Description
The present invention relates to a process for preparing a target compound using
an aldehyde and a corresponding apparatus according to the preambles of the
independent claims.
5
The project that led to this patent application was funded under Grant Agreement
No. 814557 of the European Union's Horizon 2020 research and innovation
program.
10 Prior art
The production of alcohols from aldehydes and in particular propanol via propanal
is known in principle. For example, the article "Propanols" in Ullmann's
Encyclopedia of Industrial Chemistry, 2012 edition, describes a heterogeneous
15 gas-phase process carried out at 110 to 150°C and a pressure of 0.14 to 1.0 MPa
at a hydrogen to propanal ratio of 20:1. Reduction occurs when there is excess
hydrogen and the heat of the reaction is removed by circulating the gas phases
through external heat exchangers or by cooling the reactor internally. Efficiency
with regard to hydrogen is greater than 90%, aldehyde conversion is up to 99.9%,
20 and alcohol yields greater than 99% are obtained. Commonly used commercial
catalysts include combinations of Cu, Zn, Ni, and Cr supported on alumina or
diatomaceous earth. Dipropyl ether, ethane and propyl propionate are mentioned
as major impurities.
25 Details for corresponding liquid phase processes are also given in the literature.
For example, these are carried out at a temperature of 95 to 120°C and a pressure
of 3.5 MPa. The preferred catalysts are typically Ni, Cu, Raney-Ni or supported Ni
catalysts reinforced with Mo, Mn and Na. For example, 1-propanol can be
produced in these liquid phase processes with a purity of 99.9%.
30
An important problem in the purification of propanol is the removal of water from
the product mixture of the process. If, as in one embodiment of the present
2
invention, propanol is dehydrated to propylene, i.e., if the propanol is used only as
an intermediate in the synthesis of an olefin from propanol, water is also one of the
reaction products in this dehydration step, so that no water separation has to be
carried out beforehand. The separation of propylene and water is comparatively
5 simple. However, the separation of components other than water is often more
difficult in this context as well, especially if these other components have a
significantly lower boiling point than the aldehyde and cannot be removed by
condensation.
10 The aldehyde used in a corresponding process can, for example, be provided using
hydroformylation. Typical hydroformylation processes usually assume a relatively
pure feed stream with a high alkene content on a technical scale. Therefore, the
mentioned lighter boiling components, for example carbon dioxide, lighter boiling
hydrocarbons, especially methane, but also unreacted carbon monoxide and
15 hydrogen, are usually not present in significant amounts in the product stream
either. Novel processes for the hydroformylation of alkenes that can also process
less pure and/or dilute feed streams are known, for example, from DE 10 2019 119
543, DE 10 2019 119 562 and DE 10 2019 119 540.
20 Carbon monoxide and carbon dioxide can act as inhibitors in the hydrogenation
process described at the beginning or can also lead to undesirable by-products.
In principle, there are a large number of different processes for converting
hydrocarbons and related compounds into one another, some of which are listed
25 below as examples.
For example, the conversion of alkanes to chain-length-equivalent alkenes by
oxidative dehydrogenation (ODH, also referred to as ODHE in the case of ethane)
is well known. Typically, ODH also forms a chain-length equivalent carboxylic acid,
30 i.e., acetic acid in the case of ODHE, as a co-product. Ethylene can also be
produced by oxidative coupling of methane (OCM).
The production of propylene from propane by dehydrogenation (PDH) is also
known and represents a commercially available and established process. The
3
same applies to the production of propylene from ethylene by alkene metathesis.
This process requires 2-butene as a starting product.
Finally, there are so-called methane-to-olefin or methane-to-propylene (MTO,
5 MTP) processes in which synthesis gas is first produced from methane and the
synthesis gas is then converted to alkenes such as ethylene and propylene.
Corresponding processes can be operated on the basis of methane, but also on
the basis of other hydrocarbons or carbonaceous feedstocks such as coal or
biomass.
10
The production of propylene (propene) is also described in detail in the technical
literature, for example in the article "Propylene" in Ullmann's Encyclopedia of
Industrial Chemistry, 2012 edition. Propylene is conventionally produced by steam
cracking of hydrocarbon feedstocks and conversion processes in the course of
15 refinery processes. In the latter processes, propylene is not necessarily formed in
the desired amount and only as one of several components in a mixture with other
compounds. Other processes for the production of propylene are also known, but
are not satisfactory in all cases, for example in terms of efficiency and yield.
20 An increasing demand for propylene is forecast for the future ("propylene gap"),
which requires the provision of corresponding selective processes. At the same
time, carbon dioxide emissions must be reduced or even prevented. On the other
hand, large quantities of methane are available as a potential feedstock, which is
currently only recycled to a very limited extent and is mainly incinerated.
25
Hydroformylation (also known as oxo synthesis) is, as already mentioned, another
technology used in particular for the production of oxo compounds of the type
mentioned above. Typically, ethylene or propylene are reacted in hydroformylation,
but higher hydrocarbons, in particular hydrocarbons with six to eleven carbon
30 atoms, can also be used. The conversion of hydrocarbons with four and five carbon
atoms is also possible, but of lesser importance. Hydroformylation, in which
aldehydes are first formed, can be followed by hydrogenation. This is also the case
in embodiments of a process according to the invention. Alcohols formed by such
hydrogenation can subsequently be further dehydrated to the respective alkenes.
