For decades, sulfur reduction processes have been an integral part of refinery operations. The need for reliable and effective treating technologies is even greater due to increasingly strict product specifications. A commercially proven sulfur reduction extraction unit (a) is used to treat light feeds, such as natural gas, LPG and light naphtha. It uses a caustic solution to remove mercaptan sulfur in the extraction portion of the unit. The caustic is regenerated by oxidizing the extracted mercaptan to a disulfide, using an organometallic catalyst, and removing the disulfide from the caustic solution via a separator. The type of oxidation catalyst used in this process is extremely effective, but many customers have experienced plugging in their catalyst injection systems. These plugging events cause unexpected downtime and lost revenue for refineries.
As a result, a next-generation catalyst (b) has been developed. This catalyst improves treating performance compared to previous generations and eliminates plugging issues in the catalyst injection system due to its improved flow properties.
Mercaptan oxidation extraction catalyst injection plugging. With a processing capacity of 630,000 bpd, Motiva Enterprises is the largest refinery in the US and the sixth-largest refinery in the world. Located in Port Arthur, Texas, this refinery has the largest mercaptan oxidation (a) unit. It can treat 8,760 bpd of coker LPG with 3,359 wppm of feed mercaptan sulfur. During normal operations, the feed also has an average of 17 wppm of non-extractable sulfur. The product of the mercaptan oxidation unit is fractionated to a propane and butane cut, then routed to a propylene recovery unit and an alkylation unit.
Over the past several years, Motiva has used two different catalysts--one being a previous-generation catalyst (c)--to regenerate caustic used in mercaptan extraction. Motiva experienced catalyst injection system reliability issues when using these liquid catalysts. Specifically, residual catalyst material was plugging the injection pump's discharge and small instrumentation lines. The resulting low catalyst concentration in the caustic led to incomplete mercaptide oxidation and off-spec product mercaptans. These plugging events forced the refinery to route LPG to low-value fuel gas instead of high-value propylene product. Increased product LPG sulfur concentration also caused higher acid consumption in the alkylation unit.
Frequent intervention and maintenance by the unit operations team were required to address injection system plugging and avoid inconsistent catalyst injections. As a result, operational efficiency decreased, and operational costs increased.
Motiva consulted the authors' company to identify and eliminate the cause of the catalyst injection plugging and to improve unit reliability. The catalyst injection plugging was completely resolved due to two main factors. First, a next-generation catalyst was introduced (b) into the unit....