PROCESS FOR PREPARING ALKYL-TERT-BUTYL ETHERS

Abstract: In the processes for the synthesis of alkyl-tert-butyl ethers by reacting the isobutene contained in a C4 charge with a high butadiene content with aliphatic alcohols, to the purpose of preventing polymerization phenomena in the step of separation of alkyl-tert-butyl ether product from the other unreacted components, the initial C4 charge and/or the aliphatic alcohol are submitted to an oxygen removal step, to the purpose of removing all of dissolved free oxygen, down to a maximum residual content of 1-2 ppm.

Description: "PROCESS FOR PREPARING ALKYL-TERT-BUTYL ETHERS' * The present invention relates to a process for the r preparation of a Ik L-te rt-buty L ethers in the presence of butadi ene . It is known that for the synthesis of said ethers it 5 is not • necessary to use high-purity isobutene, but a fraction is enough which contains it, even if at a Low concen ration. Suitable olefinic fractions for said syntheses are the C. cuts outcoming from catalytic cracking, or those t* 10 obtained from steam crac ing, these latter both before and after butadiene extraction. If as the feed olef nic charges, the C. fractions from catalyt c cracking, or those deriving from steam cracking after butadiene removal are used, and as the 15 alcohols methanol or ethanol are used, in the presence of catalysts constituted by macroporous sulphonated resins, of Amberlyst 15 or Lewatit SPC 108 type, the reaction can be carried out on an industrial scale, according to a wide range of schemes and of operating conditions 20 directed towards the optimization of conversion of either of reactants. In these cases, a high selectivity, as well as a good catalyst performance is always obtained, and no problems are to be faced as to the products separation 25 and recovery sections. If the process is carried out with a high-butadiene- "mt content charge (containing from 10 to 70% by weight of butadiene), such as the C. cut before butadiene extraction, the operating conditions have to be exactly defined to the purpose of recovering more than 98-99% of 4_3__ butadiene. In particular, maintaining a close relationship between temperature and space velocity is necessary, as disclosed in U.S. P. 4,039,590; this ensures a low butadiene conversion, but to the purpose of obtaining a such a catalyst performance as to allow an industrial operativity, operating is necessary with particular measures as to the reactor, and ith temperatures which do not exceed 60 C, as disclosed in French patent application 2,523,121. Even when the process is carried out under these conditions, butadiene polymerizations occur in the section wherein the fracti onati on (distillation) is carried out to the purpose of separating the tert-butyl ether from unreacted charge, and such polymerizat ons can reach such an extent as to jeopardize the operation of said fracti onati on section. The use of polymerization inhibitors, however efficacious in protecting the liquid phases, does not allow protecting the gas phases. It was surprisingly found, and this is the essence of the present invention, that it is possible to completely avoid the formation of polymers in the distillation columns, by preliminarily removing the oxygen present as dissolved gas in the charge fed to the etheri fi cation reactor. Such oxygen is basically deriving from the presence of dissolved air inside the fed aliphatic alcohol (either methanol or ethanol), as a consequence of the modalities of product transportation and storage. For example, the methanol used in the synthesis of MTBE, stored under an air atmosphere at the temperature of 20 C and under the pressure of 760 mm , dissolves about 80 ppm Hg weight/weight (w/w) of oxygen and 180 pp w/w of nitrogen; the presence of this latter, however, does not create any problems. A certain amount of dissolved oxygen can be occasionally present also in the liquified C, fraction 4- containing isobutene and isobutadiene, in particular when such a fraction has been stored at low temperatures, at which the vapour pressure of the mixture becomes lower than atmospheric. In general, however, nearly all of the oxygen dissolved in the reaction mixture fed to the etheri fi cation reactor is introduced through the alcohol. The removal of oxygen from alcohol is enough to overcome the problem of polymers formation. The procedures for oxygen removal are those known in the art for the removal of inert gases dissolved in liquid products, such as, e.g., the use of a suitable stripper, consisting in a distillation column, from the bottom of which the oxygen-free product is obtained, whilst oxygen is discharged as a overhead vent from the reflux accumulator. Summing-up, the process of the present invention is a process for the preparation of alkyl-tert-but l ethers by starting from C. charges containing isobutene and H a other C. hydrocarbons, among which butadiene at a concentration, referred to the total charge, comprised within the range of from 10% by weight to 70 by weight (C charges coming from catalytic cracking or from steam cracking without butadiene removal) and from aliphatic alcohols preferably selected from methanol and ethanol, a d comprises reacting the isobutene contained in the charge with the aliphatic alcohol in one or more reaction sections at a temperature not higher than 60 C in the presence of a macroporous sulphonated resin of Amberlyst 15 or Lewatit SPC 108 type and separating the alkyl-tert- butyl ether from the other unreacted components by dist llation, and is characterized in that the alcohol and/or the C. hydrocarbon charge are/is submitted, before 4 the reaction, to an oxygen removal step, in particular of physical type, of removal of dissolved free oxygen by stripping by outer inert agents, or distillation up to reduce the free oxygen content to 1-2 ppm or less. In special cases, the oxygen removal may be carried out by stripping by a stream of a suitable inert gas, such as nitrogen, methane, hydrogen, fuel gas, ethane, etc., or mixtures thereof. In the follo ng examples, some oxygen removal procedures are described, which are supplied to purely exemplifying purposes, and in no way are limitative of the present invention. ___ϋ_02J_≤_l 100 parts by weight of an olefinic charge (1), see Figure 1, constituted by a C. cut from steam cracking 4 before butadiene extraction, and contain ng on the average 25% by weight of isobutene and 50% by weight of butadiene, are mixed with 15.3 parts by weight of methanol (2) and the resulting charge (3) is fed to reactor (11) containing an ion-exchange resin in the acid c form, of Amberlyst 15 type. The olefinic cut (1) has a content of dissolved oxygen Lower than 1 ppm, w ilst methanol, which is stored inside a tank under an air atmosphere, has an oxygen content ranging from 30 to 60 mg/l. To the stream (2) an alcoholic solution of a polymerization inhibitor is injected (8) in such an amount that in the combined charge (3) the ihnibitor concentration ranges from 60 to 120 ppm. The reaction product (4) containing an average amount of 30.7% of MTBE, which justifies an isobutene conversion of 90%, is fed to a fractionation tray column (9) in such an intermediate point, that the rectification section represents 40% of column. From the top of said fractionation column the not-converted hydrocarbons are removed together with azeotropic methanol (6), from the bottom of the column a stream (5) is removed, which is constituted by practically pure MTBE. The column is operated under a pressure of 4.9 bars, the overhead, feed and bottom temperatures are respectively 41 C, 60 C and o 125 C. Oxygen dissolved in stream (4) fed to column (9) accumulates on column top, and is partly eliminated as an overhead vent from reflux accumulator (10); in stream (7), oxygen reaches values comprised within the range of from 3000 to 6000 ppm. Feed conditions and reflux are such that the pressure drop L ? , which is established in the column, is of 0.3 bars . After said column has been operating, in continuous run, for 860 hours under the above disclosed conditions, A.P rises up to values more than double of the initial value, while the fractionation capacity results seriously i pai red. An inspection evidences a large extent of formation of polymeric material on the trays of the rectificaton - section of the column. ^ MTBE synthesis and fractionation are carried out as in Example 1, with the difference that the polymerization inhibitor is injected, by the stream (8), Figure 1, also to the column reflux. The obtained improvement is partial only, in as much as after 2000 running hours, butadiene polymerization on rectification trays block agains column operation. Examg_e_3 MTBE synthesis and fractionation are performed under operating conditions analogous to those of Example 1, with the variant (see Figure 2) of the treatment of methanol (2),which contains an amount of dissolved oxygen ranging from 30 to 60 mg/l, and is delivered to the upper section of a stripping column (12), from the bottom of which a stream of an inert gas, such as fuel gas (13), containing an oxygen amount lower than 150 ppm, is sent in countercurrent flow. The oxygen-depleted methanol, stream (15), is recovered from the bottom of the column, with oxygen levels constantly Lower than 1 ppm. The oxygen-enriched stripping gas (14) is sent to blowdown. The efficaciously of the stripping is also confirmed by the fact that in gaseous stream (7) the oxygen level does not exceed 20 ppm. By resorting to such a measure, column (9) has been operating for 8000 hours with no drawbacks, and an inspection thereof sho ed the complete absence of polymerization phenomena. The equal find numbers have the same meaning as of Figure 1. MTBE synthesis and its fractionation are carried out as in preceding Examples, but methanol is now deprived of oxygen by sending stream (2) (Figure 3) to the head of a distillation column (12), from the bottom of which methanol is recovered with an oxygen Level lower than 0.5 ppm. The ini ially dissolved oxygen is removed as a overhead vent (14) from reflux accumulator (13). The oxygen amount found in the overhead vent (7) is lower than 10 ppm. By so doing, the column showed to be able to operate without drawbacks for more than 8000 hours. The other find numbers indicate the elements which were already illustrated in Figure 1.

