Abstract:
There is provided a novel integrated process for producing methyl tertiary butyl ether from a C 4  containing hydrocarbon feedstock comprising passing the feedstock through zones of butadiene hydrogenation, MTBE synthesis, paraffin/olefin separation and skeletal isomerization.

Description:
The present application is a continuation of application Ser. No. 08/042,477, filed Apr. 2, 1993 which is a continuation-in-part application of Ser. No. 08/001,101, filed Jan. 6, 1993, both now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a process for the production of methyl tertiary butyl ether (MTBE). More particularly, the present invention relates to a process for enhancing the ultimate yield of MTBE from a process feed stream containing a mixture of C 4  hydrocarbons. 
     BACKGROUND OF THE PRESENT INVENTION 
     In recent years, a major technical challenge to the petroleum refining industry has been the need to establish new means for producing high octane gasolines containing oxygenates in response to pollution control regulations requiring the elimination of lead from gasoline, which was previously employed as an octane enhancer. Further, the development of more efficient, higher compression ratio gasoline engines which require higher octane fuels have spurred the industry to produce new octane enhancers. 
     Initially, to meet these requirements, the industry developed non-lead octane boosters and reformulated high octane gasoline to incorporate increased fractions of aromatics and branched hydrocarbons. While these and other approaches were sufficient to meet the technical requirements of regulations requiring the elimination of lead, the economic impact on the cost of gasoline was significant. 
     Accordingly, the industry has intensified their effort to discover new processes to manufacture the gasoline products required by the marketplace. In particular, the industry has centered on blending gasoline with lower aliphatic alkyl ethers as octane enhancers. To this end, methyl tertiary butyl ether (MTBE) has been found especially useful as an octane enhancing additive. Therefore, improvements to the processes relating to the production of MTBE have become increasingly important in the petrochemical industry. 
     It is known that isobutylene may be reacted with methanol over an acidic catalyst to produce MTBE. Methanol is generally in ample supply, but the supply of isobutylene is limited since it is formed only in cracking operations, primarily catalytic cracking and olefins production. Since insufficient isobutylene is available from these sources to meet the growing industry needs, many C 4  production facilities based on dehydrogenation of isobutane are being used to prepare isobutylene feedstocks for MTBE production. See, e.g., Al-Muddarris, U.S. Pat. No. 4,329,516. However, such facilities have proven very expensive to build and operate. 
     It would therefore represent a notable advancement in the state of the art if a process which provided for the increased production of MTBE from available feedstocks could be developed. To this end, the present inventors have developed an integrated process for the production of MTBE from readily available mixed C 4  feedstocks. 
     SUMMARY OF THE PRESENT INVENTION 
     It is an object of the present invention to provide a process for the production of methyl tertiary butyl ether. 
     It is another object of the present invention to enhance the ultimate yield of methyl tertiary butyl ether from a process feed stream containing a variety of C 4  hydrocarbons. 
     It is still another object of the present invention to provide a process for the production of methyl tertiary butyl ether which has less equipment fouling. 
     It is a still further object of the present invention to provide a more economic method of producing methyl tertiary butyl ether. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 depicts in flow chart format a preferred integrated process for the production of MTBE of the present invention. 
     FIG. 2 depicts in flow chart format a process of the prior art which is employed in the comparative examples hereinbelow. 
     FIG. 3 depicts in flow chart format an alternative embodiment of the integrated process for the production of MTBE of the present invention. 
     FIG. 4 depicts in flow chart format an alternative embodiment of the integrated process for the production of MTBE of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     The present invention provides for the production of MTBE from a C 4  feedstock obtained from a catalytic cracking unit and/or an olefins production unit or mixtures thereof by conducting the following process steps, in any sequence: butadiene hydrogenation, MTBE synthesis, paraffin/olefin separation and skeletal isomerization with recycle to the MTBE synthesis or butadiene hydrogenation unit. 
     The present inventors have found that while each of these individual process steps are known in the art in their broadest sense, the use of these process steps in combination to produce MTBE is not taught or suggested by the prior art. Surprisingly, the present invention provides an unexpected significant increase in yield of MTBE from the C 4  feedstock. Further, in a preferred embodiment, by conducting the process steps of the present invention in the sequence of butadiene hydrogenation, MTBE synthesis, paraffin/olefin separation and skeletal isomerization, still further benefits are obtained. 
