WO1993011070A1

WO1993011070A1 - Dry carbonation of trona

Info

Publication number: WO1993011070A1
Application number: PCT/US1992/006321
Authority: WO
Inventors: Anthony J. Falotico
Original assignee: Church & Dwight Company, Inc.
Priority date: 1991-12-04
Filing date: 1992-08-04
Publication date: 1993-06-10
Also published as: AU2424892A

Abstract

A process for the dry carbonation of trona to form sodium bicarbonate comprises mixing in a slow blender (3) trona particles, a CO2- containing gas, and no external water except for a small amount of water added through conduit (5) to initiate the reaction and maintain the reaction if the gas contains less than 100 % CO2. The CO2 containing gas stream is passed through scrubber (6) where it is saturated with water and recycled to slow blender (3). A novel form of sodium bicarbonate having unique properties is produced thereby. The process also comprises the production of a bicarbonate sorbent useful in the desulfurization of flue gases from the carbon dioxide - containing flue gases themselves.

Description

DRY CARBONATION OF TRONA

This invention relates to a process for the dry carbonation of trona ore to produce sodium bicarbonate. Impure trona ore is generally comprised of mixtures of trona (Na₂CO₃.NaHCO-_2H₂0) with other materials, e.g., alkali minerals such as sodium chloride and sodium sulfate, as well as shales and clays. The invention also relates to such a process for the production of sodium bicarbonate useful in the desulfurization of flue gas.

BACKGROUND OF THE INVENTION

The conventional technique utilized in the commercial production of sodium bicarbonate is the solution process. In the solution process, soda ash is dissolved in spent reaction liquor from prior reaction, consisting of water and small quantities of dissolved soda ash and sodium bicarbonate. The solution is then carbonated to precipitate crystals of sodium bicarbonate. The sodium bicarbonate crystals are separated from the liquor and dried to yield highly purified, high density crystals. Disadvantages of the conventional method are that the procedure requires several steps, and necessitates the use of separation equipment, drying of the product, and the handling of large volumes of liquids.

It has also been proposed to make sodium bicarbonate by various dry carbonation techniques. In U.S. Pat. Nos. 276,990 (Carey et al.) and 574,089 (Hawliczek) , a sodium bicarbonate product is formed by placing hydrated soda ash in a revolving cylinder and then introducing carbon dioxide into the cylinder. In both patents, reaction times are of the order of five to six hours.

U.S. Pat. No 3,647,365 (Saeman) teaches a process in which hollow sodium bicarbonate beads of low density are prepared in a multistage reactor from hydrated soda ash, small amounts of water and carbon dioxide. This process requires several steps and must proceed slowly, with carbonation times exceeding one hour and drying times up to eight hours. The soda ash must first be hydrated in a separate step, and the reaction must occur at a temperature above 95.7°F. to produce commercially acceptable reaction rates.

More recently, Krieg et al. (U.S. Pat. No. 4,459,272, owned by the assignee of the present invention) described a process for the preparation of sodium bicarbonate by the reaction of a solid, particulate sodium carbonate-containing material with liquid water in a carbon dioxide-rich atmosphere. In the Krieg process, the particulate mass is mixed with the water and carbon dioxide in an internally agitated or externally rotated or vibrated reactor. The reaction is carried out at temperatures of from 125°F. to 240°F. under atmospheres containing from 20% to 90% carbon dioxide by volume. The process is carried out under reduced water vapor partial pressures to promote evaporation of water from the surfaces of the reacting carbonate particles, and to maintain high carbon dioxide partial pressures in the reactor atmosphere. Products formed by the process have apparent bulk densities as high as 50-60 lbs./ft³. On the other hand, Sarapata, et al. in U.S. Patent No. 4,664,893 (also owned by the assignee of the present invention) disclose that in the dry carbonation of sodium carbonate, it is necessary to react a substantially saturated feed gas stream (relative humidity in excess of 90%) to maintain adequate reaction rates.

Krieg, et al. in U.S. Patent No. 4,919,910 (also owned by the assignee of the present invention) disclose a process for the dry carbonation of potassium carbonate, which comprises reacting dry potassium carbonate, carbon dioxide and water vapor at atmospheric pressure and under turbulent mixing conditions to produce potassium bicarbonate.

