FI56961C - FOERFARANDE FOER TILLSAETTANDE AV SVAVEL VID EN PROCESS FOER FRAKTIONERAD DESTILLATION AV EN BLANDNING INNEHAOLLANDE POLYMERISERBARA FOERENINGAR - Google Patents

Description

Γ1 ANNOUNCEMENT

11 UTLÄGGN, NGSSKRIFT 5696 1 • jli & ljg C (45) Γ - - -1 .: t i cy JiLty 12 3 1930

Patent ceidolat 'v ^ (51) Kv.lk.' / Int.CI.1 C 07 C 7/04 B 01 D 3/34 ENGLISH —Fl N LAND (21) Ptttnttlhtktmui - PatenttiwBknlng 2218/72 (22) Hakamlipllvl - Antttknlngtdig 10.08.72 * '(23) Alkupllvl — Giltlghctsdag 10.08.72 (41) Tullut publication - Bllvlt offtntllg 12.02.73

Patent and Registration Office ....

B, (44) Date of deflection and moonshine. -

Patent and registration authorities Ansökan utlagd och utl.akriftan publlcerad 31.01.80 (32) (33) (31) Privilege claimed - Begird prloritet H.08.71 USA (US) 170861+ (71) Universal Oil Products Company, Ten U0P Plaza- Algonquin & Mt.

Prospect Roads, Des Plaines, Illinois 600l6, USA (72) Charles Vincent Berger, Western Spring, Illinois, USA (71+) Berggren Oy Ab (5I +) Method for adding sulfur in a fractionation process of a mixture containing polymerizable compounds - Förfarande för This invention relates to a process for adding sulfur in a fractional distillation process of a mixture containing polymerizable compounds, wherein the polymerizable mixture is fed to the feed point of the fractionation zone in a fractionation distillation zone

The use of elemental sulfur to inhibit the polymerization that occurs during the fractional distillation of a mixture containing a polymerizable compound is well known to those skilled in the art. U.S. Patent Nos. 2,166,125 and 2,188,772 use elemental sulfur to inhibit the polymerization of vinyl aromatic hydrocarbons, with the addition of a solid that ultimately dissolves in the hydrocarbon. In U.S. Patent No. 3,222,263, elemental sulfur is used as a polymerization inhibitor in the distillation of a vinyl aromatic hydrocarbon in which the sulfur is added in the molten state. Furthermore, in U.S. Patent No. 2,757,130, elemental sulfur is used as a polymerization inhibitor in the purification of heterocyclic nitrogen compounds, such as a vinylpyridine-type compound.

In the described systems, the addition of elemental sulfur poses several practical operational problems. When granular solid sulfur is used, the exact amount of sulfur added to the system cannot be adjusted. Due to the mixed particle size in each batch of granular sulfur and 2 h O ϋ b] since the solubility of sulfur may vary depending on the raw material composition and / or temperature, the exact amount of sulfur added when passing the inhibited flow through the sulfur-containing tank may vary from excess from insufficient to insufficient amounts of sulfur. Thus, an undesired amount of polymerization occurs. Typical sulfur addition methods using solid sulfur addition involve contacting either the feed or reflux mixture with a particular column having a solid layer of sulfur.

To overcome some of the problems associated with large variations in sulfur concentration, which in turn are due to variations in the solubility of sulfur in a distillation zone, sulfur has been added to the distillation zone in the molten state. This approach is an improvement over solid-type work, as increasing a predetermined amount of liquid sulfur is much easier to accomplish than adding a predetermined amount of sulfur by dissolving solid sulfur in a particular stream. However, this approach is not without its own operational difficulties. For example, in a typical distillation zone where styrene is fractionally distilled, the fresh feed is at a temperature of about 24-52 ° C. At this temperature, the solubility of the sulfur is low and the molten sulfur, which is insoluble, can become solid, either in the appropriate addition system or as a precipitate in the distillation system, preventing the flow of liquid through the filler or bottom used in the system. To overcome some of these precipitation and recrystallization difficulties in the art, a portion of the fresh feed to the column is drawn off and heated to a higher temperature before the molten sulfur is mixed into it. The resulting mixture is mixed with fresh feed and then passed to a distillation system.

