Process to remove olefins from light hydrocarbon stream by mercaptanization followed by MEROX removal of mercaptans from the separated stream

Abstract

A light naphtha feedstock containing olefins is introduced with hydrogen sulfide into a mercaptanization zone for conversion of the olefins into a mercaptan stream that is substantially free of olefins, after which the mercaptans are sent with an alkali caustic solution into a mercaptan oxidation treatment unit (MEROX) to produce a spent caustic stream and sweet light naphtha product stream that is substantially free of olefins and of mercaptans. Disulfide oils are produced from the wet air oxidation of the spent caustic, and the disulfide oils can be further processed to provide high purity olefin building blocks.

Claims

1. A process for treating an olefin-containing light naphtha feedstock, the process comprising: a. introducing the light naphtha feedstock containing olefins, an internally-produced mercaptan stream and an alkali caustic solution into a mercaptan oxidation treatment zone to produce a spent caustic and alkali metal alkane thiolate mixture stream and sweet light naphtha product stream that is substantially mercaptan free and comprises olefins; b. passing the spent caustic and alkali metal alkane thiolate mixture stream, catalyst, and air into a wet air oxidation zone to produce a regenerated spent caustic stream and a disulfide oils product stream; c. recovering the disulfide oils product stream; d. passing the sweet light naphtha product stream and hydrogen sulfide into a mercaptanization zone containing a catalyst and catalytically reacting hydrogen sulfide with the olefins to produce a treated effluent stream that is substantially free of olefins; e. passing the treated effluent stream to a fractionation zone and recovering a sweet light naphtha product stream and the internally-produced mercaptan stream of step (a).

2. The process as in claim 1, wherein a portion of the regenerated spent caustic stream is recycled and mixed to constitute the alkali caustic solution for introduction into the mercaptan oxidation treatment unit.

3. The process as in claim 1, wherein the olefin-containing light naphtha feedstock is selected from the group consisting of light naphtha hydrocarbon streams derived from catalytic reforming, steam cracking, fluid catalytic cracking (FCC), delayed coking or flexi-coking, isomerization, visbreaking, transalkylation, and combinations thereof.

4. The process as in claim 1, wherein the olefin-containing light naphtha feedstock has a boiling point in the range of from 10 C. to 80 C.

5. The process as in claim 1, wherein the olefin-containing light naphtha feedstock comprises C.sub.5-C.sub.6 olefins.

6. The process as in claim 1, wherein the mercaptanization zone contains a catalyst that is an active phase metal catalyst selected from Periodic Table Groups 4-11 supported by an alumina, silica, silica-alumina, titania, or zeolite support.

7. The process as in claim 1, wherein the mercaptanization zone operates at a temperature in the range of from 80 C. to 300 C., at a pressure in the range of from 10 bars to 50 bars, at a liquid hourly space volume (LHSV) in the range of from 1 h.sup.1 to 100 h.sup.1, and at hydrogen sulfide-to-olefin molar ratios in the range of from 1:1 to 100:1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The process of the present disclosure will be described in more detail below and with reference to the attached drawings in which the same number is used for the same or similar elements, and where:

(2) FIG. 1 is a simplified schematic diagram of a generalized version of the MEROX process of the prior art for the liquid-liquid extraction of a combined propane and butane stream;

(3) FIG. 2 is a simplified schematic diagram of a first embodiment of the process of the present disclosure;

(4) FIG. 3 is a simplified schematic diagram of a second embodiment of the process of the present disclosure; and

(5) FIG. 4 is a simplified schematic diagram of a third embodiment of the process of the present disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(6) Referring now to FIG. 2, an embodiment of the process and system (200) of the present disclosure that will be referred to as Embodiment 1 includes mercaptanization zone (210), a mercaptan oxidation, or MEROX zone (250), and a wet air oxidation zone (270).

(7) A light naphtha feed (202) comprising olefins and a hydrogen sulfide stream (204) are introduced into mercaptanization zone (210) to catalytically convert olefins present in the feed (202) into mercaptans and thereby produce a substantially olefin-free effluent stream (212). The substantially olefin-free effluent stream (212) is introduced with fresh alkali caustic solution (242) into a MEROX reaction zone (250) to sweeten the stream and produce a spent caustic and alkali metal alkane thiolate mixture stream (254) and sweet light naphtha stream (252) that is substantially free of olefins and of mercaptan.

(8) The sweet light naphtha stream (252) is recovered and the spent caustic and alkali metal alkane thiolate mixture stream (254) is introduced with a catalyst stream (262) and air (264) into the wet air oxidation zone (270) to provide the regenerated spent caustic (274) and to convert the alkali metal alkane thiolate compounds to disulfide oils (272), which can be recovered or passed for further downstream processing (not shown). A portion or all of the regenerated spent caustic (274) can optionally be recycled as stream (275) for mixing with fresh alkali caustic solution (242) prior to introduction into MEROX reaction zone (250). In an embodiment, the regenerated caustic and fresh caustic streams can be introduced into a mixing and storage vessel (not shown) from which it is introduced as needed into the MEROX reaction zone (250).

