GB2096121A - Apparatus for manufacturing sodium bicarbonate - Google Patents

Abstract

An apparatus for the manufacture of sodium bicarbonate is provided with a plurality of upper and lower reaction stages (1a, 1b, 1c) which are partitioned with closed partition trays (4a, 4b, 4c). These reaction stages communicate with one another by means of gas inflow pipes (8a, 8b) arranged to allow a gas containing carbonic acid gas to move from a lower stage to an upper stage and also by slurry outflow pipes (6a, 6b) arranged to allow a slurry containing sodium bicarbonate to move from an upper stage to a lower stage. Sodium bicarbonate crystals are thus arranged to remain over a longer period of time to obtain sodium bicarbonate crystals of large particle diameter. Scaling of the apparatus is thus much reduced and the apparatus can be operated continuously. <IMAGE>

Description

SPECIFICATION Apparatus for manufacturing sodium bicarbonate This invention relates to a sodium bicarbonate manufacturing apparatus and, more particularly, to an apparatus which allows the production of sodium bicarbonate crystals of large particle diameter, has a high degree of carbonic acid gas absorbing efficeiency; is easily operatable; and permits a continuous operation over a long period of time.

A tower reactor called the Solvay tower has generally been employed as apparatus for the manufacture of sodium bicarbonate by the ammonia soda process or the ammonium chloride-soda process in which an ammoniacal brine solution and an ammonium chloride separating mother liquor is arranged to contact, in counter-flow a gas containing carbonic acid gas. A tower reactor similar to this has also been employed in the manufacture of sodium bicarbonate from caustic soda.

These tower reactors consist of a plurality of cast iron rings superimposed on each other and many open partition trays which are inserted in between the rings with each partition tray being composed of a pair of a bell and a collar.

The ammoniacal brine solution, etc. are supplied to an upper part of the tower reactor.

Meanwhile, a carbonic acid-containing gas is introduced from a lower part of the tower reactor. The ammoniacal brine solution, etc.

react with the gas accordingly as they descend within the tower to precipitate crystals of sodium bicarbonate. (Hereinafter, a matter wherein sodium bicarbonate crystals are suspended in the reacting ammoniacal brine solution or an ammonium chloride separating mother liquid will be called "the slurry"). The heat of reaction which is evolved causes the temperature gradually to rise as the ammoniacal brine solution, etc. move further downward within the tower. This temperature rise of the slurry provides a favourable condition.

for the growth of the sodium bicarbonate crystals. However, this temperature rise on the other hand is not desirable in terms of carbonic acid gas-absorbing efficiency. In view of this, many cooling pipes are provided in the lower part of the reaction tower reactor for the purpose of temperature control.

However, in a tower reactor of such construction, sodium bicarbonate crystals are deposited as scale on the bells and collars arranged within the reaction tower, the inner wall surface of the tower and the outer surfaces of the cooling pipes to such an extent that the inside of the tower is closed up with the scale and renders impossible continuous operation of the reactor. Hence, at present, it is a generally practised arrangement to use several units of such a tower reactor. For example, a set of five towers is used so that four units can be operated while one is being washed in rotation. The partitioning effect of the open type partition trays which are used in the tower reactors is not sufficient. The liquid and the gas existing at the upper and lower reaction stages move upward and downward through the open parts of the partition trays. This brings about a back-mixing phenomenon.While the solid settles and successively falls to a next stage, this thus makes it difficult to hold the solid. To retain the gas-absorbing efficiency and the desired slurry concentration, therefore, each of the conventional tower reactors is provided with as many as 30 pairs of partition trays or thereabouts. However, the provision of so many partition trays still fails to give a satisfactory result. It is shortcoming of the conventional tower reactors that the mean particle diameter of the sodium bicarbonate crystals obtainable therefrom is extremely small i.e.

only about 100 y. In separating the sodium bicarbonate crystals by filtration, therefore, a great amount of liquid sticks to a filtration cake. Impurities contained in the cake, such as sodium chloride etc., must then be removed with a large quantity of washing water.