Each of the previously described dry carbonation techniques is subject to particular disadvantages. In some of these processes, the carbon dioxide concentration must be high and the reaction temperature must also be high, or the reaction rate is prohibitively low. In some, the product must be dried. Despite a passing reference to the use of calcined trona in U.S. Patent No. 4,459,272, none of the patents discloses the process of the invention for the use of trona in a dry carbonation process or the surprising benefits and properties that result from the use of trona in the dry carbonation process of the invention.

Sodium bicarbonate has also been produced, as well as utilized, in dry sorbent injection processes for removing sulfur dioxide emissions from the combustion gases of fossil fuel-fired burners. Such techniques have commanded considerable attention recently, particularly because they present the lowest "first cost" alternative for removing potentially dangerous sulfur dioxide from flue gases. Sodium bicarbonate has been demonstrated to be a very effective sorbent in the dry sorbent injection process. However, the cost of pharmaceutical grade sodium bicarbonate, as currently produced, is a major drawback to its use for such purpose.

U.S. Pat. Nos. 3,846,535 (Fonseca) and 4,385,039 (Lowell et al.) disclose methods for regenerating sodium bicarbonate from sulfate-containing solid waste formed by dry sorbent injection with sodium bicarbonate. The Fonseca regeneration step is carried out by forming an aqueous solution of the sodium sulfate-containing waste, and treating such solution with ammonium bicarbonate to precipitate sodium bicarbonate. The sodium bicarbonate is then separated, dried and recycled for further use. Lowell et al. disclose a regeneration step which involves dissolving the solid desulfurization reaction product in a basic liquor, which contains borate ions and/or ammonia. Carbonation of this liquor results in a sodium bicarbonate precipitate. The Fonseca and Lowell et al. processes thus both suffer from the use of complicated and capital intensive solution operations.

Sarapata, et al. in U.S. Patent No. 4,664,893, mentioned above, also disclose that their dry carbonation process may be used to desulfurize flue gas streams, wherein the flue gas is contacted with a solid alkali metal or ammonium bicarbonate containing sorbent to react with sulfur dioxide in the flue gas. The resulting solid waste is separated and removed from the gas stream. The cleansed gas stream, from which the solid waste has been removed, is cooled; the gas stream is saturated with water vapor; and the gas stream is thoroughly mixed with a particulate alkali metal or ammonium carbonate. The bicarbonate produced thereby is then utilized to contact the hot flue gas for further desulfurization thereof.

It is among the objects of the present invention to provide an improved process for the production of sodium bicarbonate from trona, which does not require the multiple operations required by prior art solution processes, nor is it limited to use of the high carbon dioxide-concentration gas mixtures utilized in some previous dry carbonation techniques.

A further object of the invention is to provide such a process which may be readily employed to produce bicarbonate sorbent employed in the desulfurization of flue gases, more efficiently and economically than possible utilizing previously proposed techniques.

These and other objects and advantages of the invention will be described more fully below. SUMMARY OF THE INVENTION

In accordance with the present invention, a process is provided for the dry carbonation of trona, which comprises:

(a) passing trona particles through a reaction zone (e.g., an internally agitated or externally rotated or vibrated reactor) ; (b) introducing into the reaction zone a gas stream containing from about 12% to 100% carbon dioxide by volume, any remaining percentage of the gas stream being an inert gas such as air or nitrogen, the gas stream being heated to a temperature within the range of about 140°F. to about 160°F. [about 60° to about 71.1^βC], preferably about 150°F. to about 155°F. [about 65.6° to about 68.3°C.];

(c) initiating the reaction by introducing water into the reaction zone to form a gas mixture of water vapor and the gas stream from step (b) , so that the water vapor content of the gas mixture reaches essentially 100% of saturation at the temperature of the gas stream from step (b);

(d) thereafter during the course of the reaction, intermittently introducing water into the reaction zone to form a gas mixture of water vapor and the gas stream from step (b) , so that the water vapor content of the gas mixture reaches essentially 100% of saturation at the temperature of the gas stream from step (b) , if required to maintain the reaction with a gas stream containing less than 100% C0₂; (e) maintaining the gas mixture or gas stream in direct contact with the trona particles during the reaction and continuing the reaction with production of water from the trona and no external source of water when the gas stream is close to 100% CO-, until the sodium carbonate in the trona particles is essentially all converted to sodium bicarbonate with a water content of less than about 4 percent by weight; (g) discharging the gas stream or gas mixture from the reaction zone after contact with the particles; and (h) discharging from the reaction zone reacted particles having a lower bulk density, a higher specific surface area (absorption property) than the trona feed, and a water content of less than 4 percent by weight. The operation is best carried out under atmospheric pressure or higher than atmospheric pressure in the range of about 14.7 to about 25 pounds per square inch absolute. Although higher or lower pressure may be employed, higher pressures are generally better, particularly when the process is utilizing low C0₂ concentrations, in order to increase the C0₂ partial pressure and to drive the reaction. For example, 23 psia pressure has been successfully used with 12% C0₂ concentration to increase the C0₂ partial pressure and drive the reaction.

