PROCESS FOR REMOVING HARMFUL SUBSTANCES FROM CARBON DIOXIDE AND APPARATUS FOR THE PERFORMANCE THEREOF

PROCEDE D'ELIMINATION DE POLLUANTS A PARTIR DE DIOXYDE DE CARBONE LIQUIDE ET DISPOSITIF POUR SA MISE EN OEUVRE

English Abstract

A process for removing harmful substances from a gas stream which comprises essentially carbon dioxide CO2, and also comprises substances of value, such as at least one of the gases hydrogen H2, carbon monoxide CO, nitrogen N2 or noble gases, and harmful substances such as a substance from the group of mercury, sulphur, mercury compounds or sulphur compounds, wherein a carbon dioxide condensation is performed in order to obtain liquid carbon dioxide, adsorptive removal of the harmful substances from the condensed carbon dioxide is performed to remove the harmful substances from the carbon dioxide, and a process temperature of less than -30°C but greater than -70°C is maintained.

French Abstract

L'invention porte sur un procédé de séparation de polluant à partir d'un courant gazeux, qui comprend essentiellement du dioxyde de carbone CO2, ainsi que de substances valorisables telles qu'au moins l'un des gaz hydrogène H2, monoxyde de carbone CO, azote N2 ou des gaz rares, et des polluants tels qu'une substance parmi le groupe comprenant le mercure, le soufre, des composés du mercure ou du soufre. Dans ce procédé on réalise une condensation du dioxyde de carbone pour produire du dioxyde de carbone liquide, on réalise une séparation par adsorption des polluants à partir du dioxyde de carbone condensé pour éliminer les polluants du dioxyde de carbone et on respecte une température de procédé inférieure à -30°C mais supérieure à -70°C.

Claims

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CLAIMS:
1. A process for separating a harmful substance from a
gas mixture which essentially consists of carbon dioxide as
well as a recyclable substance, comprising:
carrying out a carbon dioxide condensation in order
to produce and to separate liquid carbon dioxide;
carrying out an adsorptive separation of the harmful
substance from the liquid carbon dioxide in order to remove the
harmful substance from the liquid carbon dioxide;
wherein the recyclable substance remains in the
liquid carbon dioxide exiting the adsorptive separation; and
setting a process temperature in a range from
-30°C to -70°C.
2. The process as claimed in claim 1, wherein the
recyclable substance contained in the gas mixture is selected
from the group consisting of hydrogen, carbon monoxide,
nitrogen, and a noble gas.
3. The process as claimed in any one of claims 1 and
2, wherein the harmful substance contained in the gas mixture
is selected from the group consisting of sulfur, mercury,
sulfur compounds and mercury compounds.
4. The process as claimed in any one of claims 1 to 3,
wherein at least one substance for an adsorbent for the
adsorptive separation of the harmful substance is selected from
the group consisting of activated carbon, zeolite, silica gel,
alumina and at least one polymer having a large surface area.

9
5. The process as claimed in anyone of claims 1 to 4,
wherein the separation of at least one harmful substance from
carbon dioxide takes place in low- or zero-carbon dioxide power
stations.
6. A power station comprising:
a cryogenic carbon dioxide separation unit to provide
cryogenically separated liquid carbon dioxide;
an apparatus for separating a harmful substance from a mixture
which essentially consists of liquid carbon dioxide as well as
a recyclable substance, the apparatus comprising:
a fixed-bed adsorber for the adsorptive separation of
the harmful substance from liquid carbon dioxide for the
purpose of removing the harmful substances therefrom,
wherein the recyclable substance remains in the
liquid carbon dioxide exiting the adsorptive separation, and
wherein the operating temperature of the apparatus
lies in a range from -70°C to -30°C.
7. The power station as claimed in claim 6, wherein for
the purpose of the adsorptive separation, the fixed-bed
adsorber includes at least one adsorbent selected from the
group consisting of activated carbon, zeolite, alumina and a
polymer having a large surface area.
8. The power station as claimed in any one of claims 6
and 7, wherein a plurality of fixed-bed adsorbers having
adsorbents are present, and

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wherein the plurality of fixed-bed adsorbers are
connectable in parallel in groups, such that adsorbers or
groups of adsorbers are capable of being switched over to allow
regeneration of the adsorbents.
9. The power station as claimed in any one of claim 6
to 8, wherein the recyclable substance is selected from the
group consisting of hydrogen, carbon monoxide, nitrogen, and a
noble gas.
10. The power station as claimed in any one of claims 6
to 9, wherein the harmful substance is selected from the group
consisting of mercury, sulfur, and compounds thereof.

