Principles and history of batch cooking
Traditional or conventional batch cooking
Displacement batch cooking
SuperBatch
Present SuperBatch process
RDH
Enerbatch and Cold Blow
Calcium scaling

 

 

 

 

 

 

 

 

 

 

 

 

Principles and history of batch cooking

In batch cooking pulp is produced in several different digesters . Each digester in turn undergoes chip filling, liquor filling, a heating phase, cooking and blow. In general there are at least four digesters to ensure adequate, uniform production. The size of the digester has grown considerably over the years and at present the biggest digesters are about 400m3.

The batch cooking method was the only cooking method until beginning of the 1950s, but with the energy crisis of the 1970s, conventional batch cooking lost its competitive edge to continuous cooking due to its poorer energy economy. In the early 80s, suppliers of equipment based on batch cooking processes were forced to make batch cooking more competitive with regarding energy consumption. This resulted in the development and construction of batch cooking plants in the 80s that could compete on energy efficiency with continuous cooking. Energy savings were made with liquor exchange. It was noticed fairly quickly that liquor exchange had a positive effect on cooking results.

 

Traditional or conventional batch cooking

In traditional batch cooking the stages are as follows:

After chip filling, the chips are heated with steam to remove air from them and the air is evacuated from the digester simultaneously. After that, white liquor at a temperature below 100°C and the required amount of black liquor are pumped to the digester to achieve the correct liquor-to-wood ratio.

In the heating step, the liquor is circulated through a heat exchanger and then back to the digester, for use as indirect heating . Heating can also be carried out by direct steam. However, in conventional cooking steam consumption is very high, so it can be quite uneconomic to heat with direct steam. The heating phase is quite long up to an hour and a half.

After heating there is a cooking phase,which continues until the necessary H-factor is achieved . During cooking, the liquor is recirculated through a circulation screen to the circulation pump and heat exchangers and is then returned to the digester.

After the cooking cycle, the digester is degassed by decreasing the pressure and reducing the temperature and the pulp is discharged with the residual pressure to a blow tank.

The advantages of conventional batch cooking are production flexibility and ease of operation. The drawbacks include a high consumption of steam and lower strength delivery compared with current cooking techniques.

 

Displacement batch cooking

Displacement batch cooking is based on the exploitation of the heat from the previous cooks for the heating of a subsequent cook. At the same time the residual chemicals of the black liquor are utilized; mainly the high sulfidity . This happens by storing the hot black liquor in high-pressure accumulator tanks and feeding it to the impregnation and heating steps of the following cooks.

In addition, the cooking stage in displacement batch cooking is terminated by pumping cooler washing filtrate to the bottom of the digester, which displaces the hot black liquor. This gives the method a quality advantage over conventional batch cooking, where blowing the hot pulp damages the fibers.

Liquor displacement reduces the high steam consumption of conventional batch cooking to a competitive level. Furthermore, several factors are achieved in the cooking chemistry that are desirable for the pulp . For these reasons, displacement batch cooking has, in practice, completely displaced conventional batch cooking.

Several consecutive liquor displacements are typical for displacement batch cooking. It can generally be divided into the following stages:

 

SuperBatch

SuperBatch is the most common form of batch cooking. In the SB-method, the chips are impregnated with washing filtrates. This first liquor the chips encounter, the impregnation liquor, is cooled to about 90°C. A low impregnation temperature is beneficial regarding pulp strength, and in addition, alkali consumption in the impregnation stage is low.

In early SuperBatch, the hot black liquor was displaced at the end of the cook by washing liquor to two separate high-pressure storage tanks, i.e. hot black liquor accumulators (HBL1 and HBL2) . A certain amount of hot black liquor from the top of the digester was displaced into its own accumulator, HBL accumulator 1. Then, the part of the black liquor that was diluted with washing liquor and thus cooled was displaced to HBL accumulator 2.

Later on, SuperBatch cooking has been developed to reduce the build-up of scale due to calcium and into a simpler structure.

 

Present SuperBatch process

The SuperBatch cooking plant has one hot black liquor accumulator , a white liquor accumulator, a displacement tank and a black liquor tank. The liquor accumulators are in horizontal position .