(9) Referring now to FIG. 3, an embodiment of the process and system (300) of the present disclosure that will be referred to as Embodiment 2, includes mercaptanization zone (310), a fractionation zone (330) a mercaptan oxidation, or MEROX zone (350), and a wet air oxidation zone (370).

(10) A light naphtha feed (302) comprising olefins and hydrogen sulfide stream (304) are introduced into mercaptanization zone (310) to catalytically convert olefins present in the feed (302) into mercaptans and thereby produce a substantially olefin-free effluent stream (312). The substantially olefin-free effluent stream (312) is introduced into a fractionation zone (330) to separate a light naphtha stream (334) comprising paraffins, naphthenes and aromatics which is substantially olefin free from a mercaptan stream (332) that is also substantially olefin free. The light naphtha stream (334) is recovered.

(11) The mercaptan stream (332) is introduced with an alkali caustic solution (342) into a MEROX zone (350) to sweeten the stream and produce a spent caustic and alkali metal alkane thiolate mixture stream (354) and sweet light naphtha stream (352) that is substantially free of both olefins and mercaptans.

(12) The sweet light naphtha stream (352) is recovered, and can optionally be combined with light naphtha stream (334) (not shown). The spent caustic and alkali metal alkane thiolate mixed stream (354) is introduced with a catalyst stream (362) and air (364) into the wet air oxidation zone (370) to provide the regenerated spent caustic stream (374) and to convert the alkali metal alkane thiolate compounds to disulfide oils (372), which can be recovered or further processed downstream (not shown). A portion or all of the regenerated spent caustic (374) can optionally be recycled as stream (375) for mixing with fresh alkali caustic solution (342) prior to its introduction into MEROX zone (350).

(13) Referring now to FIG. 4, an embodiment of the process and system (400) of the present disclosure that which will be referred to as Embodiment 3, includes mercaptanization zone (410), a fractionation zone (430) a mercaptan oxidation, or MEROX reaction zone (450), and a wet air oxidation zone (470).

(14) A light naphtha feed (402) comprising olefins is mixed with internally-generated mercaptan stream (432) to form a mixture (436) that is introduced with an alkali caustic solution (442) into a MEROX reaction zone (450) to sweeten the stream and produce a spent caustic and alkali metal alkane thiolate mixture stream (454) and sweet light naphtha stream (456) that is substantially mercaptan free and comprises olefins.

(15) The spent caustic and alkali metal alkane thiolate mixture stream (454) is introduced with a catalyst stream (462) and air (464) into the wet air oxidation zone (470) to provide the regenerated spent caustic (474) and to convert the alkali metal alkane thiolate compounds to disulfide oils (472), which can be recovered as a product, or further processed downstream (not shown). A portion or all of the regenerated spent caustic (474) can optionally be recycled as stream (475) for mixing with alkali caustic solution (442) prior to introduction with MEROX zone (450).

(16) The sweetened light naphtha stream (456) is introduced and hydrogen sulfide stream (404) are introduced into mercaptanization zone (410) to catalytically convert olefins present in the feed (402) into mercaptans and thereby produce a substantially olefin-free effluent stream (412). The substantially olefin-free effluent stream (412) is introduced into a fractionation zone (430) to separate a light naphtha stream (434) comprising paraffins, naphthenes and aromatics and that is substantially olefin free from the mercaptan stream (432) that is substantially olefin free. The light naphtha stream (434) is recovered. The mercaptan stream (432) is internally recycled and mixed with light naphtha feed (402).

(17) As will be understood by one of skill in the art, the above processes are described in terms of steady-state continuous operating conditions which follow a start-up period that is required for each of the unit operations.

(18) The fractionation zones can include units such as atmospheric columns, distillation columns, flash columns, gas strippers, steam strippers, alone or in combination.

(19) Suitable reactors used in the mercaptanization zone include, but are not limited to fixed bed, ebullated bed, slurry, moving bed and continuous stirred-tank reactors (CSTR).

(20) The mercaptanization unit can operate at temperatures in the range of from 80 C. to 300 C., 150 C. to 300 C., or 200 C. to 300 C.; at pressures in the range of from 10 bars to 50 bars, 10 bars to 30 bars, or 10 bars to 20 bars; at a liquid hourly space volume (LHSV) in the range of from 1 h.sup.1 to 100 h.sup.1, 2 h.sup.1 to 40 h.sup.1, or 5 h.sup.1 to 30 h.sup.1; and at hydrogen sulfide-to-olefin molar ratios in the range of from 1:1 to 100:1, 1:1 to 5:1, or 1:1 to 2:1.