4
In Green et al, Catal. Lett. 1992, 13, 341, a process for the production of propanal
from methane and air is described. In the process presented, generally low yields
relative to methane are recorded. The process involves oxidative coupling of
5 methane (OCM) and partial oxidation of methane (POX) to hydrogen and carbon
monoxide, followed by hydroformylation. The target product is the aforementioned
propanal, which must be isolated as such. A limitation arises from the oxidative
coupling of methane to ethylene, for which only lower conversions and limited
selectivities are typically achieved at present.
10
The hydroformylation reaction in the process just mentioned is carried out on a
typical catalyst at 115 °C and 1 bar in an organic solvent. The selectivity to the
(undesired) by-product ethane is in the range of about 1% to 4%, whereas the
selectivity to propanal is reported to reach more than 95%, typically more than
15 98%. Extensive integration of process steps or the use of the carbon dioxide
formed as a by-product in large quantities, especially in the oxidative coupling of
methane, is not further described here, so that there are disadvantages compared
to classical processes. Because partial oxidation is used in the process as a
downstream step to oxidative coupling, i.e., there is a sequential interconnection,
20 large amounts of unreacted methane must be handled in the partial oxidation from
the oxidative coupling or separated at great expense.
US 6,049,011 A describes a process for the hydroformylation of ethylene in dilute
streams. The ethylene can be formed in particular from ethane. Propane can be
25 produced as the target product in addition to propanal. Alcohols formed can also
be further dehydrated to alkenes. However, this publication also does not disclose
any further integration or advantageous solution for separating lighter boiling
compounds.
30 The present invention sets out to provide an improved process for the preparation
of target products such as alcohols or alkenes from aldehydes.
Disclosure of the invention
5
Against this background, the present invention proposes a process for preparing a
target compound using an aldehyde and a corresponding apparatus having the
respective features of the independent patent claims. Preferred embodiments of
the present invention are the subject matter of the dependent patent claims and
5 the following description.
According to the invention, the aldehyde serves as a starting compound for the
synthesis of the target compound, optionally via one or more intermediates. The
aldehyde may be provided by means of process steps which may also be part of
10 the present invention. The target compound may in particular be an alcohol, which
may be formed from the aldehyde, in particular by hydrogenation, but may also be
an olefin, which in turn may be prepared from such an alcohol, in particular by
dehydration. In the latter case, therefore, an olefin is produced as the target
product, the aldehyde being converted to the olefin via an alcohol as an
15 intermediate. More generally, in the context of the present invention, the target
product represents an alcohol formed from said aldehyde or a compound formed
in turn from said alcohol. Therefore, when "the alcohol" is referred to hereinafter, it
is the target product or the intermediate product.
20 Regardless of the specific embodiments of the invention, i.e., regardless of which
target product is provided, the aldehyde is thereby provided in the context of the
present invention in a component mixture comprising components boiling lower
than the aldehyde, in particular lower than 0°C.
25 According to the invention, an (alcohol) synthesis feed enriched in the aldehyde
and depleted in the components boiling lower than the aldehyde is formed from the
component mixture using an extractive distillation and subjected to a reaction to
form the alcohol as a target or intermediate product. According to the invention, at
least a portion of the alcohol is thereby used to form an entrainer used in extractive
30 distillation. The reaction to form the target product may also be one of several
synthesis steps on the way to the target product.
In the context of the present invention, the aldehyde and the alcohol are in
particular compounds with linear carbon chains of the same chain length with a
6
terminal aldehyde or alcohol group. The chain length can be 2 to 20, in particular
2 to 10, further in particular 2, 3, 4, 5, 6, or 7. In particular, the aldehyde is propanal
and the alcohol is 1-propanol. If the target compound is a compound formed from
the alcohol and not the alcohol itself, the target compound may in particular be an
5 olefin, in particular propylene in the case just explained. However, the invention is
not limited to this, although the following explanations refer predominantly to these
compounds.
If it is further mentioned here that "an" aldehyde is converted to "an" alcohol and
10 possibly further to "an" olefin, it is understood that the corresponding process
variants may also comprise the processing of several corresponding compounds.
The compounds boiling lower than the aldehyde may in particular be lighter
hydrocarbons, for example methane, ethane or ethylene, and non-hydrocarbons
boiling lower than the aldehyde, such as carbon dioxide, carbon monoxide and
15 hydrogen. In particular, the compounds mentioned boil lower than water in liquid
form.
The invention comprises, as mentioned, providing the aldehyde in a component
mixture containing said components boiling lower than the aldehyde. In particular,
20 the oxidative dehydrogenation and/or oxidative coupling of methane, each followed
by hydroformylation, as explained below, may be used in this regard, but also any
suitable other process that provides an appropriate mixture of components. The
invention is not limited hereby, but the present invention may comprise
corresponding process steps, for example according to the already mentioned DE
25 10 2019 119 543, DE 10 2019 119 562 and DE 10 2019 119 540, as part of the
proposed process and is particularly advantageously also suitable for processes
in which significant amounts of the mentioned components boiling lower than the
aldehyde are contained in the component mixture, for example for processes
comprising a step of oxidative coupling of methane.
