CA1322767C - Process for the production of pure dimethylether - Google Patents

Abstract

Wesseling, 27.10.1987 NL/Dr.H?-Me -2115-Unser Zeichen: UK 372/373 A process for the production of pure dimethylether Abstract of the disclosure This invention relates to a process for the production of pure dimethylether by catalytic dehydration of methanol in the presence of a ?-Al2O3-catalyst which preferably con-tains a small quantity of SiO2 and purification of the de-hydration product by feeding it at definite trays of the column into a distillation Column for the production of pure dimethylether and withdrawal of pure dimethylether as a side -stream and withdrawal of contaminations as a sidestream at definite trays of the same column.

Description

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Backaround of the invention Up to the development of low pressure methanol synthesis processes dimethylether has been obtained in a quantity of 2 - 5 weight-% as a side-product in high-pressure methanol production units and has been isolated by distillation from mixtures r which contained additional low boiling side-pro-ducts.

After introduction of lo~ pressure methanol processes, which yield only negligible quantities of ~imethylether, special synthetic processes have been developed, based on the cata-lytic dehydration of methanol.
Numerous processes have been disclosed in the patent litera-ture. For example, according to DE-PS 680 328, aliphatic ethers are obtained by heatinq alcohols in the presence of zinc chloride.

~ 1 ~32~

Other suitable catalysts for the production of ethers from alcohols are according to GB-PS 332 756, Gs-PS 350 010l GB-PS 403 402, US-PS 1 873 537 and FR-PS 701 335, ferrous and ferric chloride~ copper sulfate, stannic and stannous chloride, manganese chloride, aluminum chloride and -sulfate, chromic sulfate, alum, thorium con~pounds, aluminum oxide, titanium oxide, bariu~n oxide, silica or aluminum phospate.

In "Industrial and Enginnering Chemistry", Vol. 41, No. 12, page 2928 (1949) use of bauxite with a SiO2 portion of 4,40 -13,99 weight-% is described.
In US-PS 3 036 134 an aluminum silicate-catalyst is dis-closed for the production of dimethylether from methanol, with a ratio of Al2O3 : SiO2 of 1 part : 1,35 - 0,3 parts.
The synthesis of dimethylether directly from synthesis gas ~CO ~ H2) has also been described (DE-PS 23 62 944, DE-PS
27 57 788 and DE-PS 32 20 547).

The technically most important cataLysts have turned out to be aeeording to DE-PS 28 18 831, DE-OS 32 01 155, EP-A
0 099 676 and EP-A 0 124 078 in parl:icular, aluminum oxide and aluminum silieate eatalysts with and without doping.
ïn DE-PS 28 18 831 a eatalyst fox the production of di-methylether is disclosed, whieh can contain any aluminum oxide as a base material, as far as it possesses a suffi-eiently large surfae~ and additives of 1 to 30 weight-~ of rare earthes.

Finally in EP-A 0 ~99 676 a eatalyst is disclosed, which eontains 1 - 20 weight-~ of SiO2, preferably 1 - 10 weight-%
of SiO2 and more preferably 6 weight-% of SiO2.

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_ 3 _ UK 372/373 Crude dimethylether thus obtained contains reaction water, unreacted methanol as well as small quantities of contami-nations, like methyl formate, hydrocarbons, amines and sul-fides, carboxylic acids and esters, amides, acetales ana others.

In synthesis units of the state of the art, crude dimethyl-ether is worked up in two distillation col~mns connected in series. In the first column,~hich operates under pressure, pure dimethylether is obtained. In the s~cond one unreacted methanol is distilled off. Thus in EP-A 0 124 078 a process is described, according to which dimethyleth~r is drawn off in a first column, which is operated under pressure as a sidestream, where as in a second column, which is operated under lower pressure, contaminations with boiling points between dimethyletherand methanol,are drawn off overhead Methanol is obtained in the same column as a sidestream.
Catalysts are ~l2O3, SiO2,aluminum silicates and preferably ~ -Al2O3.

Although by this process dimethylet:her of higher purity com-pared to the state of the art is obtained, it has; from an economical point of view, a considerable disadvantage, be-cause not only the first column, but also the second one have to be equipped with a high number of trays.This leads to high in~estment costs and in particular to high costs of operation. Furthermore, there exists a considerable risk, that the contaminations, boiling between dimethylether and methanol, are not completely transferred to the second column, but accumulate in the first column and are drawn off with d;methylether. As a consequence a dimtheylether quality is obtained, which is not free of odour.

