PRIOR ART
[0002] Dimethyl ether (DME) is the structurally simplest ether. Dimethyl ether
contains two methyl groups as organic moieties. Dimethyl ether is polar and is
15 conventionally used in liquid form as solvent. Dimethyl ether can also be used as
coolant and replace conventional chlorofluorocarbons.
[0003] Recently, dimethyl ether is increasingly used as a substitute for fuel gas
(liquefied gas) and conventional fuels such as diesel. Due to its comparatively
20 high cetane number of 55 to 60, conventional diesel engines, for example, only
have to be modified slightly for operation with dimethyl ether. Dimethyl ether
burns comparatively cleanly and without soot formation. If dimethyl ether is
prepared from biomass, it is considered to be a so-called biofuel and can therefore
be marketed at lower tax rates.
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[0004] Dimethyl ether can be produced either directly from methanol or indirectly
from natural gas or biogas. In the latter case, the natural gas or biogas is first
reformed to form synthesis gas. Synthesis gas is a gas mixture which contains at
least carbon monoxide and hydrogen in changing portions, but which together
30 make up the predominant part of the gas mixture. Synthesis gas can also be
obtained by means of other methods, for example by pyrolysis of coal, oil,
carbonaceous waste, biomass or other carbon-containing starting materials, by dry
reforming of natural gas with carbon dioxide, by steam reforming of natural gas,
by autothermal reforming (ATR) of natural gas, by partial oxidation (POX) of
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hydrocarbons, in particular natural gas or methane, or combinations of the
aforementioned methods. The synthesis gas is then converted either in a two-stage
reaction into methanol and then into dimethyl ether or in a single-stage reaction
directly into dimethyl ether.
5
[0005] The synthesis of dimethyl ether from synthesis gas is thermodynamically
and economically more advantageous than synthesis from methanol.
[0006] The present invention relates in particular to the single-stage synthesis of
10 dimethyl ether, wherein a “single-stage” synthesis is understood to mean a
synthesis in which all reactions proceed in the same reactor. The single-stage
synthesis of dimethyl ether is known, for example, from US 4,536,485 A and US
5,189,203 A. Hybrid catalysts are conventionally used in this case. The reaction is
exothermic and typically takes place at a temperature of 200 to 300°C and at a
15 pressure of 20 to 100 bar.
[0007] For single-stage synthesis of dimethyl ether, normally upright tubular
reactors are used, which are charged from below with pressurized and heated
synthesis gas. A product stream obtained in the tubular reactor is withdrawn from
20 the top, cooled and fed to a separation process.
[0008] Besides dimethyl ether, the product stream contains unreacted components
of the synthesis gas and further reaction products. Typically, the product stream
comprises, in addition to dimethyl ether, at least methanol, water, carbon dioxide,
25 carbon monoxide and hydrogen, and, in lower amounts, methane, ethane, organic
acids and higher alcohols.
[0009] In a gas mixture formed from the product stream, carbon dioxide and
components which boil more easily than carbon dioxide, such as hydrogen and
30 carbon monoxide, are therefore typically present in addition to dimethyl ether.
These must be at least partially separated out to obtain dimethyl ether conforming
to specifications. Methods used for this purpose, however, prove to be
unsatisfactory, in particular in terms of energy.
4
[0010] To obtain dimethyl ether from the product stream, the latter must be cooled
to temperatures significantly below 0°C. It may be necessary here to separate out
relatively large amounts of methanol and water before cooling. However, WO
2015/104290 A1 also discloses methods in which such a separation is not
5 necessary.
[0011] An improved recycling concept for unreacted synthesis gas is to be
specified.
10 DISCLOSURE OF THE INVENTION
[0012] This object is achieved by a method for preparing dimethyl ether (DME), a
plant for the separation processing of a gas mixture and a plant for carrying out
such a method, having the features of the respective independent claims.
