Method and apparatus for the separation by distillation of a three- or multi-component mixture

Abstract

A method and apparatus for distillative separation of a mixture comprising three or more components including at least one low boiler, at least one medium boiler, and at least one high boiler, the method comprising feeding the mixture of three or more components to a first distillation column, removing the at least one high boiler as a bottom fraction from the first distillation column, feeding a top fraction of the first distillation column to a second distillation column, removing the at least one medium boiler via a sidestream takeoff from the second distillation column, removing the at least one low boiler as a top fraction from the second distillation column, and feeding a bottom takeoff stream from the second distillation column to the first distillation column as a reflux, wherein the first and the second distillation columns have vertical dividing walls.

Claims

1. An apparatus for distillative separation of a mixture comprising three or more components, the apparatus comprising: a first distillation column coupled to a second distillation column to allow material transfer; at least one vapor tube of the first distillation column connected to a bottom of the second distillation column such that vapor(s) from the first distillation column are communicatively connected to the bottom of the second distillation column; and at least one bottom takeoff stream of the second distillation column connected to a reflux section of the first distillation column, wherein the first and the second distillation columns both have vertical dividing walls which in the first distillation column extend to a top end of the interior of the column, and which in the second distillation column extend to a bottom end of the interior of the column, the first distillation column having a combined lower stripping section and the second distillation column having a combined upper recitifying section, wherein the second distillation column has one or more sidestream takeoffs below a top takeoff stream and above the at least one bottom takeoff stream, and wherein at least the first distillation column comprises one or more evaporators for evaporating a liquid bottom stream.

2. The apparatus of claim 1, wherein the first and the second distillation columns have 1-200 theoretical plates.

3. The apparatus of claim 2, wherein both the first distillation column and the second distillation column comprise one or more evaporators for evaporating liquid bottom stream.

4. The apparatus of claim 1, wherein at least the second distillation column comprises one or more condensers for condensing vapor stream(s).

5. The apparatus of claim 2, wherein at least the second distillation column comprises one or more condensers for condensing vapor stream(s).

6. A method for distillative separation of a mixture comprising three or more components including at least one low boiler, at least one medium boiler, and at least one high boiler in an apparatus of claim 1, the method comprising: feeding the mixture of three or more components to the first distillation column, removing the at least one high boiler as a bottom fraction from the first distillation column; feeding a top fraction of the first distillation column to the bottom of the second distillation column; removing the at least one medium boiler via a sidestream takeoff from the second distillation column; removing the at least one low boiler as a top fraction from the second distillation column; and feeding a bottom takeoff stream from the second distillation column to the first distillation column as a reflux.

7. The method of claim 6, further comprising operating the first and the second distillation columns at an offgas pressure of from about 1 to 10 bar and a boiling temperature range of from about 20 to 200 C.

8. The method of claim 6, wherein the first and the second distillation columns comprise one or more evaporators that utilize water vapor or thermal oil(s) of various pressure and temperature ratings as heat transfer agents.

9. The method of claim 6, wherein the first and the second distillation columns comprise one or more condensers that utilize cooling water or cooling brine of various pressure and temperature ratings as heat transfer agents.

10. The method of claim 7, wherein the first and the second distillation columns comprise one or more condensers that utilize cooling water or cooling brine of various pressure and temperature ratings as heat transfer agents.

11. The method of claim 6, further comprising feeding uncondensable top stream component(s) from a first condensation stage to a second condensation stage and/or to a scrubber system.

12. The method of claim 7, further comprising feeding uncondensable top stream component(s) from a first condensation stage to a second condensation stage and/or to a scrubber system.

13. The method of claim 6, wherein the mixture comprising three or more components comprises chlorosilanes as the medium boiler.

14. The method of claim 7, wherein the mixture comprising three or more components comprises chlorosilanes as the medium boiler.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIGS. 1A-1D show interconnections of two or more columns and also a dividing wall column according to the prior art.

(2) FIG. 2 shows the interconnection of two columns provided with dividing walls according to the invention.

(3) FIG. 3 shows a conventional interconnection of distillation columns composed of a stripping column with evaporator and condenser and also a second distillation column with evaporator and condenser.

(4) FIG. 4 shows a dividing wall tandem column with evaporator and condenser according to the invention, as used in the example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) The invention provides for coupling of distillation columns with one another so as to allow material transfer. In addition, vertical dividing walls are incorporated in each of the distillation columns.

(6) The coupling so as to allow material transfer achieves an adding of the number of theoretical plates of the two columns.

(7) Thus if two identically constructed columns are used, a doubling of the number of theoretical plates results.

(8) The coupling so as to allow material transfer is accomplished in that each of the columns has at least two linkages with the respective other column at spatially separate locations.

