RU2331652C1 - Method of obtaining cellulose nitrates - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention pertains to the technology of production of cellulose nitrates and can be used when producing them using continuous and combined technologies. The method involves preliminary and final nitration of cellulose with subsequent recovery of acid. Final nitration of squeezed out and partially nitrated cellulose is done in a centrifuge through short-time flowing-irrigative treatment with a nitrating mixture in the field of centrifugal forces. As the nitrating mixture a fresh nitrating acidic mixture or concentrated nitric acid used is. After final nitration, the nitro-mass is squeezed from the acid to moisture of 45-60 mass % and is taken for desaturation in a "НУОК" apparatus directly in form of squeezed nitro-mass or in form of acid with 5-12% suspension. For dilution of the nitro-mass and its transportation, the more concentrated fraction of acid, desaturated in the "НУОК" apparatus, is used. Desaturation of acid in the "НУОК" apparatus is done with extra use of flowing etherification zones of the apparatus.

EFFECT: design of a highly productive method of obtaining cellulose nitrates.

4 cl, 1 tbl, 15 ex

Description

This invention relates to the field of manufacturing technology of cellulose nitrates (pyroxylins and colloxylins), in particular stages of cellulose nitration and recovery of waste acids, and can be used in their preparation by continuous and combined technologies.

In the scientific and technical literature, a periodic method for the production of cellulose nitrates is known and widely used in industrial practice (see the book: Gindich V.I., Zabelin L.V., Marchenko G.N. Production of cellulose nitrates. Technology and equipment. TsNIINTI, 1984 pg. 55-59). This method produces mainly high-nitrogen cellulose nitrates (pyroxylins), and the nitration of cellulose from CA, XC and RB grades is carried out in batch nitrators with a working acid mixture (SCS) and, after its completion, the nitromass is squeezed from spent acid mixtures (ACS) in centrifuges . Further, cellulose nitrates (SC), unloaded from there, are transported through a mutilator to a displacer. In the displacer, the acid mixtures sorbed by cellulose nitrate are periodically displaced (recuperated) in two stages: first with weak irrigating acid, then with irrigating water. Displaced acid mixtures as they strengthen up to 50 wt.% Are allocated for regeneration. From the displacer, the SC comes for further processing at the stage of preliminary stabilization. SC unloading is carried out through a centrifuge flushing apparatus into a rotary kiln.

The main disadvantages of this method of manufacturing SC are the following.

1. Low productivity of the method of manufacturing SC (0.13 ÷ 0.25 t / h).

2. The heterogeneity of nitration of cellulose fibers, which causes the heterogeneity of the physicochemical properties of the resulting SC.

3. High probability of decomposition of the mass of the SC in a centrifuge.

4. The formation of a large number of low concentrated displaced acid mixtures.

Also known is the high-speed method of periodic recovery (displacement) of acid mixtures in acid-squeezing centrifuges (see the book: Gindich V.I., Zabelin L.V., Marchenko G.N. Cellulose nitrate production. Technology and equipment. TsNIINTI, 1984, p. 117-119), according to which the recovery of acids from SCs made by a batch process is carried out in a centrifuge in two stages: first, with a weaker recovery (concentration of 40 ÷ 45 wt.%), Then with water. The time of a two-stage recovery in a centrifuge does not exceed 2 minutes. The main disadvantage of this method is that it reduces the performance of the centrifuge.

Among other methods, it is necessary to emphasize the known method for producing mixed pyroxylins according to a continuous technological scheme (see the book: Gindich V.I., Zabelin L.V., Marchenko G.N. Cellulose nitrate production. Technology and equipment. TsNIINTI, 1984, p. 59-62), which is widely used in industry. According to this method, the nitration of cellulose is carried out in two stages: in the nitrator-dispenser and in the zone of flow irrigation esterification of the apparatus for the continuous removal of waste acids (NUOK).

