Desulfurization

Removal of sulfur from a substance

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Desulfurization or desulphurisation is a chemical process for the removal of sulfur from a material. This involves either the removal of sulfur from a molecule (e.g. A=S → A:) or the removal of sulfur compounds from a mixture such as oil refinery streams.[1]

These processes are of great industrial and environmental importance as they provide the bulk of sulfur used in industry (Claus process and Contact process), sulfur-free compounds that could otherwise not be used in a great number of catalytic processes, and also reduce the release of harmful sulfur compounds into the environment, particularly sulfur dioxide (SO2) which leads to acid rain.

Processes used for desulfurization include hydrodesulfurization, the SNOX process and the wet sulfuric acid process (WSA process).

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References

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1. ^Babich, I (April 2003). "Science and technology of novel processes for deep desulfurization of oil refinery streams: a review". Fuel. 82 (6): 607–631. doi:10.1016/S0016-2361(02)00324-1.

Hydrodynamic cavitation for sulphur removal

Initial experiments were carried out to identify point of cavitation inception. Pressure drop measurements as a function of flow rate of two phase mixture (organic phase and water) were carried out. The contribution of bypass valve in the overall cavitation can be considered to be insignificant due to its partial closing in most cases (especially at very low pressure drop across vortex diode) and also due to the fact that mere cavity formation is not sufficient and requires effective cavity collapse for cavitation phenomenon as depicted in Fig. 1. It was established in earlier studies that cavitation inception can be identified from the deviation of measured pressure drop from the usual square law (ΔP proportional to square of flow rate or mean velocity). It was established that for the case of octanol – water mixture (up to 10% volume percent of octanol), the cavitation inception occurs just before the pressure drop across vortex diode reaches 0.5 bar. The details of pressure drop and identification of inception point are provided in Fig. 3. All the further experiments were carried out at two values of pressure drop across vortex diode (0.5 bar and 2 bar with flow rate of ~330 and 680 LPH respectively).

Inception of Cavitation; (a) Effect of solvent; (b) Calculations to demonstrate cavitation occurring at a ΔP of 0.5 bar; (c) Prediction of inception of cavitation based on deviation from square law.

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The initial sulphur content of organic phase (e.g. octanol) was adjusted to predetermined concentration (e.g. 300 ppm) by adding appropriate quantity of thiophene. The thiophene containing organic solvent was mixed with measured quantity of water to generate two phase mixtures with organic solvent volume fraction of 2.5% and 10%. The sulphur content in organic phase (obtained by separating organic layer from the treated mixture) was monitored as a function of time. The experiments were carried out to quantitatively understand influence of various process parameters. Some of these results are discussed in the following to elucidate influence of key process parameters.

Influence of pressure drop, initial concentration and volume fraction

For the developed process, the effect of parameters such as pressure drop, initial concentration of the sulphur compound, organic volume fraction and nature of organic phase/solvent is believed to be most crucial and some important results pertaining to these are given in Fig. 4. Further details on the results of the above parameters are given in supplementary materials (Supplementary Figs S1–S7). Pressure drop across the vortex diode or for that matter any cavitating device, is an important parameter that contributes towards the extent of cavitation. The number density of cavities and effective intensity of cavity collapse are governed by pressure drop across cavitating devices (for a specified configuration of device and downstream piping). It is evident from Fig. 4, that the effect of pressure drop was found to be rather negligible, especially at low values of organic to aqueous phase volume ratio. Influence of initial concentration on sulphur removal is also shown in Fig. 4. It was observed that sulphur removal was generally better when initial sulphur concentration was low for high organic phase volume. The trend depends on nature of organic solvent and ratio (Fig. 4), though as seen from the figures, a very high removal close to 100% can be obtained at low organic fraction of 2.5%. In the developed process, there are two distinct liquid phases viz. aqueous and organic (containing sulphur compounds). Influence of nature of organic phase is expected to be important. Realized cavities and cavity collapse intensities are expected to vary with respect to the nature of organic phase and volume ratio of organic to aqueous phase. Experiments were therefore carried out for different organic solvents and using different volume ratios of organic to aqueous phase. Sample of results obtained with three different organic phases for different organic volume fractions are shown in Fig. 4. It can be seen that the removal of sulphur compounds is drastically different with different organic phases. A very high removal of sulphur was observed with n-octanol compared to other solvents. The order of sulphur removal with respect to the three organics studied in this work was as below:

Effect of different parameters on desulphurization by Hydrodynamic cavitation; (a) Pressure drop; (b) Initial concentration; (c) Volume fraction.

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The initial concentration of sulphur in the organics and effectiveness of the developed process at low pressure drop have important bearing from the view point of commercial operations. Since conventional hydrodesulphurization process has limitations in bringing down sulphur levels below 350 ppm, a process that can take care of sulphur removal from this point can be highly beneficial. The results of this work clearly indicate effectiveness for initial concentrations up to 500 ppm. Further, effective deep desulphurization at low pressure drop of just 0.5 bar across the cavitating device indicates lower cost of operation and significant ease of operation. Another important aspect of the process is that it is effective without using any catalyst or requiring high temperatures/pressures.

It is also important to note that the insolubility of the thiophene in water and the huge difference with respect to the organic solvent practically eliminates possibility of physical transfer of the thiophene in the aqueous phase. In order to establish that sulphur removal is because of hydrodynamic cavitation and not because of vigorous contact with aqueous phase, the results of stirred tank experiments were also compared. The measured sulphur content of the different organics samples collected from these tanks (Initial sulphur concentration: 300 ppm;without cavitation) and those using hydrodynamic cavitation are shown in Fig. 5.

Cavitation is essential for reduction of sulphur content; (a) Sulphur removal using cavitation(Initial S: 300 ppm); (b) FTIR spectra of aqueous phase after cavitation.

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Hydrodynamic cavitation is known to generate hydroxyl radicals through cleaving of water molecules- an active oxidant and this can be effectively utilized for removal/degradation of organics30,31. A plausible mechanism for the removal of sulphur would require cleavage of the sulphur bond with the attack from the oxidant and release of sulphur dioxide. Alternatively, it can also form other oxidation products such as sulphone that would go in the aqueous phase, thereby effecting sulphur removal. A postulated mechanism can be thought to involve mainly reaction of the free radicals with the sulphur compounds resulting into removal of sulphur as SO2 and mineralization of the organic skeleton to final products as carbon dioxide and water.

It is also possible that the cavitation here works as a specific form of extractive, but not catalytic, oxidative desulphurization with water as a solvent and without employing any conventional catalyst of the type reported in the literature for ECOD. A number of other possibilities such as formation of SO2, HSO3, H2SO4 etc. may also be listed. No change in the pH of the aqueous solution was observed during or after hydrodynamic cavitation experiments, indicating that formation of acid or acidic species may not occur. The aqueous phase after the cavitation was analysed for increase in the organic carbon using COD/TOC test. It was observed that when toluene was used as the organic phase in cavitation, there was practically no increase in the COD/TOC values during the progress of cavitation indicating that no organic species entering into the aqueous phase due to cavitation. However, there was increase in the COD/TOC when n-octane was used as solvent in cavitation experiments, though the COD values were not high, but showed approximately 4 times increase (from 130 ppm to 650 ppm) indicating presence of organics in the aqueous phase which can be attributed to mainly organic solvent though possibility of entering solubilised forms of organic sulphur (e.g. sulphones) in the aqueous phase cannot be ruled out. Preliminary analysis of organic phase using gas chromatography, however, indicated no presence of any other compound than the solvent or thiophene. FTIR analysis of the aqueous samples (Fig. 5) indicated no appreciable presence of sulfones in the aqueous phase.