.
.i Since dimethylether gains increasing importance as a propellant for sprays, very high demands are made with regard to purity.
Thus, no irritating substanc~.s in dimethylether are permitted in applications like cosmetic, human and household sprays.
Furthermore dimethylether has to be free of odour for these applications~

Object of the instant invention therefore was the production of a highly pure dimethylether by a more economlcal process comparea to the state of the art and to convert methanol nearly quantitatively into a highly pure product which is suitable for the above named applications.

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Preferred embodim~nt Applicant has succeeded in solving the problems desc~ibed above by the instant inventive process. According to~the in-vention highly pure dimethylether is continuously produced by catalytic dehydration of methanol a temperature of 140-500C
and a pressure of 1-50 bar and distillative workup of the de-hydration product, Characterized in that the dehydration product is fed to the distillation column for the production of pure dimethylether below the 25th tray (from the head of the column) at one tray or several trays, I
that pure dimethylether i5 dxawn off above the 15th tray (rom the head of the column), p:referably above the 10th tray, at one tray or several trays, that a fraction, which contains contaminations boiling bet ween dimethylether and methanol, is drawn off at one tray or several trays, which is (are) situated at least 5 trays above the highest feed (dehydration product) tray and at least one tray below the 15tll tray (from the head of the column) and : that the dehydration product is fed to the column at least 1 tray abo~e the bottom of the column, whereby in case of feed, which contains 1-S weight-% of dimethyl-ether, the distillation is operated at a reflux ratio of 1:1 to 1:25, whareby in case of feed, which contains 20-80 weight-~ of dimethylether, the distillation is operated at a reflux ratio of 1:0,4 to 1:5, preferably of 1:1 to 1:2,5, ~ one Qspect ~

- 6 - L3~ 2~ 6 whereby in case of feed, which contains 6-19 weight-% oE dimethyl-ether, the distillation is operated at a reflux ratio between 1:0.4 and 1:25 and whereby in case of feed, which contains more than 80 weight-% and < 99 weight-% of dimethylether, the distillation is operated at a reflux ratio between 1:0.01 to 1:5.
According to another aspect of the invention there is provided a process for the production of pure dimethylether by catalytic dehydration of methanol at a temperature of 140-500C
and a pressure o~ 1 to 50 bar, comprising dehydrating methanol in contact with a y-A12O3-catalyst, which contains 0.0001 to < 1 weight % of SiO2.
According to the state of the art, the determination of substances wi-th an unpleasant odour is carried out predominantly by empirlcal methods, in particular by sensory determination by a trained team. Thus, the limit of inconvenience in case of H2S
has for example been identified in the Federal Republic of Germany by 150 persons as 45 ~g/m3. (Schriftenreihe der Landesanstalt fur Immissionsschutz des ~andes Nordrhein-Westfalen, Heft 49 (1979), pa~e 77.) In those cases, when the limit of perception of odour can be analyzed ~y instruments, gaschromotography, electric conductivity, photometry or fluorescence measurement are applied l''Erdol und Kohle-Erdgas-Petrochemie, Vol. 32, Nr. 2, Febr. 1975, page 86). The determina-tions of odour in the instant application are based on sensory methods. Crude methanol from methanol synthesis units as well as dimethylether produced catalytically from methanol, contain as outlined above, numerous contaminations, with, in some cases, strong odour. Whereas from high-pressure-methanol synthesis i~

- 6a - 1322767 units crude methanol is obtained, which in general contains 2-3, however up to 5 welght % of dimethylether, crude dimethylether which ~.

7 ~

is produced from crude or pure methanol in dimethylether syn-thesis units, consits of 20 to 80 weight-% of dimethylether and in addition the above named contaminations, reaction water and unconverted methanol.
Since the boi.ling points of the contaminations, for example of dimethylamine (b.p. 6,9C), dimethyl sulfid~ (b.p. 37,3C), methyl mercaptane (b.p. 5,8C), formic acid (b.p. 100,75C), formic acld methylester (b.p. 31,5C), formaldehyde (b.p.-21C) formaldehyde dimethyl acetale (b.p. 45,5C) or acetic acid methylester (b.p. 56,95C), as well as solubilities and vapor pressures are very different and since the intensity of odour of the individual compounds is also very different and in ad~ition numerous azeotropic mixtures are formed, the object of obtaining highly pure dimethylether in high yield by a more economical process compared to the state of the art, is very difficult to achieve.

