Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1176A
1 METHOD FOR THE PREPARATION OF
2 METHYL TERTIARY BUTYL ETHER
4 BACKGROUND OF THE INVENTION
8 Field of the Invention
9 The present invention relates to an improvement in the
manner of conducting concurrent reactions and distillations
11 wherein the catalyst is also the distillation structure.
12 Related art
13 Recently a new method of carrying out catalytic reactions
14 has been developed, wherein the components of the reaction system
are concurrently separable by distillation, using the catalyst
16 structures as the distillation structures. This method is now
17 generally known as catalytic distillation and any reference to
18 catalytic distillation herein will be taken to mean this method
19 or process. Such systems are described variously in U.S. Patents
4,215,011; 4,232,177; 4,242,530; 4,302,356; 4,307,254;
21 4,336,407; 4,439,350; 4,443,559; and 4,482,775 commonly assigned
22 herewith.
23 Briefly, a preferred and commercial catalyst structure
24 described in the above patents comprises a cloth belt with a
plurality of pockets spaced along the belt and containing
26 particulate catalyst material, said cloth belt being wound in a
.~ crl.pat~1176A.app
~. . . .........
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~ helix about a spacing material such as stainless steel knitted
2 mesh. These units are then disposed in the distillation column
3 reactor. In addition, commonly assigned U.S. Patent Nos.
4 4,443,559 and 4,250,052 disclose a variety of catalyst
structures for this use and are incorporated herein.
6 The success of catalytic distillation lies in an
7 understanding of the principles associated with distillation.
8 First, because the reaction is occurring concurrently with
9 distillation, the initial reaction product is removed from the
reaction zone as quickly as it is formed. The removal of the
11 reaction product minimizes further reaction, decomposition,
12 polymerization and the like. Second, because in a distillation
13 the compounds are boiling, the temperature of the reaction is
14 controlled by the boiling point of the mixture at the system
lS pressure. The heat of the reaction simply creates more boil up,
16 but no increase in temperature. Third, the reaction has an
17 increased driving force because the reaction products have been
18 removed and cannot contribute to a reverse reaction (Le
19 Chatelier's Principle).
The distillation parts of the above disclosures have been
21 conventional, i.e., counter-current vapor liquid flow in the
22 packed catalyst bed with the catalyst acting as the contact
23 structure, at least in the reaction zone. The reaction zone
24 having the catalyst packing is designated the reaction
distillation zone to distinguish it from other distillation zones
26 which contain either inert packing or conventional distillation
crl.pat\1176A.app 2
5~
~_ trays. The conventional distillation zones may be above or below
2 the distillation reaction zone according to the separation
3 desired.
4 In one particular embodiment for making methyl tertiary
butyl ether, the physical embodiment of the distillation column
6 reactor includes a separate distillation zone below the
7 distillation reaction zone to insure that the unreacted feed
8 components are removed from the ether product which is taken off
9 as bottoms product. In at least one case the lower distillation
zone is a separate distillation column connected to another
11 distillation column which contains the catalyst. Vapor and
12 liquid flow lines are provided so that essentially the two
13 columns act as one.
14 Because of the nature of the distillation the reactants and
products are separated. Depending upon the components, however,
16 the reactants may be separated before the desired reaction is
17 completed requiring recycle. It was thus seen to be desirable to
18 retain the reactants in contact with the catalyst while still
19 separating out the products.
SUMMARY OF THE INVENTION
21 Briefly the present invention is the discovery that the
22 reaction rate can be increased by improving the contact of the
23 liquid with the catalyst, which is accomplished by increasing the
24 liquid level in the reaction distillation zone. This is
achieved by a liquid flow restrictor between the distillation
26 reaction zone and the lower distillation zone. That is, the
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57a~
r- vapor from below may rise up to (and through) the reaction
2 distillation zone as in a conventional or prior operation but a
3 portion of the liquid is maintained there. If a single
4 distillation column reactor is used, a conventional distillation
tray with the downcomer area blocked is located between the
6 reaction distillation zone and the distillation zone. A by pass
7 line for liquid flow is provided about the tray and a valve is
8 provided in the liquid flow conduit to restrict liquid downflow
9 and thereby to build up a liquid level above that tray just below
the catalyst bed. Alternatively a perforated plate may be used
11 to support the catalyst and cause a liquid pressure drop in the
12 column thus building up a level in the catalyst. If the two
13 column system is used, then a valve or other restriction means is
14 placed in the liquid flow line between the two columns.
While the particular position of the liquid level has been
16 described above to be at the lower end of the distillation
17 reaction zone, it could just as easily be placed anywhere in the
18 catalyst bed depending upon the desired reactions.
