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Improvements to tailgas treatment process

A review of configurations in tailgas treating to achieve 99.9% sulphur recovery with smaller reactors

Jean-Pierre Ballaguet and Cécile Barrère-Tricca, IFP
Christian Streicher, Prosernat
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Article Summary
Elimination of problems with corrosion, catalyst stability and improved sulphur quality to IFP’s Clauspol process has made this 30-year-old wet sub-dew point Claus tailgas  treatment (TGT) process simpler, smoother running and more cost effective in achieving over 99.9% sulphur recovery. The most recent developments have been to the desalter. The principle of this “packaged” desalter is to withdraw the salt-rich solvent in small amounts, treating the slipstream in a small package that operates independently of the Clauspol unit.

This package generates a solvent, essentially free of salts, which is returned to the reactor. Periodically, a salt solution can be sent to the waste water treatment plant. The desalter’s primary advantages are:
• Clauspol unit is expected to operate five years in between scheduled turnarounds, thus minimising SO2 emissions
• Cost of the system is marginal and largely compensated by the savings due to reduced reactor size
• Reactor size can be reduced because the role of the reactor packing is now simply that of ensuring the reaction without the need to act as a salt or storage medium that avoids sulphur-solvent settling problems. By changing the reactor’s gas/liquid contactor with a more efficient design, IFP plans to reduce the reactor size even further. 

Two Clauspol process versions have been available since the middle of 1998: the basic process for up to 99.8% sulphur recovery; and the Clauspol Booster 99.9+ for over 99.9% sulphur recovery. Both configurations are based on the continuation, in a non-volatile liquid organic solvent, of the Claus reaction between the residual H2S and SO2 present in Claus tailgases as shown in the equation:

2 H2S + SO2 ⇔ 3/x Sx + 2 H2O   
The Claus tailgas enters the bottom of the reactor and flows up through a packed bed (Figure 1) countercurrently through the solvent. H2S and SO2 are absorbed by the solvent, which contains a proprietary catalyst. The reaction is performed at temperatures slightly above the sulphur melting point (120°C). The produced sulphur is only slightly miscible with the solvent and, owing to its higher density, is recovered as a separate liquid phase from the bottom of the reactor.

Very high conversion rates are achieved if the H2S:SO2 ratio is properly controlled and maintained near the stoichiometric value of two. Under such conditions the residual levels of H2S + SO2 in the Clauspol off-gas can be tailored down to 150ppmv S (parts per million in volume expressed as elemental sulphur), depending on the size of the reactor. Levels as low as 100ppmv S (H2S + SO2) have been commonly obtained in industrial plants.

The process reaction rate is limited primarily by the transfer of H2S and SO2 from the gas phase to the solvent. Therefore, the reactor is packed with high surface area ceramic intalox saddles. The process is not very sensitive to feed inlet H2S and SO2 concentrations, as long as the average H2S:SO2 ratio remains constant. The process has therefore a marked buffer capacity towards fluctuations of the Claus tailgas composition and towards short term fluctuations of the H2S:SO2 ratio. Other sulphur components that are advantageously removed from Claus tailgas include elemental sulphur as vapour or as entrained liquid, and COS and CS2, which are partially hydrolysed to H2S in the reactor.

The other compounds most frequently encountered in Claus tailgas have very little effect on the process. Only residual oxygen might have some deleterious effect on the solvent but at the levels encountered in Claus tailgases (typically below 100ppmv), such effects are too small to be observed. IFP’s industrial experience with the process has shown that, under proper operation, the solvent remains stable and behaves satisfactorily for many years.

The packing induces very low-pressure drop on the gas phase and avoids the need for any additional blower. The only rotating equipment needed by the process is the solvent circulation pump. Its flow-rate needs to be kept at the minimum level necessary for complete wetting of the packing. Catalyst degradation compounds that accumulate on the packing, mainly sodium sulphate salts, need to be removed periodically (every two to four years) by a water wash.

The process needs no hydrogen, a decided cost and operating reliability advantage when compared to processes that convert the sulphur compounds to H2S and recycle it back to the Claus unit. Clauspol, then, frees existing Claus capacity by avoiding recycle streams to the Claus unit as well as not being dependent on hydrogen supply. A final advantage of the process is its very low operating cost. In fact, the process only requires some power to drive the solvent circulation pump, a small amount of continuous catalyst make-up in order to maintain the catalytic activity in the reactor, and some cooling water for temperature control. Among these items, electrical power represents more than half of the total operating cost. Table 1 provides economic information for a Clauspol unit treating a typical Claus unit tailgas (combined production of 100 tonnes/day of sulphur).

Sulphur recoveries of 99.8% can be reached in the basic Clauspol process provided that the COS and CS2 have been hydrolysed to below 200ppmv in the catalytic stages of the Claus plant. Only the most active specialised TiO2 Claus catalysts (Axens CRS 31) can fulfil this requirement.

Booster process
Elemental sulphur in reactor effluent is present only as vapour in equilibrium with liquid sulphur formed in the reactor. This corresponds to around 350ppmv S. Under the normal operating conditions of the basic Clauspol, the solvent in equilibrium with liquid sulphur contains around 2–3 wt% of elemental sulphur. Contacting the offgas with an unsaturated solvent (containing less sulphur than at equilibrium with liquid sulphur) results in a much lower elemental sulphur concentration in the gas.

The proprietary solvent desaturation loop, developed by IFP, enables the reactor to be operated using such an unsaturated solvent, without modifying the basic reactor features. It is thus possible to reduce the amount of sulphur vapour in the reactor effluent to levels as low as 50ppmv or if required lower. Therefore, sulphur recoveries over 99.9% can be reached with the Clauspol Booster 99.9+, with only marginal additional cost.

The process with the desaturation loop is illustrated in Figure 2. A fraction of the solvent from the re-circulation loop is withdrawn and cooled in a heat exchanger, to temperatures where solid sulphur precipitates (typically 50–70°C). Solid sulphur is then separated from the cold solvent with a filter. The clarified solvent from the filter is heated up and sent back to the solvent circulation loop. The slurry resulting from the discharge of the filter is heated to melt the sulphur in a heat exchanger and sent to the bottom of the reactor where the liquid sulphur settles.

The equipment needed for the desaturation loop is comparatively small, resulting in a minor additional investment cost for the installation of the desaturation loop. This additional investment increases the cost of the TGT unit by approximately 10%. For the same reason, operating costs of the desaturation loop (mainly cooling water and low-pressure steam) is also comparatively low, increasing those of the entire unit by around 20%.
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