The investigations of applicant, which have been carried out during several years in numerous test sequences, in laboratory, pilot plant and technical unit, have non-obviously led to the result that highly pure dimethylether can be produced nearly quantitatively in an particularly economical way by the in-ventive catalysts and the inventive distillative purification.

Figur 1 represents exemplarily a unit for the production and purification of dimethylether.

In Figur 2 exemplarily a distillation column for the pro-duction of pure dimethylether is shown.

In Figur 3 exemplarily a column is shown which is equipped with a side-stripper.

A 7 _ .~

- 8 - ~ 3 2 ~ 76 J

Preferred inventive catalysts are catalysts of the ~-Al2O3-type which contain 0.0001 to < l weight-% of SiO2. Unlike the catalysts of the state of the art, they contain only very small quantities of SiO2 and lead to considerably better results than the known catalysts. The preferred SiO2-concentration is 0.001 to 0.5 weight-% and a particularly preferred concentration is 0.001 to 0.2 weight~6 SiO2.
~dditionally the inventive catalysts may contain other components in small and very small quantities, for example ~a2 or other alkali- and alkaline earth oxides, alkali, alkaline earth- or aluminum sul~ate, iron oxide, cobalt oxide, nickel oxide and other compounds.
The reaction in the presence of the inventive catalysts is carried out at a temperature of 1~0 to 500C~ preferably at 140 or 150 to 450C and pressure of 1 to 50 bar, preferably of l to 25 bar. The reaction may be carried out in the gas or li~uid phase, prefe:rably in the ~as phase. Preferably the pressure in the synthesis reactor and in the distillation columns are ad~usted to each other. Operation is carried out at a liquid hourly space velocity (LHSV) of 0.2-16 1/l h, preferably of 0.5-13.5 l/l h.
The inventive process can be operated discontinuously however preferably continuous.
Reactors, which can be used for the inventive process may be the known reactors of the state of the art/ like fixed bed-, fluid bed- or fluidized bed-reactors, but also modified, new, and improved reactors, which are suited for catalytic reactions.

~ 3 2 2 ~ ~ r~

The reaction in -the presence of catalysts, resp. inventive catalysts can be kinetically or thermo-dynamically controlled, dependent on the Ye~c~ion parameters. As a consequence the corresponding quantities of dimethylether are obtained at the reactor exit besides -the respective unconverted quantities of methanol.

The investigations of applicant have led to the result, that in order to obtain highly pure, odourless dimethylether by a particularly economical process in nearly quantitative yield, crude product from the reactor exit has to be fed to the distillation column for the production of pure dimethylether below the 25th tray (from the head of the column) at one or several trays. If the feed product is introduced into the column at a higher tray, the desired product purity can not be obtained, in particular a product free oE
odour can not be obtained According to the invention crude dimethylether can be fed to the column, depending on the t:otal number of trays in the column, between the 25th and 50th tray (from the head of the column~. For example in case of a column with a total number of 60 trays, feed is introduced between the 32nd and ~6th tray.
Since, according to -the invention, the feed must be introduced at least one trav, preferably 5 trays and particularly 10 trays above the bottom, the feed tray(s) can be chosen between the first tray from the bottom and the 26th tray from the top of the column, however preferably above the 5th and particularly preferable above the 10th tray from the bottom.

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- 9a - 23769-43 Furthexmore~ according to the invention, pure dimethylether is withdrawn from the column from the 15-th tray included up to the top of the col.umn, preferably above the 10th tray at one or several trays.

