CANADIAN RETROFIT MEETS STIFFER SULFUR RECOVERY REGULATIONS

Elmo Nasato
Mobil Oil Canada
Calgary

B. Gene Goar
Goar,Allison & Associates Inc.
Tyler, Tex.

J. Borsboom
Comprimo B.V.
Amsterdam

Retrofitting Mobil Oil Canada's Lone Pine Creek gas plant with the Superclaus-99 process has led to SO2 emissions being reduced by 30-45%. Commissioning of the retrofit occurred in late 1990.

The process was chosen because of its relatively low capital investment requirements and its operational simplicity and flexibility. Retrofit requirements included minor equipment and piping modifications and a new catalyst for the third converter.

Operationally, Superclaus-99 (OGJ, Oct. 10, 1988, p. 68) is a straight-through, continuous process with a more flexible air-to-acid-gas control than the conventional Claus process. This retrofit was the first of its kind in North America.

TOUGHER REQUIREMENTS

The Mobil Oil Lone Pine Creek (LPC) gas plant, located 35 miles northeast of Calgary, is a sour-gas facility designed to process 30 MMscfd of raw gas and licensed to process 165 tons/ day of sulfur. The original sulfur-recovery license specified 90%, but the three-stage Claus facility consistently operated with a recovery of between 96.5% and 97.5%.

The acid gas, recovered by a diethanolamine (DEA) unit, was 65 mole % H2S and 35 mole % CO2. Typical loading of the plant in recent years had been about 60% of design capacity.

In March 1988, a four-well development program was initiated to utilize excess capacity at LPC in order to base load the facility. This program triggered new sulfur-recovery guidelines (Fig. 1) introduced in August 1988 by the provincial regulatory body, Alberta's Energy Resources Conservation Board (ERCB).

The new requirements stipulated a recovery level of 98.6% and a quarterly reporting average of 98.3%. These requirements exceeded the capabilities of the original conventional Claus plant.

The new Superclaus-99 process is a development of Comprimo B.V. and VEG Gasinstituut of The Netherlands. It was chosen to achieve the much higher recovery.

At the time, two Superclaus-99 retrofits were operating worldwide; this was to be the first in North America. Relative to alternate processes, Superclaus-99 offered higher recoveries at lower capital investment and operating costs.

Comprimo, however, would only guarantee a recovery level of 98.2% for the LPC plant, a level below the 98.6% required by the ERCB.

Mobil requested and was granted a relaxation period for an air license to operate the facility at 98.2% with a quarterly reporting, average of 98.0% in order to prove out this new technology.

Pending results of the basic retrofit, several enhancements were available if needed to improve recovery. These improvements were proposed and accepted as future alternatives to be executed in a step-wise procedure in order to gather the most scientific data possible and to verify the capabilities of the new Superclaus-99 process.

As a result of an internal Mobil hazard assessment (June 1988), the original LPC instrumentation and control system was to be replaced with a state-of-the-art distributed control system (DCS). The DCS project was scheduled for execution in conjunction with the maintenance shutdown and Superclaus retrofit in September 1990.

PROCESS ALTERNATIVES

To achieve the new recovery levels, two process retrofits were considered:

• The MCRC (Minerals Chemical Research Corp.) process

• Comprimo B.V.'s Superclaus-99 process.

Introduced in 1980, MCRC is a sub-dew point process that requires cyclic operation alternatively to regenerate the catalytic converter reactors. Estimate for the MCRC retrofit exceeded $4 million with a 15% quality estimate.

The Superclaus-99 process, commercially introduced in 1988, is a straight-through continuous operation that involves a new catalyst in the final catalytic converter that permits selective, partial oxidation of H2S directly to elemental sulfur.

The cost estimate for the Superclaus-99 retrofit was $1.4 million (Table 1), also with a 15% quality estimate.

Economically, the Superclaus process was the better option. But when Comprimo B.V., through Goar, Allison & Associates, could only guarantee a recovery level of 98.2% for the LPC retrofit, Mobil in November 1989 pursued the relaxation period from the ERCB to prove out this new technology.