19 The term "liquid level" is used herein to mean an increased
density of the material in the reaction distillation zone over
21 that of a pure distillation as distinguished to a continuous
22 liquid phase. The phase system as present in the reaction
23 distillation zone is physically a froth. This is the result of
24 the vapor traveling up through the liquid retained in the zone.
Another way of viewing this is that in normal distillation
26 there is a vapor with liquid (internal reflux) trickling down
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201 5574
1 through the vapor and contracting the catalyst whereas in the
2 present "flooded" system the vapor is traveling up through a
3 liquid phase to create the froth or foam.
4 Hence in essence the benefits of the distillation are still
obtained, i.e., separating the various components by the
6 distillation whereas the increased liquid volume in contact with
7 the catalyst improves the synthesis reaction.
8 BRIE~ DESCRIPTION OF THE DRAWING
9 Fig. 1 is a flow diagram of one embodiment of the invention
showing separate columns for the distillation and reaction
11 zones.
12 Fig. 2 is a plan view of the liquid flow restriction in a single
13 column.
14 Fig. 3 is top view of a perforated plate useful in the column of
Fig. 2.
16 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
17 Referring now to the Fig.s a detailed description of the
18 preferred embodiments can be appreciated.
19 Fig. 1 shows a simple flow diagram of a process using the
present invention. The particular process shown is for the
21 production of methyl tertiary butyl ether (MTBE) from the
22 reaction of methanol and isobutene in a mixed butene/butane feed
23 stream. For a detailed description of the process the reader is
24 referred to U.S. Patent 4,307,254. Generally there is shown a -
first distillation column 1 which contains an acid cation ex-
26 change resin packing 7 suitable
cr l . pa t~11 76A . app 5
for the reaction. The acid cation exchange resin catal
2 suitably disposed in the column 1 as described in U.S. Patent
3 4,307,254 to act as both a catalyst and distillation structure.
4 The methanol and mixed butene/butane stream is fed to the first
5 column 1 into the catalyst 7 in a feed zone via flow lines 5 and
6 6. The methanol reacts with the isobutene in the catalyst bed or
7 reaction distillation zone to form MTBE. The unreacted
8 components of the mixed butene/butane stream are distilled off
9 overhead and recovered via flow line 8. At the same time the
10 MTBE product is distilled off toward the bottom since the
11 temperature of the catalyst (reaction distillation) zone is
12 maintained at the boiling of the reactants at the operating
13 pressure of the column, which is lower than the boiling point of
14 the MTBE.
The bottoms liquid product containing MTBE and some
16 dissolved unreacted methanol and C4 hydrocarbons is carried out
17 the bottom of the first column 1 via flow line 3 to the top of
18 second column 2 where the MTBE is more completely separated from
19 any dissolved methanol or C4's in a conventional distillation
20 column 2 having trays as shown or inert packing and recovered via
21 flow line 9. The unreacted materials are recovered overhead via
22 flow line 4 which carries them back as vapors to the bottom of
2 3 the first column 1. A level controller 10 is secured to the
24 first column l and senses a liquid level in the first column
25 (as by a differential pressure) and operates flow control valve
26 11 which acts as a liquid flow restriction between the two
crl.pat\1176A.app 6
2~ S~
~Sr columns and maintains a desired preset liquid level in the
2 catalyst bed 7 of column 1. Note the level control 10 may be
3 positioned to detect the level over any portion of the column 1.
4 Pumps, compressors, and other operating equipment are not
shown as they are conventional and readily selected by those of
6 ordinary skill in the art of distillation column design. Example
7 I shows a comparison of one such unit operated with and without
8 the liquid level in the catalyst bed.
9 Fig. 2 illustrates an arrangement which may be used if only
one column is used. Only that portion of the column is
11 illustrated that is used to maintain the liquid level in the
12 catalyst bed.
13 In Fig. 2 the column 200 has a bed of catalyst 201 which
14 acts as a distillation structure. Directly below the catalyst
bed 201 is shown a perforated plate 202 which supports the
16 catalyst bed 201. The plate 202 as indicated in Fig. 3 is
17 perforated to allow gas passage upward into the catalyst bed 201
18 yet provides a sufficient pressure drop to allow a liquid level
19 to build up above the plate in the bed 201. The plate is
approximately 5-20 percent open space. A liquid bypass flow
21 line 203 is provided about the plate 202 to give added control of
22 the level. Valve 204 in bypass 203 may be opened or closed in
23 response to a differential pressure (indicating liquid level) to
24 control the liquid level. If desired the valve can be part of a
control loop (not shown) responding to a liquid level controller.