This addition system, described in U.S. Patent No. 3,222,263, requires the liquid stream to be heated to a temperature of about 38-121 ° C and thus requires a relatively high surface temperature of the heat exchanger (i.e., 1-1 ° C) to efficiently heat the mixture without the need for a large lämmönvaihdinpintaa. Heating the liquid flow close to the surface temperature of the heat exchanger not only increases the surface of the heat exchanger but also increases the residence time in the heat exchanger. Furthermore, the method described in this patent makes it possible to use a mixing tank to obtain a homogeneous, concentrated sulfur solution. The long residence time at elevated temperature in this container may explain the exceptional behavior mentioned in this patent, where an increase in the amount of sulfur above about 0.4% by weight actually caused an increase in total styrene loss. This finding is in contrast to the predicted results of polymerization alone, as indicated by Figure 4 of that patent, showing a continuous decrease in polymerization rate, 3 GGS6 as sulfur concentration increases. Because the extended time and elevated temperature lead to the formation of an additional polymer, this molten sulfur addition system is not entirely satisfactory. Thus, although in this system, where the sulfur-containing mixture is mixed into the feed after the removal point, some sulfur is present during heating due to recycling, an unnecessary amount of polymerization still occurs.

Accordingly, it is an object of the present invention to provide a system for adding sulfur in either a molten or solid state to a distillation system to separate a mixture containing a polymerizable compound without unnecessarily increasing the tendency to form a polymer.

The invention therefore relates to a process for adding sulfur in a fractional distillation process of a mixture containing polymerizable compounds, wherein the polymerizable mixture is fed to a fractionation zone feedstock and divided upstream and downstream in the zone, and in which elemental sulfur is used from the bottom stream to provide a bottom stream containing added sulfur, and (b) directing the sulfur-containing bottom stream from step (a) to the feed point of the fractionation zone. A preferred embodiment of the invention is characterized in that the sulfur is molten sulfur, that the bottom stream is heated before said sulfur is mixed with it, and that the sulfur-containing bottom stream obtained from step (a) is fed to a polymerizable mixture mixed at the feed point of the fractionation zone.

The process of the invention is preferably applicable to a process for adding sulfur to a fractional distillation process of a polymerizable mixture obtained from the effluent of an ethylbenzene dehydrogenation unit containing benzene, toluene, ethylbenzene, styrene, styrenic polymers and tars using sulfurization. Using the addition method of the invention, this polymerizable mixture is fed to the fractionation zone at the fresh feed feed point to provide a benzene-toluene overflow and an ethylbenzene-styrene bottom stream. A portion of the bottom stream thus obtained is diverted and heated and then molten sulfur is added to provide a bottom stream containing dissolved sulfur. This bottom stream containing added sulfur is then mixed with a polymerizable mixture which is fed as a feed to the fractionation zone.

The ethylbenzene-styrene bottom stream is preferably diverted from the distillation zone at a temperature between n.

i \ S G 96 1 93_113 ° C and is heated to a temperature between about 121 and 9 ° C · Preferably about 1-10 / 5 of the styrene-ethylbenzene bottom stream originally removed as bottom stream from the fractionation zone is set aside and recycled back to the distillation zone after addition of sulfur.

Other objects, embodiments, and a more detailed description of the above embodiments can be found in the following more detailed description of the present invention.

The sulfur addition system of the present invention can be used whenever elemental sulfur is used as a polymerization inhibitor and the polymerizable compound is fractionally distilled. Examples of fractional distillation systems in which sulfur is used as polymerization inhibitors and in which the improved addition system of this invention finds use include the polymerization of polymerizable vinyl aromatic hydrocarbons such as styrene, 4-chlorostyrene, 4-ethylstyrene, methylethylstyrene, ethyl vinylbenzene, ethyl vinylbenzene, ethyl vinylbenzene, Likewise, the present invention is applicable to the distillation of heterocyclic nitrogen compounds such as 2-methyl-5-vinylpyridine, etc.

However, the present invention finds particular use in fractional distillation systems in the presence of fractional distillation of a mixture containing styrene, and in particular styrene, obtained from the effluent of an ethylbenzene dewatering unit. In a typical styrene recovery system, the normally liquid hydrocarbon portion of the ethylbenzene dehydrogenation unit effluent containing benzene, toluene, ethylbenzene, styrene, styrene polymers, and tars is separated in the presence of a three column separator. In the first column, the C8 fraction (i.e. mainly benzene and toluene and not C8 hydrocarbons) is removed from above and the C8 fraction, mainly ethylbenzene and styrene (i.e. 90% by weight) is recovered as a bottom stream. This Cg bottoms stream is fractionally distilled in a second fractionation column to provide an ethylbenzene overflow that is predominantly ethylbenzene (i.e., below styrene) and to produce an impure styrene bottoms stream containing styrene, polymers, tars, and sulfur. This impure styrene bottom stream is further fractionated in a third fractionation column from the pure styrene product. to produce an upstream and a residual stream consisting of polymers, tars, sulfur, etc.