We have found that a number of surprising benefits result from the process of the invention: A. The starting material is natural trona (which need not be calcined) , not the more expensive processed sodium sesquicarbonate. B. There is no need for liquid water addition, except possibly for a small amount at the beginning of the reaction to initiate it. However, if the C0₂ content of the gas stream is less than 100%, intermittent injections of water into the reactor may be required to maintain the reaction, depending upon the balance of the stoichiometry/C0₂ partial pressure.

C. There is no need to use a fine particle size trona starting material.

D. Dry carbonization leads to a useful reduction in particle size.

E. Dry carbonization leads to a useful increase in surface area, e.g., to about 0.3 m²/gm.

F. No separate drying or cooling process is required.

G. Dry carbonated trona can be screened or air classified in order to reduce insolubles.

H. The final product is more free flowing and/or has less of a tendency to cake than bicarbonate produced from pure sodium sesquicarbonate. The sodium bicarbonate of the invention is novel because of these unique properties. While not wishing to be bound by any particular theory for the superior properties of the novel sodium bicarbonate of the invention, certain differences can be emphasized. The morphology of the novel sodium bicarbonate shown in FIGS. 5a and 5b is unique. The surface area of the novel sodium bicarbonate is greater than prior art sodium bicarbonates. In addition, there is usually some shale or clay or both present in the novel sodium bicarbonate that remains from the trona starting material.

In accordance with another embodiment of the invention, the dry carbonation process hereof is utilized in connection with desulfurizing low carbon dioxide-content flue gas streams, wherein the flue gas is contacted with a sodium bicarbonate-containing sorbent to react with sulfur dioxide in the flue gas, and the resulting solid waste is separated and removed from the gas stream. In accordance with the present invention, the cleansed gas stream, from which the solid waste has been removed, is cooled (to a temperature as low as about 140°F. [60°C.]), water is injected into the gas stream, and the gas stream is thoroughly mixed with a particulate trona in the manner indicated above and then utilized to contact the hot flue gas for further desulfurization thereof.

Flue gas streams from the combustion of sulfur-containing carbonaceous fuels, such as oil, coal, and coke, contain low concentrations of carbon dioxide, typically about 8-17% by volume. Flue gases also contain amounts of about 3-18% water vapor, 2-4% oxygen, 68-77% nitrogen, and up to about 0.5% sulfur dioxide, by volume. Therefore, it is apparent that two of the three ingredients required to form sodium bicarbonate from trona — carbon dioxide and water — are already present in flue gas streams.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flow diagram of one embodiment of the process of the invention. FIG. 2 is a graph of the results from continuous reactions in accordance with the invention. The graph shows the relationship between the lbs. NaHC0₃/min.-ft.³ made and the CO- partial pressure (pounds per square inch absolute) in the exit gas from the reactor.

FIG. 3 is a graph of the results of two batch reactions of trona with a gas mixture containing 12% C0₂. The percent of NaHCO- in the product (insoluble-free basis) is plotted against the reaction time in hours. For comparison purposes, a data point from a 150 lbs./hr. continuous reaction ^' is also shown.

FIGS. 4a and 4b show the surfaces of trona feed samples enlarged to 5,000 and 10,000 times magnifications respectively.

FIGS. 5a and 5b show the surfaces of sodium bicarbonate product samples enlarged to 2,000 and 10,000 times magnifications respectively.

PREFERRED EMBODIMENTS OF THE INVENTION The process of the invention can be conducted in a batch or continuous manner. For most purposes, the continuous manner will be preferred.