26 Alternatively a standard distillation tray may be
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~~ substituted for the perforated plate 202. The downcomer area of
2 the standard tray is blocked and the by pass flow line 203 used
3 to control the liquid level in the bed.
4 EXAMPLE I
A commercial catalytic distillation process for the
6 production of MTBE was operated according to that disclosed in
7 U.S. Patent 4,307,254. Following an incident in which the
8 catalyst was partially deactivated, the operation was changed to
9 maintain the liquid level at the top of the catalyst zone. The
arrangement was similar to that shown in Fig. 1. A control
11 valve acted as a restriction in the liquid flow line 3 to control
12 the liquid level which was sensed by a differential pressure in
13 the distillation reaction column 1. Unexpectedly, the
14 performance of the commercial unit with the damaged catalyst was
almost equal to the unit with undamaged catalyst.
16
17 EXAMPLE II
18 The method and structure have been found to be particularly
19 useful for conducting the catalytic distillation production of
tertiary butyl alcohol (TBA) from the hydration of isobutylene.
21 In the TBA process a stream containing isobutylene is fed to the
22 column below the catalyst bed and water is fed above the
23 catalyst bed. The catalyst bed contains an acid cation exchange
24 resin as described in U.S. Patent 4,307,254 and is placed into
the one inch laboratory column in the manner described therein.
26 Unreacted butylene, water and inerts (such as other C4's) are
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201 557~
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1 taken overhead and the TBA product is recovered as bottoms.
2 Water must be present in amounts sufficient to maintain the
3 catalyst in a hydrated state plus enough for the reaction and to
4 accommodate the water azeotropes in the system. One method of
control is to measure the amount of water present in the TBA
6 fraction within the column and to maintain that amount above
7 zero but below the azeotropic concentration at the temperature
8 and pressure used.
9 ~ithout the liquid level, the catalyst performs
satisfactorily at first but quickly loses its selectlvity due to
ll loss of water despite the control technique outlined above. This
12 may be attributed to mass transfer and distribution problems
13 within the catalyst bed. It has been found that maintaining a
14 liquid level in the catalyst bed using the technique of Fig.s 2
and 3 maintains the wetted state of the catalyst and allows high
16 selectivity toward tertiary butyl alcohol production. Table I
17 below compares the results of the process with and without the
18 liquid level in the bed. The liquid level in the catalyst bed is
19 indicated by the high differential pressure across the bed. In
the test runs, a 1" diameter tower was used ten feet in length.
21 Four feet of Rohm and Haas AMBE~L~ST* 15 catalyst was inserted
22 into the column in a pocketed belt twisted with wire mesh.
* Trademark
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2f)l 5574
1 _ TABLE I
2 PRODUCTION OF T-BUTYL ALCOHOL
4 Standard Process L i q u i d i n
Catalyst Bed
6 Overhead pressure,psig 160 165
7 Feed Rates, ml/min.liq
8 C 4~S (42%IB) 5.0 5.0
9 H2O 0.68 0.78
Column Temp., ~F
11 Overhead 168 165
12 Cat. Zone 165 185
13 Bottoms 230 315
14 lDiff. Press. Across
Cat. Zone 0.0 72
16 Bottoms analysis,wt.%
17 Lt. Ends (C4 + Cs) 46.9 5.3
18 TBA 18.0 93.7
19 DIB 35.1 1.0
21 1Differential pressure is measure as % change in pressure in
22 normal distillation pressure in catalyst zone and when totally
23 flooded with liquid in catalyst zone.
24
EXAMPLE III
26 In one other example the pilot plant was run as described
27 in commonly assigned U.S. Patent No. 4,849,569 issued July 18,
28 1989 using a 3" pilot plant column with Union Carbide LZY-82
29 molecular sieve in the pockets of the catalyst structure for the
production of cumene form the alkylation of benzene with
31 propylene.- Again, the use of the liquid level as measured by the
32 differential pressure across the bed improved performance of the
33 catalyst and process. Table II below shows comparative data
34 between the normal operation and with the liquid level.
36
37
38
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2TABLE II
3PRODUCTION OF CUMENE
5Standard Process L i q u i d i n
6 Catalyst Bed
7 Overhead Pressure, psig . 109 109
8 Differential across bed, psi 1.7 7.2
9 Feed rate, lb/hr
Benzene 15.1 16.6
11 Propylene 12.1 13.1
12 Reaction Temp. ~F 336 340
13 Propylene conversion,% 73.7 91.0
14 While particular configurations have been shown, it should
be understood that the liquid level may be maintained at any
16 location within the catalyst bed using the techniques disclosed
17 in either Fig. 2 or 3.
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