As shown, the improved sulfur addition system involves mixing sulfur with a portion of the bottom stream obtained by fractional distillation of a mixture containing 5 & 6961 polymerizable compounds, where the use of sulfur as a polymerization inhibitor is accepted in the art. Thus, in accordance with the present invention, sulfur can be added to any of the three column sets described for recovering styrene from the effluent of an ethylbenzene dehydrogenation reactor. However, sulfur is preferably added to the first distillation column in the series by the process of this invention.

Sulfur can be added either as a solid by passing a portion of the bottom stream over the solid sulfur bed or, preferably, by mixing molten sulfur with the bottom stream. The resulting bottom stream, which contains the newly added sulfur, is then directed to the same point in the fractionation zone as the point to which the feed is directed, whereby the sulfur is distributed throughout the system. Preferably, the bottom stream is mixed directly to the feed to the column to ensure thorough sulfur distribution throughout the bottom of the column, where high temperatures and associated polymerization problems are more prevalent.

In a typical distillation system as described, the bottom temperature is usually the highest temperature in the distillation system. Thus, it is not always necessary to heat the part of the base to which the sulfur is to be added further, since the amount of sulfur to be added is often easily dissolved in the base parts so that it can be led to the feed point. Whether or not additional heating is required depends on the composition, temperature and amount of the bottom stream to which sulfur is to be added and is readily apparent to those skilled in the art. The amount of bottom stream thus recycled should not significantly interfere with the otherwise normal operation of the distillation zone and generally amounts to about 1 to about $ 10 of the total bottom flow.

The exact amount of sulfur to be maintained is well known to those skilled in the art, as shown in U.S. Patent Nos. 2,166,125; 2,188,772; 2,757,130 and 3,222,265, the instructions of which are incorporated herein by reference in their entirety. The amount added is preferably below the bottom flow saturation level. The temperature of the molten sulfur should be below about 149 ° C. to make it easy to handle. Higher temperatures rapidly increase the viscosity of liquid sulfur and can cause polymerization due to the heat contained in the sulfur when added to the polymerizable mixture.

Specifically, in the case of a fractional distillation process for recovering styrene from the effluent stream of a dehydrogenation reactor, sulfur is preferably added, as mentioned above, to a portion of the bottom stream of the first fractionator distillation column. In such a separation, in which benzene and toluene are removed from above and C8 hydrocarbons are removed from the bottom, pressures below normal pressure are used so that temperatures as low as possible can be used to prevent polymerization. Bottom temperatures of about 93 to about 113 ° C are usually used. In this system, it has been found that raising the temperature of the bottom flow section to which the sulfur is to be added to about 121-1.1 ° C and adding sulfur at a temperature of about 132 ° C provides efficient operation. More specifically, about 1 to about 10% of the bottom flow is recycled and the amount of sulfur in the total feed to the column is considered to be about 0.2-1% by weight. This generally requires about 20 wt.% Sulfur for the recycle stream, which at the recommended temperature of about 132 ° C does not precipitate sulfur at any point in the system.

A particular advantage of this addition system is that the recyclable bottom stream portion must be heated prior to the addition of sulfur. Since the bottom flow is already at an elevated temperature and contains sulfur, when the operation of the column reaches equilibrium, the tendency to form a polymer due to the heating step is reduced. Further, since the temperature difference between the base and the desired final temperature is smaller than when adding sulfur to the fresh feed, either lower surface temperatures or shorter residence times are required in the heat exchanger. Thus, the tendency to polymerize is reduced.

The addition system of the present invention can best be explained by reference to the accompanying schematic drawing showing the dehydrogenation of ethylbenzene to form styrene and the associated product recovery equipment necessary to provide a relatively pure styrene product.

A schematic drawing of the type shown must necessarily be subject to certain limitations and is not intended to limit the generally broad scope of this invention to any particular feedstock, flow rates, operating conditions, catalysts, etc. Miscellaneous accessories such as valves, controllers, pumps, compressors, separators, separators, separators. is omitted and only those containers and tubes necessary for a complete and clear understanding of the various embodiments of this invention are shown.

