Process and apparatus for removing heat and water from flue gas

Abstract

Disclosed is a process for use with flue gas having a moisture content M. The flue gas is introduced to strong brine adapted to exothermically absorb moisture. Simultaneously, heat is withdrawn. This produces heat, water-enriched brine and a gas having a moisture concentration less than M. The strong brine can be recovered by distillation from enriched brine to produce water. The brine temperature throughout absorption can remain within 2 F. of a temperature T in the range 220 F.-300 F. The heat withdrawal can be associated with gas-liquid phase change of a working fluid. The terminus of the heat flow can be associated with gas-liquid phase change of the working fluid. The working fluid can: as liquid, flow only by gravity, convection or wicking; and, as gas, flow only by diffusion or convection. The heat flow can drive a boiler producing steam. M can be greater than 15 wt. % water.

Claims

1. A process for use with a flue gas having a moisture content M, the process comprising the steps of: i. simultaneously introducing said flue gas to a flow of strong brine adapted to exothermically absorb moisture from the flue gas to produce heat; and withdrawing heat, thereby to produce a flow of heat, a flow of water-enriched brine and a gas having a moisture concentration less than M; and ii. recovering the strong brine from the water-enriched brine to produce a flow of steam, characterized in that a. the temperature of the brine throughout the absorption step remains within 2 F. of a temperature T that lies in the range 220 F.-300 F.; and/or b. the withdrawal of heat is associated with the phase change of a working fluid from the liquid state to a gaseous state; the terminus of the heat flow is associated with the phase change of the working fluid from the gaseous state to the liquid state; in the liquid state, the working fluid flows only by one or more of gravity, convection and wicking; and in the gaseous state, the working fluid flows only by one or more of diffusion and convection; and/or c. the flow of heat directly drives a boiler to produce positive pressure steam; and/or d. M is greater than 10 wt. % water; and/or e. the strong brine is recovered through multiple-effect distillation of the water-enriched brine.

2. The process according to claim 1, further comprised in that the temperature of the brine throughout the absorption step remains within 2 F. of the temperature T.

3. The process according to claim 1, further comprised in that: the withdrawal of heat is associated with the phase change of a working fluid from the liquid state to a gaseous state; the terminus of the heat flow is associated with the phase change of the working fluid from the gaseous state to the liquid state; in the liquid state, the working fluid flows only by one or more of gravity, convection and wicking; and in the gaseous state, the working fluid flows only by one or more of diffusion and convection.

4. The process according to claim 1, further comprised in that the flow of heat directly drives a boiler to produce positive pressure steam.

5. The process according to claim 1, further comprised in that M is greater than 12 wt. % water.

6. The process according to claim 1, further comprised in that the strong brine is recovered through multiple-effect distillation of the water-enriched brine.

7. The process according to claim 3, wherein the working fluid is contained within a plurality of heat pipes.

8. The process according to claim 7, wherein the heat pipes operate in use as a packed absorption column.

9. The process according to claim 7, wherein the water-enriched brine is in vapor-liquid equilibrium with the flue gas.

10. The process according to claim 7, wherein the brine is in vapor-liquid equilibrium with the gas having a moisture concentration less than M.

11. Use of the process of claim 7 with a flue gas that originates from a solids dryer.

12. Use of the process of claim 7 with a flue gas that originates from a solids dryer and is delivered via a thermal oxidizer.

13. The process according to claim 7, wherein the flow of heat drives a boiler to produce process steam which is combined with the flow of steam from the recovering step to produce process steam of an accordingly greater magnitude.

14. The process according to claim 13, wherein a thermocompressor is used to convert the relatively low pressure process steam to higher pressure process steam.

15. The process according to claim 7, wherein the strong brine is recovered in a desorber having two or more stages and a thermocompressor is used to increase the pressure of the steam generated between the stages.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of apparatus according to an exemplary embodiment of the invention;

(2) FIG. 2 is a perspective view of a portion of the structure of the exemplary embodiment

(3) FIG. 3 is a view of the structure of FIG. 2 with the flue illustrated in phantom lines to show the interior;

(4) FIG. 4 is an enlarged view of encircled area 4 of FIG. 3;

(5) FIG. 5 is an enlarged view of encircled area 5 of FIG. 3;

(6) FIG. 6 is an enlarged view of a portion of FIG. 4, showing the grid of the exemplary embodiment in isolation; and

(7) FIG. 7 is a schematic view showing an exemplary embodiment of the process;

(8) FIG. 8 is a schematic view showing another exemplary embodiment of the process;

(9) FIG. 9 is a schematic showing an exemplary conventional starch ethanol plant; and

(10) FIG. 10 is a schematic showing another embodiment of encircled area A of FIG. 8.