The standard Claus process has certain limitations because of an approach to thermodynamic equilibrium, an increase in the amount of water vapor in the process gas as it flows through the train, a decrease in the concentration of H2S and SO2, and formation of nonrecoverable sulfur compounds resulting from side reactions.1

Also, the Claus process has the stringent requirement of maintaining the H2S/SO2 ratio at exactly 2:1 for optimum recovery.

The Superclaus process addresses the limitations of the Claus process.

A simpler, more flexible air-to-acid-gas control is incorporated. Also, a new catalyst in the final converter stage selectively oxidizes the H2S to sulfur directly.

Schematically, the Superclaus process (Fig. 2) varies only slightly from the Claus process (Fig. 3).

Operation of the sulfur-recovery unit (SRU) with the Superclaus process abandons the requirement of an H2S/SO2 ratio of 2:1. With Superclaus, the acid gas is burned with a substoichiometric amount of air so that the process gas leaving the second Claus stage will contain 0.8-3.0 vol % H2S.

Main air flow is controlled by ratio control on the inlet acid gas to the SRU, and an H2S tail-gas, air-demand analyzer controls trim air to maintain the 0.8-3.0 vol % H2S out of the second converter stage.

Process air is introduced upstream of the Superclaus stage. In this stage a non-equilibrium, partial oxidation reaction occurs, as shown in the following:

H2S + 1/2 O2 1/n Sn + H2O (1) This is a partial oxidation reaction of H2S to sulfur and incomplete oxidation to SO2. The complete oxidation is minimized by maintenance of a proper inlet temperature to the Superclaus converter. Oxidation of the H2S is made possible through use of the new Superclaus catalyst. The catalyst is alumina based, like the Claus catalyst, but coated with an iron oxide and chromium oxide layer. This catalyst promotes 80-90% of the H2S to be oxidized to sulfur. Compounds such as COS, CS2, H2, and CO are unaffected by this catalyst. Sulfur compounds other than H2S will pass through this catalyst as lost recovery.

Depending upon acid-gas quality and given an SRU with one thermal stage and two catalytic Claus stages upstream of the Superclaus stage, a recovery level of 99.0% can be achieved.

Lone Pine Creek, with an acid gas of 65% H2S, was guaranteed by Comprimo to achieve a recovery of 98.2%.

CHANGES, MODIFICATIONS

To facilitate the retrofit, certain equipment modifications were necessary. These consisted of equipment change outs, process piping rerouting, and the addition of instrumentation and control equipment.

As indicated earlier, the Superclaus retrofit was constructed in conjunction with the DCS retrofit. The DCS facilitated the centralization of burner control of the reaction furnace and all three in-line burners and Superclaus-oxidation air control. DCS also enabled better data gathering of pressure and specifically temperature data throughout the entire SRU.

Equipment and piping modifications of the original SRU included the following (Fig. 4):

• Conversion of Reheater No. 2 (RH2) from acid-gas firing to fuel-gas firing

• Replacement of RH2 burner with an LD Duiker burner

• Installation of a fuel gas-fired RH3 with a Duiker burner to supplement the gas/gas reheat exchanger

• Replacement of Condenser No. 4 with a larger unit

• Installation of a steam-jacketed oxidation air line and static mixer upstream of the Superclaus converter

• Installation of a Western Research air-demand analyzer (ADA) and oxygen analyzer

• Catalyst change out of all three converters, with Superclaus catalyst being used in Converter No. 3.

RH2 was converted from acid-gas firing to fuel-gas firing to avoid by-passing acid gas around Converter No. 1 and to prevent excessive SO2 in RH2 (lost conversion).

Computer simulations indicated that acid-gas firing of RH2 would result in a significant reduction of sulfur-recovery efficiency.

