ENERGY CONSERVATION STUDIES FOR THE MULTIPLE EFFECT EVAPORATOR HOUSE OF PULP AND PAPER MILLS

Abstract

The present investigation deals with the theoretical studies pertaining to concentration of weak kraft black liquor in multiple effect evaporators in the pulp and paper mills for recovery of chemicals with the distinct aim to make the evaporation system achieve high gains in energy efficiency. Thus, in essence, it is an attempt to provide short term measures to conserve steam energy in the evaporator house of the mills without incorporating any hardware which necessitates capital input burden on the part of management. In other words, this investigation considers the variations in input parameters, viz., the liquor feed temperature, liquor feed concentration, steam temperature, temperature in the last effect, and, liquor feed rate, to evaluate their influence on steam consumption, steam economy along with heating surface area requirements. The effect of choice of the liquor flow sequence has also been investigated on steam energy requirements. Hence, it represents, to some extent, a short term measure to solve the complicated issue of energy requirements in black liquor evaporators. Equations based on material- and energy- balances and heat transfer rate have been formulated for this purpose. For calculation of overall heat transfer coefficient, the Gudmundson's model is employed. It is important to note that these equations are of general nature and not necessarily applicable to black liquor of any set of physico-thermal properties. The above equations are capable of computing quantities like; boiling point rise, temperature and concentration of liquor in each effect, steam consumption, steam economy, heating surface area requirements, etc. of the evaporators having any number of effects. For the solution of these equations, Newton-Raphson method has been employed. A set of computer programme in FORTRAN 77 has been developed and executed using an IBM PC/AT 486 machine. General applicability of the equations to solve for various quantities has been supported by an excellent agreement between the predicted quantities and the reported values of McDonald in the available literature. Sensitivity analysis has been performed to screen out qualitatively the input parameters having appreciable effect on the output parameters, viz.^ saturation temperature and concentration of each effect, steam consumption, steam economy and heating surface area of evaporators for improved design and control strategies of multiple effect evaporators. The investigation has categorized the liquor flow sequences into two main groups, namely; backward-and mixed-feeds. As regards the backward feed, it has been further classified into sequences with and without split of liquor feed and steam. This facilitates a systematic study of the effect of liquor flow sequence on the steam consumption, steam economy and heating surface area of evaporators. 11 Finally, it may be mentioned that this investigation suggests the use of sextuple effect evaporator with split of liquor feed evenly among 5th and 6th effects and steam in the 1st 5 effect, leading to a sequence >4- 3- 2- IB- 1A. This 6 provides the best steam economy, along with the smallest heating surface area requirements for concentrating the weak kraft black liquor from its concentration lying between 0.12 and 0.20 to a final liquor product concentration of 0.45. This observation holds good practically for the entire range of each operating parameter used in the mills, as given in Table 3.3 A comparison between the above sequence and the Scandinavian sequence also suggests that the latter is inferior to the former as regards the steam consumption, steam economy and requirement of heating surface area.