|Title of Invention
A METHOD FOR REMOVING AND PREVENTING DISCHARGE OF CARBON DIOXIDE
|A method for removing and preventing discharge of carbon dioxide The present invention relates to a method for removing and preventing discharge of carbon dioxide from combustion gases and natural gas into the atmosphere from installations for production of oil and/or gas, wherein the combustion gas is passed to an absorber containing a solvent, where carbon dioxide is absorbed in the said solvent, and the thereby purified combustion gas, largely free of carbon dioxide, is released into the atmosphere, where the C02-rich solvent is passed to a desorber where C02 is removed from the solvent, and the thereby largely C02-free solvent is recycled to the absorber, and the separated CO2 gas is passed to a compression stage for compression and utilization and/or disposal in a suitable manner, characterized in that membrane gas/liquid contactors having a packing factor in the range 500-1000 m2 /m3 are employed in both the absorber and the desorber, and that an external stripping steam is supplied to the desorber.
|The present invention relates to a method for removing and preventing discharge of carbon dioxide from combustion gases and natural gas, into the atomosphere.
The removal of carbon dioxide or other compounds from gases may be desirable or necessary for a number of reasons. If a gas is to be burned as fuel or emitted into the atmosphere as a waste flow, the removal of carbon dioxide from the gas is necessary in order to satisfy the carbon dioxide emission requirements which are set by air pollution control authorities. By removing CO2 from natural gas, a natural gas is obtained which satisfies sales specifications or other process-dependent requirements.
Several processes for removing carbon dioxide from gases are known, including those from EP patent applications nos. 410845, 502596, 537593, 551876, 553643, 558019 and 588175. In the applicant's Norwegian patent application no. 940527 there is further disclosed a method for removing and preventing discharge into the atmosphere of carbon dioxide from combustion gases from thermal power machines, especially gas turbines, for production of oil and/or gas where about 40% of the combustion gas is recycled to the compressor inlet for the said gas turbine before the combustion gas is passed to the absorption stage of the process.
Gas absorption is a unit operation where one or more components in a gas mixture are dissolved in a liquid (solvent). The absorption may either be a purely physical phenomenon or involve a chemical reaction, such as the reaction between CO2 and mono ethanolamine (MEA).
An absorbed component is normally removed from the solvent by means of a distillation or stripping process, see figure 1.
An example of an absorption process is the process for removing CO2 from flue gas by means of the monoethanolamine. The flue gas is led into an absorption column where it comes into contact with MEA which absorbs the CO2 molecules. The solvent is then led to a desorption process where the liquid is heated, and the CO2 molecules are removed from the solvent by means of a desorption column. The solvent is cooled and passed back to the absorption column, while the concentrated CO2 is removed.
When an absorption column is designed, there are two important factors which determine the size:
i) The amount of gas which has to be treated often or in most cases determines the diameter of the column. If the rate of the gas flow upwards in the column becomes too high due to a too small tower diameter, it will bring with it the solvent which is intended to run downwards in the column, resulting in flooding.
ii) The degree of purification determines the height of the column. In
order to have components from the flue gas absorbed, the components have to meet the solvent. In other words what is required is a certain liquid surface (m ) in contact with the gas. Inside the absorption column there is equipment which is designed in such a manner that the gas which flows upwards comes into the best possible contact with solvent which is running downwards (highest possible packing factor m 2/m3). This means that the height of the column is determined by the degree of purification/required liquid area. If the physical absorption goes slowly or the chemical reaction in the absorption column has a low reaction rate, it may be the residence time required for the solvent which determines the height of the column.
When a desorption column is designed the same rules/restrictions apply in principle. The diameter of the desorption column is also determined in many cases by the amount of stripping gas required, while the height is determined by the purity desired in the solvent which is employed.
In connection with absorption processes there will be a consumption of solvent, principally due to evaporation of the solvent in the absorption column; evaporation of the solvent in the desorption column; degrading of the solvent, particularly in connection with the boiler/reboiler where the solvent is degraded due to high surface temperature on the surfaces which transfer the heat to the solvent; chemical degradation due to impurities in the system; and/or carry-over of drops of solvent which accompany the gas.