30
Oxidative dehydrogenation is, as already mentioned at the outset, a process
known in principle from the prior art. In the context of the present invention, known
process concepts may be used for oxidative dehydrogenation. For example, in
oxidative dehydrogenation in the context of the present invention, a process such
7
as that disclosed in Cavani et al, Catal. Today 2007, 127, 113, is disclosed. In
particular, V, Sr, Mo, Ni, Nb, Co, Pt and/or Ce and other metals containing catalysts
may be used in conjunction with silicate, alumina, molecular sieve, membrane
and/or monolith supports. For example, combinations and/or oxides of
5 corresponding metals, for example MoVTeNb oxides and mixed oxides of Ni with
Nb, Cr and V can also be used in the context of the present invention. Examples
are disclosed in Melzer et al, Angew. Chem. 2016, 128, 9019, Gärtner et al,
ChemCatChem 2013, 5, 3196, and Meiswinkel, "Oxidative Dehydrogenation of
Short Chain Paraffins", DGMK-Tagungsbericht 2017-2, ISBN 978-3-941721-74-6,
10 as well as various patents and patent applications of the applicant.
A typical by-product of oxidative dehydrogenation in essentially all process variants
is the respective carboxylic acid, i.e. in the case of the oxidative dehydrogenation
of ethane, acetic acid, which may have to be separated, but may represent a further
15 value product and is typically present in contents of a few percent (up to the low
two-digit percentage range). Carbon monoxide and carbon dioxide are also formed
in the low percentage range. A typical product mixture of the oxidative
dehydrogenation of ethane has, for example, the following mixture proportions:
20 Ethylene 25 to 75 mole percent
Ethane 25 to 75 mole percent
Acetic acid 1 to 20 mole percent
Carbon monoxide 0.5 to 10 mole percent
Carbon dioxide 0.5 to 10 mole percent
25
In comparison, a typical product mixture of the oxidative coupling of methane has,
for example, the following mixture proportions:
Hydrogen 0.1 to 10 mole percent
30 Methane 20 to 90 mole percent
Ethane 0.5 to 30 mole percent
Ethylene 5 to 60 mole percent
Carbon monoxide 0.5 to 30 mole percent
Carbon dioxide 0.5 to 30 mole percent
8
These figures refer in each case to the dry portion of the product mixture, which
may contain water vapor depending on the process. Other components may be
present in trace amounts, i.e. typically less than 1%. It should be mentioned here
5 that all proportions and quantity ratios described in this disclosure refer to the
amount of substance (which, in the case of gases, usually also corresponds to the
volume fraction), unless explicitly stated otherwise.
Since oxidative coupling of methane is used in a preferred embodiment of the
10 present invention, it is first explained in more detail below. Oxidative coupling of
methane is described in the literature, for example, in J.D. Idol et al, "Natural Gas",
in: J.A. Kent (ed.), "Handbook of Industrial Chemistry and Biotechnology", Volume
2, 12th edition, Springer, New York 2012. However, in principle, within the scope
of the present invention it is also possible to process other, i.e. not provided by the
15 oxidative coupling, is possible and advantageous if these gas mixtures contain one
or more olefins in an appreciable content, for example in excess of 10, 20, 30, 40
or 50 mole percent and up to 80 mole percent (as an individual or sum value) and
carbon monoxide in just such quantity ranges. Where the present invention is
described below with specific reference to the oxidative coupling of methane and
20 ethylene formed in the oxidative coupling, no limitation is intended to be implied
thereby.
According to current knowledge, the oxidative coupling of methane involves a
catalyzed gas-phase reaction of methane with oxygen, in which one hydrogen
25 atom is split off from each of two methane molecules. Oxygen and methane are
activated at the catalyst surface. The resulting methyl radicals first react to form an
ethane molecule. A water molecule is also formed during the reaction. With suitable
molar ratios of methane to oxygen, suitable reaction temperatures and the
selection of suitable catalytic conditions, oxydehydrogenation of the ethane to
30 ethylene, a target compound in the oxidative coupling of methane, then takes
place. In this process, another water molecule is formed. The oxygen used is
typically completely converted in the above reactions.
9
Reaction conditions for the oxidative coupling of methane classically include a
temperature of 500 °C to 900 °C, a pressure of 0.5 MPa to 1 MPa, and high space
velocities. More recent developments are also moving toward the use of lower
temperatures in particular. The reaction can be carried out homogeneously and
5 heterogeneously catalytically in a fixed bed or in a fluidized bed. In the oxidative
coupling of methane, higher hydrocarbons with up to six or eight carbon atoms can
also be formed, but the focus is on ethane or ethylene and possibly also propane
or propylene.
10 Particularly due to the high binding energy between carbon and hydrogen in the
methane molecule, the yields in the oxidative coupling of methane are
comparatively low. Typically, no more than 10 % to 15 % of the methane used is
converted. Moreover, the comparatively harsh reaction conditions and
temperatures required to cleave these bonds also favor further oxidation of the
15 methyl radicals and other intermediates to carbon monoxide and carbon dioxide.
