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A paradox pacified
On a global level, refiners will face significant challenges in the next few years. The immediate impact of the soaring global population will be a heightened demand for oil, particularly from emerging economies. The problem is aggravated by the diminishing availability of sweet crudes which means refiners are increasingly required to process sour crudes containing higher levels of sulphur.
The essence of the sulphur paradox lies in the stringent product specifications and emissions legislation, which dictate that less sulphur leave refineries both in the finished product and as harmful emissions. Processing heavier sour crudes makes meeting these specifications incredibly challenging for refiners. As a result, refineries are thus faced with the inevitable task of having to evolve their operations to accommodate the effects of this emerging sulphur paradox.
There is no single, universally applicable solution to this problem. Instead, refinery managers must adopt an approach tailored to the idiosyncrasies of their facility.
Robust technological solutions and a future proof sulphur management strategy are essential in order for refineries to meet the challenges they will face over and beyond the next five years.
Yet there are technologies that can help refiners successfully execute their sulphur management strategies. One such aid is the Sulphur Technology Platform: a portfolio of technologies which aims to enable refiners to deal effectively with sulphur in all its forms, from sulphur in crude to sulphur in products, such as liquefied petroleum gas (LPG); residue; hydrogen sulphide (H2S); sulphur dioxide (SO2); mercaptans; and solid sulphur.
Its developer, Shell Global Solutions, then augments the technology by providing support to refineries throughout the project lifecycle. Through this combination, refineries are offered optimised technical solutions that can help them to meet product specifications by reducing the sulphur levels of their products, whether they be gasoline, diesel, LPG or fuel oil.
The solutions also enable refiners to remove H2S, SO2, and CO2 from their off gases to meet emissions targets whilst processing opportunity crudes that represent an opportunity for additional revenue.
Case Study: diesel hydrotreater
Through phased expenditure, the revamp of an existing diesel hydrotreater enabled a refinery in the Russian Federation to position itself competitively in the region in the long term and helped the complex to meet the regional demand for winter grade diesel. The supply and demand orbit in the region had historically been short of winter grade diesel and forecasted trends show that this shortfall would potentially grow even greater.
The phase I feasibility study aimed to assess the performance of the diesel hydrotreater reactor section to meet the Euro 5 diesel specifications. A future capacity increase of 30% compared to design was the target.
The phase II revamp had the objective of installing a second stage dewaxing section to achieve a further cloud point reduction of 20°C, enabling production of a winter diesel product for the local market.
Originally, the diesel hydrotreating unit was designed to process straight run diesel and kerosene with a maximum sulphur content of 50 to 350 weight ppm.
In terms of the unit’s capabilities, a maximum capacity test run revealed that the unit could operate at 115% of the design throughput at the premised feedstock, having a sulphur content of approximately 100 weight ppm in the diesel product.
The premised feedstock is a blend of 85 wt% straight run diesel and 15% visbreaker gasoil and light catalytic cracked cycle oil (LCO). The unit was further constrained by a maximum pressure differential across the reactor section, set by the fixed speed recycle gas compressor capacity and heat integration limitations.
To achieve sulphur content in the diesel product of less than 10 weight ppm, an additional hydrodesulphurisation (HDS) reactor downstream of the existing reactor needed to be installed, while the increased feed rate would necessitate the installation of a maximum diameter impeller and replacement of the pump driver for the diesel hydrotreater feed charge pump.
For the refinery, this solution was a minimum capex solution, meeting the premised Euro 5 diesel specifications and the target capacity increase of 30% compared to design.
Processing of gas oils to very low sulphur levels (especially when they contain LCO) requires hydrogenation of the difficult sterically hindered sulphur species (dibenzothiophenes), which is found in the heavy end of the feed.
In order to achieve the required sulphur in product, a catalyst system based around Criterion Catalysts & Technologies‘s ASCENT™ platform DN-3531 and DC-2531 in a sandwich structure was proposed.
The nickel molybdenum (NiMo) based catalyst was applied in order to reduce the residual nitrogen to low enough levels for the (future) second stage noble metal hydroisomerisation dewaxing (HDW) catalyst.
The cobalt molybdenum (CoMo) based catalyst was included to meet the diesel sulphur specification, while minimising hydrogen consumption.
The heat integration of the reactor section was optimised by the installation of a new hot high pressure separator (HHPS) scrubber. This provided the opportunity to minimise regret investment for the dewaxing expansion in phase II of the revamp project.
This combination would make substantial heat available from the new HHPS pump to be used for feed preheat, while simultaneously enabling the reduction of the condensing duty requirement of the high pressure vapour condenser.