In most absorption/desorption processes, particularly processes where amines are used, corrosion is a problem. Corrosion arises mainly in the absorption column, the desorption column and the boiler/reboiler, and the corrosion products which are formed must be removed by means of filters in order to avoid problems in the process.
In connection with the operation of absorption and desorption columns foaming can be a major problem. Foaming can occur for many reasons including particles in the solvent (e.g. corrosion products). In present processes a careful watch is kept on possible foaming, which is combated with the use of filters, alteration in the operation of the actual column and/or by means of chemicals.
If the packing material in the columns is not packed in a completely uniform fashion, channels will be formed where the gas can move with low pressure loss, with the result that a part of the gas will remain untreated, or pass through the column with a reduced degree of absorption. This applies both to the absorption and the desorption column.
With regard to the operation of absorption processes the greatest possible flexibility is to be desired with a view to the amount of gas which has to be treated, circulation rate and steam which is employed. When columns are used the flexibility is limited due, amongst other reasons, to the carry-over of solvents, flooding and wetting of the packing surfaces.
In the absorption processes which are used in the treatment of natural gas hydrocarbons and BTX aromatics from the natural gas are absorbed by the solvent and stripped from the solvent in the desorption column. The loss of hydrocarbons requires to be minimised for economic reasons, while the discharge of BTX aromatics from the desorption column requires to be minimised for environmental reasons.
In connection with the choice/development of solvent the viscosity and surface tension of the liquid are important in order to ensure that the packing material in the absorption and desorption columns is wetted/covered by liquid, thus causing the liquid to run downwards in the column in an optimum manner. In order to ensure this, it is not always possible to employ the solvent which is optimal for the process. The reactant(s) which are active for the absorption process are normally dissolved in a liquid which does not itself participate directly in the absorption reactions, e.g. MEA is often dissolved in water. Such physical solvents (in some cases a large percentage of liquids are employed which do not affect the process, such as water) are necessary in order to give the solvent the optimum total characteristic. This use of liquids which are "unnecessary or neutral" for the process increases
the energy consumption of the process due to a high circulation rate (liquid flow in circulation) which gives increased pump work and high energy consumption in the boiler/reboiler, thus requiring unnecessary liquid to be heated up to the desorption temperature.
When absorption and desorption columns are employed solvent must be chosen which has good mass transfer properties for the component, such as C02, which has to be absorbed and desorbed, in order thereby to keep the size of the absorption and desorption columns at an acceptable level. A contactor with a large contact area between gas and liquid per volume unit will open the way for the use of more stable, economical and environmentally correct solvents.
With regard to the removal of C02 from natural gas, for environmental reasons the depositing of C02 has become a subject of current interest. In commercial processes where amines are employed for separation of C02 from natural gas, C02 is desorbed at or very close to atmospheric pressure. It is desirable to be able to desorb C02 at a somewhat higher pressure in order to save compression energy. Due to the degradation of amine it is difficult to implement this since the temperature has to be increased in the desorption column. The amount of amine which degrades in the boiler/reboiler increases exponentially with the amine temperature in the boiler/reboiler.
US 4147754 relates to a method of selectively removing hydrogen sulfide from a mixture of gases including carbon dioxide, such as gasified coal. As carbon dioxide is one constituent of gasified coal in addition to hydrogen sulfide, some carbon dioxide may be removed during the process of removing hydrogen sulfide, Hence, the main purpose of US 4147754 is to remove as much hydrogen sulfide as possible and as little carbon dioxide as possible, making hydrogen sulfide in the stripper effluent as concentrated as possible in order to make a Claus plant less costly and more efficient. It is explicitly disclosed in column 4, lines 63-68 of US 4147754 that by reducing the removal of carbon dioxide from the coal gases, the power cycle efficiency is increased. Hence, US 4147754 clearly does not relate to a method for removing and preventing discharge into the atmosphere of carbon dioxide.
It is noted that gasified coal is treated in US 4147754, and not flue gas or natural gas from installations for production of oil and/or gas. Gasified coal gas, flue gas, and natural gas from installations for production of oil and/or gas are gases of different compositions, so that "gasified coal gas" cannot possibly anticipate "flue gas" or "natural gas".
US 4147754 fails to disclose a membrane gas/liquid contactor having a packing factor in the range of 500-1000 m2/m3.