L32~7~'~
- lo - UK 372/373 This may be for example the 6th tray, it may be alter~
natively a tray between the first and 5th or between the 6th and 15th tray. However it is also possible to with-draw head condensate depending on the feed. As mentioned above, withdrawel of pure dimethylether can also take place at several trays.
Furthermore the investigations of applicant have led to the result that components with boiling points between methanol and dimethylether are withdrawn in the same column at one or several trays, which are located at least 5 trays above the highest feed tray for crude dimethylether resp. feed mixtures of dimethylether, methanol and contaminations resp. water, however below the 15th tray from the top of the column.
In case of a feed product, which for example contains 1 S weight-~ of dimethylether, besides methanol, other components with boiling points between those of methanol and dimethylether and optionally water and other oxygen containing hydrocarbons, like for example alcohols with a number of C-atoms ~1, the distillation column is operated with a reflux ratio of 1:1 to 1:25V depending on the quantity of dimethylether. Thus in case of a dimethylether concentration of 1 weight-%, the reflux ratio may be 1:20.
A reflux ratio of 1:1 means, that the quantity of dimethyl-ether withdrawn, is 1 part and the quantitiy of dimethyl-ether vapour which is condensed at the top condenser of the column is 1+1 part (the first number represents the quantity withdrawn).
A reflux ratio of 1: 5-8 is preferred for example in case of a dimethylether concentration of 3-4 weight-%. These quantities of dimethylether correspond to the concentrations, which are present in crude high-pressure methanol.

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~ UK 372 / 373 Dimethylether can only be obtained in highly pure, nearly ~uantitative yield, if the inventive feed trays for crude dimethylether and the trays for withdrawal of pure dimethylether and contaminations boiling between methanol and dimethylether, are chosen according to the inverltion .
If the concentration of dimethylether is 20-~o weight-%
in the feed and also methanol, contaminations boiling between methanol and dimethylether and optionally water and other ~xygen ccntaining hydrocarbons like for example alcohols with a number of C-atoms ~1 are present, the reflux ratio is 1:o,4 to 1:5, preferably 1:1 to 1:2,5 depending on the concentration of dimethylether. For example in case of a mixture of 60 weight~-~ of methanol, besides water and contaminations, a reflux ratio of 1:1,5 to 1:2,5 may be chosen.
Such feeds are typical mixtures, which are obtained at the reactor exit in synthesis-processes for the catalytic pro-duction of dimethylether from methanol in the presence of catalysts like A1203- and A1203/SiO2-Catalysts.
In case of dimethylether concent.rations between 5 to 20 weight-~ respectively ~80 weight-%, the inventive reflux ratios have to be selected on the basis of the data out-lined above. For example, in case of a dimethylether con-centration of 5-20 weight-% r the reflux ratio is chosen between 1:o,4 and 1:25 and in case of ~80 weight-% between l:o,ol and 1:5.
The distillation column for the production of pure dimethyl-ether is gen.erally operated at a pressure of 5-lo bar, where-by the pressure is preferably adapted to the preceeding syn-thesis reactor. However pressure beyond 5-lo bar can also be applied according to the invention.
Throughput is determined as usual by column design, heat supply and reflux ratio.
~' , , .
.~ I I ,i ~27~7 In order to obtain pure dimethylether nearly quantitatively, in partieular in case of small concentrations in the feed product, waste gas, which leaves the column at the top and which contains small quantities of dimethylether mixed essentially with C02, N2 and hydrocarbons, is washed in a washing device. The washing liquid can be recycled to the distillation column or to the synthesis reactor.
Suitable washing liquids are for example methanol or bottoms of the dimethylether distillation column.
The waslling process can be operated in parallel flow or countercurrently.
The distillate, which is withdrawn as a sidestream and which contains the contaminations can be stripped i~n a side-stripper.
The stripping product, containing small quantities of dimethyl-ether, ean be recycled to the column.
Methanol can be separated from wa-ter in a stripping column, which succeeds the dimethylether column.
Aeeording to the invention, distillation eolumns of the state of the art with the eapacity desired, can be used.
Trays can be those of the state of the art, for example valve trays, sieve trays, bubble trays and other trays of the state of the art.
In prineiple also paeked eolumns ean be used, whieh are equipped for example with ceramic materials, glass materials, wire mesh and other materials of the state of the art.
In case of packed eolumns the inventive locations for feeding erude produet and withdrawal of dimethylether and contamin~
ation, ean be caleulated from the required number of trays aeeording to the invention.
The invention is described in more detail with the aid of figures 1 to 3.