FIG. 1 is a schematic flow diagram of one embodiment of the invention. Dry trona particles are fed from hopper 1 through feed conduit 2 having an in-line flow valve to heat traced and insulated plow blender 3, which has an internal agitator, not shown, that is rotated about its axis by motor 4, which preferably rotates the agitator at a slow speed on the order of 50 to 70 rpm. The rpm is a function of the diameter of the plow blender 3 and may be optimized to produce a "falling curtain" of powder inside the plow blender 3. The plow blender 3 is heated to maintain the temperature of the trona particles within the range of about 140° to about 160°F. [60° to 71.1°C], preferably about 150° to about 155°F. [65.6° to 68.3°C.]. Temperatures above about 160°F. [71.1°C] for any material length of time can cause decomposition of the particles, producing an undesired product. The reaction is unduly long at temperatures materially below about 130°F. [54.4°C.].

The particulate trona reactant employed in the present process may comprise any trona (and need not be calcined) . The materials used are impure ores, or mixtures of trona (Na₂CO-.NaHCO-.2H₂0) with other materials, e.g., alkali minerals such as sodium chloride and sodium sulfate, as well as shales and clays. In the following description, the process of the invention will be illustrated in connection with the preferred carbonation of trona. It will, however, be understood that the invention is not limited to the use of trona, as any of the other sodium carbonate-containing ores that contain excess water of hydration can be employed therein.

Preferably, the trona feed has particles generally of sizes in the range of about 200 to 20 mesh with more than 95 cumulative percent of the particles being retained on 40, 60 and 100 mesh, U.S. sieve size.

CO- at temperature T₁ from scrubber 6 is introduced to the plow blender 3 through conduit 7 by blower 8. If desired, an inert gas, such as air or nitrogen, may be included in the gas mixture in an amount up to about 88 percent by volume.

In plow blender 3, the initial water injection and any subsequent intermittent water injection that may be needed to maintain the reaction is made through conduit containing an in-line valve. That water and some of the C0₂ react with the carbonate on the surface of the trona feed to yield a relatively dry bicarbonate. The dry bicarbonate exits the plow blender 3 through conduit 9, which contains an in-line valve. Unreacted humid C0₂ at temperature T₂ exits the plow blender 3 through heat-traced and insulated conduit 10, which conducts it back to the scrubber 6.

Valve 15 directs steam or cooling water to heat exchanger 16. Heat exchanger 16 heats or cools the recycle water from the bottom of scrubber 6 that is pumped to heat exchanger 16 by pump 17 through pipeline 18. From heat exchanger 16, the heated or cooled recycle water flows through pipeline 19 to the scrubber nozzle 20 inside at the top of scrubber 6. Water is sprayed through the scrubber nozzle 20 downward and countercurrent to the flow of C0₂, which enters the scrubber 6 at a lower point. The water spray and the countercurrent flow of C0₂ serve to saturate the C0₂ with water vapor at temperature T

Generally, during the course of the reaction, water vapor in the system that is reacted is replaced by makeup water from source 21 and is supplied to pipeline 18 by pipeline 22, and C0₂ in the system that is reacted is replaced by makeup C0₂ from source 23 and is supplied to conduit 7 by conduit 24. However, in the case of high C0₂ concentrations where the moles of water available equal the moles of water required for saturation plus reaction, no additional water is required. In fact, net water condensed in the system may have to be eliminated from the process. In addition, there is a possibility that at high pressures, the correct molar balance-temperature-humidity equilibrium may be achieved so that little or no water addition is required even with low concentrations of C0₂.

Although water is released from the particles of trona during the reaction, it is important to maintain an atmosphere of moisture in the gases during the initiation of the reaction. During the continued reaction, particularly towards the end of the reaction, there is sufficient moisture generated to provide an atmosphere of moisture in the carbon dioxide gas surrounding the particles and indeed there may be a net production of moisture.

In accordance with another important feature of the invention, the carbonating-gas stream in the reactor may be maintained initially under essentially saturated conditions, i.e., the moisture content is maintained at about 100% of saturation at the reaction temperature utilized, either by feeding the carbonating gas stream solely through a saturation tank and/or by vaporizing some liquid water sprayed into the reactor. In this manner, the initial presence of sufficient water on the surfaces of the reacting particles is assured, and the carbonation reaction proceeds at commercially acceptable rates.

The extent of the reaction can be determined by periodically taking samples of the product being discharged from plow blender 3, analyzing the samples by pH or any other convenient analytical method to determine the percentage of sodium bicarbonate as well as by examination under the microscope to determine the nature of the product particles.

The continuous process of the invention can be controlled by suitable means. For example, one can use temperature or pressure sensors in the control of the process. It has been convenient and simple to control the reaction with a timer and the temperature of the exhaust stream.