Claims: C_j._a_i_m_._i 1. Process for the preparation of a Ik L-tert-butyI ethers by starting from C. charges containing isobutene 4 and other C, hydrocarbons, among which butadiene at a 4 concentration, referred to the total charge, comprised within the range of from 10% by weight to 70% by weight and from aliphatic alcohols preferably selected from methanol and ethanol, comprising reacting the isobutene contained in the charge with the aliphatic alcohol in one or more reaction sections at a temperature not higher than 60 C in the presence of a macroporous sulphonated resin of Amberlyst 15 or Lewatit SPC 108 type and separating the alkyl-tert-butyl ether from the other unreacted components by distillation, characterized in that the alcohol and/or the C. hydrocarbon charge are/is 4 submitted, before the reaction, to an oxygen removal step, in particular of physical type, of removal of dissolved free oxygen by stripping with outer inert agents, or distillation up to reduce the free oxygen content to 1-2 ppm or less. 2. Process according to claim 1, wherein the oxygen , removal is of physical type. 3. Process according to claim 2, characterized in that the oxygen removal is carried out by stripping with inert outer agents. 4. Process according to claim 3, characterized in that the inert outer agents are selected from nitrogen, methane, hydrogen, fuel gas, ethane or mixtures thereof. 5. Process according to claim 2, characterized in that the oxygen removal is carried out by distillation, by s end i ng t he s t r eam do be dep r i ved o f oxyg en t o t h e head of a di sti l l ation column.

Inventors: ANCILLOTTI FRANCESCO, PESCAROLLO ERMANNO, ANCILLOTTI, FRANCESCO, PESCAROLLO, ERMANNO
Applicants: SNAM PROGETTI, SNAMPROGETTI S.P.A.
Application year: 1987
Main classification: C07C43
IPC codes: B01J31, C07B61, C07C41, C07C43, C07C67