DESCRIPTION
[0001] The invention relates to a method and to a system for separating a feed flow
according to the preambles of the independent claims.
10 PRIOR ART
[0002] In the industrial production and processing of hydrocarbons and other
organochemical compounds, it is frequently necessary to separate product flows of a
method step into different components before further steps can be carried out, for
15 example in order to separate such components which interfere in the following steps
or to emit a product with a required purity.
[0003] For such separations, so-called cryogenic separation methods are frequently
used, in which a gaseous feed flow is cooled, wherein the feed flow is at least
20 partially liquefied. By means of such partial condensations, various components
present in the feed flow can be separated from one another in accordance with their
respective boiling points or vapor pressures at the respectively prevailing pressures
or temperatures. For this purpose, so-called C2-refrigerants are frequently used
which consist of hydrocarbon mixtures which are composed essentially of
25 compounds having two carbon atoms per molecule. The use of pure C2-refrigerants
such as ethane or ethylene is also possible.
[0004] Furthermore, separation methods are known which operate without C2
refrigerants. Such a cryogenic separation method is known, for example, from U.S.
30 Pat. No. 6,333,445 B1. There, a gaseous feed flow originating from an alkane
dehydrogenation process and which therefore contains hydrogen and the unconverted
alkane and an alkene produced therefrom is first compressed and then cooled and
3
partially condensed using two heat exchangers. A liquid raw material flow containing
the alkane required for the dehydrogenation process is evaporated and heated as the
coolant. A “feed flow” is understood below to mean a gaseous flow which is fed to a
separation method. This originates from an organochemical conversion method, to
which in turn a “raw material flow” is fed, according 5 to the terminology used below.
In the example explained, the raw material flow is thus fed to the alkane
dehydrogenation process and converted therein into the feed flow of the separation
process.
10 [0005] The condensates or liquid flows produced from the feed flow are separated
from the remaining residual gas flows. The cooling capacity is applied primarily by
the evaporation heat absorbed by the liquid raw material flow. The process
conditions, in particular a positive pressure difference between the gaseous feed flow
and the liquid raw material flow, bring about a temperature difference and thus
15 enable the heat transfer and the partial condensation of the feed flow or evaporation
of the raw material flow.
[0006] Depending on the configuration of the alkane dehydrogenation process, a
situation can thereby arise in which the amount of heat which is withdrawn from the
20 evaporation of the raw material flow is not sufficient to achieve the separation or
partial condensation of the feed flow.
[0007] The object of the present invention is therefore to provide an improved
separation method which, without C2 refrigerant, ensures a corresponding separation
25 even in unfavorable situations with respect to the amount of heat withdrawn by the
evaporation of the raw material flow.
DISCLOSURE OF THE INVENTION
30 [0008] This object is achieved by methods and systems according to the respective
independent claims. Advantageous designs and developments result from the features
of the dependent claims and from the following description.
4
[0009] Before the features and advantages of the invention are shown, their
principles and terms used within the scope of the disclosure will be explained.
[0010] A “compressor” is a device which is configured 5 for compressing at least one
gaseous flow from at least one inlet pressure (also referred to as suction pressure) at
which it is supplied to the compressor to at least one end pressure at which it is
withdrawn from the compressor. A compressor forms a structural unit which,
however, can have several “compressor stages” in the form of rows of pistons,
10 screws and/or blades (i.e., axial or radial compressor stages). In particular,
corresponding compressor stages are driven by means of a common drive, for
example via a common shaft. A compressor in the described sense or a compressor
stage of such a compressor performs a “compression step” according to the
terminology of the present disclosure.
15
[0011] A “cooler” is a device which is configured for cooling a (for example gaseous
or liquid) fluid flow from at least one inlet temperature at which it is supplied to the
cooler to at least one end temperature at which it is withdrawn from the cooler. A
cooler forms a structural unit which, however, can have several “cooling stages” in
20 the form of (e. g., plate, pipe, counter-flow) heat exchangers and/or expanders (for
example throttle valves or turbines). In particular, corresponding cooling stages can
be realized using a single heat exchanger. A cooler in the described sense or a cooling
stage of such a cooler performs a “cooling step” in the terminology of this disclosure.
25 [0012] A “thermal separation” is characterized in the language of this disclosure in
that a gas mixture is separated under at least partial liquefaction in the same, and in
this case a suitable refrigerant is used. Known heat exchangers are used for this
purpose. The separation is effected by means of known phase separation devices, for
example by means of gas separators. In a thermal separation, in particular so-called
30 C3 and/or C2-refrigerants are used. These are conducted between different pressure
levels, wherein the aforementioned compressors and, for example, known expansion
turbines or expansion or throttle valves are used.
5
[0013] If in the present disclosure it is stated that a flow or a mixture of substances is
“enriched” with one or more components in relation to another flow or another
mixture, this is to be understood in such a way that the concentration of this/these
component(s) in the flow or mixture enriched in this way 5 is higher by at least a factor
of 1.1, 1.2, 1.5, 2, 3, 5, 10, 30, 100, 300 or 1000 in comparison to the reference flow
or mixture. A “depleted” material flow accordingly has a lower concentration than
the reference flow and in particular has a concentration of the component which is
lower compared to the reference flow, i.e., at most 90%, 80%, 50%, 30%, 10%, 3%,
10 1%, 0.3% or 0.1% of the concentration of the component in the reference flow.