The prototype of this invention, the authors chose the above method for producing pyroxylins in a continuous process flow (see book: Gindich V.I., Zabelin L.V., Marchenko G.N. Cellulose nitrate production. Technology and equipment. TsNIINTI, 1984, p. 59 -62). According to the prototype, the method of obtaining SC is as follows.

Accordingly, prepared cellulose of grades ХЦ, ЦА, RB with residual moisture up to 5 wt.% Is fed through four hopper-batchers installed directly above the NUOK through the hopper-dispenser. An appropriate amount of CSW is fed there and the cellulose is pre-nitrated. After its completion, the reaction mixture from cyclically operating metering nitrators is poured into the zone of flow irrigation esterification of the NUOK apparatus, where the final nitration of cellulose is performed. In the subsequent sections of this apparatus, the spent acid mixtures are removed from the mass of the SC in four stages with acid mixtures of decreasing concentration and water. By this technology, pyroxylins No. 1 and No. 2 are obtained.

The above method of manufacturing SC (prototype) however has a number of significant drawbacks.

1. The method is not universal, it is used only for the manufacture of SC from traditional types of cellulose raw materials (cotton and wood pulp of the grades ХЦ, ЦА and RB) and does not provide for the use of other types of raw materials - cotton lint and dense wood pulp in the form of a “folder”.

2. Cellulose nitrates produced by this method are unevenly penetrated and therefore have inhomogeneous physicochemical properties (in terms of nitrogen content, chemical resistance, viscosity, etc.), due to the uneven treatment of cellulose with a nitrating mixture in the area of flow irrigation esterification of the apparatus WELL, OK.

3. High denitration of the obtained SC during the recovery of acids (up to 1.5–5.0 ml of NO / g).

4. The duration of the nitration process (28 ÷ 50 min).

5. The impossibility of obtaining, according to this method, individual grades of SC (for example, low esterified alcohol-soluble colloxylins).

6. The formation in the process of recovery in the NUOK apparatus of a large number of dilute acid mixtures (from 3 to 6 tons per 1 ton of production), i.e. the manufacture of NC is accompanied by a large number of acids in the acid-turnover.

7. Additional production costs for the regeneration of a significant amount of dilute acid mixtures from NUOK, which this method of obtaining SC makes expensive and uneconomical.

8. The process of regeneration of a large amount of acids, in turn, entails environmental pollution, including the air basin around the enterprise.

The technical result of the present invention is the creation of a universal, high-performance and economical technology for the production of low and high nitrogen cellulose nitrates, which also provides high quality products and environmental safety of production.

The technical result is achieved by the fact that upon receipt of the SC, the stage of the final nitration of partially nitrated cellulose is carried out in a centrifuge by successive operations of squeezing it from excess acid, flow-irrigation processing of the pressed nitromass layer with a nitrating mixture in the field of centrifugal forces and final squeezing of cellulose nitrates from acids up to 45 ÷ 60 wt.%. In the final nitration, an acidic mixture of increased concentration is used as the nitrating mixture — fresh nitrating acidic mixture or concentrated (up to 97–98 wt.%) Nitric acid, i.e. realize the countercurrent principle of processing (nitration) of cellulose with nitrating acid mixtures of increasing concentration.

The squeezed cellulose nitrates are transferred to the loading zone of the NUOK apparatus for displacing sorbed acids either directly in the form of a mass with an acid content of 45–60 wt% or as a suspension obtained by diluting the squeezed out nitrate cellulose displaced during recovery in the NUOK unit with the most concentrated fraction of acid mixtures .