     However, depending upon the particular chemistry of the individual process steps, the sequence of the steps can be varied. For example, the chemistry of the skeletal isomerization step involves a number of competing reactions occurring simultaneously, such as: (1) the isomerization of butene-2 to isobutylene; (2) the isomerization of butene-2 to butene-1; (3) dimerization of butene-1 and butene-2 to various C 8  olefins; (4) transalkylation of normal butenes to produce propylene and C 5  olefins; (5) hydrogen transfer which results in the formation of n-butane; (6) polymerization which produces gasoline and distillate constituents; and (7) cracking with produces low molecular weight gas and coke. Thus, depending upon the catalyst system employed in the skeletal isomerization step, the compositional make-up of the effluent from the skeletal isomerization reactor will vary significantly. Accordingly, the integration of the various process steps can be altered to accommodate the change in chemistry in the skeletal isomerization unit. 
     Employing the process steps of the present invention, substantially all of the normal butene compounds are ultimately converted to MTBE, C 4  paraffins which are non-reactive, can be effectively purged from the process stream; butadienes and acetylenes are converted to butene feedstock and are eliminated upstream of the MTBE synthesis, paraffin/olefin separation and skeletal isomerization units preventing fouling and coking from occurring; and the need for employing a light ends distillation column within the skeletal isomerization unit is eliminated. 
     The present invention will be described in more detail hereinbelow with reference to a preferred sequence of process steps. However, it is to be understood that other sequences of process steps may be employed in the practice of the present invention. 
     Referring to FIG. 1, a C 4  feedstock in a line 2 is first fed to a hydroisomerization unit 4. Alternatively, the hydroisomerization unit can comprise any butadiene hydrogenation unit known to those of ordinary skill in the art which converts butadiene to butenes, such as a hydroisomerization unit or selective hydrogenation unit. Preferred is a hydroisomerization unit which in addition to converting butadiene to butenes also isomerizes at least a portion of the butene-1 component to the butene-2 components. The hydroisomerization unit 4 is fed with hydrogen by a line 6. The C 4  feedstock is typically a mixed C 4  feedstock from either a catalytic cracking or olefins production process, or mixtures thereof, and which comprises all of the C 4  isomers (acetylenes, dienes, olefins and paraffins), and small quantities of C 3  and C 5  hydrocarbons. The C 4  isomers are as follows: ethyl acetylene, vinyl acetylene, 1,3-butadiene, 1,2-butadiene, isobutylene, cis-2-butene, trans-2-butene, 1-butene, isobutane and n-butane. The actual composition of the C 4  stream will vary considerably between cracking and olefin sources and will also differ if a butadiene extraction unit is employed upstream to recover butadiene from olefins feedstock. 
     In the hydroisomerization unit 4, most of the acetylenes and dienes are catalytically converted to butenes and most of the 1-butene is catalytically converted to the 2-butenes, i.e., cis-2-butene and trans-2-butene, in the presence of hydrogen. 
     Hydroisomerization is a process which is well known to those of ordinary skill in the art and any particular hydroisomerization process may be employed. Typically, the hydroisomerization step is carried out in the presence of a catalyst comprising at least one hydrogenating metal and a moderately acidic carrier. A preferred catalyst can comprise a Group VIII metal, such as platinum, palladium and/or nickel, on a microporous crystalline silicate, such as a mordenite with a surface area of from 100 to 800 m 2  /g. 
     Suitable hydroisomerization conditions may include a temperature of from 40° to 400° C., a pressure from 1-100 bar and a space velocity from 0.5 to 20 kg hydrocarbon feed/kg catalyst hour. Preferred conditions are a mixed phase process at a temperature of from 40° to 150° C., a pressure of from 10 to 40 bar and a space velocity of from 1 to 15 kg feed/kg catalyst hour. See, e.g., Grandvallet et al., U.S. Pat. No. 5,023,389. 
