The CHDA (1,4-cyclohexanedicarboxylic acid) production plant that I work at experienced a significant decrease in conversion efficiency due to oil contamination of a fixed bed reactor catalyst. The source of the oil proved to be a pneumatic convey system blower. A specialty filter was installed to prevent the possibility of future catalyst contamination.
After noticing the decline in reactor conversion efficiency, we analyzed samples of the used catalyst and found they were contaminated with oil. A process review yielded several potential sources, which were narrowed down by inspection. We identified the culprit as a pneumatic convey blower used to transfer PTA (purified terephthalic acid) from dry bulk pneumatic trailers to a mixing tank. Oil was coating the inside of the discharge piping of the Roots rotary lobe blower, and this had contaminated the PTA as it was conveyed from the trailer to the tank.
PTA Unloading Blower Package
The area lubricator confirmed that he had topped off the oil reservoir on the blower for several months due to low oil levels. A review of the CMMS system revealed that the blower had no maintenance performed on it in the last eight years. The blower was removed from service and replaced with a spare. Teardown of the blower showed that the mechanical seals that separate the oil-lubricated bearings from the process gas side of the blower had leaked, allowing oil to enter the process gas side of the blower.
Since the cost to change out the catalyst exceeded $150,000, area operations management wanted a solution that would prevent this from reoccurring. Some of the alternatives included a close-coupled regenerative blower with permanently sealed bearings, a different lobe-style positive displacement blower with improved seals, and a filter to capture any oil leakage that might occur.
We agreed that the use of a ZC filter was the most cost-effective solution from a life-cycle cost standpoint. The filter has a 1-micron rated media that handles the 800 cfm of hot 20 psig nitrogen with very little pressure drop. It is advertised to capture 100 percent of hot oil or oil mist from blower bearing or seal failures. We also initiated a preventive maintenance program for the blower that includes monthly vibration readings, periodic oil changes and inspection for leaks, and filter inspection. In addition, all area lubricators were informed to contact maintenance when they noticed equipment requiring abnormal oil replenishment.
The filter installation involved a simple change to the blower discharge piping, and a differential pressure gauge was installed to allow operations to monitor the filter element. The filter cost less than $1,000; the total project cost close to $3,000. No contamination in the discharge has occurred since the filter was installed.
Kevin G. Bellamy is a principal reliability engineer at Eastman Chemical Company.