Method for preparing isobutene and alkyl-tret.-butylether

Alkyl tert-butyl ethers are used as high octane gasoline additives, which are obtained by the interaction of isobutene with the corresponding alcohols (methanol-MTB, ethanol-ETB, etc.) in the liquid phase under suitable catalyst at 15-40 atmospheres and 60-100 atmospheres. ° C (see IT Patent No. 1012,690).

The current tendency to use increasing amounts of especially MTBE in gasoline extraction technology and the almost complete utilization of refined streams containing isobutene have required the expansion of isobutene extraction complexes by isobutane dehydration.

The raw material is usually a blend of butanes, so a typical block diagram of the MTBE Unit is shown in FIG. 1.

The feed stream 1 containing normal butane and isobutane is fed to a distillation column 2 from which isobutane 3 is virtually exited and stream 4 containing HC₄ and high hydrocarbons released from the lower part. The bottom stream portion 5 isomerizes in reactor 6 and recycles to column 2 through 7.

Isobutane 3 is dehydrated in Unit 8, thereby guaranteeing a stream of light gas 9 and a stream of isobutane and isobutene 10 which is directed to MTBE synthesis 11 where it reacts with methanol 12 until MTBE 13 is obtained.

In addition to MTBE, unit 11 also produces a stream 14 containing isobutane, which is recycled to a point upstream of the dehydrogenation reactor 8.

It should be noted that the source of the dehydrogenation process is not related to the production of MTBE.

However, it has to be taken into account that most of the MTBE facilities will use isobutene obtained by dehydrogenation of isobutane, and by analogy, a larger proportion of isobutane dehydrogenation plants will supply isobutene to MTBE plants.

Any integration of these two devices will start at the cost of capital investments and / or service cost and therefore of great interest.

An isobutane dehydrogenation device, the scheme of which is illustrated in FIG. 2 based on a method analogous to that currently used in commerce, that is, gas preparation, compression, and purification. More specifically, isobutane 21 is fed to the dehydrogenation reactor 22, followed by a compression step 23 and a purification step 24.

Purification involves the separation of hydrogen, nitrogen and light hydrocarbons 25 from the reaction product 26 hydrocarbon component C₄.

One of the most problematic moments is the hydrocarbon C₄ separation from the stream of light gas, which remains non-condensable after pressure.

In modern facilities, this separation is obtained by cryogenic techniques. This can also be accomplished by absorption in a suitable solvent followed by evaporation of C₄ and solvent recovery.

Thus, for example, in isobutane dehydrogenation plants, the solvent is Ph 2 C₁₀ a mixture of hydrocarbons. FIG. 3 shows a typical isobutane dehydrogenation scheme for SnamprogettiYarsintez by cryogenic isolation (see Octane Week, October 8, 1990, pp. 7-8).

Before being fed to the dehydrogenation reactor 34, which is connected by lines 36 and 37 to the dehydrogenation catalyst regenerator 35, the isobutane 31 is preheated in the heat exchangers 32 and 33.

The gas stream 38 leaves the upper portion of the reactor 34 and, after cooling 40, compression 41, and partial condensation 42, enters the separator 39.

Separator 39 produces two streams, one of them 43, consisting mainly of C₄ hydrocarbons and the other 44, predominantly hydrogen and C₃ hydrocarbons.

The stream 43 is fed to a depropanizer 45, the lower portion of which is a stream 46 consisting mainly of iso-C₄. Stream 44 is fed to a low-temperature regeneration system 47 to recover isobutene and isobutane 48 therein, then added to stream 43. The stream 49, exiting 47 and containing predominantly hydrogen and C 1 -C 6 hydrocarbons, is added to the stream 50 leaving the top. part 45 of the depropanizer.

After compression 52 and heating 53, air 51 is supplied to the regeneration column 39.

The gas stream 54 exits the upper portion of column 39 and is cooled 53 before being used as fuel gas 56, it is filtered through 55.

FIG. Figure 4 shows a typical dehydrogenation scheme of isobutane by isolation by absorption and evaporation.

We will describe only one part related to the purification, as the others are analogous to those shown in FIGS. 3.

The stream 38 leaving the reactor 34 is cooled 33, compressed 41, and partially condensed in a condenser 42 before being fed to a separator 39 to separate the heavy hydrocarbons 43 from the light hydrocarbons 44, the latter eventually entering the absorber 60.

The light gas and hydrocarbons 61 exit the upper portion of the indicated absorber while the remainder is absorbed by the solvent supplied by line 62 and discharged from the bottom 63.