(9) Two such columns coupled so as to allow material transfer are equivalent to a single dividing wall column regarding energy requirements. Large energy savings can thus be realized, where, however, lower capital costs are incurred in comparison to the new acquisition of a conventional single dividing wall column since conventional pre-existing distillation columns can be converted into dividing wall columns in the context of a revamp and interconnected with one another in such a way that the two mentioned distillation columns provided with dividing walls perform the function of a prior art dividing wall column.

(10) The columns coupled so as to allow material transfer may each be equipped with a dedicated evaporator and/or condenser.

(11) The low boiler fraction and the high boiler fraction may be removed from different columns. The operating pressures of the columns are adjusted in such a way that the prescribed direction of flow is maintained. It is also possible to partially or completely evaporate the bottom stream of the first column in an evaporator and subsequently feed it to the second column in biphasic form or in the form of a gaseous stream and a liquid stream.

(12) The mixture of three or more components is preferably a mixture comprising chlorosilanes or a mixture comprising methylchlorosilanes. Preference is given to mixtures from the TCS synthesis or the MCS synthesis (TCS=trichlorosilane, MCS=methylchlorosilane), or from the deposition of polycrystalline silicon. Preference is given to a mixture composed of chlorosilanes comprising TCS, STC, DCS, and also traces of further impurities (methylchlorosilanes, hydrocarbons, high boilers) as is obtained via the reaction of commercially available metallurgical silicon with HCl in a fluidized-bed reactor at 350-400 C.

(13) In an assembly for producing polycristalline silicon, TCS is generated as crude silane either from metallurgical silicon and HCl or from metallurgical silicon with STC/H.sub.2 (STC=silicon tetrachloride) in a fluidized-bed reactor. Subsequently, the crude silane is purified by distillation/purification to form TCS. Polycrystalline silicon is deposited from the purified TCS, whereupon, inter alia, STC is formed. The subsequent utilization of the STC (e.g. via hydrogenation to form trichlorosilane or by combustion to produce finely divided silica or silicic esters) is common.

(14) During the deposition of polycrystalline silicon from a mixture of chlorosilane, in particular TCS, and hydrogen, a fraction of high boiling chlorosilanes is formed in addition to STC. The term high boiling chlorosilanes describes compounds composed of silicon, chlorine, optionally hydrogen, oxygen, and carbon and having a boiling point higher than that of STC (57 C. at 1013 hPa). Preference is given to the disilanes H.sub.nCl.sub.6-nSi.sub.2 (n=0-4) and higher oligo(chloro)silanes preferably having 2 to 4 Si atoms, and also to the disiloxanes H.sub.nCl.sub.6-nSi.sub.2O (n=0-4) and higher siloxanes preferably having 2 to 4 Si atoms including cyclic oligosiloxanes and also their methyl derivatives.

(15) The residues (high boilers) of the Mller-Rochow process are principally tetrachlorodimethyldisilane, trichlorotrimethyl-disilane and dichlorotetramethyldisilane, that is methylchloro-disilanes of the general composition Me.sub.6-xCl.sub.xSi.sub.2. These can be treated with metallurgical silicon and HCl at a temperature of at least 300 C. TCS and STC are formed in the process.

(16) The high boilers in offgas from the deposition of polycrystalline silicon (Siemens process) are mainly chlorodisilanes of the general composition H.sub.6-xCl.sub.xSi.sub.2 and, as the case may be, chlorodisiloxanes H.sub.6-xCl.sub.xSi.sub.2O. In addition, the offgases comprise TCS, STC, and DCS.

(17) The invention and its differences compared to the prior art are illustrated below with the aid of figures.

(18) FIG. 2 shows a dividing wall column via coupling of two existing distillation columns TWK1 and TWK2 according to the invention. The invention provides for coupling of the existing distillation columns so as to allow material transfer, i.e. feeding the vapors of one column directly to the column bottom of the second column and also providing the bottom takeoff stream and/or bottom takeoff streams of the other column to the first column as reflux. The two individual apparatuses are equipped with the necessary dividing walls, internals, and sidestream takeoffs in the context of the conversion. The design according to FIG. 2 is energetically equivalent to the dividing wall column 14 in FIG. 1D. Compared to new investment, the conversion can be carried out with reduced capital expenditure.

(19) The distillation columns are preferably equipped with separating plates of different types such as separating trays (e.g. sieve trays, fixed valves), random packings (packing bodies), or structured packings. The internals are critical determinants of separation performance and also of the pressure drop over the distillation columns.

(20) The mentioned distillation columns preferably have 1-200 theoretical plates, where the number of theoretical plates necessary is dependent on the quality/degree of contamination of the starting mixture to be separated, the specified purity requirements for the target product, and also on the relative volatility of the individual components of the multicomponent mixture (with respect to the key components).

(21) The distillation columns are preferably operated at an offgas pressure of from 1 to +10 bar and a boiling temperature range of from 20 to +200 C.

