Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2000-06-09
2001-05-15
Keys, Rosalynd (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S699000
Reexamination Certificate
active
06232509
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the etherification of C
4
and C
5
isoolefins, with an alcohol such as methanol, to produce the corresponding tertiary ether. More particularly the invention relates to a process wherein a catalytic distillation process is used in the process and wherein both isobutene and isoamylene are contained in the charge and wherein methanol is used to produce methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME).
2. Related Information
The increased governmental restriction on gasoline has strained the availability of feed stocks for etherification. The use of the isoamylenes for the preparation of octane improvers for gasoline has increased. It is highly desirable to be able to employ mixed isobutene/isoamylene streams. In a conventional separation of the reaction components from the etherification of a mixed C
4
/C
5
stream the separation of the unreacted C
5
components from the MTBE is quite difficult because of their close boiling points. Even small amounts of MTBE in the C
5
raffinate is detrimental to its use in a downstream gasoline alkylate plant.
U.S. Pat. No. 4,661,209 discloses the separation of MTBE from close boiling hydrocarbons by extractive distillation using a higher boiling oxygenated, nitrogenous and/or sulfur containing compound. The past necessity for such costly procedures has inhibited the actual feeding of C
4
's and C
5
's together to a reactor to co-produce MTBE and TAME. It has been found, however, that using a catalytic distillation system operated in a specific manner the separation can be achieved easily and concurrently in the reaction.
The reaction of an alcohol and an olefin and concurrent separation of the reactants from the reaction products by fractional distillation has been practiced for some time. The process is variously described in U.S. Pat. Nos. 4,232,177; 4,307,254; 4,336,407; 4,504,687; 4,987,807; and 5,118,873 all commonly assigned herewith. The isoolefins preferably react with the alcohol to form ethers. Briefly the alcohol and isoolefin are fed to a distillation column reactor having a distillation reaction zone containing a suitable catalyst, such as an acid cation exchange resin, in the form of catalytic distillation structure, and also having a distillation zone containing inert distillation structure.
U.S. Pat. No. 5,248,837 discloses a method for controlling catalytic distillation etherifications wherein the methanol concentration below the catalyst bed is controlled to a point that maximizes ether production and prevents alcohol from leaving with the bottoms ether produce. The patent discloses only the reaction of one isoolefin and one alcohol at a time. In particular the invention is embodied by the production of TAME by the reaction of methanol and isoamylene.
Commonly assigned patent application Ser. No. 08/120,644 discloses a similar process to U.S. Pat. No. 5,248,837 wherein the alcohol content is controlled by total oxygen in the form of OH. This system is particularly useful when a mixture of alcohols is used.
Neither U.S. Pat. No. 5,248,837 nor application Ser. No. 08/120,644 address the problem of separation of MTBE from the unreacted overheads containing the unreacted C
5
's.
It is an advantage of the present invention that MTBE product can be separated from the reactants when charging a mixed C
4
/C
5
stream to produce MTBE and TAME.
SUMMARY OF THE INVENTION
Briefly, the present invention is the discovery that control of the methanol concentration in a mixture of C
4
's, C
5
's, and MTBE (or MTBE and TAME) in a distillation column, preferably a distillation column reactor where the ethers are co-produced, to maintain substantially an azeotrope of methanol and C
5
in the distillate allows for the separation of MTBE (or MTBE and TAME) from the unreacted C
5
's by fractionation. The separation is made possible because it has been found surprisingly that it is substantially easier to separate MTBE from the C
5
/methanol azeotrope than it is to separate MTBE from C
5
hydrocarbons alone. The catalytic distillation column may be used alone or more desirably combined with a front end straight pass reactor. The upper limit on the C
4
content is determined by economics. Isobutene is a better reactant than isoamylene in this reaction, however, if the intent of the process is to react isoamylene then isoamylene should be maximized.
A preferred embodiment is the process for the co-production of MTBE and TAME from the reaction of the isobutene and isoamylenes contained in a mixed C
4
/C
5
stream comprises the steps of:
(a) feeding methanol and a mixture comprising C
5
hydrocarbons including pentanes, n-pentenes and isoamylenes and C
4
hydrocarbons including butanes, n-butenes and isobutene to a down flow fixed bed straight pass reactor preferably at least 15 mole % substantially inert C
4
hydrocarbons including butanes and isobutene;
(b) reacting a portion of said isobutene and said isoamylene with a portion of said methanol in said down flow fixed bed reactor to form a first mixture containing methyl tertiary butyl ether, tertiary amyl methyl ether, unreacted methanol, isobutene, isoamylenes, unreacted C
4
's and unreacted C
5
's;
(c) feeding said first reaction mixture to a distillation column reactor;
(d) concurrently in said distillation column reactor
(i) reacting a substantial portion of the said unreacted isobutene and said unreacted isoamylenes with unreacted methanol to form additional methyl tertiary butyl ether and tertiary amyl methyl ether in a second reaction mixture containing methyl tertiary butyl ether, tertiary amyl methyl ether and unreacted methanol, and
(ii) separating said methyl tertiary butyl ether and said tertiary amyl methyl ether contained in said second reaction mixture from said unreacted C
4
's, unreacted C
5
's and unreacted methanol contained in said second reaction mixture by fractional distillation;
(e) removing said unreacted methanol, C
4
's and C
5
's from said distillation column reactor as overheads;
(f) controlling the methanol concentration in said column to maintain substantially a methanol/C
5
azeotrope in said overheads,
(g) removing said methyl tertiary butyl ether and said tertiary amyl methyl ether from said distillation column as bottoms; and
(h) condensing a portion of said overheads and returning a portion of said condensed overheads to said distillation column reactor as reflux.
Surprisingly it has been found that when the methanol concentration profile is appropriately controlled to allow only an azeotrope amount of methanol overhead the separation of the MTBE from the C
5
's is facilitated and methanol does not exit in the bottoms ether products.
In the etherification reaction, especially the TAME reaction, the presence of an excess of methanol is necessary to direct the otherwise reversible reaction toward the ether.
The reflux ratio may be adjusted to optimize the effect described above, preferably in range of 1:2 to 2:1.
REFERENCES:
patent: 4504687 (1985-03-01), Jones, Jr.
patent: 5248837 (1993-09-01), Smith, Jr. et al.
Groten Willibrord A.
Semerak Henry J.
Smith, Jr. Lawrence A.
Catalytic Distillation Technologies
Johnson Kenneth H.
Keys Rosalynd
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