A proper assessment of the issue of novelty of the subject-matter of claim 1 allows only one conclusion, namely that US 4147754 does not anticipate the subject matter of this claim.
WO 9521683 is an application of the same applicant and partially of the same inventors. WO 9521683 relates to a method for removing and preventing emissions into the atmosphere of carbon dioxide from exhaust gases from heat engines installed on offshore platforms for the production of oil and/or gas.
The objective technical problem is that costs of compression energy should be reduced. This corresponds to the provision of a process of a desorption step which may be operated at a higher pressure in order to save compression energy (please see page 5, lines 15-20 of the present application). This objective technical problem in view of WO 9521683 is solved by using a membrane gas/liquid contactor having a packing factor in the range of 500-1000 m2/m3 in the desorber instead of the conventional stripping column for rotating gas/liquid contactor which is called HIGEE, as disclosed in WO 9521683. That is, it is the solution provided by the present application to provide a method employing membrane gas/liquid contactors having specific packing factors in both the absorber and the desorber.
There is no hint in WO 9521683 that a membrane gas/liquid contactor may be used in the desorber and that its use could even be advantageous. While WO 9521683 discloses several advantages of using gas absorption membranes, such as compactness of the equipment, no entrainment, flooding, channelling or foaming, see page 7, lines 23-32, WO 9521683 considers the use of a conventional stripping column with a boiler/reboiler to be essential for desorption. Hence, although the inventors of WO 9521683 were aware of a number of advantages of using membrane gas/liquid contactors for absorption, it did not appear to them to use these contactors for desorption despite the same potential problems relating to it. Hence, the use of membrane gas/liquid contactors for desorption was not even obvious to the inventors of WO 9521683 whose skills surely go beyond those of the average skilled person.
The only prior art document disclosing the use of a membrane gas/liquid contactor in a desorber unit is US 4147754. Hence, for the issue of inventive step, it is to be asked whether a skilled person, starting from the disclosure of WO 9521683, would refer to the disclosure of US 4147754 and then arrive at the teaching of the present application. As will be appreciated from the following, this is clearly not the case.
Firstly, it is again emphasized that US 4147754 refers to a different field of application, namely the removal of hydrogen sulfide, and not to the removal of carbon dioxide. As indicated in US 4147754, preventing air pollution necessitates removal of hydrogen sulfide from coal gas utilized in a combustion process in order to minimize release of sulfur dioxide into the atmosphere (see column 1, lines 16-19). The sour off-gas from such hydrogen sulfide recovery process is normally sent to a Claus plant for conversion to sulfur. It is the object of the invention of US 4147754 to provide a method for efficiently transferring hydrogen sulfide out of a mixture of gases including carbon dioxide (column 1, lines 65-67). In other words, US 4147754 attempts to discriminate the removal of hydrogen sulfide against carbon dioxide passage (column 1, line 40). This is achieved by the use of a membrane system comprising an immobilized liquid membrane of CCV'/HCCV solution in combination with a porous gas permeable barrier. A schematic diagram of stripping apparatus for removing hydrogen sulfide previously absorbed by the amine solution is shown in Fig. 6. However, this stripping apparatus permits that only a small amount of carbon dioxide passes into the steam sweep fluid (see column 6, lines 41-42).
Consequently, in view of the contrary teaching of US 4147754, the skilled person, starting from WO 9521683, would not even refer to the disclosure of US 4147754 to solve the above-mentioned objective problem.
Moreover, even if one were to assume that the skilled person considered the teaching of US 4141754 to solve the above-mentioned objective problem, he would have failed, it is implicit to said problem that the process to be provided have to be efficient, a process showing a reduced compression energy while being considerably less effective than the one of the closest prior art would not solve the objective problem. However, when using the liquid membrane used for desorption according to US 4147754 in the process of the present invention, a process showing such a decreased effectivity would be obtained since the immobilized liquid membrane of C0327HC03" merely does not allow an effective passage of C02.
Furthermore, neither US 4147754 nor WO 9521683 disclose a membrane gas/liquid contactor having a packing factor in the range of 500-1000 m2/m3.
With a view to achieving more optimal absorption/desorption processes an absorption/desorption technology has now been developed which can be used in several absorption/desorption processes. The technology developed provides an optimized process with regard to weight, cost, energy consumption and environmental aspects.
