Sorbent regeneration in a heated hollow-fiber assembly

Methods and apparatus relate to recovery of carbon dioxide and/or hydrogen sulfide from a gas mixture. Separating of the carbon dioxide, for example, from the gas mixture utilizes a liquid sorbent for the carbon dioxide. The liquid sorbent contacts the gas mixture for transfer of the carbon dioxide from the gas mixture to the liquid sorbent. The carbon dioxide then desorbs from the liquid sorbent using hollow-fiber contactors as a source of heat to liberate the carbon dioxide further separated by the hollow-fiber contactors from the liquid sorbent.

FIELD OF THE INVENTION

Embodiments of the invention relate to methods of regenerating sorbent with heated hollow-fiber contactors that also facilitate separation of gases desorbed from the sorbent.

BACKGROUND OF THE INVENTION

Desire to reduce greenhouse gas emissions in various industrial processes requires viable carbon dioxide mitigation strategies. Capture of the carbon dioxide depends on ability to separate the carbon dioxide from a mixture. Separation of the carbon dioxide from the mixture enables transport of the carbon dioxide and subsequent handling or sequestering of the carbon dioxide.

Absorption processes utilize a sorbent to remove the carbon dioxide from the mixture followed by regeneration of the sorbent to liberate the carbon dioxide. The regeneration relies on heating of the sorbent to a temperature at which the carbon dioxide desorbs from the sorbent. Separation of the sorbent from the carbon dioxide released from the sorbent thereby isolates the carbon dioxide.

The regeneration of the sorbent thus contributes to costs and energy requirements associated with such carbon dioxide recovery. Previous energy intense and inefficient approaches for the regeneration utilize steam to transfer heat to the solvent. Factors such as equipment size, operating expense and capital expense contribute to making these past desorption units undesirable.

Therefore, a need exists for methods of desorbing gases from sorbent fluids to regenerate the sorbent fluids used in recovering the gases.

BRIEF SUMMARY OF THE DISCLOSURE

In one embodiment, a method of recovering carbon dioxide and/or hydrogen sulfide includes passing a sorbent loaded with at least one of carbon dioxide and hydrogen sulfide along hollow-fiber contactors of a desorption unit. The method further includes heating the sorbent by applying a DC or AC electrical voltage across the hollow-fiber contactors heated by electrical resistance and/or directing at least one of microwave and radiofrequency energy toward the hollow-fiber contactors which are heated upon absorbing the energy. The heating of the sorbent through thermal contact with the heated hollow-fiber contactors results in at least one of the absorbed gases, the carbon dioxide and the hydrogen sulfide, being released from the sorbent and transferred across walls of the hollow-fiber contactors through pores of the hollow-fiber contactors to regenerate the sorbent.

According to one embodiment, a method of recovering carbon dioxide includes transferring carbon dioxide from a gas mixture to a liquid sorbent and transferring the carbon dioxide from the liquid sorbent into a sweep gas, such as steam. The carbon dioxide transfers from the liquid sorbent to the steam through a hollow-fiber contactor that provides a heat source in addition to phase separation. Condensing the steam separates the carbon dioxide transferred to the steam.

For one embodiment, a system for recovering carbon dioxide and/or hydrogen sulfide includes a sorption unit in fluid communication with a sorbent and a mixture containing at least one of carbon dioxide and hydrogen sulfide for transfer of at least one of the carbon dioxide and the hydrogen sulfide to the sorbent. The system further includes a desorption unit having a heater including at least one of an electromagnetic generator for at least one of microwave and radiofrequency heating of hollow-fiber contactors disposed in the desorption unit and a DC or AC voltage source coupled to the hollow-fiber contactors for resistive heating of the hollow-fiber contactors. A circulation flow path loop couples the sorption and desorption units with the sorbent in fluid communication with a flow path along the hollow-fiber contactors.

DETAILED DESCRIPTION

Embodiments of the invention relate to recovery of carbon dioxide and/or hydrogen sulfide from a gas mixture, such as flue gas or natural gas that may be recovered as hydrocarbon production from a sour gas field. While described herein with respect to carbon dioxide recovery, systems and methods disclosed also enable hydrogen sulfide (H2S) recovery along with the carbon dioxide or in a same manner as the carbon dioxide recovery, if the hydrogen sulfide is present in the gas mixture either with or without the carbon dioxide. Separating of the carbon dioxide from the gas mixture utilizes a liquid sorbent for the carbon dioxide. The liquid sorbent contacts the gas mixture for transfer of the carbon dioxide from the gas mixture to the liquid sorbent, which may be aqueous amine solutions or ionic liquids. The carbon dioxide then desorbs from the liquid sorbent using hollow-fiber contactors as a source of heat to liberate the carbon dioxide further separated by the hollow-fiber contactors from the liquid sorbent.