     The effluent stream 8 from the hydroisomerization unit 4 substantially comprising isobutylene, cis-2-butene, trans-2-butene, isobutane, n-butane, unconverted 1-butene and C 3  and C 5  components is combined with recycle stream 28, described hereinbelow, and directed to an MTBE synthesis unit 10. 
     The synthesis of methyl tertiary butyl ether (MTBE) from isobutylene and methanol is a process which is well known to those of ordinary skill in the art and any particular synthesis process may be employed in the practice of the present invention. The general reaction scheme is set forth below. ##STR1## 
     Typically, the synthesis step is carried out at mild temperatures, in the liquid phase, in the presence of a sulfonated polystyrene resin. See, Hatch and Matar, &#34;From Hydrocarbons to Petrochemicals,&#34; Gulf Publishing Co., 1981, pp. 128-29. 
     The reaction usually employs an acid type ion exchange resin, such as a high molecular weight carbonaceous material containing sulfonate groups --SO 3  H. Sulfonated resins of various types are available such as the sulfonated coals, phenol formaldehyde resins reacted with sulfuric acid, sulfonated resinous polymers of cumarone-indene with cyclopentadiene, strongly acidic cationic exchange resins such as sulfonated divinylbenzene polystyrene copolymers, and others, under various commercial names. The catalyst can be employed in a particulate solid form with sizes ranging from about 10 to about 50 U.S. sieve employing about 0.5 to 50 percent dry weight of catalyst relative to liquid content of the reactor. A fixed bed of particulate solid ion exchange resin catalyst, e.g., such as Amberlyst 15 from Rohm &amp; Haas Co., or Dowex® M31 or M32 from Dow Chemical Co., may be employed. The same catalyst may also be employed in tubular reactors or supported in bags or other devices which permit catalytic distillation to be practiced in the reactor. 
     The reaction of the isobutylene with methanol can be carried out under any suitable reaction conditions. The mole ratio of methanol to isobutylene generally is in the range of about 0.05 to 10, preferably about 0.1 to 5, and still more usually about 1 to 1, at a temperature in the range of about 100° F. to about 250° F., more usually about 100° F. to 250° F., employing a pressure sufficient to maintain the reactants substantially in the liquid state, typically in the range of about 80 to 400 psig. The liquid hourly space velocity, volume of feed per volume of catalyst per hour, is preferably about 0.5 to 10. 
     More specific processes of MTBE synthesis are described in Childs, U.S. Pat. No. 4,440,963, Wentzheimer et al., U.S. Pat. No. 4,198,530, Masilamani et al., U.S. Pat. No. 4,792,639, Smith, Jr. et al., U.S. Pat. No. 4,950,803, Lee, U.S. Pat. No. 3,946,450 and Leum et al., U.S. Pat. No. 2,480,940. 
     The resultant product MTBE, along with the C 5  and heavier components, are withdrawn from the MTBE unit through a line 14 by fractionation, as is well known to those skilled in the art. The remaining components of the MTBE synthesis feed, the cis-2-butene, trans-2-butene, isobutane, n-butane, unconverted 1-butene and C 3  components are then directed through a line 16 to a paraffin/olefin separation unit. The paraffin/olefin separation can be carried out by a wide variety of separation processes known to those skilled in the art, including, but not limited to, extractive distillation, selective membrane separation and/or molecular sieve separation. Particularly suitable for use in the practice of the present invention is an extractive distillation unit 18 to remove paraffins and C 3  components. 
     Extractive distillation is a well known process, and has been employed in the past to separate butadiene from C 4  feedstreams, as well as other separations such as separating MTBE from cyclopentane. See, e.g., Berg, U.S. Pat. No. 4,661,209. Extractive distillation generally refers to processes where a higher boiling selective solvent is added to alter the relative volatilities of the components in the feed mixture. See, generally, Perry and Chilton, &#34;Chemical Engineers&#39; Handbook,&#34; McGraw Hill, 5th ed., 1973, pp. 13-43 to 13-48. 
