FIELD OF THE INVENTION
The present invention relates to a process for selective removal of sulfur compounds from LPG using non-regenerative caustic treatment. The invention also relates to optimization of use of fresh caustic. The present invention further relates to an apparatus for selective removal of sulfur compounds from LPG in which conventional regeneration of caustic is avoided.
BACKGROUND OF INVENTION
LPG in the refinery is sourced from the primary and secondary conversion units. LPG is basically a mixture of hydrocarbons such as propane, butane and pentane. Depending on the origin i.e. crude distillation unit, coker or FCCU, LPG contains different species of sulfur. These are hydrogen sulphide, mercaptans, carbonyl sulphide, carbon di-sulphide etc. The extraction of mercaptans from hydrocarbon streams (and LPG) is widely practiced in the petroleum refining industry and is probably performed in most of the world's major petroleum refineries. It is described in basic reference sources such as Volume 15 of the second edition of the Kirk-Offner Encyclopedia of Chemical Technology. This reference shows the basic mercaptan extraction process in which a hydrocarbon feed stream is passed through an extraction column countercurrent to a descending stream of lean aqueous alkaline solution normally referred to in the art as caustic. The treated product is removed from the top of the extraction column.
A mercaptan-containing caustic solution referred to as a "rich" caustic solution is removed from the bottom of the extraction column and passed into an oxidation zone in admixture with air. An oxidation catalyst dissolved in the caustic solution promotes the oxidation of the extracted mercaptans to disulfide compounds within the oxidation zone. The effluent stream of the oxidation zone is passed into a phase separation vessel from which the disulfide compounds are decanted. This procedure serves to remove the mercaptan compounds from the rich caustic stream and is therefore referred to as "regeneration" of the caustic. The resultant "lean" caustic is removed from the separation vessel and recycled to the extraction column.
Most of the sulfur species can be extracted by caustic treatment process which requires an equipment in which caustic and LPG are intimately contacted. Caustic of desired concentration is to be used for extracting mercaptans. This is conventionally achieved by using

a caustic regeneration process in which rich caustic containing sodium mercaptides is reacted with air in the presence of a soluble catalyst to regenerate caustic. The regenerated caustic is returned back to the sulfur extraction process. Fresh LPG reacts with the regenerated caustic.
It is known to the those skilled in the art that a good separation of the disulfide compounds from the caustic solution is required in order to minimize the content of disulfides in the caustic being recirculated to the extraction zone. The disulfide oils are soluble in hydrocarbon streams. Therefore, disulfide compounds present in the regenerated caustic being fed to the top of extraction column will become dissolved in the hydrocarbon stream which is being treated. This will raise the sulfur content of the treated hydrocarbon stream and may be totally unacceptable. It is known in the art to counteract this effect by removing disulfide compounds from the regenerated caustic. The regenerated caustic or regenerated aqueous alkaline solution may therefore be processed as in US Patent 2921020 which describes the use of a disulfide removal zone 26. In this zone, the regenerated caustic is contacted with a hydrocarbon distillate such as pentane or hexane. The caustic solution is then passed into the extraction zone.
US Patents 2853432 and 3408287 also describe the regeneration of alkaline solution through the oxidation of dissolved mercaptans to disulfide compounds are presented. These patents references illustrate the common practices of admixing air with a mercaptan-containing extract stream to support the catalyzed oxidation of the mercaptans, the subsequent passage of the oxidation zone effluent stream into a phase separation zone from which excess air is vented and a regenerated alkaline solution is removed, and the usage of the preferred oxidation catalyst. The references also describe the overall flow of the alkaline stream including the removal of this stream from the phase separation zone and its recirculation to the extraction zone. US Patents 4040947 and 4104155 is pertinent to the subject inventive concept since these references illustrate the prior art method of handling a sidestream of alkaline solution which is processed in a contacting zone for the purpose of water removal.
This process is expensive as it involves a catalyst, a multiphase reactor, a separator with or without hydrocarbon solvent having low sulfur content, caustic recirculation system, air compressor, off-gas disposal facilities for caustic regeneration. Additionally, disulphide oil

