As one of the four great inventions of China, papermaking technology starts from crop stalk pulping process. Due to the lack of advanced technology about straw pulp making and wastewater treatment, crop stalk pulp or other kinds of straw pulp making develop slowly. Facing the situation of fiber material shortage, how to develop non-wood fiber pulp making become a vital topic for everyone in the paper and pulp industry.
There is no doubt that pulp cooking machines are the main part of the paper pulping process. The capacity and performance of pulp cooking equipment directly affect the whole pulp making system. For solving the problem of pollution and capacity of crop stalk pulping, high-efficiency pulp cooking equipment will be the key point.
Classification of pulp cooking equipment
Pulp cooking equipment has a variety of structure due to various chemical pulping processes, different kinds of fiber material and production capacity. In general, pulp cooking equipment is divided into batch cooking and continuous cooking according to pulp cooking process. The main equipment of batching cooking is digester. The rotary digester is mainly used in the small-scale pulp mill. The vertical digester has a large capacity and mostly applies for large-scale paper and pulp mill. Based on the chemicals and pulping process, the vertical digester can be classified into sulfate cooking digester and sulfite cooking digester.
Continuous cooking system has many types, such as Kamyr continuous cooking system for sulfate wood pulp and Pandia continuous digester for sulfate straw pulp. In conclusion, pulp cooking equipment goes to the direction of low energy consumption, low pollution, high efficient, high capacity, high auto control and pluralism.
Digester selection for crop stalk pulping
After the long-term study, the digester selection for crop stalk pulping is mainly based on the fiber material characters, high-efficiency equipment. How to select digester among various types? CNBM list some typical digesters for crop stalk pulping process, let’s analyze all of them together.
The rotary digester is a spherical thin-walled pressure vessel. The rotary digester is almost applied for all kinds of material and process because of its strong suitability, especially for crop stalk. However, with the development of economic and technology, rotary digester has disadvantages of high labor intensity, high energy consumption, and low auto-control, it is not suitable for large-scale paper pulping making.
Vertical batch digester
The vertical batch digester is only applied to fiber material with good drainability, such as wood chip, reed, bamboo, and etc. these materials will not lead to cyclic temperature rise. The vertical batch digester is not suitable for wheat straw pulp making, because chemical liquor cannot recycle, which can’t meet the requirement of pulping process.
Continuous tubular digester
Continuous tubular digester belongs to the high-temperature and high-speed cooking equipment. The fiber materials enter into the digester, and steam rapidly heat it up to highest temperature(175℃), the cooking cycle is relatively shorter, according to different material, impregnation, pulp quality requirement, the cooking time is about 10-50min.
In recent year, the processing of continuous tubular digester can be summed up into three types:
- Apply for fiber materials that have low-density, high-compressibility and strong liquor absorptivity, such as wheat straw, rice straw, reed and so on. This system uses horizontal screw steamer, feeder with a large compression ratio to add chemical liquor. It is also known as soft non-wood fiber continuous digester.
- Use pre-steaming of vertical live bottom bin, feeder with small compression ratio and pressure macerator to impregnate chemical liquor. This system is more suitable for bamboo, cotton stalk and wood chips with poor absorption.
- The last one adopts double-blowing continuous tubular digester, apply for sugarcane bagasse, wheat straw and bamboo pulp making.
Vertical continuous digester
Kamyr continuous cooking system is a representative of vertical continuous digester. It is applied for fiber materials with good filtration, including wood chips and bamboo. For reed, bagasse, wheat straw, rice straw, bad filtration result in difficulty of liquor circulation, which can’t achieve displacement washing and heat recovery. Compared to continuous tubular digester, it has almost no advantages.