(11) FIG. 11 is a schematic of a variant of FIG. 8.

(12) FIG. 12 is a schematic of a variant of FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(13) Reference is now made to the Figures which show exemplary embodiments of the invention.

(14) With reference to FIG. 1, this shows, in schematic form, the collector 20, flue 22, vessel 24, distributor 26, plurality of heat pipes 28, conduit 30 and regenerator 32 which collectively form the exemplary apparatus 34 and have the functions indicated in the summary of the invention to be characteristic of the apparatus in all embodiments.

(15) The reservoir 20, flue 22, vessel 24, distributor 26, plurality of heat pipes 28 and conduit 30, which collectively define a stripping absorption module 36, are shown in detail in FIGS. 2-6. In this document, the stripping absorption module is sometimes referenced as a SAM.

(16) Herein, it will be seen that this collector 20 takes the form herein of a shallow dish-shaped reservoir having a drain 38.

(17) This flue 22 will be seen to include a stack 40 extending vertically from the collector 20, the stack 40 being defined by an annular wall 42 which forms an extension of the collector 20 and which has a gas inlet 44 defined therethrough adjacent the collector 20 and further has a brine inlet 46 therethrough adjacent the top of the stack 40.

(18) This vessel 24 is an obround with vertical sides 48 and is disposed in concentric, spaced relation to the stack 40 to define a flow channel 50 between the vessel 24 and the stack 40. Interior of the vessel 24 is a headspace 52. A liquid inlet 54 is provided near the base of the vessel 24, as indicated in FIG. 1.

(19) This distributor 26 is defined by a plate 56 and a plurality of chimneys 58. The plate 56 is annular and is disposed proximal to the top of the vessel 24 and beneath the brine inlet 46 to occlude the flow channel 50. The chimneys 58 each extend upwardly from the plate 56, to a height above the brine inlet 46. At the base of each chimney 58 is a gas passage 60 that extends through the plate 56. The surface of the plate 56 between the chimneys 58 is provided with perforations 62, to provide for fluid communication across the plate 56.

(20) The heat pipes 28 are of the conventional type, i.e. each pipe 28 is a partially evacuated tube used for heat transfer and containing a working fluid. At the hot interface within a heat pipe, the working fluid, which is in liquid form, contacts a thermally conductive solid surface and turns into a vapor by absorbing heat from that surface. The vapor flows by one or more of diffusion and convection to a cold interface in the heat pipe, whereat it condenses back into a liquid, releasing the latent heat. The liquid then returns to the hot interface through either capillary action, wicking or gravity action where it evaporates once more and repeats the cycle.

(21) These heat pipes 28 are arranged in layers 64, with each heat pipe 28 leading between the flow channel 50 and the interior of the vessel 24 and having the shape of the ogee, and the heat pipes 28 in each layer collectively defining a grid 66.

(22) As best seen in FIG. 6, each grid 66 has the general shape of a frustum of an onion dome the onion dome being of the architectural type [not shown] having a bulbous lower part defined by a convex surface and an upper part having a concave surface that tapers to an apex; and the horizontal planes defining the frustum lying, respectively, between widest part of the lower part and the transition of the lower and upper parts; and between the transition of the lower and upper parts and the apex.

(23) Although an onion dome is not shown, the portions of the grids that correspond to the upper part and lower part are indicated, respectively, by reference numerals 68, 70 in FIG. 6

(24) The arrangement of the heat pipes 28 is such that the vessel 24 projects above and below the plurality of heat pipes 28.

(25) This conduit 30 extends through the sidewall 42 of the stack 40, proximal to the collector 20, thence upwardly through the base of the vessel 24, and thence interiorly of the vessel 24 to the headspace 52.

(26) It will be understood that this exemplary apparatus can be used with a moist feed gas, such as flue gas, with a supply of liquid water to vessel 24 and with a flow of strong brine adapted to exothermically absorb moisture from the feed gas to produce heat.

