12000m3/h air separation equipment in a certain factory was described and analyzed. The fault was dealt with and the problems that occurred during the transformation process were summarized. keywords: Air separation equipment; main heat exchanger; hot end temperature difference; bias flow 1. The phenomenon of large temperature difference at the hot end of the main heat exchanger. A factory's 12000m3 claw air separation equipment adopts molecular sieve adsorption purification, pressurized turbine expansion, structured packing on the tower and full distillation hydrogen-free nitrogen production external compression process. Since the start-up of the air separation plant, users have been reporting that the temperature difference at the hot end of the main heat exchanger is too large. One group of heat exchangers has a temperature difference of 7°C to 8°C. Through various adjustment means, the liquid output still cannot reach the design value required by the contract. Looking at the flow chart of the air separation equipment and the general diagram of the distillation tower, it is found that the six main heat exchangers (El-E6) are divided into two groups. The air at the outlet of the molecular sieve adsorber enters the two groups of heat exchangers El-E3 and E4-E6 respectively through the positive flow air valves Vln and vllZ. The hot-end temperature difference between the two sets of heat exchangers can be adjusted by using vChuan and vllZ valves, and the distribution of airflow is balanced by piping in the cold box between the three units in each group. Each unit of the main heat exchanger for pressurized air leaves has two outlets in the middle and the bottom. The temperature of the air entering the expander is adjusted by the central outlet main valve V31 and the bottom outlet main valve V31. The flow of return gas can only be adjusted through the regulating valves on the respective main pipes. The actual operation of the DCS control system data on site shows that when the VllZ valve is fully open and the Vlll valve opening is small, the temperature difference at the hot end of the El-E3 heat exchanger is small, about 4°C, while the temperature difference between the E4-E6 heat exchanger is The end temperature difference is large, with a temperature difference of nearly 8°C, and the mid-pump temperature of the pressurized air leaving the main heat exchanger is also very different. The mid-pump gas temperature of the El-E3 heat exchanger is significantly lower than the mid-pump gas temperature of the E4-E6 heat exchanger. 2. Cause analysis In order to find the reason for the excessive temperature difference at the hot end of the main heat exchanger, the following work was done: (l) Re-examine the design brief and heat exchanger design calculations, and through parameter analysis of similar products, it is believed that the heat exchange area of ​​the heat exchanger of this set of air separation equipment has sufficient margin, and the low temperature of the return gas is an isolated phenomenon. (2) Based on the actual situation that leakage occurred in the water cooler after supercharging during the start-up of the air separation equipment, the air separation equipment was greatly heated and purged. After the start-up, the temperature difference at the hot end of the main heat exchanger was slightly reduced, indicating that ice or dry ice blockage was not the main reason for the large temperature difference at the hot end; at the same time, during the blowing process, no molecular sieve and pearlescent sand powder were found, indicating that the molecular sieve and pearlescent sand powder did not enter the main heat exchanger. (3) Backflush the air flow path of the main heat exchanger. When the backflush pressure reaches 0.45MPa, no obvious impurities are blown out, indicating that there are no impurities in the air flow path, or that there are no impurities that can be purged out by backflushing. (4) According to the data displayed on the OCS control system, it can be considered that the temperature difference at the hot end of the main heat exchanger is too large, which is caused by the bias flow of the main heat exchanger. However, comparing the air resistance test report of the main heat exchanger when it leaves the factory, it is believed that the air resistance of each unit of the main heat exchanger is basically consistent with the design value. These differences will not cause obvious deviation of the main heat exchanger, which will cause the temperature difference at the hot end of the main heat exchanger to be too large. Through the above analysis and processing, it is believed that it is very likely that there are large impurities in some return gas pipelines, resulting in uneven air resistance in each unit of the main heat exchanger, causing bias flow in the main heat exchanger, and thus causing a large temperature difference at the hot end of the main heat exchanger. 