A number of organic chemical hydrocarbon conversions, such as alkylation isomerization, disproportionation, oligomerisation are acid-catalyzed. Such reactions give rise to by-products, which either comprise acidic groups per se (e. g. sulphates), or are by-products, which upon hydrolysis or thermal decomposition, produce acids, such as organic chlorides or fluorides. These by-products can cause corrosion, and are therefore dangerous to expensive columns or pipelines. To avoid problems arising from corrosion, it is often necessary to manufacture any process equipment which is potentially exposable to acid, out of corrosion resistant metal. This can greatly increase costs.

In alkylation, an olefin (e. g. a C3 or C4 olefin) is reacted with C3 5 branched alkane to form a mixture of branched alkanes, called alkylate. Alkylate has a high Octane Number, and accordingly, is useful for aviation gasoline. The alkylation reaction may be catalysed by hydrofluoric acid, or other Lewis or protic acid species. When fluorine- containing catalysts are employed, side-products, such as alkyl fluorides, tend to be formed in low concentrations. After the alkylation, the crude product is purified to recover hydrogen fluoride and then distilled to produce a fraction for use in avgas. In the distillation column the alkyl fluoride is converted to hydrogen fluoride, which ends up in the overhead together with water and lower boiling hydrocarbon. The condensate from the overhead separates into two liquid layers: an organic layer of hydrocarbon, and

an aqueous layer of water and hydrogen fluoride. Hydrogen fluoride is corrosive both to the pipes downstream and the column upstream. Water is corrosive because it encourages the formation of rust.

It is among the objects of the present invention to reduce the problems of the corrosive condensate.

According to the present invention, there is provided a process for purifying a product, said product comprising at least a first hydrocarbon and at least one second hydrocarbon of differing boiling points, which process comprises: passing a feed comprising said product (and usually water) to a distillation column to separate a first fraction, usually a one-phase fraction of vapour or liquid, comprising said first hydrocarbon (and usually water), and at least one second fraction, usually a vapour fraction, of boiling point different from said first fraction and comprising said second hydrocarbon, recovering said second hydrocarbon, cooling said first fraction, and before any formation therefrom (e. g. by condensation) of a liquid phase comprising water in contact with a separate liquid phase of said first hydrocarbon, having present with said water, a water miscible organic solvent to produce a single liquid phase comprising the first hydrocarbon, and solvent (and usually water).

Prior to treatment by the process of the present invention, the product (hereinafter crude product) may comprise at least one of (i) an acid from said process and (ii) a by- product of said process which is convertible to an acid. The acid and/or by-product usually forms part of the first fraction. The crude product may be from an acid catalyzed hydrocarbon conversion process. However, the process of the present invention may also be suitable for purifying a product comprising trace amounts of any corrosive substance; for example, the process may be employed to treat crude oil.

As mentioned above, the crude product contains at least a first hydrocarbon and at least one second hydrocarbon of different boiling point. The difference in boiling point is usually at least 10 °C, for example, 10 to 80°C, preferably, 20 to 50°C. The first and second hydrocarbons usually differ in carbon content by 1 to 4 carbon atoms. The first fraction may either be distilled from the column as an overhead or a middle fraction, or alternatively, be removed as a bottom stream, depending on its boiling point relative to

the second hydrocarbon and water.

There may be two or more second fractions, which may be separated from the first fraction, and additionally, each other.

In a preferred embodiment, the crude product is derived from a hydrocarbon conversion process. This hydrocarbon conversion process may involve a rearrangement reaction, in which a single hydrocarbon is isomerized primarily to another single hydrocarbon. Examples of such conversion processes include the conversion of ethyl benzene to a xylene, and the conversion of n-butane to isobutene.

Alternatively, the hydrocarbon conversion process may be an oligomerisation reaction, for example, of an alkane or alkene. The alkane or alkene may have 3 to 12 carbon atoms, preferably, 3 to 8 carbon atoms. Linear, branched and cyclic (eg of 5 to 12 carbon atoms) alkanes and alkenes may be employed. Aromatic hydrocarbons, for example, of 6 to 14 carbon atoms, may also be employed. Such aromatics may comprise one or more aromatic rings, for example, one or two rings. Examples of suitable feeds include butane, pentane, hexane, octane, ethylene, butene-1, butene-2, and benzene.

Mixtures of hydrocarbons may also be used as feeds. Suitable mixtures include crude oil, crude oil distillates, atmospheric or vacuum residues from the distillation of crude oil, and products therefrom.

