respresented the ‘sweetening’ process for
this gas plant.
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I
53occur in the extractor; the disulfides will then return to the LPG phase and increase the product’s total sulfur content.
2) Small levels of mercaptides in the caustic (30 ppmw–50 ppmw) keep the catalyst dispersed. Thus, the catalyst does not accumulate at the rich-disulfide caustic interface in the disulfides separator.
Consequently, regenerated caustic must hold 30 ppmw–50 ppmw sodium mercaptide. In Fig. 6, the present unit operates with 8,234-ppmw sodium mercaptide concentration at the inlet of the oxidizer, and 108%–110% excess air is the optimum value.
The oxygen level in the air leaving DSO separator must range between 1.5% and 2%.
Caustic concentration and regeneration efficiency.
Caustic solution as a reaction medium has an optimum concen-tration of 1.8–1.9 molar, which supports 75% conversion. While increasing the caustic concentration to 3.8 molar is still practical;
the high levels only yield 70% conversion. Consequently, very high caustic concentrations are not beneficial to regenerating NaOH. To explain the regeneration reaction kinetics, there are two points. First, when increasing the concentration of caustic solution, the solubility of CoSPc catalyst will decrease catalyst dispersion in the solution. Second, higher alkaline solutions have greater viscosities, which hinders the transfer of free radical in the radical oxidation reaction of mercaptides.7,8
Experimental results suggest an appropriate alkaline concen-tration of 2.75–4.25mol/dm3 for the sweetening of LPG. Fig. 7 shows the simulation results over the effect of caustic concentra-tion on the rate of mercaptide oxidaconcentra-tion. Rich caustic soluconcentra-tion in the oxidizer is mixed with air as oxidant. Thus, variations of
caustic molarity by mass fraction of NaOH in solution are not the same as molarity variations of pure caustic solution by its composition, as listed in Table 9.
Since regenerated caustic is recycled from the oxidizer to the extractors, the concentration of regenerated caustic at the reactor outlet must be the same as the caustic concentration entering the extractor. Caustic concentration at the reactor inlet is specified as a function of the sodium mercaptides concentration to be oxidized to NaOH and the caustic concentration at the inlet of the extractors.
For present plant conditions, 8,200 ppmw of sodium mercap-tide is oxidized to NaOH. The optimum caustic concentration to the extractors and, thus, recycling from the oxidizer is 14.93%.
Consequently, the caustic concentration from the extractors to the oxidizer must be increased from 12.2 wt% to 14.5 wt% at the oxidizer inlet. Accordingly, 884.41 kg/hr of fresh caustic (solution of 40% wt) makeup is mixed with the rich-caustic solutions from the extractors. Referring to Fig. 7, 30 ppmw of sodium mercaptide will remain in caustic solution, which is a desirable level. Note:
SPGC Phases 4 and 5 propane is not prewashed; thus, a large volume of fresh caustic is required.
Temperature and efficiency of caustic regeneration.
Temperature is one of the most important factors influencing reactions. To oxidize propane mercaptide, the optimum tempera-ture based on oxidizer performance ranges between 40°C–50°C.
0.001 Excess air, mole air injected/mole air stochiometric Ethyl mercaptide remaining in regenerated caustic, ppmw
Ethyl mercaptide into the oxidizer: 8,234 ppmw Ethyl mercaptide into the oxidizer: 35,770 ppmw For caustic solution containing 8,234 ppm ethyl mercaptide:
Required Inlet concentration of caustic:12.5 wt% (dealing with not prewashed propane)
Outlet concentration of caustic: 14.93 wt%, oxidizer top to bottom temperature:40°C-50°C
Mass flowrate of caustic solution:13,851.9 kg/hr
For caustic solution containing 35,770 ppm ethyl mercaptide:
Required Inlet concentration of caustic:14.54 wt%
(if propane is prewashed)
Outlet concentration of caustic: 14.93 wt%, oxidizer top to bottom temperature: 40°C-50°C
Mass flowrate of caustic solution: 14,300.3 kg/hr
Sodium mercaptide in regenerated caustic as a function of excess air based from simulation results.
FIG. 6
0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18
Mass fraction of NaOH in the caustic solution, xw Ethyl mercaptide remaining in the regenerated caustic, ppmw
Inlet caustic containing 8,234 ppm ethyl mercaptide:
Oxidizer temperature from top to bottom:40°C-50°C Excess air: 110%
Mass flowrate of caustic solution: 13,851.9 kg/hr Sodium mercaptide in regenerated caustic vs. caustic concentration based on the simulation results.
FIG. 7
TABLE 9. Variations of caustic molarity by mass fraction of sodium hydroxide in solution
Caustic solution mixed with 110%
Pure caustic solution excess air under conditions of Fig. 7 Mass fraction Molarity of NaOH Mass fraction Molarity of NaOH of NaOHⴛ100 in the solution of NaOHⴛ100 in the solution
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I
MARCH 2009 HYDROCARBON PROCESSINGHowever, the oxidizer temperature should always be kept as low as possible considering catalyst activity while still maintaining the desired degree of mercaptans regeneration. In any event, 55°C would be considered as an absolute maximum temperature because of metallurgical limitations and also the possibility of disulfide oils decomposing into sulfonic acids.
