Predicting SCOT catalyst activity.(CLAUS TAIL GAS TREATING)

A new reactor model that relates traditional catalyst activity assessment to SCOT reactor performance has been developed by Shell Global Solutions. The new predictive tool enables better planning of SCOT turnarounds, thus avoiding unnecessary catalyst replacement and reducing the risk and costs of unplanned outages for unscheduled catalyst change-out. Fred Stone, Michael Huffmaster and Steve Massie discuss the new methodology and report on Shell's Puget Sound Refinery, which served as the test case for model development.

**********

Environmental performance of Shell Oil Company's Puget Sound Refinery (PSR) is critical in the environmentally sensitive, northwest US area of Puget Sound. Maximum allowed sulphur emission is 250 ppm S[O.sub.2] in incinerator stacks, on a dry, oxygen free basis, with incineration at 1200[degrees]F (649[degrees]C). Rolling hourly averages and 12-hour averages cannot exceed the sulphur limit. Violations require formally notifying the State of Washington. These criteria are fairly typical for US refineries; however, their enforcement in the Northwest is more stringent than in most parts of the country.

The Shell Claus Off-Gas Treatment (SCOT) process is used extensively to clean up Claus sulphur recovery plant off-gas streams to meet environmental requirements. The Claus process achieves 94 to 97% sulphur recovery. SCOT improves overall sulphur recovery, is the most widely selected tail gas clean-up process, and is the industry standard when tail gas recoveries of 99% to 99.95% are required. Today more than 190 SCOT units are licensed to operate throughout the world, with upstream Claus unit sulphur capacities ranging from 3 to over 2400 long tons per day.

With a standard SCOT unit, an overall sulphur recovery efficiency of 99.7% or better can be achieved. This standard SCOT process can be enhanced to meet more stringent environmental requirements or reduce operating cost:

* LS-SCOT delivers a [H.sub.2]S specification of less than 10 ppmv in the SCOT vent gas, which can be achieved by utilising a small concentration of an additive in the solvent. Overall sulphur recovery exceeds 99.95%.

* LT-SCOT achieves performance level of traditional SCOT but with more active catalyst that functions at lower bed inlet temperatures, reducing capital costs or energy required.

The SCOT process includes a reaction step, reducing sulphur compounds in SRU off-gas to [H.sub.2]S, and then an [H.sub.2]S capture step, selectively removing [H.sub.2]S in amine treating section. In the reaction step, catalyst activity governs the reducing function of non-[H.sub.2]S sulphur compounds, converting them to [H.sub.2]S by hydrogenation and hydrolysis. There are several operating variables that affect reactor performance, but the key parameters are the activity condition of SCOT catalyst and bed temperature.

Declining performance level for SCOT catalyst is quantified in a catalyst testing standard for End-Of-Run (EOR). From a design standpoint it is the lowest level of performance for the catalyst, but the condition for which the unit is designed to meet performance guarantees. In practical terms, it represents a decline to 50% of fresh activity. In pragmatic terms, it means that catalytic activity to meet unit performance has to be achieved by increasing reactor bed temperatures. It is the beginning of the end, as the catalyst will age more rapidly and lose activity more rapidly. Catalyst at EOR should be replaced before beginning a normal three to five year cycle for turnaround. As a result, it is critical to accurately forecast the catalyst life of SCOT units.

PSR is not allowed to operate with sulphur emissions exceeding permitted limits, thus having SRU 3 SCOT off-line for a catalyst change has significant economic consequences. Refinery operation without SCOT requires a reduced diet of alternative sweet crude and/or significant slow down of CCU and coker units. SRU-3 SCOT was scheduled for a turnaround in 2004, during which refinery operation would have been curtailed. Extending the current run for a year to 2005 meant avoiding refinery curtailment all together. A second SCOT unit scheduled to be on-line in 2005 will provide redundancy for tail gas cleanup and, after it is operational, will enable SRU-3 SCOT shutdown for catalyst replacement without a refinery operational slowdown for sulphur emission constraints.

Operation of SRU-3 SCOT catalyst to 2005 requires a five-year catalyst run, which is longer than the typical planned four-year run, but not an unreasonable expectation for SCOT catalyst in refinery service. As a result, there was considerable incentive for extending the SCOT run and keeping SRU-3 and SCOT on-line until the new SRU-4 (Claus & SCOT) was on-line, so no refinery slow down would be required. A high probability for satisfactory performance must be established for the fifth year of …