Optimising Sulphur Recovery Rate versus Carbon Dioxide Footprint
Figure 2: Sulphur Recovery Rate in a SubDewPoint Process as a Function of Outlet Temperature
100
99.9 99.8
99.7 99.6 99.5 99.4 99.3 99.2
100 105 110 115 120 125 130 135
Influence of last reactor outlet temperature on sulphur recovery rate in SubDewPoint operation. SRR = sulphur recovery rate.
Figure 3: Sulphur Recovery Unit at the NYNÄS Refinery, Sweden 140 Solid sulphur Liquid sulphur
be lower than ca 125 °C. Due to the Claus reaction the temperature in the reactor rises slightly so that the outlet temperature from the reactor is close to or even above 130 °C. This corresponds to an optimum SRR of ca 99.5 %. Fluctuations in feed gas composition and/or flow plus fluctuations of the controllers reduce the SRR so that in practice the long-term average of SRR in these plants is typically 99.0 to 99.2 %.
In SMARTSULF, the heat exchanger in the converter reduces the temperature from the typical inlet temperature of 190 °C to the outlet of down to ca 100 °C. In the catalyst bed, the sulphur formed is adsorbed on the catalyst faster than on the heat exchanger surface. This allows a reduction in the outlet temperature of the converter to lower values, even below the sulphur solidification point and that results in SRRs up to 99.85 %. Again, this value is under ideal conditions. The long-term average is typically by 0.2 to 0.3 % lower, as in the conventional SubDewPoint processes.
Such high values for the SRR could be achieved in the past only by much more complicated processes, such as a complete Claus plant with a downstream hydrogenation plus amine scrubber, as SCOT or equivalent for tail gas treatment. As the two-reactor process contains much less equipment and process steps, it is substantially cheaper and – being less complex – it tends to be more reliable as well.
The first commercial-scale plant using this two-reactor process was started up in December 1995 at the Nynäs Refinery, Sweden. Two gas streams from an amine unit and a sour water stripper had to be treated. The plant proved to be very reliable, easy to operate and
Conventional sulphur recovery processes with better than
99.0 % recovery rates have one disadvantage in common: they add tailgas clean-up processes to the Claus process.
new approach as it combines the catalytic Claus process with a SubDewPoint tail gas treatment in just two catalytic reactors.
The process principle of SubDewPoint tail gas treatment is well proven and is known from the CBA and SULFREEN processes. Different from these older processes SMARTSULF requires only two catalytic converters that are cooled internally. This allows a rather low temperature at the outlet of the reactors. As a direct consequence, the SRR rises, which is shown in Figure 2.
Thermodynamics clearly drive the SRR up as the temperature goes down. Lower limits are the water dewpoint of the gas, which must not be transcended, and kinetics, which might render the reaction too slow. This thermodynamic effect has been known for a very long time. But in the conventional processes it is not possible to operate the outlet temperature below ca 130 °C. The reason is that sulphur is a very good insulator. Therefore, the outlet temperature of the heat exchanger upstream of a SULFREEN or CBA reactor has to be kept above the sulphur solidification temperature, i.e. it cannot
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cheap to maintain. The onstream factor was always better than 99.5 %/year. The customer claims that even now, after more than 15 years in operation, this unit is the most reliable in the whole refinery. It showed SRRs as calculated, the optimum values reaching up to 99.85 %, with aged catalyst. At low-load operation, the SRR dropped by only 0.1 % at a turndown ratio of 6:1. Figure 3 shows a photograph of the plant.
Based on these positive results, the process was further improved over the years in some details and now a second generation is available. It is even lower in capital cost and some minor children’s diseases were eliminated. Essentially three modifications were introduced:
1. For internal cooling, a special type of plate heat exchanger, the thermoplate exchanger, is now applied, rather than the expensive coiled tubular heat exchangers used before. This reactor system is patented.
HYDROCARBON WORLD – VOLUME 6 ISSUE 2
SRR (%)
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