GAS-DESULFURIZATION PLANT HANDLES WIDE RANGE OF SOUR GAS COMPOSITIONS

Nigel A. Taylor, Jim A. Hugill
Shell Internationale Petroleum Maatschappij B.V.
The Hague

Mathieu M. van Kessel, Rene P. J. Verburg
Nederlandse Aardolie Maatschappij B.V.
Emmen, The Netherlands

The Nederlandse Aardolie Maatschappij B.V. (NAM) gas-desulfurization facilities at Emmen treat a natural gas feed containing H2S, CO2, and mercaptans, to tight pipeline specifications.

The highly selective Sulfinol-M solvent enables the plant to treat natural gases with a CO2/H2S ratio as high as 25:1, while producing an acid-gas feed suitable for a conventional Claus unit.

To help meet the stringent environmental regulations, an integrated Shell Claus off gas treating (SCOT) unit achieves an overall sulfur recovery of better than 99.8%.

INTRODUCTION

Shell's Sulfinol process removes contaminants from gas streams by countercurrent contacting with a regenerable liquid solvent. The process has proven especially successful in the treatment of high-pressure natural gas or syngas streams.'

The Sulfinol solvent is of the "mixed solvent" type, being a mixture of a chemical solvent (an aqueous amine) with a physical solvent (sulfolane). The addition of sulfolane to the aqueous amine gives a number of advantages, notably:

Better solubility for organic sulfur compounds
Easier regeneration
Lower foaming tendency
Less water vapor in treated gas.

On the other hand, a potential disadvantage is the somewhat higher solubility for hydrocarbons. However, this solubility is still far less than that of a purely physical solvent, and is normally considered acceptable.

The original Sulfinol process, introduced in the early sixties, made use of the solvent now known as Sulfinol-D, of which the amine component is diisopropanolamine (DIPA). There are now about 140 Sulfinol-D units worldwide.

The eighties saw the advent of Sulfinol-M, of which the amine component is methyldiethanolamine (MDEA). Sulfinol-M retains the aforementioned advantages of Sulfinol-D and, in addition, has a much greater selectivity for H2S in the presence Of CO2.

This makes it especially attractive when it is unnecessary to remove all the CO2 from the gas, or when it is necessary to increase to a maximum the H2S/CO2 ratio of the acid gas that is removed.

The Emmen plant was the fifth Sulfinol-M plant to be started up, and is by far the most sophisticated.

EMMEN PLANT

The Emmen gas-desulfurization plant came on stream on July 18, 1988. The plant processes sour gas from a variety of small gas fields in the northeast portion of The Netherlands. It has a capacity of 300 MMscfd. A simplified block scheme is shown in Fig. 1.

After condensate removal and compression, the sour gas is treated in two parallel trains of Sulfinol and molsieve units to remove H2S, mercaptans, and water. Each of the two parallel Sulfinol trains has five Sulfinol-M absorbers sharing a common regenerator.

The sweet sales gas is fed to the grid of Nederlandse Gasunie N.V., a Dutch gas distribution company.

The plant has been designed to cope with a wide range of sour gas compositions (H2S, 0-1-1-0 Vol %; CO2, 2.0-5.0 vol %; and mercaptans, up to about 90 ppm [vol]) while meeting stringent product specifications (H2S < 3.5 ppm [vol], mercaptans < 7 ppm [vol]).

The feedstock is characterized by a high CO2/H2S ratio (up to 25, vol/vol). This imposes special requirements on the design and operation of the treating units, in view of the need to provide a suitable feed to the Claus sulfur-recovery unit-preferably 40 vol % H2S.

The plant is located in a rural area, and is subject to tight environmental regulations, concerning not only SO2 emissions, but also such trace sulfur components as H2S, COS, and CS2. Fig. 2 shows the gas desulfurization facilities at the Emmen plant.

PROCESS DESCRIPTION

Details of the Sulfinol lineup are shown in the process flow scheme, Fig. 3.

The two key features of the plant are:

The high H2S/CO2 selectivity of Sulfinol-M, which is exploited not only in the main absorber, but also in the "flash enrichment" step. In this step, the H2S/CO2 ratio of the fat solvent is enhanced by selective flash regeneration.
The overall selectivity is increased to a maximum by optimum column design and by a judicious selection of operating conditions and solvent formulation. This enables the processing of sour gas with a high CO2/H2S ratio to produce a suitable Claus feed.
The high degree of integration, with five Sulfinol absorbers-including the SCOT absorber-sharing a common regenerator. This reduces the capital cost of the plant to a minimum.

