BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an improved, integrated process for the production of alkyl tertiary alkyl ethers, such as methyl tertiary butyl ether (MTBE), by reacting isoalkene with alcohol. More particularly, the invention relates to a process wherein a stream containing isoalkene and normal alkenes is treated to remove dienes and/or isomerize normal alkenes to separate the higher boiling normal alkenes from the isoalkene, thereby concentrating the isoalkenes. Related Art

Methyl tertiary butyl ether (MTBE) , methyl tertiary amyl ether (TAME) and the ethyl analogues thereof are useful as a component for improving the octane of gasolines and have commonly been prepared by the acid catalyzed reaction of the appropriate alcohol with isobutene or isoa ylene. Examples of such a process are disclosed in U.S. Pat. Nos. 4,039,590 and 4,198,530.

The use of oxygenated motor fuels, as mandated by the Clean Air Act, has substantially increased the demand for ethers - especially MTBE. The feed stocks for MTBE are isobutylene and methanol. Isobutylene may be obtained by the dehydration of tertiary butyl alcohol or by the skeletal isomerization of linear butenes. The preferable source of isobutylene and isoamylenes for many refiners is the C4 raffinate obtained from a catalytic cracker because it is available on site. However, the C ₄ and C ₅ raffinate streams are comparatively dilute or lean sources of isoalkene and the entire stream is conventionally fed to the etherification reactor. The non-isoalkene components of these streams, including paraffins such as butane and isobutane and mixed linear butenes, normal pentenes, isopentanes and pentane are essentially inert under etherification conditions but do increase the hydraulic load on the reactors. For example, the C ₄ raffinate stream from a catalytic cracker

typically consists of about 50 weight % paraffins, 35 weight % mixed linear butenes and only 15 weight % isobutylene. Nonetheless, this raffinate stream is an important source of isobutylene for MTBE production. Accordingly, an improved process to use dilute isoolefins for the production of ethers would be useful.

As commercialized, hydroisomerization is a process used to upgrade C ₄ streams, usually from fluid catalytic cracking units. In the fixed bed process as practiced by some, butadiene contaminating the feed is hydrogenated to butenes, and the normal butenes are isomerized to the equilibrium mixture which is predominately butene-2. The advantage of that process is to remove butadiene which causes the loss of acid used in the alkylation process and improvement of the alkylate octane number in HF alkylation by using mostly butene-2 in the feed rather than butene-l.

Double bond isomerization can be carried out during the hydrotreating. For example palladium hydrogenation catalysts are known and used for the butene-1 to butene-2 isomerization. Isomerization occurs only after hydrogenation.

Hydroisomerization has been practiced in a catalytic distillation column, particularly the bond shift of butene- 2 to butene-1, as illustrated in commonly assigned U.S. Pat. No. 5,087,780. The preferred catalyst structure for the hydroisomerization reaction is a more open mesh like structure as disclosed in commonly assigned U.S Pat. Nos. 5,266,546; 5,348,710; and 5,431,890.

In catalytic reactions the components of the reaction system are concurrently separable by distillation using the catalyst structures as the distillation structures. Such systems are described variously in U.S. Pat. Nos. 4,215,011; 4,232,177; 4,242,530; 4,302,356; 4,307,254; 4,336,407; 4,439,350; 4,443,559; and 4,482,775. SUMMARY OF THE INVENTION

In the present invention the isoalkene containing stream for reaction in an etherification is first hydrotreated in a hydrotreating zone by feeding a hydrocarbon stream

containing isoalkene and highly unsaturated compounds which comprise diolefins and/or acetylenes along with a hydrogen stream at an effectuating hydrogen partial pressure of at least about 0.1 psia to less than 70 psia, preferably less than 50 psia to a distillation column reactor containing a hydrogenation catalyst which is a component of a distillation structure and selectively hydrogenating a portion of the highly unsaturated compounds and concurrently separating by fractionation an overhead stream having a higher concentration of the isoalkene than the hydrocarbon stream and feeding the overhead stream to an etherification reactor with alcohol to produce ether.

The success of catalytic distillation lies in an understanding of the principles associated with distillation. Because the reaction is occurring concurrently with distillation, the initial reaction product is removed from the reaction zone as quickly as it is formed which minimizes further reaction. The heat of the reaction, if any, simply creates more boil up, but no increase in temperature. Additionally the removal of the reaction products in equilibrium limited reactions, such as etherification and the bond shift isomerization, provides for greater conversions.