In conducting and controlling the reaction, it is desirable to stoichiometrically balance the available water from the trona with the total gas volume (i.e., % C0₂) to maintain a super saturated condition and a "rain storm" of condensing water inside the reactor. The "rain" should equal the amount of water required to effect conversion of the sodium carbonate to sodium bicarbonate.

The chemical make-up of the product is, in general, sodium bicarbonate plus trona plus sodium sesquicarbonate plus insolubles. If the reaction is carried to completion, sodium bicarbonate is essentially the sole product mixed with insolubles. Differences in bulk density may, in general, be achieved by controlling the degree of reaction. The water content of the product will vary depending generally on the degree of reaction. When low C0₂ concentrations are used in the reaction, from about 2 to about 4 percent by weight of the water may be present in the product sodium bicarbonate. Generally, when high C0₂ concentrations are used in the reaction, substantially no water is present in the sodium bicarbonate product.

Another embodiment of the invention effects thorough mixing of the gas stream and particulate trona feed in a turbulent fluidized bed under conditions which produce thorough contact between the solid and gaseous reactants with substantially complete back mixing and heat transfer therebetween. Such conditions are preferably insured by mechanically fluidizing the bed. The conditions can also be effected by introducing the fluidizing gas into the fluidized bed at rates varying as required by the particle size and density of the trona feed. However, the use of fluidizing gas to effect a fluidized bed results in the undesirable "dilution" of the available water with the excess gas required to maintain fluidization. Of course, this is not a problem with mechanical fluidization.

Fluidizing conditions may be provided in either a conventional gas fluidized bed reactor in which the energy required to fluidize the trona particles is imparted to the carbonating-gas stream, or preferably in a mechanically fluidized bed wherein the solid particles are mechanically accelerated through the gaseous medium to effect turbulent fluidization thereof. In a mechanically fluidized bed, the flow rate of the carbonating-gas stream must at least be equal to that necessary to supply the gaseous reactants and to remove the heat of reaction. In a gas fluidized bed, the gas feed rate must also be sufficient to produce turbulent fluidization; in most instances, such feed rate is significantly greater than that required for adequate feed of the reactants and heat removal. Employing such conditions, commercially acceptable carbonation rates are obtained, employing gas streams containing as little as about 12% C0₂ by volume, at temperatures as low as about 140°F. [60°C] and up to about 155°F. [68.3°F.], preferably about 150° to 155°F. [65.6° to 68.3°C.].

The process of the present invention produces a bicarbonate of a quality which is suited for a number of applications where a U.S.P. grade is not necessary, e.g., neutralization of acidic lakes and the dry sorbent injection process for desulfurizing flue gas because its particles are coarse. The use of sodium bicarbonate having such properties is desirable because the sorption of sulfur oxides is believed to be surface related. The process of the invention leads to a useful increase in surface area, e.g., to about 0.3 m²/gm. On the other hand, commercially produced sodium bicarbonate has a surface area of about 0.1 meter²/g., and a specific density of about 50-60 lb/ft³.

A preferred embodiment of the carbonation process hereof resides in the desulfurization of flue gases by the dry injection technique. The invention makes possible the direct use of low carbon dioxide-content lue gas containing at least about 12%, typically about 12-17%, C0₂ by volume. For example, in one preferred embodiment, a boiler flue gas stream containing fly ash and sulfur dioxide, is recovered from a boiler at approximately 300°F. [148.9°C.]. Such a stream may typically incorporate about 8-17% carbon dioxide, 2-4% oxygen, 68-77% nitrogen, 3-18% water vapor, and up to 0.5% sulfur dioxide, by volume. The flue gas is mixed with a sodium bicarbonate-based sorbent which may also contain, for example, sodium carbonate and sodium sulfate, metered from a storage bin into the flue gas stream, the sorbent reacting with the sulfur dioxide in a particulate collection device.

Trona is metered from a storage bin into the reactor, and the gas stream is intimately mixed with the trona in the fluidized bed. As and when needed, liquid water may also be metered into the reactor, forming a film on the trona particles in the bed. Water is generally required more frequently at low CO- concentrations than with high CO- concentrations. The bicarbonate reaction product is removed, and waste gas is vented after the removal of particulates.

The carbonation reaction is thought to occur only when an aqueous, C0₂ containing film forms around the trona particles. Such a film forms more rapidly when liquid water is added directly to the trona particles rather than waiting for the carbonate to adsorb sufficient water from the gas stream. Accordingly, liquid water is initially sprayed or sparged into the reactor.