The use for the final nitration of partially nitrated cellulose of a more active, increased concentration of the nitrating mixture (fresh nitrating mixture or concentrated nitric acid) is explained by the need for a very quick (in tens of seconds) stage of the final nitration of cellulose in the field of centrifugal forces. Moreover, the latter is carried out in a working centrifuge by sequentially spinning the nitromass received in the centrifuge to an acid moisture of 45 ÷ 60 wt.%, Flow irrigation and impregnating it with a nitrating acid mixture in the ratio of sorbed to the irrigated acid mixture as 1: 0.5 ÷ 1.5 and the final extraction of nitromass to an acid humidity of 45 ÷ 60 wt.%. Moreover, this combination of operations allows you to quickly displace the old ACS from the SC, replace it with a fresh nitrating acid mixture and complete the final nitration in 12-18 seconds. Carrying out this stage under the influence of the field of centrifugal forces (in a centrifuge) ensures high uniformity of the physicochemical properties of the obtained SCs, a significant reduction in the duration of the nitration process, a significant increase in the productivity of the nitration phase, minimal consumption of nitrating acid mixtures, and improvement of the sanitary condition of the working zone due to nitration in a closed centrifuge.

Another advantage of the final nitration of cellulose in the field of centrifugal forces is that the long-fiber cellulose used as raw material, in particular dense wood pulp in the form of a “folder”, is well penetrated and has homogeneous physicochemical properties (nitrogen content, viscosity, chemical resistance, etc.). When nitrating such cellulose according to the prototype method, uneven nitration occurs.

On the other hand, the use of long-fiber and dense cellulose as cellulose raw materials, due to the presence of lumps and agglomerates, complicates the subsequent processing of nanocrystals: nitromasses that are finally made and squeezed from acids have poor fluidity and self-decomposition and denitration of nanocrystals are possible. To avoid this, during the transportation of the SC to the NUOK apparatus, the nitromass extracted after completion of the esterification is diluted to a concentration of 5-12 wt% with the appropriate acid and gravity or transferred to the NUOK loading zone through an intermediate tank for displacement.

In the absence of clumps and agglomerates of fibers in the squeezed nitromass and its better transportability (for example, using short-fiber cellulose — cotton lint with a fiber length of 0.5–3.5 mm as a raw material), it is transferred to the displacement unit in the NUOK without dilution, in the form pressed to 45 ÷ 60 wt.% nitromass. At the same time, the amount of acids used in the total acid turnover of production decreases sharply.

Due to the sharp reduction in the amount of acids used in the preparation of SC by the proposed method, the amount of acids recovered in the NUOK apparatus is also reduced. At the same time, the introduction into the NIOC apparatus due to the free zone of flowing irrigation esterification (nitration) of an additional displacement stage performed with a recycle of irrigating acid mixtures (i.e., feeding the acid mixture displaced from the same stage through a pressure tank for re-irrigation of nitromass), instead of four steps (according to the prototype), it allows to actually carry out six stages of displacement. This, in turn, contributes to a significant increase in the concentration of the displaced acid mixture at the outlet of the apparatus, to a decrease in the amount of acid mixtures allocated for regeneration, and to a sharp increase in the completeness of acid displacement from the NC: the residual acidity of the NC decreases to 1.5–3.0 wt.% Instead 6 ÷ 8 wt.% According to the prototype.

The method of continuous production of cellulose nitrates according to the invention is as follows.

Cellulose prepared in an appropriate way with a moisture content of not more than 5 wt.% Enters the metering hopper, from where it is transferred to the nitrator-metering device.

There, in advance, a measured amount of a heated nitrating acid mixture is pumped from the supply tanks through heat exchangers, in accordance with the adopted nitration module. To ensure a continuous circuit, the technology uses four nitrator-dispensers, which, working cyclically (on a schedule) ensure the continuous operation of a system consisting of a centrifuge and an NUOK apparatus.

After the pre-nitration time has elapsed, the reaction mixture is automatically poured into a centrifuge and, after the nitromass is pressed (squeezed) to an acid moisture of 45 ÷ 60 wt.%, Flow irrigation is performed (with a ratio of sorbed to irrigated acids mainly within 1: 1 ÷ 1.5) the pressed SC layer with a fresh nitrating acid mixture or concentrated (97 ÷ 98 wt.%) nitric acid for about 6 ÷ 10 sec, then nitromass is pressed for 7 ÷ 9 sec to an acid moisture content of 45 ÷ 60 wt.%. Full (total) completion of the final nitration of cellulose occurs in 12 ÷ 25 sec.

