US1955836A - Process of treating cellulose - Google Patents

Description

April 1934- G. SCHNEIDER PROCESS OF TREATING CELLULOSE Original Filed Jan. 4, 1929 INVENTOR George Schneider Y ATTORNEY Patented Apr. 24, 1934 PA Er FFlCE PROCESS OF TREATING CELLULOSE George Schneider, Mcntclair, N. 5., assignor to Celanese Corporation of America, a corporation of Delaware Original application January 4, 1929, Serial No. 330,245. Divided and this application July 15, 1931, Serial No. 550,931. In Canada December Claims.

This invention relates generally to an improved process of the treatment of cellulose prior to the esteriiication of the same and particularly to the treatment of cellulosic material prior to acetyla- 5 tion. 7

This application is a division of my application Serial No. 330,245 filed January 4, 1929.

An object of my invention is to provide a process for the pretreatment of cellulose which will .10 not only facilitate its esterification but will also yield superior products.

Another object of my invention is to devise'a process for the pretreatment of cellulosic material which will consume but a small fraction of the time heretofore required for such operations.

Another object of my invention is to devise a process whereby the steps of pretreating the cellulosic material and esterifying the same can be combined into a single and continuous operas tion.

Another object of my invention is to employ an apparatus for the pretreating of cellulosic material which is compact and efficient. Other objects and advantages will appear from the following detailed description.

The pretreatment of cellulosic material to facilitate the esterification thereof has been proposed heretofore and has'been used to some extent with varying degrees of success. The processes employed have been rather cumbersome and involve the use of unwieldy and uneconomical apparatus. These processes were conducted as separate and distinct operations from that of acetylation and involved a large amount of efiort and required considerable time.

Generally heretofore, the pretreatment of cellulosic material has been effected in a rotary apparatus constructed to give thorough tumbling and/or mixing or in a stationary apparatus provided with agitating means. These types of apparatus were objectionable for various reasons.

The rotating apparatus in order to insure thorough tumbling and mixing ofthe cellulose must be of such size that its volume is about three times that of the cotton or cellulose being treated. In order for such an apparatus to take an amount of cellulose as is actually employed as an acetylizing charge, its size would be so large that it would preclude the installation of such an apparatus with each acetylizer. Another disadvantagearising from the use of such rotating apparatus is the fact that the discharge of the pretreated material from the apparatus when 55 placed over the acetylizer is mechanical but not uniformly constant. This gives a nonuniformly acetylized product.

The stationary type of apparatus heretofore employed has also been found objectionable. Its use necessitated the removal of the pretreated 68 material therefrom by hand and also the subse- -'quent charging of the acetylizer by hand.

Naturally these operations are slow and greatly curtail the productivity of an acetylizing unit. Not only is this method and apparatus inefiicient but it is also hazardous to the health of the employees since it entails disagreeable exposure to the strong acetic acid vapors and fumes. I

To overcome these disadvantages and draw- 7c backs I have invented the present apparatus and process. My apparatus is of simple construction and consists of a pretreater which ispreferably rectangular in shape andprovided with agitators. This rectangular container is arranged in combination with an acetylizer so that the contents of the former are discharged directly into the latter. The container is also provided with means for introducing the pretreated liquid and also with an opening for charging the cotton or other cellulosic material thereinto and a discharge means into the acetylizer.

The pretreatment process comprises the. treatmentof the cellulosic material with a relatively small proportion of a lower fatty acid such as formic acid or acetic acid or with a mineral acid such as hydrochloric acid. a The formic or acetic acid may be applied to the cellulosic material in any suitable manner, either in the form of a liquid, or in the form of a vapor. One mode of pretreatment comprises the agitation of the cellulosic material with a relatively small amount of glacial acetic acid in a suitable apparatus, say a horizontal type of agitator. Instead of glacial acetic acid, weaker acetic acid of from 50%. to strength may be used.

The temperature of the pretreatment may be boiling temperatures of the organic acid used, or it may be below such temperature, and may be ordinary temperature or even below ordinary r temperature. The time of treatment may be varied from one hour to five hours or more, depending on the concentration, amount and nature of the organic acid used, and the temperature employed. The amount of the lower fatty acid used may be varied within wide limits, but we have found that if glacial acetic acid is used, excellent results are obtained by employing 10 to 30 parts of the glacial acetic acid to parts of the cellulose.