A ~

., :~3`227~33 In figure 1 ~3) represents the dehydration (synthesis) reactor. Fresh methanol is fed to the reactor through pump (4~, pip~ (18) and heat exchangers (8) and (7).
(1) is the distillation column for the production of pure dimethylether. The column usually is operated at a pressure, which is related to the synthesis reactor, in general at a lower pressure. If for example, the synthesis pressure is 8-12 bar, the column can advantageously be operated for ex-ample at 6 11,5 ~ar.
These data are however not to be considered as limiting~ In general it is preferred to operate reactor and distillation Column at a pressure di~ference of 0 to 10 bar.
Highly pure dimethylether is wi.thdrawn at (9). Head vapours are fed through (11) tolcondenser (6). Reflux is recycled through (12) to column (1).
Waste gas is introduced through (15) .into washing device (5), where .it i9 washed with methanol (16). In principle other washing liquids can also be used like for example crude methanol or bottoms of column (1~. In the latter case the washing liquid~ which contains d.imethylether is recycled to column (1).
Small quantities of dimethylether are fed to reactor ~3) through (17), mixed with washing methanol.
~aste gas is withdrawn at (23).
Synthesis product (19) is fed to (1) through heat exchanger(~l.
At (20) contaminations, boiling between methanol and dimethyl-ether are withdrawn and can ~e incinerated for example.
Bottoms of (1), essentially containing methanol and water are fed to stripping column (2) through (14).
The stripping column usually is operated at normal pressure, however it may also be operated at higher or lower pressure.
In general the pressure is lower than in column (1). Methanol is withdrawn through (lo) and recycled to the synthesis r~actor.

~3 :., ~ 3227 f~ ~1 From the bottom of 12), waste water is withdrawn. If necessary, contaminations with boiling points higher than the boiling point of methanol, can be withdrawn at (21).
In _gure 2, (l) represents the distillation column for the pro-duction of pure dimethylether. Feed product is introduced at 12).
Pure dimethylether is withdrawn at (3).
Contaminations are withdrawn at ~4~.
Head vapours are fed through (5~ to condenser (lo).
Reflux is recycled through (7) to column (l).
~aste gas is discharged at t6). (8) represents the reboiler cycle. At (9) bottoms are withdrawn.
In igure 3 the fraction, which contains the contaminations, is passed to stripper t5) through (4). (5) is equipped with reboiler cycle (7).
Stripped dimethylether is recycled to column (l) through (6).
~t (2) feed product is introduced. At (3) pure dimethylether is withdrawn. Head vapours pass through (ll) to condenser tlo~.
Reflu~ is recycled through (8) to co:Lumn (l).
Waste gas is dischar~ed at (9).
At (12) contaminations free of dimethylether are withdrawn.
According to the instant invention h:ighly pure dimethylether is obtained from the distillation column nearly quantitatively.
D methylether thus obtained is free of odour~ contains less than lo ppm of methanol, a maximum of o,l weight-~ of hydrocarbons and is of a purity of up to 99,9 wei~ht~% of dimethvlether.
Dimethylether obtained according to the invention is excellently suited for any application in the field of aerosol sprays.
Furthermore compared to the state of the art, only one highly efficient distillation column is needed, whereas it is sufficient to operate a second column for the separation of methanol from water with only low separation efficiency.
In addition, in the second column optionally contaminations with boiling points between methanol and water can be withdrawn as a side-stream. As a consequence waste water is obtained with only small quantities of contaminations, which can be~purified more A eaSily ~ - ' ~ 3~?~ 7~ P~

Although in principle, operation of additional columns is possible, a less economical process would result.
Examples Work-up of crude synthesis product (3000 kg/h with 65 weight-%
of dimethylether) was carried out in examples 1-9 in-two succeeding continuously operating distillation columns, the first of which was e~uipped with 50 valve trays, whereas the second one was a column packed with ceramic Raschig rings.
In the first column which was operated at 7 bar, highly pure dimethylether was withdrawn at the 12th tray (from the top of the column) at a reflux ratio of 1:2.
Crude product was fed to the column at the 22nd tray. In the second column, unconverted methanol was recovered.
The dimethylether synthesis was carried out at 27c-290C and a pressure of lo bar.
In example 1 a ~-A1203-catalyst was used, whieh contained o,ol8 weight-% of SiO2.
Pure dimethylether was obtained nearly quantitatively, (99,8 weight-% based on methanol corlverted) which was free of odour.
In example 2 a~CAl 0 -catalyst was used with a SiO -content of o,ooS weight-%, however a reflux ratio of l:o,4 was applied and the crude feed to the distillation column contained 80 weight-% of dimethylether.
As in example 1 pure, odourless dimethylether was obtained nearly ~uantitatively.
In example 3 the synthesis reaction was carried out with a ~-A1203-catalyst, containing o,4 weight-% of SiO2.
The same xesult as in examples 1 and 2 was obtained.
In e~ample 4 the ~-~1203-catalyst used, contained o,o25 weight % of SiO2 and o,c2 weight--~ of Fe2o3.