In the main absorber (C-1, Fig. 3), the H2S and part of the mercaptans are removed by washing with Sulfinol-M at high pressure (about 940 psia, or 65 bara).

Most of the CO2 present in the feed gas slips through the absorber.

The remaining mercaptans, as well as water, are removed by the downstream molsieve unit.

This unit comprises two parallel absorbers with common auxiliary equipment so that one bed can be regenerated while the other is being used for adsorption.

Mercaptans are removed from the molsieve regeneration offgas by means of the Sulfinol absorber (C-3, Fig. 3).

The fat solvent from the main absorber is first flashed at medium pressure in the vessel (V-1) to remove dissolved/entrained hydrocarbons. The hydrocarbon stream is used as fuel gas after desulfurization by recontracting with lean solvent in the absorber (C-2).

The solvent from V-1 is heated and then flashed at low pressure in the "enrichment flash" vessel (V-2) to remove most of the dissolved CO2-

The CO2-rich offgas, which contains some H2S and mercaptans, is treated with lean solvent in the enrichment absorber (C-4) prior to catalytic incineration of the remaining trace components.

The enriched solvent from V-2 is sent to the regenerator (C-5). The resultant acid gas contains more than 40 vol % H2S and is thus suitable for routing to a conventional Claus unit (no split-flow or oxygen-enrichment scheme is required). The Claus tail gas is routed via the SCOT unit to the catalytic incinerator .2 The SCOT absorber (C-6) is integrated with the Sulfinol system. The Claus/SCOT combination has an overall sulfur recovery efficiency of greater than 99.8%.

Most of the electricity and steam are generated in a gas-fired gas turbine/wasteheat boiler cogeneration unit. Because an uninterrupted steam supply to the Sulfinol unit is vital to the operation of the plant, steam production capacity is spared by two standby boilers.

OPERATING EXPERIENCE

In November 1988, two acceptance test runs were carried out. A summary of the results is presented in Table 1. It can be seen that actual performance concurred with design. The plant has now been in operation for over 2 years, including two inspection and maintenance shutdowns.

It was not possible to maintain design capacity throughout the first year, because of insufficient adsorption capacity of the molsieves. This was attributed to carryover of Sulfinol.

Accordingly, during the first maintenance shutdown in July 1989, some remedial modifications were implemented in the top of the main absorber and in the downstream knockout vessel to reduce entrainment and thus restore production capacity.

In addition, the capacity of the molsieves was increased by means of a stacked-bed configuration (comprising a top section for selective water removal and a bottom section for mercaptan removal), and by applying densebed loading.

Test runs following the 1989 shutdown confirmed that the design production capacity of 287 MMscfd at an intake of 300 MMscfd had been completely restored. Throughout the year, design capacity could be achieved within the constraints of product quality and SO2 emissions.

Actual test-run data fitted very well with the design limitations in terms of H2S and CO2 contents of the plant feedstocks. Moreover, operation was characterized by a high on stream time (plant availability 99.5%).

The quantity of hydrocarbons dissolved in the solvent and recovered as fuel gas amounts to some 0.3% on intake. Nonrecoverable hydrocarbon losses are found to be less than 0.05% on intake.

After 2 years of operation, no significant solvent degradation has been observed. Furthermore, the low foaming tendency of the solvent has been maintained with minimum use of antifoam agent.

Equipment inspection carried out during the maintenance shutdown in July 1990 did not reveal any significant corrosion.

FUTURE DEVELOPMENTS

Following the minor modifications implemented during the first maintenance shutdown, operation of the Emmen plant is considered satisfactory in terms of plant capacity and product quality. The plant also complies with the stringent environmental permits.

Current efforts are essentially directed toward further optimization of plant performance with respect to operating costs and compliance with requirements resulting from evolving environmental legislation.

Experience with the Emmen plant over more than 2 years has proven the ability of the Sulfinol-M process to meet a tough treating challenge in an economical, flexible, and reliable way.

Two further Sulfinol-M plants, both similar to the Emmen lineup, are now under construction:

Natural gas treating and conditioning plant, south of Mobile, Ala. (Shell Offshore Inc.'s Yellowhammer plant)
A syngas treating plant at Buggenum, The Netherlands (part of the Demkolec demonstration plant for the Shell coal gasification process).

REFERENCES

Nasir, P., "A mixed solvent for a low total sulfur specification," American Institute of Chemical Engineers summer national meeting, Aug. 19-22,1990, San Diego.
Wetzels, M.L.J.A., Dam, W., and Hugill, J.A., "The SCOT Process: still very much alive nearly twenty years after its first introduction," Western Research Seminar, November 1990. Budapest.