US2254237A - Process - Google Patents

Description

Sept. 2, 1941. R; w, NASH 2,254,237

PROCESS Filed July 1, 19:59

Rflg INVENTOR BY v i) I v ATTORNEY ment, ribbon, web or the like).

Patented Sept. 2, 1941 PROCESS Roy W. Nash, Kenmore, N. Y., assignor to E. L du Pont de Nemours & Company, Wilmingt Del., a corporation of Delaware application July 1, 1939, Serial No. 282,415

6 Claims. This invention relates to an eflicient method for maintaining solutions of sulfuric acid at a constant given concentration, when such solutions are used in a manner which depletes the acid, and more particularly to an especially economical procedure for reconditioning the coagulating and/or regenerating baths used in the viscose process for the manufacture of regenerated cellulose.

In the manufacture of filaments, ribbons or webs from viscose (cellulose 'xanthate solution), it is customary to extrude the viscose, through suitable orifices which give it the desired shape, directly into a bath comprising a salt and an acid. The first function of such a bath is to coagulate the viscose in the desired shape (fila- This action is immediately followed by one of regeneration in which the precipitated cellulose xanthate is decomposed to pure cellulose.

In order to produce a satisfactory product, it

is necessary to control very accurately the various factors, such asconcentration of bath constituents, bath temperature, etc.,' involved in this coagulation and regeneration process. This is generally done by circulating the bath continuously through the viscose treating vat, and. other vats and apparatus for correcting the changes caused by contact with the viscose.

Of the various reactions which take place between the viscose and the coagulating and/or regenerating bath, the two having the greatest effect upon the constitution of this bath are (a) dilution by the large amount of water present a in the viscose, and (b) neutralization of the acid by the alkali of the viscose. The reaction between the alkali of the viscose and the acid of the bath results not only in' a depletion of acid, but also in a building up in the bath of salt formed by the reaction and a further dilution by the water liberated in the neutralization. It will be seen, therefore, that this reaction results in (1) a depletion of acid in the bath, (2) a building up of salt in the bath, and (3) an increase in'water content in the bath.

In' the manufacture of webs of regenerated cellulose, it is common to extrude the viscose through a slot into the coagulating and/or regenerating bath, to allow the resulting web as it passes through such a bath to remain in con-- tact therewith for a sufilcient period of time to acquire the mechanical strength necessary for further handling, and then to remove the web from this first vat orbath (due care being taken to prevent the web from carrying of! undesirably large quantities of surface adherent liquid from the'bath) and pass it into subsequent vats, where it is further treated. a

In general, the amount of liquid which is transported out of the bath (and consequently the circulating system. of which it is a part) by adherence to the surface of the film is controlled by means of squeeze rolls and rods which scrape or otherwise remove the liquid from the surface of the film. In one mode of procedure these removal devices are regulated so that the salt formed by the process is continuously removed at the same rate at which it is formed.

As a result there is no increase in salt concentration of the bath (or its circulatory system). Part of the water which is added to the bath liquid as a result of the neutralization of the acid by the alkali and the addition of water through the water content of the viscose is likewise removed, but not all of the water added to the bath from these sources can be so removed in a satisfactory manner. Viewed generally then, the actual bath system (tank #1) suffers a depletion of acid and an increase in water concentration, while the amount of salt remains constant.

It would be possible to circulate the bath after contact with the viscose through a large reservoir to an evaporator, which would remove part of the water. The latent heat of vaporization of the water removed in the evaporator could be made up by heating with steam coils in the evaporator. Such a process of evaporation would likely be carried out at reduced pressure, with the result that the temperature of the bath after to nearly the boiling point, and it would be necessary to cool by means of water coils or refrigeration apparatus before this portion of the bath could be returned through circulation to the viscose treating bath. The refrigeration required to cool this bath material, and the heat required to evaporate the water in the evaporator, constitute expensive items, and a general reconditioning process employing the described pro /ad the trough to bath 2|.

ure would be uneconomical both from apparatus and operation points of view.