The present invention provides an efficient technique for producing a sodium bicarbonate-based sorbent in the very desulfurizing process in which the sorbent is required. The cost of producing, for example, a sodium bicarbonate-based sorbent by the present technique is far below that of producing a conventional pharmaceutical grade sodium bicarbonate sorbent, since trona is the only extrinsic raw material required for use in the process. As noted above, the other reactants required, carbon dioxide and water, are contained in the flue gas and, therefore, do not have to be purchased or added to the carbonation reaction in a separate step. However, in some cases, it may be desirable to augment the carbon dioxide content of the flue gas by adding carbon dioxide from an extrinsic source. The bicarbonate product may thus be directly and ef iciently produced from trona with minimum processing.

The following examples further illustrate preferred embodiments of the invention. It should be understood that the unit operations mentioned herein may be varied widely without departing from the scope of this invention.

EXAMPLE 1. Continuous Process for the Dry Carbonation of Trona.

A continuous reaction in accordance with the invention was carried out in pilot plant apparatus and in a manner similar to that illustrated in the flow diagram of FIG. 1.

The trona feed had the following composition:

Sodium Bicarbonate 34%

Soda Ash 44%

Water 15%

Insolubles 7%

The trona particles had pH values as a 1.0 percent aqueous solution at 25°C as shown in Table I. Also shown in the table are the corresponding pH values for 1% aqueous solutions of the reaction product at the times indicated. With the exception of the trona, the percents in Table I represent the amount of sodium bicarbonate in the products.

TABLE I

Percents stated are on a dry, insoluble-free basis. The trona feed rate was 8.33 pounds per minute. The reactor exit gas temperature was 145°F. [62.78°C.]. The reactor pressure was 14.8 psia. The feed gas stream consisted of essentially 100 percent C0₂ (dry basis) and a trace percent air. Water was injected into the reactor at a rate of 1.328 pounds per minute for about 10 to 15 minutes to initiate the reaction. The water given off in the reaction was condensed by passing some gas from the reactor through line 10 into the scrubber 6 and recycling the gas back into the reactor through line 7. The blender residence time at 60% full was 104 minutes.

FIG. 2 is a graph of the results from continuous reactions in accordance with the invention. The graph shows the relationship between the lbs. NaHC0₃/min.-ft.³ made and the C0₂ partial pressure (pounds per square inch absolute) in the exit gas from the reactor. It is probably possible to improve the throughput by making subtle changes in the process variables, e.g., smaller trona feed particle size, pressure, control of leaks, etc.

Additional data about the experimental results are given in Tables II, III and IV.

TABLE II

CONTINUOUS DRY CARBONATION OF TRONA PILOT PLANT

TYPICAL PARTICLE SIZE DISTRIBUTION

(CUMULATIVE PERCENT RETAINED)

+60 +100 +170 +200 +325

Dry

Carbonated 37 56 72 70 82 average

Trona (21-53) (30-76) (88-76) (40-87) (55-91) Range

Trona 72 95 — 99 100

TABLE III DENSITY #/FT^g %INSOLUBLE BET2 H2/G Dry Carbonated 59 7.6— Average Trona (3.0-7.6) (0.29-0.30) Range

Trona 73 8.0

² Brunauer-Emmet-Teller Method. This standard method was used to measure the surface area.

TABLE IV HEAT AND MATERIAL BALANCE DATA FROM CONTINUOUS CARBONATION REACTION 500 PPH TRONA FEED

Total Production Rate NaHCO- _ Na₂C0₃ 6.16 LB/MIN

C0₂ Makeup Flow 6.01 LB/MIN

Scrubber Makeup Flow 0 LB/MIN

Scrubber Net H-0 OUT 1.23 LB/MIN

CO- in Blender Plenum Exit Gas 1.336 LB/MIN (Calc.)

Total Blender Plenum Exit Flow 1.503 LB/MIN (Calc.) + H₂0 vapor and condensate

Blender Exit Gas Temp 145°F. [62.8°C]

Scrubber Body Temp °F 80 [26.7°C]

C0₂ Recycle Temp Exit Blower 95°F [35°C]

C0₂ Makeup Temp °F 69 [20.6°C]

Scrubber Heat Exchange Process Exit °F 84 [28.9°C]

EXAMPLE 2. Batch Process for the Dry Carbonation of Trona.

