Anaerobic degradation of methanethiol in a process for Liquefied Petroleum Gas (LPG) biodesulfurization

Due to increasingly stringent environmental legislation car fuels have to be desulfurized to levels below 10 ppm in order to minimize negative effects on the environment as sulfur-containing emissions contribute to acid deposition (‘acid rain’) and to reduce the amount of particulates formed during the burning of the fuel. Moreover, low sulfur specifications are also needed to lengthen the lifetime of car exhaust catalysts. The research presented in this thesis focuses on the biological desulfurization of Liquefied Petrol Gas (LPG). Currently, LPG is mainly desulfurized by physical-chemical methods that absorb volatile sulfur compounds present (mainly hydrogen sulfide and thiols) into a strong caustic solution, whereafter the thiols are partially oxidized to disulfides whilst the dissolved hydrogen sulfide is discharged as a ‘spent sulfidic caustic’. Disadvantages of this physical-chemical method are the relatively high energy and caustic consumption and the production of a hazardous waste stream. As an alternative, a new three-step biotechnological LPG desulfurization technology has been studied, that produces elemental sulfur as an end-product from the bio-conversion of hydrogen sulfide (H2S) and methanethiol (MT). The new process involves: (i) extraction of the sulfur compounds from the LPG phase into a (bi)carbonate-containing solution; (ii) anaerobic degradation of MT to H2S, CO2 and CH4 and (iii) partial oxidation of H2S to elemental sulfur. The formed sulfur particles are removed from the system whilst the sulfur-free alkaline process water is re-used in the extraction process. The sulfur can be used for the production of sulfuric acid and hydrogen sulfide or for agricultural applications. In this research attention is paid to the feasibility of the second process step, i.e. the anaerobic treatment step as the first and third process step are already well described. Anaerobic degradation of MT appeared to be possible with a variety of anaerobic (reactor) sludges and sediments, both under methanogenic and sulfate-reducing conditions. The related compounds dimethyl disulfide and dimethyl sulfide were degraded as well, in contrast to ethanethiol and propanethiol, which were not degraded anaerobically. In the new LPG biodesulfurization process higher thiols are converted to their corresponding oily disulfides that have to be skimmed off from the reactor solution and can be sent for disposal, e.g. to an incinerator. The fifty percent inhibition concentration of MT, ethanethiol and propanethiol for methanogenic activity of anaerobic granular sludge on methanol and acetate was found between 6 and 10 mM (pH 7.2, 30°C). Hydrogen sulfide inhibited anaerobic MT degradation at concentrations below 10 mM, depending on the pH and the source of the inoculum. Dimethyl disulfide inhibited MT degradation already at concentrations below 2 mM. In a lab-scale upflow anaerobic sludge blanket (UASB) reactor that was inoculated with anaerobic granular sludge originating from a full-scale reactor treating paper mill wastewater, MT degradation was possible up to a volumetric loading rate of 17 mmol MT∙L-1∙day-1 (pH 7.0-7.5, 30°C, < 0.03 M total salts). MT degradation with this inoculum was inhibited by sodium concentrations exceeding 0.2 M. Initially, MT-degrading methanogenic archaea related to the genus Methanolobus were enriched in the reactor. Later, they were outcompeted by methanogens belonging to the genus Methanomethylovorans, which were mainly present in small aggregates (10-100 μm) in between larger particles. Estuarine sediment from the Wadden Sea was used to inoculate an anaerobic reactor operated at Na+ concentrations of 0.5 M. The maximum volumetric degradation rate achieved amounted to 37 mmol MT∙L-1∙day-1 at pH 8.2-8.4 and 22 mmol MT∙L-1∙day-1 at pH 8.9-9.1 (30°C). MT degradation at pH 10 was not possible with this inoculum. In activity tests, no inhibition of MT degradation was observed till 0.8 M Na+. Initially, Methanosarcina mazei was the dominant MT-degrading methanogen, but after about 1.5 years of continuous reactor operation, methanogens related to Methanolobus taylorii became dominant, probably due to the pH shift to pH 9.0 in the reactor. In a UASB reactor inoculated with a mixture of estuarine and salt lake sediments from the Soap Lake (USA) and the Kalunda Steppe (Russia) it was possible to degrade MT at pH 10, at a maximum volumetric loading rate of 13 mmol MT∙L-1∙day-1 (30°C, 0.8 M Na+) in the presence of methanol as a co-substrate. The methanogenic archaea responsible for the degradation of MT were related to Methanolobus oregonensis. Thiols that are not degraded in the anaerobic reactor of the novel LPG desulfurization process are directed to the third process step, i.e. the aerobic bioreactor. Our research shows that here MT will react with biologically produced sulfur (both 1-16 mM; pH 8.7 and 10.3; 30-60ºC) to form poly-sulfur compounds, i.e. polysulfide ions and dimethyl polysulfides. The first reaction step is a S8 ring opening by nucleophilic attack to form CH3S9-. The reaction rate depends on the MT and bio-sulfur concentrations, pH and temperature. The activation energy of this reaction was determined to be 70 kJ·mol-1 at pH 8.7 and 16 kJ·mol-1 at pH 10.3. The CH3S9- ion is unstable and leads to shorter-chain sulfur compounds. The main end-products formed are polysulfides (S32-, S42-, S52-), dimethyl polysulfides [(CH3)2S2, (CH3)2S3] and H2S. Also long-chain dimethyl polysulfides [(CH3)2S4-7] are formed in trace amounts (μM level). Excess MT results in complete methylation of the initially formed inorganic polysulfides. An increased molar MT/S ratio results in the formation of relatively more (CH3)2S2 over (CH3)2S3. Flowsheet simulations of the new LPG desulfurization process reveal that for an acceptable degree of desulfurization (i.e. less than 10 ppm in the treated LPG product) the pH in the recycle stream to the extractor column must be higher than 9. This means that the used inocula (estuarine and salt lake sediments) provide good opportunities to be applied in the process.