Note: Descriptions are shown in the official language in which they were submitted.
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Process and apparatus for the separation of light-boiling components from
hydrocarbon mixtures
[0001] The invention relates to a process and an apparatus for the separation
of light-
boiling components from a hydrocarbon stream, particularly for the separation
of a C2 fraction
from a C3+ fraction to be obtained as target product, e.g. in the
dehydrogenation of light
hydrocarbons, but also suitable for other separation tasks within the range of
C, to C4-
[0002] Usually, such separations are performed by means of a so-called
"coldbox" in
combination with a distillation column. The distillation column is referred to
and operated as a
de-ethanizer because all substances of boiling points less or equal to that of
ethane are
separated overhead by this de-ethanizer.
[0003] Before entering the coldbox, the feed mixture is cooled down to approx.
-25 C.
The condensate gained is passed directly into the de-ethanizer. Vapours that
have not
condensed are further cooled to approx. -90 C in the coldbox, the product-
enriched
condensate thus obtained also being directed to the de-ethanizer after heat
exchange. The
coldbox therefore performs a one-step rough separation.
[0004] The remaining vapour phase basically comprising uncondensable
components,
e.g. hydrogen, is depressurised after heat exchange in the flow entering the
coldbox. The
light-boiling substances cool down to approx. -110 C due to the Joule-
Thompson effect. This
temperature level is used to partially condense the stream entering the
coldbox. The
uncondensed light-boiling substances are basically free from C3+ components.
[0005] Finally all product-enriched condensate phases are led to a final
separation in the
de-ethanizer where the remaining light-boiling components are separated form
the heavier
boiling components. For this purpose a temperature of about -20 C is necessary
at the head
of the column.
[0006] Evaporating propane or propene can be used at -30 C as a cooling agent
for
cooling the feed mixture before it enters the coldbox, for operating the
coldbox and for
cooling the de-ethanizer.
[0007] The generation of cold for such a process is extremely expensive. The
aim of the
invention is, therefore, to provide a process and an apparatus by which the
consumption of
cold can be reduced to a significant degree.
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[0008] The aim of the invention is achieved according to the main claim by a
process for
the separation of a feed mixture
= into a higher-boiling and a lower boiling fraction in a continuously
operated distillation
apparatus comprising at least one inlet pipe for feeding one or more feed
mixtures, an
outlet means for the lower-boiling fraction, an outlet means for the higher-
boiling fraction
and a heating device, in which
= the distillation apparatus includes at least two condensation stages, each
of a different
temperature level,
= the condensation stages provided upstream in direction of the vapour flow
having a
higher temperature level than the downstream condensation stages,
= separation-effective internals are installed between the condensation
stages,
= partial condensation takes place in the condensation stages,
= partial amounts that have not condensed in these stages are fed to
downstream
separation-effective internals or condensation stages of lower temperature
level and
partial amounts that have condensed are passed via separation-effective
internals in
direction of the outlet means for the higher-boiling fraction,
= a basically vaporous fluid is obtained at the condensation stage of the
lowest temperature
level, where it partially condenses,
= the uncondensed part of the fluid being recycled to the outlet means for the
lower-boiling
fraction and the condensed part being recycled to a section of the
distillation apparatus
which is upstream of the condensation stage having the lowest temperature
level,
= and the condensation stage of the lowest temperature level having a
temperature of
below -40 C.
[0009] In embodiments of the invention, the distillation apparatus comprises
three to five
successive condensation stages, each of a different temperature level.
[0010] In a further embodiment of the invention the condensation stage of the
lowest
temperature level is operated at a temperature between -120 C and -70 C and
a pressure
of at least 2 MPa absolute, preferably at least 3 MPa.
[0011] In a further embodiment of the invention the mixture which leaves the
distillation
apparatus as lower-boiling fraction is depressurised, the mixture thereby
cooling down further
under exploitation of the Joule-Thompson-effect, and thus being used to cool
the
condensation stage with the lowest temperature level.
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[0012] In a further embodiment of the invention the depressurisation is
performed by an
expansion turbine.
[0013] Further embodiments of the invention relate to the use of suitable feed
mixtures
which are of particular advantage to the process for the purpose of obtaining
useful products.