A batch reaction in accordance with the invention was carried out in pilot plant apparatus and in a manner similar to that illustrated in the flow diagram of FIG. 1. The plow blender reactor and trona used were as described in Example 1.

The trona particles charge to the reactor was about 500 pounds. The temperature of the reactor exit gas was 149°F. [65°C.]. The pressure was 22.3 psia. The feed gas stream consisting of 12 percent C0₂ and 88 percent air (dry basis) was introduced through line 7 into the reactor at such a rate as to provide an excess of C0₂. The object was to provide the excess so that the reaction would not be C0₂ limited. The reactor exit gas was passed from the reactor through line 10 into scrubber 6 and recycled to the reactor through line 7. No cooling water was used in this batch reaction because it was not required to maintain temperature. The concentration of C0₂ in the recycle gas was about 12 percent on a dry basis. As required, water was injected into the reactor to maintain the reaction by pulsing the water into the reactor 0.5 minute on, 4.5 minutes off, to 0.75 minute on, 4.25 minutes off.

FIG. 3 is a graph of the results of two batch reactions of trona with a gas mixture containing 12% C0₂. The percent of NaHC0₃ in the product (insoluble-free basis) is plotted against the reaction time in hours. For comparison purposes, a data point from a 150 lbs./hr. continuous reaction is also shown. EXAMPLE 3. Scanning Electron Microσraphic Analysis of Trona and Dry Carbonated Trona.

Representative samples of trona feed for the reaction of the invention and three representative powder samples of 95% sodium bicarbonate product from the reaction were submitted for analysis to an independent laboratory. Scanning electron microscopy was chosen for the study to document the morphology of the powder.

Scanning electron microscopy (SEM) uses a highly focused electron beam (less than lOnm diameter) which can be scanned in a raster on the sample surface. The intensity of secondary electrons produced at each point is used to form a picture of the sample. Magnification factors from 10X to 100,000X can be obtained. The depth of field is inherently quite large, which allows the micrographs to be in focus at all points across a rough surface. In addition, the SEM does not suffer from the light microscope problem of light reflecting off at odd angles and being lost from view.^"

FIGS. 4a and 4b show the surfaces of trona feed samples enlarged to 5,000 and 10,000 times magnifications respectively. FIGS. 5a and 5b show the surfaces of sodium bicarbonate product samples enlarged to 2,000 and 10,000 times magnifications respectively. The increase in surface area can readily be seen by comparing the two sets of scanning electron micrographs.

It will be understood that various changes may be made in the preferred embodiments of the process described hereinabove without departing from the scope of the present invention. Accordingly, the preceding description should be interpreted as illustrative only.

Claims

What is claimed is:

1. A process for the production of sodium bicarbonate from trona which comprises:

(a) passing trona particles through a turbulent reaction zone;

(b) introducing into the turbulent reaction zone a gas stream containing from about 12% to 100% carbon dioxide by volume (dry basis) , any remaining percentage of the gas stream being an inert gas, the gas stream being heated to a temperature within the range of about 140°F. to about 160°F. [60° to 71.1^βC.];

(d) thereafter during the course of the reaction, intermittently introducing water into the reaction zone to form a gas mixture of water vapor and the gas stream from step (b) , so that the water vapor content of the gas mixture reaches essentially 100% of saturation at the temperature of the gas stream from step (b) , if required to maintain the reaction with a gas stream containing less than 100% C0₂; (e) maintaining the gas mixture or gas stream in direct contact with the trona particles during the reaction and continuing the reaction with production of water for the reaction from the trona and no additional water from an external source, until the sodium carbonate in the trona particles is essentially all converted to sodium bicarbonate;

(g) discharging the gas stream or gas mixture from the reaction zone after contact with the particles; and

(h) discharging from the reaction zone reacted particles having a lower bulk density, a higher specific surface area than the trona feed, and a water content of less than 4 percent by weight.

2. A process as claimed in claim 1, in which the trona feed has particle sizes in the range of about 200 to 20 mesh with more than 95 cumulative percent of the particles being retained on 40, 60 and 100 mesh, U.S. sieve size.

3. A process as claimed in claim I wherein the gas mixture introduced into the reaction zone contains in addition an inert gas such as air or nitrogen in an amount up to about 88 percent by volume of the total gas mixture.

4. A process as claimed in claim 1 wherein the turbulent reaction zone is in an internally agitated or externally rotated or vibrated reactor.