Consequently, a loss by dissolution in the washing water results in a lowered yield of sodium bicarbonate. It is a further shortcoming of conventional tower reactors that a calcining process which must be carried out later necessitates consumption of a large amount of energy. U.S.P. 3,511,097 has proposed an apparatus for producing sodium bicarbonate by reacting carbon dioxide gas with ammonia and sodium chloride, which comprising passing the gas upwardly into and through a series of connecting compartments each of which has a base with a plurality of perforations for passing the gas therethrough and a conduit disposed in the base.The solution and the product suspended therein flow downwardly into and then laterally through each compartment, the solution and the product suspended therein being passed from one compartment to the next thereunder by the conduit, the gas being flowed through the perforated base at a rate sufficient to agitate the solution and keep the solid product suspended therein and to prevent any substantial amount of solution or product suspended therein from passing through the perforations of the perforated base.The solution and product suspended therein flow laterally in at least one of the compartments and is passed over a bundle of cooling tubes disposed therein, the cooling tubes being disposed in a lateral direction and the gaseous phases are caused to pass upwardly through some parts of the bundle of cooling tubes, then downwardly through the other portions of the cooling bundle and then upwardly by means of baffles disposed in the compartments. According to the patent, the perforated base is used as partition. A circulative flow of slurry is arranged to form by gas lift caused by a baffle within the compartment which has the bundle of cooling tubes disposed therein. The use of the perforated base as partition and insufficient circulative flow of the slurry create a dead space on the perforated base.This tends to cause a selid to percipitate and deposit there to close up the partition. Accordingly, this 'might eventually makes a long continuous operation impossible. Further, in order to enhance gas-absorption efficiency, this method necessitates use of a large number of reaction plates (stages) and makes the apparatus too complicated. Further, British Patent No.

1,520,249 has proposed a sodium bicarbonate manufacturing apparatus which has reaction parts arranged in a plurality of stages.

The reaction is carried out by wallowing the reaction liquid to circulate through every reaction part by virtue of a gas lift action of carbonic acid gas. This apparatus is provided with open type partition trays which are called "funnel shaped bodies" and are interposed between the reaction stages. The partition trays of this apparatus also inevitably have the back-mixing of the liquid and the shortcutting of crystals through the open parts of the partition trays in'the same manner as in the case of the above stated partition tray consist ing of the bell-collar pair. Therefore, this apparatus is also incapable of preventing the slurry concentration from lowering to a considerable extent.

The method proposed, therefore, does not always give good results in respect of the particle diameter of crystals and the gas-absorbing efficiency. Further, it necessitates control over the liquid level of each reaction stage through the pressure balance between the gas portion of the stage and the reaction liquid of an upper stage. This seems to render the operation of the apparatus complicated.

To eliminate these shortcomings of the prior art, the present inventors strenuously conducted studies. As a result of the studies, they have completed a sodium bicarbonate manu fracturing apparatus which eliminates these shortcomings. We have therefore sought to provide an apparatus for the manufacture of sodium bicarbonate which is capable of giving sodium bicarbonate crystals of large particle diameter; has a high degree of carbonic acidabsorbing efficiency; is easily operable; and permits a continuous operation thereof.

The apparatus according to the invention is divided with closed type partition trays. Each of the partition trays has a conical protrusion which is located in the middle portion thereof.

Each of the partitioned sections is arranged to serve as a reaction zone and is provided with a guide tube which is disposed in the middle of the section With each partitioned section arranged in this manner, the apparatus comprises a plurality of reaction stages. The apparatus further includes slurry outflow pipes which are arranged to allow an ammoniacal brine solution containing sodium bicarbonate formed or an ammonium chloride separating mother liquid to move from the upper portion of the reaction zone of one stage to the reaction zone of a lower stage (at the lower most stage, from the reaction zone to a slurry receiving tank);; and gas inflow pipes which are arranged to allow a gas which contains carbonic acid gas to move from a gaseous phase zone of each reaction stage, with the exception of the uppermost reaction stage, to the lower portion of the reaction zone of an upper reaction stageXand also to the lower portion of the reaction zone of the lowermost stage. At the reaction zone of each stage, the ammoniacal brine solution or the ammonium chloride separating mother liquid is contacted with the carbonic acid gas in counter current and is thus allowed to react with the carbonic acid gas.

The invention is further illustrated with refe rence to the accompanying Fig. 1.

Figure 1 is a schematic view showing a sodium bicarbonate manufacturing apparatus as a preferred embodiment of the present invention.