As a result of the need for fuel-gas firing, the original burner was replaced with a high-efficiency, multiple-vortex LD Duiker burner. This burner was chosen in an attempt to ensure complete combustion of the fuel gas so that oxygen break-through or soot formation would be held to a minimum.

The required inlet temperature for the Superclaus converter is higher than for a conventional Claus converter. This higher temperature requirement was beyond the capabilities of the existing gas/gas reheat exchanger.

It was decided to supplement the gas/gas exchanger with a fuel gas-fired reheater for the Superclaus stage; the gas/gas exchanger would act as an economizer to reduce fuel-gas consumption in the reheater. The burner on this reheater (RH3) is an LD Duiker, identical to RH2.

Exit temperatures of the Superclaus converter are significantly higher than for the original Claus process. Hence, the original Condenser No. 4 was replaced with a new unit having about twice the duty of the original exchanger.

Designing the new exchanger to utilize the existing sulfur rundown line process-gas outlet, and one of the two existing concrete supports simplified construction. The other concrete support was constructed 40 days before the maintenance shutdown, allowing adequate time for concrete curing.

The oxidation air line is a 152 mm (6 in.) line inside a 202 mm (8 in.) line, steam-jacketed with a 1,750 kPa gauge (230 psig) steam. The oxidation air is heated to a minimum of 120 C. (248 F.) before mixing with the main process stream upstream of the Superclaus converter.

To ensure proper mixing, the oxidation air is introduced to the process stream upstream of a 610 mm (24 in.) diameter stainless steel static mixer. The combined stream then enters the Superclaus converter.

The single most important SRU control-system modification was addition of the Western Research ADA and Western Research oxygen analyzer.

The ADA sample location is upstream of the Superclaus converter. The plant is operated to hold an H2S concentration in the tail gas leaving the last Claus converter stage between 0.8-3.0 vol % H2S.

The oxygen analyzer sample location is downstream of the Superclaus converter. Oxidation air is controlled to ensure excess oxygen in the Superclaus converter in the range of 0.5-0.8 vol %.

Excess oxygen is required to enhance the selective oxidation reaction in the presence of Superclaus catalyst. These analyzers provide the closed-loop fine tuning control required to optimize the sulfur recovery.

To incorporate the equipment modifications, piping demolition and rerouting of a major 610 mm (24 in.) process-gas line were necessary. Construction for the retrofit required 14 days with two full-time cranes and a work force of up to 90 people. The final SRU equipment layout is presented in Fig. 4c.

The original 20 tons of Claus catalysts support material, and carbon-steel mesh in the third converter were removed. Two layers of stainless-steel mesh were installed, 4 x 4 mesh covered by 10 x 10 mesh. Subsequently, two layers of inert ceramic balls were installed.

Finally, the Superclaus catalyst was installed by manual loading to reduce potential damage.

LPC RESULTS

The Mobil Canada Lone Pine Creek Superclaus process was commissioned Oct. 4, 1990, on schedule and on budget. This represents a success from a project execution perspective.

From the date of commissioning to August 1991, the Lone Pine Creek sulfur-recovery level was consistently between 98.2 and 98.5% (Fig. 5). These results have been verified by gas analysis and material-balance calculations.

Relative to the pre-Superclaus level of 97.0-97.5%, this improved recovery represents a 30-45% reduction in SO2 emissions, a significant environmental benefit.

A future enhancement of the process may be the development of a second-generation Superclaus catalyst by Comprimo and VEG. Commercially introduced in December 1990, this new catalyst can be added to an existing Superclaus converter.

The catalyst also promotes partial oxidation of H2S to sulfur but at a lower activation temperature. This would reduce the duty requirements of the reheater for the Superclaus stage and would permit other process and recovery benefits.

REFERENCE

1. Goar, B.G., Lagas, J.A., Borsboom, J., "Superclaus-A Fresh New Approach to Achieving Higher Claus Plant Efficiencies," Canadian Gas Processors Association Third Quarterly meeting, Sept. 15, 1988, Calgary.

Copyright 1992 Oil & Gas Journal. All Rights Reserved.