Pulp Yield as a Function of Process Parameters

Pulp yield is a very decisive economical factor, as the wood cost dominates the

total production cost of a kraft pulp. Consequently, the knowledge of the relationship

between process conditions and pulp yield is an important prerequisite for

economical process optimization. Based on the numerous published reports on

conventional kraft pulping, it is known that the pulp yield generally increases by

0.14% per increase of one kappa unit for softwood in the kappa number range of

30 to 90, and by 0.16% for hardwood in the kappa number range of 10 to 90,

respectively [1]. In the higher and lower kappa number range, the influence on

yield is slightly more pronounced. Kappa numbers below 28 should be avoided

when using conventional kraft pulping technology, because the yield and the viscosity

losses increase considerably. The pulp yield is also influenced by the effective

alkali charge (EA). It is reported that in pulping of softwood an increase in the

EA charge of 1% NaOH on wood, will decrease the total yield by 0.15% [2]. The

small overall drop in yield is explained by two oppositely directed effects, namely

an increase in the retention of glucomannan and a decrease in xylan due to

increased peeling reactions. The influence of EA charge is much more pronounced

in case of hardwoods due to the very small amounts of glucomannans

present. An increase of 1% EA charge results in a total yield loss of about 0.4%

(Fig. 4.37) [3].

4.2 Kraft Pulping Processes 229

(127) and (130), the DAEs simplify to a system of ordinary differential equations

(ODE) which can be solved by any standard numerical ODE solver that has good

stability properties, for example, an implicit Runge Kutta method. Euler’s – which

has excellent stability properties – is used in the sample code, and although a set

of linear equations must be solved for every time step, the method is very fast

because the system matrix is almost trigonal.

4.2.6