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Continuous Tubular Digester Selection Guide

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continuous tubular digester


The continuous tubular digester is common pulping equipment that is suitable for non-wood fiber such as bagasse, rice straw, reed, stalk, and bamboo. These raw materials are characterized by lightweight, looseness, poor fluidity, easy bridging, poor water filtration and easy pulping.

The continuous tubular digester is mainly composed of a quantifier, a pre-steamer or a wet pipe, a screw feeder, a chemical liquid impregnator or a spray pipe, a cooking pipe, an intermediate material level control pipe, a discharge, and a discharge valve. The modern horizontal tube continuous digester has been continuously improved and equipped with advanced and reliable monitoring instruments and automatic control devices. It has high automation, stable and reliable operation, low operating labor intensity, compact layout, low steam consumption and balanced steam and electric load. The continuous tubular digester also has advantages of high cooking yield and good pulp quality, and it is widely used in the paper and pulp industry. It can be said that the continuous tubular digester has become a typical non-wood fiber pulping equipment.

How to select continuous tubular digester?

Equipment selection

There are many types of non-wood fiber raw materials, and their bulk density, fluidity, compressibility and liquid permeability are very different. Even if the same kind of raw material, the production areas, the preparation methods, and effects are different, and the various properties of the raw materials are also affected. For example, bamboo, using the two methods of shaving and tearing, respectively, the bulk density, fluidity, compressibility and absorption of the liquid are completely different, so different equipment should be selected according to the nature of the raw materials. The equipment that is greatly affected by the nature of the material mainly includes quantifiers, screw feeders, and chemical add equipment.

Quantifier selection

An important condition for the normal operation of a continuous cooking system is the requirement for uniform continuous dosing, so the choice of the quantifier has a great impact on the overall system. Improper selection will not only affect the accuracy of the measurement but even affect the uniform continuous feeding. Two volumetric quantifiers are usually used: pin drum quantifiers and double helix quantifiers. It is generally considered that softer bulk materials such as straw, bagasse, bamboo, and soft rafts are preferred to use pin drum quantifiers; and hard materials such as stalks, hoes, and bamboos should use double screw quantifiers. For bamboo chips, when using a PREX pressure impregnator, a screw feeder can be used for metering. A screw feeder should equip with a variable speed transmission.

Steaming equipment selection

In order to improve the absorption capacity of raw material such as bamboo chips, the vertical steamer can be used before the impregnation, which requires less room and ensures sufficient residence time. Steaming raw materials and make it soft. Vertical steaming silos are generally used in conjunction with pressure impregnators, but they are not suitable for most non-wood fiber materials with light bulk density and poor fluidity, otherwise, they are easy to bridge. In a continuous steaming system, in order to avoid bridging, it should generally be designed as an inverted tapered structure. In fact, for materials with good absorption of liquids such as rice straw, bagasse, reed, and bamboo, the screw conveyor steamer can meet the processing requirements.

Screw feeder selection

The screw feeder is the most critical equipment for the continuous tubular cooking system. Whether its selection is correct or not, it determines the performance of the entire system. The two most important parameters that determine the performance of the screw feeder are the compression ratio and the length of the plug tube. When the compression ratio is too small, and the plug tube is too short, it is difficult to form a tight plug, which is prone to back spray. On the contrary, if the compression ratio is too large, and the plug tube is too long, which not only consumes too much power but also easily causes clogging, which aggravates the wear of the screw shaft, the conical shell and the plug, increase the fiber damage, thereby reducing the strength of the pulp. .

The compression ratio and the length of the plug tube are mainly related to the specific proportion, hardness, and compressibility of the raw material. Generally, the raw material has light bulk density, large compressibility, and easy to form a tightly stuffed, such as rice straw, bagasse, bamboo silk, soft reed and etc. Screw feeder with a short plug tube and a large compression ratio of 3:1 to 3.5:1 can be selected. For the bamboo chips, the bulk density is large, compressibility is small and it is not easy to form a tight plug, so this situation should select a long plug tube screw feeder with a compression ratio of 2:1. For hard reed, cotton stalk and other material that have compressibility and bulk density between the above two types of raw materials, select a suitable feeder according to specific conditions.

Selection of chemical add equipment

The selections of chemical add equipment have a great influence on the quality of cooking. The selection depends on the liquid absorbing ability of the raw material. For some raw materials like rice straw, bagasse, reed and bamboo, because of their strong ability to absorb liquid medicine, it is suitable to select T-tube and spray the liquid on the material, while the bamboo with a poor absorption capacity should use a vertical pressure impregnator.