D. Chaudhuri and A. DASGUPTA, Fluor, New Delhi, India
Green refineries produce transportation fuels, such as green diesel and sustainable aviation fuel (SAF), from renewable biomass sources like waste cooking oil and animal fat. These refineries offer several benefits:
Reducing carbon emissions by converting waste materials into fuel
Enhancing energy security by reducing dependence on imports through local production
Ensuring compatibility with existing infrastructure to facilitate the transition to greener fuels
Helping meet environmental standards while benefiting from government incentives.
The conversion of biomass to transportation fuel involves two key processes: hydrodeoxygenation (HDO) and isomerization. HDO is a chemical reaction that removes oxygen from biomass feedstocks such as vegetable oils, animal fats and used cooking oils by mixing the feedstock with hydrogen (H2) gas and passing it over a catalyst at elevated temperatures and pressures. This catalyst, typically made of metals like nickel, molybdenum or cobalt, helps remove oxygen atoms, resulting in hydrocarbon products similar to conventional diesel. The removal of oxygen also helps increase the fuel's energy density and reduce its corrosiveness, making it compatible with existing diesel engines and infrastructure. Conversely, isomerization rearranges the molecular structure of hydrocarbons to improve their cold flow properties by converting straight-chain hydrocarbons from the HDO process into branched-chain hydrocarbons through a different catalyst, often having platinum or zeolite, at elevated temperatures. The branched-chain hydrocarbons have lower freezing points and better flow characteristics at low temperatures, making the fuel suitable for use in colder environments.
Sulfur dilemma in a green refinery. HDO reactors need a hydrogen sulfide (H₂S)-rich environment for optimal catalyst activation and stabilization. This is achieved by injecting dimethyl disulfide (DMDS) into the feed, as biomass feedstocks lack sulfur. However, the continuous use of DMDS poses environmental challenges due to H₂S emissions since the sulfur comes out as H₂S from the reaction stage of HDO. This H₂S must be recovered or managed to prevent harm to the environment and human health. Additionally, because of the HDO reaction, a significant amount of carbon dioxide (CO2) is also produced and must also be removed from the reaction loop.
In conventional refineries, a sulfur recovery unit (SRU) captures and converts H₂S into elemental sulfur. However, this is economically infeasible in a standalone green refinery due to the low and diluted H₂S quantities. Technologies like a proprietary sulfur recovery solutiona can recover sulfur from highly diluted acid gas streams, but incorporating these units into a green refinery would result in continuous DMDS consumption and produce tiny amounts of sulfur byproduct, which induces further complication in storage and handling.
This article discusses an innovative solution that eliminates the need for an SRU and continuous DMDS requirements. It uses a two-unit approach combining a conventional amine recovery unit (ARU) with a specialized H₂S enrichment unit (H2SEU) unique to green refineries. This article is based on a case study of a standalone green refinery project.
Details from a case study. This case study describes a standalone renewable diesel unit (RDU) that processes renewable feedstocks like waste cooking oil and animal fats. It produces green diesel, SAF, green naphtha and green liquefied petroleum gas (LPG). The unit can process 18,000 bpd of feedstock.
FIG. 1 shows a simplified flow diagram of the recycle gas section of the HDO unit. A brief and simplified description of the process is provided here.
The feed is pre-heated and sent to the HDO reactor. DMDS is added intermittently to create an H₂S-rich environment in the reactor, which is crucial for catalyst functioning. The reactor products are cooled in the reactor effluent cooler (REC) and sent to the hot separator, which separates vapor from liquid. The vapor is further cooled in the hot separator vapor cooler (HSVC) and then sent to a cold separator. The cold separator, a three-phase separator, generates vapor, hydrocarbon liquid and sour water streams. The sour water is sent to the sour water stripper (SWS) unit. The vapor from the cold separator mainly contains H₂, CO2, H₂S and light hydrocarbons. This gas undergoes amine treatment in an amine absorber, which removes both H₂S and CO2. The rich amine is then sent to the ARU. The treated recycled gas is compressed by the recycle gas compressor and sent to the HDO reactor inlet. The recovered H₂S from the H2SEU joins the recycle gas before entering the reactor. This makes the addition of DMDS to the feed intermittent.
FIG. 2 shows a simplified block flow diagram of acid gas stream management. The rich amine from the HDO unit is sent to the ARU. The ARU regenerates the amine and directs the acid gases (CO2 and H₂S) to the H₂SEU, while the lean amine returns to the HDO unit.
Sour water from the HDO section—containing CO2, H₂S and ammonia (NH₃)—is processed in a two-stage SWS unit. This unit produces an acid gas stream with CO2 and H₂S, and an 18% aqueous NH₃ solution. The acid gas is sent to the H2SEU.