15 Advantageous developments of the invention form the subject matter of the
dependent claims and the description below. Before explaining the features and
advantages of the present invention, the principles and the terms used are
explained.
20 [0013] A fluid (the term “fluid” is also used below for short for corresponding
streams, fractions, etc.) is “derived” from another fluid (which is also referred to
as the starting fluid) or “formed” from such a fluid when it has at least some
components contained in the starting fluid or obtained therefrom. A fluid derived
or formed in this sense can be obtained or formed from the starting fluid by
25 separating or branching off a portion or one or more components, enriching or
depleting with respect to one or more components, chemically or physically
reacting one or more components, heating, cooling, pressurizing, and the like. A
stream can also be simply “formed”, for example, by being drawn off from a
storage tank.
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[0014] Fluids can, in the terminology used herein, be rich or low in one or more
contained components, wherein “rich” can refer to a content of at least >50%,
75%, 90%, 95%, 99%, 99.5%, 99.9%, 99.99% or 99.999%, and “low” can refer to
a content of at most 10%, 5%, 1%, 0.1%, 0.01% or 0.001% on a molar, weight, or
5
volume basis. In the terminology used herein, they can be enriched with or
depleted of one or more components, wherein these terms relate to a
corresponding content in a starting fluid from which the fluid was formed. The
fluid is “enriched” if it contains at least 1.1 times, 1.5 times, 2 times, 5 times, 10
times, 100 times or 1000 times the content, 5 and “depleted” if it contains at most
0.9 times, 0.5 times, 0.1 times, 0.01 times or 0.001 times the content of a
corresponding component, in relation to the starting fluid. A fluid containing
“predominantly” one or more components contains said one or more components
in a percentage of at least >50%, 75%, 90%, 95%, 98% or 99% or is rich in them.
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[0015] The terms “pressure level” and “temperature level” are used below to
characterize pressures and temperatures, whereby it is intended to be expressed
that pressures and temperatures do not need to be used in the form of exact
pressure or temperature values in order to realize an inventive concept. However,
15 such pressures and temperatures typically fall within certain ranges that are, for
example, ± 1%, 5%, 10%, 20% or even 50% around an average. Different
pressure levels and temperature levels may be in disjoint ranges or in ranges
which overlap one another. In particular, pressure levels, for example, include
unavoidable or expected pressure losses, for example due to cooling effects. The
20 same applies to temperature levels. The pressure levels indicated here in bar are
absolute pressures.
[0016] In the terminology used herein, a “distillation column” is a separation unit
which is configured to separate at least partially a substance mixture (fluid)
25 provided in gaseous or liquid form or in the form of a two-phase mixture with
liquid and gaseous portions, possibly also in the supercritical state, i.e., to produce
pure substances or substance mixtures from the substance mixture, which are
enriched or depleted in relation to the substance mixture with respect to at least
one component or are rich or low in the sense explained above. Distillation
30 columns are well-known from the field of separation technology.
[0017] Distillation columns are typically designed as cylindrical metal containers
which are equipped with fittings, such as sieve trays or ordered or disordered
packings. A distillation column is characterized, inter alia, in that a liquid fraction
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is deposited in its lower region, also referred to as the bottom. This liquid fraction,
which is referred to here as the bottom liquid, is heated in a distillation column by
means of a bottom evaporator, so that some of the bottom liquid is continuously
evaporated and rises in gaseous form in the distillation column. A distillation
column is also typically provided with a 5 so-called top condenser, into which at
least some of a gas mixture enriched in an upper region of the distillation column
or a corresponding pure gas, referred to here as top gas, is fed, liquefied in part to
form a condensate, and provided as a liquid return at the top of the distillation
column. Some of the condensate obtained from the top gas can be used elsewhere.