In a most preferred embodiment, the crude product is derived from an alkylation reaction, for example, the alkylation of an aliphatic or an aromatic hydrocarbon. Any suitable feedstock may be used for the alkylation reaction. Examples include isobutane, isopentane, isobutene, propylene, and mixtures thereof. Preferably, a pair of feedstocks are employed; examples of which are (i) isobutane and a C3 5 alkene, such as propylene and butene; (ii) propylene and a branched C4 or C5 alkane, such as isobutane or isopentane; and (iii) benzene and an alkene, such as ethylene or propylene.

As noted above, the crude product comprises a first hydrocarbon and a second hydrocarbon. The first and second hydrocarbons may each independently be an alkane or alkene of, for example, 3 to 12 carbons, preferably, 4 to 10 carbons. Preferred examples of such alkanes and alkenes include isobutane, isobutene, butene-2, iso- pentane, iso-octane, other trimethyl pentanes, and mixtures thereof. Other examples of first and second hydrocarbons include aromatic hydrocarbons of, for example, 6 to 12 carbons, such as benzene, ethyl benzene, o-, m-and p-xylene and toluene. Depending

on the nature of the other hydrocarbon and their boiling points relative to water, any particular hydrocarbon may be a first or second hydrocarbon.

These first and second hydrocarbons are separated using a distillation column. In one embodiment, the first hydrocarbon is contained in a narrow cut alkylation fraction, which may be recovered from the crude product by distillation. This narrow alkylate cut may be removed, for example, from the middle of the column, whilst a higher boiling fraction containing, for example, the second hydrocarbon may be removed as a column residue or bottom stream. This residue or bottom stream may comprise C, o+ alkanes. It may be possible to purify the narrow alkylate cut further, for example, by further distillation using the same or a separate column. Lower boiling fractions of isobutane and/or isopentane, for instance, may be distilled from the narrow alkylate cut as an overhead stream. Preferably, the narrow alkylate cut has a boiling point of 25 to 135°C , preferably, 50 to 125°C, more preferably, 90 to 125°C, and most preferably 95 to 105°C.

As noted above, the crude product may comprise an acid from said process, and/or a by-product which is convertible to an acid. The crude product may also comprise water. This water may be present in the crude product as an impurity, or as a decomposition product.

When the crude product contains an acid, the acid may be the acid catalyst employed in the production of the crude product itself. Suitable acid catalysts include proton and Lewis acids. Examples of such acids include phosphoric, sulphuric, hydrochloric, hydrofluoric acid, and the chlorides and bromides of metals, such as aluminum and zinc. In the case of hydrofluoric acid, this may be present as such, or in the form of a complex, such as HF-BF3. The acids may be unsupported, or supported, for example, on supports of silica or alumina.

When the crude product comprises a by-product which is convertible to an acid, such by-products may be capable of hydrolytic and/or thermal decomposition to form an acid. Such by-products include alkyl halides. For example, in an alkylation reaction catalysed by hydrogen fluoride, such as the reaction between isobutane and butene-2, alkyl fluorides are produced. These alkyl fluorides tend to decompose under the process conditions to produce the corresponding alcohol and olefin, as well as HF. The latter is corrosive, and is usually entrained in the first fraction, as it leaves the distillation

column. The concentration of HF and/or its alkyl halide precursor in the first fraction is usually I to 200ppm based on the total amount of first hydrocarbon, water and acid present in the first fraction. Preferably, the concentration of HF and/or HF precursor I to 50 ppm, for example, 5 to 40 ppm. As mentioned above, the first fraction may be a single vapour or liquid phase. Where the fraction is a vapour, it is usually condensed to a liquid. Part of this liquid may be recycled to the distillation column to maintain steady state operation, whilst the remainder of the liquid is usually removed as product.

Before this condensation step, a water miscible solvent is added, so as to ensure that the condensate is a single liquid phase. This single liquid phase usually contains less than 1000ppm, preferably, less than 100ppm, for example, 5 to 50ppm acid. Water may also be present in the single liquid phase. Usually, the amount of water present is 5-100ppm (based on the weight of the first hydrocarbon).

The water miscible solvent employed usually has a boiling point below that of the desired fraction when the latter is above that of water.

Preferably, the water miscible solvent employed is miscible with at least 2% water, more preferably, at least 50% water and most preferably, miscible with water in all proportions at 25°C. Alternatively or additionally, the solvent may be at least partially miscible with IM aqueous sodium chloride solution. For example, the solvent may be 5 to 50% miscible with IM aqueous sodium chloride solution. The solvent may also dissolve at 25°C in alkanes (e. g. of 4-10 carbons such as isooctane or isopentane) to an extent of at least l Oppm, preferably, at least 100 ppm, more preferably, at least 1000ppm, for example, to an extent of 10 to 10000ppm.

