TECHNOLOGY AND COST FOR SULFITE PULPrNG
SPENT LIQUOR RECOVERY
Howard Edde, President
Howard Edde, Inc. Consulting Engineers
Bellevue, Washington 98004
INTRODUCTION
The recovery of spent chemical pulping liquors is a cost optimization problem involving energy and environmental considerations.    In earlier years of the pulping industry the economic considerations were mainly based on the difference between the cost
incurred with chemical recovery and non-recovery.    In the Kraft industry it was recognized at an early date that a return on investment in the form of both energy and
chemicals savings could be realized with a recovery system.    This was not the case for
the sulfite industry where economics have often not favored recovery.    For the sulfite
industry the impetus for installing an efficient liquor removal system has largely been
environmental requirements.
Higher fuel prices have also recently made liquor removal more economically attractive [ 1 ].    In some cases, the cost of recovering energy may be as high as that of
purchasing it, but it may be necessary to follow this concept because of the expected
tight energy supply [2].    Recent increased interest in high yield pulping processes has
further complicated the technical and economic  analysis of the problem since high yield
sulfite pulp production results in a large fraction of pulpwood in the form of pulp and
less organics remaining in the pulping spent liquor.    Since sulfite liquor must be evaporated, which requires energy prior to incineration, recovery of liquor from a sulfite pulp
of approximately 75% yield results in zero net energy [3].
The purpose of this paper is to present current technology and cost information
applicable   to this complex problem.    Emphasis is given to the latest developments in
liquor removal concepts and the influence of high yield pulping on the economics.   The
technology considered is mainly applicable to older, small and medium size high yield
sulfite mills.
THE TECHNOLOGY
When considering all the processes developed to date for pulping liquor removal,
only liquor combustion in conjunction with heat or chemical recovery, or both has been
technically and economically competitive [4].   With very few exceptions, under special
conditions, all operating recovery systems the world over are of this type.   Thus, the
subsequent sections of this paper deal only with liquor evaporation and combustion
systems.
Liquor evaporation is usually performed in multi-stage units, using indirect steam as
a heat source.    Each stage consists basically of a flash tank from which liquor is circulated through a heat exchanger where it is heated by steam.    Because of the corrosive
nature of sulfite liquor, it is impractical to use anything less resistant than stainless steel.
Therefore, capital cost of a sulfite evaporator is much higher than for the Kraft process.
Also, the number of evaporator effects are reduced to a minimum.
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