FIG. 1illustrates a schematic of a contactor system including an exemplary sorption unit100and a desorption unit102coupled to a condenser and steam generator104. The desorption unit102as described herein provides sorbent regeneration regardless of specific configuration of the sorption unit100. For example, the sorption unit100may employ other configurations that do not include a plurality of hollow-fiber sorption contactors (represented by a dotted line)101.

In operation, a gas mixture106, such as flue gas that contains nitrogen (N2) and carbon dioxide (CO2), enters the sorption unit100. The gas mixture106passes through the sorption unit100along a flow path defined by the plurality of hollow-fiber sorption contactors101that enable contact of the gas mixture106with a liquid stream of lean sorbent108passing through the sorption unit100. Regardless of whether the sorption contactors101are utilized, the sorption unit100functions to contact the gas mixture106with the lean sorbent108.

The carbon dioxide in the gas mixture106diffuses across the hollow-fiber sorption contactors101. This diffusion at least reduces concentration of the carbon dioxide in a resulting treated output110of the sorption unit100relative to concentration of the carbon dioxide in the gas mixture106that is input into the sorption unit100. The lean sorbent108that sorbs the carbon dioxide transferred through the sorption contactors101exits the sorption unit100as rich sorbent112for feeding into the desorption unit102.

A sweep gas, such as steam114, passes through the desorption unit102along a flow path defined by a plurality of heated hollow-fiber desorption contactors (represented by a dotted line)103that separate the steam114from the rich sorbent112. The desorption unit102includes a contactor heating device120such that the desorption contactors103provide a source of heat for heating the rich sorbent112. As used herein, the source of heat or heat source refers to origination of heat from another energy form as opposed to transference of heat already generated. The desorption contactors103thus heat the rich sorbent112without relying on heat transfer from the steam114across the desorption contactors103. In some embodiments, the heating provided by operation of the contactor heating device120alone raises temperature of the rich sorbent112to above 100° C. or between 120° C. and 140° C. by heat transfer from the desorption contactors103.

Examples of the heating device120include at least one of an electromagnetic generator for microwave and/or radiofrequency heating of the desorption contactors103and a voltage source coupled to the desorption contactors103for resistive heating of the desorption contactors103. The resistive heating may utilize either direct and/or alternating current. If employing a microwave generator, the heating device120may generate electromagnetic energy at 915 megahertz (MHz) or 2,450 MHz, for some embodiments, or at another frequency selected to correspond with a microwave absorption spectrum of the desorption contactors103. Some embodiments heat the desorption contactors103through a combination of the resistive heating and the microwave heating. Further, the resistive heating and the microwave heating of the desorption contactors103may to a lesser extent increase temperature of the rich sorbent112as a result of direct microwave absorption by the rich sorbent112and/or induced heating from electrical resistance of the rich sorbent112.

Heat transfers from the desorption contactors103to the rich sorbent112for liberating the carbon dioxide from the rich sorbent112thereby regenerating the lean sorbent108supplied to the sorption unit100. The carbon dioxide carried by the steam114exits the desorption unit102as a combined vapor stream116input into the condenser and steam generator104. In the condenser and steam generator104, the carbon dioxide separates from liquid water upon cooling of the combined vapor stream116and steam condensation. Overhead of the carbon dioxide separated from the water forms a recovered carbon dioxide output118. Subsequent heating of the water by the condenser and steam generator104produces the steam114that is recycled for feeding to the desorption unit102.

In comparison to using the steam114for sweeping the carbon dioxide liberated from the rich sorbent112, relying on steam-based heating for all heating requirements of desorption requires extra expense of a relative higher quality steam. Some embodiments may further utilize gases other than steam as the sweep gas or generate a vacuum to carry away for processing any of the carbon dioxide transferred between inside and outside of the desorption contactors103. Suitable gases for the sweep gas include hydrocarbons, such as pentane, or other gases condensable to provide relative easier separation from the carbon dioxide based on boiling points than separation of the carbon dioxide from the gas mixture106.

