When Bapco commissioned a cutting-edge refinery gas desulphurisation project (RGDP) in January this year, it marked another milestone in its vision to turn the refinery into one the greenest and cleanest in the world.
“Undertaken at a massive investment of $151 million, the RGDP was executed over a period of nearly 4 million man-hours without a lost-time accident, which is a significant safety achievement for such a complex construction project,” says Essa Al-Ansari, the general manager of Bapco’s engineering division and chairman of its environmental affairs committee. “This is incontrovertible proof of Bapco’s strong safety culture, which is very high on its list of priorities.”
The commissioning of the RGDP comes a little less than a year after Bapco launched its low sulphur diesel production (LSDP) project, which enabled it to produce some of the cleanest diesel in the world.
Foster Wheeler Italiana, the contractor for the RGDP, took 32 months to complete the project and hand it over to Bapco. Pre-commissioning took place in the fourth quarter of 2008 and the RDGP was commissioned on January 20, 2009.
“The RGDP is one of Bahrain’s most important environmental projects, further improving the quality of air and water to meet the most stringent environmental regulations. Several environmental design excellence features in the RGDP reflect our commitment to invest and fully comply with international regulations achieving the highest levels of environmental performance,” Al-Ansari says, adding that the project has now led to a massive reduction in sulphur dioxide (SO2) emissions from the refinery.
Elaborating on the history behind the RGDP, Al-Ansari says: “Bapco first met the Bahrain government’s environmental authorities in 1999 and agreed on an environmental plan for the refinery with a number of key projects to be completed over a 10-year period as part of its broader strategic investment programme (SIP). These included the RGDP, production of unleaded gasoline, an environmentally friendly method of treating kerosene and jet fuel, hazardous solid waste management, a waste-water treatment plant, sewage treatment plant, doubled-sealed tanks to control VOCs (volatile organic compounds), and the LSDP project.
These projects were prioritised according to their impact on the environment and their potential to generate additional revenues for the kingdom.
The RGDP was selected to be completed as early as possible to further enhance ambient air quality through the use of cutting-edge technology to desulphurise process gas and treat sour water.
The RGDP had two main goals: The first was to remove hydrogen sulphide (H2S) from refinery off-gas streams to less than 150 parts per million (ppm) thus ensuring that SO2 emissions meet Bahrain government and World Health Organization (WHO) standards; the second was to collect all refinery sour water streams and strip them in state-of-the-art facilities to remove H2S and ammonia (NH3) to levels compatible with Bapco’s biological waste-water treatment facilities.
Following completion of the RGDP, the refinery treated sour water now contains less than 1 ppm of H2S and less than 10 ppm of ammonia (NH3).
GREENER ENVIRONMENT
The completion of the RGDP will result in significant benefits to the environment, says Al-Ansari.
“By reducing hydrogen sulphide to one-fourth of permissable levels, sulphur dioxide emissions is now reduced from 120 tonnes to less than 5 tonnes per day (tpd), a massive reduction of more than 90 per cent. This reduction in SO2 emissions will have a dramatic effect on ground level concentration, which will be well within Bahrain and WHO limits,” he explains. “The emission reduction will have a dramatic and positive effect on human health by improving the ambient air quality.”
He continues: “Effluent water quality, meanwhile, has been improved significantly to levels attainable with the latest stream stripping technology (for H2S and NH3) to provide an acceptable feedstock for the biological waste water treatment project which is currently under way.
“The RGDP has eliminated flaring of sour water stripper off-gas through a state-of-the-art sulphur recovery unit which destroys NH3 and recovers sulphur from H2S present in the stripper off-gas. It has also enabled the recovery of over 50 tpd of incremental sulphur for sale of international markets.”
ELEMENTS OF THE RGDP
The RGDP involved installation of a new 225 tpd sulphur recovery unit (SRU), a new tail gas treating unit (TGTU), two new 300 gallons-per-minute sour water stripping (SWS) units, revamping the existing amine unit, and installation of a new olefinic gas treatment unit (OGTU).
Sulphur recovery unit (SRU)
In addition to an existing sulphur recovery unit (SRU) which has a nominal capacity of 230 tpd, a new 225 tpd sulphur recovery unit has been installed to operate in parallel with the existing unit. The acid gas streams from existing and new amine units and the tail gas treating unit (TGTU) will be collected in a common header. The total acid gas will be distributed between these SRUs. When one unit is shut down, the other unit will take the entire acid gas load, providing SRU redundancy. The efficiency of the new unit is 96 per cent before the TGTU.
The acid gas from the new sour water stripping (SWS) units will also be processed in the old and new SRUs, so that NH3 in the off-gas stream will be completely destroyed and sulphur recovered from the H2S in this stream.
The technology selection for the sulphur recovery and tail gas treating focussed on obtaining the best proven technology available to ensure safe, reliable operations and to meet all aspects of environmental performance.
Tail gas treating unit (TGTU)
The tail gas streams from the new and the existing SRUs will be sent to the new TGTU. Tail gas treating increases the total sulphur recovery to 99.5 per cent and meets SO2 limits for the unrecovered tail gas routed to the incinerator.
Sour water stripping (SWS) units
Two new sour water stripping units have been installed to treat the refinery sour water streams. All sour water streams will be collected in a common header. This header will also be connected with the second sour water stripper (SWS) on the LSDP project. Under normal circumstances all refinery sour water will be treated in the RGDP with the two SWS units equally haring the load. When one of the units is down, the other unit will operate at the maximum capacity with any remaining water routed to the LSDP complex. Two sour water strippers will allow H2S and NH3 specifications for effluent water to be met all of the time.
Now, following completion of the LSDP and RGD projects, the refinery has four SWSs which will have adequate capacity to process all sour water generated in the refinery, even when one SWS is down. The required water stripping redundancy will also be met with the new configuration. The redundancy in sour water stripping is required because the H2S and NH3 content of the stripped sour water must be reduced before it can be processed in the waste water treatment plant or upgraded for reuse. With the previous single SWS, the refinery’s sour water could not be treated once storage tank capacity was used up which would result in the shutdown of operational units and hence loss of revenue.
Amine unit revamp
The LSDP project included converting an existing mild hydrocracking unit to a diesel hydrotreater. After the conversion, the gas load on the existing amine unit decreased and this spare capacity/load was needed to process RGDP gases. The amine unit was thus revamped to treat the LPG-rich paraffinic gas streams from a number of process units. After treatment these paraffinic gas streams will be sent to Banagas for LPG recovery. The recovery of LPG from the gas is very profitable and the revamp of the amine unit will now allow this profit to be realised.
Olefinic gas treatment unit (OGTU)
Off-gas streams from process plants that generate olefinic gas, including the fluid catalytic cracking unit (FCCU), will be collected and treated in the new OGTU to have a maximum of 150 ppm of H2S. The treated gas will be sent to the refinery fuel gas system for consumption.
Off-gas streams from three vacuum distillation units, normally burnt in fired heaters, will now be treated in new amine absorbers. The lean amine to these absorbers will be provided by the OGTU.
The OGTU consists of typical amine absorption and regeneration equipment using amine as the circulating fluid. Surplus capacity has been added to the OGTU amine regenerator to accommodate future requirements of a proposed LPG amine treating unit.
Summing up the project, Al-Ansari says: “The RGDP will lead to a significant reduction in sulphur emissions from our refinery. As a result, ambient ground level concentrations will be reduced so that the Bahrain’s national standards for this pollutant will not be violated due to emissions from the refinery.”
In essence, a greener Bahrain, and cleaner world!