produced in the reaction is immiscible in caustic and comes out as very fine droplets. These droplets are either removed through coalescing or by dissolving in hydrocarbons. Total removal of disulphide oil from caustic is very difficult and therefore efficiency of removal is always less than 100%. Sulfur contaminated hydrocarbons is disposed of to hydrotreater to remove sulfur compounds.
Prior art conventional process for the treatment of LPG involving caustic regeneration is carried out in two stages. In Stage-I, primarily hydrogen sulphide is removed by reacting with caustic in a batch process. Caustic concentration gets depleted to 1-2% from about 10%. In Stage-II, LPG from Stage-I is continuously treated with caustic received from a caustic regenerator. In Stage-II, mercaptans from LPG are extracted (Fig. 1A). Caustic regeneration process, shown in Fig. 1B is expensive and contributes to 30-40% of the LPG treatment cost. However, for moderate level of mercaptan in the LPG and also for moderate extent of removal of mercaptan such elaborate process can be avoided and a simplified process as shown in Fig. 2 can be adopted.
OBJECTS OF THE INVENTION
An important object of the present invention is to provide a process for selective removal of sulfur compounds from Liquefied Petroleum Gas.
Another object of the present invention is to provide an apparatus for selective removal of sulfur compounds from Liquefied Petroleum Gas, the apparatus comprising of a batch column reactor B1; specially designed distributor; continuous extraction column B2; contacting vessel B3; extracting vessel B4; storage vessel B5 and associated pumps and control system.
Still another object of the invention is to provide an apparatus and a process for removal sulfur compounds from LPG at reduced cost without involving caustic regeneration.
SUMMARY OF THE INVENTION
The above and other objects of the present invention are achieved by a process for selective removal of sulfur compounds from Liquefied Petroleum Gas claims comprising the steps of distributing LPG in batch column reactor B1 through a specially designed distributor
continuously through a pool of caustic; distributing LPG from batch column reactor B1 at

bottom of continuous extraction column reactor B2 through a specially designed distributor and feeding caustic from top; distributing LPG and caustic from continuous extraction column reactor B2 to the contacting column B3; feeding LPG and caustic from contacting column reactor B3 to extracting vessel B4; and feeding LPG and caustic from extracting vessel B4 to storage vessel B5 separating the layers of caustic and LPG.
Preferably concentration of said caustic in batch column reactor Bl gradually decreases as incoming sulfur impurities continue to consume caustic.
Preferably said continuous extraction column B2 facilitates counter-current contacting of LPG and caustic.
Preferably said contacting column B3 facilitates co-current contacting of caustic and LPG.
Preferably said extracting vessel B4 acts as contactor where the caustic is contacted with the solvent.
Preferably said solvent extracts immiscible fine droplets of disulphide oil from the caustic.
Preferably said storage vessel B5 has an upper layer of LPG and lower layer of caustic and acts as caustic holding vessel in which excess caustic from continuous extraction column B2 accumulates and after predetermined time is discharged to batch column reactor B1.
In another embodiment of the invention, the process consists of batch process for extraction of hydrogen sulphide and continuous process for removal of mercaptans.
In still embodiment of the invention, the process of the present invention avoids the step of caustic regeneration, thereby reducing the cost of treatment of LPG.
In yet embodiment of the invention, an apparatus for selective removal of sulfur compounds from Liquefied Petroleum Gas comprising of batch column reactor B1; continuous extraction column B2; specially designed distributor, contacting vessel B3; extracting vessel B4; storage vessel B5; and associated pumps and control system is disclosed.
Preferably said batch column reactor B1 has a sheet metal structured packing or random packing.
Preferably said continuous extraction column B2 has single or multiple beds of
structured packing or random packing.


Preferably said wherein contacting vessel B3 has a structured packing.
In yet embodiment of the invention, the hold up volumes of continuous extraction column B2, contacting vessel B3 and extracting vessel B4/ storage vessel B5 is 20.36 m3, 42.47 m3, and 15.7 m3, respectively.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIGURE 1A: Conventional apparatus for treatment of LPG for removal of sulfur impurities.
FIGURE 1B: Conventional apparatus for caustic regeneration.
FIGURE 2: The apparatus of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Liquefied Petroleum Gas (LPG) which consists of mainly butane/propane is produced in Petroleum refinery. LPG normally contains sulfur impurities e.g. hydrogen sulphide, mercaptans etc. which are normally extracted by strong caustic solution. The caustic after reaction forms sodium sulphide and sodium mercaptide which require regeneration in the presence of catalyst and air in the conventional treating/extraction process. Regeneration of caustic is expensive as it involves elaborate plant & machinery, catalyst and chemicals and disposal of sulfur products. In the present method, regeneration of used caustic can be avoided by innovatively designing the system after taking into consideration of inlet concentration of sulphur impurities in the LPG. However, dilute caustic generated in the plant is discharged to effluent treatment plant as spent caustic. Figure 2 illustrates the apparatus of the present invention.
LPG from battery limit is distributed through a specially designed distributor into column B1 containing structured packing. The column is partially filled with caustic. Essentially this is a batch contactor. LPG from this column is again distributed in column B2 containing structured packing. Caustic is distributed from top of the column such that LPG & caustic contacts counter-currently in the presence of structured packing. Finally, LPG from B2 is contacted co-currently with fresh caustic in contactor B3 containing structured packing. Both are collected in vessel B4 and these separates out in two layers: upper layer of LPG and lower layer of caustic.


