Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through immobilized liquid
Reexamination Certificate
1999-08-20
2001-05-08
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through immobilized liquid
C095S046000, C095S051000, C096S005000, C096S006000, C096S007000
Reexamination Certificate
active
06228145
ABSTRACT:
The present invention relates to a method for removing carbon dioxide from combustion gases and natural gas.
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 CO
2
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 gas turbine before the combustion gas if 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 CO
2
and monoethanolamine (MEA).
An absorbed component is normally removed from the solvent by means of a distillation or stripping process, see FIG.
1
.
An example of an absorption process is the process for removing CO
2
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 CO
2
molecules. The solvent is then led to a desorption process where the liquid is heated, and the CO
2
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 CO
2
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
2
) 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
/m
3
). 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 CO
2
, 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 CO
2
from natural gas, for environmental reasons the depositing of CO
2
has become a subject of current interest. In commercial processes where amines are employed for separation of CO
2
from natural gas, CO
2
is desorbed at or very close to atmospheric pressure. It is desirable to be able to desorb CO
2
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 in
Dannström Henrik
Falk-Pedersen Olav
Arons Edward M.
Fish & Neave
Kvaerner ASA
Morris Robert W.
Spitzer Robert H.
LandOfFree
Method for removing carbon dioxide from gases does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for removing carbon dioxide from gases, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for removing carbon dioxide from gases will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2563159