Referring to the attached schematic drawing, a relatively pure ethylbenzene stream enters via pipe 1 and is mixed with the ethylbenzene recycle stream described below coming from pipe 2, and the resulting mixture is passed along pipe 1 to a well-known zone , catalyst and operating conditions, etc. are not critical to the practice of this invention. In a typical dehydrogenation system, zone 3 comprises a plurality of dehydrogenation reactors with internal heating between each reaction section. Zone 3 produces a reactor effluent at a temperature of about 593 ° C, while dehydrogenation zone 3 consists of a steam dehydration zone. This effluent, which is discharged along pipe 4, is cooled by equipment which are not visible to a temperature of about 38 ° C and led to the settling tank 5 of the reactor products »

In the settling tank 5 of the reactor products, the reaction effluent separates into a gas phase, a hydrocarbon phase and an aqueous phase. The aqueous phase is removed along line 6 and the gas phase containing carbon dioxide, carbon monoxide, hydrogen, methane, ethane, ethylene, etc. is removed along line 7. The resulting hydrocarbon phase is then removed for further separation by fractional distillation along line 8 and mixed with the sulfur-containing stream described below coming through line 9 and passed to a benzene-toluene column 10. More specifically, in a commercial embodiment, 16900 kg / h of hydrocarbon is removed from separation zone 8. and contains about * 10 moles of # ethylbenzene and about 52 moles of # styrene. Generally, the column 10 contains about 30-50 midsoles and the feed point is in the center of the column.

The resulting hydrocarbon-sulfur mixture, which is passed to the center of the benzene-toluene column 10, is separated into a Cy stream which is removed from above via line 11 and contains about 310 kg / h of benzene and toluene, about 11 kg / h of normally gaseous compounds has not been removed through pipe 7, and about 12 kg / h of water vapor. A mixture of C8 containing ethylbenzene, styrene, styrene polymers, tars and ice is removed from the bottom via line 12, the origin of which will be described later. The top of the benzene-toluene column 10 is maintained at a temperature and pressure of about 38 ° C and 100 mm / g absolute pressure and the bottom at a temperature of about 103 ° C and a pressure of 210 mmHg absolute pressure. Thermal equilibrium is maintained by removing a portion of the bottom stream along line 12 and passing that portion to a preheater 13 to maintain a constant bottom temperature of 103 ° C by evaporating at least a portion of the bottom material and passing the resulting vaporized mixture to a lower portion of the distillation column.

The other part of the bottom flow is removed from the pipe 12 at a rate of approx. * <5 * <kg / h along a pipe 1 * 1 and contains approx. 0.63 wt. This stream is heated to a temperature of about 132 ° C by an indirect heat exchanger 15 and the resulting heated mixture is removed via a pipe 9 and mixed with about 101 kg / h of molten elemental sulfur, 8 56961, which is led from a sulfur addition tank 16 via a pipe 17. This sulfur-containing mixture is then introduced into pipe 9 and mixed with hydrocarbons removed from the settling tank of the reactor products via pipe 8. Complete mixing is achieved with a mixing nozzle in tube 9. The addition of sulfur as shown and in the amounts described gives a sulfur concentration of 0.6% w / w in the hydrocarbons passed to the distillation column 10 and is sufficient to inhibit the polymerization of the styrene therein.

The total bottom flow of the benzene-toluene column is removed along line 18 at a rate of about 16200 kg / h and passed to an ethylbenzene fractionation column 19> the upper end of which is maintained at a temperature of about 52 ° C and an absolute pressure of 35 mmHg and the lower end at about 105 ° C. temperature and an absolute pressure of 240 mmHg. The upstream stream removed from the fractionation column 19 via line 2 is an ethylbenzene recycle stream which is led to dehydration zone 3, as described above. The part removed as a bottom flow along the pipe 20 is an impure styrene flow containing styrene, styrene polymers, tars and sulfur.

This impure styrene stream is then passed through line 20 to a styrene purification column 21, where the impure styrene mixture is separated into an upstream stream 22 containing a relatively pure styrene product and a bottom stream 23 containing styrene polymers, tars and sulfur. More specifically, the upper end of the fractionation column is maintained at a temperature of about 54 ° C and an absolute pressure of 30 mmHg and the bottom at a temperature of about 99 ° C and an absolute pressure of 100 mmHg.

As described, fresh elemental sulfur is added to the benzene-toluene column 10 by adding sulfur to a portion of the bottom stream produced therein and mixing together the resulting fresh sulfur-containing mixture and the feed stream to the distillation column. The same scheme, although not described, could be used in an ethylbenzene column 19 by mixing sulfur with a portion of the impure styrene bottom stream, or similarly the same scheme could be used in a fractionation column 21 by mixing sulfur with a polymer tar residue.