27. 1 Introduction
The black liquor (lignin-rich) produced from the Sulfate pulp production contains 95-98% of digested chemicals. To reduce air and water pollution and to balance economy of operation, these chemicals should be removed before disposal.
In chemical recovery process, black liquor is concentrated, burned and limed.27. 1 Introduction The black liquor (lignin-rich) produced from the Sulfate pulp production contains 95-98% of digested chemicals. To reduce air and water pollution and to balance economy of operation, these chemicals should be removed before disposal.
In chemical recovery process, black liquor is concentrated, burned and limed.
Figure 27.1 Flow sheet of chemical recovery from black liquor
27.2 Functional role of various units (Figure 27.1)
(a) Multi effect evaporator
(b) Mix tank
Na2SO4 + 2C→ Na2S + 2CO2
(d) Dissolving tank
(f) Washing tank
(g) Causticizing tank
27.3 Production of Paper (Figure 27.2)
Paper is a sheet material made of fibers which are held together by hydrogen bonds. In wet process, fiber suspension in water is made and it is then made into sheet and dried. Paper making process is generally done on Fourdrinier machine.
Figure 27.2 Flow sheet of production of paper
27.4 Functional role of various units
(c) Web forming
27.5 Technical Questions
1 Why are make-up chemicals added to the mix tank before the smelter?
Ans: White liquor chemicals (Na2S+ NaOH) are lost in the pulp manufacturing process in various ways. These include loss along with pulp, waste sludge, wood knots and undigested residue. Though significant portion of the make-up chemicals are recovered from the recovery plant, the make-up chemicals are still required to add up for the loss. These chemicals are added to the mix tank for smelting reaction. This is because Na2SO4 is not desired in the digester and Na2S is desired which is generated from the smelting reactions.
2 Discuss energy economy in the paper and pulp manufacturing process
3 The smelt produced by boiler is in solid, liquid or gaseous phase?
Ans: Smelt is a molten salt mixture.
4 How areenvironmental issues taken care in pulp and paper production?
5What is white water?
Ans:White water consists of flour (minute fibers) that drains from the wire part of the paper manufacturing machines. It consists of about 0.5 % of pulp.
6 Why is bleed stream taken out?
Ans: White water contributes to the slurry viscosity and blending issues of the pulp + water mixture. Since beyond a desired viscosity the pulp + water mixture would not provide good quality paper for fixed choice of other operating parameters of the paper manufacturing process, white water needs to be bleed out. Also, usually, white water from the machinery where web forming is carried out is usually recycled and white water emanating from the pressing machines is sent for wastewater treatment and appropriate disposal.
7 Explain how dryer performance can be controlled to obtain good paper quality in quantity.
Ans:The dryer needs to operate in such a way that the temperatures of different rolls and their speeds be optimized. This is due to the fact that paper with about 60 % water enters the dryer and leaves with about 5 % moisture in the paper. Therefore, the drying rolls need to be efficient to quickly remove the moisture using steam heated rolls. A variation in the steam heated rolls temperature is also expected, as progressively variant temperatures along the rolls will provide maximum removal efficiency of the paper. In summary, there exists trade-offs with respect to the roller speed and roller temperature. Subsequently, the roller temperature is controlled using steam flow rate. In other words, both steam flow rate and roller speed contribute towards the dryer efficiency and providing required quality control for the manufactured paper.
8. Why is web forming process supported by the endless belt ?
Ans:Aslurry of 99.5 % cannot withstand its own weight anddisperses away. Therefore, the endless belt would act as a support system to gradually form the web as the solution progresses ahead.