Apr-2008

New direct oxidation process for sulphur recovery

First tests are due to be carried out this year on a newly developed catalyst. An offshore demonstration unit is to be used to recover sulphur from acid gas containing H2S, currently being sent to flare.

G Srinivas and M E Karpuk, TDA Research Inc
C T Ratcliffe, Unocal Corporation
D Leppin, Gas Research Institute
W Kensell and M L Raven, The M W Kellogg Company (Now KBR Technology)

Article Summary

A new catalyst for the direct oxydation of hydrogen sulphide to sulphur has been developed in the USA by TDA Research with the aim of recovering H2S as bright yellow sulphur instead of incinerating it to SO2 and emitting it to atmosphere.

As part of a project funded by the Gas Research Institute (GRI) and the Department of Energy (DOE), Spirit Energy 76 – a business unit of Unocal Corporation – will host and co-fund work for a demonstration unit on one of its offshore platforms in the Gulf of Mexico. The installation is scheduled to be completed by June this year, with testing later in the summer. It is hoped that final results of the tests should be ready for collation by 1999.

The catalyst test evolved from the identification by Spirit Energy of a sour tailgas stream from an existing offshore platform that contained CO2 and a low concentration of H2S, varying from 0.5–1.5 per cent that had to be treated by a process that could offer minimum footprint and weight.

TDA Research’s Catalyst/Technology for selective oxidation of H2S to sulphur offers these characteristics. It provides simplicity, small footprint and high recovery capability with a very dilute H2S feed stream. The conversion process involves a single gas-phase reactor chamber, so high onstream time and reduced maintenance should be possible, as compared to conventional sulphur recovery processes.

Redox processes are the proven technology for this application, but they are inherently higher in capital cost, chemical replacement cost and maintenance. Other small-scale sulphur recovery units have limitations for this application. The required treatment in an offshore environment, and the rather large quantity of H2S, places a high cost on replacement and disposal of adsorbents used in scavenger processes. Modified Claus has many commercial applications, but requires a more concentrated H2S stream.

The process chemistry for the TDA Direct Oxidation Process is described as follows:
Primary reaction (desirable):
H2S + 1/2O2 Æ H2O + S
    (Selective oxidation)       (1   
Secondary Reactions (undesirable):
H2S + 11/2 O2 Æ    H2O + SO2      (2
S + O2 Æ SO2              (3
3 S + 2 H2O Æ 2 H2S + SO2      (4

The first reaction, which is promoted by the catalyst, is selective to the formation of sulphur, rather than to the production of SO2. The following summarises the test results obtained by TDA Research during catalyst performance tests using gas compositions typically produced by amine units.

Temperature has a significant impact on the catalyst performance. Lower catalyst temperatures generally favour higher selectivity. (Selectivity refers to the conversion of H2S to sulphur via equation 1. H2S conversion refers to H2S being converted to either sulphur or SO2 via equations 1, 2 and 4).

However, the temperature of the catalyst bed must be higher than the sulphur dew point. If the catalyst temperature is below the sulphur dew point, the sulphur formed during the reaction condenses in the pores and deactivates the catalyst. Heating the catalyst to temperatures above the sulphur dew point restores the catalyst activity.

Figure 1 shows the measured conversion of H2S and the selectivity at temperatures between 180°C and 210°C. This temperature range was optimum for this feed gas composition, which is typical for the offshore production platform.

The figure shows that raising the temperature from 180°C to 200°C causes the H2S conversion to increase from 93 per cent to 96.5 per cent. However, the conversion decreases slightly when the temperature is raised to 210°C. The selectivity was highest at 180°C and decreased gradually as the temperature was increased to 210°C.

Water reduced the catalyst activity. With no water present in the feed stream, the conversion and selectivity to sulphur were nearly 100 per cent at 190°C. But when 6 per cent water was introduced into the feed stream, the conversion dropped to 96 per cent, as shown in Figure 1. It is thought that water promotes reaction 4 as written.

Gas flow affects conversion but has little impact on selectivity. H2S conversion was measured at gas hourly space velocity (GHSV) ranging from 18000h-1 to 6000h-1 at a temperature of 180°C and using a feed containing 0.26 per cent H2S.

As the GHSV was reduced from 18000–1 to 12000h-1, the conversion of H2S increased from 81 to 91 per cent. A further reduction in GHSV, to 6000h-1, only increased the conversion by about 2 per cent. The selectivity was between 95 and 98 per cent over the entire range of flow rates tested.

O2:H2S ratio affects both activity and selectivity. Tests were conducted at 240°C and using a feed containing 8 per cent H2S. Under rich conditions (O2:H2S < 0.5), nearly 100 per cent selectivity was measured, while conversion was limited by the lack of oxygen. Under lean conditions (O2:H2S > 0.5), maximum conversion occurs but the selectivity decreases.

Durability testing showed constant activity over a 250-hour period. The catalyst was tested at a temperature of 240°C using a feed containing 5 per cent H2S and flowing at a GHSV of 9883h-1. Both conversion and selectivity remained greater than 96 per cent over the 250-hour test period.