Because of the greater speed of reaction, the output of a given esterifying apparatus is increased from 50 to 60%. Moreover, since by this process, the amount of cooling required is less than in the former processes, a great saving is effected. Moreover, by my process, it is not necessary to precool the esterifying mixture in a separate vessel as is now the common practice, so that the expensive precoolers now used may be eliminated by the substitution of mere measuring devices which are much cheaper.

The process may be employed for preparing various organic esters of cellulose such as cellulose acetate, cellulose formate, cellulose propionate or cellulose butyrate. Fowever, further details will be given with respect to the manufacture of cellulose acetate and the properties of cellulose acetate produced in accordance with my invention.

Cellulose acetate prepared from cellulosic material pretreated in accordance with this invention has a clarity of about 20 whereas cellulose acetate prepared from cellulosic material under identical conditions but without pretreatment has a clarity of only 12. (The units given as a measure of the clarity are determined by dissolving 6 grams of cellulose acetate in ice parts of acetone and are expressed in terms of centimeters of depth of the solution, which, when placed in the tube is so opaque as to just prevent a black spot at the bottom of the tube to be discernible when illuminated by a bright light.)

In the accompanying drawing The figure illustrates, diagrammatically one embodiment of my apparatus. In the drawing a pretreater is represented at 1, the pretreater being preferably rectangular in shape and provided with a hinged door 2 through which the material to be treated is charged. To agitate the cellulosic material being treated, I provide a stirrer comprising a horizontal shaft 3 provided along its length with a plurality of arms 5, the arms 5 being supplied or formed at their outer ends with blades 4. These blades 4 are arranged in such a manner and at such angles that the rotation of the shaft 3 will cause the blades to deliver the cellulosic material to the opening 6 of the pretreater.

The opening 6 at the bottom of the pretreater is adapted to be closed by the door 7 hinged as at 8. Attached to the pretreater so as to receive the material delivered through the opening 6 is a chute 9. This chute is preferably constructed in two sections, the lower portion 10 being hinged as at 11 to enable its being swung into and out of alignment with the upper rigid portion 9. The chute 9 is arranged to deliver the pretreated cellulosic material into the acetylizer or reaction chamber 12. The acetylizer 12 is provided with any suitable stirrers or agitator 14 mounted on shafts 15. The acetylizer is supported on the standards 16 and is adapted to be tipped or tilted about the pivot 13 for discharging its contents. Suitable covers for the acetylizer are also provided, but for the sake of simplicity these have not been shown in the drawing. Obviously the hinged doors 2 and 7 can be replaced by sliding doors if desired. Also a cloth or other flexible tube or means may be substituted for the chute 9 for delivering the pretreated material to the acetylizer.

To introduce the pretreating liquid into the pretreater, I provide a pot or kettle 17 which has a discharge pipe 19 terminating, within the pretreater, in a plurality of outlets 20. The supply of liquid to this pipe 19 and the outlets 20 is regulated by the valve 18. The shaft 3 is rotated by means of the belt and pulley 21 which is connected with any suitable source of power, the shaft 3 being provided with suitable bearings 22.

The operation of the apparatus is as follows: The desired charge of cellulosic material is introduced into the pretreater through 2 and the shaft 3 rotated, the door '7 being in the closed position. The pretreating liquid, (acetic acid, formic acid, acetic and nitric acids, or other pretreating liquid) is put into the pot 17 and introduced into the pretreater at a suitable rate by opening the valve 18. Due to the comparatively large volume of the pretreater and the constant agitation, the liquid introduced is vaporized and uniformly distributed throughout the cellulosic material. After the desired degree of activity of the cellulose has been attained the chute 10 is put into operative position and the door 7 opened. The rotation of the shaft 3 delivers the cellulosic materials to the opening 6 and the material discharges into the acetylizer 12 at a regular rate which can be governed by the speed of rotation of the shaft 3. Before the pretreatment of the cellulosic material is completed, the acetylizing mixture is introduced into the acetylizer and cooled to the desired temperature, for example to 0 to 2 C. The pretreated cellulose is then introduced into the acetylizer 12.

After all of the cellulose has been delivered to 12, the chute 10 is raised to the non-operative position as shown in dotted lines on the drawing, the door 7 closed and a new charge of cellulosic material introduced into the pretreater for pretreatment. The cellulosic material and acetylizing mixture are thoroughly agitated and mixed until the desired amount of acetyl groups have been introduced into the cellulose molecule when the reaction mixture is removed from the acetylizer by tilting the latter at 13. The reaction mixture is then subjected to the usual ripening, secondary and purification treatments.