     A wide variety of solvents may be employed in the extractive distillation step of the present invention, including, but not limited to, tetrahydrofuran, diethyl ketone, diethyl carbonate, methyl ethyl ketone, pentanedione, cyclopentanone, acetone, butyronitrile, acetyl piperidine, acetophenone, pyridine, diethyl oxalate, propionitrile, dimethyl acetamide, n-methyl pyrrolidone, acetonyl acetone, tetrahydrofurfuryl alcohol, dimethyl sulfolane, dimethyl cyanamide, methyl carbitol, dimethyl formamide, methyl cellosolve, furfural, acetonitrile, ethylene chlorhydrin, gamma-butyrolactone, methanol, beta-chloropropionitrile, pyrrolidone, propylene carbonate, nitromethane, ethylene diamine and mixtures of any of the foregoing. Especially preferred is acetonitrile. Further, these solvents may also be employed with a water diluent. 
     The solvent, in a line 20, is introduced near the top of the extractive distillation column or tower (not shown), usually a few plates from the top, and flows downward until it reaches the stillpot or reboiler. Its presence on each plate of the distillation column alters the relative volatility of the close boiling compounds to make the separation on each plate greater than would be possible without the solvent and thus requires either fewer plates to effect the same separation, makes possible a greater degree of separation with the same number of plates and also makes possible separation which could not be achieved with conventional distillation. 
     The light compounds, C 3  and lighter boiling hydrocarbons, as well as the C 4  paraffins isobutane and n-butane, are thereby removed from the top of the extractive distillation unit through a line 22. The bottoms from the extractive distillation are directed to a stripper (not shown) wherein the cis-2-butene, trans-2-butene and unconverted 1-butene are recovered from the overhead of the stripper, withdrawn through a line 24 and fed to the skeletal isomerization unit 26. 
     Skeletal isomerization is a process by which the 2-butenes, cis-2-butene and trans-2-butene, are converted to isobutylene and the 1-butene is isomerized to a 2-butene and which can then be further isomerized to isobutylene. Skeletal isomerization of olefins is known to be conducted by contacting unbranched olefins with acidic catalysts at pressures near atmospheric and temperatures ranging from about 600° to 1100° F. The isomerization of olefins is well known to be limited by the thermodynamic equilibrium of reacting species. Useful catalysts and processes are described in the patent literature, inter alia, Smith, Jr., U.S. Pat. No. 4,482,775, Sun, U.S. Pat. No. 4,778,943, Schwerdtel et al., U.S. Pat. No. 4,548,913, Del Rossi et al., U.S. Pat. No. 5,107,047 and Chih-Cheng, et al., EP 0 508 008. 
     Accordingly, a portion of the 2-butenes are essentially converted to isobutylene with a small amount of light and heavy hydrocarbon by-products (gasoline) and the effluent from the skeletal isomerization unit is recycled in a line 28 to the MTBE synthesis unit for conversion of the isobutylene to MTBE product. Further, the heavy hydrocarbon by-products (gasoline) can be withdrawn from the skeletal isomerization unit through a line 29. 
     When a relatively minor quantity of heavy hydrocarbon (gasoline) by-product is produced in the skeletal isomerization unit the heavy hydrocarbon takeoff stream 29 can be omitted and the heavy hydrocarbon can be recycled with the rest of the effluent from the skeletal isomerization unit in streams 28 to the hydroisomerization (see FIG. 4) or MTBE synthesis unit where it can be purged or removed from the process with the methyl tertiary butyl ether product stream 14. 
     By employing the integrated process of the present invention, significant advantages are achieved over the teachings of the prior art. See FIG. 2. By employing the process of the present invention, catalyst fouling in the MTBE synthesis step is reduced by upstream butadiene conversion to butenes, the capital and operating costs of the extractive distillation step are reduced since the 2-butene components require less trays and utilities to effect the equivalent separation than 1-butene, fouling in the extractive distillation unit is reduced due to upstream conversion of butadiene, and the overall yield of the process is improved since additional isobutylene is available for conversion. The process of the present invention enables an operator to convert from about 80 to about 90% or greater of the entire raw C 4  stream derived from ethylene production plant to MTBE. Additionally, the removal of C 5  and heavier components in the MTBE synthesis step prevents contamination, and facilitates recovery, of the extractive solvent in the extractive distillation unit. 