Description

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Process for removing harmful substances from carbon dioxide
and apparatus for the performance thereof
FIELD OF INVENTION
The invention relates to a process and to an apparatus for
= separating harmful substances from a carbon dioxide stream.
Compounds of sulfur or mercury are examples of harmful
substances.
BACKGROUND
On account of the so-called greenhouse effect and the global
temperature rise associated therewith, the emission of
greenhouse gases into the atmosphere is subject to increasing
criticism. A considerable proportion of the greenhouse effect
is attributed to carbon dioxide CO2 which is produced during
the combustion of fossil fuels.
There is currently a consensus across society that separating
out carbon dioxide from power station waste gases may
counteract an increase in the greenhouse effect. For this
reason, corresponding further developments are directed at
making possible plants having low levels of carbon dioxide
emissions or even none at all. Accordingly, the latest
concepts for power stations having low levels of carbon
dioxide emissions or which are free of carbon dioxide
emissions are at the development stage. All of these concepts
pursue the common objective of separating the carbon dioxide
as completely as possible and at a high level of purity with
minimal energy consumption.
Here the focus is on the separation of the carbon dioxide in
the first instance. In addition, however, further undesirable
substances are present which occur depending on the type of

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fuel and which need to be removed. Sulfur, mercury or their
compounds are the most commonly encountered substances in
terms of quantity in this context.
The latest power station concepts include general conditions
which in each case provide the most advanced techniques for
separating harmful substances according to the current prior
art.
According to the prior art, harmful substances which
accumulate in power stations, such as sulfur or mercury
compounds, are removed directly from the gas phase. Thus,
hydrogen sulfide, for example, is separated by means of gas
scrubbing. This entails routing the gas stream through an
absorber in which a liquid medium absorbs the harmful
substances. Liquid absorber media are for example aqueous
alkanolamine solutions, specifically aqueous methyl
diethanolamine, or for example cold methanol, employed in the
Rectisol process. Such processes using liquid absorber media
are known and established in various technologies.
Disadvantages in the prior art consist primarily in the fact
that gas scrubbing processes involve an increased energy
requirement because they may be associated with process steps
such as compression and/or cooling.
The regeneration of absorber media is accomplished by means of
distillation, with which a substantial degree of energy
consumption is associated.
Furthermore, processes of said type are relatively complicated
in terms of infrastructure and setup, which has
disadvantageous implications in relation to process

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implementation and investment costs.
SUMMARY
The object of some embodiments of the invention is to describe an energy-
saving option for removing harmful substances in power stations which
are substantially fired by fossil fuels. It is furthermore
intended to disclose an easy-to-use apparatus for performing
the process.
Some embodiments of the invention are based on the fact that in the case
of a gas mixture which in large part consists of carbon dioxide CO2 and
includes proportions of valuable gases and harmful substances,
particularly in the case of condensation of the carbon
dioxide, the harmful substances preferably accumulate in the
liquid carbon dioxide. This knowledge is exploited to the
effect that the separation of the harmful substances from the
liquid phase of the carbon dioxide takes place at low
temperatures through the use of adsorber materials/adsorbents,
preferably solid adsorber materials. In this case it is
particularly advantageous that the energy balance associated
with separating harmful substances at low temperatures turns
out to be positive, in other words the overall energy
requirement is less.
The separation of the harmful substances from the liquid phase
can take place at low temperatures very advantageously by
means of adsorber materials because the materials employed
here have large surface areas which are available for the
absorption of the harmful substances.
If the process for separating harmful substances from a gas
stream consisting substantially of carbon dioxide is combined
with a process in which carbon dioxide is already present in
liquid form, synergistic effects in the overall energy balance

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can thereby be achieved.
It is particularly advantageous to set and maintain the
process temperature at less than -30 C. It is not beneficial
to use temperatures below -70 C because carbon dioxide exists
in the solid phase at such low temperatures.
The process pressure must in principle lie above the triple
point of carbon dioxide in accordance with the
temperature/pressure diagram. It is thus at least 5 bar. A
temperature range which can likewise be used for the process
temperature begins at -5 C and extends toward lower
temperatures.
The use of the process for separating harmful substances from
a gas mixture which substantially consists of carbon dioxide
can be implemented particularly advantageously in so-called
zero-carbon dioxide power stations. In said power stations the
separation of the carbon dioxide can be carried out
cryogenically. With this approach the carbon dioxide is
brought to a low temperature, liquefied and separated out. The
liquid phase is very well suited for the adsorptive separation
of harmful substances because the latter by preference
accumulate in the liquid carbon dioxide. Furthermore, low
temperatures promote adsorption, which means for example that
a gas scrubbing stage as provided in the prior art is no
longer necessary. The use of fixed-bed adsorbers for the
adsorption of harmful substances is advantageous because
solids having a large surface area are employed here. These
are in particular alumina (aluminum oxide), activated carbon,
silica gel, zeolites or polymers having a large surface area.
Since the surface area of the adsorbents becomes populated