Flow 63 containing spent solvent and C₄ hydrocarbons fed to the distillation column 64 from the lower part 65 which provides the recovered solvent and the upper part which produces the stream containing most₄ hydrocarbons 66 and is further supplied to the depropanization column 67 after addition to stream 43. The stream consisting mainly of iso-C₄, discharged from the bottom portion 68 of column 67, and having a stream having a maximum of C₃of hydrocarbons, passing from the upper part 69.

Such an extraction procedure is very expensive and complex. In particular, a cryogenic system requires high capital installation and maintenance costs, as the refrigerator cycle operates at very low temperatures and requires expensive equipment (such as turbo expansions).

Absorption isolation and targeting have the disadvantage that the solvent used is additive. in terms of the production cycle and thereafter must be carefully distinguished, which requires additional technological costs and C₄ the fraction produces excess heavy hydrocarbons. The high cost of C must also be borne in mind₄ for the desorption of the fraction from the solvent.

Unexpectedly, it was found that C₄ the hydrocarbons can be isolated from the vapors formed after the first condensation due to the absorption of the alkyl tert-butyl ether and / or the corresponding alcohol used without reducing the yields to values ​​lower than those described above, despite the high vapor pressure of these compounds under process conditions seemingly hinder their use.

Compared to the absorption of heavy hydrocarbons according to the scheme shown in FIG. 4, the main advantage of using these solvents is that these compounds are present either as reagents (methanol, ethanol, etc.) or as products (MTBE, ETBE, etc.) and that the streams containing the isolated C₄ and the solvent can be directly applied to the work equipment already provided in the factories where alkyl tert-butyl ether is obtained without any further treatment.

Given the versatility of the cryogenic scheme used, it is obvious that a system operating at very low temperatures is more complex than an absorption column operating at 40-60 ° C.

The combined process for the preparation of isobutene and alkyl tert-butyl ethers according to the present invention essentially comprises the following steps:

a) dehydrogenation of the stream containing isobutane followed by compression and partial condensation of the gas, which allows for a stream of gas containing hydrogen, nitrogen and C;₂-C₄ hydrocarbon distillation to produce a liquid stream having a maximum content of C₄ hydrocarbons;

b) feeding the gas stream to an absorption column using a solvent to obtain from the top a gaseous mixture containing mainly hydrogen, nitrogen and C 1 -C 4;₃ of hydrocarbons, the lower part being predominantly C₄ a liquid mixture of hydrocarbons and spent solvent;

(c) at most C₄ directing a stream of hydrocarbon-containing liquid to the distillation column until a gaseous mixture containing most₃ a hydrocarbon and a liquid blend of isobutane with isobutene at the base;

d) supplying a liquid mixture containing the isobutane and isobutene obtained from the reactor step or the first reactor, if two or more reactors are used, together with the appropriate alcohol, until alkyl tert-butyl ether is obtained;

(e) supplying the product from the reactor to the distillation column to obtain from the upper part a stream containing the most unreacted gas and a lower part containing the liquid containing the most alkyltret. -butyl ether;

f) feeding the stream containing the most unreacted gas from step e) directly to the wash column if only one reactor is used or to a second reactor if two or more reactors are used, followed by supplying the product from said second reactor to a distillation column until a liquid mixture from the lower part containing alkyl tert-butyl ether which is recycled to the distillation column of step (e) or to the third reactor if several reactors are used, and from the top, the mixture containing the most unreacted gas, further feeding this stream to the wash column;

(g) Cleavage in the wash column to obtain substantially unreacted hydrocarbons from the top C₄and, at the bottom, a liquid mixture containing mainly water and the alcohol used, furthermore, they are isolated in a distillation column, characterized in that the solvent used in the absorption column of step (b) is the liquid containing the most alkyl tert-butyl ether. from step (e) and / or the proportion of alcohol used in the process.

Liquid mixture containing mainly C₄ hydrocarbons and alkyl-tert-butyl ether as the spent solvent from the oxygen portion of the adsorption column b) may be partially or completely supplied to one or more of the following units:

- to a distillation column of stage (c),

- to the distillation column of stage (e),

- to the reactor of stage d).

It should be noted that alkyl tert-butyl ether recovery is not necessary as it is stated that the indicated liquid mixture does not need to be fed to the distillation column of step c).

If the alcohol is used in the process and is still used as a solvent in the absorption column of step b), the liquid mixture leaving the column is fed to the reactor of step d).

The spirits isolated from the distillation column may be recycled from the washing column of step g) to the reactor of step d) and / or to the reactor of step f) if two or more reactors are used and / or the absorption column of step ib).