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[0018] In contrast to a distillation column, an “absorption column” typically does
not have a bottom evaporator. Absorption columns are also generally known from
the field of separation technology. Absorption columns are used for absorption in
the phase counter-current and are therefore also referred to as counter-current
15 columns. During absorption in the countercurrent, the releasing gas phase flows
upwards through an absorption column. The receiving solution phase, provided
from above and withdrawn at the bottom, flows towards the gas phase. The gas
phase is "washed" with the solution phase. Also typically provided in a
corresponding absorption column are built-in components which ensure a step-by20
step (trays, spray zones, rotating plates, etc.) or continuous (random filling of
filling material, packings, etc.) phase contact. At the top of such an absorption
column, a gaseous fluid is obtained which can be drawn off therefrom as a “top
product.” A liquid which can be drawn off as “bottom product” is deposited in the
bottom of the absorption column. The gas phase is depleted in the absorption
25 column with respect to one or more components, which pass into the bottom
product.
[0019] For the design and specific configuration of distillation columns and
absorption columns, reference is made to relevant textbooks (see, for example,
30 Sattler, K.: Thermische Trennverfahren: Grundlagen, Auslegung, Apparate, 3rd
edition 2001, Weinheim, Wiley-VCH).
[0020] Where a “synthesis” of dimethyl ether is mentioned for short below, this is
understood to mean a process in which a synthesis gas or syngas-containing input,
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i.e., a gas mixture which contains at least carbon monoxide and hydrogen in
suitable portions, is converted into a corresponding product stream containing
dimethyl ether. The portion of hydrogen in the synthesis gas can be increased, for
example, using a water-gas shift reaction. In this case, carbon monoxide present in
the synthesis gas reacts with water (steam) 5 added for this purpose to form carbon
dioxide, which can be separated out if necessary. A synthesis gas aftertreated in
this way is also referred to as a shifted synthesis gas. A corresponding product
stream of the synthesis of dimethyl ether contains not only dimethyl ether but also
further compounds, due to the incomplete reaction and due to the occurrence of
10 side reactions during the synthesis of dimethyl ether, in particular depending on
the characteristics of the catalysts used and the respective contents of the
components of the synthesis gas. These are at least methanol, water, carbon
dioxide, carbon monoxide and hydrogen, but typically also lower amounts of
methane, ethane, organic acids and higher alcohols. Said further compounds have
15 to be separated out, as mentioned. The separation is carried out on the one hand to
enable subsequent separation steps and on the other hand to obtain dimethyl ether
in the required purity, i.e., “conforming to specification.”
Advantages of the invention

We Claim:
1. A method for preparing dimethyl 5 ether (abbreviation: DME) from
synthesis gas, wherein an input (c), which is formed using shifted and/or nonshifted
synthesis gas (b), undergoes a catalytic conversion, thereby forming a
product stream (d), wherein the product stream (d) undergoes a first separation,
wherein a gas mixture (k) is formed by at least partial separation of methanol
10 and/or water from the product stream (d), and the gas mixture (k) is partially
condensed at a first pressure level by means of cooling from a first to a second
temperature level, characterized in that a portion (s) of the gas mixture (k)
remaining in gaseous form at the second temperature level is washed in an
absorption column (16) with a return (v) predominantly containing dimethyl ether,
15 wherein the return (v) predominantly containing dimethyl ether is formed at least
partially from a portion of the gas mixture (k) condensed during cooling, and
wherein the gaseous portion (x) of the gas mixture (k) not washed out in the
absorption column (16) is at least partially transferred into the input (c), and a
dimethyl ether product (z) is formed using the portion (I, q, r, w) of the gas
20 mixture (k) condensed during cooling.
2. The method according to claim 1, in which the gas mixture (k) is cooled to
the second temperature level via multiple intermediate temperature levels,
wherein multiple condensates (I, q, r, w) are deposited.
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3. The method according to any of the preceding claims, in which the portion
(s) of the gas mixture (k) remaining in gaseous form at the second temperature
level is washed in the absorption column (16) with the return (v) predominantly
containing dimethyl ether, thereby obtaining a top product (x) and a bottom
30 product (w), wherein the return (v) predominantly containing dimethyl ether is
formed at least partially from the bottom product (w) and/or partially from the top
product (x).
