Principle and history of continuous cooking
Conventional cooking
Modified cooking methods
MCC
EMCC, ITC and BLI+ITC
Lo-Solids
Downflow Lo-Solids
EAPC
CompactCooking
Procedures for increasing digester capacity
In continuous cooking all the pulp cooking reactions take place in a single
cooking container 
. Chips are fed at the top
of the digester and move downwards in the digester. Circulating hot liquor
turns the chips into pulp by the time the chips reach the bottom of the digester.
A continuous digester has typically several circulations where liquor is sucked
through a screen 
, pumped to the heat exchanger and returned through
the central distribution chamber (center pipe) to the center of the digester
. At certain screens, black liquor is extracted to
the evaporation plant. At some points the liquids flow in the same direction
as the chips and at some points in the opposite direction. The flows are based
on the empty space between the chips, enabling the liquor to flow at a different
speed and, if necessary, in opposite direction than chips. The liquor is made
to flow up or down by changing the pressure differential.
The first continuous digesters were introduced in the beginning of the 50's. The
big breakthrough for the continuous cooking method came in the 1960's when cold
blow was invented and the continuous cooking had become competitive compared
with batch cooking. In addition to hard competition between equipment manufacturers,
the need to decrease chemical use has accelerated the development of the cooking
methods. Therefore the aim is to continue cooking towards a lower kappa number
without decreasing yield and strength too much.
To improve yield, pulp properties and energy economics, the alkali and dry
solids profile of continuous cooking has been changed towards a direction ideal
for the pulp . A smaller amount of dissolved dry solids in the impregnation
and cooking phase liquor has enabled shortening the washing zone and lengthening
the cooking zone. This, in turn, enables lowering the cooking temperatures
(while reaching the same H factor 
),
which improves pulp properties.
Digesters have also become wider and somewhat shorter. Widening the digester increases the volume and capacity and slows down the speed of the chip flow in the digester, thus enabling longer delays and/or increased production. The reason for shortening the digester is the fact that too tall digester causes chip column packing at the bottom of the digester, pressed by the chip column on top (hydraulic load). This makes the flow of chips and liquor in counter-current zones considerably more difficult.
In continuous digesters, Kamyr-type digesters dominate. Until 1980's the run model was based on so called conventional cooking, where all the white liquor was fed at the beginning of the cooking and the cooking temperature was increased after a short impregnation. A so called con-current cooking was used, or the liquor was flowing downwards, in the same direction as the chips. At the end of the cooking, a counter-current washing was performed in the hi-heat zone. In the washing zone the pulp is also cooled from the cooking temperature to a temperature below one hundred degrees (so called cold blowing).
Aiming at a lower cooking kappa and stronger pulp has led to modifications in cooking conditions, with several commercial names. The basic equipment in modified cooking is often the same as in conventional cooking, but there can be more screen zones and the feeding equipment is usually simpler. Usually the modifications aim at lower cooking temperatures and steady alkali profile.
The first modification was MCC (Modified Continuous Cooking). In this model some of the white liquor was fed into the MCC circulation between the extraction screen and washing screen. This way the final cooking stages could be implemented in counter-current direction, and the concentration of dissolved lignin was lower.
MCC cooking soon evolved into EMCC (Extended Modified Counter-Current cooking) cooking, where white liquor and heat is increased in the wash circulation as well, enabling cooking in the hi-heat zone in addition to washing. The cooking time becomes considerably longer, and the cooking temperature can be lowered.
The principle of ITC (IsoThermal Cooking) is the same as in EMCC but the temperature of the digester washing has been chosen so that the digester is isothermal up till the washing screens.
BLI+ITC cooking is like ITC but equipped with a separate impregnation tower, where hot black liquor and counter-current are used to perform black liquor impregnation and liquor replacement.
Lo-Solids cooking maintains an even alkali profile with minimal cooking temperatures throughout the cook. It minimizes the dissolved solids concentration throughout the bulk phase of delignification and to the end of the cook.
This profiling of dissolved solids concentrations is achieved by adding white
liquor and wash filtrate at multiple points and extracting spent liquor at multiple
points . Profiles of dissolved solids concentration, alkali
concentration and sulfidity are manipulated by adjusting the relative flows
of extraction and wash filtrate additions during the cooking process. The multiple
points of extraction and multiple wash filtrate additions result in improved
chip column movement and more uniform kappa number control.
The principle of Downflow Lo-Solids is similar to Lo-Solids: white liquor is fed and black liquor extracted in several locations of the digester.
Machinery has been simplified and there is fewer liquor circulations compared with Lo-Solids system. Long counter current zones have been eliminated, which has advantages considering digester capacity.
Also conventional Kamyr digester can be modified to downflow Lo-Solids principle
. Then the digester counter-current wash zone is modified
to con-current cooking zone. Because of longer cooking zone the cooking temperatures
can be decreased and therefore, for example, improve pulp strength.
EAPC (Enhanced Alkali Profile Cooking) process was developed from Lo-Solids. The main idea was to control the alkali concentration in cooking phase without increasing white liquor consumption. The process uses black liquor with a high alkali residue from an earlier flashing in the impregnation zone, thus utilizing the remaining alkali. This enables increasing the alkali concentration at the final stages of cooking, if necessary. This has been found to improve the quality of the pulp, especially in softwood. In addition, it enables a high sulfidity in the impregnation phase.
The development of the CompactCooking G2 has focused on simplifying 
the cooking system by achieving a better understanding of the cooking
chemistry/process.
Process features:
System parts and their function :
.
The production in the existing continuous digester systems is increased annually by 2-3 percent. The resulting reduction in cooking time can be compensated to some extent increasing cooking temperature. This however increases the H-factor exponentially and makes the process more difficult to control. The most common issues for overloaded digesters are reduced cooking time, deteriorated digester wash and limitations in the extraction capacity.
Lowering of the extraction screens
One solution for increasing the limited capacity of existing digesters is to modify the digester by lowering the extraction screens. The extraction screens can be simultaneously replaced to meet increased capacity. Lowering the extraction screens inside the digester means that the cooking zone becomes longer and thus the cooking time is increased. This modification is exemplified for instance in following ways:
Prolonging the cooking zone means that the counter-current cooking and washing zones beneath the extraction screens are shortened correspondingly. Decrease in the length of the counter-current washing zone does not have a major impact on the result of the cooking, when the actual cooking zone has been simultaneously prolonged. Additionally the decrease in counter-current liquor flow allows the chip column to move down with less resistance, which favours the digester capacity increase.
The aforementioned shortening of the counter-current zones weakens the efficiency
of the in-digester wash. This has to be accounted for at the brown stock washing
brown
stock washing 
following the cooking, as it may have to be increased.
Also the filtrate from brown stock washing, which is fed to the final washing
zone beneath the extraction screens, might be more contaminated.
Addition of a impregnation vessel to a single-vessel digester
The limited digester capacity can also be increased by moving the cooking chemical
impregnation, taking place at the top of the digester, to a separate impregnation
vessel . The impregnation zone in the digester can be removed
as a result, and the digester can be used completely for cooking. This improves
the cooking process and enables increase in the chip flow through the digester.
The impregnation vessel is situated right before the digester , and it operates at similar pressure and temperature
(100-135 °C) with the digester.
Feedback to maintainers:
|
feedback@knowpulp.com