FIG. 2shows a cross-sectional representation of a single fiber203referred to herein as hollow-fiber due to having a cylindrical structure with an open interior bore and a diameter between about 200 microns and about 2 millimeters. Multiple lengths of the fiber203assembled together may exemplify the heated hollow-fiber desorption contactors103disposed in the desorption unit102depicted inFIG. 1. In some embodiments, a voltage source220applies a voltage across the fiber203to provide resistive heating of the fiber203.

The rich sorbent112containing the carbon dioxide (CO2), monoethanolamine (MEA) and liquid phase water (H2O(l)), for example, may flow through the interior bore of the fiber203without being dispersed in the steam114flowing along an exterior of the fiber203. Since the fiber203is permeable to gas, the carbon dioxide and some vaporized water (H2O(v)) resulting from the heating pass through pores of the fiber203leaving behind within the fiber203the lean sorbent108that is thereby regenerated. In some embodiments, the steam114may flow through the interior bore of the fiber203without being dispersed in the rich sorbent112flowing along the exterior of the fiber203.

Packing density for multiple lengths of the fiber203and surface area of the fiber203enable efficient thermal transfer. The fiber203achieves surface areas of 1500 m2/m3or more. The surface area achieved with use of the fiber203correlates to heat source size since the fiber203is heated.

Pore size and hydrophobic properties of the fiber203maintain separation of a bulk gas phase (e.g., the steam114) from a bulk liquid phase (e.g., the rich sorbent112). This non-dispersive flow through the desorption unit102avoids problematic issues including flooding, entrainment, channeling and foaming that often occur with dispersive contacting. The non-dispersive flow further enables control of gas and/or liquid flow rates without limitations of fluid-mechanics within towers that utilize the dispersive contacting.

In some embodiments, a hydrophobic polymeric material that can be processed into the hollow-fiber thus forms the fiber203. Suitable materials that form the fiber203can dissolve in a spinning solvent, be cast into the fiber203and be compatible with sorbent formulations desired for use in removing the carbon dioxide. Various exemplary compositions of polymer that may form the fiber203satisfy foregoing criteria and include but are not limited to polysulfones, polyimides, polyethers, polycarbonates, fluoropolymers, or polymers of amides with various other constituent monomer combinations.

For some embodiments, a spinning process fabricates the fiber203from an extrusion mixture of the polymer (e.g., polyimide) and the solvent for the polymer. Examples of suitable solvents include N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc) and dimethyl sulfoxide (DMSO). These solvents provide miscibility in water that is high enough to promote phase separation during quenching of the extrusion mixture in an aqueous media while spinning. Concentration of the polymer in the extrusion mixture influences pore size and porosity given that pore size and porosity increase as the concentration of the polymer in the extrusion mixture decreases.

In some embodiments, the extrusion mixture includes an additive, such as a conductive agent or microwave absorbent, for incorporation into the fiber203to facilitate the heating described herein. Examples of the additive include metallic or carbonaceous particles, such as graphite powder. Particle size of the additive when applied to the extrusion mixture may range from one nanometer to one micron. Preparing the fiber203by spinning a composition doped with the conducting agent enables heating of the fiber203with the voltage source220upon passing current through a network of the conductive material within the fiber203. The carbonaceous particles may also function as the microwave absorbent to facilitate heating of the fiber203with microwave energy.

Some embodiments apply the additive as a coating on the fiber203. Such coating techniques include incipient wetness deposition or vapor deposition. The additive may coat an inside surface of the fiber203, an outside surface of the fiber203or both the inside and outside surfaces of the fiber203.

The extrusion mixture used in some embodiments includes the solvent and carbonaceous material dispersed in the solvent along with the polymer or instead of the polymer. The carbonaceous material, such as pitch, results in the fiber203formed upon the spinning containing carbon. Following the spinning, the carbonaceous material may further be carbonized to prepare the fiber203having a carbon content that aides in the heating as described herein.

For some embodiments, preparing the fiber203includes preheating the carbonaceous material or a polymeric material, such as the polymer described herein, to a temperature sufficient to carbonize the material forming the fiber203that is thereby made conductive due to its carbon content. The materials suitable for such carbonization decompose to carbon without melting and may be those used in commercial hollow-fiber contactors without the carbonization. By way of example, a hollow-fiber contactor made from the material, such as polyimides, may carbonize upon preheating the polymeric material to a temperature above 400° C. in order to form the fiber203. As used herein, carbonization may result in the fiber203having at least 85%, at least 90% or at least 95% carbon content by weight.