5. A process as claimed in claim 1, wherein the reaction is carried out in a mechanically agitated reactor.

6. A process as claimed in claim l wherein the pressure in the reaction zone is between about 14.7 and about 25 pounds per square inch absolute.

7. A process as claimed in claim 1 wherein the particles passing through the reaction zone are heated to a temperature within the range of about 150° to about 155°F. [65.6° to 68.3°C.].

8. A process for the dry carbonation of trona, comprising:

(a) substantially saturating a flue gas stream containing at least about 12% carbon dioxide by volume with water vapor, the moisture content of the gas stream being essentially saturated at the temperature at which the gas stream reacts in step

(b) ; and

(b) reacting solid particles of the trona with the gas stream in a turbulent fluidized bed at temperatures of from about 140°F. to about 160°F. [60° to 71.1°C] while initially maintaining a relative humidity of at least 100% in the vapor phase adjacent to the trona particles to initiate the reaction to produce the sodium bicarbonate;

(c) thereafter during the course of the reaction, intermittently introducing water into the reaction zone to form a gas mixture of water vapor and the gas stream so that the water vapor content of the gas mixture reaches essentially 100% of saturation at the temperature of the gas stream, if required to maintain the reaction;

(d) maintaining the gas mixture or gas stream in direct contact with the trona particles during the reaction and continuing the reaction with production of water for the reaction from the trona and no additional water from an external source, until the sodium carbonate in the trona particles is essentially all converted to sodium bicarbonate;

(e) discharging the gas stream or gas mixture from the reaction zone after contact with the particles; and

(f) discharging from the reaction zone reacted particles having a lower bulk density, a higher specific surface area than the trona feed, and a water content of less than 4 percent by weight.

9. The process of claim 8, wherein the gas stream saturated in step (a) is a flue gas stream prepared from a sulfur-containing carbonaceous fuel.

10. The process of claim 9, wherein the carbonaceous fuel is coke breeze, Eastern Coal, or #6 fuel oil.

11. The process of claim 8, wherein the bed is mechanically fluidized in step (b) to produce thorough contact between the solid and gaseous reactants with substantially complete back mixing and heat transfer between the reactants.

12. The process of claim 8, wherein the reaction is carried out in step (b) at temperatures of from about 150°F. to about 155°F. [65.6° to 68.3°C.].

13. A process for desulfurizing hot sulfur-containing flue gas containing at least about 12% carbon dioxide by volume, comprising:

(a) contacting the flue gas with the sodium bicarbonate-containing sorbent from step (f) to react with sulfur dioxide in the flue gas;

(b) separating the resulting solid waste and removing it from the gas stream;

(c) cooling the cleansed gas stream, from which the solid waste has been removed, to a temperature of from about 140°F. to about 160°F. [60° to 71.1°C.];

(d) initially substantially saturating the gas stream from step (c) with water vapor, the moisture content of the gas stream being about 100% of saturation at the temperature at which the gas stream reacts in step (e) ;

(e) thoroughly mixing the gas stream with trona, in essentially dry, particulate form in a turbulent fluidized bed at temperatures of from about 140°F. to about 160°F. [60° to 71.1°C], while initially maintaining a relative humidity of about 100% in the vapor phase adjacent to the trona particles to initiate the reaction to produce the sodium bicarbonate and thereafter maintaining the reaction by intermittently injecting water into the reactor if necessary; and

(f) supplying the bicarbonate thus produced to step (a) to contact the hot flue gas in an amount sufficient for desulfurization thereof.

14. The process of claim 13, wherein the bed is mechanically fluidized in step (e) to produce thorough contact between the solid and gaseous reactants with substantially complete back mixing and heat transfer therebetween.

15. The process of claim 13, wherein the reaction is carried out in step (e) at temperatures of from about 150°F. to about 155°F. [65.6° to 68.3°C.].

16. The process of claim 13, wherein the gas stream saturated in step (d) is a flue gas stream prepared from a sulfur-containing carbonaceous fuel.

17. Sodium bicarbonate produced from dry carbonation of trona and having greater surface area, greater free flowing properties and less of a tendency to cake than sodium bicarbonate produced from pure sodium sesquicarbonate.

18. Sodium bicarbonate as claimed in claim 17 which contains residual shale or clay from the trona starting material from which it was prepared and which has a moisture content of up to about 4 percent.

19. Sodium bicarbonate as claimed in claim 17 which has a surface area of about 0.3 m²/gm.

20. Sodium bicarbonate having the morphology substantially as represented in FIGS. 5a and 5b.