(21) A suitable catalyst for use in the mercaptanization unit is an active phase metal catalyst that is selected from Periodic Table IUPAC Groups 4-11 and is supported by an alumina, silica, silica-alumina, titania, or zeolite support.

(22) In all embodiments, the mercaptanization unit can include gas-liquid separators for separation of the hydrogen sulfide from the liquid effluent stream (not shown). The recovered hydrogen sulfide can optionally be recycled to the mercaptanization unit. The liquid effluent stream is a substantially olefin-free effluent stream that is introduced into either the MEROX zone (Embodiment 1) or the fractionation zone (Embodiments 2 and 3).

(23) Disulfide oils produced can optionally be catalytically cracked to recover substantially pure olefins that can be used as chemical building blocks to make other fuel components or chemicals. These substantially pure olefins are of higher value than the olefins present in the original light naphtha stream, which due to their impurities, cannot be used effectively as a building block for other high value products.

(24) In preferred embodiments, the feedstream to the process can include light naphtha hydrocarbon streams derived from catalytic reforming, steam cracking, fluid catalytic cracking (FCC), delayed coking or flexi-coking, isomerization, visbreaking, transalkylation, cracking in the presence of water and other types of non-conventional hydrocarbon processing, alone or in combination. In some embodiments, the feedstream to the process boils in the range of from about 10 C. to 220 C. A light naphtha stream containing C.sub.4 olefins can have an initial boiling point of 10 C. In these embodiments, the feedstream has a boiling point in the range of from about 10 C. and up to 85 C. The feedstream can contain from 0.1 to 50 W %, from 0.1 to 30 W %, or from 0.1 to 10 W % of olefinic constituents. It should also be understood that the presence of paraffins will not adversely affect the processs.

EXAMPLE 1

(25) A light naphtha stream recovered from a delayed coking unit operation was subjected to mercaptanization with hydrogen sulfide in a fixed-bed reactor at a temperature of 200 C. and a pressure of 15 bars. The hydrogen sulfide was generated in situ by the decomposition of dimethyldisulfide (DMDS) with hydrogen over a catalyst bed in the same reactor. The mercaptanized stream was subjected to the MEROX process steps as described above to provide an olefin-free feedstock. Table 3 includes the composition and properties of a typical light naphtha stream. The total sulfur content of the light naphtha stream is 4,000 ppmw of which 2,848 ppmw is mercaptans. The original disulfide content is negligible and 20.2. W % of total sulfur of the light naphtha stream is thiophenic sulfur. The light naphtha stream contains 35.8 V % of olefins and has the very low aromatics content of only 2.1 V %.

(26) TABLE-US-00003 TABLE 3 PROPERTY Unit Value Boiling Point Range C. 32-115 Yield (total coker Naphtha basis) V % 54.0 Gravity API 72.2 Density @60 F./15.6 C. Kg/Lt 0.695 Sulfur ppmw 4,000 Basic Nitrogen ppmw 1.0 Nitrogen ppmw 69 n-paraffins V % 25.4 i-paraffins V % 21.5 Olefins V % 35.8 Naphthenes V % 10.7 Aromatics V % 2.1 Unknowns V % 4.4 Reid Vapor Pressure psi 2.9 Maleic Anhydride Value 19.7 Diene Value 0.020 Sulfur Distribution (of total sulfur) Mercaptans W % 71.2 Dialkyl Sulfides W % 8.4 Disulfides W % 0.2 Thiophenes W % 20.2

(27) The light naphtha stream was processed in accordance with Embodiment 1 as schematically illustrated in FIG. 2. The material balance for the process is shown in Table 4. When 1000 kg of light naphtha is processed, 639 kg of olefin-free sweet light naphtha and 511 kg of disulfide oil are recovered. The olefin-free hydrocarbon can be sent to a steam cracking unit to produce ethylene. The disulfide oils produced can be catalytically cracked to produce high purity light olefins.

(28) TABLE-US-00004 TABLE 4 Stream # Description Mass Flow, Kg/h 202 light naphtha 1000.0 204 hydrogen sulfide 158.1 212 olefin-free effluent 1158.1 242 alkali caustic solution (NaOH) 4757.5 252 sweet light naphtha 639.2 254 spent causticand alkali metal 5273.6 alkane thiolate 262 catalyst Negligible 264 air Negligible 272 disulfide oil 511.4

(29) The processes of the present disclosure have been described above and in the attached figures; process modifications and variations will be apparent to those of ordinary skill in the art from this description and the scope of protection is to be determined by the claims that follow.