Squeezed out after the final nitration, the nitromass is transferred for displacement to the NIOC apparatus according to two options: according to the first variant, the squeezed nitromass at the outlet of the centrifuge is diluted with acid to an acid suspension concentration of 5-12 wt% and transferred directly or via an intermediate container to the loading zone of the NUOC for stage displacement of sorbed acids. At lower values of the concentration of the suspension (less than 5 wt.%), Good transportation is ensured, but a large overspending of acids, with concentrations of the suspension of more than 12 wt.%, The fluidity and transportability of the product in the NUOK are reduced. According to the second option, the transportation of nitromass to the NUOK apparatus is carried out in the form pressed to 45 ÷ 60 wt.%.

In this case, the first transmission option is usually used to exclude the formation of lumps and agglomerates when receiving SCs from long-fiber and dense cellulose raw materials (CA, HC, RB and wood in the form of a dense “folder”), and the second option - when using more technological cellulose raw materials (cotton short fiber lint).

In addition, to dilute the squeezed nitromass and transport it in the form of a 5–12 wt.% Acid suspension, the most concentrated fraction of acid mixtures displaced in the NUOK apparatus is used (within 75–80 wt.%), Which allows to reduce the amount of acids in total acid circulation and exclude the receipt at the outlet of the NUOK of a low-concentrated fraction of acids.

Next, the process of multi-stage displacement of sorbed acids from the SC is carried out in the NUOK apparatus by successive irrigation with acid mixtures of decreasing concentration and water, and in the free zone of the flow irrigation nitration of the apparatus, an additional recirculation irrigation of nitromass previously displaced at the same stage with an acid mixture is arranged, which allows to obtain the effect of a two-stage crowding out and ensure high efficiency of the NUOK apparatus: the acidity of the SC at the outlet of the apparatus decreases about 1.5 ÷ 3.0 wt.%, and the concentration of the final displaced fraction of acid mixtures discharged from the NUOK reaches 75 ÷ 80 wt.%, which is then used both for diluting and transporting the squeezed mass of SC to the NUOK unit, and for preparing fresh nitrating acid mixture.

After the displacement stage, cellulose nitrates with an acidity of not more than 3 wt.% Are sent to the mutilator and then to preliminary stabilization.

In the process of obtaining SC by the proposed method, the disposal of spent acid mixtures is carried out in the form of centrifuges (after extraction of the nitromass before and after the final nitration), which are used in the preparation of fresh acid mixtures or sent for regeneration.

The main distinguishing features of the new method of obtaining SC from the prototype are the following.

1. The final nitration of cellulose in a centrifuge by flow-irrigation treatment with a nitrating mixture in a field of centrifugal forces.

2. Use for the final nitration of pre-squeezed partially nitrated cellulose.

3. Transfer pre-squeezed up to 45 ÷ 60 wt.% Nitromass for displacement in the form of a squeezed product or in the form of a dilute acid suspension with a concentration of 5 ÷ 12 wt.%.

4. The use for dilution of squeezed nitromass and transportation of its displaced during recovery in NUOK the most concentrated fraction of acid mixtures.

5. Use at the stage of final nitration of cellulose nitrating acid mixture of high concentration (fresh nitrating acid mixture or concentrated nitric acid).

6. The stage of the final nitration of cellulose at low values of the irrigation module: 1: 1 ÷ 1,5 according to the invention and 1: 10 ÷ 20 according to the prototype.

7. The use of a zone of flow-irrigation nitration with recirculating irrigation of nitromass for additional displacement of sorbed acids in the NUOK apparatus.

Examples confirming the possibility of obtaining cellulose nitrates by the proposed method are shown in the table.

