Keith Eddington
BP Exploration
Welton, U.K.
Peter Carnell
ICI Katalco
Cleveland, U.K.
Compact fixed-bed reactors can control H2S emissions from hydrocarbon vent gases.
Once installed there is no operating requirement other than routine analysis.
This type of reactor is now in service on oil platforms in the North Sea and in the U.S. The design was first tested in the Welton oil field, located in Lincolnshire, U.K.
In this field, oil produced at the well site travels through underground flow lines to a gathering center situated about 6 miles from Lincoln. The center processes up to 6,000 b/d of stabilized crude oil that is shipped by rail tank car.
The crude oil has a sulfur content of 0.01% wt/wt and although the gas-to-oil ratio is low, the associated gas can have a high sulfur content, up to 15,000 ppm H2S by volume.
The field was designed and is operated to meet very tight environmental constraints. The operations must be hidden from view and there must be no odors.
These constraints pose problems for the operation of the Welton site. Local planning constraints precluded the use of vents or flares on the well sites; therefore, the Welton site has had to develop a novel approach to deal with minor gas venting.
ACCEPTABLE H2S LEVELS
An environmental impact assessment report considered the odor threshold and nuisance concentrations for H2S emissions. A gas-dispersion computer model enabled back calculating the maximum level of H2S permissible in a low-level vent, 10-m high.
This indicated that the peak H2S emission for a 10-m high vent stack should not exceed 200 ppm.
Potential sources of H2S odor nuisance are:
- Rail car vent gases
- Process area blowdown sump
- Manifold blowdown sump
- Well site blowdown sumps.
All of these sources are characterized by having intermittent discharges of hydrocarbon gas containing high levels of H2S. The gas rates are low and discharge pressure is close to atmospheric.
The rail car vent gases are controlled by a closed-circuit caustic scrubber system. This system was not considered suitable for the other vents.
Over recent years ICI Chemicals & Polymers Ltd. has developed processes for the removal of sulfur species such as H2S from hydrocarbon gases and liquids at temperatures down to ambient. This technology (marketed as Puraspec technology) uses fixed beds of catalytic absorbents to remove the impurities by chemical reaction.
In 1988, BP Exploration and ICI decided to carry out trials to see if this technology could be adapted to control vent gases and therefore solve an environmental problem.
Accordingly, a test reactor was installed on the process area blowdown sump. The main concerns to be resolved were the resistance to flow of a fixed-bed system, the effect of wetting with heavy hydrocarbons, and the effect of low winter temperatures (-5 C.).
TEST REACTOR TRIALS
A test reactor was formed from a radial-flow carbon gas filter. It was felt that a radial-flow reactor would minimize the pressure drop.
The reactor was installed between the sump manifold and the vent stack. Gas samples were collected before and after the reactor during normal operations and analyzed for H2S. These results were very encouraging.
Inlet H2S contents typically varied from 5,000 to 10,000 ppm with peaks as high as 15,000 ppm. The reactor reduced these to acceptable levels for a reasonable life. The tests were continued through the winter of 1987-88 with different cartridges fitted to the reactor. Typical results for the test reactor and the permanent reactor are listed in the table.
These tests showed that the fixed-bed approach was feasible. The bed did not offer too much resistance to gas flow.
No attempt was made to dry the gas. The bed was wetted with a small amount of hydrocarbons but this did not affect performance.
Typical hydrocarbon loading was around 2%. Low ambient temperatures caused no problems. However, the radial-flow design was found to have limitations. Flow rates were different across the bed so that the absorbent could not be used to greatest efficiency and the reactor was relatively bulky for the capacity of the cartridge.
PERMANENT REACTOR
The test results were sufficiently encouraging to justify the installation of permanent desulfurization reactors. It was decided to change from radial flow to axial flow for the reasons previously given.
Normally, fixed-bed systems use down flow but because the gas was wet and the pressure low, an up-flow design was selected.
It was assumed that the maximum gas rate will be about 10 cu m/hr (8.5 Mcfd). The cartridge principle was followed to allow easy operation and this was sized for a manageable weight of about 30 kg.
Construction material was carbon steel with welding procedures to limit hardness in the heat affected zone to Rockwell Hardness HRC 22.
The first permanent unit (Fig. 1) was installed in April 1989 on the process-sump blowdown vent (Fig. 2).
Regular testing (see table) has shown that this reactor completely controls H2S emissions. This unit has been in service for over a year.
Similar units have now been installed on the other three vents and two are being used on a nearby development well site,
The units have been found easy to install on existing plants. Recharging the reactor is a simple, safe operation taking only a few minutes. For these applications it is estimated that both capital and operating costs are less than 10% of alternative systems.
The installed cost of the reactor was about $9,000, of which the cost of the reactor itself was around $2,000. Operating costs of the absorbents are less than $500/year.
The H2S is removed from the gas stream by reaction with the absorbent to form stable metal sufides. Hence, no impurities are added to the gas stream. The spent absorbent is nonhazardous and is discharged in the form of free-flowing granules. It can be disposed of through the metal recovery industry.
Further tests need to be carried out to ascertain the maximum amount of hydrocarbon wetting which would be tolerated by the absorbents before efficiency became unacceptable.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
