METHOD FOR REMOVAL OF CO2 FROM EXHAUST GAS USING FACILITATED TRANSPORT MEMBRANES AND STEAM SWEEPING

The invention relates to methods for separating CO2 from mixed gases. A stream of mixed gases passes one side of a facilitated transport membrane, while a sweep fluid, such as steam, passes the other side of the membrane, removing the CO2. The method is especially useful in the removal of CO2 from gases produced by internal combustion engines on mobile devices.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now toFIG. 1, an embodiment of the invention is shown.

An engine, such as an internal combustion engine “10” is provided with both an air stream containing oxygen “11,” and a feed stream of a hydrocarbon fuel “12.” In operation, the engine produces exhaust gas “13” (which is cooled down to a suitable temperature for proper operation of the membrane module.) Such a practice is standard in the art and also used to produce steam “14.” Steam production can be achieved by tapping into the heat available in the hot exhaust gas heat exchanger and/or by tapping into the heat available in the hot coolant fluid of the engine, in each case via use of a heat exchanger. The exhaust gas is channeled to one side of an FT membrane “15,” which selectively removes CO2therefrom, while the steam produced is directed to the other side of the membrane, to remove the permeated gases. The steam and permeated gases stream leaving the membrane are directed to the knock down stage (16) where steam—water gas—is condensed and precipitated down by virtue of heat exchange, and is directed back to the engine (10) for steam production while the resultant CO2-rich stream is directed to next stage for densification and storage. The CO2lean exhaust gas then escapes to the atmosphere “17.” Separation of the CO2, or other gas of interest, occurs when the exhaust gas is passed on one side of the membrane (the so-called “feed” or retentate side), at appropriate conditions of temperature, pressure and flow rate. The CO2or other gas permeates the membrane and passes to the other side (the so-called “permeate side”). Any required driving force necessary to facilitate this can be created as a result of, creating a vacuum on the permeate side, increasing pressure on the gas on the feed or retentate side, and/or, preferably, via sweeping the permeate with a gas, such as steam, at constant pressure.

Note that in operation, the exhaust gas and steam travel in opposite directions; however, the CO2enriched steam then moves to an appropriate point for further removal of the CO2or other action.

While not wishing to be bound by any theory, performance for separation of any two gases, e.g., CO2and N2, is governed by (i) the permeability coefficient, or “PA,” and the selectivity or separation factor, or αA/B. The former is the product of gas flux and the thickness of the membrane divided by partial pressure difference across the membrane. The latter results from the ratios of gas permeability (“PA/PB”), where PAis the permeability of the more permeable gas, and PBthat of the lesser. It is desirable to have both high permeability and selectivity, because a higher permeability decreases the size of membrane necessary to treat a given amount of gas, while higher selectivity results in a more highly purified product.

Operation of the invention will be seen in the examples which follow.

EXAMPLES

The following examples detail a simulation of a facilitated transport membrane in combination with steam sweeping, for removing CO2from a mixed gas exhaust feed.

The exhaust gas composition was CO2(˜(13%), N2(˜74%), and H2O (˜13%). This is representative of exhaust gas produced by combustion engines using hydrocarbon fuels.

The simulation was set up for 30% recovery of CO2from a mixed gas, with a composition as described supra, and an exhaust gas flow rate of 28.9 gmol/min. Feed and permeate pressures of 1.5 atm and 1.0 atm, respectively were used, and the results are shown inFIG. 5, where steam was used for the sweeping step.FIGS. 2-4, in contrast, present results with no sweep conditions and with different permeate pressures but a fixed feed pressure (1.5 atm).

The theoretical membrane of the simulation had a CO2permeability of 4000 Barrer (1 Barrer=10-10 cm3.(STP).cm.cm-2s-1 cm Hg-1), a CO2/N2 selectivity of about 400, and water permeability of 15000 Barrer. Two coating thicknesses, i.e., 10.0 um and 1.0 um were tested.

Criteria evaluated included the effect of feed/permeate pressure ration (Pf/Pp) and, as noted, the coating thickness.

FIG. 3shows that high purity CO2 (greater than 90%) can be obtained at a Pf/Pp ratio of 4 or greater. This experiment, however, did not use steam sweeping on the permeate side.

FIGS. 2 and 3shows the very high permeability of water and CO2 mimics the effect of using sweep steam on the permeate side.

FIG. 4shows the area, in m2, needed to recover 30% of CO2from exhaust gas, for the two different coating thicknesses discussed supra. The figure shows that there was a sharp reduction in the required membrane area as the ratio increases, and the membrane thickness decreases.

In follow-up experiments, a simulation was carried out testing a steam sweep flow rate/gas permeate flow rate ratio on separating CO2from the mixed gas described supra. The results are shown inFIGS. 5 and 6, with the ratio plotted as the X-axis (Qw/Qd). The exhaust gas flow rate, and the feed and permeate pressures were fixed at 1.5 and 1.0 atm.

In total, the results of the simulation shown that the theoretical, highly permeable facilitated transport membrane, when employed in the steam sweep methodology discussed herein, resulted in high CO2concentration (ranging up to 97% pure CO2FIG. 5when the sweep steam flow ratio with respect to the dry permeate is set at 4.5 or higher.

FIG. 6shows the area, in m2, needed to recover 30% of CO2from exhaust gas, for the two different coating thicknesses discussed. The figure shows that there was a sharp reduction in the required membrane area as the sweep steam ratio increases, and the membrane thickness decreases.

FIGS. 4 and 6shows that high permeability membranes are necessary in this methodology, as less membrane area was required for low membrane thickness and high permeability of the membrane.

The foregoing experiments set forth aspects of the invention, which is, inter alia, a method for removing a gas, CO2in particular, from a mixed gas stream, using a facilitated transport membrane in combination with pressure driven and steam sweep technologies. In practice, the mixed gas stream, such as exhaust gas from an internal combustion engine, follows a path along a first side of a facilitated transport membrane, where the membrane is specifically permeable to a specific gas, such as CO2. For CO2, the “FT” membrane preferably has a permeability for CO2of at least 1000 barrers.

A sweep fluid, preferably steam, is provided via e.g., action of a cooling system on the source of the mixed gas, such as the internal combustion engine. The steam, be it from this configuration or another, is directed along the side opposite the side on which the mixer gas stream passes, and in the opposite direction. CO2or some other gas moves into the sweep liquid and is carried away to, e.g., a temporary storage unit for further processing.

Different conditions of pressure, membrane thickness, gas flow, and other factors may be employed with the invention remaining operative, as the figures show.

Other features of the invention will be clear to the skilled artisan and need not be reiterated here.