The inlet acid gas stream to the H2SEU contains ~3% H₂S and 95% CO2. The H2SEU produces two streams: a CO2-rich stream with ~100 ppmv H₂S, and an H₂S-enriched stream with 51% H₂S and 44% CO2. The CO2-rich stream is sent to the incineration and sulfur dioxide (SO2) scrubber unit, where it is incinerated to convert H₂S to SO2. The gas is then cooled and scrubbed with caustic, absorbing the SO2. The treated gas—containing nitrogen, CO2 and oxygen—is released into the atmosphere. The H₂S-rich stream is compressed and sent to the HDO reactor. Further details of the H2SEU are provided in the following section.
The H2SEU specifics. As discussed above, the H2SEU receives the amine acid gas generated from the ARU and an H₂S and CO2-rich stream from the first stage of the two-stage SWS unit. The H₂S concentration is enriched in the H2SEU by rejecting the required amount of CO2, and then the enriched stream (with H₂S) is recycled back to the HDO.
FIG. 3 shows the H₂SEU, illustrating the two stages of the absorber-regeneration system and the gas compression system. The H2SEU enriches H₂S content from 3% to 51% in two stages using a specialized amine that is highly selective to H₂S and lets most of the CO2 pass through. The acid gases from the amine regeneration unit and the SWS unit enter the first-stage absorber. The gas is treated with lean amine, resulting in an overhead gas with mostly CO2 and < 100 ppmv H₂S. The rich amine is regenerated in the first-stage amine regenerator, producing a gas with 17 vol% H₂S and 78 vol% CO2. The second stage works similarly, producing a gas with 51 vol% H₂S and 44 vol% CO2. The recovered H₂S-rich gas is compressed and recycled to the HDO reactor. If the HDO unit needs less H₂S than available, excess H₂S is sent to the incinerator. If more H₂S is needed, fresh DMDS is added. In steady-state operation, no DMDS addition is needed, and no H₂S is sent to the incinerator. This allows the green refinery to run with net-zero sulfur.
The acid gas recycled to the HDO reactor—containing approximately 51% H₂S, 44% CO2 and the rest water—must be pressurized from a suction pressure of 5 psig to about 1,000 psig. Given the highly toxic and corrosive nature of this gas, it is essential to use a compressor made from corrosion-resistant materials with minimal leakage. To meet these stringent requirements, a two-stage diaphragm compressor was selected, as it is well-suited for such challenging conditions.
Selection of amine solvent. Two types of amine solvents were used in the process based on specific requirements. In the HDO section, a formulated amine solvent from a reputable vendor, with equal affinity for both CO2 and H₂S, was used to ensure the removal of both these gases, helping the reaction in the HDO reactor. Conversely, the amine solvent needed in the H2SEU required high selectivity towards H₂S to absorb most of the H₂S while allowing most of the CO2 to pass through. For this purpose, a highly selective formulated amine from a reputable vendor was chosen.
Takeaways. In a green refinery, acid gas from the amine regeneration unit has low H₂S content, originating from the addition of DMDS in the HDO section. Separating and recycling H₂S produced in the HDO unit reduces the need for continuous DMDS, helping both the environment and economic efficiency. The H2SEU increases H₂S concentration to required levels for recycling. This process ensures efficient gas treatment, compliance with environmental standards and optimal use of chemicals and resources. HP
ACKNOWLEDGEMENT
The conclusions presented in this article are solely those of the authors and cannot be ascribed to Fluor Corp. and/or any of its subsidiaries.
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DEBOPAM CHAUDHURI is a Process Engineer with Fluor Daniel India in New Delhi, and is subject matter expert and a Fluor Fellow of sulfur recovery processes. He has 24 yr of experience in petroleum refining, petrochemical complexes and upstream projects. His experience ranges across all phases of a project for an SRU: he has worked in licensor selection, as a licensor, executed front-end engineering and design (FEED) and completed detailed design. His other expertise includes H2 generation via the conventional SMR route and hydrotreaters for middle distillates. Chaudhuri earned BSc and BTech degrees in chemistry and chemical engineering from the University of Calcutta. The author can be reached at Debopam.Chaudhuri@fluor.com.
AYAN DASGUPTA is a Process Engineer at Fluor Daniel India in New Delhi, and a subject matter expert in acid gas treatment. With more than 22 yr of experience in petroleum refining and gas processing, he has been involved in all project phases, from conceptual design to FEED, detailed engineering and lump-sum engineering, procurement and construction (EPC) projects. Dasgupta holds a B.E. degree in Chemical Engineering from Jadavpur University, Kolkata. The author can be reached at Ayan.Dasgupta@fluor.com.