It was, therefore, an object of this invention to provide new and improved methods of processing aqueous solutions of materials having high heats of dilution and solution. Further objects were to provide improved methods of regenerating sulfuric acid baths used in the viscose process and economies in the regeneration of acid coagulating and/or regenerating baths for viscose.- A still further object was to devise an economical process for maintaining constant the composition of the first coagulating bath of a cellulose web regenerating system. A general advance in the art, and other objects which will appear hereinafter, are also contemplated.

In general, the objects of this invention are attained by passing a treating bath used in the coagulation and/or regeneration of viscose and comprising sulfuric acid and certain salts, which bath has suffered dilution and depletion of acid, into an evaporator chamber, adding additional concentrated sulfuric acid directly to the portion in the evaporator chamber, removing part of the -water at reduced pressure by the evaporation caused by the heat of solution of the sulfuric acid, returning the concentrated bath directly to the main circulating system, further mixing with attenuated sulfuric acid solution until the resulting bath has the desired concentration for processing of viscose, and circulating such treated bath through the viscose treating vat.

From the following description and specific ments of the invention as well as details of what is believed to be the best mode for carrying out the invention, it will be apparent how the foregoing objects and related ends are accomplished.

' for the removal of water vapor from this charge.

examples, in which are disclosed certain embodi tion of the fluid of the bath passes through pipe 33,'through valve 34 (which controls the amount) and into. the reservoir 32. In order to decrease the proportion of water in the material entering from the line 33, a portion of the liquid 3| is removed through line 35 (the amount being controlled by a valve 36), and conducted through line 43 to the evaporator 42. Ordinarily the vacuum in the evaporator is sufiicient to draw the liquid through the lines.35 and 43, but a pump 31 may be used as a booster 'if desired. Concentrated sulfuric acid in an amount equal to that consumed in'the process of regeneration, from a supply tank 52 is supplied through line 53 to the evaporator 42. The acid may enter the evaporator directly through the valve 54 or may be fed into the stream of spent coagulating liquor in line 43 at the-Junction of the line 55 with the line 43, i. e., immediately before the spent liquor enters the evaporator. A valve 56 regulates the amount (if any) of the acid supplied through the line 55, thereby performing a function similar to valve 54. Obviously acid -may be supplied simultaneously through both lines if desired, by suitable regulation of the valves 54 and 56. Ordinarily it is preferred to keep the valve 56 closed and control the amount of acid entering the evaporator by means of the valve 54.

The addition of the acid in the manner de-v scribed raises the temperature of the charge of the evaporator, and supplies the heat required This removal of water vapor is accomplished at reduced pressure, and the concentrated bath solution is removed by pump 48 through line 41.

From the pump it flows through line 45 containing valve 46 into'reservoir 12. Here by means of an agitator having blades 18 and motor drive 13, the material from the evaporator is thoroughly mixed with a large volume of liquor of the circulating bath system which has entered the reservoir 12 through line 13 and valve I4. This mixing brings the whole back to the required constitution for further processing of viscose web. It will be appreciated that prior to the addition of the-material from the evaporator indicates an initial coagulating bath which is contained in vat l2. From a hopper l3, whose lips extend below the surface of the coagulating liquid, viscose is extruded through an elongated orifice into the bath I I, where it is coagulated to a Web ID. This web is formed continuously, and is guided through this bath by means of rolls l4 and I5. After the web has acquired sufficient mechanical strength, it is drawn up out of the bath II between doctor rods l6 and 26 and between squeeze rolls 21 and 28. It then passes over a supporting roll 29 into the second processing bath 2! in tank 22. During its passage through the second bath it is guided by means of lower rolls 24' and upper rolls 25. A drainboard 23 collects the liquid removed from the surface of the web by scraper rod 26 and rolls 2'! and 23, and conveys the same into bath 2|. The doctor rods, squeeze rolls and drain-board are so arranged that part of the fluid scraped off returns to bath II, and part drains through The part draining into the vat 22 eventually passes to the sewer or other place of disposal, and never again enters the vat I2. of acid is also removed. I

In order to keep the bath II in constant circulation and at a constant concentration, a por- In ordinary practice a small amount the liquid in the reservoir was at a slightly reduced acid concentration. A portion of the reconditioned bath liquid H is then withdrawn from the tank 12 through line 63, control valve 64,pump 65, line 66 and valve 68 to the open top constant level feed tank 62. From this feed tank 62 the conditioned bath material is supplied through control valve 61 and line 66 to the vat l2, thereby completing the circulatory travel of the bath material.

