Nitrocellulose dehydration

Manufacture of nitrocellulose pi wdcr The dehydration of nitrocellulose Dehydration with centrifuges Dehydration in presses RcL tification of alcohol from dehydration The preparation of nitrocellulose mixtures Partial dissolution of nitrocellulose Shaping the dough Predrying Solvent recovery... 

M. R. Olsen and R. K. Major, Comparative Study of Dehydrating Processes in the Manufacture of Nitrocellulose, United Technology Chemical Systems Division, Suimyvale, Calif., 1975. 

The main features in which the Radford process differs from the batch operation are in thermal dehydration and compounding. Water-wet nitrocellulose on a continuous vacuum belt filter is vacuum-dried followed by hot air transfusion (80°C) to reduce the moisture to less than 2%. After cooling, alcohol is sprayed on the nitrocellulose to a concentration of 15—20%. The alcohol-wet nitrocellulose is then transferred from a surge feeder to a compounder by a continuous weigh-belt along with the other ingredients of the composition, which are also weighed and added automatically. 

The nitrocellulose is first dehydrated, that is the water present is replaced by alcohol. This is done by compressing the wet nitrocellulose in a hydraulic press and passing alcohol through the press until the strength in the block is about 92%. The resulting block of alchohol wet nitrocellulose is broken down to small pieces with toothed rolls. 

Uses Manufacture of artificial leathers, plastics, safety glass, photographic films, lacquers as a solvent in the production of perfumes, natural gums, and synthetic resins solvent for nitrocellulose lacquers dehydrating agent. 

Uses Solvent for nitrocellulose, ethyl cellulose, polyvinyl butyral, rosin, shellac, manila resin, dyes fuel for utility plants home heating oil extender preparation of methyl esters, formaldehyde, methacrylates, methylamines, dimethyl terephthalate, polyformaldehydes methyl halides, ethylene glycol in gasoline and diesel oil antifreezes octane booster in gasoline source of hydrocarbon for fuel cells extractant for animal and vegetable oils denaturant for ethanol in formaldehyde solutions to inhibit polymerization softening agent for certain plastics dehydrator for natural gas intermediate in production of methyl terLbutyl ether. 

In the manufacture of nitrocellulose powders the water is displaced with alcohol. This method was proposed by Lundholm and Sayers [3] and widely used in many countries [4, 5]. Despite the simplicity of the idea the dehydration process is rather complicated. It is influenced by such factors as the solubility of nitrocellulose in alcohol and the ability of nitrocellulose to swell under the influence of alcohol the lower the solubility of nitrocellulose in alcohol, the more easy dehydrated with alcohol. Since, however, the solubility of nitrocellulose depends primarily on its nitrogen content dehydration is easier with the higher nitrated types of nitrocellulose. 

The advantage of dehydrating nitrocellulose with alcohol lies in the fact that any residual alcohol may be subsequently included in the solvent. The amount of residual alcohol in nitrocellulose depends not only upon the pressure applied in the dehydration press, but also on the type of nitrocellulose, i.e. it is somewhat larger in more highly nitrated nitrocellulose. Nitrocellulose made from wood cellulose swells in alcohol more readily than that made from cotton which is why the former retains more alcohol and more water. 

The dehydration process is based primarily on the ability of alcohol to displace water. Since, however, the water is not always perfectly displaced the alcohol becomes partly mixed with water. In addition, some of the water is adsorbed by the nitrocellulose and cannot readily be removed, which causes a further dilution of the alcohol. The subsequent portions of fresh alcohol displace the dilute alcohol, the residual alcohol adsorbed by nitrocellulose is mixed with concentrated alcohol, the latter is displaced by fresh alcohol etc. This course of the operation is illustrated by variations in the concentration of alcohol in the liquid flowing out of the dehydration apparatus (Figs. 203-205). 

The nitrocellulose is then centrifuged to a definite alcohol content. The dehydration of one batch of nitrocellulose requires approximately 1 hr. 

The best method for recovering the alcohol is to collect the 60-70% alcohol and rectify it and to collect the 80% alcohol separately and use it for the dehydration of the next batch of nitrocellulose. 1251. of 95% alcohol is consumed per 100 kg of dehydrated nitrocellulose of this 30-35 1. remain in the nitrocellulose and about 90 1. are recovered by rectification. 

After dehydration 30-35% alcohol remains in the nitrocellulose and is subsequently utilized as a part of the solvent. The final centrifuging should last until the content of residual alcohol remains constant. It is found that with certain forms of nitrocellulose (e.g. those made from wood cellulose) the removal of excess alcohol by centrifuging is difficult. In such cases centrifuging should last somewhat longer. If the content of alcohol in the nitrocellulose is too high, more solvent will be put at a later stage of dilution, or its composition is altered to increase the content of alcohol in it. 

Another method for dehydrating nitrocellulose is based on the use of pneumatic or hydraulic presses. Dehydration in pneumatic presses (diffusers) (Fig. 200) consists of filling the cylinder of the press with wet guncotton squeezed out of the tank under... 

Fig. 200. Schematic view of a pneumatic press (a pressure diffuser) for dehydration of nitrocellulose with alcohol.