(27) In use: the feed gas is directed into the flue 22 through the inlet 44; liquid from said supply is introduced into the vessel 24 via the liquid inlet 54; strong brine is introduced to the distributor 26 via the brine inlet 46; the strong brine passes through the perforations 62 to enter the flow channel 50 from the top thereof; contemporaneously, the feed gases enter the flow channel 50 via the bottom thereof, as the brine and feed gases pass one another in counterflow, the surfaces of the heat pipes 28 function in a manner analogous to packing in a packed absorption column, thereby to facilitate the absorption by the brine of moisture from the feed gas; this generates water-enriched brine and heat, the former being in thermal equilibrium with the flue gas and near the corresponding liquid phase equilibrium concentration relative to the gas phase moisture content and the latter being contemporaneously withdrawn from the flow channel 50 by the heat pipes 28 and delivered to liquid in the vessel 24 to produce process gas; the process steam produced interiorly of the vessel 24 travels to the vessel headspace 52 and is thence vented through the conduit 30; the water-enriched brine falls to the reservoir 20 and is directed to the regenerator 32 via the drain 38; and within the regenerator 32, the water-enriched brine is flashed to recreate the strong brine and produce a flow of surplus water and/or Steam, indicated by arrow 98 in FIG. 1.

(28) Reference is now made to FIG. 7, which shows an exemplary use of the process and the apparatus.

(29) Herein, portions of a typical starch ethanol plant are shown.

(30) With initial reference to that portion of FIG. 7 within encircled area 8, namely, the beer column 72, evaporators 74, 76, dewatering facility 78, rotary drier 80 and thermal oxidizer 82, it will be understood that this structure is representative of a conventional starch ethanol plant as has been described in U.S. Pat. No. 7,572,353.

(31) In use, the evaporators 74, 76 which form part of the ethanol refining system (not shown in its entirety) are arranged, with the beer column 72, for multiple effect distillation, i.e. positive pressure stream is used to drive the first column 74; vapors from the first column 74 are used to drive the second column 76; and vapors from the second column 76 are used to drive the beer column 72. Whole stillage from the bottom of the beer column 72 is pumped to the dewatering facility 78 and separated into wet cake which is transferred together with syrup concentrated in evaporators to the rotary drier 80, to produce DDGS and moist offgas. The offgas passes through the oxidizer 82, to remove organics. In a conventional facility (not shown), fuel such as natural gas would be used to provide the energy to produce the entire stream of positive pressure steam to the first evaporator 74, and the flue gas generated in the oxidizer discharged to the air. In the exemplary embodiment, this functionality is instead provided by the balance of FIG. 7, which will be understood to correspond generally to the structure of FIG. 1 and include a boiler vessel 24 operating at 225 F., a SAM 36 operating at 260 F., a pump 86, a multiple effect desorption unit 88 and an evaporative cooling tower 90. In use, approximately 300-F flue gas from the thermal oxidizer 82, which contains approximately 40 wt. % water, is fed to the bottoms of the SAM 36 and 65% LiBr brine is fed to the distributor 26. The sensible heat of the flue gas, as well as the latent heat associated with the heat of absorption, is transferred via the heat pipes 28 to the boiler 24, to create 5 psig steam for first column 74. The 58 wt. % LiBr brine from the SAM 36 collector is desorbed in the multiple effect desorption unit 88 using plant steam, to recover the strong brine and produce low quality steam which is dewatered via the evaporative cooling facility 90. About 5,500 BTU/gallons of ethanol produced in plant steam is required to fire desorption unit 88, which generates about 10,400 BTU/gallon in 5 psig steam, leading to net savings of 4,900 BTU/gallon. In this application, about 0.33 gallons of water are saved for each gallon of ethanol produced.

(32) FIG. 8 shows a variation of the structure of FIG. 7. In this structure, thermocompressors 92 will be seen to have been introduced. This allows for operation at pressures lower than 5 psig, which has operational advantages which include the option of using a less concentrated brine [benefits energy performance], the option of extracting more moisture from the flue gas or the option of using a higher T across the heat pipes, to reduce the number of pipes and capital cost. Thermocompressors 92 can bring the lower pressure steam up to a level usable by the plant, i.e. up to about 5 psig or higher.

(33) It will be understood that, in FIG. 8, structure 24 can be defined by a vacuum boiler.