BRIEF DESCRIPTION OF THE DRAWING The figure is a process flow diagram in schematic form of the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The hydrocarbon stream typically comprises C to Cg aliphatic compounds, which may be narrow cuts or include a range of carbon content. Refining techniques are such that very accurate cuts of carbon groups can be achieved. Preferably the feeds are C ₄ and C5 cuts in which the diene and/or acetylene impurities, normal -alkenes, -alkanes and the isoalkanes correspond in carbon number to isoalkene. HYDROTREATING

The catalytic material employed in the hydrogenation process is in a form to serve as distillation packing. Broadly stated, the catalytic material is a component of a

distillation system functioning as both a catalyst and distillation packing, i.e., a packing for a distillation column having both a distillation function and a catalytic function. The reaction system can be described as heterogenous since the catalyst remains a distinct entity. Any suitable hydrogenation catalyst may be used, for example Group VIII metals of the Periodic Table of Elements as the principal catalytic component, alone or with promoters and modifiers such as palladium/gold, palladium/silver, cobalt/zirconium, nickel preferably deposited on a support such as alumina, fire brick, pumice, carbon, silica, resin or the like.

A preferred catalytic material comprises palladium oxide, preferably 0.1 to 5.0 weight %, supported on an appropriate support medium such as alumina, carbon or silica, e.g., 1/8" alumina extrudates. In a preferred catalytic distillation structure the particulate catalyst material is disposed within a porous plate or screen to contain the catalyst and provide distillation surfaces, in the form of a wire mesh structure, such as a wire mesh tubular structure or any other similar structure.

A catalyst suitable for the hydroisomerization process is 0.5% PdO on 1/8" AI ₂ O ₃ (alumina) extrudates, hydroisomerization catalyst, supplied by Engelhard Industries. The catalyst is believed to be the hydride of palladium which is produced during operation. The hydrogen rate to the reactor must be sufficient to maintain the catalyst in the active form because hydrogen is lost from the catalyst by hydrogenation, especially when butadiene is contained in the feed. The hydrogen rate must be adjusted such that there is sufficient to support the butadiene hydrogenation reaction and replace hydrogen lost from the catalyst but kept below that required for hydrogenation of butenes and to prevent flooding of the column which is understood to be the "effectuating amount of hydrogen" as that term is used herein. Generally the mole ratio of hydrogen to butadiene contained in the C ₄ hydrocarbon fed to the fixed bed of the present invention

will be about 1.0/1.0 to 10/1, preferably 2/1 to 6/1.

A preferred catalyst structure for use in the distillation column reactors for the present hydroisomerizations comprises flexible, semi-rigid open mesh tubular material, such as stainless steel wire mesh, filled with a particulate catalytic material

A structure developed for use in hydrogen processes is described in U.S. Pat. No. 5,266,546 which is incorporated herein in its entirety. Another catalyst structure particularly suited for hydrogen processes is described in U.S. Pat. No. 5,431,890 which is incorporated herein in its entirety.

The hydrotreating is carried out in a catalyst packed column which can be appreciated to contain a vapor phase and some liquid phase as in any distillation and is described in detail in commonly assigned U.S. patent application Serial No. 08/163,311 filed 12/08/93 and incorporated herein. The distillation column reactor is operated at a pressure such that the reaction mixture is boiling in the bed of catalyst. The present process operates at overhead pressure of said distillation column reactor in the range between 0 and 350 psig, preferably 250 or less and temperatures within said distillation reaction zone in the range of 40 to 300°F, preferably 110 to 270°F at the requisite hydrogen partial pressures. The feed weight hourly space velocity (WHSV) , which is herein understood to mean the unit weight of feed per hour entering the reaction distillation column per unit weight of catalyst in the catalytic distillation structures, may vary over a very wide range within the other condition perimeters, e.g. 0.1 to 35.

ETHERIFICATION

The preferred catalysts for the etherification is an acidic cation exchange resin such as Amberlyst as manufactured by Rohm & Haas Co. Concentration of the isoalkene in the feed to the etherification will enhance the operation of any of the known reaction systems, e.g, fixed bed straight pass, fluidized beds, ebulating bed and

catalytic distillation.

