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Comparison Of Continuous Tubular Digester And Spherical Digester

Home » Comparison Of Continuous Tubular Digester And Spherical Digester
comparison of continuous tubular digester and spherical digester


The chemical pulping is most popular pulping process on the market. The pulp cooking machine of chemical pulping can be divided into the batch digester and continuous cooking digester. The batching digesters include spherical digester and vertical digester. Continuous cooking digesters include continuous tubular digester and continuous digester, m and d digester. Non-wood fiber pulp making generally adopt spherical digester or continuous tubular digester. The spherical digester belongs to traditional cooking equipment, the continuous tubular digester is developed from the 1990s.

As a leading paper pulper machine manufacturer, CNBM can supply pulp digester, blow tank, twin roll press, vacuum drum washer, single screw press, disc filter, and related pulping machines. Today we will compare continuous tubular digester and spherical digester, and find the best solution for chemical pulping.

Paper pulping process

Pulping process of continuous tubular digester: feeding belt, straw cutter, feeding conveyor, dedusting system, belt conveyor gallery, stock bin, belt conveyor gallery, Hydraulic grass crusher, grass piece pump, oblique dehydration, feedback screw, counter, pre-boiling screw, screw conveyor, t-finned tube, cooking tube, vertical discharger, blow tank.

Pulping process of spherical digester: feeding belt, straw cutter, Biconical dust remover, traveling belt, preimpregnation, spherical digester, blow tank.

Advantages and disadvantages

The continuous tubular cooking system


  • Use counter, alkali and grass liquid mix evenly, cooked pulp quality is uniform and stable.
  • Adopt a cold blow to protect the fiber efficiently.
  • Continuous operation reduces power and steam consumption.
  • DCS control is applied to all production, the high degree of automation, less labor required and low labor strength.
  • Stable power and steam consumption, no peak load.
  • Compact structure and less area occupied.
  • Wet preparation, effectively solves the problem of moisture variation.


  • The pulping process is fixed, the material application is narrow;
  • Cooked Pulp has low strength, hard to wash;
  • Produce a large amount of washing water, it is difficult to process;
  • Maintenance is more difficult, high maintenance cost, non-stop operation of the cooking system;
  • High requirement on the operation of the continuous cooking system;
  • High accident probability of continuous cooking system;
  • Compared to the spherical digester, the continuous cooking system needs higher investment.

Spherical digester

The advantages and disadvantages of the spherical digester are contrary to that of the continuous tubular digester.

Operating cost

Cost of raw material (based on wheat straw)

Continuous tubular digester and spherical digester have little difference in the cost of raw material.

Alkali charge of pulp cooking

Due to continuous tubular digester adopt wet preparation, which effectively remove the dust of wheat straw, reduce the dust content, and avoid alkali consumption caused by dust, so the alkali charge of continuous cooking system is less than that of spherical digester, volume id about 19.5kg/t (bleached pulp), the alkali is calculated in terms of 1.23 yuan/kg, for every ton of coarse pulp, alkali cost can save 19.12-20.39 yuan (the loss of pulp washing and pulp bleaching section is different, so total loss is based on 15%-20%).

Power consumption of pulp cooking

The continuous cooking system is more complex than spherical digester. Installed capacity of total motors is 818.2kW; the installed capacity of the total spherical digester is 459kW, the spherical digester don’t require continuous operation, so the power consumption is lower than the continuous cooking system.

Steam consumption of pulp cooking

Generally, the steam consumption of the continuous cooking system is 2.4t/ ton (coarse pulp). Because the steaming time of the continuous cooking system is only 15-25min, it requires the fast reaction time and high temperature, and the spherical digester can be adjusted greatly to establish a false cooking curve. The spherical digester saves 88.45 yuan per ton of pulp.

Water consumption of pulp cooking

Spherical digester adopts dry preparation. The spherical digester has no need of water, and cooking liquid comes from alkali liquid and black liquid. The continuous cooking system requires a large amount of washing water, about 100m3per ton of coarse pulp. Even if 70% comes from the reuse water of the pulp washing and pulp bleaching section, also need 30m3 of fresh water. Grass washing water not only contains a large number of sediment, SS, COD, high turbidity, and complex composition but also contains so much cold water extract and organic silicon, difficult to deal with. Even if the processing cost is 0.56 yuan /m3, the additional cost per ton of pulp is 16.8 yuan.

Labor cost

Six workers are needed for continuous steaming system and 48 workers are needed for spherical digesters. The monthly salary is 2000 yuan per person and the daily output is 130 tons of coarse pulp. The additional cost of the spherical digester is 21.53 yuan per ton pulp.

Other costs

Because the continuous cooking system has more equipment, complex structure, its maintenance cost is higher than that of spherical digesters. screw shaft of screw conveyor, wear-resistant alloy steel blade pump is expensive and easy to wear. Because the investment of continuous cooking is bigger, even the depreciation cost is higher than spherical digester.

In conclusion, the operation cost of the spherical digester is lower than that of the continuous tubular digester, except for the cost of wheat straw raw material. The best solution for paper pulp making is based on various aspects, raw material, equipment, cost, pulp quality and etc. You should choose suitable paper pulper machines as you need, if you have any problems, welcome to contact us!