Unresponsive C₄ the hydrocarbons isolated in the washing column of step (g) can be readily mixed with the stream containing isobutane of step (a) for dehydrogenation.

The methods described above can also be carried out using a reactor column, where the reactor and the distillation column are combined into one unit. In this case, part of the liquid containing the most alkyl tert-butyl ether leaving the reactor column directly is recycled to the absorption column of step ib). The amount of solvent used in the absorption column of step (b) is mainly within the following ranges:

- as the sole solvent for alkyl tert-butyl ether, 0.5 to 2 moles C₄ hydrocarbon moles in the absorption column, and more preferably from 1 to 1.5;

- 1 to 3 moles C of the corresponding alcohol as the sole solvent₄ for a hydrocarbon mole in the absorption column, and more preferably from 1.5 to 2.

In the case of a mixed solvent, the amounts of alkyl tert-butyl ether and the corresponding alcohol can, of course, be reduced more than the ratios described.

If an alkyl tert-butyl ether is used as the unit solvent, the liquid portions thereof which can be fed to the p absorption column comprise from 15 to 50% by volume, and more preferably from 30 to 40% of the total amount of liquid leaving the distillation column (e). stages.

The invention will be more readily understood by reference to the accompanying drawings, which show some of the more valuable, but not limiting, embodiments thereof.

FIG. 5 shows a schematic diagram of a MTBE mining combined process using MTBE as an absorbent.

The feed stream 101, containing normal and iso-butanes, is preheated in heat exchangers 102 and 103 and fed to a dehydrogenation reactor 104 which is connected in lines 106 and 107 to a dehydrogenation catalyst regenerator 105. The gas stream 108 exits the upper reactor portion 104 and is fed to a separator 109, pre-cooled 103, filtered through 110, post-compression 111 and partial condensation 112 to separate heavy hydrocarbons 113 from light hydrocarbons 114 fed to absorber 115, in which MTBE 116 is used as a solvent. The light gas 117 exiting the top of the absorber 115 is cooled 118 and separated 119 to separate it from the gas MTBE 120 which is recycled to the absorber.

The fluid flow 113 is fed to the distillation column 122

C₃ to obtain hydrocarbons from the top 123 and isobutene and isobutane from the bottom 124.

io

Iso-C₄ 124 is supplied to the first reactor 125 with methanol 126 to obtain a stream containing MTBE 127 which is fed to a distillation column 128 to receive all MTBE 129 from the bottom and unreacted gas (methanol, isobutene and isobutane) 130 from the top.

The gas stream 130, together with the methanol, is fed to the second reactor 131 for further flow containing MTBE 132 (where MTBE is less than 127 in the stream), which is fed to a distillation column 133, from which a stream 134 containing mainly MTBE is recycled to column 128, and a stream 135 containing methanol, isobutane and isobutene is fed from the upper portion and fed to a washing column 136 to which water 137 is added.

Isobutane 138 exits from the top of column 136 and is recyclable by subsequent addition to stream 101. Methanol and water leaving the bottom portion will be separated in column 140.

The portion of stream 129 containing the most MTBE is recycled 141 as the solvent in the absorption column 115. Flow of liquid 142 having C₄ of hydrocarbons and MTBE supplied to the distillation column 128. However, part of it can be supplied to the reactor 125.

After compression 145 and heating 146, air is introduced into regeneration column 105. The gas stream 147 exits the top of column 105, cools it 146 and filters 148 before using it as a fuel gas 149.

FIG. Fig. 6 shows a schematic diagram of the combined process for obtaining MTBE using MTBE as an absorbent, as shown in Fig. 6. 5, the only difference being that from the absorbent 142, the bottom

the inlet stream is supplied with the liquid 113 from the separator 109 to the distillation column 122.

In this way, the partially absorbed hydrocarbons C are further removed₃.

FIG. The numeric characters in Scheme 6 have the same meanings as in FIG.

FIG. Figure 7 shows a possible scheme for the MTBE extraction process using methanol as solvent. These schemes and figs. The difference in the scheme shown in Figure 5 is that the methanol portion is fed to the absorption column 115 instead of the MTBE portion, and the numerical marks have the same meaning as in FIG. 5.

The following is a better illustration of the invention with two examples.

EXAMPLE

100 kmol / h isobutane is fed to a dehydrogenation reactor treating gas at 580 ° C under atmospheric pressure and Cr-Al catalyst.

From.

.

49.

3.8

<td>of the reactor</td><td>outgoing</td>

<td>0 kmol / h</td><td>isobutane,</td>

<td>9 kmol / h</td><td>isobutene,</td>

<td>0 kmol / h</td><td>hydrogen</td>

<td>kmol / h</td><td>methane,</td>

the stream must:

2.3 kmol / h C₃ hydrocarbons,

1.5 kmol / h C₅ of higher hydrocarbons.