[0014] The present disclosure uses the terms “pressure level” and “temperature
level” to characterize pressures and temperatures, which means that corresponding
pressures and temperatures in a corresponding plant do not have to be used in the
15 form of exact pressure or temperature values in order to realize the inventive
concept. However, such pressures and temperatures typically fall within certain
ranges that are, for example, 1%, 5%, 10%, 20% or even 50% around an average. In
this case, corresponding pressure levels and temperature levels can be in disjointed
ranges or in ranges that overlap one another. In particular, pressure levels, for
20 example, include unavoidable or expected pressure losses. The same applies to
temperature levels.
[0015] If it is stated in this description that a mixture contains at least one liquid
phase, this is to be understood as meaning that the mixture can contain one or more
25 liquid phases which are completely, partly or not miscible with one another.
ADVANTAGES OF THE INVENTION
[0016] In the following, features and advantages of the invention are explained
30 primarily with respect to the method mentioned. The corresponding statements also
apply analogously to systems according to the invention and advantageous
embodiments thereof, which profit accordingly from the advantages. For example, it
6
can be mentioned that a flow is subjected to a method step. This is to be understood
in relation to a corresponding system in such a way that components which are
designed to carry out a corresponding method step are provided and means, for
example pipelines, valves and the like, are provided for feeding the respective flow
into the component. The explanations relating to a system 5 accordingly also apply to a
corresponding method.
[0017] According to the invention, a method for separating a feed flow comprising at
least hydrogen and a hydrocarbon having three carbon atoms per molecule is
10 proposed. In the compressed state, the feed flow is partially liquefied by means of at
least two coolers, which are operated at different temperature levels, via at least two
cooling steps to obtain at least one first and one second condensate flow and at least
one first and one second residual gas flow. The residual gas flow of a cooling step is
respectively fed into the subsequent cooling step. In the case of partial liquefaction,
15 each condensate flow is depleted with regard to hydrogen and enriched with respect
to hydrocarbon relative to the feed flow and each residual gas flow is enriched with
respect to hydrogen and depleted with respect to the hydrocarbon in relation to the
feed flow. A liquid C3 product flow predominantly consisting of the hydrocarbon is
formed from the condensate flows and a gaseous gas product flow consisting
20 predominantly of hydrogen is formed using at least one of the residual gas flows. A
part of at least one of the condensate flows is combined with a part of at least one of
the residual gas flows and used as an internal refrigerant for at least one of the
cooling steps (or coolers) under expansion. The expansion can take place before
and/or after the combination.
25 Analogously to the method described above, in the method according to the
invention, a raw material flow is also supercooled in at least one of the coolers,
combined with a part of the gas product flow, expanded and used as refrigerant for
the cooler. The advantage of the invention over the conventional method is that,
irrespective of the amount, the pressure and the composition of the raw material flow,
30 the separation of the feed flow can always be carried out, since the energy balance
can be closed by means of compression of the feed flow upstream of the cooler and
by cooling to a natural ambient temperature level. The expanded internal refrigerant
7
can advantageously be returned to the feed flow, optionally with further gaseous
extraction flows, before it is compressed. As a result, the method can be controlled
significantly more flexibly than conventional methods and can, for example, be
adapted to fluctuating amounts in terms of gaseous feed flow and/or liquid raw
material flow and other fluctuating process 5 conditions such as, for example,
unfavorable pressure and/or temperature levels, which impede the heat exchange
between condensing feed flow and evaporating raw material flow.
Preferably, at least one first cooling step takes place from a high inlet temperature
10 level (for example temperature of the natural atmosphere or environment, e.g., 10°C
to 50°C) to an average temperature level which is in a range from -40°C to 10°C,
preferably from -40°C to -10°C, and at least one second cooling step to a low
temperature level, which is in a range from −130°C to 80°C, preferably from -110°C
to -90°C. This makes it possible to substantially separate the hydrocarbon from the
15 hydrogen already at the average temperature level, for example in the first
condensate flow.
[0018] In some embodiments of the invention, only flows of materials formed from
the liquid raw material flow are used to achieve the low temperature level. In order to
20 achieve the average temperature level, an externally generated or externally supplied
refrigerant can additionally be used. As a result, any available process cooling can
advantageously be integrated and the energy balance can be closed in a simple
manner without requiring refrigerant at the low temperature level.
25 [0019] Advantageously, the first and second cooling steps take place by counterflow
heat exchange, in particular of the condensing feed flow and the evaporating raw
material flow. As a result, the energy balance can be closed essentially within the
method.
30 [0020] Preferably, the second residual gas flow of the second cooling step is
subjected to at least one expansion to obtain further condensate and residual gas
8
flows. As a result, energy can be extracted from the method and this can be used in
the form of mechanical energy, for example in order to drive pumps or compressors.
