DESIGNING MOLECULAR SIEVE DEHYDRATION UNITS TO PREVENT UPSETS IN DOWNSTREAM NGL/LPG RECOVERY PLANTS

摘要

Molecular sieve dehydration is the industry-standard method of removing water from natural gas upstream of cryogenic NGL/LPG recovery units when significant recovery of light hydrocarbons (ethane and propane) is desired. The design of the dehydration system regeneration and subsequent cool-down operations can, however, have negative unforeseen impacts on the downstream cryogenic processing unit. Ortloff has designed four cryogenic NGL/LPG recovery units over the last 10 years where undesirable transient effects due to the upstream dehydration system design have been observed. In each of these facilities, the switching of a freshly regenerated mole sieve bed led to temperature and, in some cases, compositional disturbances at the inlet to the cryogenic unit, which then propagated through the entire unit over a 15-30 minute period. These disturbances caused process excursions which affected the recovery level of the unit, plus introduced some undesirable temperature variations at the heat exchangers. This paper analyzes data from the plants in question to show the effects on the cryogenic processing unit associated with the upstream dehydration unit design, and presents recommendations for the design of molecular sieve dehydration units which can minimize the disturbances.