3. Treatment measures Since the most likely cause of bias flow in the main heat exchanger is the presence of impurities in the heat exchanger, first consider how to remove the impurities. By backflushing the air flow path, it is proved that the impurities are not in the air flow path. The purging of the return gas flow path requires the same high pressure as the backflow of the air flow path. However, the heat exchanger and the return flow pipe cannot withstand such a high pressure; if the backflow gas is backflushed according to the design pressure of the heat exchanger, the effect will not be achieved. Therefore, the only way is to cut the pipes where impurities are believed to be present and remove the impurities manually. Since the design only installed thermometers on the two main pipes of oxygen, nitrogen and waste nitrogen, and did not install thermometers on each branch of the return gas, it was impossible to accurately determine which pipe was blocked. If every pipeline is cut and inspected on site, it will not only consume a lot of manpower and material resources, but also take a long time, which is obviously inoperable. In addition, if impurities have entered the internal channels of the main heat exchanger, then this method will not be able to do anything. Since impurities are difficult to remove, can the bias flow be improved by artificially changing the air resistance of each unit of the main heat exchanger? So the following two solutions were proposed: one is to add valves on the pipes leading to each heat exchange unit from air, and adjust the valve opening according to the temperature indication of a certain return gas; the other is to install valves on the flow path of a certain return gas out of each unit of the main heat exchanger, and adjust the valve opening according to the temperature indication of a certain return gas. The initial solution choice is to add a valve to the oxygen outlet pipe, because oxygen is the product gas that users are most concerned about. Taking into account the increase in backflow gas resistance, it will * * The energy consumption of the entire air separation system is increased, and in China, the method of adding valves to the return gas is rarely used to improve the bias flow, so the method of adding valves to the air flow path was chosen. The specific method is: after the air passes through the vlll and VllZ valves, add valves to the pipes leading to each heat exchange unit, install thermometers on the branches where the dirty nitrogen gas leaves each unit of the main heat exchanger, and adjust the valve opening according to the temperature of the dirty nitrogen gas leaving each unit of the main heat exchanger. The temperature of the dirty nitrogen gas is selected as the adjustment reference point because among all the return gases, the flow rate of the dirty nitrogen gas is the largest and can most accurately reflect the heat exchange effect of each unit of the main heat exchanger. After obtaining the user's consent, the air inlet main heat exchanger pipe was modified by taking advantage of the user's opportunity to perform mid-term repairs on the air separation equipment. After the actual transformation was completed, the opening of the newly added valve was repeatedly adjusted. In the end, when the forward air temperature was 19°C, the temperature of the waste nitrogen main pipe was 1.2°C, and the forward and return gas temperature difference was 3.8°C. Compared with before the transformation, the temperature difference was greatly improved, and the liquid output also reached the contract value. 4. Problems discovered during the actual transformation Some problems were also discovered during the actual transformation, which are worth learning from in similar projects in the future.: (l) Since the main heat exchanger cold box and the main cold box are not separated, in order to modify the main heat exchanger pipeline, the main cold box has to be sanded. On the one hand, it consumes a lot of time, manpower and material resources; on the other hand, it also causes certain damage to the equipment and pipelines in the cold box. If the main heat exchanger cold box and the main cold box are separated during design, obviously the above two problems can be avoided or alleviated. Of course, the direct result of this is an increase in the cold box footprint and cold box manufacturing costs. Therefore, whether to separate the main cold box and the main heat exchanger cold box needs to be implemented based on the actual situation of the user. (2) Since the newly added valve is a manual valve, and the control of the total air flow requires a regulating valve, there are double valves on the air flow path, which additionally increases the resistance of the air flow path. If one considers setting a regulating valve on each heat exchanger branch during design, only one type of valve is needed, and the valve opening will be automatically adjusted during operation. When unexpected heat exchanger bias occurs, there is an additional adjustment method. This method is feasible when the number of heat exchangers is small, but when the number of heat exchangers is large, the cost increases due to the increase in the number of regulating valves and is subject to space constraints, so it is still worthy of discussion. 5. Summary: There are many reasons for excessive temperature difference in the main heat exchanger. This situation is common in many air separation equipment and should be paid attention to. Carefully adjust the operating conditions of the air separation equipment to minimize the operating cost of the entire air separation equipment; when it is found that the temperature difference in the main heat exchanger is too large, the cause of the excessive temperature difference must first be correctly analyzed; adding valves to each branch of the air flow path can In order to effectively improve the bias flow and temperature difference, although there are some troubles in actual operation, it is an effective method to deal with the large hot end temperature difference caused by the bias flow of the main heat exchanger. It can be used when multiple methods are still ineffective.