[0014] In a further embodiment of the invention the process is used for feed
mixtures
basically containing hydrogen, hydrocarbons of up to two carbon atoms and
hydrocarbons of
at least three carbon atoms. At the outlet means for the lower-boiling
fraction a mixture is
obtained which basically contains hydrogen and hydrocarbons of up to two
carbon atoms
and basically no hydrocarbons of at least three carbon atoms. At the outlet
means for the
higher-boiling fraction a mixture is obtained, which basically contains
hydrocarbons of at
least three carbon atoms and basically neither hydrogen nor hydrocarbons of up
to two
carbon atoms.
[0015] In a further embodiment of the invention the process is used for feed
mixtures
containing less than 2 mol-% each of carbon dioxide and water or water vapour.
[0016] In a further embodiment of the invention the feed mixture used is a
reaction
mixture from the catalytic dehydrogenation of hydrocarbons.
[0017] In a further embodiment of the invention the segment of the
distillation apparatus,
through which that part of the feed mixture that has condensed in the
condensation stage of
the highest temperature level is passed to the outlet means for the higher-
boiling fraction, is
provided as a stripping section of the distillation apparatus.
[0018] In further embodiments of the invention feed mixtures of a relatively
small content
of components of low boiling point are preferably fed at a point below the
condensation stage
of the highest temperature level and feed mixtures of a relatively high
content of components
of low boiling point are preferably fed at a point above the condensation
stage of the highest
temperature level.
[0019] In a further embodiment of the invention the condensation stages are
provided in
the form of condensers.
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[0020] In a further embodiment of the invention the condensation stages are
cooled by
cooling water, evaporating ammonia, propane, propene and/or by exploitation of
the Joule-
Thompson effect when depressurising process gases.
[0021] In a further embodiment of the invention the heating device is operated
by
external waste heat.
[0022] The invention achieves the aim also by a distillation apparatus
suitable for the
separation of a feed mixture, comprising:
= one or more inlet pipes for feeding one or more feed mixtures,
= an outlet means for the lower-boiling fraction,
= an outlet means for the higher-boiling fraction,
= at least one heating device,
= at least two successive condensers, and
= separation-effective internals installed between the condensers.
[0023] In alternative inventive embodiments of the distillation apparatus the
latter
consists of one single distillation column or consists of a cascade of several
distillation
columns in which condensers are installed between the distillation columns. In
particular it
may be provided that the distillation apparatus comprises 3, 4 or 5 successive
condensers,
each being operated at a different temperature level.
[0024] What was achieved in two process steps in the past, can now be
implemented in
one single process step according to the invention, without providing an
upstream coldbox,
which is an advantage of the invention.
[0025] By the use of the distillation apparatus according to the process
involving several
condensers operating at different temperature levels it is achieved that it is
not required to
provide the total amount of cooling fluid for the condensation at the lowest
temperature level
and therefore at the highest costs. Instead, the intermediate condensers are
operated at
temperature levels of approx. +45 C, +15 C and -30 C, respectively, which is
a further
advantage of the invention.
[0026] The main part of the rising vapours in the column, if used, therefore
condenses
before entering the column head condenser and flows downwards as liquid. The
coldness
level necessary for operating the column head condenser, about -80 C, and the
related
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condensation performance can be produced by depressurising the high-boiling
substances
within the apparatus itself, which is a further advantage of the invention.
[0027] In the following the invention is illustrated in detail by means of
three examples:
Fig. 1 shows an embodiment of the inventive process, in which the distillation
apparatus consists of one single distillation column and several intermediate
condensers;
Fig. 2 shows an embodiment of the inventive process, in which the distillation
apparatus comprises a stripping section and a rectifying section separately of
each other;
Fig. 3 shows an embodiment of the inventive process, in which the distillation
apparatus comprises three sections with intermediate condensers between
these sections.
[0028] In all three examples, vaporous feed mixture 1 can be cooled down first
to 15 C
in an ammonia vaporiser 2, for example. In another heat exchanger 3 the
obtained C2
fraction 4 is cooled down further to about 10 C, making part of the vapour
condense. Vapour
phase 5 and condensate 6 are fed separately into the distillation apparatus.
Depending on
the composition of the feed mixture the inlet can also be above the
condensation stage of the
highest temperature level.