In particular, the use of oxygen plays a dual role here. Thus, methane conversion
is dependent on the oxygen concentration in the mixture. The formation of byproducts is coupled with the reaction temperature, since the total oxidation of
methane, ethane and ethylene occurs preferentially at high temperatures.
20
Although the low yields and the formation of carbon monoxide and carbon dioxide
can be partially counteracted by selecting optimized catalysts and adapted reaction
conditions, a gas mixture formed during the oxidative coupling of methane
contains, in addition to the target compounds such as ethylene and possibly
25 propylene, predominantly unreacted methane as well as carbon dioxide, carbon
monoxide and water. As a result of any non-catalytic cracking reactions that may
take place, considerable amounts of hydrogen may also be present. In the
terminology used here, such a gas mixture is also referred to as the "product
mixture" of the oxidative coupling of methane, although it predominantly contains
30 not the desired products but also the unreacted educt methane and the by-products
just explained.
In the oxidative coupling of methane, reactors can be used in which a catalytic
zone is followed by a non-catalytic zone. The gas mixture flowing out of the
10
catalytic zone is transferred to the non-catalytic zone, where it is initially still present
at the comparatively high temperatures used in the catalytic zone. In particular,
due to the presence of the water formed during the oxidative coupling of methane,
the reaction conditions here resemble those of conventional steam cracking
5 processes. Therefore, ethane and higher kerosenes can be converted to olefins
here. Further kerosenes can also be fed into the non-catalytic zone, so that the
residual heat of the oxidative coupling of methane can be utilized in a particularly
advantageous manner.
10 Such targeted vapor cracking in a non-catalytic zone downstream of the catalytic
zone is also referred to as "post bed cracking". The term "post-catalytic vapor
cracking" will also be used hereafter. Where reference is made below to the fact
that a starting gas mixture used in accordance with the invention is formed or
provided "using" or "with the use of" an oxidative coupling of methane, this
15 specification is not to be understood in such a way that only the oxidative coupling
itself must be used in the provision. Rather, further process steps, in particular
post-catalytic steam cracking, can also be included in the provision of the starting
gas mixture.
20 In the context of the present invention, the alkene formed, for example, in the
oxidative dehydrogenation and/or oxidative coupling of methane can be subjected
to hydroformylation with carbon monoxide and hydrogen to obtain an aldehyde.
Hydroformylation processes are also known from the prior art. Currently, Rh-based
25 catalysts are typically used in such processes, as described in the literature cited
below. Older processes also use Co-based catalysts.
For example, homogeneous Rh(I)-based catalysts with phosphine and/or
phosphite ligands can be used. These can be monodentate or bidentate
30 complexes. Reaction temperatures of 80 to 150 °C and corresponding catalysts
are typically used for the production of propanal. All processes known from the
prior art can also be used within the scope of the present invention.
11
Hydroformylation typically operates with a hydrogen to carbon monoxide ratio of
1:1, but this ratio can generally range from 0.5:1 to 10:1. The Rh-based catalysts
used may have an Rh content of 0.01 to 1.00 wt%, and the ligands may be present
in excess. Further details are described in the article "Propanal" in Ullmann's
5 Encyclopedia of Industrial Chemistry, 2012 edition. The invention is not limited by
the above process conditions.
In another process, as described for example in the chapter "Hydroformylation" in
Moulijn, Makee & van Diepen, Chemical Process Technology, 2012, 235, a
10 pressure of 20 to 50 bar is used for an Rh-based catalyst and a pressure of 70 to
200 bar for a Co-based catalyst. Co also appears to be relevant to hydroformylation
in metallic form. Other metals are more or less insignificant, in particular Ru, Mn
and Fe. The temperature range used in the above process is between 370 K to
440 K.
15
In the process disclosed in the chapter "Synthesis involving Carbon Monoxides" in
Weissermel & Arpe, Industrial Organic Chemistry 2003, 135, mainly Co- and Rhphosphine complexes are used. With specific ligands, hydroformylation can be
carried out in aqueous medium and recovery of the catalyst is straightforward.
20
According to Navid et al, Appl. Catal. A 2014, 469, 357, in principle all transition
metals capable of forming carbonyls can be used as potential hydroformulation
catalysts, with activity according to this publication being observed according to Rh
> Co > Ir, Ru > Os > Pt > Pd > Fe > Ni.
25
By-products of hydroformylation are formed in particular by the hydrogenation of
the alkene to the corresponding alkane, e.g. from ethylene to ethane, or the
hydrogenation of the aldehyde to the alcohol, i.e. from propanal to propanol.
According to the article "Propanols" in Ullmann's Encyclopedia of Industrial
30 Chemistry, 2012 edition, propanal formed by hydroformylation can be used as the
main source of 1-propanol in industry. In a second step, propanal is to be
hydrogenated to 1-propanol.
12
In the context of the present invention, the provision of the aldehyde in the
component mixture may thus comprise, for example, forming an alkene from an
alkane by means of oxidative dehydrogenation and/or oxidative coupling,
subsequently subjecting at least part of the alkene formed in these reactions to
5 hydroformylation with carbon monoxide and hydrogen to obtain the aldehyde, and
using at least part of a product mixture formed in the hydroformylation to form the
component mixture. The component mixture thus contains, in particular, unreacted
reactants and by-products in addition to products from oxidative dehydrogenation
and/or oxidative coupling and hydroformylation. For example, if ethane is used as
10 the reactant in oxidative dehydrogenation, the component mixture may contain
unreacted ethane. In the case of oxidative coupling, unreacted methane in
particular may be present in significant amounts in the component mixture.
