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Newly completed sulfur project at SAMREF refinery

Case study of newly completed sulphur project at SAMREF refinery. SAMREF (Saudi Aramco ExxonMobil Refinery) recently completed the design and successful start-up of a new retrofit to EUROCLAUS unit.

Marco van Son and Rien van Grinsven, Jacobs Comprimo Sulfur Solutions
Khalid S. Ghazal, Saudi Aramco ExxonMobil Refinery
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Article Summary
Jacobs provided the required engineering, inspection, pre-commissioning, ABC controllers tuning, start-up and performance test support. Marco van Son and Rien van Grinsven of Jacobs Comprimo Sulphur Solutions and Khalid S Ghazal of SAMREF report on the development of the project and how the project resulted in the highest confirmed overall sulphur recovery efficiency from a facility with the EUROCLAUS technology.    

SAMREF was running two three-stage 220 tpd sulphur recovery units achieving a sulphur recovery of up to 97.5%. Both SRU-1 and SRU-2 were designed to process amine acid gas (AAG) and sour water acid gas (SWAG), but normally only one unit processes SWAG. A third SRU-3 had recently been taken into operation as part of the Clean Fuels Project, processing AAG only and was designed to achieve a guaranteed sulphur recovery efficiency of 97% with a designed capacity of 220 tpd. Based upon SAMREF’s agreement with the Local Environmental Regulator, the refinery’s sulphur recovery efficiency needed to be increased from +97.5% to 99.2% by the end of 2015. As a result, SAMREF evaluated several technologies and after thorough evaluation it was decided to install a tail gas clean-up (TGCU) EUROCLAUS retrofit unit.

Jacobs Comprimo Sulphur Solutions Group provided the engineering study and design basis considering SAMREF’s design improvements; additionally, Jacobs guaranteed a SRE of 99.4% start of run (SOR) and 99.3% end of run (EOR) of 99.3% exceeding the requirement of the local environmental regulator. Jacobs also provided engineering, pre-commissioning, ABC controllers tuning, start-up and performance test support.

The EUROCLAUS process is based on the selective reduction of SO2 present in the Claus process gas to elemental sulphur and H2S in a selective reduction stage (EUROCLAUS stage). This stage is operated on H2S control followed by the selective oxidation of H2S present in the Claus tail gas to elemental sulphur in a selective oxidation stage by leading H2S containing gas over a special selective oxidation catalyst. The typical configuration of a EUROCLAUS unit is shown in Figure 1.
The SAMREF TGCU Project consisted of converting SRU-1, SRU-2 and SRU-3 to the EUROCLAUS operation and required following modifications to meet the required overall sulphur recovery.

• Installation of a common 660 tpd selective oxidation stage with bypass line and triple offset butterfly valve.
• Installing the EUROCLAUS hydrogenation catalyst in the third Claus catalytic reactors in all SRUs.
• Utilisation of titania catalyst in the upstream SRUs.
• Installation of Jacobs’ Advanced Burner Control System (ABC) in all three SRUs.
• Using and upgrading the SRU-3 thermal incinerator during the design phase for the total tail gas flow from the new selective oxidation stage or SRU-1, SRU-2 and SRU-3.
• Resolving the following bad actors, which is essential for the Jacobs ABC system:
  −  Upgrading the main air control valves for SRU-1 and SRU-2.
  −  Resolve the deficiency with the SRU-1 and SRU-2 main air flow element (annubars).
  −  Restore the automation of SRU-1 and SRU-2 vent controllers of the combustion air blowers.
  −  Major improvement in the reliability and performance of all the three SRU tail gas analysers by closely working with Ametek/AIMS Analyser Experts.
  −  Restoring the functionality of the SWAG flowmeters to the SRU-1 & SRU-2.
  −  Reduction of the ammonia content in the amine acid gas
• Triple offset butterfly valve in the tail gas lines from SRU-1 and SRU-2 to their dedicated incinerator.
• Addition of rupture pin safety valves to existing SRU-1, SRU-2, with discharge into the existing incinerator inlet lines, as well as rupture discs on SRU-3 with discharge into the new incinerator inlet line. These devices protect the upstream equipment, on each SRU, in the event that the TGCU inlet and bypass valves are both closed.

One of the main components in a successful operation of the Jacobs EUROCLAUS technology is proper control of the H2S concentration from the third stages of each SRU. As in this particular design there were three SRUs feeding into a common TGCU stage, the control of the H2S concentration from each SRU was handled independently, with the existing tail gas analysers, which were converted to control on H2S concentration instead of conventional 2:1 ratio control.

The Jacobs advanced burner control (ABC) system was installed in all three SRUs to maintain a tight control of the H2S concentrations in the tail gas from each unit to the TGCU. In addition, a common tail gas analyser was installed at the inlet of the TGCU to be able to properly control the oxidation air required for the selective oxidation reaction with high accuracy, as well as verify the operation of the upstream individual analysers. In order to have a continuous calculation of the performance of the TGCU, SAMREF elected to install a tail gas analyser at the outlet of the common TGCU stage, which analysed for SO2, H2S, COS and CS2. This analyser, which is often not installed due to cost savings, provides operator insight into the operation and proven to be an excellent process engineering troubleshooting tool.

It is also worth highlighting that SAMREF’s project team deployed the following design improvements to the TGCU retrofit EUROCLAUS unit as result of learnings by Saudi Aramco and Jacobs:
• Design improvements in the TGCU inlet feed and bypass valve to overcome sticking and corrosion issues.
• ControTracing of the coalescer outlet piping from the new TGCU to improve maintenance of pipe wall temperatures as the process gas is operated near the freezing point of sulphur.
• Implementation of a revised line up of the fourth reheater and inlet and bypass lines around the selective oxidation stage.
• Automation of the nitrogen purge upstream of the TGCU reactor, condenser and coalescer.
• Elimination of the requirements of sulphur pit eductors for the TGCU facility as a sulphur collection vessel was incorporated into the design instead of sulphur pit. This increased the reliability of the vent gas system.
• Using sulphur trapping devices (SulTraps™) instead of seal legs.

The Results
As a result of the project, the downtime in SAMREF’s TGCU is considerably lower due to:
• Reduced risks of sulphur fires due to the revised line-up.
• Reduced TGCU bypasses, due to a more stable control and therefore, less temperature excursions.
• Reduced risk of catalyst abuse/damage avoiding catalyst sulphidation which will call for repeated rejuvenation, shorter catalyst lifetime and unexpected maintenance shutdowns.
The overall sulphur recovery efficiency is continuously calculated and displayed using the following inputs:
• The feed gas flows to each SRU.
• The calculated tail gas flows of each SRU (part of the ABC control).
• The calculated tail gas flow to the TGCU.
• The oxidation air flow.
• H2S and SO2 losses in the TGCU off-gas.
• COS and CS2 losses in the TGCU off-gas (initially assumed values and updated after the performance test run).
• Calculated sulphur losses in the TGCU off-gas, based on the outlet temperature from the final condenser.
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