Organic compounds -- part of the class 532-570 series – Organic compounds – Carbohydrates or derivatives
Reexamination Certificate
2002-09-11
2003-12-23
Barts, Samuel (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carbohydrates or derivatives
Reexamination Certificate
active
06667395
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process and an apparatus for the industrial preparation of methylhydroxyalkylcelluloses (hereinafter referred to as MHACs), and more specifically methylhydroxylethylcellulose (MHEC) and methylhydroxypropylcellulose (MHPC).
2. Brief Description of the Prior Art
It is known that MHACs and their specified mixed ethers can be prepared in a multistage process. In the first stage, the cellulose used is gently milled to a desired particle size spectrum. In the second stage, the milled cellulose is intimately mixed with a concentrated aqueous solution of an alkali metal hydroxide, in particular sodium hydroxide, in a mixer, and activated to form the alkali metal cellulose salt. This pretreatment is critical for the properties of the resulting cellulose ethers. The known processes are spray alkalization in a suitable mixing apparatus in which the milled cellulose is sprayed with alkali solution. In the slurry process, the milled cellulose is slurried in a nonsolvent and the alkali is then added. Alkalization in a slurry proceeds more uniformly, and more uniformly substituted products are obtained. In the mash alkalization process, the cellulose is slurried in aqueous sodium hydroxide and subsequently passed through screw presses or sieve drum presses.
In the third stage, the heterogeneous reaction with the halide of the alkyl radical to be added on as etherifying agent, e.g. methyl chloride, and the hydroxyalkylation agents such as ethylene oxide and/or propylene oxide occurs. The reaction is exothermic and proceeds under pressure.
The reaction sequence in the process can be such that partial alkalization, then partial etherification, repeated partial alkalization or etherification, etc., are carried out.
The difficulty is that the alkalization and etherification are, as exothermic reaction stages, associated with considerable liberation of heat and a simultaneous increase in pressure. Furthermore, there is a risk that uncontrolled temperature peaks can lead to degradation of the molecular weight of the cellulose.
Furthermore, to achieve good economics in an industrial production process, it is necessary for the reaction to proceed in a high space-time yield and give a high throughput combined with a uniform substitution pattern, characterized by the average degree of substitution of methyl DS(M) and the average molar degree of substitution of hydroxyalkyl MS(HAC) for methyl and hydroxyalkyl substitution, respectively.
Various properties of the products, e.g. the thermal flocculation point, the solubility, the viscosity, the film formation capability and the adhesive strength, are set via the degree of etherification and the type of substituents.
The further process stages comprise the purification of the cellulose ethers, milling and drying.
The preparation of cellulose ethers, their properties and applications are described in general terms in: Ullmann's Encyclopedia Of Industrial Chemistry, 5th Edition 1986, Volume A5, 461-488, VCH Verlagsgesellschaft, Weinheim, Encyclopedia Of Polymer Science and Engineering, 2nd Edition 1985, Volume 3, 226-269.
It is known from DE-A-2 635 403 that cellulose ethers can be prepared without use of separate reaction vessels for the preparation of the alkali metal cellulose salts or the heterogeneous etherification in a single-stage process. The preparation is done by carrying out the reaction of cellulose to the cellulose ether in a mechanical mixer with fast-running mixing element comprising a ploughshare mixer with choppers in a closed vessel with an adjustable internal pressure and cooling of the interior wall of the vessel. Heat of reaction liberated in the alkalization is substantially absorbed by the vaporization of the alkyl halide and the vaporized alkyl halide is condensed on the cooled wall of the vessel. The mixing vessel described in DE-A-2635403 having a total volume of 20 m
3
has a batch time of 4 hours, calculated from filling of the reactor with milled cellulose to the end of the discharge of the methylcellulose having a DS=1.3 at a 75% conversion. For ethylcellulose, this document describes a reactor having a total volume of 25 m
3
which allows ethylcellulose having a DS=2.45 to be prepared in 4.5 hours at a conversion of 75%. The maximum achievable capacity is 6000 tonnes per annum at an availability of 8400 h/a.
A further increase in the capacity and thus an improvement in the economics is not possible using the 20 or 25 m
3
reactors described in DE-A-2635403. The pressure and temperature rise caused by the exothermic reaction cannot be controlled by the methods described because sufficient rapid mixing, a high mixing effectiveness and sufficient great cooling power cannot be achieved industrially for relatively large reactors by means of the processes described. Thus, the reactor sizes claimed according to the prior art represent an upper limit for reactions which can be controlled safely.
EP-A-023692 describes a process for preparing polysaccharide ethers using a reactor having a multistage agitator and baffles. The universal mixer for different raw materials comprises a stirred vessel with a central vertical mixer shaft with a multistage agitator and baffles. The design is restricted to a shaft mounted at one end, and the torques for mixing of the starting materials and reaction products are consequently limited, thus restricting the capacity.
EP-A-0347653 describes a stirred vessel having radially pumping stirrers and at least one baffle and also a method of mixing liquids with the aid of the stirred vessel. The rapid axial mixing of liquids, even of different densities, is achieved by means of a vertically mounted central stirrer with baffles which are configured as a hydrofoil profile. This arrangement, too, is restricted to the concept of a shaft mounted at one end and thus has a restricted capacity.
EP-A-0470493 describes an upright vessel having a central agitator and baffles for rapid and uniform mixing, even of highly viscous media. A disadvantage thereof is the restricted torque which can be introduced via the shaft mounted at one end, so that the volume of the reaction products to be mixed and homogenized is restricted. This also applies to the apparatus described in SE 940 1144 A for mixing solid/liquid or liquid/liquid substances to produce suspensions, which has a built-in dividing tool transverse to the flow direction which can be operated at different rotational speeds.
U.S. Pat. No. 4,199,266 describes an apparatus for dispersing shear-sensitive solids in a liquid by means of a horizontally installed shaft or an obliquely mounted shaft. Compared with vertical mixers, this gives better mixing of suspensions since heavy particles are lifted. Disadvantages of this arrangement are the restricted torque which can be introduced via the shaft mounted at one end and the limitation of the throughputs.
It is known that, in industrial production processes, an increase in the throughput can be achieved by increasing the volume of the reactor. However, in the preparation of MHAC, the exothermic reaction of the alkalization and etherification makes it necessary to remove the heat via the wall of the reactor, as is described, for example, in DE-A-2 635 403. The reactors of the prior art consequently have a length/cross section ratio of >2.5.
U.S. Pat. No. 4,015,067 describes a continuous process for preparing polysaccharide ethers in which a slurry of finely divided polysaccharide, aqueous alkali metal hydroxide and an etherifying agent are introduced approximately continuously into a tube reactor (with coiled tubes) which is free of obstacles to the flow of the slurry and in which the slurry is conveyed through the reactor during the reaction. The length/cross section ratio (L/D) of the tube reactors is from 5 to 2000, preferably from about 100 to 800. Capacities of more than 6000 tons per annum were able to be achieved by means of this reactor, but the large L/D ratio makes such tube reactors disadvantageous in th
Holtkötter Torsten
Michel Stefan
Sonnenberg Gerd
Akorli Godfried R.
Barts Samuel
Eyl Diderico van
Krishnan Ganapathy
Wolff Cellulosics GmbH & Co. KG
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