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4. The method according to any of the preceding claims, in which the return
(v) predominantly containing dimethyl ether is formed using a dimethyl ether
carbon dioxide distillation column (9).
5
5. The method according to claim 4, in which the dimethyl ether carbon
dioxide distillation column (9) is operated in such a way that a top gas
predominantly containing carbon dioxide forms at the top, and a bottom liquid
enriched with dimethyl ether forms at the bottom.
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6. The method according to claim 5, in which some of the dimethyl ether
product (z) which is formed from at least some of the bottom liquid of the
dimethyl ether carbon dioxide distillation column (9) is used as the return (v)
predominantly containing dimethyl ether.
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7. The method according to claim 5 or 6, in which at least some of the
bottom liquid of the dimethyl ether carbon dioxide distillation column (9) is
drawn off as a dimethyl ether product (z) having a dimethyl ether content of more
than 90 mole percent, in particular more than 95 mole percent, in particular more
20 than 98.5 mole percent.
8. The method according to any one of claims 4 to 7, in which the dimethyl
ether carbon dioxide distillation column (9) is operated at a second pressure level
below the first pressure level.
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9. The method according to any of the preceding claims, in which the return
(v) predominantly containing dimethyl ether has a carbon dioxide content of at
most 5 percent by weight, in particular at most 2 percent by weight, at most 1
percent by weight, at most 0.5 percent by weight or at most 0.1 percent by weight.
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10. The method according to any of the preceding claims, wherein a portion,
not transferred into the input (c), of the gaseous portion (x) of the gas mixture (k)
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not washed out is discharged in part from the method as a purge stream (i),
wherein the purge stream (i) is subjected to a wash with at least partially liquid
carbon dioxide, which is provided in particular using the dimethyl ether carbon
dioxide distillation column (9), thereby obtaining a wash stream, and the wash
stream is at least partially conveyed back 5 into the dimethyl ether carbon dioxide
distillation column (9).
11. The method according to any of the preceding claims, in which the first
temperature level is 10 to 50°C, in particular 20 to 40°C, and/or in which the
10 second temperature level is 0.5 to 20°C, in particular 1 to 10°C above the melting
temperature of carbon dioxide at the first pressure level, and/or in which the first
pressure level is 20 to 100 bar, in particular 30 to 80 bar.
12. The method according to any of the preceding claims, wherein the
15 synthesis gas (b) is transferred into the input (c) via the absorption column (16)
together with the gaseous portion (x) of the gas mixture (k) not washed out.
13. The method according to any of the preceding claims, wherein a top gas
(x') of the absorption column (16) is at least partially condensed and the
20 condensed portion of the top gas x' is at least partially conveyed back into the top
region of the absorption column (16).
14. A separation plant which is configured for the separation processing of a
gas mixture (k) which can be formed from a product stream (d) of a reactor (4) for
25 the synthesis of dimethyl ether from synthesis gas (b), and which contains at least
dimethyl ether, carbon dioxide and at least one further component which boils
more easily than carbon dioxide, with means which are configured to provide the
gas mixture (k) while at least partially separating methanol and/or water from the
product stream (d) and to cool it at a first pressure level from a first to a second
30 temperature level and to wash a portion (s) of the gas mixture (k) remaining in
gaseous form at the second temperature level in an absorption column (16) with a
return (v) predominantly containing dimethyl ether, wherein means are further
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provided which are configured to form the return (v) predominantly containing
dimethyl ether at least partially from a portion (I, q, r, w) of the gas mixture (k)
condensed during cooling.
15. A plant (100) for preparing dimethyl ether, 5 comprising at least one reactor
(4) configured for the synthesis of dimethyl ether from synthesis gas (b) and a
separation plant according to claim 14.