The pump around could also be used as liquid quench in between the catalyst beds of the new reactor.
Phase II of the revamp project involved the application of hydroisomerisation dewaxing technology, due to yield advantage over catalytic cracking dewaxing. This was integrated within the existing HDS reactor section. The basis of isomerisation dewaxing involves a catalyst with a zeolite structure, impregnated with noble metal platinum.
The integrated second stage hydroisomerisation dewaxing reactor section also comprises a high-pressure interstage stripper, where the product from the first stage HDS reactor is stripped from dissolved H2S and NH3, using hydrogen rich effluent vapour from the second stage hydroisomerisation dewaxing effluent. This was necessary as the performance of the dewaxing catalysts was affected by the presence of these compounds.
The feasibility study yielded several benefits for the refinery’s revamp project. The refinery was able to expand capacity by 30% and produce diesel product that met Euro 5 standards through phased capital expenditure with no replacement of capital intensive equipment. The refinery was also able to produce winter diesel grade with a cloud point specification of -25 ⁰C. Finally, the complex achieved increased diesel yield (optimisation of gasoil stripper in work-up section) and greater unit reliability (reinstate wash water application).
Figure 1: Final diesel hydrotreater flow scheme following the revamp with dewaxing technology
Case Study: Integrated MHC/HDS Configuration
Added value was provided to an existing European refinery, generating Euro 5 diesel quality products and feedstock for the fluid catalytic cracking (FCC) unit, through the integration of mild hydrocracking and diesel desulphurisation units. The intended outcome of the configuration review was upgrading heavy gas oil (HGO) / vacuum gas oil (VGO) to diesel whilst producing unconverted oil (UCO) as feedstock to the FCC, and hydrotreating excess light gas oil (LGO) to produce diesel. It was necessary to determine the most economical and reliable solution through coprocessing or post treatment while meeting diesel Euro 5 specification.
Several configuration options were studied, with focus on capital cost reduction, maximum diesel yield, lowest hydrogen consumption and optimal energy efficiency.
The unconverted oil bleed from the common fractionation section went to the FCC and the remainder was recycled to a hydrocracker unit (operating at higher conversion).
This demonstrated the infeasibility of an integrated HCT/HCU configuration at all of the three investigated pressure levels (80 bar, 100 bar and 120 bar).
This was because the sulphur content of the middle distillate (kerosene and diesel combined) could not meet the 10 weight ppm sulphur specification. Even at the 120 bar pressure level, the S slip in ultra low level sulphur diesel was estimated at 65 weight ppm.
Table1: MHC complex: Configurations Assessment
For both the single stage full range mild hydrocracker (MHC) and the integrated MHC/HDS configurations, the cetane number specification could not be achieved at the lower pressure levels of 80 bar and 100 bar.
The density specification of less than 845 kg/m3 could only be met at SOR conditions for the single stage full range.
However for both these configurations, at the highest pressure level of 120 bar, all diesel specifications could be met. The middle distillate yield from the single-stage full range MHC configuration compared with the integrated MHC/HDS is approximately 5%lower as a result of extra cracking of the LGO feed.
Although the single stage full range MHC would have the higher operating flexibility of independent turnarounds for maintenance and/or catalyst change compared with the HDS from MHC functions in the integrated MHC/HDS configuration, the higher capital cost involvement for the two MHC reactors versus the HDS and MHC reactors justifies the loss in operating flexibility.
After consultation with the refinery, the most economically attractive and energy efficient integrated configuration was accepted. It consisted of a single-stage MHC reactor in once through operation.
In parallel, a single stage hydrodesulphurisation (HDS) reactor in once-through operation would operate at the same pressure level. The configuration involves a common high pressure gas and recycle section, common cold separation section and common work up section.
The configuration review and optimisation studies yielded several benefits to the refinery. The complex achieved its highest diesel and kerosene yields, met Euro 5 standards and produced unconverted oil requirement as feed for FCC. The project provided optimal energy efficiency and reliability of operations, yet remained a low capex solution.
With numerous options available to deal with sulphur throughout the refinery, it is important to develop a suitable sulphur management strategy following deliberate evaluation of the application of technologies and their integration within a refinery.
For refiners, flexibility is key, as it facilitates an ability to respond to market variations, clean fuel legislation and more stringent product specifications.
Depending on business goals and the existing refinery configuration, unique, customised technical solutions are available that can encompass the whole refinery or, where necessary, focus on individual units. With this is mind the Sulphur Technology Platform is a platform that can provide refiners with the means to handle sulphur in any form and manage the sulphur paradox effectively.
Figure 2: Sulphur Technology Platform diagram