The present invention relates to the production of blends of tertiary alkyl ethers and hydrocarbons.

It is known to incorporate tertiary alkyl ethers into gasoline in order to improve the octane number of the fuel and otherwise improve its combustion characteristics. There is an increasing demand for such tertiary alkyl ethers as gasoline additives in view of moves to retrict or eliminate the use of lead anti-knock agents in gasoline fuels for motor vehicle engines.

One approach to providing tertiary alkyl ethers for addition to gasoline is to make a relatively pure material such as methyl tertiary butyl ether (MTBE), which is then blended into the gasoline. The MTBE may be made by reacting methanol and isobutene over an appropriate catalyst. An example of such a process is that disclosed in GB 2 047 706. Isobutene is a reactive material and it is possible to obtain very high yields of MTBE by reacting together isobutene and methanol in a single reactor. Thus conversions of 93-98% of the isobutene are mentioned in GB 2 047 706. The product from the first reactor is subjected to a distillation step in which substantially pure MTBE is recovered from the bottom of the distillation column while unreacted hydrocarbons and methanol are taken overhead.

The conversion in the first reaction stage is very high and there would normally be little benefit in see i^s to obtain the reaction of the last few % of isobutene left in the hydrocarbon mixture which leaves the initial reactor. However for some

processes it is necessary to provide feeds containing very low levels of unreacted isoolefin. This is the case if the n-butenes left in the C4 feed are to be used for the synthesis of maleic anhydride or the production of butene-1 polymers. An alternative approach to preparing blends of tertiary alkyl ethers and hydrocarbons is to use mixtures containing less reactive olefins, for example isopentenes or isohexenes. Instead of trying to obtain substantially complete reaction and a substantially pure product a process is used which reacts a high proportion, but not all, of the higher isoolefins and produces a mixed product consisting of unreacted hydrocarbons and the tertiary alkyl ethers which is then used as the basis for a motor vehicle fuel. In such processes it is not realistic to consider obtaining a hydrocarbon residue which is substantially fee of isoolefins. Disclosures relating to processes which substantially eliminate isoolefins such as those in GB 2 047 706 are not relevant to processes which use higher isoolefins.

There is a need to maximise the yield of higher tertiary alkyl ethers such as methyl tertiary pentyl ether (otherwise known as tertiary amyl ether (TAME)) and methyl isohexyl ether.

According to the present invention there is provided a process for making a mixture of tertiary alkyl ethers and hydrocarbons which comprises the successive steps of:

(a) feeding methanol and a hydrocarbon feed containing C ^~ and Cg tertiary alkenes to a first reactor,

(b) feeding product from the first reactor to a distillation column operated so as to recover a bottoms product consisting predominantly of ethers containing tertiary C5 and C - alkyl groups while recovering an overhead product containing unreacted methanol, unreacted hydrocarbon, and any lower tertiary alkyl ethers present, and

(c) feeding the overhead product from the distillation column into a second reactor where further reaction of unreacted C ^~ and Cg isoolefin takes place. The hydrocarbon feed to the first reactor may consist

substantially only of C ^~ and higher hydrocarbons. Alternatively it may also contain a C stream containing isobutene. Processes for making MTBE and higher alkyl ethers such as TAME are well known and persons skilled in the art will be familiar with the types of hydrocarbon feed suitable for use in such processes. Similarly the reaction conditions for etherification reactions and for catalyst to be used are well known to those skilled in the art. In general it will be desirable to provide for a recycle of some product from the first reactor but with the major portion being taken off into the distillation column.

Persons skilled in distillation technology will be readily able to design and operate a distillation column so as to leave the ethers derived from the C ^~ and higher tertiary olefins in the bottoms product from the distillation column while the remaining products pass overhead. As is well known in the art a distillation column will be operated with a reflux stream.

The second reactor will be operated under conditions generally similar to those used in the first reactor. If the feed to the first reactor contained isobutene then the feed to the second reactor will contain MTBE in addition to the unreacted isoolefins (mainly C5 and higher) .

The feed from the second reactor may be processed so as to recover unreacted methanol and a product which is a mixture of unreacted hydrocarbons, including unreacted tertiary isoolefins, as well as tertiary alkyl ethers.

The invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic representation of a flow sheet for a process not according to the invention, Figure 2 is a diagrammatic representation of a flow sheet of a process according to the invention.

As will be apparent to those skilled in the art the detailed flowsheet may contain various pumps and valves to maintain pressure and various heat exchangers to maximise heat recovery in addition to the main features shown in the diagram.

A mixture of methanol and a hydrocarbon fraction containing C ^~ and C - isoolefins is fed into reactor (1) through line (2). Part of the product from reactor (1) is fed through line (3) to reactor (4). The remainder is recycled to reactor (1). The product from reactor (4) is recovered through line (5).

Referring now to Figure 2 we find the same units as in Figure 1. These are identified by the same numerals as in Figure 1. In addition however a further distillation column (6) is interposed between reactors (1) and (4). From the base of this distillation column a mixture consisting predominately of tertiary alkyl ethers derived from C5 and higher isoolefins is taken off through line (7). An overhead product containing unreacted carbons, methanol, and any MTBE which may be present is taken off through line (8) and passed to reactor (4). Comparative Test A

A process corresponding to that illustrated in Figure 1 was simulated using a process simulator. This process simulator had been validated through extensive comparisons with various types of plant at oil refineries. It is believed that the results obtained are a reliable indication of the results obtained in a real plant. The hydrocarbon feed stock assumed for the process was a stabilized light catalytically cracked spirit with a typical composition as follows: Component wt% i-butane 4.1 n-butene 4.1 i-butene * 1.0 n-butane 1.6 i-pentane 13.9 i-pentene 14.0 n-pentane 21.4

3τmethyl butene-1 * 0.9 2-methyl butene-1 * 6.1 2-methyl butene-2 * 12.6 Cyclopentane 5.5

100.0 * Reactive Olefins

The catalyst was assumed to be an ion-exchange resin type catalyst as is commonly used in the production of tertiary alkyl ethers. Reaction conditions assumed were conventional reaction conditions.

The yields of methyl isoalkyl ethers were as follows

Example 1

The results of operating a process in accordance with the invention as disclosed in Figure 2 were determined. The conditions used were as in Comparative Test A. The distillation column (6) was operated at about 140"C and about 3 bar pressure.

The product from the base of column (6) taken off through line (13) was determined to have the composition:

Component wt% n-hexane 1.1

Benzene 4 n-heptane 17

Cycloheptane 0.7

C5 ethers 81.1

C - ethers 13.0 100.0

The remainder of the product taken off through line (5) was determined to have the composition

100.0 The total yields of tertiary alkyl ether were determined to be

A comparison of the results given for the experiments above shows that the yield of MTBE is not substantially increased by the process of the invention. However there is a marked increase in

the production of the higher alkyl ethers. It would be noted that the final product from the process still contains significant quantities of unreacted isoolefins, despite the increase in the production of the higher tertiary alkyl ethers.