Thus, the present invention is a significant improvement over conventional process/apparatus for removal of sulfur impurities using caustic as shown in Fig 1A. Typical compositions of impurities in raw and treated LPG are shown in Table 1. To meet this, the quantity of caustic (15 wt%) required is 100 Kg/hr. The concentration of caustic purged is 2 wt%. In the process of the present invention, LPG is first treated in stage-I (B1) to remove hydrogen sulphide. In stage-II, a contactor B2 is used for proper contacting of LPG & caustic. Caustic extracts mercaptans from LPG and then sent to regenerator B3. From regenerator B3, caustic is contacted with solvent in contactor B4. Caustic containing immiscible fine droplets of disulphide oil are extracted with solvent. Sulfur free caustic is collected in separator B5. This caustic is returned back to the process.
The present invention thus provides an improved apparatus and process for selective removal of sulfur species from LPG without conventional regeneration of caustic. The process is optimized for reduction of operating and capital cost. The process consists of batch process for extraction of hydrogen sulphide and continuous process for removal of mercaptans and results in total removal of hydrogen sulphide and limited removal of mercaptans. A mathematical model has been developed to calculate volume of different equipments as shown in Table 2.
ADVANTAGES OF THE PRESENT INVENTION
• The process of the present invention is optimized for reduction of operating and capital cost.
• The process is suitable for total removal of hydrogen sulphide and limited removal of mercaptans.
• The process consists of batch process for extraction of hydrogen sulphide and continuous process for removal of mercaptans.
• The storage vessel integrates continuous to batch process. Hold up of the vessel is decided based on precise calculations involving material balance of sulfur impurities present in the feed.
• In the present invention, caustic is discharged in batches after specified time such that concentration reduces to 1-2% from about 8-10% of caustic transferred from vessel B5.
• Equipment volumes/hold up are calculated using a proprietary process model specifically developed for this process.
• A mathematical model has been developed to calculate volume of different equipments.

We claim:
1. A process for selective removal of sulfur compounds from Liquefied Petroleum Gas
comprising the steps of -
(a) distributing LPG in batch column reactor B1 through a specially designed distributor continuously through a pool of caustic;
(b) distributing LPG from batch column reactor Bl at bottom of continuous extraction column reactor B2 through a specially designed distributor and feeding caustic from top;
(c) distributing LPG and caustic from continuous extraction column reactor B2 to the contacting column B3;
(d) feeding LPG and caustic from contacting column reactor B3 to extracting vessel B4;
(e) feeding LPG and caustic from extracting vessel B4 to storage vessel B5 separating the layers of caustic and LPG.

2. The process as claimed in claim 1, wherein concentration of said caustic in batch column reactor Bl of step (a) gradually decreases as incoming sulfur impurities continue to consume caustic.
3. The process as claimed in claim 1, wherein said continuous extraction column B2 of step (b) facilitates counter-current contacting of LPG and caustic.
4. The process as claimed in claim 1, wherein said contacting column B3 of step (c) facilitates co-current contacting of caustic and LPG.
5. The process as claimed in claim 1, wherein said extracting vessel B4 of step (d) acts as contactor where the caustic is contacted with the solvent.
6. The process as claimed in claim 5, wherein said solvent extracts immiscible fine droplets of disulphide oil from the caustic.

7. The process as claimed in claim 5 or 6, wherein fresh caustic at higher strength is fed to said extracting vessel B4 continuously at a predetermined rate.
8. The process as claimed in claim 1, wherein said storage vessel B5 of step (e) has an upper layer of LPG and lower layer of caustic.
9. The process as claimed in claim 8, wherein said storage vessel B5 acts as caustic holding vessel in which excess caustic from continuous extraction column B2 accumulates and after predetermined time is discharged to batch column reactor B1.
10. The process as claimed in claims 8 or 9, wherein said storage vessel B5 integrates continuous to batch process.
11. The process as claimed in claims 1 to 10, wherein said process consists of batch process for extraction of hydrogen sulphide and continuous process for removal of mercaptans.
12. The process as claimed in claim 11, wherein said process results in total removal of hydrogen sulphide and limited removal of mercaptans.
13. The process as claimed in claims 1 to 12, wherein said process avoids the step of caustic regeneration, thereby reducing the cost of treatment of LPG.
14. An apparatus for selective removal of sulfur compounds from Liquefied Petroleum Gas, the apparatus comprising of batch column reactor Bl; continuous extraction column B2; specially designed distributor, contacting vessel B3; extracting vessel B4; storage vessel B5; and associated pumps and control system.
15. The apparatus as claimed in 14, wherein said batch column reactor Bl has a sheet metal structured packing or random packing.
16. The apparatus as claimed in 14, wherein said continuous extraction column B2 has single or multiple beds of structured packing or random packing.


17. The apparatus as claimed in 14, wherein said contacting vessel B3 has a structured packing.
18. The apparatus as claimed in claims 14 to 17, wherein the hold up volumes of continuous extraction column B2, contacting vessel B3 and extracting vessel B4/ storage vessel B5 is 20.36 m3, 42.47 m3, and 15.7 m3, respectively.
19. A specially designed distributor substantially as herein described and as illustrated by the accompanying drawings.