Production Process of Dissolving Pulp
There are two kinds of production processes of dissolving pulp: sulfate pre-hydrolysis process and sulfite process. Generally, the acid sulfite process can be applied to low content hemi-cellulose of raw material; for high content resin of raw material, the sulfate pre-hydrolysis process must be adopted. Compared to two processes, the content of α-cellulose of dissolving pulp made by the sulfate pre-hydrolysis process is higher than that made by the sulfite process. Pre-hydrolysis process is similar to sulfite cooking. From this viewpoint, the sulfate pre-hydrolysis process is a comprehensive process in combination with the advantages of both sulfate process and sulfite process with a wide adaptability to raw materials. So it is usually applied to new-built projects much more.
The sulfate pre-hydrolysis process
The sulfate pre-hydrolysis process mainly includes two working procedures for making dissolving pulp: pre-hydrolysis process and sulfate cooking process. The pre-hydrolysis process can be divided into three types: acid pre-hydrolysis process, water pre-hydrolysis process and steam pre-hydrolysis process. Though the acid pre-hydrolysis process can be operated at low temperature, it needs to be equipped with acid liquid recycling device just with high construction costs and complex operation. The water pre-hydrolysis process, also named hot water extraction, needs longer heating time and temperature rise time. It has to bring people higher steam consumption and lower time efficiency. Comparatively, the steam pre-hydrolysis process is more mature and economic. So it is popularly used on the new-built systems.
The pre-hydrolysis process is usually carried out under the acid conditions. Under the conditions, the hemi-cellulose will dissolute out by the acid hydrolysis process, which can make its content of raw materials (Especially pentosan) greatly decreased. The pre-hydrolysis process can improve the reaction ability to make pulp as well. It is similar to sulfite cooking process, which can damage the primary wall of fibers, and make it stripped down. And then the secondary wall of fibers that is rich in cellulose will be exposed, which can contact with chemicals to improve the reaction ability of dissolving pulp.
The pre-hydrolysis process can flexibly control the viscosity and polymerization of the dissolving pulp. In addition, because the hydrolysis of hemicelluloses can destroy or weaken the connection between carbohydrates and lignin, the part of the lignin can be removed during the process. After the process, raw material will continue to be cooked by the sulfate process. The process is similar to the cooking method of the sulfate process for common pulp making. But during the pre-hydrolysis process, the part of lignin reacts with condensation and the other material have been changed. The pre-hydrolysis process is an acidic reaction, so in the cooking time, some of acidic materials are needed to neutralize, alkali usage amount is higher than that of traditional sulfate pulping.
Production Procedures of Dissolving Pulp
The procedures for producing dissolving pulp by steam sulfate pre-hydrolysis process are briefly described as follows:
1. Tank filling
When wood chips are loaded, passing through low pressure steam can not only improve the density and the temperature of tank filling, but also make partial air in wood chips be removed.
2. Temperature rise
Low pressure steam can help make the temperature increased. Generally, 0.6MPa low pressure steam can just help reach 159℃. When the pressure difference is decreased and the steam is reduced, medium pressure steam shall be used to heat to reach the required temperature. The corresponding temperature of 0.8MPa medium pressure steam in saturation is 170℃.The pre-hydrolysis temperature always varies from 165℃-172℃ depending on the different raw materials.
As heating up to the required pre-hydrolysis temperature, the time shall be kept about 60mins. Whether it is to raise the temperature or extend time, P factors must be crucial to be used for control. Specifically, it is determined by test results.
After the completion of pre-hydrolysis process, the tank is in acidic state with PH value of about 2-3. At this time, waste liquid after pre-hydrolysis process can be discharged and collected for the producing butanol, furfural and the other by-products. But the treatment needs special technology. Thus, generally recycle is at high cost. In order to neutralize the acid liquid, about 30% of total alkali amount of white liquid will be filled into the bottom of the digester.
5. Alkali filling
According to the proportion of the remaining liquid, white liquid and black liquid will be added evenly into digester bottom. This procedure has two key steps: first, white liquid and black liquid must be added evenly, if not, due to low alkalinity of black liquid, lignin condensation and precipitation will occur; second, they must be added from the bottom of the digester, this can avoid that the cooking liquid contact with non-neutralized chips.
Through two-time temperature rise, the cooking by the sulfate process is on. As the liquid level in the tank is beyond medium circulation position, the circulating pump starts for forced circulation of cooking liquid. When heat exchanger gets increased at the temperature after the inlet of medium pressure steam, began conventional cooking operations shall be carried out.
7. Tank discharging
When the H factor reaches the standard value of the process, tank discharging begins.