Kraft pulping process

Sulfate or Kraft pulp is one of the chemical pulping processes. It is manufactured by alkaline method. Kraft pulping process is the dominating pulp manufacturing process in the world. Today this process is most widely used with a production of 70-80% of the total Pulp.

Raw material:
Although soft wood are mostly used but any type of soft or hard pulp wood can be used for Kraft pulp manufacturing. The benefits of soft wood are that cellulose fiber is thin and long.

Cooking chemicals, this is known as white liquor. The white liquor contains the active cooking chemicals NaoH and Na2S. NaoH and Na2S are used for cooking the wood chips in the digester. There is no fixed percentage of the chemicals NaoH and Na2S in the white liquor, but it is better to keeping 25-35% sulfidity (based on Total Titratable Alkali). Low percentage of sulfidity (especially below 15%) may affect on the reaction rate and pulp quality.

Black liquor is used as makeup liquor to balance the liquid requirement. The ratio of the liquor to wood might be between 3 and 5 approximately. The concentration of the liquor should not be more dilute, it may be affect on the reaction rate. Furthermore steam is used for receiving temperature and pressure. Pressure and temperature are very important for cooking process. During Kraft pulping process above 360 ⁰F temperature the strength of the fiber and yield percentage are greatly affected.

Below there is a Kraft pulping process diagram
Kraft pulping process
Digester cooking process is two type; batch type and continuous type process. The capacity of the digester may be 10-20 ton pulp.

In batch type process, chips are taken a certain volume of the digester. Then a calculated amount of white liquor (contains NaOH and Na2S) or caustic soda and black liquor (if needed) are taken into the digester, so that the cooking liquor soak and cover the chips. After the digester filled with chips and liquor, then the heat is applied on contents by cooking liquor circulation method though a heat exchanger. The circulating liquor is collecting from the middle point of the vessel through a pump and deliver to the top and bottom point of the vessel. Temperature is control from 320 to 350 ⁰F. And pressure is control from 110 PSI to 150 PSI. To reach maximum temperature and pressure, it is needed from 1.5 hours to 3.0 hours. After reaching the maximum temperature and pressure then, the content is allowed to stay one to three hours to complete the cooking reaction. This time is called cooking time. The cooking time varies basis on uses purpose of the pulp. At this time the chips becomes softened. After completing the cooking time the content are ready to discharge into blow tank.

Pressure release control valve: At the top of the vessel, there is a pressure release control valve to release air and other non-condensable gases.

The main benefit of the Kraft pulping process is that used pulping chemicals can be recovered economically. About 90% of the chemicals can be recovered.

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