In the interest of more satisfactory control of the circulating operation, various by-passes are provided. The normally closed valve 86 connects the lines 33 and 35, and the normally closed valve 84 connects the lines 63 and 45. A line 99 and normally closed valve 94 are provided for by-passing the material from pump 65 to the The line 3| with the header 32 main- Lines; 29l and 263 perform functions equivalent to lines SI and B3, respectively. This arrangement is especially suited to procedures where only small portions of the bath liquid are circulated through the evaporator.

It is believed clear from a consideration of the apparatus set-up illustrated in the-drawing that the heat of the solution and dilution of the sulfuric acid is expended in the evaporator, thereby furnishing all or a part of the heat which would ordinarily have to lie-supplied to the evaporator by means of steam coils or the like. In addition, .the employment of cooling coils or other forms of refrigeration to remove the heat of solution and dilution of the acid is no longer necessary.

Exxmmn I Viscose was extruded into a coagulating and/or regenerating bath at such a rate that it required 2.9 pounds per minute of sulfuric acid for the reaction in this initial treatment vat. As a result'of the reaction between the viscose and the bath, 31.0 pounds of water were added to the bath and 4.2 pounds of salt (sodium sulfate) were formed. The squeeze rolls and scraper rods were so adjusted that 35.7 pounds of bath fluid, containing 4.2 pounds of sodium sulfate and 1.5 pounds of acid, were removed each minute by the film. The total amount of acid removed from the system, therefore, was 4.4 pounds. Each minute 566 pounds of the bath were supplied to the reservoir. Solution from the reservoir was passed at the rate of 566 pounds per minute into the evaporator, and 94% sulfuric acid added to the evaporator at th rate of 4.7

pounds per minute. All of the figures for transfer of liquid from one part of the system to the other during the processing of the bath are on a continuou basis, and refer to a controlled, uniform and constant flow.

The heat produced by the addition of the sulfuric acid to the bath amounted to 1300 B. t. u., which was suflicient for the evaporation of 1.26 pounds of water from the bath. This water was removed from the bath at a pressure of 2 inches of mercury, and there resulted a solution containing 13.2% Of sulfuric acid, 18.3% of sodium sulfate, the remainder being water. This solution was supplied at the rate of 569.4 pounds per hour to the reservoir. The solution feed tank was supplied with the reconditioned solution from the reservoir. From the solution feed tank the reconditioned solutionpassed into the processing tank (for the viscose web) at the rate of 569.4 pounds per minute.

- EXAMPLE II may be varied within rather wide limits, depend- .ing upon such factors as the preferred temperature for processing the viscose. In this example,

-the heat of reaction between the viscose and the bath is substantially completely dissipated by evaporation from the bath surface and by radiation from the tank walls, piping, etc., so that little variation in temperature results in the viscose treating tank. Water vapor, being removed from the evaporator at a pressure 015.2 inches of mercury, enables the temperature of the bath in the evaporator to be kept at approximately the same temperature as the system as a whole. The bath fluid from the evaporator is supplied through a pipe to the reservoir at a temperature of 40+ C., which is just sufiiciently higher to supply heat to the system at the same rate at which it is lost to the surrounding atmosphere.

It is necessary that the temperature of the bath as supplied to the web processing vat be maintained uniform, and for this purpose it may be necessary to supply either a-small amount of heat to the system as a whole, either in the evaporator by changing the evaporator pressure or in the reservoir, in the feed tank or some intermediate point, or, if necessary, a slight amount of cooling. The amount of such heating or cooling required will be dependent upon the temperature of the surroundings and the nature or the area of the bath system. In any event, it is very small compared to the amount of heat that would have to have beensupplied for the evaporation of the water and the amount of refrigeration necessary to cool the bath after the addition of the sulfuric acid thereto.