The solvent may be a polar oxygen containing organic compound, in particular, one containing CH and O atoms only. It may be an organic alcohol, for example, of 1 to 4 carbons, such as methanol, ethanol, n-propanol isopropanol or tert butanol.

Alternatively, it may be a dialkylketone, for example, of 3 to 6 carbons such as acetone, methyl ethyl ketone, methyl isopropyl ketone and methyl isobutyl ketone. It may also be cyclopentanone or cyclohexanone, or an ether, for example, a cyclic ether such as tetrahydrofuran.

Preferably, the solvent is chosen to have a boiling point below, for example, at least 10°C below the boiling point of the desired product. Thus, for an aviation alkylate having a boiling point of 95 to 105°C, an alcohol having a boiling point of 50 to 85°C,

such as methanol, ethanol, isopropanol or acetone may be used.

Instead of the solvent having a lower boiling point than the desired fraction, the solvent may have a higher boiling point, provided that the water in the distillation column also has a higher boiling point than the desired fraction. An example of this operation is the distillation of isobutane from alkylate, in which the desired fraction, isobutene, is removed up the column or as an overhead, leaving a lower fraction or bottom stream comprising isopentane, highers and water. In this case, examples of suitable solvents may be the ones described above, especially those boiling at 50 to 110°C. Preferably, however, solvents boiling at or above 110°C, for example, at 110 to 200°C are employed. Such solvents include glycols and glycol ethers, for example, ethylene or propylene glycols. Oligomers (including co-oligomers thereof) such as diethylene glycol, and their mono or dialkyl ethers, for example, mono or di-methyl, ethyl or butyl ethers may also be employed. Mono butyl or monoethyl diethylene glycols are preferred for this use.

Sufficient solvent is employed to ensure that the condensate contains at least 20ppm, preferably, 20 to 500ppm, more preferably, 100 to 400 ppm of the solvent. The weight ratio of solvent to water is preferably at least 0.5: 1, for example, 0.5 to 50: 1, preferably, 1 to 10: 1.

The solvent may be introduced on one or more occasion. It may be mixed with water present, for example, as a vapour in an overhead or intermediate vapour cut. Acid may also be present in such a cut. Alternatively, the solvent may be mixed with water when the water is in a single liquid phase comprising water and the first and/or second hydrocarbon. Acid may also be present in this liquid phase. Thus, before or at the time of condensation of water, the solvent is preferably present, either as a liquid, or preferably a gas.

The solvent may be introduced at any stage prior to any separation of water (and acid) as a liquid phase. For example, when the first fraction is an overhead stream, the solvent may be added to the first fraction before or after the overhead stream is cooled to effect condensation. When the solvent is added after the cooling step, however, the cooling must not have been sufficient to produce a separate aqueous liquid phase. The addition may also take place before or after a portion of the overhead stream is recycled back to the column. When the first fraction is a single phase liquid (such as a bottom

product), the solvent may also be added to the first fraction at any time, as long as at the time of addition there is not an aqueous liquid phase comprising a majority of water.

Alternatively or additionally, the solvent may be added to the column, preferably at a point in the column at which the vapour temperature is above that of the solvent. The solvent may also be added with the feed to the column. In the case of the latter, the water-containing feed to the column may be a two phase system. Preferably, however, the feed is a single phase.

In a preferred process, an alkylation of isobutane with mixed C3-5 olefins catalysed by HF forms a mixture of hydrocarbons and alkyl fluoride among which hydrocarbons are isopentane and C3 5 aliphatic hydrocarbons, C7 9 alkanes mostly isooctane and other trimethyl pentanes and C, 0+ alkanes. Distillation produces the Coo+ alkanes as column bottoms, the C7 9 alkanes as an intermediate cut of bp90-130°C e. g.

95-105°C and a lower boiling fraction e. g. overhead of isopentane and other C3 s aliphatics. Presence of water in the column and/or the heat of the column converts at least some and usually substantially all of the alkyl fluoride to HF which leaves the column off in the lower bp fraction. Without addition of the solvent, the overhead condensate forms 2 phases, the lower of which is water with the HF, and is thus very corrosive. With the solvent, the condensate is one liquid phase of much reduced corrosivity so a column, and the condenser and associated pipework of less corrosion resistant material can be used. The same benefit applies if the first fraction comprising water is a liquid bottoms product. Even if the first fraction comprises water but no acid, then the presence of the solvent in the one liquid phase of first hydrocarbon, water and solvent reduces rusting in the pipework.