A preferred structure for use in a catalytic distillation etherification is to dispose the resin beads in pockets of a cloth belt which is then wound into a spiral with demister wire which supports and separates the belts in the column. Such a system has been described in commonly assigned U.S. Pat. Nos. 4,215,011; 4,232,177; 4,242,530; 4,302,356; 4,307,254 4,336,407; 4,439,350 and 4,482,775. In addition U.S. Pat. Nos. 4,443,559 and 4,250,052 disclose a variety of catalyst structures for this use and are incorporated herein.

Typical conditions for the catalytic distillation MTBE reaction include catalyst bed temperatures of about 150-170 °F, overhead pressures of about 90-110 psig and equivalent liquid hourly space velocities of about 1.0 to 2.0 hr- ¹ . Typically the methanol and C *s are first fed to a down flow guard bed reactor operated as a boiling point reactor wherein considerable etherification occurs prior to the distillation column reactor. The operation of the boiling point reactor for etherification is detailed in U.S. Pat. No. 4,950,803 which is herein incorporated by reference.

Fixed bed etherifications are described in U.S. Pat. Nos. 4,475,005 and 4,336,407. A fluidized bed operation is disclosed in U.S. Pat. No. 4,471,154 and an ebulating bed is disclosed in U.S. Pat. No. 4,774,364, any of which may be used for the etherification. SKELETAL ISOMERIZATION

An additional feature of the process is that a portion of the mono-olefins contained within the stream or produced by the selective hydrogenation of the diolefins may be isomerized to more desirable products. Isomerization can be achieved with the same family of catalysts as used in hydrogenations. Generally the relative rates of reaction for various compounds are in the order of from faster to slower:

(1) hydrogenation of diolefins

(2) isomerization of the mono-olefins

(3) hydrogenation of the mono-olefins.

It has been shown generally that in a stream containing diolefins, the diolefins will be hydrogenated before isomerization occurs. It has also been found that very low total pressures may be used for optimal results in some of the present hydrogenations, preferably in the range of 50 to 150 psig with the same excellent results. Both higher and lower pressures within the broad range may be used with satisfactory results.

The normal mono-olefins can be recovered or preferably subjected to skeletal isomerization to produce more of the isoalkene. The product from the skeletal isomerization can be combined with the feed to the etherification or returned to the hydrotreating zone for separation of the isoalkene and further double bond isomerization of normal mono- olefin.

For example in the case of C feed streams the hydrotreating/isomerization of the C ₄ stream containing butanes, normal butenes, isobutylene and butadiene in a distillation column reactor containing a hydroisomerization catalyst in the form of a catalytic distillation structure is preferably operated to reduce butadiene content and isomerize the butene-1 to a higher boiling butene-2. The higher boiling butene-2 is concurrently separated by fractional distillation and removed as bottoms. The overheads, thus enriched by the removal of the original butene-2 and the converted butene-1, is fed to an etherification reactor preferably comprising a second distillation column reactor containing an acid etherification catalyst in the form of a catalytic distillation structure to produce MTBE. The MTBE is concurrently separated as bottoms from the second distillation column reactor with unreacted methanol and inerts (essentially the butanes and butene-1 not isomerized during the hydrotreating) removed as overheads. After the methanol has been removed the butene-1 may be used as feed to a cold acid alkylation unit for the production of the important gasoline component 2,2,4 tri-methyl pentane (isooctane) or as a co-monomer.

The bottoms from the first distillation column reactor containing the butene-2 may be recovered or is preferably passed on to a skeletal isomerization reactor containing a skeletal isomerization catalyst, such as a gamma alumina, for conversion to isobutylene. The isobutylene from the isomerization reactor is then combined with the overheads from the first distillation column reactor for feed to the etherification reactor.

The preferred catalysts for the skeletal isomerization is ZSM 35 and its isotypes, such as ferrierite. Other zeolitic molecular sieves such as ZSM-22 and ZSM-23 will carry out the isomerization although less effectively. These catalysts and their use in skeletal isomerization are described in U.S. Pat. No. 5,449,851, European Appln. 87304550.4, British Appln. 8612815 and European Appln. 92305090.0

Other catalysts which may be used for skeletal isomerization are aluminas having a surface area of at least about 100 m ² /g, preferably the surface area is greater than 150 m ² /g. Techniques of preparing such alumina catalysts are well known in the art. Examples of such alumina catalysts include eta-alumina and gamma- alumina.