The output from the reactor is pressurized to 20 atm. and cooled to 40 ° C to separate the flow of liquid and the flow of gaseous products. The fluid flow has a maximum C₃, C₄ and higher hydrocarbons.

The gas flow still has about 25% C₄ of hydrocarbons and its composition:

12.2 km / h isobutane,

9.4 kmol / h isobutene,

47.7 kmol / a. hydrogen,

3.4 kmol / h methane,

1.0 kmol / h C₃ hydrocarbons.

This stream is supplied to the lower part of the absorption column, to which the liquid part of MTBE is supplied at a temperature of 35 ° C such that the amount of molar MTBE and C₄ the hydrocarbon ratio would be 1: 1.

The column temperature is maintained between 35 and 60 ° C. In this way, 99.6% of the isobutane and isobutene present in the feedstock are isolated; 0.5% solvent loss with vapor flow.

The composition of the fluid flow from the bottom is as follows:

12.15 kmol / h isobutane,

9.36 kmol / h isobutene,

<td></td><td> 13</td>

<td>0.65 kmol / h</td><td>hydrogen,</td>

<td>0.20 kmol / h</td><td>methane,</td>

<td>0.62 kmol / h</td><td>C₃ hydrocarbons,</td>

<td>21.49 kmol / h</td><td>. MTBE.</td>

This flow is mixed with the flow of liquid from the condensation at 40 ° C until the flow is as follows:

<td>51.95 kmol / h</td><td>. isobutane,</td>

<td>43.86 kmol / h</td><td>. isobutene,</td>

<td>1.95 kmol / h</td><td>hydrogen,</td>

<td>0.60 kmol / h</td><td>methane,</td>

<td>1.92 kmol / h</td><td>C₃ hydrocarbons,</td>

<td>1.50 kmol / h</td><td>C₅ and higher hydrocarbons,</td>

<td>21.49 kmol / h</td><td>. MTBE.</td>

This mixture is fed to a distillation column separating the residue containing MTBE, C₄ hydrocarbons and low levels of propane and propylene.

This residue is mixed with a 1: 1 methanol to isobutane ratio and fed at ZHSV5 (fluid volume per hour) into a primary MTBE synthesis reactor where it reacts on Amberlite 15 resin at 60 ° C. and 15 atmospheres for pressure.

Composition of reactor outlet:

51.95 kmol / h. isobutane,

<td> 5.48</td><td>kmol / h</td><td>isobutene,</td>

<td> 5.48</td><td>kmol / h</td><td>methanol,</td>

<td> 0.42</td><td>kmol / or.</td><td>C₃ hydrocarbons,</td>

<td> 1 . 50</td><td>kmol / h,</td><td>C₅ and higher hydrocarbons,</td>

59.87 kmol / h. MTBE.

This stream is fed to a fractionation column from the bottom to obtain 98% purity MTBE and from the top a liquid distillate which is fed to the second reactor at a methanol to isobutene molar ratio of 1.3: 1.

Again, treatment with Amberlite 15 at 60 ° C and ZHSV5 yields the following flow composition:

51.95 kmol / h. isobutane,

<td> 0.60</td><td>kmol / h</td><td>isobutene,</td>

<td> 2.24</td><td>kmol / h</td><td>methanol,</td>

<td> 0.42</td><td>kmol / h</td><td>C₃ hydrocarbons,</td>

<td> 4.88</td><td>kmol / h</td><td>MTBE.</td>

This is how MTBE is supplied to the first reactor

98.4% isobutene yield.

EXAMPLE

It is identical to example 1 with respect to the raw material fed to the absorption column.

In this case, the solvent used is methanol in the ratio C₄ hydrocarbons are 1.5: 1.

98% isobutane and 98.5% isobutene are isolated and 0.1% solvent is lost.

The liquid flow from the bottom of the absorber is of the following composition:

12.02 kmol / h

9.35 kmol / h

32.37 kmol / h

0.25 kmol / h

0.35 kmol / h

0.65 kmol / h

isobutane, isobutene, methanol, hydrogen, methane,

C₃ hydrocarbons.

This stream is blended with a liquid hydrocarbon residue from a fractionation column fed to a fluid stream condensed at 40 ° C after compression, degassed and mixed with methanol in a 1: 1 methanol / isobutene reactor operating under the conditions of Example 1. .

In this case, a full 99% isobutene yield is obtained without delivering the product to the MTBE unit.