The pretreater 3 may be constructed of acid resistant metal or of wood or both, as is deemed most economical under existing conditions and according to the pretreating agent used. The shaft 3 and the stirrer arms and blades are preferably made of acid resistant metal. The blades 4 may be replaced by a continuous helical ribbon stirrer but I have found that the use of fingers for segments of such a helical ribbon in their outer ends, as shown in the drawing, is preferable. The acetylizer should preferably be surrounded by a hollow jacket through which a fluid heating or cooling medium can be passed to control the temperature of the reaction mixture. Instead of the tilting type of acetylizer shown, I may employ an acetylizer provided in its bottom with a valve for discharging the reaction mixture or other type of reactor.

Example The cellulosic material is charged into the pretreater through the top door 2 and the pretreating liquid, in this case, glacial acetic acid in amount of 20 lbs. per 100 lbs. of cotton is sprayed on it through the nozzles 20.

While the pretreatment is going on, the acetylizer 12 is charged with 500 to 600 lbs. of a mixture of acetic acid and acetic anhydride, the acetic anhydride comprising 37 to 39% of the mixture. This mixture of acetic anhydride and acetic acid also contains about 2 oz. of H2SO4. The mixture is then-cooled to about 0 to 5 C.

After the cellulosic material has been pretreated for a period of about 3 hours the bottom discharge door '7 is opened and the pretreated material is fed through the chute 12 into the prepared cooled acid mixture in the acetylizer, 12.

The temperature of the reaction mixture rises to about 12 to 15 C. When the latter point is reached 2 ounces of sodium carbonate are added to the mass, with continued stirring for about 1 hour. The temperature is reduced to about 5 to 8 C. and a mixture containing 13 to 16 lbs. of sulfuric acid and 100 lbs. of a mixture of glacial acetic acid and acetic anhydride of the strength above mentioned is added. ihe temperature rises to about 43 C. in about half an hour, and the acetylation is now virtually complete. The mass is new cooled and ripened in any desired manner to normal acetone or any other desired solubility.

The pretreatment of the cellulosic material in the manner above indicated yields a final esterified product superior to that obtained from the esterification of untreated cellulosic material. It has a higher clarity and is more stable than the product obtained from untreated cellulosic material and in addition possesses more desirable viscosity characteristics. A further advantage, perhaps the most important, is the fact that a pretreating operation as above set out shortens considerably the time required for esterification.

Having described my invention what I claim and desire to secure by Letters Patent is:

1. Process for the production of uniformly esterified cellulose which comprises pretreating cellulose in a fixed pretreatment chamber by introducing a lower fatty acid into said chamber at a regular rate, agitating the cellulose within the chamber and then gradually discharging the pretreated cellulose directly into an esterification mixture.

2. Process for the production of uniformly acetylated cellulose which comprises pretreating cellulose in a fixed pretreatment chamber by introducing acetic acid into said chamber at a regular rate, agitating the cellulose within the chamber and then gradually discharging the pretreated cellulose directly into an acetylating mixture.

3. Process for the production of uniformly esterified cellulose which comprises pretreating cellulose in a fixed pretreatment chamber by introducing a lower fatty acid into said chamber at a regular rate, agitating the cellulose by lifting the same and allowing it to fall within the chamber and then discharging the pretreated cellulose at a uniform rate directly into an esterification mixture.

4. Process for the production of uniformly acetylated cellulose which comprises pretreating cellulose in a fixed pretreatment chamber by introducing acetic acid into said chamber at a regular rate, agitating the cellulose by lifting the same and allowing it to fall within the chamber and then discharging the pretreated cellulose at a uniform rate directly into an acetylating mixture.

5. Process for the production of uniformly acetylated cellulose which comprises pretreating cellulose in a fixed pretreatment chamber by introducing acetic acid into said chamber at a regular rate, agitating the cellulose by lifting the same and allowing it to fall within the chamber and then discharging the pretreated cellulose at a uniform rate directly into an acetylating vessel containing 875 parts of an acetylating mixture comprising about 15 lbs. of sulphuric acid, 37 to 39% of acetic anhydride and the balance glacial

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