Furthermore, the component mixture may also contain ethylene from the stream of
matter subjected to hydroformylation that was not converted in the
15 hydroformylation, as well as components used in the hydroformylation but not
converted, such as hydrogen, carbon dioxide and/or carbon monoxide.
Extractive distillation (extractive rectification) is known to be a distillation process
for separating liquid mixtures using a comparatively high-boiling, in particular
20 selective, solvent, also referred to here as an entrainer. Extractive distillation is
based on the fact that the relative volatility of the components to be separated is
influenced by the entrainer. In particular, the relative volatility of one of the
components can be increased or the activity coefficients of the components to be
separated can be changed significantly in different directions. The result is a
25 positive change in the separation factor in terms of separation technology.
As mentioned, the present invention comprises the conversion of the aldehyde
formed, for example, in a hydroformylation, which is contained in the component
mixture used, to a corresponding alcohol and, if necessary, a further conversion,
30 in particular to an alkene, if the alcohol does not represent the final target product.
The conversion of the aldehyde to the alcohol takes place in particular in the form
of a catalytic hydrogenation and a further conversion of the alcohol to the alkene
in the case last explained in the form of a dehydration with formation of water.
13
The hydrogenation of unsaturated components is a well-known and established
technology for the conversion of components with a double bond into the
corresponding saturated compounds. Typically, very high or complete conversions
with selectivities well above 90% can be achieved. Typical catalysts for the
5 hydrogenation of carbonyl compounds are based on Ni, as also described, for
example, in the article "Hydrogenation and Dehydrogenation" in Ullmann's
Encyclopedia of Industrial Chemistry, 2012 edition. Noble metal catalysts can also
be used specifically for olefinic components. Hydrogenations are among the
standard reactions in technical chemistry, as also shown, for example, in M. Baerns
10 et al, "Example 11.6.1: Hydrogenation of double bonds," Technical Chemistry
2006, 439. In addition to unsaturated compounds, other groups of substances are
also hydrogenated, for example aldehydes and ketones. Low-boiling substances
such as butyraldehyde from hydroformylation are hydrogenated in the gas phase.
Ni and certain noble metals such as Pt and Pd, typically in supported form, are
15 used here as hydrogenation catalysts.
The dehydration of alcohols on suitable catalysts to produce the corresponding
alkenes is also known. In particular, the production of ethylene (from ethanol) is
common and is gaining importance in the context of increasing production volumes
20 of (bio)ethanol. Commercial application has been realized by different companies.
For example, reference should be made to the previously mentioned article
"Propanols" in Ullmann's Encyclopedia of Industrial Chemistry as well as Intratec
Solutions, "Ethylene Production via Ethanol Dehydration," Chemical Engineering
120, 2013, 29. Dehydration of 1- or 2-propanol to propylene has no practical value.
25 Nevertheless, the dehydration of 2-propanol in the presence of mineral acid
catalysts at room temperature or above is the simplest. The reaction itself is
endothermic and equilibrium limited. High conversions are favored by low
pressures and high temperatures. Heterogeneous catalysts based on Al2 O3 or
SiO2 are typically used. In general, several types of acid catalysts are suitable and
30 also e.g. molecular sieves and zeolites can be used. Typical temperatures are in
the range of 200 to 250 °C for the dehydration of ethanol or 300 to 400 °C for the
dehydration of 2-propanol or butanol. Due to the equilibrium limitation, typically the
product stream is separated (separation of the alkene product and also at least
14
partially of the water by e.g. distillation) and the stream containing unconverted
alcohol is recycled to the reactor inlet.
In embodiments, the present invention thus proposes overall the coupling of an
5 aldehyde preparation process, which in particular comprises the preparation of the
aldehyde, and (at least) a downstream hydrogenation, wherein the alcohol product
of the hydrogenation or a part thereof is used to form an entrainer for an extraction
or extractive distillation of the crude aldehyde and is recycled accordingly in this
way. In general, however, the alcohol product can also be used to form an entrainer
10 in the extractive distillation when processes other than those described are used
to provide a mixture of components to be processed accordingly. The use "to form"
an entrainer does not preclude the prior use of a corresponding alcohol elsewhere,
for example as an absorbent as explained below, and therefore the composition of
the entrainer differs.
15
In the context of the present invention, particular advantages result in particular
from the fact that the extractive distillation can be carried out using the alcohol
formed in particular in the hydrogenation as intermediate or target product, and
that the remaining components can be separated from a product mixture of the
20 aldehyde production process ("crude aldehyde") in the extractive distillation without
complex cryogenic separation steps. In particular, components that do not
participate in the reaction, such as alkanes and carbon dioxide, but also educts
that have not been completely reacted, such as alkenes, carbon monoxide and
hydrogen, can be carried along and separated more easily. In particular, the
25 unreacted reactants can be easily recycled in this way, for example, and used
again in the reaction feed to produce the aldehyde. Also, hydrogen formed or not
reacted in previous reaction steps can be used for subsequent hydrogenation
steps. In this context, hydrogen can also be separated and/or enriched, for
example, by separation steps known per se, such as pressure swing adsorption.