     Further, there is no need for including a depropanizer in the skeletal isomerization process to remove light by-products. These light ends are inert in the MTBE synthesis and are removed in the extractive distillation overhead. The heavy hydrocarbon by-products (gasoline) are removed from the process stream in the MTBE product stream or via a takeoff in the skeletal isomerization unit. 
     Alternatively, as shown in FIG. 3, it is contemplated that the process of the present invention provides for a by-pass of a portion of the hydrocarbon feed around the extractive distillation unit. For ease of understanding, the parts of the process of FIG. 3 have been given part numbers similar to the corresponding parts of the process of FIG. 1, except in the 100 series. The use of a by-pass line 130 around the extractive distillation unit 118 results in the feed line 124 to the skeletal isomerization unit 126 comprising both the bypass stream 130 and the olefin enriched stream 123 from the extractive distillation unit. 
     The sequence of FIG. 3, comprising a by-pass of the extractive distillation with a portion of the MTBE synthesis effluent, is particularly useful in embodiments where the feed to the integrated MTBE process contains low concentrations of normal and isobutane. With low concentrations of normal and isobutane in the feed, it is desirable to by-pass the extractive distillation with a portion of the effluent from the MTBE synthesis unit since the paraffin purge stream is relatively small compared to the quantity of material to be processed. Moreover, the use of the by-pass results in an increase in MTBE product. Olefin loss in the extractive distillation unit from olefins in the by-pass stream is avoided and the additional olefins are then converted to MTBE. 
     Another alternative embodiment is shown in FIG. 4. For ease of understanding, the parts of the process of FIG. 4 have been given part numbers similar to the corresponding parts of the process of FIG. 3, including an optional by-pass stream, except in the 200 series. The process of FIG. 4 comprises directing the recycle stream 228 from the skeletal isomerization unit 226 to the feed stream 202 of the hydroisomerization unit 204. This embodiment is particularly useful where the skeletal isomerization catalysts isomerize a significant portion of the butene-2 hydrocarbons to butene-1. By recycling the effluent from the skeletal isomerization unit to the hydroisomerization unit, additional butene-2 is provided for the extractive distillation unit. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following examples illustrate the present invention. They are not to be construed to limit the scope of the appended claims in any manner whatsoever. 
     EXAMPLE 
     Material balances were performed on a process according to the present invention comprising, in sequence, a hydroisomerization unit (operating at 77.8% conversion), an MTBE synthesis unit, an extractive distillation unit and a skeletal isomerization unit (operating at 37.4% n-butene conversion). For comparative purposes, processes without the extractive distillation, but with 15, 30 and 60% purge, respectively, were also analyzed. The results are set forth below in Tables 1-4. All values are reported in kg/hr. 
     
                                           TABLE 1__________________________________________________________________________PROCESS WITH EXTRACTIVE DISTILLATIONCompo-Stream No.nent 2     6 8     8 + 28                    16    24    28    22   12   14    29__________________________________________________________________________H.sub.20     9 0     0     0     0     0     0    0    0     0Fuel Gas0     0 0     63    63    0     63    63   0    0     0C.sub.2 --C.sub.320    0 20    565   565   0     545   565  0    0     0Hydro-carbons1,3-buta-84    0 0     0     0     0     0     0    0    0     0diene1-butene5655  0 1260  2589  2589  2123  1329  466  0    0     02-butenes2795  0 7114  19001 19001 18989 11887 12   0    0     0i-butene8093  0 8093  14465 145   95    6372  50   0    0     0n +  3255  0 3424  4073  4073  531   699   3542 0    0     0i-butanepentane53    0 53    53    0     0     0     0    0    53    0gasoline0     0 0     0     0     0     0     0    0    0     893fuel oil0     0 0     0     0     0     0     0    0    0     0methanol0     0 0     0     0     0     0     0    8209 41    0MTBE 0     0 0     0     0     0     0     0    0    22488 0Total19955 9 19964 40809 26436 21738 20845 4698 8209 22582 893__________________________________________________________________________ Notes: 1. Minor byproducts of MTBE reaction are not shown. 2. Material balance is on a water free basis. 3. Butene2 isomerization to butene1 in the skeletal isomerization reactio is not shown. 