Depending on the type of cellulose nitrates obtained, nitration was carried out with the following compositions of the working acid mixture:

Components Pyroxylin No. 1 Pyroxylin No. 2 Colloxylin Pyroxylin No. 1 Examples 1-6 Examples 7-9 Examples 10-12 Examples 13-15 (prototype) Nitric acid 27.0 ÷ 27.4 23.0 ÷ 23.5 23.3 ÷ 23.7 27.1 ÷ 27.4 Sulphuric acid 63.3 ÷ 63.6 60.5 ÷ 60.7 61.0 ÷ 61.2 63.5 ÷ 63.6 Water 9.3 ÷ 9.4 16.0 ÷ 16.3 15.3 ÷ 15.5 9.1 ÷ 9.3

From the above examples it follows that cellulose nitrate samples are obtained with homogeneous physicochemical properties (in terms of nitrogen content, chemical resistance and viscosity).

From the same examples it is also seen that according to the proposed method, it is possible to obtain cellulose nitrates with any nitrogen content and from any type and form of cellulosic raw materials, including and short-fiber cotton lint and wood pulp in the form of a “folder”.

Using the proposed method for the production of cellulose nitrates, compared with the prototype, has the following advantages.

1. The universality of the production method, ie it allows you to make any brand of cellulose nitrate from any type and form of cellulose raw materials, including from wood and cotton cellulose (HC, CA, RB), dense cellulose “folder” and cotton lint.

2. The manufacture of cellulose nitrates with good uniformity of physico-chemical properties (nitrogen content, chemical resistance, viscosity, etc.).

3. Reduced residual acidity obtained after NUOK SC (no more than 3 wt.% Instead of 6 ÷ 8 wt.% Of the prototype).

4. The low value of denitration of the obtained NC (an average of 0.25 ml NO / g instead of 2.1 ml NO / g of the prototype).

5. Less than the prototype, the acid consumption during nitration and, accordingly, less than the prototype, acid turnover in production.

6. A high concentration of displaced acids discharged from the NUOK (75 ÷ 80 wt.% Instead of 60 ÷ 70 wt.% According to the prototype).

7. High productivity of the technological installation, exceeding the productivity of the prototype by 1.5 ÷ 1.7 times, which is caused, first of all, by a significantly shorter duration of the nitration process.

8. High production efficiency and less environmental pollution by acid and gas emissions are due to reduced production acidity and a small amount of acid mixtures allocated from NPAs for the regeneration of acid mixtures.

Based on the proposed method, it is possible to create modern universal, economical and environmentally friendly industrial technologies for the production of cellulose nitrates of a wide range of grades.

Claims (4)

1. The method of producing cellulose nitrates, including preliminary and final cellulose nitration and acid recovery, characterized in that the final nitration of partially nitrated cellulose is carried out in a centrifuge by flow-irrigation treatment, pre-squeezed to 45-60 wt.% Nitromass nitrating acid mixture in the field centrifugal forces and the final extraction of cellulose nitrates from acids to humidity 45-60 wt.% with their subsequent transfer to the NUOK apparatus for the displacement of sorbed acids. 2. The method of producing cellulose nitrates according to claim 1, characterized in that for the final nitration use nitrating acid mixture of high concentration, which is used as a fresh nitrating acid mixture or concentrated nitric acid. 3. The method of producing cellulose nitrate according to claim 1, characterized in that the transfer of the squeezed cellulose nitrate to the NUOK apparatus for displacing the sorbed acids is carried out in the form of a mass pressed to an acid moisture content of 45-60 wt.% Or in the form of a suspension with a concentration of 5-12 wt.% obtained by diluting the squeezed cellulose nitrates displaced in the recovery process at the NUOK the most concentrated acid mixture. 4. The method of producing cellulose nitrate according to claim 1, characterized in that in the zone of flowing irrigation nitration of the NIOC apparatus, an additional displacement is organized by recirculating irrigation of cellulose nitrates displaced by acid mixtures previously displaced at the same stage.