30
As mentioned, in the aldehyde production process, in particular in the form of
oxidative dehydrogenation with subsequent hydroformylation, a carboxylic acid in
particular can be formed as a further by-product, in the case of ethane as a feed
into the oxidative dehydrogenation in particular acetic acid. These and other by15
products and/or unreacted feedstocks from a process for providing the feed stream
used for the hydroformylation can, together with reaction water, be at least partially
separated comparatively easily, for example by a condensation and/or a water
wash, from a corresponding feed or product stream upstream or downstream of
5 the step in which the aldehyde is formed, in particular the hydroformylation. Carbon
dioxide can likewise still be removed comparatively easily from a corresponding
mixture due to its polarity, whereby known processes for carbon dioxide removal,
in particular corresponding washes (for example, amine and/or caustic washes)
can be used. Cryogenic separation is not required, so that the entire process of the
10 present invention, at least including the aldehyde production process, in particular
hydroformylation, and extractive distillation, does not require cryogenic separation
steps.
This omission of cryogenic separation is also advantageous for the reason that no
15 drying of the aldehyde-containing component mixture upstream of the separation
is required. This advantage applies equally to CO2 removal from the component
mixture containing the aldehyde upstream of the separation. In certain
embodiments, drying and/or CO2 removal, possibly partial, may be provided, but it
is not absolutely necessary with regard to the extractive distillation and also the
20 subsequent hydrogenation process.
The separation of the by-products and/or not or not completely reacted reactants
mentioned above is advantageously carried out completely non-cryogenically and
is therefore extremely simple in terms of equipment and energy consumption. This
25 represents a significant advantage of the present invention over processes
according to the prior art, which typically require a complex separation of
undesirable components in subsequent process steps. The use of a stream
internal to the process as an entrainer also avoids the introduction into the process
of foreign materials that may be difficult to separate from a product mixture. This
30 therefore has a beneficial effect on product purity or process economy.
The term "non-cryogenic" separation refers to a separation or a separation step
which is carried out at a temperature level above 0 °C, in particular above ambient
temperature. In any case, however, "non-cryogenic" in the context of this
16
disclosure also means in particular that no C3 and/or C2 refrigerant has to be used,
and thus at least temperatures above -30 °C, in particular above -20 °C, are meant.
The aldehyde used in the present invention has a relatively high vapor pressure
5 compared to the alkenes and/or alkanes additionally contained in the component
mixture, so that a simple distillation results in corresponding losses to the overhead
product or the distillation column requires very many theoretical trays. In this way,
the apparatus complexity and thus the costs increase significantly. This is
particularly true in the case of the lightest aldehyde propanal used in the present
10 invention. Especially here, the high proportions of methane and/or ethane have a
particularly negative effect due to the comparatively close vapor pressures and/or
boiling points. According to the invention, this disadvantage is overcome by
designing the separation as an extractive distillation using the alcohol formed in
the hydrogenation as the extraction or entrainer. According to the invention, only
15 very little aldehyde is lost in the separation step, since most of the aldehyde that
would otherwise remain gaseous is dissolved in the entrainer or passes into the
bottom liquid.
Frequently, the components present in a component mixture, such as can also be
20 used according to the invention, cannot be mixed with each other indefinitely. In
particular, it depends to a large extent on the set conditions whether miscibility
gaps with two liquid phases occur. The choice of suitable process conditions can
therefore be severely restricted. In particular, the use of light alcohols, such as
propanol, i.e. an alcohol formed as a product or intermediate in the context of the
25 present invention, again has a favorable effect here, because it is then possible to
mix these with both water and hydrocarbons. Possible miscibility gaps are thus
significantly reduced or even avoided by the use of the present invention.
In particular, for a separation of propanal and propanol from the components
30 boiling lower than the aldehyde, a reflux has to be generated from these
components in the case of a classical distillation. This requires a correspondingly
low, mostly cryogenic head temperature, which in turn provokes mixing gaps. In
addition, as already explained, at such low temperatures a prior separation of
carbon dioxide and/or water is typically required to prevent the formation of solid
17
deposits (dry ice/ice). This disadvantage is also overcome by the use of the
invention.
In particular, the extractive distillation is carried out in such a way that at least 60%,
5 70%, 80%, 90%, 95%, 99% or 99.9% of the aldehyde contained in the component
mixture, but possibly also contained water, is separated into a bottom stream, i.e.
is transferred into a liquid fraction formed there, and in particular can be transferred
into the subsequent hydrogenation.
10 The extractive distillation is advantageously carried out in such a way that at most
40%, 30%, 20%, 10%, 5%, 1% or 0.1% of the components boiling lower than the
aldehyde are separated into the sump stream, i.e. are transferred to a liquid fraction
formed there.
15 The bottom stream thus advantageously consists to a predominant extent of the
aldehyde and the alcohol as entrainer as well as possibly water, whereas a
corresponding top stream of the extractive distillation consists in particular of
compounds boiling lower than the aldehyde, which are also less soluble in the
alcohol or the entrainer compared to the aldehyde, as well as possibly water
20 contained therein.