    
     
                                           TABLE 2__________________________________________________________________________PROCESS WITH 15 PERCENT PURGE      Stream No.Component  2&#39;  6&#39;            8&#39;  8&#39; + 28&#39;                     16&#39; 24&#39; 28&#39; 22&#39;                                    27&#39;                                       29&#39;                                          12&#39;                                             14&#39;__________________________________________________________________________H.sub.2    0   4 0   0    0   0   0   0  0  0  0  0Fuel Gas   0   0 0   0    0   0   0   0  46 0  0  0C.sub.2 --C.sub.3 Hydrocarbons      20  0 20  20   20  17  0   3  416                                       0  0  01,3-butadiene      84  0 0   0    0   0   0   0  0  0  0  01-butene   5655          0 5675                12128                     12128                         10309                             6453                                 1819                                    0  0  0  02-butenes  2795          0 2838                6066 6066                         5156                             3228                                 910                                    0  0  0  0i-butene   8093          0 8093                12796                     124 105 4703                                 19 0  0  0  0n + i-butane      3255          0 3280                22445                     22445                         19078                             19165                                 3367                                    0  0  0  0pentane    53  0 53  53   0   0   0   0  0  0  0  53gasoline   0   0 0   0    0   0   0   0  0  654                                          0  0fuel oil   0   0 0   0    0   0   0   0  0  0  0  0methanol   0   0 0   0    0   0   0   0  0  0  7264                                             36MTBE       0   0 0   0    0   0   0   0  0  0  0  19900Total      19955          4 19959                53508                     40783                         34665                             33549                                 6118                                    462                                       654                                          7264                                             19989__________________________________________________________________________ Notes: 1. Minor byproducts of MTBE reaction are not shown. 2. Material balance is on a waterfree basis. 3. Butene2 isomerization to butene1 in the skeletal isomerization reactio is not shown. 
    
     
                                           TABLE 3__________________________________________________________________________PROCESS WITH 30 PERCENT PURGE      Stream No.Component  2&#39;  6&#39;            8&#39;  8&#39; + 28&#39;                     16&#39; 24&#39; 28&#39; 22&#39;                                    27&#39;                                       29&#39;                                          12&#39;                                             14&#39;__________________________________________________________________________H.sub.2    0   4 0   0    0   0   0   0  0  0  0  0Fuel Gas   0   0 0   0    0   0   0   0  32 0  0  0C.sub.2 --C.sub.3 Hydrocarbons      20  0 20  20   20  14  0   6  287                                       0  0  01,3-butadiene      84  0 0   0    0   0   0   0  0  0  0  01-butene   5655          0 5575                10101                     10101                         7071                             4426                                 3030                                    0  0  0  02-butenes  2795          0 2838                5052 5052                         3536                             2214                                 1516                                    0  0  0  0i-butene   8093          0 8093                11324                     113 77  3231                                 34 0  0  0  0n + i-butane      3255          0 3280                11132                     11132                         7792                             7852                                 3340                                    0  0  0  0pentane    53  0 53  53   0   0   0   0  0  0  0  53gasoline   0   0 0   0    0   0   0   0  0  448                                          0  0fuel oil   0   0 0   0    0   0   0   0  0  0  0  0methanol   0   0 0   0    0   0   0   0  0  0  6427                                             32MTBE       0   0 0   0    0   0   0   0  0  0  0  17606Total      19955          4 19959                37682                     26418                         18490                             17723                                 7928                                    319                                       448                                          6427                                             17691__________________________________________________________________________ Notes: 1. Minor byproducts of MTBE reaction are not shown. 2. Material balance is on a waterfree basis. 3. Butene2 isomerization to butene1 in the skeletal isomerization reactio is not shown. 