[0021] The internal refrigerant is preferably formed from a part of the second
condensate flow and a part of the residual 5 gas flow which has been expanded. As a
result, the obtained internal refrigerant has particularly advantageous process
conditions with respect to temperature and composition and can be optimally used
for the supporting cooling of the second condensate flow.
10 [0022] Advantageously, several or all condensate flows are combined to form an
aggregate flow, and the aggregate flow is subjected to a gas separation forming a
flash gas and the C3 product flow. This results in synergy effects and potential
savings in relation to required pipelines, insulations and the like.
15 [0023] In particular, the C3 product flow, the gas product flow and the flash gas are
heated to a temperature level corresponding to the temperature level of the feed flow.
As a result, no cold is lost, which is particularly favorable in terms of energy.
[0024] Since it is desired both for the flash gas and for the internal refrigerant to
20 return the fluid back into the feed flow in order to increase the yield of the method,
these two flows can be combined at a suitable point and fed back together to the feed
flow before it is compressed.

We Claim:
1. A method for separating a feed flow (1) 5 which contains at least hydrogen and
a hydrocarbon with three to four carbon atoms per molecule, which feed flow is
formed from a raw material flow (18),
wherein the feed flow (1) is partially liquefied in the compressed state using
at least two coolers (120, 130) which are operated at different temperature levels, via
10 at least two cooling steps, to obtain at least two condensate flows (7, 8, 9) and at least
two residual gas flows (3, 5, 11),
wherein the residual gas flow (3, 5) of a cooling step is respectively fed into
the subsequent cooling step, and wherein each condensate flow (7, 8, 9) is depleted
in hydrogen and enriched in the hydrocarbon with respect to the feed flow (1) and
15 each residual gas flow (3, 5, 11) is enriched in hydrogen and depleted in hydrocarbon
with respect to the feed flow (1),
characterized in that
a part (13) of at least one of the condensate flows (7, 8, 9) is combined with a
part (14) of at least one of the residual gas flows (11) under expansion and used as
20 internal refrigerant (15) for at least one of the cooling steps or coolers (120, 130) and
is at least partially recycled to the feed flow (1).
2. The method according to claim 1, wherein at least a first cooling step takes
place at an average temperature level in a range from -40°C to +10°C, preferably
25 from -40°C to -10°C, and at least a second cooling step takes place at a low
temperature level in a range from -130°C to -80°C, preferably from -110°C to -90°C.
3. The method according to claim 2, wherein only material flows (11, 13, 14,
15, 16, 18) formed from the raw material flow (18) are used to achieve at least the
30 low temperature level.
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4. The method according to claim 2 or 3, wherein a refrigerant (21) which is not
provided internally in the process is used to achieve the average temperature level.
5. The method according to any of claims 2 to 4, wherein the first and second
cooling steps are performed b 5 y counterflow heat exchange.
6. The method according to any of claims 2 to 5, wherein the residual gas flow
(5) of the second cooling step is subjected to at least one expansion to obtain further
condensate flows (9) and residual gas flows (11).
10
7. The method according to one of the preceding claims, wherein the internal
refrigerant (15) is formed of a part (13) of at least one of the condensate flows (8)
and a part (14) of at least one of the residual gas flows (11), wherein the condensate
flow (8) is formed in a step which takes place upstream of the step in which the
15 residual gas flow (11) is formed.
8. The method according to one of the preceding claims, wherein a plurality of
or all condensate flows (7, 8, 9) are each at least partially combined to form a
combined flow, and the combined flow is subjected to a gas separation to form a
20 flash gas (12) and a liquid product flow (10).
9. The method according to one of the preceding claims, wherein the liquid
product flow (10), a predominantly hydrogen-containing gas product flow (20),
formed from a part of at least one of the residual gas flows, and the flash gas (12) are
25 heated to a temperature level corresponding to the temperature level of the feed flow
(1).
10. The method according to one of the preceding claims, wherein the flash gas
(12) is at least partially recycled into the feed flow (1).
30
11. A system (100) for separating a feed flow (1) which contains at least
hydrogen and a hydrocarbon with three to four carbon atoms per molecule,
17
having at least two heat exchangers (120, 130), of which at least one (120)
can be operated at an average temperature level and at least one (130) can be
operated at a low temperature level, and which are configured to cool the feed flow
(1) according to the counterflow principle, and
having at least two phase separation 5 devices (142, 144, 146, 148), which are
each configured to split a partially liquefied flow (2, 4, 6) into a condensate flow (7,
8, 9) and a residual gas flow (3, 5, 11),
characterized by
means configured to combine a part (13) of at least one of the condensate
10 flows (7, 8, 9) with a part (14) of at least one of the residual gas flows (5, 11 to form
an internal refrigerant (15) under expansion, and to feed the internal refrigerant (15)
to at least one of the heat exchangers (120, 130).
12. The system (100) according to claim 11, also having means which enable the
15 system (100) to carry out a method according to any one of claims 1 to 10.