Modifying a Mol-sieve Dryer Sequence Control to Enable Two Beds in Simultaneous Adsorption Mode during Unplanned Maintenance Activity

In a Liquefied Natural Gas (LNG) Plant, natural gas pre-treatment prior to liquefaction process typically includes a molecular-sieve (mol-sieve) based dehydration unit divided into two or three beds mode depending on the designated capacity.In a case study in LNG Plant "A", the mol-sieve dryers consist of 3 x 50% beds full rate capacity, normally with two beds in adsorption mode while the third one is being regenerated or stand-by. One of the dryer beds had an integrity problem and required it to be isolated and repaired within a week. On the other hand, the Mercury Removal Unit (MRU), the column to dissipate mercury content in natural gas, also indicated a plugging which requires a defrosting process and requires full capacity of natural gas flowing into it for certain period.With one of the drier beds being completely out of operation and the requirement of full capacity to test the MRU, some changes in dehydration sequence control are required to keep the production capacity stable and ensure the MRU capacity while waiting the dryer bed repair to be completed.The changes made to the dehydration sequence control was included the changes in sequence table, control drawing programming script, and graphic interface to allow new "maintenance mode" can be activated for bypassing the existing sequence control limitation so the two drier beds can be operated in adsorption mode in at least one cycle.