In this context, it should be noted that water, when present in low concentrations,
has a significantly higher vapor pressure than pure water. This is due to the fact
that the vapor pressure or the associated boiling temperature of pure or at least
25 highly concentrated water is attributable to the strong hydrogen bonds between
individual neighboring water molecules. In low concentrations, only a few water
molecules are adjacent to each other, so that only a few hydrogen bonds can be
formed. In such a situation, the vapor pressure or boiling temperature of the water
component is dominated by the molecular mass, resulting in a comparatively low
30 boiling temperature or vapor pressure. Therefore, depending on the composition
of the distillation feed, water present in the distillation feed may be separated either
preferentially into the overhead stream or preferentially into the bottom stream of
the extractive distillation. The present invention avoids the formation of azeotropes
by the extractive distillation designed according to the invention.
18
The by-products of oxidative dehydrogenation in the component mixture, if formed
in this way, are typically unreacted alkane, carbon dioxide and carbon monoxide.
In the case of oxidative coupling, due to typically low conversions, unreacted
5 methane in particular is usually a major component of the component mixture
(along with carbon dioxide and carbon monoxide). These compounds can be
transferred to the subsequent hydroformylation without any problems. Carbon
monoxide can herein be reacted with the alkene together with additionally fed
carbon monoxide, which can, for example, originate from a dry reforming. The
10 alkane is typically not reacted in the hydroformylation. Since the aldehydes formed
in the hydroformylation are heavier compounds with a higher boiling point or
different polarity, they can, as already mentioned, be separated comparatively
easily, and also non-cryogenically, from the remaining alkane in the extractive
distillation according to the invention.
15
In one embodiment of the invention, the conversion of the aldehyde to the alcohol
by hydrogenation is particularly advantageous because excess hydrogen
contained in a product mixture of the hydroformylation can be used for this, which
can already be present in a feed mixture upstream of the hydroformylation and can
20 be passed through the hydroformylation. A content of hydrogen and carbon
monoxide in a feed mixture of the hydroformylation can thereby be adjusted within
the scope of the present invention, in particular in a water gas shift of a basically
known type.
25 At any suitable point in the process according to the invention and its embodiments,
hydrogen can be fed in, in particular upstream of the hydrogenation. Hydrogen is
thus available for this hydrogenation. The feed need not take place directly
upstream of the hydrogenation; rather, hydrogen can also be fed by process or
separation steps present or carried out upstream of the hydrogenation.
30
In a particularly preferred embodiment of the process according to the invention,
additional absorption is further provided, wherein an overhead gas of the extractive
distillation is subjected to absorption using an absorption liquid formed using at
least part of the alcohol and obtaining a liquid fraction. In this way, a further
19
improved depletion of the undesirable components is achieved with lower aldehyde
losses at the same time. In this embodiment of the invention, the liquid fraction
formed in the absorption is advantageously used at least in part to form the
entrainer for the extractive distillation. In other words, the alcohol is used here first
5 as an absorbent and then, in an already partially loaded state, as an entrainer in
the extractive distillation. Absorbent and entrainer phases have different
compositions here due to the loading of the alcohol in the absorption.
In particular, the alcohol formed during the reaction of the aldehyde can be
10 separated comparatively easily from unreacted alkane. In this way, a recycle
stream of the alkane can also be formed non-cryogenically here and recycled, for
example, to oxidative dehydrogenation and/or oxidative coupling.
The present invention also extends to an apparatus for producing a target
15 compound, with respect to which express reference is made to the corresponding
independent patent claim. A corresponding apparatus, which is preferably set up
for carrying out a process as previously explained in various embodiments,
benefits from the previously already mentioned advantages in the same way.
20 The following examples, which are intended to contribute to a better understanding
of the general explanations, represent various advantageous embodiments of an
extractive distillation according to the invention (Examples 3 and 7a/b/c each
without additional absorber, Examples 4 and 8a/b/c each with additional absorber).
Examples 2 and 6 each serve as a comparative case of a conventional distillation
25 without extraction and absorption. The pressure is set to a pressure level of 2.0
MPa in each case. The theoretical plate number of the distillation or extractive
distillation is given by n(stages column), while the plate number of the absorber
n(absorber) is only given if an absorber is additionally provided (examples 4 and
8a/b/c).
30
Examples 1 and 5 each serve to define an exemplary input stream (also referred
to as a feed stream) for the other examples that follow.
20
In each case, only mass flows (in kg/h) or proportions (in mol.%) are listed for
hydrocarbons, propanal, hydrogen, carbon monoxide and carbon dioxide. Other
trace components and, in particular, proportions of water are not included.
5 The distillations in the examples are each characterized by the corresponding
temperature levels in the bottom (T_Bottom), at the top condenser (T_Condenser)
and that of the outgoing top stream (T_OVHD). The temperature T_Feed indicates
the temperature level of the fed feed stream from example 1 or 5 into the column
and the temperature T_Propanol indicates the temperature level at which the
10 entrainer propanol is fed into the column. The boil-up ratio value describes the ratio
of the liquid boiled up and then sent back to the separation as a gas and the product
stream removed in liquid form as the bottom product of the distillation (in kg/h in
each case).