(22) Regarding a distillation assembly composed of two or more individual apparatuses, the offgas pressures canbearing in mind economic aspectsbe selected independently of one another. The mentioned distillation columns/individual apparatuses are also preferably equipped with one or more evaporator systems for supplying the heat energy.

(23) In a conventional evaporator system, one or more heat generators are flanged to the column body of an individual apparatus via connectors/adapters. The heat generator can also be designed in a wide variety of forms from a process technology standpointpreferably, however, it is designed as a natural circulation evaporator.

(24) The column body is preferably equipped with a further connection for a second evaporator system.

(25) If two distillation columns are coupled to one another as in FIG. 2, then at least one vapor tube of the first column is directly flanged to the column body of the second column. An existing flange connection on the column body can, where present, be used for such purpose. Should this connection be absent, then it must be retrofitted.

(26) The vapor tube is preferably designed as a double tube. This allows heating by the bottom takeoff stream of a different column in the assembly. Condensation in the vapor tube can thereby be avoided.

(27) The bottom takeoff stream of the second column is used for the reflux of the first column. To this end, for example, the reflux pumps of the first column can be connected to the bottom takeoff of the second column, as long as the pumps prove useful.

(28) Preferred heat transfer agents for the evaporation are water vapor and/or thermal oils of various pressure and temperature ratings. The choice of the various operating materials for the evaporation is determined primarily by economic aspects and also by availability.

(29) It is preferred that the mentioned distillation columns/individual apparatuses are additionally equipped with one or more condensing systems for condensing the vapor to supply the reflux amount to the respective column.

(30) In the first condensing stage, uncondensable vapor portions composed of components having low boiling points and/or inert gas are fed to a further condensing stage and/or a further workup/other use (preferably a scrubber system).

(31) Preferred heat transfer agents for the condensation are cooling water and/or cooling brine of various pressure and temperature ratings. The choice of the various operating materials for the condensation is determined primarily by economic aspects and also by availability.

(32) Operation as a dividing wall tandem column requires one or preferably two or more side stream takeoffs on one of the two columns, at which the target product is withdrawn when the target product is a pure medium boiler B.

(33) The correct position on the circumference and height of the column body should be selected according to the thermodynamic design. If, according to this thermal design, the takeoff is located between the two columns, then the target product can be withdrawn from the reflux line to the right of the dividing wall of the one column.

Example and Comparative Example

Comparative ExampleConventional Interconnection

(34) FIG. 3 shows a conventional distillation arrangement composed of a stripping column including an evaporator and a condenser and also a rectifying column including an evaporator and a condenser.

(35) The material stream F is composed of a chlorosilane-containing mixture having a low boiler fraction, medium boiler fraction, and high boiler fraction. In column K1, the low boiler fraction is removed via the material stream D1. The material stream B1 is fed into the second column K2 in which the high boiler fraction is withdrawn via material stream B2 and in which the target product (medium boiler fraction) is withdrawn via material stream D2.

(36) Table 1 shows the mass fractions of the individual components in the respective substreams according to the comparative example

(37) TABLE-US-00001 TABLE 1 Material stream Component F D1 B1 D2 B2 TCS 99.5% 90% 99.9% 99.99% 99.99% DCS 0.5% 10% C1 <10 ppmw 10 ppmw 1 ppmw 300 ppmw C2 <0.5 ppmw 10 ppmw .sup.0.04 ppmw .sup.0.04 ppmw C3 <10 ppmw 20 ppmw
C1-C3 are trace impurities such as methylchlorosilanes, hydrocarbons, and dopant compounds.

ExampleDividing Wall Tandem Column

(38) FIG. 4 shows a preferred embodiment of a distillation column according to the invention comprising a first distillation column TWK1, designed as a dividing wall column, including an evaporator, and also a second distillation column TWK2, likewise designed as a dividing wall column, including a condenser.

(39) The material stream F is composed of a chlorosilane-containing mixture having a low boiler fraction, a medium boiler fraction, and a high boiler fraction. In column TWK1, the high boiler fraction (comprising C1) is removed via material stream B. In the second column TWK2, the low boiler fraction (comprising DCS and C3) is withdrawn via material stream D and the target product (medium boiler fraction comprising TCS) is withdrawn via material stream M.

(40) Table 2 shows the mass fractions of the individual components in the respective substreams according to the example.

(41) TABLE-US-00002 TABLE 2 Material stream Component F D B M TCS 99.5% 90% 99.9% 99.99% DCS 0.5% 10% C1 <10 ppmw 300 ppmw 1 ppmw C2 <0.5 ppmw 10 ppmw .sup.0.04 ppmw C3 <10 ppmw 20 ppmw

(42) It can be seen in FIG. 4 that both an evaporator and a condenser are dispensed with compared to the comparative example.