PROCESS FOR THE SYNTHESIS OF METHYL-TERT-ALKYL ETHERS WITH SUPPRESSION OF CORROSION

Abstract: Methyl-tert-alkyl ethers coming from a synthesis section are separated from unreacted hydrocarbons by means of a rectification operation, in which the unreacted hydrocarbons are drawn as overhead fraction, in the form of an azeotropic mixture with methanol. To the purpose of removing methanol, and of therefore allowing a suitable use of said unreacted hydrocarbons, the azeotropic mixture is washed with water inside a washing column, which gets very rapidly corroded. The invention solves the corrosion problem by resorting to the removal of oxygen from the hydrocarbon charge and/or of methanol, before these are sent to the synthesis.

Description: "PROCESS FOR THE SYNTHESIS OF METHYL-TERT-ALKYL ETHERS WITH SUPPRESSION OF CORROSION" * SH The present invention relates to a process for the preparation of methyl-tert-alkyl ethers by starting from methanol and branched C -C β olefins, with the double bond H 8 being on a tertiary C atom. In a preferred form of the 5 present invention, the branched olefins are isobutene and isoamylenes (2-methyl-butene-2 and 2-meth l-butene-1) and produced ethers are MTBE and TAME. It is known that for the synthesis of sa d ethers it not necessary to use the reactive olefins at a igh 10 purity level, but it is enough to use hydrocarbon cuts wh ch contain them even at a low concentration. The reaction between alcohols and tertiary olefins is catalyzed by acidic catalysts and in the presence of the most suitable catalysts / for example the ion-exchange 15 resins in the acidic form, such as Amberlyst 15, Dowex 50, Lewatit SPC, the thermodynaππ c equilibrium is reached also at relatively low temperatures of 50-80 C. It is known that the excess of either of reactants allows an increase in conversion of the other reactant, 20 so that a wide range exists of operating schemes and conditions directed to the optimization of either of unconverted reactants. The separation of produced ether from unconverted reactants is carried out by distillation in a fractionation column, from the bottom of which the 25 ether, and from the top of which the hydrocarbons are separated. It is known that unconverted methanol is either totally or partly recovered from the column head as an azeotropic mixture with the hydrocarbons. This behaviour allows methanol-free ethers to be obtained even when from the reactor a reaction product containing an unconverted methanol portion exits. This fact is exploited to the purpose of operating ith a certain alcohol excess, so to force, by so doing, the conversion rate, as referred to the olefin. However, the presence of methanol in the hydrocarbons often jeopardizes the further exploitat on the ' reof, such as, e.g., their use as a charge for processes of the oli gomeri zation or alkylation type, so that, often, it has to be removed. For methanol removal, several techniques have been described, which range from an absorption on beds of solid absorbents, to countercurreπt-f low liquid-liquid extract on. As the extraction solvents, water or organic glycol- type solvents are described, but water is undoubtedly the choice solvent. However, the use of water in the liquid- liquid extraction column causes severe corrosion problems, so that for the construction of 'such a column, such valuable materials as stainless steel must be used, or burdensome procedures of lining by inert materials, or of coating by special paints must be adopted. It was surprisingly found, and is the basis of our invention, that it is possible to overcome all of corrosion problems affecting the washing column, even when carbon steel is used as the construction material, on condition that oxygen present as dissolved gas inside the charges fed to the extraction column is preliminarily removed. Such oxygen is basically arising 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, it methanol used in the synthesis of MTBE, stored under an air atmosphere at the temperature of 20 C and under the 5 pressure of 760 mm , dissolves about 80 ppm Hg weight/weight (w/w) of oxygen and 180 ppm w/w of nitrogen; the presence of this Latter, however, does not create any problems. A certain amount of dissolved oxygen can be 10 occasionally present also in the C, fraction containing liquified 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. 15 A last source of oxygen is the wash water, but, in as much as the process is normally run by feeding it with the water removed from the bottom of methanol recovery column, such an entry way is nullified. Occasional and short openings of water cycle do not normally cause 20 negative effects. To the purpose of controlling the corrosive effects, it is generally enough to eliminate the main oxygen entry point, i.e., that occurr ng through methanol. The object of the present invention s a process for 25 the synthesis of methyl-tert-alkyl ethers by starting from methanol and branched C,-C β olefins, preferably **\ 4 8 sobutene, in hydrocarbon charges containing, besides said branched olefins, straight olefins and saturated hydrocarbons, which comprises reacting the branched 30 olefins with an excess of methanol, relatively to the stoichiometric amount thereof, in a reaction section " provided with an acid c catalyst in the form of an ion- exchange resin of Amberlyst 15, Dowex 50 or Lewatit SPC type, at a temperature of from 50 to 80 C and furthermore comprising the separation of formed methyl-tert-a IkyI ether from the mixture of unreacted hydrocarbon components and from the excess of methanol, by distillation, and furthermore comprising the washing, with water, of the mixture of unreacted hydrocarbon components and excess methanol to the purpose of separating sa d methanol from said unreacted hydrocarbon components, characterized in that methanol and/or the hydrocarbon charges are deprived of the therein contained oxygen gas before they are fed to the reaction section. 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 of a distillation column, from the bottom of which the oxygen-free product is obtained, whilst oxygen is discharged as the overhead vent from the reflux accumulator. In some cases, the oxygen removal can be obtained by a stripping with a stream of a suitable inert gas, such as nitrogen, • methane, hydrogen, fuel gas, ethane or mixtures thereof. In the following examples, some oxygen removal procedures are described, which are however supplied to purely exemplifying purposes, and in no way are limitative of the present invention. §2_-_---_---_-_l Reference is made to Figure 1. 100 parts by weight of a C. olefinic charge (1), containing 22,0% by weight of isobutene, is mi ed with 14.33 parts of methanol (2) and the resulting charge (3) is fed to a reactor (20) containing an ion-exchange resin in the acidic form, of Dowex 50 type. The olefinic cut (1) has a content of dissolved oxygen lower than 1 ppm, whilst methanol, which is stored inside a tank under an air atmosphere, has an oxygen content ranging from 30 to 60 mg/l. The reaction product (4) containing an average amount of 27.8% of MT8E, which justifies an isobutene conversion of 92%, is fed to a fractionation tray column (11), in such an intermediate point, that the rectification section represents 40% of column. From the top of said fractionation column, the unconverted hydrocarbons are removed together with azeotropic methanol (6), from the bottom of the column a stream C5) be ng removed, wh ch 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 125 C. Oxygen dissolved in stream (4) fed to column (11) is only partly eliminated as an overhead vent from reflux accumulator (12); most of oxygen remains dissolved in stream (6), which is fed to a perforated-trays column (13) for liquid-liquid extraction, totally made from carbon steel. Inside said column, the azeotropic methanol-containing hydrocarbon stream is treated in countercurrent-f low with water (10) drawn from the bottom of methanol recovery column (14). Methanol inside C, 4 fraction is extracted by water, exits the bottom of column (13 ) as a water-alcohol solution (8), and is sent to a recovery column (14) (distillation column equipped with reflux accumulator (15), the overhead product of which is methanol (9), which is sent to the synthesis). Overhead column (13), C. hydrocarbons are recovered 4 <7), with a methanol content lower than 5 ppm. By operating under the above-said conditions, after about 1000 operating hours an increase is observed in pressure drop, with the apperance of C, products together with 4 stream (8) . The malfunctioning is so serious as to jeopardize the operativity of the facility. Inspection of column evidences an extensive forma on of a mixture of iron oxides identified as Fe_0, j 4 (magnetite), Fe 0..H.0 (goethite) and Fe_0 .H_0 (lepidocrocite). The deposit of such oxides is such as to considerably reduce the free passage opening through the tray holes, and from here the difficulties arise for C. 4 products to move upwards along the column. I__3IE._.____2 MTBE synthesis and fractionat on are carried out under analogous operating conditions as of Example 1, with the variant that methanol is treated as reported in Figure 2. Stream (2), which contains an amount of oxygen ranging from 30 to 60 mg/l, is delivered to the upper section of a stripping column (18), to the bottom of which a stream of nitrogen (16), containing an oxygen level lower than 150 ppm, is sent in countercurrent flow. The oxygen-depleted methanol, stream (3), is recovered from the bottom of the column, with oxygen levels constantly lower than 1.mg/l, and is sent to the reaction, after to it recycled methanol (9) being added. The oxygen-enriched stripping gas (17) is sent to blowdown. By resorting to such a measure, column (13) has been, operating for 8000 hours ith no drawbacks, and an inspection thereof showed the complete absence of polymerization phenomena. In Figure 2, 'the other find numbers have the same meaning as in F gure 1.