After dehydration in a diffuser, the nitrocellulose contains about 50% of alcohol. This is obviously too much and on removal from the cylinder the nitrocellulose is again subjected to pressure in a hydraulic press (100-200 kg/cm2) to reduce the content of alcohol to 30-35%. The method described above requires an expensive installation, more numerous staff and takes longer than dehydration in centrifuges or in hydraulic presses. It does not seem to be in use any more. 

After the upper piston has touched the layer of nitrocellulose, i.e. after the alcohol has been forced through the dehydrated material, the upper piston rises, while the lower one moves down and the set of cylinders again turns through 90°, so that the cylinder which started from position (/) through (2) now takes position (5). Here the lower piston is again fitted with conduits to drain away the alcohol. The upper... 

Ponchon [9] studied the course of dehydration of nitrocellulose in a Champigneul press, and drew the graphs indicating how water and alcohol are displaced in each position of the cylinder. The graphs in Fig. 203 denote the process of dehydration... 

Fig. 203. Characteristics of the dehydration process of nitrocellulose in a Champigneul press (position (2) of the cylinder).

Ponchon also produced a graph (Fig. 205) showing the difference between the dehydration of CPt nitrocellulose (containing 1.5% of soluble in alcohol)... 

According to Ponchon 110 kg of 93% alcohol are used for the dehydration of 100 kg of nitrocellulose (dry weight). In French factories (1917) an average of 33 kg of 42-52% waste alcohol was recovered and returned for rectification. 

The Becker and van Hiillen (Krefeld) press is of another design. In this press the wet non-Centrifuged nitrocellulose is first dehydrated by removal of up to 20-30% of its water content, and then subjected to further dehydration with alcohol. Partial dehydration of the nitrocellulose takes place in the feeding screw used with this press. 

Alcohol from dehydration contains a certain quantity of nitrocellulose, i.e. its soluble fractions, mostly degraded, and a certain amount in suspension. Experiments have shown that about 2.2 g of dissolved nitrocellulose and 1.3 g of nitrocellulose in suspension—a total 3.5 g—occur in 11. of 70% alcohol from centrifuges. Sometimes however, the content of nitrocellulose in the alcohol may reach 10-12 g/1. 

After analysis each form of nitrocellulose (guncotton, collodion cotton, pyror collodion cotton) is batched separately into a linen bag which for convenience and safety is stored in an air-tight iron vessel (to protect the nitrocellulose from drying up and becoming dusty). Nitrocellulose is batched by charges, the size of which depends on the dimensions and the type of apparatus used. In France, for instance, a total charge of nitrocellulose (CPt and CP2) is 20 kg when dehydrated in a hydraulic press or 30 kg when dehydrated in a centrifuge. 

The more modem approach to the problem of mixing nitrocellulose consists of mixing defined types of nitrocellulose in the nitrocellulose factory itself. In this case the two forms of nitrocellulose are mixed under water in mixers as described in Vol. II, p. 374. The water is then centrifuged and the mixture dehydrated with alcohol. This method, however, creates certain inconvenience to the powder factory which loses the possibility of changing (within certain limits) the composition of the mixtures, i.e. nitrogen content and total solubility of nitrocellulose. The powder factory is therefore compelled to limit the number of factors which can be varied to obtain the powder of required ballistic properties. 

In principle the manufacture of cordite RDB was similar to that of ordinary cordite with the difference that instead of drying the damp nitrocellulose it was dehydrated with alcohol and the required amount of ether and additional alcohol were introduced in the kneader. The manufacture of cordite RDB was discontinued after the w3r since its ballistic properties were inferior to those of common cordite. 

Finely ground nitrocellulose of low viscosity was used. It was impregnated with DGDN under water in the usual way. After dehydration in a centrifuge to a moisture content of 18-22%, the blend was stirred into molten TNT. The vat was evacuated to evaporate the water and to produce a homogeneously gelatinized grain. 

The first attempts to esterify cellulose by means of nitric acid consisted in treating cellulose with the acid without any dehydrating agents. Vieille [1] discovered in 1882 that on nitration with nitric acid alone the rate of reaction and the nitrogen Content of the nitrocellulose increased as the concentration of HN03 was increased ... 

Acetic acid, or acetic anhydride can be used in the manufacture of nitrocellulose as dehydrating agents. Berl and Smith [42] showed that increasing the proportion of acetic acid mixed with 88% nitric acid favours the nitrating capacity of the mixed acid, since they obtained products containing 13.7-14.0% of nitrogen. 

Cotton or wood pulp as delivered to nitrocellulose plants contains 6% or more of moisture. This moisture should be removed prior to nitration so that it does not bring about any dilution of the mixed acid and thereby change the course of the reaction. Cotton is usually transported in the form of compressed bales, which are opened and shredded before drying. Wood cellulose, in the form of pressed cellulose pulp or of crepe paper, is dealt with similarly. Dryers for dehydrating cotton or cellulose may be of various types. The simplest is the shelf drier, equipped with mechanical ventilation and heaters situated at the bottom, supplied with water or steam so as to maintain a temperature of 80-100°C inside the drier. 

The water content of the thickened nitrocellulose pulp is reduced to 30-40% in centrifuges of the type similar as those used in the manufacture of sugar. In many plants, additional dehydration is carried out in hydraulic presses, in which the nitrocellulose is subjected to a pressure of about 20 kg/cm2 and formed into blocks or cakes in which the water content is 20-30%. Clearly this is advantageous from the point of view of transport. 

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