It is not necessary to use the precise apparatus shown in the drawing. Nor is it necessary to employ the precise steps indicated in handling the bath; for example, rather than pass all of the bath from the viscose processing vat to the reservoir, part of it may be passed directly tothe evaporator and the other part to the. reservoir. It is possible to mix th sulfuric acid with a portion of the bath before passing to the evaporator, but it is preferable to mix directly in the evaporator, since in general the process of evaporation will prevent the temperature from increasing to the same extent that would occur if the mixture were made outside the evaporator.

The feed tank may be considered a non-essential in that the bath might be supplied directly from the reservoir to the viscose treating bath.

The temperature of processing in the evaporator may vary through wide limits, and is governed chiefly by the viscose treating process itself. For relatively low temperatures it will be necessary to maintain a high vacuum upon the evaporator in order to remove water vapor. At very high temperaturesthere will be considerable heat losses from the system as a whole, unless suitable insulation is provided. The temperature does not otherwise affect very materially the heat relations in the dilution of sulfuric acid and the evaporation of the water. The range of 35". C. to 60 C. is especially satisfactory for large scale operations.

results in very considerable savings in such processes by the elimination of heat necessary to evaporate water from the solutions and refrigeration to cool solutions to operating temperatures. Furthermore, it simplifies the apparatus required for handling such solutions.

As many apparently widely different embodifrom the coagulating vat, part of the water thereof evaporated, new acid supplied and the reconditioned liquor returned to the coagulating vat, the step which comprises adding the fresh sulfuric acid to the bath liquor while the water is being evaporated to recondition said bath liquor.

2. In a cellulose xanthate regeneration process in which spent coagulating bath liquor is removed from the coagulating bath, part of the water removed therefrom, sulfuric acid added thereto to recondition it and the reconditioned bath liquor returned to the coagulating bath; the procedure which comprises simultaneously adding sulfuric acid to and evaporating water from the spent bath liquor which has been removed from the coagulating bath.

3. The process which comprises coagulating viscose in a bath containing sulfuric acid and a salt, allowing the coagulated material to carry out of the coagulating bath an amount of salt equivalent to -that formed by the reaction of the acid in the coagulation, removing the aciddepleted bath liquor, simultaneously adding fresh acid toand evaporating water from thedepleted bath liquor to'recondition the same, and returning the reconditioned tion bath.

liquor to the coagula- 4. The process which comprises continuously coagulating viscose in a bath at such a rate that 2.9 pounds of sulfuric acid are consumed per minute, removing the coagulated material'in such a manner that it carries out 35.7 pounds of the bath material containing 4.2 pounds of sodium sulfate and 1.5 pounds of sulfuric acid per minute, removing 566 pounds of the bath material per minute for reconditioning, passing the material to be reconditioned into an evaporator and adding 4.4 pounds of 94% sulfuric acid to the material in the evaporator per minute, removing the water in the evaporator so that a solution containing 13.2% of sulfuric acid and 18.3% of sodium sulfate leaves the evaporator, adding the material leaving the evaporator to a body of the material leaving the coagulating bath to recondition the same, and supplying the reconditioned material to the coagulating bath at the rate of 569.4 pounds per minute.

5. The process of claim 3 when the salt is sodium sulfate.

6. In a. viscose cellulose regeneration process, in which sulfuric acid is used for regeneration, the acid of the coagulating bath is depleted, the water of the coagulating bath is increased and the salt content of the coagulating bath remains constant, the procedure which involves continuously withdrawing acid depleted bath liquor, continuously reconditioning the withdrawn liquor by simultaneously adding fresh sulfuric acid to and evaporating water from the depleted bath liquor, and continuously returning the reconditioned liquor to the bath.

ROY W. NASH.

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