Optimising Sulphur Recovery Rate versus Carbon Dioxide Footprint a report by Michael Paul Heisel General Manager, ITS Reaktortechnik GmbH
Refineries these days need to minimise all their emissions, including
CO2, which was of less importance or even totally neglected only a decade ago. As a consequence, processes in the past were not
optimised with respect to CO2. This also holds true for processes for sulphur recovery and Claus tailgas treatment. In light of today’s priorities the conventional and well-known processes should be evaluated. This will be done in this article and suggestions for a more adequate sulphur recovery process are made.
Evaluation of the Different Types of Tailgas Treatment Processes for their Carbon Footprint
For evaluation of the CO2 footprint, the most applied and well-known tailgas treatment principles of hydrogenation plus amine scrubber, SubDewPoint operation and direct oxidation were considered, represented by the processes Shell Claus Off-gas Treatment (SCOT), SULFREEN and SUPERCLAUS. For these processes, the utility
consumptions were calculated and converted to CO2 emissions. The conversion factors applied are listed in Table 1.
With the data listed in Table 1, the various tailgas treatment processes can be compared and their respective CO2 footprints quantified. In Table 2, the CO2 emissions of the different processes are listed together with their respective SO2 emission from a 100 t/d sulphur recovery unit.
The energy trends in the different processes can easily be understood when considering their set-up. Hence SCOT consists of a two-stage Claus plant with a hydrogenation and a complete amine scrubber downstream. The scrubbing step requires a lot of energy for regeneration of the solvent and for pumping it around. Therefore it is not surprising that it requires more energy than the other three processes in the comparison that contain no scrubber.
SUPERCLAUS is a three-stage Claus plus the direct oxidation step downstream. Every one of these four steps has a reheat by medium pressure (MP) steam to the catalyst reaction temperature followed by a
Michael P Heisel has more than 30 years’ experience with sulpur recovery plants. He started his career in the engineering division of Linde Gas and Engineering. He then became in charge of process selection for Claus plants. He invented a number of novel processes in that field. Two of them received Kirkpatrick Awards, being considered to rank among the worldwide best innovations in the chemical industry. He has published numerous papers and patents used in licensed plants.
In 2000, he founded ITS Reaktortechnik GmbH, a company engineering sulpur recovery plants and licensing out his patents. He holds a PhD from the Technical University of Munich/Germany.
E:
m.heisel@its-reaktortechnik.de
cooling step to condense the sulphur formed, raising low pressure (LP) steam. So in essence each step consumes MP steam and produces LP steam, or in other words converts MP steam to LP steam, which means a loss of energy.
SULFREEN is a two-stage Claus plant with a SubDewPoint reactor downstream. No reheat is required for this third reactor. But after having charged the SubDewPoint reactor with sulphur it has to be regenerated, which is again done by heating the reactor by MP steam and recovering the sulphur released in a condenser and raising LP steam. Overall, SULFREEN is slightly more energy efficient than SUPERCLAUS as there are only three steps to convert MP steam to LP steam.
SMARTSULF contains only two reactors so that there is only twice the reheat and downstream sulphur condensation, i.e. the converting of MP steam to LP steam occurs only twice. Accordingly this process is the most energy efficient in the comparison. For the detailed calculation of the data in Table 2 please see Heisel, 2011.1
Table 2 highlights the fact that SCOT is the most efficient process
to reduce SO2 emission. In comparison to SMARTSULF, SCOT emits only ~8 kg/h of SO2 versus ~13 kg/h of SMARTSULF. But these ~5kg/h less SO2 come at a cost of (349 + 932) = 1,281 kg/h of CO2. It is rather questionable whether this small SO2 amount recovered is worth the ca 150-times higher CO2 flow emitted. Even though these are theoretical values and both SCOT and SMARTSULF will be a bit less
efficient in practice, the difference in SO2 emissions will remain small, while CO2 emissions are very different.
As SMARTSULF is still not well known in the industry it will be described in some detail below.
SMARTSULF – a Two-reactor Process for up to 99.9 % Sulphur Recovery Rate and Minimum CO2 Emission
SMARTSULF is a two-reactor process with a rather simple plant configuration, as shown in the process flow diagram in Figure 1. The process as shown can reach up to 99.9 % sulphur recovery rate (SRR). To achieve such values the more conventional sulphur recovery processes require much more complex plants.
Process Description
The acid gas to the sulphur recovery unit is burned sub-stoichiometrically, exactly as in any Claus plant. Downstream follows a waste heat boiler, a sulphur separator and a reheat to the first reactor. This reactor is different from the conventional ones.
The fundamental idea of SMARTSULF is the removal of reaction heat of the Claus reaction directly in the catalyst bed rather than in a
40 © TOUCH BRIEFINGS 2011
Sulphur
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