15 Furthermore, the corresponding mass flows for the feed stream from example 1 or
5 into the respective distillation and for the entrainer propanol are given. The tables
Flow OVHD [kg/h] and Flow Bottom [kg/h] specify the proportional composition of
the resulting top and bottom streams of the respective separation.
20 For each of the distillations, the propanal distribution between the head and sump
streams was calculated as the relevant efficiency criterion. Accordingly:
1. Efficiency OVHD [split top:bottom in wt.%] is the ratio of propanal over head
to propanal over sump
25 2. Efficiency Bottom [split bottom:top in wt.-%] is the ratio of propanal over
sump to propanal over head
I.e. the value for Efficiency Bottom [split bottom:top in wt.-%] should be as close as
possible to 100 wt.-%, i.e. in this case propanal is completely transferred to the
30 sump stream of the column. Correspondingly, the value for Efficiency OVHD [split
bottom:top in wt.-%] should be as close as possible to 0 wt.-%, i.e. in this case the
top stream contains little or no propanal

I/We Claim:
1. A process for preparing a target compound (6) using an aldehyde, wherein the
target compound is an alcohol formed from the aldehyde or another compound
formed from at least a part of the alcohol, and wherein the aldehyde is provided
5 in a component mixture (1) comprising components boiling lower than the
aldehyde, characterized in that that an (alcohol) synthesis feed (2) enriched
in the aldehyde and depleted in the components boiling lower than the
aldehyde is formed from the component mixture (1) using extractive distillation
(E) and is subjected to a reaction to form the alcohol, the alcohol being used
10 at least in part to form an entrainer (4) for the extractive distillation (E).
2. The process of claim 1, wherein the aldehyde and the alcohol are compounds
having linear carbon chains of the same chain length with terminal aldehyde
and alcohol groups, respectively.
15
3. The process according to claim 2, wherein a chain length of the carbon chains
of the aldehyde and the alcohol is 2 to 20.
4. The process according to claim 3, wherein the aldehyde is propanal, the
20 alcohol is propanol, and the target compound is propanol or propylene formed
from the propanol.
5. The process according to any one of the preceding claims, wherein the
compounds boiling lower than the aldehyde include at least one of the group
25 comprising carbon dioxide, carbon monoxide, water, hydrogen, one or more
alkanes, and one or more alkenes.
6. The process according to any one of the preceding claims, wherein the
provision (B) of the aldehyde in the component mixture (1) comprises forming
30 an alkene from an alkane by means of oxidative dehydrogenation and/or
oxidative coupling, subjecting at least part of the alkene thus formed to
34
hydroformylation to obtain the aldehyde, and using at least part of a product
mixture formed in the hydroformylation to form the component mixture (1).
7. The process according to any one of the preceding claims, wherein the
5 aldehyde is converted by hydrogenation at least in part to the alcohol and the
alcohol is converted by dehydration at least in part to an olefin when the
alcohol is not the target product.
8. The process according to any one of the preceding claims, wherein at most
10 40%, 30%, 20%, 10%, 5%, 1% or 0.1% of the components boiling lower than
the aldehyde contained in the component mixture (1) are converted into a
liquid fraction formed in the extractive distillation (E).
9. The process according to any one of the preceding claims, wherein in the
15 extractive distillation at least 60%, 70%, 70%, 80%, 90%, 95%, 99% or 99.9%
of the aldehyde contained in the component mixture (1) is transferred from the
component mixture (1) to a liquid fraction formed in the extractive distillation
(E).
20 10. The process according to any one of the preceding claims, further comprising
an absorption (A), wherein an overhead gas of extractive distillation (E) is
subjected to the absorption (A) using an absorption liquid formed using at least
a part of the alcohol and obtaining a liquid fraction.
25 11. The process according to claim 10, wherein at least part of the liquid fraction
formed in the absorption is used to form the entrainer for the extractive
distillation (4).
12. The process according to any one of the preceding claims, wherein the
30 extractive distillation (E) is carried out with a bottom temperature in the range
from 50 °C to 300 °C, preferably from 100 °C to 280 °C, in particular from 150
°C to 250 °C, a top temperature in the range from -30 °C to 50 °C, preferably
from 0 °C to 40 °C, in particular from 15 °C to 35 °C, and at a pressure in the
35
range from 0.5 MPa to 10 MPa, preferably from 1.5 MPa to 5 MPa, in particular
from 1.5 MPa to 3.5 MPa.
13. An apparatus (100) for producing a target compound (6) using an aldehyde,
5 the target compound being an alcohol formed from the aldehyde or a further
compound formed from at least part of the alcohol, the apparatus (100)
comprising means adapted to provide the aldehyde in a component mixture
(1) comprising components boiling lower than the aldehyde, characterized by
an extractive distillation column (E), adapted to form from the component
10 mixture (1) an (alcohol) synthesis feed (2) enriched in the aldehyde and
depleted in the components boiling lower than the aldehyde and to subject it
to a reaction to form the alcohol, wherein a recycle means (R) is provided
adapted to utilize the alcohol from the to form an entrainer (4) fed to the
extractive distillation column (E).
15
14. The apparatus (100) according to claim 13, further adapted to perform a
process according to any one of claims 1 to 12