1S-, ., :~32 ~P~

Dimethylether ~hus obtained was ~ree of odour. The yield was nearly quantitiative based on methanol converted.
In example S the same catalyst was used as in example 4, however the dimethylether distillation column was packed with wire mesh. Feed product was introduced at the calcu-lated position and dimethylether and contaminations were withdrawn at the calculated positions.
A pure, odourless dimethylether could be obtained in nearly quantitative yield.
In comparative example 6 a ~-A12-03-catalyst with o,o2 weight-% of SiO2 was used.
The contaminations boiling between dimethylether and methanol were withdrawn at the top of the second column.
No dimethylether free of odour could be obtained in this case.
In comparative example 7 , example 6 was repeated, however the catalys-t contained 6 weight-~ of SiO2.
~o dimethylether ~ree of odour could be obtained.
In example 8 pure dimethylether was withdrawn at the 3rd tray (~rom the top of the column) at a distillation pressure of 7 bar and a reflux ratio of 1:3.
Feed was introduced at the 30th (~as phase) and at the 36th tray (liquid) (from the top of the column).
Contaminations were withdrawn at the 18th tray (from the top of the column ).
d~ A1203-catalyst with 6 weight-~ of SiO2 was used.
Pure odourless dimethylether was obtained.
In example 9 , example 8 was repeated, howe~er re~lux in the distillation column was 1:4.
As a ~atalyst aluminum silicate was used. Pure, odourless dimethylether was obtained.
In the following examples 10-14, dimethylether synthesis was carried out at a temperature of 260-280C and a pressure which was 1-~ bar higher than in the dimethylether distillation column.
The latter was operated continuously.

16 , .i ~ 3 ~ J

Example lo 4000 kgJh consisting of a mixture of 2400 kg of dimethyl-ether , 580 kg of methanol, 91o kg of water and llo kg of contaminations was fed to a distillation column equipped with 65 valve trays at the 49th tray (from the top of the column).
The column was operated at 8 bar and a reflux ratio of 1 : 1 , 9 . , From the top of the column approx. 30 m3 of waste gas were withdrawn, consisting essentially of C02, N2, hydrocarbons and a small quantity of dimethylether.
At the 6th tray from the top, 2385 kg/h of pure dimethylether were withdrawn with lo ppm of methanol.
At the 35th tray from the top, 9o kg/h of contaminations were withdrawn.
From the bottom a mixture of 580 kg/h of methanol, lo kg/h of higher boiling components and 91o kg/h of water were withdrawn and fed to a second column, where methanol was distilled off.

Example 11 Example lo was repeated, however the contaminations were passed to a side~stripper.
2395 kg/h of pure dimethylether and 80 kg/h of contaminations at the stripper exit were obtained.

Example 12 A mixture of 61 ooo kg/h consisting of 55000 kg of methanol, 2000 kg of dimethylether, 3500 kg of water and 500 kg of con-taminations were fed to a distillation column, equipped with loo valve trays.
The column was operated at a reflux ratio of 1:7 and a pressure of 6 to 8 bar. Feed was passed to the 35th tray from the top.
~7 , ;~22~