Details of mechanisms have been reported in the case of oxidative desulphurization in presence of various catalysts such as hydrogen peroxide and other acid catalysts26,27. The conventional approach involves reaction of hydrogen peroxide with the acid resulting into formation of acid peroxide which subsequently reacts with the organic sulphur resulting into formation of sulfone or sulfoxide. Otsuki et al.27 reported formation of sulfones in the organic phase during oxidative desulphurization using IR spectra analysis. The sulfone/sulfoxide, thus formed, can be extracted in different phase. It is important to note that the authors indicated difficulty in oxidising thiophene at 50 °C due to low electron density though benzothiophene or dibenzothiophene could be easily oxidised using hydrogen peroxide and formic acid mixture as catalyst. Thus, in the developed method, removal of organic sulphur is possible by both mineralization as well as oxidation mechanism (Fig. 6). However, since the process here does not employ acid catalyst, the contribution of later mechanism may not be significant. In principle, the cavities of organic, water or both (mixture) can form, the contribution of which would depend on the nature, vapour pressure of the organic solvent and composition. In the present work, the solvents, including water have vapour pressure in the range of 0.2–38 mm Hg at 30 °C; e.g. octanol (0.21), octane (17.65), toluene (37.7) and water (32) while thiophene has vapour pressure close to 92.6 mm Hg at 30 °C. However, in view of very low mol fraction of thiophene, its partial vapour pressure in the aqueous-organic two phase mixture of this study is significantly low. The probability of cavity formation for any phase is expected to increase with increase in its relative volume percent. However, even if organics cavities are more in number on the basis of low vapour pressure of the solvent, the degradation of sulphur species is believed to be difficult, if oxidative reaction by hydroxyl radical is assumed. The mechanism here is certainly much more complex and needs to be investigated in detail. In this context also, it is instructive to evaluate inception of the cavitation under different conditions/solvents and this can be done using the data of Fig. 3 for water alone and using different solvents for the two solvent ratios. The data on inception of cavities with addition of different solvents and with different ratios did not reveal significant difference from that of water alone, indicating that the role of solvent can be viewed predominantly as facilitator in oxidative interfacial reactions through effective transfer of sulphur moiety in cavities that provide predominantly oxidising species. However, this again needs further detailed investigation.

Proposed cavitative oxidation mechanism for desulphurization.

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Results with diesel

It is instructive to evaluate the impact of nature of solvent from commercial application point of view for deep desulphurization of fuels. For this purpose, a commercial diesel was tested for the removal of sulphur (thiophene) similar to that discussed in the study. The commercial diesel had initial sulphur content of 30 ppm (probably in the form of refractory sulphur compounds). A known amount of sulphur using thiophene was added in this diesel and the effect of cavitation process was studied for pressure drop and for the extent of sulphur removal. The results of sulphur removal (thiophene in diesel) for the two different pressure drop conditions and using an intermediate organic to aqueous ratio (Organic phase, 6.5%) are shown in Fig. 7 along with a comparison with other organic solvents (Fig. 7). The results indicate a very high removal of sulphur even from the commercial diesel which is a mixture of aliphatic and aromatic organic compounds. Similar to that observed in certain cases earlier, here, the removal was better at higher pressure drop condition of 2 bar as compared to 0.5 bar. The results clearly demonstrate that proposed method is successful in removing sulphur from commercial diesel.

Comparison of deep desulphurization results using commercial diesel; (a) commercial diesel; (b) comparison with other organics.

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The proposed method can be effectively employed to reduce sulphur content of transportation fuels or other organic streams. The aqueous phase used in the proposed method can be recycled after removing a purge stream (with corresponding make-up water). The proposed method can be implemented in a compact set-up with effective removal of sulphur. The process may be further intensified using a number of ways e.g. aeration, catalyst etc. Hydrodynamic cavitation usually improves performance with scale-up. Therefore the proposed method can be effectively implemented for large scale deep desulphurization operation.

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