Claims: C_j._a_i ι _m_s 1. Process for the synthesis of methyl-tert-a IkyI ethers by star ing from methanol and branched C -C 0 4 o olefins, preferably isobutene, in hydrocarbon charges containing, besides said branched olefins, straight olefins and saturated hydrocarbons, which comprises reacting the branched olefins with an excess of methanol relatively to the stoichiometric amount thereof, in a reaction section provided with an acidic catalyst in the form of an ion-exchange resin of A berlyst 15, Dowex 50 o or Lewatit SPC type, at a temperature of from 50 to 80 C and furthermore comprising the separation of formed meth l-tert-a IkyI ether from the mixture of unreacted hydrocarbon components and from the excess of methanol, by dist llation, and furthermore comprising the washing, with water, of the mixture of unreacted hydrocarbon components and excess methanol to the purpose of separating said methanol from said unreacted hydrocarbon components, characterized in that methanol and/or the hydrocarbon charges are deprived of the therein contained oxygen gas before they are fed to the reaction section. 2. Process according to claim 1, character zed in that oxygen is removed in a stripper consisting of a distillation column, said oxygen being discharged as a overhead vent from the reflux accumulator. 3. Process according to claim 1, character zed in that oxygen is removed by means of stripping by a stream of an inert gas. 4. Process according to claim 3, characterized in that the inert gas is selected from nitrogen, methane, hydrogen, fuel gas, ethane, or mixtures thereof.

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