After having evaluated different solutions, an optimized process has now been produced which utilizes membrane gas/liquid contactors both in the absorber and the desorber.
If a membrane is placed between the gas and the solvent, the solvent will not be in direct contact with
the gas which is in motion. This division between the gas and solvent phases makes it possible to
employ a high gas rate in the absorber without the liquid being carried along by the gas, and in fig. 2 a
principle drawing is shown of the current technology. The size of the pores in the membrane is
selected according to the following reasoning: the pores are
so large that the X molecules (e.g. C02) move (diffuse) rapidly through the pores and into the solvent, and the pores are so small that solvent does not penetrate into the pores and through the membrane.
It is an object of the present invention to provide membrane gas/liquid contactors both in the absorber and desorber, as illustrated in fig. 3 for the present process, where the extent of the absorption process islubstantially reduced since the membrane contactor has a high packing factor (m /m ). This gives a reduction in weight and volume for both the absorber and the desorber compared with conventional columns.
It is a second object of the invention to avoid foaming. Since there is not contact between gas and solvent, no foaming will occur. It will be possible to reduce the number of filters and the use of defoaming agent when membrane contactors are employed both in the absorber and the desorber.
It is a further object of the invention to avoid channelling, since the membranes are assembled with a uniform pitch, the distance between the membranes being uniform.
Yet another object of the invention is to avoid carry-over of the solvent. The solvent is not pushed out of the absorber or the desorber due to a high gas rate.
A further object of the invention is to achieve a process which in total is very flexible and thereby simpler to operate since there is no contact between the gas and the solvent in both the absorber and the desorber. The amount of feed gas, circulation rate and amount of stripper gas may be varied independently of one another.
It is a further object of the invention to reduce the absorption of hydro¬carbons and BTX aromatics, thus reducing the loss of hydrocarbons and the discharge of BTX aromatics, by optimization of the membrane type, pore size, surface tension, etc.
It is a further object of the invention to permit optimization of the solvent by the use of the membrane gas/liquid contactor in both the absorber and the desorber, since more flexible requirements are placed on the solvent's physical properties, such as viscosity and surface tension. The liquid is pumped through the membrane modules' liquid channels. The amount of
passive liquid in the solvent whose only function is to supply the liquid with the desired physical properties can be reduced or completely removed. The use of process-optimal solvents will reduce the total energy consumption of the process.
It is a further object of the invention to be able to use solvents with lower mass transfer coefficients due to the membrane gas/liquid contactor's high packing factor. The size of the absorber and the desorber are reduced to such an extent that it is acceptable to use solvents which, e.g., only have half as good mass transfer as today's solvents, without the absorber and the desorber becoming unacceptably large. This effect opens the way for the use of new or other solvents which provide a lower energy consumption, less consumption of solvent and fewer environmental problems.
A further object of the invention is to substantially reduce the corrosion problem by means of membrane contactors in both the absorber and the desorber, since the absorber and the desorber are principally constructed of polymers. Due to the reduced equipment size by using a membrane gas/liquid contactor both in the absorber and the desorber, it may be economically justified to use high steel in the connecting pipework. This will reduce the corrosion rate and thereby further minimise operational problems, and reduce the consumption of corrosion inhibitors.
It is a further object of the invention to reduce the consumption of solvent, since the correct choice of membranes reduces the evaporation from the absorber and the desorber. It is particularly important that the process employs external steam, i.e. a boiler/reboiler should not be used. The membrane contactor is also used in order to transfer necessary desorption heat, i.e. the membrane contactor both transfers molecules from the solvent to the stripper gas while at the same time transferring heat from the stripper gas to the solvent. This eliminates the degradation and the corrosion which normally take place in the boiler/desorber. If necessary, a preheater may be installed upstream of the desorber.
A further object of the invention is to permit the implementation of desorption at a somewhat higher pressure than atmospheric by means of membrane contactors in the desorber in amine processes for separation of C02. This is possible due to the use of stripper steam instead of the
boiler/reboiler, which would cause an unacceptable degree of degradation of the amine. The degradation rate of the amine may also be reduced by eliminating the reboiler encountered in conventional processes. Instead, the membrane contactor may act as a heat exchanger and provide the necessary heat for desorption through heat transfer from externally supplied vapor through the membrane to the amine solution.