Referring to Fig. 1, the reaction tower 1 includes reaction stages 1 a, 1 b and 1 c. The reaction stage 1 a is the uppermost reaction stage and the reaction stage 1 c the lowermost reaction stage. While this particular embodi ment consists of three stages, the present invention is not limited to three stages but the number of the reaction stages may be in creased as desired. The reaction stages 1 a, 1 b and 1 c are completely separated from each other by closed partition trays 4a, 4b and 4c each of which has a conical protrusion 3a, 3b or 3c respectively disposed in the middle9 part thereof.In the sectional middle portion'-6f the inside of each reaction stage, there is pr- vided a guide tube 2a, 2b or 2c which is mwqunted with suitable spacing from the parti tion tray 4a, 4b or,4c. These parts jointly form each of reaction zones 5a, 5b and 5c.

Meanwhile, each of slurry outflow pipes''6a and 6b is connected between the upper' por tion of the reaction stage 1 a or the reaction stage 1 b and the lower reaction stage 1'b ar 1 c. Further, there are provided gas inflow pipes 8a and 8b for causing a gas containing carbonic acid gas to move from the upper gaseous phase portions 7b and 7c of the reaction zones Sb.and Sc-of the reaction stages 1 b and 1 c to the lower zones of the outsides of the guide tubes 2a and 2b dis posed in the upper reaction zones 5a and 5b respectively. A supply port 9 is provided at the uppermost reaction stage la for the sup ply of either an ammoniacal brine solution or an ammonium chloride separating mother li quid. A tank 1 2 is provided for receiving a sodium bicarbonate slurry and includes a slurry discharge port 1 3. A supply port 10 is provided in the lower part of the lowermost reaction stage 1 c for supplying the carbonic acid gas containing gas and a gas inflow pipe 8c is provided for supplying the gas containing carbonic acid gas from the supply port 10 to the outside of the guide tube 2 in the lower portion of the reaction part 5c. An outlet 11 is provided at the top of the reaction stage 1 a for an exhaust gas containing carbonic acid gas.

With the tower reactor arranged as described in the foregoing, an ammoniacal brine solution or an ammonium chloride mother liquor which is continuously introduced via the supply port 9 reacts at the reaction zone 5a of reaction stage 1 a with the carbonic acid gas which is separated at the reaction zone 5b of the next stage and is introduced from the gaseous phase zone 7b through the gas inflow pipe 8a. Then, sodium bicarbonate crystals are precipitated by this reaction.

A sodium bicarbonate crystal slurry thus obtained is introduced preferably by an overflow process to the reaction zone 5b of the next stage 1 b via the slurry outflow pipe 6a. The slurry reacts there with the carbonic acid gas which is also introduced there from the gaseous phase zone 7c of the next stage through the gas inflow pipe 8b in the same manner as described above. After that, the slurry is further allowed to move to the reaction zone Sc of the last reaction stage to react there with the carbonic acid gas which is introduced from the carbonic acid gas containing gas supply port 10 through the gas inflow pipe 8c.After completion of the reaction, the slurry is guided to the sodium bicarbonate slurry receiving tank 1 2 via the slurry outflow pipe 6c and is removed from the slurry discharge port 1 3.

At each of the reaction zones 5a, 5b and 5c, the introduced gas containing carbonic acid gas causes a specific gravity difference between the fluid located within the guide tube 2a, 2b or 2c and the fluid located outside of the guide tube. This brings about a gas lift action which causes an upward flow of the slurry on the outside of the guide tube and a downward flow on the inside. Thus, there takes place a circulative fiowing movement of the slurry. This circulative flow of the slurry may be aided by some suitable forced circulation process. In one example of such a process, a pump is operated to take out a portion of the slurry within the guide tube from above and then to reintroduce the portion of the slurry into the guide tube from the lower part thereof.

It is a feature of the present invention that the slurry which flows down through the insides of the guide tubes 2a, 2b and 2c collides against the conical protrusions 3a, 3b and 3c of the partition trays 4a, 4b and 4c which are respectively formed into a closed unique shape; this collision causes a reverse upward flow resulting from radial dispersion from the middle parts of these protrusions; and then an extremely favourable circulative flowing movement of the slurry is brought about by this reverse upward flow and an upward flow which takes place outside of the guide tubes 2a, 2b and 2c.

Therefore, the slurry of the precipitating sodium bicarbonate crystals vigorously circulates throughout the entire region of each reaction part, so that the super-saturation brought about by a carbonic acid gas absorbing reaction can be adequately absorbed. The supersaturation is thus advantageously utilized for growth of the crystals. Further, the flow of the slurry very smoothly takes place at the bottom of each reaction part to leave no dead space therein. Therefore, no crystalline particles nor massed crystals are sedimented and left accumulated at the bottom of each reaction part. Accordingly, the wall face and the partition tray of each reaction stage will be free from scaling and permit satisfactory continuous operation of the apparatus for a few months.

