3 Accessory Equipment for Pulp Digester

paper pulp cooking process

As we all know, pulp digester works with twin roll press, drum washer, disc filter and other paper pulping machine in the paper pulp production. As the main equipment of pulp cooking process, pulp digester is a big pressure vessel for chemicals reaction. There is three accessory equipment for pulp digester includes blow tank, heat exchanger, and digester distributor. They give support for pulp digester and complete the pulp cooking process together. These accessory equipment have different functions in the process.

Digester circulating system

Based on the mode of heating, pulp digester can be divided into direct-heated forced circulation and indirect-heated forced circulation. In China, pulp mills usually adopt indirect-heated forces circulation, often accessorized with direct heating. Indirect heating can keep stable liquid ratio, increase the concentration of the chemical, save the cooking time, and make sure the evenness in pulp quality. Besides, the indirect heating mode will have an influence on black liquor, specifically, black liquor has a high concentration and steam consumption for alkali recovery will be reduced.


For increasing the production of pulp digester, using circulating device install the digester with press fitting. Alkaline pulp digester generally uses two types of the direct heating circulating system include leaching liquor in the lower part of the cylinder and leaching liquor in the middle part of the cylinder.

Discharge device

The discharge device refers the pulp storage containers for holding pulp from the digester, including blow pit, blow bin and blow tank. In general, the small-scale digester is equipped with blow pit or blow bin, while the big-scale digester should be equipped with blow tank. The gross capacity of blow tank is 1.5~1.8 times of digester gross capacity. Every blow tank (or blow bin) capacity should be 2.5~3.0 times of every digester.

Blow bin

The blow bin is the high-density pulp storage container, the top and bottom of blow bin are tapered, and the middle part is cylindrical.  The blow bin is usually applied to the small and medium-size straw pulp mills.

Blow tank

blow tank

The blow tank is relatively the large-scale pulp storage container. Cone-bottom blow tank is a traditional type. Currently, flat-bottomed blow tank is more popular in the paper pulp production. Compared to the cone-bottom blow tank, flat-bottomed blow tank have several advantages, such as simple dasher, convenient maintenance, low cost, easy installation and less space occupied.

As for the structure, the top of blow tank is the gas-liquid separator. There are a number of nozzles on the ring pipe of cone bottom for diluted pulp. The screw-type dasher is installed on the bottom part of cone bottom, and its rotation rate is 110r/min. The pulp passes through the blowpipe, along with the tangential direction enter into the blow tank from the inlet. The waste gas separated from the pulp is usually cleared out of exhaust steam pipe, and sent to the heat recovery system. The pulp is out from the discharge pipe. Cone-bottom blow tank is high-density pulp storage container, the concentration of pulp storage is about 11~14%.

Filler for digester

The filler for digester is used to increase the charge capacity and pulp yield, improve the distribution of material in the digester, raise the temperature of materials and remove the air in the materials. Most paper pulp mills adopt filler. There are many charger types, common types include chip charging with backpacker, simple filler and steam chip distributor.

filler for digester

Chip charging with backpacker

The chip charging with backpacker can increase 10~40% of filled volume. How does the backpacker work? Wood chip passes the funnel and rotary disk and falls into the digester. The rotary disk is driven by the reducer, gear case, and electromotor, and control the speed of digester charging. The rotate speed of the rotary disk is 20~30r/min.

Steam chip distributor

Steam chip distributor has fixed and mobile type, to avoid the trouble with reloading and movement, the fixed type is more applicable. The steam pressure required by the steam chip distributor is 250~350kPa, steam consumption is 0.10~0.20t per ton pulp. The steam chip distributor can enhance 25%~30% of filled volume.

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