    
     
                                           TABLE 4__________________________________________________________________________PROCESS WITH 60 PERCENT PURGE      Stream No.Component  2&#39;  6&#39;            8&#39;  8&#39; + 28&#39;                     16&#39; 24&#39;                            28&#39;                               22&#39; 27&#39;                                      29&#39;                                         12&#39;                                            14&#39;__________________________________________________________________________H.sub.2    0   4 0   0    0   0  0  0   0  0  0  0Fuel Gas   0   0 0   0    0   0  0  0   14 0  0  0C.sub.2 --C.sub.3 Hydrocarbons      20  0 20  20   20  8  0  12  125                                      0  0  01,3-butadiene      84  0 0   0    0   0  0  0   0  0  0  01-butene   5655          0 5675                7571 7571                         3028                            1896                               4543                                   0  0  0  02-butenes  2795          0 2838                3786 3786                         1514                            948                               2272                                   0  0  0  0i-butene   8093          0 8093                9480 95  37 1387                               58  0  0  0  0n + i-butane      3255          0 3280                5509 5509                         2204                            2229                               3305                                   0  0  0  0pentane    53  0 53  53   0   0  0  0   0  0  0  53gasoline   0   0 0   0    0   0  0  0   0  192                                         0  0fuel oil   0   0 0   0    0   0  0  0   0  0  0  0methanol   0   0 0   0    0   0  0  0   0  0  5380                                            26MTBE       0   0 0   0    0   0  0  0   0  0  0  14738Total      19955          4 19959                26419                     16981                         6791                            6460                               10190                                   139                                      192                                         5380                                            14818__________________________________________________________________________ Notes: 1. Minor byproducts of MTBE reaction are not shown. 2. Material balance is on a waterfree basis. 3. Butene2 isomerization to butene1 in the skeletal isomerization reactio is not shown. 
    
     In Tables 2-4, the primed stream numbers generally correspond to the stream numbers in FIG. 1. Further, stream no. 22&#39; refers to a purge stream removed from the MTBE synthesis unit effluent which includes both paraffins and valuable olefins; stream no. 24&#39; refers to the stream which is fed to the skeletal isomerization unit and comprises the MTBE synthesis effluent minus the purge stream 22&#39;; stream 27&#39; refers to a light hydrocarbon takeoff from the skeletal isomerization unit; and stream 29&#39; refers to a heavy hydrocarbon (gasoline) takeoff from the skeletal isomerization unit. Additionally, unit 4&#39; is normally a selective hydrogenation unit which would convert butadiene to olefins but would not perform the olefin isomerization. See FIG. 2. 
     For convenience, the results of the material balances are summarized below in Table 5. 
     
                       TABLE 5______________________________________Summary of Results              MTBE     MTBE     SIProcess            Produced Feed     Feed______________________________________Table 1 - with extractive distillation              22488    40809    21738Table 2 - 15% Purge              19900    53508    34665Table 3 - 30% Purge              17606    37682    18490Table 4 - 60% Purge              14738    26419     6791______________________________________ SI  Skeletal Isomerization 
    
     From Table 5 it can be seen that the process according to the appended claims with the extractive distillation unit provides significant advantages over the processes without the extractive distillation step. As compared to the process employing the 15% purge, the process of the present invention provides improved yields of MTBE and significantly reduces the feed to the MTBE synthesis unit and skeletal isomerization unit. As compared to the process with the 30% purge, while the feed rates to the MTBE synthesis and skeletal isomerization units are similar to the process of the present invention, the process of the present invention provides a significant increase in MTBE yield. Further, as compared to the process with the 60% purge, while the 60% purge process has reduced feed rates to the MTBE synthesis unit and the skeletal isomerization unit, the MTBE yield for the 60% purge process is only abut 66% of the MTBE yield of the present process. 
     Accordingly, it can be seen that the process of the present invention concurrently provides both increased yields and reduced feed rates to the MTBE synthesis and skeletal isomerization units. 
     Many variations of the present invention will suggest themselves to those skilled in the art in light of the above-detailed description. For example, the specific conditions and catalysts for each of the process steps may be varied. Further, molecular sieve separation and selective membrane separation units may be employed in place of or in addition to the extractive distillation unit; and selective hydrogenation units may be employed in place of or in addition to the hydroisomerization unit. All such obvious modifications are within the full intended scope of the appended claims. 
     All of the above-referenced patents and publications are hereby incorporated by reference.