1996 kg/h of pure dimethylether were withdrawn at the 9th tray from the top.
From the bottom a mixture was withdrawn, consisting of 55000 kg/h of methanol, 3500 kg/h of water and l9o kg/h of higher boiling contaminations, consis~i~g mainly of alcohols with a number of C-atoms ~1.
12 kg/h of waste gas were washed with methanol counter-currently.
Example 13 4000 kg/h of a mixture consisting of 800 kg of dimethylether, 2825 kg of methanol, 300 kg of water and 75 kg of conta-minations boiling between methanol and dimethylether, was fed to a distillation column, equipped with 70 bubble trays at the 48th tray from the top.
The reflux ratio was 1:2.
At the 4th tray from the top, 796 kg/h of pure dimethylether and at the 40th tray, 77kg/h of contaminations were wi-thdrawn.
2825 kg/h of methanol and 300 kg/h of water were withdrawn from the bottom of the column.
Example 14 2000 kg/h of a mixture consisting of 1750 kg of dimethylether, loo kg of methanol, loo kg of water and 50 kg of contaminations were fed to a distillation column, equipped with 45 bubble trays at the 34th tray from the topO
The reflux ratio was l:o,5.
1745 kg/h of pure dimethylether were withdrawn at khe 3rd tray from the top. 48 kg/h of contaminations were withdrawn at the 29th tray from the top.
loo kg/h of water, loo kg/h of methanol and 5 kg/h of higher boiling contaminations were withdrawn from the bottom.
In examples lo - 14, a highly pure odourless dimethylether in practically quantitative yield was obtained.

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Example 15 Example 13 was repeated, however dimethylether was withdrawn at the 18th tray from the top.
Contaminations were withdrawn at the 45th tray from the top.
No pure, odourless dimethylether could be obtained.

Claims (14)

1. A process for the continuous production of pure dimethylether by catalytic dehydration of methanol at a tempera-ture of 140-500°C and a pressure of 1- 50 bar and distillative work-up of the dehydration product, which process comprises introducing the dehydration product feed into the purification column below the 25th tray (from the top of the column) at one or several trays, wherein pure dimethylether is withdrawn from the column from the 15th tray included up to the top of the column, at one or several trays, a fraction, containing impurities, boiling between dimethylether and methanol, is with-drawn at one or several trays which are located at least 5 trays above the highest feed tray and at least one tray below the 15th tray and that the dehydration product is fed to the column at least one tray above the bottom of the column wherein in the case of a feed product which contains 1-5 weight-% of dimethyl-ether, the column is operated at a reflux ratio of 1:1 to 1:25 and wherein in the case of a feed product which contains 20-80 weight-% of dimethylether, the column is operated at a reflux ratio of 1:0.4 to 1:5 and wherein in the case of a feed product which contains 6-19 weight-% of dimethylether, the column is operated at a reflux ratio of 1:0.4 to 1:25 and wherein in the case of a feed product which contains > 80 weight-% and < 99 weight-% of dimethylether, the column is operated at a reflux ratio of 1:0.01 to 1:5.

2. A process according to claim 1, wherein the dehydration is carried out in the presence of a .gamma.-Al2O3-catalyst, which contains, 0.0001 to < 1 weight-% of SiO2.

3. A process according to claim 1, wherein the dehydration is carried out in the presence of a .gamma.-Al2O3-catalyst which contains 0.001 to 0.5 weight-% of SiO2.

4. A process according to claim 1, wherein the dehydration is carried out in the presence of a .gamma.-Al2O3-catalyst which contains 0.001 to 0.2 weight-% of SiO2.

5. A process according to claim 1, wherein the dehydration is carried out at a temperature of 140°C to 450°C and a pressure of 1 to 25 bar.

6. A process according to claim 1, wherein the liquid hourly space velocity (LHSV) is 0.2 to 16 1/1h.

7. A process according to claim 1, wherein the liquid hourly space velocity (LHSV) is 0.5 to 13.5 1/1h.

8. A process according to claim 1, wherein the light components which are withdrawn from the top of the column are washed with methanol in parallel flow or countercurrently.

9. A process according to claim 1, wherein the fraction, which contains impurities and which is withdrawn from the column below the tray from which dimethylether is withdrawn, is stripped in a side-stripper and dimethylether separated in the side-stripper, is recycled to the column.

10. A process according to claim 1, wherein the dehydration product is fed to the column at one or several trays between the 25th and 50th tray (from the top of the column) depending on the total number of trays.

11. A process according to any one of claims 1 to 10, wherein the dehydration product is fed to the column at least 5 trays above the bottom of the column.

12. A process according to any one of claims 1 to 10, wherein the dehydration product is fed to the column at least 10 trays above the bottom of the column.

13. A process according to any one of claims 1 to 10, wherein pure dimethylether is withdrawn from the column above the 10th tray (from the top of the column).

14. A process according to any one of claims 1 to 10, wherein in the case in which the feed to the purification column contains 20-80 weight-% of dimethylether, the column is operated at a reflux ratio of 1:1 to 1:2.5.