In accordance with the present invention, the conventional partition tray which allows crystals to take a shortcut through the bottom of each reaction stage is replaced with a closed type partition tray. The use of the closed type partition tray permits the crystals of sodium bicarbonate to remain at each reaction part for a sufficiently long period of time.

This enables sodium bicarbonate crystals to be obtained having very large particle diameters, for example 1 70 ju, which have not previously been readily pbtainable.

The guide tubes according to the invention are not particularly limited to one shape. The shape of the guide tube may be selected as desired in so far as it permits circulation, though it is normally of a cylindrical shape or of an inverse conical tubular shape. However, for better circulation efficiency, it is preferable to use guide tubes of an inverse conical tubular shape which have a wide upper end a narrow lower end.

As for the shape of the partition trays which, in the embodiment illustrated, have conical protrusions, any shape may be employed in so far as the slurry can be radially dispersed to bring about a reverse upward flow. Where a reaction tower is divided into reaction stages with partition trays of the open type as in the case of a conventional Solvay tower, back-mixing of a reaction liquid takes place through the partition tray between one reaction part and another. Whereas, in accordance with the present invention on the other hand, the arrangement to move the slurry through the slurry outflow pipe from the upper portion of one reaction stage to the subsequent reaction stage precludes the possibility of back-mixing of the liquid. Iri particular, in accordance with the present invention, the slurry is preferably arranged to overflow from the gas-liquid interface of the reaction zone.

This arrangement complemently obviates the back-mixing of the liquid because the liquid within the reaction zone and the liquid within the slurry outflow pipe are separated from each other.

Therefore, in accordance with the invention, the same partitioning effect as the efect hith erto Obtained by the Solvay tower can be obtained with much fewer closed partition trays. Accordingly, it is an advaritage of the invention that the carbonic acid gas absorbing efficiency is extremely high.

A cooler may be used for controlling a temperature rise caused by reaction heat. In such a case, it is an effective method, for example, to extract a portion of the liquid from the reaction zone and then to return it to the reaction zone after cooling.

Claims (5)

1. An apparatus for manufacturing sodium bicarbqna.te comprising in combination: a plu- rality of closed partition trays each of which has à centrally disposed conical protrusion arranged to partition reaction stages to form a multi-stage reaction tower; a plurality of slurry outflow pipes arranged to meve an ammoniacal brine solution containing sodium bicarbo- nate produced or an ammonium chloride sep- arating mother liquid from an upper portiOn of the reaction zone of each reaction stage to the reaction zone of a lower reaction stage, or, in thecase of the slurry outflow pipe provided for the reaction zone of the lowest stage, arranged to move a slurry of sodium bicarbonate to a slurry receiving tank, and a plurality of gas inflow pipes arranged to move a gas containing carbonic acid gas from a gaseous phase zone in the upper portion of the reaction zone of each reaction stage, except the reaction zone of the uppermost stag,% to th'e lower portion of the, reaction zone of each upper reaction stage and Blso to the lower portion of the reaction zone of the lowest reaction stage, the ammoniacal brine solution or an ammonium chloride separating mother liquid thus being arranged to contact in counter-current and react with the carbonic acid gas.

2. An apparatus according to claim 1 wherein, at each of the reaction stages, there is provided a vertical guide tube which is open at both the upper and lower ends thereof and is arranged in combination with one of the closed partition trays horizontally disposed below the guide tube symmetrically in relation to the centre axis of the guide tube; and the combination of the guide tube and the partition tray is such that the downward flow of the slurry through the inside of the guide tube, a reverse upward flow of the slurry resulting from radial dispersion thereof caused by a protrusion provided in the middle part of the partition tray and an upward flow of the slurry taking place outside of the guide tube joimtly cuase a circulative flowing movement of the slurry.

3. An apparatus according to claim 1 or 2, wherein are arranged to move a gas containing which are arranged to move a gas containing carbonic acid gas form a gaseous phase zone in the upper portion of the reaction of each reaction stage to the lower portion of the reaction zone of an upper reaction stage and to the lower portion of the reaction zone of the lowest reaction stage if arranged to have the upper end thereof open to lower end of each of the guide tubes on the outside ther- eof.

4. An apparatus according to claim 2 por 3, wherein each of the guide tubes is formed either into a cylindrical shape or into an inverse conical tubular shape.

5. An apparatus according to claim 1 substantially as herein described with reference to the accompanying Figure.