After the separation of condensate, the condensate e. g. isopentane, water, solvent (preferably isopropanol) with acid e. g. HF can be passed to a neutralizer in which the acid can be neutralized, e. g. with an organic amine or alkali metal base, such as a hydroxide or carbonate especially sodium hydroxide to form a salt, preferably in solution. The salt formation preferably causes the condensate to separate into 2 liquid phases (and optionally a solid salt phase). The organic phase can then be recovered substantially free of acid. Thus preferably the solvent is of a nature and amount such that it homogenizes the condensate, but forms 2 liquid phase in the presence of the salt; isopropanol and tertbutanol are preferred. If desired the solvent may be distilled from

the neutralised solvent/condensate mixture to produce two liquid phases (and the salt one), or from the separated organic phase. The hydrocarbon e. g. isopentane can be recovered from the organic phase.

Alternatively or in addition to the use of base, the acid e. g. HF can be removed from the condensate comprising first hydrocarbon by dilution with water, especially at least 5 or 10 times the weight of water in the condensate, in order to produce two liquid phases the acid being in dilute form in the lower aqueous phase which is separated from the organic phase. The solvent can be recovered from either or both of the organic and aqueous phases e. g. by distillation.

The recovered solvent may be recycled for reuse, or if desired may be retained in the purified lower boiling fraction.

The same principle for work up of the condensate from the overhead applies also in respect of work up of condensate from an intermediate column position, or for separation from the liquid at the column bottom but in this case the hot liquid first fraction from the column is one phase and would form 2 liquid phases on cooling in the absence of the solvent.

The desired product from the distillation may be the second fraction from the column or the first fraction, after removal of water and usually the solvent or both.

Thus from an alkylation reaction may be recovered low boiling hydrocarbon e. g. isobutane or isopentane and/or higher boiling hydrocarbon e. g. alkylate cut bp 90-105°C or isooctane for aviation or motor gasoline use.

The present invention is illustrated with reference to the attached drawings which show a flow diagram of the process.

The figure shows an alkylation reactor 1 with a product exit line 2 feeding crude alkylate into a distillation column 3, which has an overhead removal line 4 and an overhead return line 5 separated by a condenser 6, and from the return line extends an overhead exit line 7. At the bottom of the column 3 is a bottoms removal line 8, and a bottoms return line 9, and separated by a bottoms reboiler 10, with a bottoms line 11 leading from removal line 8 to a second condenser 15 and bottoms exit line 14.

Between lines 5 and 9 on column 3 is an intermediate removal line 12 also leading to a condenser 16 and intermediate exit line 17. Also shown are the possible locations of solvent addition lines; these locations are marked 13A, 13B, 13C, 13D, 13E and 13F

and 13G and 13H but usually only one such addition line is present especially 13B or 13G.

In use alkylation product comprising first and second hydrocarbons with alkyl fluoride passes as feed (with water) into column 3. In the column 3 and/or in the reboiler 10, the alkyl fluorides are converted into HF and by products. Distillation proceeds so narrow alkylate cut is removed in line 12, while into line 4 is removed a blend comprising isopentane, water and HF, which is cooled in condenser 6. Lower boiling polar solvent is added at one or more locations 13A-E especially B, C, D or E, and the condensate after 13A is a one phase system, some of which is returned to the column 3 and the rest passed for purification in line 7. Higher boiling polar solvent than the cut in line 12 may be added in 13D, 13E, 13F or 13G, as also can happen when the fraction in line 12 is a light one which has a boiling point significantly below (e. g. at least 50°C below that of water). In this case also, intermediate line 12 may be missing and the light fraction (second hydrocarbon) can be distilled as overhead from line 4.

Example In comparison experiments were measured the degree of rusting of carbon steel strips in crude isopentane (simulating the overhead from a distillation of 90-105°C alkylate fraction). In each of 3 sealed flasks A, B and C were 500ml of the crude isopentane and the strip, while in sealed flask B was also 1. 5ml water and in sealed flask C also 1. 5ml water and 12ml isopropanol. The flasks were kept at 34°C for 2 days, and rusting was found on the steel in flask B, but not in A or C.

2 drops of 50% aqueous HF were then added to each flask. After 8hrs at 24°C, there was no rusting in A or C but accelerated rusting in flask B.