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with harmful substances in the course of the process, a
periodic regeneration of individual fixed-bed adsorbers is
advantageously provided.
According to one aspect of the present invention, there is
5 provided a process for separating a harmful substance from a
gas mixture which essentially consists of carbon dioxide as
well as a recyclable substance, comprising: carrying out a
carbon dioxide condensation in order to produce and to separate
liquid carbon dioxide; carrying out an adsorptive separation of
the harmful substance from the liquid carbon dioxide in order
to remove the harmful substance from the liquid carbon dioxide;
wherein the recyclable substance remains in the liquid carbon
dioxide exiting the adsorptive separation; and setting a
process temperature in a range from
-30 C to -70 C.
According to another aspect of the present invention, there is
provided an apparatus for separating a harmful substance from a
gas mixture which essentially consists of carbon dioxide as
well as a recyclable substance, comprising: a carbon dioxide
condensation unit to produce and to separate liquid carbon
dioxide; and a fixed-bed adsorber for carrying out an
adsorptive separation of the harmful substance from liquid
carbon dioxide in order to remove the harmful substances from
the liquid carbon dioxide, wherein the recyclable substance
remains in the liquid carbon dioxide exiting the adsorptive
separation, and wherein the operating temperature of the
apparatus lies in a range from -70 C to -30 C.

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5a
According to another aspect of the present invention, there is
provided a power station comprising: a cryogenic carbon dioxide
separation unit to provide cryogenically separated liquid
carbon dioxide; an apparatus for separating a harmful substance
from a mixture which essentially consists of liquid carbon
dioxide as well as a recyclable substance, the apparatus
comprising: a fixed-bed adsorber for the adsorptive separation
of the harmful substance from liquid carbon dioxide for the
purpose of removing the harmful substances therefrom, wherein
the recyclable substance remains in the liquid carbon dioxide
exiting the adsorptive separation, and wherein the operating
temperature of the apparatus lies in a range from -70 C to -
30 C.
BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments will be described in the following with
reference to schematic accompanying figures, in which
specifically:

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Figure 1 schematically illustrates a fixed-bed adsorber
through which liquid carbon dioxide containing
harmful substances is fed, the harmful substances
being adsorbed in the fixed-bed adsorber.
Figure 2 illustrates adsorbers connected in parallel which
can be switched over individually or in groups
between the adsorption and regeneration operating
states by means of corresponding valves.
DETAILED DESCRIPTION
The adsorption of harmful substances is carried out in fixed-
bed adsorbers, such as are illustrated in Figures 1 and 2.
After a certain period of use it is necessary to regenerate
the adsorbents. This is effected by means of a reduction in
pressure, an increase in temperature, or by feeding through a
gas or vapor, or by a combination thereof. For this reason
there exist a plurality of adsorbers which are in operation or
can be regenerated on an alternating basis. An arrangement of
three fixed-bed adsorbers is illustrated in Figure 2.
Analogous configurations for two or more than three adsorbers
are possible.
During the carbon dioxide condensation a substantial
accumulation of harmful substances contained in a gas mixture
occurs automatically in the liquid carbon dioxide. It has been

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possible to demonstrate this with the aid of simulation
calculations taking as example hydrogen sulfide H2S and also
carbonyl sulfide COS. The harmful substances can subsequently
be separated out from the liquid phase at low temperatures by
mans of adsorber materials.
The energy-saving separation of the harmful substances at
lower temperatures in the overall concept is advantageous. A
process according to the invention can be excellently combined
with a power station concept which already provides cryogenic
carbon dioxide separation.
The temperature range for operating the process can lie
between -5 C and -70 C. The lower temperature limit lies in
the region of the solidification of carbon dioxide from the
liquid phase, which means that solids can block the process.
As a general rule it is necessary to ensure that the fixed-bed
adsorbers are kept open for the liquid reaction stream.
However, the energy balance of the entire process reveals that
temperatures of -30 C and lower are advantageous for the
separation of harmful substances.
The valuable gases such as hydrogen, carbon monoxide, nitrogen
or noble gases which may be present in the gaseous or liquid
stream in the process are not adsorbed in the course of the
described process approach. Harmful substances such as
mercury, sulfur or their compounds, which are to be found
substantially in the liquid carbon dioxide stream, accumulate
on the surface in the fixed-bed adsorbers, in other words are
adsorbed by the adsorbents, and are thus retained in the
fixed-bed adsorbers. A further harmful substance which can be
separated out in this manner is carbonyl sulfide COS.

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Figure 1 shows a fixed-bed adsorber 1 into which a liquid
carbon dioxide stream 2 containing harmful substances is
introduced. A liquid carbon dioxide stream 3 free of harmful
substances emerges at the outlet of the fixed-bed adsorber 1.
Figure 2 shows three fixed-bed adsorbers 1 connected in
parallel. The liquid carbon dioxide stream 2 is supplied at
the top, harmful substances are captured in the fixed-bed
adsorbers 1, and the liquid carbon dioxide stream 3 free of
harmful substances can be removed at the bottom. Individual
fixed-bed adsorbers 1 can be switched over in each case for
regeneration purposes 4.
While a direct catalytic conversion may be associated with a
favorable state of equilibrium on account of the low
temperatures, it does however exhibit very unfavorable
kinetics. Very large quantities of catalysts would be
required, which would probably become very rapidly deactivated
due to the presence of sulfur.

Representative Drawing