Based on simulation results, Fig. 8 shows the effect of oxi-dizer temperature on the conversion of sodium mercaptide. The results are presented for two cases—design and actual operating conditions. The extraction of 2,500 ppmw of ethyl mercaptan from butane and 330 ppmw of methyl mercaptans from pro-pane by caustic will yield 8,234 ppmw of sodium mercaptide in the caustic solution at the oxidizer inlet (Fig 8). Since this is an endothermic reaction, if the sodium mercaptide content of the caustic at the reactor inlet is 8,234 ppmw, then the reactor top and bottom optimum temperatures should be approximately 45°C and 50°C, respectively under mentioned conditions in Fig. 8. Remember: At least 30 ppm of RSNa must remain in regenerated caustic.
Outlook. According to the results, caustic concentration of 14.93 wt% and temperatures of 40°C are optimum values for extractors. The required amount of caustic to extract mercaptans can be selected according to the purity of the product, as shown in Tables 7 and 8. When considering caustic regeneration con-ditions, amount of air injection to the oxidizer is a key factor affecting the sweetening process efficiency. Approximately 30 ppm–50 ppm of sodium mercaptide must be included in the circulating caustic. Fig. 6 shows the required air amount for specified conditions. The optimum log mean temperature of the oxidizer is 40°C to 45°C depending on the impurities con-centration. The optimum caustic concentration of 1.9 molar after mixing with air is the optimum value within the oxidizer, which can be adjusted by fresh caustic makeup. However, the
required concentration of recycling caustic to the extractor as well as the amount of sodium mercaptide impurities in the rich caustic are limiting factors for the unit and should be considered when defining the required concentration of the inlet caustic to the oxidizer.
High-purity propane and butane products were obtained in SPGC Phases 4 and 5 when operating variables were adjusted.
LPG with mercaptan content less than 10 ppm is sold at $ 3/
ton to $4/ton—more than present LPG prices. Consequently, optimizing this unit resulted in a total net income increase of
$2.9–$3.9 million/yr. This task is achieved without new equip-ment installed or equipequip-ment modifications. The results were possible by only fine-tuning operational process parameters with some extra caustic consumption reduction. HP
NOMENCLATURE Kp Partition coefficient KE Extraction coefficient
So Solubility in water Sc Solubility in salt solution C Salt concentration in water K Salting-out constant
rRSNa Reaction kinetic of sodium mercaptide oxidation KW Water ionization constant
KA Mercaptan ionization constant Kh Hydrolysis constant
Y2 Saturation of the caustic solution for averaging (moles S–2/mole NaOH)
Y´2 Saturation of the caustic solution for breakthrough (mole S–2/mole NaOH)
X1 NaOH weight fraction in caustic solution⫻100 T Temperature
xw Weight fraction
ACKNOWLEDGMENT
The authors thank South Pars Gas Company R&D for their support and their permission to publish this article.
LITERATURE CITED
1 C. P. D., Propane Treatment, Operating Manual, Chapter 2, Process Section 2, Iran South Gas Field, Phases 4 and 5, Unit 114, June 2003.
2 C. P. D., E. L., Butane Treatment, Operating Manual, Chapter 2, Process Section 2, Iran South Gas Field, Phases 4 and 5, Unit 115, June 2003.
3 d’ESTEVE, C., “Sulfrex Process, Process Data Book, South Pars Phases 4 and 5,” On Shore Facilities, Assaluyeh, p. 7, pp. 20–21, 2001.
4 Aminian, H., “Chemical refining of condensate produced by Iran’s Razi Complex,” M Sc. Thesis, Sharif University of Technology, pp. 25–34, 1996.
5 Mazgarov, A., “Desulfurization of Oil, Gas, Petroleum Products and Wastewater,” Volga Research Institute of Hydrocarbon Feed, Kazan, Russia, 2005.
6 Mazgarov, A. M., “A selective treatment of various oils and gas condensates to remove light mercaptans and hydrogen sulfide,” World Petroleum Congress, 2006.
7 Ruiting, L., X. Daohong and X. Yuzhi, “Oxidation of sodium mercaptide with sulfonated cobalt phthalocyanine as catalyst,” American Chemical Society, Vol. 48, No. 2, pp. 74–76, March 2003.
8 Ruiting, L., X. Daohong and X. Yuzhi, “ Study on the Stability of CoSPc in LPG Sweetening,” American Chemical Society, Vol. 48, No. 4, pp. 338–340, August 2003.
9 Ruiting, L., X. Daohong, X. Yuzhi and T. Yongliang, “Effects of caustic con-centration on the LPG sweetening,” Petroleum Science and Technology, Vol. 23, No. 5–6, pp. 71–72, May/June 2005.
10 Tukov, G. V., N. N. Ivanova, A. N. Sadykov, A. M. Polotskii and N.
A. Glebova, “Establishing Standards for Consumption of Caustic Soda in Treating Liquefied Petroleum Gases (LPG) to Remove Mercaptans,”
Chemistry and Technology of Fuels and Oils, Vol. 11, No. 11–12, pp 869–872, November/December 1975.
11 Savary, L., “Gas Processing with Axens’ Technology, From Purification to Liquefaction,” Axens, 1996.
1 10 100
20 25 30 35 40 45 50 55
Mean log temperature, °C Ethyl mercaptide in caustic leaving the reactor, ppm
Inlet caustic containing 8,234 ppm ethyl mercaptide:
Required inlet concentration of custic: 12.5 wt (if propane is not prewashed
Excess air: 110%
Mass flowrate of caustic solution: 13,851.9 kg/hr Inlet caustic containing 35,770 ppm ethyl mercaptide:
Inlet concentration of caustic: 14.54 wt% (if propane is prewashed)
Excess air: 200%
Mass flowrate of caustic solution: 14,300.3 kg/hr
Sodium mercaptide in regenerated caustic as a function of temperature based on the simulation results.
FIG. 8
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HYDROCARBON PROCESSING MARCH 2009