Modification of the sieve filter element

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Modification of the molecular sieve filter elements in cryogenic boxes Zhong Shicai, Xu Jiafu (Sichuan Tianhua Co., Ltd., Hejiang, Sichuan 646207) Abstract: By analyzing the reasons for the short operating cycle of cryogenic boxes, and by modifying the originally designed filter elements of the molecular sieve filters installed in these boxes, the filtering accuracy and the reliability of sealing were improved, resulting in a significant enhancement in the performance of the cryogenic boxes. 　　Keywords: Modification of molecular sieve filter elements 1. Phenomena and causes. Since the 300kt synthetic ammonia plant operated by Tianhua Co., Ltd. came online in 1995, it was observed that over time, the temperature difference of the waste gas at the outlet of the cold box increased gradually, as did the pressure difference between the inlet and outlet of the expander. These issues significantly affected the normal operation of the cold box. Most of the time, the operation cycle of the cryogenic tank is only about 2 months, and each purge due to a rupture in the cryogenic tank results in a reduction in ammonia synthesis output of 350 to 500 tons. Online blasting and purging not only pose significant risks but also require substantial manpower and resources, while also having a considerable impact on the service life of cryogenic tank equipment. After reheating the cryogenic tank and the expander multiple times, the inlet pipeline of the cryogenic tank was inspected and a large amount of molecular sieve dust was found; considerable dust as well as trace amounts of oil were also detected in the expander and its pipelines. By analyzing the composition of the gas discharged from the expander’s outlet drain, the causes of poor operation of the cryogenic tank due to oil leakage from the expander and excessive water content in the process gas were ruled out. After further analysis and verification, and through a comparison of the design and operation conditions of molecular sieves in ammonia synthesis plants of the same type, it was ultimately found that the short circuit of the molecular sieve filters (F-4A/B) located upstream of the cold box was the main cause of the deterioration in the performance of the cold box, as it allowed molecular sieve dust to enter the cold box. 2 Structural Features and Existing Problems of F-4A/B 2.1 Structure The design structure of the F-4 A/B is shown in Figure 1. Air enters through port A; after being filtered by the filter elements to remove dust, the clean air exits through port B and proceeds to the cold box. The filter elements originally used in the cold box have a mesh size of 10μm and are made of AISI316; a total of 74 such elements are used per unit. The installation of the filter elements within the equipment is shown in Figure 1. Each filter has 37 fastening rods for the filter elements; two filter elements are mounted on each rod. After that, a cover plate is placed on top, and the fixing bolts are tightened. A polytetrafluoroethylene gasket is used at each end of each filter element. 2.2 Existing problems 1) The filter elements are installed in series using a single tie rod; in this configuration, there are 7 sealing points per tie rod. For each filter, the total number of sealing points is 74x7=259. With such a large number of sealing points, the likelihood of leakage is relatively high. 2) Since the F-4A/B are subjected to alternating loads, misalignment is likely to occur at the junctions of the upper and lower filter elements during operation, resulting in a short circuit in the filter. 3) The two sealed end faces of the filter element are fixed by spot welding a sealing panel to an inner frame, and the parallelism accuracy of these two end faces is not high. 4) In the original design of the filter element structure, as shown in Figure 2, the inner diameter of the rolled inner skeleton is 70 tmn; due to the low rolling precision, the roundness error is significant, and it is absolutely impossible to ensure an accuracy of 70 tern for the inner diameter. Therefore, when the filter element is installed, the inner hole cannot fit properly into the shoulder present on the flange, which prevents the sealing gasket from making effective contact with the sealing surface on the flange, resulting in a short circuit. 5) The sealing gasket is designed as a PTFE gasket, which has its advantages, but its poor compensatory capacity also presents some drawbacks; in particular, short circuits can occur when the parallelism of the sealing panels is poor or when the sealing surfaces are uneven. 3 Renovation measures 1) In March 1997, the original filter element specification was changed from 095/70H300 to 095/70H600. As a result, the number of filter elements per unit decreased from 74 to 37, and the number of sealing points dropped from 259 to 148, thereby reducing the rate of seal leakage and eliminating the possibility of leakage due to misalignment. 2) In November 1997, a request was made to the manufacturer to improve the structure of the filter element. As shown in Figure 3, the sealing panel is machined to ensure both the parallelism of the sealing surfaces at both ends and a precision within 0.70 mm for the inner hole; this allows the filter element to be easily inserted into the socket during installation, thereby completely resolving the issue of sealing reliability. 3) Replace the polytetrafluoroethylene gasket with a polytetrafluoroelastomer gasket to enhance the compensatory ability of sealing, thereby improving the reliability of sealing in the presence of defects such as irregularities on the sealing surface. 4) During the major overhaul in June 1998, 203.2 mm manholes were added at the bottom of the F-4A/B to facilitate the removal of molecular sieve dust that settles at the bottom when cleaning and replacing the filter elements. 5) As a result of the above modifications, the operating cycle of the cold box increased to about 100 days, but this still did not meet the requirements. In August 1998, the filtration precision of the filter element was changed from 10µ to 5µ. 4 Effects of the modifications: Through these improvements to the filter element, the reliability of the sealing was enhanced, and the possibility of short circuits caused by sealing leaks was eliminated. Since the upgraded filter elements were put into use in October 1998, the production risk associated with short operation cycles of the cryogenic tank has been completely eliminated; now the tank is only purged by blasting during the annual major maintenance. By modifying the filter element, an additional approximately 4,000 tons of synthetic ammonia can be produced each year.