According to the present invention a method is provided for removing and preventing discharge into the atmosphere of carbon dioxide from combustion gases and natural gas from installations for production of oil and/or gas, where the combustion gas is passed to an absorber containing a solvent, where carbon dioxide is absorbed in the said solvent, and the thereby purified combustion gas, largely free of carbon dioxide, is released into the atmosphere, where the C02-rich solvent is passed to a desorber where C02 is removed from the solvent, and the thereby largely C02-free solvent is recycled to the absorber, and the separated C02 gas is passed to a compression stage for compression and utilization and/or disposal in a suitable manner, which is characterized by the use of membrane gas/liquid contactors in both the absorber and the desorber, and that an external stripping steam is supplied to the desorber.
This and other features of the present invention are presented in the following patent claims.
The theoretical calculations and tests which have been conducted for the present invention have demonstrated that membrane gas/liquid contactors can advantageously replace both absorption and desorption columns. Figure 4 shows a typical comparison of a conventional gas/liquid contactor and a membrane gas/liquid contactor of the present invention.
The mass transfer coefficient is defined/calculated by means of the following equation:
k = mass transfer coefficient (m/s)
Q = volume flow (Nm3/s)
Am = membrane area (m2)
yin = mol fraction X into the absorber/desorber
yout = mol fraction X out of the absorber/desorber
The mass transfer coefficient will vary from process to process, but for the tested absorption/desorption processes the combination of mass transfer coefficient and packing factor for the membrane system gives a reduction of between 40-95% in the size and weight of the absorber and the desorber compared with conventional towers with standard tower packings.
For separation of C02 by means of amine systems the following values have been measured and calculated: The absorber: (0.1-8.0)10"3 m/s y The desorber: (0.1-2.0)10"3 m/s y
Theoretically a membrane gas/liquid contactor with a high membrane packing density (m /m ) will reduce the required equipment size to carry out a contacting process providing the mass transfer for the said process is good enough. It is proven that the packing density for a membrane contactor can be 500-1000 m2/m3 compared to typically 100-200 m2/m3 for traditional structured packing columns.
The mass transfer coefficent has been measured for a number of processes and running conditions by laboratory work at various locations. These numbers show that by using a membrane gas/liquid contactor both in the absorber and desorber, significant reductions in both equipment size and weight can be achieved.
This example is intended to be illustrative of the invention and is not meant to be construed as limiting the scope of the invention.
Desorption of C02 from monoethanolamine through water vapor stripping, sized to desorb 233 kmole/h C02:
For an offshore application, further weight reduction may be achieved by the decreased amount of structural steel needed to support the unit.
1. A method for removing and preventing discharge of carbon dioxide from
combustion gases and natural gas into the atmosphere from installations for
production of oil and/or gas, wherein the combustion gas is passed to an absorber
containing a solvent, where carbon dioxide is absorbed in the said solvent, and the
thereby purified combustion gas, largely free of carbon dioxide, is released into the
atmosphere, where the CCVrich solvent is passed to a desorber where CO2 is removed
from the solvent, and the thereby largely C02-free solvent is recycled to the absorber,
and the separated C02 gas is passed to a compression stage for compression and
utilization and/or disposal in a suitable manner, characterized in that membrane
gas/liquid contactors having a packing factor in the range 500-1000 m2/m3 are
employed in both the absorber and the desorber, and that an external stripping steam is
supplied to the desorber.
2. The method according to claim 1, wherein solvent is employed with a mass transfer coefficient in the range (0.1-8.0)10"3 m/s for the absorber and (0.1-2.0)10"3 m/s for the desorber.
3. The method according to claim 2, wherein the solvent which is employed is an amine.
4. The method according to claims 1 to 3, wherein the combustion gas/natural gas which is passed to the absorber's membrane gas/liquid contactor has a temperature in the range 20°C-70°C.
5. The method according to claims 1 to 4, wherein C02 is removed from the desorber's membrane gas/liquid contactor by means of heating to a temperature of 120°C-150°C.
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|Name of Patentee
|POSTBOX 169, N-1324 LYSAKER
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