Pulp Digester of Batch Cooking

pulp digester for batch cooking

The pulp digester is a large vessel that is used for paper pulp making. Just like blow tank, disc filter, drum washer, the pulp digester is a necessary equipment of paper pulp production line. Generally, the pulp cooking process can be divided into batching cooking and continuous cooking. So we can classify the pulp cooking machines into batch-type and continuous-type according to the operating process. As the main equipment in the pulp cooking process, there are different forms of pulp digester. Today, we discuss the pulp digester for batch cooking, also called batch digester.

Vertical pulp digester and rotary spherical digester are two typical kinds of the digester. The rotary spherical digester usually applied to the medium and small-sized paper factory. While the vertical digester has larger capacity, it is used in the Large and medium-sized paper plant. By comparison, the vertical digester has wider applications.

  • The rotary spherical digester is mainly used for the soda pulping process or sulfate pulping process in the small paper mill.
  • Vertical pulp digester is fixed equipment. Because the cooking liquors differ in the corrosion and process features, the structure, form, and capacity of digester has differences.

Except for the pulp digester, batch cooking pulp machines should equipped accessory equipment like blow tank, circulating pump, heat exchanger, exhaust heat recovery apparatus, etc.

Vertical pulp digester features

Raw materials: The sulfate digester is a typical type of digester. In China, large and medium-sized sulfate pulp mill mostly use fixed vertical pulp digester to cook wood, bamboo, silvergrass, reed and etc.

Cycle period: The sulfate pulp cooking has a short cycle period, generally up to 4~5h. Actually, the cycle period can be shortening to 3~4h by automatic digester cover, automatic control in the cooking process.

Capacity: The digester has common sizes like 50m³, 70m³, 110m³, 125m³, 160m³, 225m³, etc. To increase the temperature evenly and quickly, the sulfate digester capacity should not be too large.

Structure: As for the structure, the digester is made up of digester body, cover, charger, and blow off valve, heat exchanger and supporting seat.

Pulp digester advantages and disadvantages

  • Advantages: large capacity; high pulp yield; higher productivity; less area occupied (compare to rotary spherical digester with the same size).
  • Disadvantages: less accessory equipment; complex structure; high manufacturing requirement; high investment.

Body of pulp digester

  • The body of digester consists of three parts including the upper, middle and lower. The upper part is mostly a combination of sphere and cone, also could be The middle part is cylindrical and the lower part is same as the upper part.
  • The height, diameter and the sizes of top or bottom taper angle is the important indicator for form and size of pulp digester. When the ratio of height and diameter is too big, the cost of the workshop will increase. But when the ratio of height and diameter is too small, the cycle liquor will distribute on the digester section unevenly, even form the cross flow, which make the temperature rise of materials uneven and result in bad pulp quality and low pulp yield. For the sulfate digester, the proper ratio of height and diameter is between 3.3 and 4.
  • The top or bottom taper angle has a certain influence on the charge, blowing, charging of liquor and steaming. The top taper angle is about 90°and the bottom taper angle is about 60°. If the top taper angle is too big, and the materials in the digester silo can’t be compacted, result in decreasing the capacity. If the bottom taper angle is too big, and the blow incompletely and heat unevenly. Otherwise, if the top taper angle or bottom taper angle is too small, the height of digester will increase.
  • The connection between upper and bottom taper angle and cylinder shell of the middle part adopt circular arc transition, which reduces or relieve edge stress.
  • To the heat cycling of cooking liquor, there are two symmetrical pipes on the cylindrical part of digester or transition part between cylinder and hypocentrum. Two circular filter screens are respectively installed on the corresponding pipe wall, which is used to avoid tablet or pulp extraction during liquor extraction.

sheel of digester and filter screen

  • Shell of digester and filter screen: screen holder is welded on the wall of the shell, and the upper end and the lower end of the filter screen is welded on the shell by the access panel. The filter screen is fixed on the screen holder and access panel by screw or welding. The distance between the shell of digester and filter screen is generally 120~150mm. the filter screen is made of 4~6mm steel plate. The opening is φ 3~8mm, it is better a tapered hole to avoid locking the mesh with pieces of wood or sundries. For keep enough circulation volume of cooking liquor, the effective filtration area of filter screen ten times higher than inlet cross-section of circulating tube.
  • The digester neck is wholly made of cast steel, on its surface, there is the filter screen fixed in the screen holder. There are two pipes on the side of digester neck. The thicker pipe is used to transfer the heated liquor to the hydronic heating system. Another pipe is used to transfer the steam and handle auxiliary heating during the heating. The lower port of digester neck connects emptying syphon, emptying valve and emptying pipe. The diameter of the lower port of digester neck is decided by the structure of emptying syphon. While the diameter of the emptying valve and emptying pipe is decided by the emptying time of pulp (about 10~20min). In general, the diameter of emptying pipe is 200~350mm, but also depend on the capacity of the digester.

In conclusion

Although pulp digester is the most important equipment in the pulp cooking process, some accessory equipment is also necessary. For the whole set of paper pulp process, there are many other paper pulp machines like disc filter, vacuum drum washer, twin roll press and etc. CNBM provide the whole set of paper pulp production customization or single machine of whole pulp line.

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