Those aluminas having high purity, particularly with respect to their content of alkali metals are preferred. Thus generally the aluminas should contain less than about 0.1 weight percent alkali metal, preferably less than 0.05 weight percent, based on the weight of the catalyst. The alkali metals include lithium, sodium, potassium, rubidium, and cesium. The aluminas generally are less efficient than the specified zeolites and tend to coke up faster.

Such catalysts can be employed in the manner known in the art for the skeletal isomerization of olefins to more highly branched olefins. Such catalysts are particularly suitable for the isomerization of n-olefins having 4 to 10 carbon atoms per molecule or mixtures thereof. Mixtures comprising olefins and essentially inert hydrocarbons can also be treated according to this invention to provide an

isomerized olefinic product in admixture with the inert hydrocarbons. Particularly useful isomerizations with such catalysts are conducted on feed-streams of gasoline, especially catalytically cracked gasoline. This isomerization with such catalysts can be conducted under any conditions sufficient to produce the desired isomers. Generally the isomerization is conducted at a temperature in the range of about 315°C. to about 510"C., preferably about 343 " C. to about 454°C. Isomerization is generally not affected significantly by pressure, but elevated pressures can accelerate some undesirable olefin reactions, such as polymerization. Hence low reaction pressure is generally favored. Partial pressure of the hydrocarbon during the isomerization generally will be in the range of about atmospheric to about 200 psig (i.e. about 1.03 X 10 ⁵ Pa to about 148 x 10 ⁶ Pa). More preferably the reaction pressure for isomerization does not exceed 100 psig (7.9 x 10 ⁵ Pa). Typically the contact time for the hydrocarbon in such isomerization reactions, expressed in volumes of liquid feed stock per volume of catalyst per hour (LHSV) , is in the range of about 0.1 to about 15, preferably about 0.5 to about 5.

Referring now to the figure there is shown a process flow diagram in schematic form of one embodiment of the present invention.

A mixed C ₄ cut from a FCCU is fed via flow line 1 and an effectuating amount of hydrogen is fed via flow line 2 both being combined in flow line 3 and fed to first distillation column reactor 10 containing a bed 12 of hydroisomerization catalyst in the form of a catalytic distillation structure. In the first distillation column reactor 10 the butadienes are hydrogenated to butenes. In this embodiment the hydrotreating is operated to isomerize a portion of the butene-1 to higher boiling butene-2. The butene-2 in the feed along with that formed from the isomerization is taken as bottoms via flow line 4 and fed via line 4a to skeletal isomerization reactor 20 containing a fixed bed 22 of skeletal isomerization catalyst wherein a portion of the

butene-2 is skeletally isomerized to isobutylene. Alternatively, the butene-2 can be recovered via line 4b. The skeletal isomerization products are removed via flow line 5. The overheads from the first distillation column hydrotreating reactor 10 containing the isobutylene and inerts including butanes and butene-1, are taken via flow line 6 and combined with skeletal reactor products from flow line 5a into flow line 7. Methanol is added via flow line 13 and the combined stream fed down flow to guard bed reactor 30 containing a fixed bed 32 of etherification catalyst wherein a considerable amount (80-90%) of the isobutylene reacts with methanol to form MTBE. The effluent from the guard bed reactor 30 is fed via flow line 8 to a second distillation column reactor 40 containing a bed 42 of etherification catalyst in the form of a catalytic distillation structure wherein the remainder of the isobutylene is reacted with methanol to form MTBE which, being higher boiling, is taken as bottoms via flow line 11.

The overheads, being essentially depleted of isobutylene and containing butene-1, butanes and unreacted methanol, is taken via flow line 9. The methanol is recovered and recycled (not shown) and the stream, now having little butadiene, is suitable for feed to a cold acid alkylation process.

An alternative to combining the skeletal isomerization reactor effluent with the overheads from the first distillation column reactor is also shown as dotted flow line 5b. The effluent from the skeletal isomerization reactor 20 may be recycled back to the first distillation column reactor 10 via flow line 5b where the isobutylene is taken along with the overheads for feeding to the etherification reactor. The unconverted butene-2 in the effluent from the skeletal isomerization reactor is taken as bottoms and recycled to the skeletal isomerization reactor. A slip stream (not shown) would be required to prevent build up of butene-2 in the system.

Generally, there is a reflux for each distillation column reactor, but those along with other standard equipment such as reboilers and condensers are not shown, their design and operation being in the knowledge of one of ordinary skill in the art of distillation column design and operation.