[0029] In the example shown in E4,1 the liquid flows downwards in distillation
column 7
and is partly evaporated again. The non-evaporated part is drawn-off as a C3+
product 8 at
the bottom of distillation column 7. The light-boiling substances rise upward
as vapour and
partly condense in first condenser 9, which is designed as a two-piece
condenser located
above the feed tray, cooling water and ammonia being used one after the other
as cooling
agents. Further rising vapours are partly liquefied in second condenser 10,
which is operated
with propane or propene as cooling agent so that only a small part of the
vapours arrives at
head condenser 11. The vapours not condensed in head condenser 11 constitute
the C2
fractions 12 and 13 which are depressurised after the condensation in expander
14, hereby
cooling down to about -125 C. This cooled vapour 15 is used as cooling agent
for the cold
side of head condenser 11, where it heats up to about -50 C. Subsequently C2
fraction 4 is
conveyed through heat exchanger 3 for cooling feed mixture 1.
[0030] In the example shown in Fig. 2 the liquid flows downwards in stripping
column 16
and is partly evaporated again. The non-evaporated part is drawn-off as a C3+
product 8 at
the bottom of stripping column 16. The vaporous light-boiling substances 17
flow into first
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condenser 9, which is designed as a two-piece condenser, where they partly
condense,
cooling water and ammonia being used as cooling agents one after the other.
Condensate 18 and vapours 19 are given into rectification column 20. Part of
the bottom
product of rectification column 20 is used as reflux 21 for stripping column
16. Further rising
vapours are liquefied in part in second condenser 10, which is operated with
propane as
cooling agent so that only a small part of the vapours arrives at head
condenser 11. Vapours
not being condensed in head condenser 11 constitute the C2 fractions 12 and 13
which are
depressurised after the condensation in expander 14, hereby cooling down to
about -125 C.
This cooled vapour 15 is used as cooling agent for the cold side of head
condenser 11,
where it heats up to about -50 C. Subsequently this C2 fraction 4 is conveyed
through heat
exchanger 3 for cooling feed mixture 1.
[0031] In the example shown in Fig. 3 the liquid flows downwards in stripping
column 16
and is partly evaporated again. The non-evaporated part is drawn-off as a C3+
product 8 at
the bottom of stripping column 16. Vaporous light-boiling substances 17 flow
to first
condenser 9, which is designed as a two-piece condenser, where they partly
condense,
cooling water and ammonia being used one after the other as cooling agents.
Condensate 18 and vapours 19 are given into first rectification column 22.
Part of the bottom
product of first rectification column 22 is used as reflux for stripping
column 16. Further rising
vapours are liquefied in part in second condenser 10, which is operated with
propane or
propene as cooling agent, condensate and vapours are led into second
rectification
column 23. The bottom product of the second rectification column 23 serves as
reflux for the
first rectification column 22, in this way only a small part of the vapours
arrives at head
condenser 11. Vapours not having condensed in head condenser 11 constitute C2
fractions 12 and 13 which are depressurised after the condensation in expander
14, hereby
cooling down to about -125 C. This cooled vapour 15 is used as cooling agent
for the cold
side of head condenser 11, where it heats up to about -50 C. Subsequently
this C2
fraction 4 is conveyed through heat exchanger 3 for cooling feed mixture 1.
[0032] In contrast to the preceding variants, this last variant involves the
advantage that
only the upper part of the second rectification column must be of a design
resistant against
low temperature and insulated more strongly against heat transfer, which is
one of the
advantages of the invention.
[0033] In all three examples the heat amount necessary for the operation of
the
vaporisers exceeds those of the state of the art, but this heat is to be
provided at only about
75 C, which is why usually waste heat from other process sections of a plant
complex are
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used which would otherwise have to be removed expensively by means of air
coolers, which
is another advantage of the invention. In total, a higher refrigerating
capacity will be required
but the heat can be removed at a less expensive cooling level, which is
another advantage of
the invention.
[0034] Further advantages of the process of the invention in comparison to the
processes mentioned at the beginning are:
By using cooling water, about 25 % of the ammonia refrigeration capacity can
be saved, so
that a smaller ammonia refrigeration unit is necessary.
The propane refrigeration capacity is about 55 % less so that the compressor
capacity for the
propane refrigeration circuit is about 50 % less and the compressor can be of
a
correspondingly smaller size.