(34) As yet another option, not independently shown, the brine regenerator can be designed in such a manner that its final stage operates at roughly the same pressure as the SAM's vessel 24, so that its vaporous product can be combined with that of the SAM's vessel 24. This arrangement avoids the use of a condenser, and an evaporative cooling tower, in the brine regenerator. The steam produced in this final stage of the brine regenerator can either be combined with that of the SAM or be equipped with its own thermocompressor, and the discharge combined with that of the SAM's thermocompressor discharge. To restate, this would involve a modification to structure 88 of FIG. 8 and removal of structure 90; last stage steam (vacuum or positive pressure) would be sent to process rather than to tower 90. An advantage of this combination embodiment is that, for every pound of high pressure plant steam used by the desorber, roughly two pounds of low pressure steam are generated with half the energy being extracted from the flue gas's energy content associated with the moisture removed from the flue gas.

(35) The above thermocompressor modification allows for the potential of producing relatively higher pressure process steam.

(36) As another option, the above modification allows for the potential of operating the SAM's boiler vessel 24 at or near atmospheric pressure, which can reduce capital costs.

(37) A yet further embodiment, in the form of a an exemplary conventional starch ethanol plant as described in U.S. Pat. No. 7,572,353, is shown in FIG. 9. Various mass and energy flows are labelled, using lb/bu, which is short hand for lb-mass/bushel. Bushel is a bushel of corn (unit of measure that is used in production of starch-based ethanol). A change from the former implementation is that the desorber is a single effect, atmospheric desorber, with a single economizer heat exchanger. Three atmospheric steam streams supply the thermocompressor and the discharge therefrom is 5 psig steam. There are 2 high pressure (150 psig) steam sources (from the plant's boiler) which drive the desorber and the thermocompressor. Calculations show that the plant's energy consumption may be reduced by 1340 btu/gal ethanol, and save 5 lb/bu water from the flue gas; a 40.5% recovery of moisture from the flue gas.

(38) FIG. 10. shows a variation of encircled area A of FIG. 8, which includes a two effect desorber and a thermocompressor which is used to increase the pressure of the steam generated between the effects.

(39) FIG. 11 shows a variation of FIG. 8, wherein the steam from the desorber is scrubbed through the SAM boiler vessel 24.

(40) FIG. 12 is a schematic of a variant of FIG. 11 wherein the steam from the desorber is condensed in a supplementary boiler and heat exchanger in communication with boiler vessel 24, transferring its latent heat to produce clean steam to be combined with the process steam exiting boiler vessel 24.

(41) Whereas specific exemplary embodiments are described and illustrated, it will be appreciated that the invention is not so limited.

(42) Rather, the invention should be understood as advantageous in the context of any application wherein there exists: a use for positive pressure steam; and a moist flue gas

(43) Although the invention is contemplated to be useful with flue gas having a moisture content as low as 8 wt. %, a minimum moisture content of about 10 wt. % provides enhanced utility; at concentrations significantly below this, the energy associated with the latent heat of the contained water vapor will normally be insufficient to maintain the temperature of the absorber at 220 F., and below this temperature, useful steam heat will not normally be available.

(44) Persons of ordinary skill will readily appreciate the manner in which the invention can be deployed in other applications and accordingly, a detailed description is neither required nor provided. However, it will be appreciated that, at elevated temperatures, absorption capacity of LiBr brine falls off; accordingly, it will be expected that the stripping absorption module will normally be designed to operate in the 220-300 F. range. As well, LiBr brine has a tendency to crystallize at elevated concentrations, which could cause facility damage; for this reason, the strong brine will normally be designed to enter the SAM at no higher than 70 wt. % LiBr.

(45) By adjusting the flow rate of the brine, and providing adequate heat withdrawal and mass transfer area via the grid of heat pipes the equilibrium temperature of the strong brine at the top of the absorber and the equilibrium temperature at the bottom of the absorber can be the same and the actual operating temperature of the brine as it passes through the absorber can be substantially constant, i.e. within about 2 F. Without intending to be bound by theory, it is believed that this has advantage in terms of heat exchange efficiency, as heat exchangers in general are most efficient use under constant temperature differential conditions.

(46) Yet further, whereas LiBr brine is specifically mentioned, other brines, such as calcium chloride, can be utilized, although this has corrosion consequences, and affects equilibrium brine concentrations and other physical properties.

(47) Accordingly, the invention should be limited only by the accompanying claims, purposively construed.