The SuperBatch cooking cycle has the following steps :

  1. Chip filling and simultaneous impregnation liquor fill. Filtrate from washing is used as impregnation liquor.
  2. Hot liquor fill, at first hot black liquor from the high-pressure accumulator is pumped to the digester and after a certain time white liquor is mixed with the black liquor before pumping it to the digester. The displaced liquor is first taken to the black liquor tank in softwood cooking and to the displacement liquor tank in hardwood cooking, and finally to the HBL accumulator in both cases.
  3. Heating (with direct steam) and cooking. Some of the white liquor is added during the cooking stage.
  4. Displacement, where the cooking liquor is displaced by pumping first warm black liquor and later displacement liquor in softwood cooking. In hardwood cooking, only displacement liquor is used. Displaced liquor is pumped mainly to hot black liquor accumulator, it is also possible to pump the additional amount which corresponds to dilution factor at the washing plant to the black liquor tank.
  5. Discharge. Washing filtrate is used for diluting the bottom of the digester and the digester is pumped empty.

Other displacement batch cooking versions differ mainly from SuperBatch with regard to the displacement arrangement and to a lesser degree to the equipment. SuperBatch cooking plants may also differ a little, for example in accordance with the wood species to be cooked , but the principle is the same.

 

RDH

RDH cooking (Rapid Displacement Heating) makes use of the black liquor from previous cooks in nearly the same way as SuperBatch. Thus, a better level of heat economy and pulp quality is achieved than in conventional batch cooking.

The most significant difference between RDH and SuperBatch is that in the former, the first warm black liquor to come into contact with the chips is about 125 - 130°C compared with about 90°C for SuperBatch. Therefore, the alkali consumption in the RDH impregnation stage is greater and more white liquor needs to be added to the warm black liquor.

 

Enerbatch and Cold Blow

In the Enerbatch method, the chips are impregnated with white liquor, which is displaced with hot black liquor and white liquor. After this, cooking and wash liquor displacement take place in the same way as the other methods of displacement cooking.

In the Cold Blow method, first white liquor at 90°C is conveyed to the digester filled with chips and then black liquor at 165°C. Heating is done using indirect steam. Cooking can be carried out in two stages so that in the middle of the cook there is a displacement with hot black and white liquor.

 

Calcium scaling

Typical features of displacement batch cooking are “cold” impregnation and hot liquor pre-treatment steps prior to cooking, as well as displacement with cool wash filtrate and cold, low-pressure discharge. Various mixtures of black liquor, green liquor and white liquor are used in the modifications of the early stages of cooking and the spent liquor sent for evaporation is typically collected during the displacements of the “cold” impregnation liquors by hotter liquors .

“Cold” impregnation is used to enhance pulp quality by impregnation under conditions of low reaction rate enabling uniform mass-transfer of cooking chemicals into the chips. However, the route of calcium and its scaling tendency in the above liquor-exchange sequences is significantly different compared with ,e.g., conventional cooking without liquor exchanges.

The main source of calcium in kraft cooking is wood raw material . In a conventional kraft cook with Scandinavian pine chips, calcium concentration in cooking liquor increases during the heat-up and is at its maximum after the cooking temperature has been reached, after which the calcium concentration decreases rapidly. The calcium content in the cooking liquor can be higher than the theoretical solubility of calcium carbonate. This can be attributed to the presence of complex-forming compounds giving rise to supersaturated forms of calcium. These forms of calcium are thermodynamically unstable with respect to calcium carbonate formation. Liquors with high amounts of these complexes may form severe scaling on hot surfaces, e.g., digester heat exchangers or evaporation units resulting in an increased need for washing and thus limiting pulp production. In the traditional displacement batch cooking system, calcium scaling has been recognized to appear in the mill’s evaporator plant. The scaling can be so intense that condensate washing is needed every 2-3 days.

In studies with full-scale digesters as well as in the laboratory, the calcium contents in the black liquors in the various steps of displacement batch cooking were found to differ . The calcium content was highest in the outcoming liquors in the early stages of a cook, i.e., a high amount of calcium was dissolved in the impregnation liquor, which was displaced out from the digester during the subsequent hot liquor stage.

This displaced cool liquor, with a high amount of soluble calcium, is then fed to the evaporation plant. As the liquor is a mixture of cold (below 100°C) liquors, the evaporation liquor has not undergone any heat treatment before the evaporation plant. Thus the calcium complexes break down and form calcium carbonate scaling on hot surfaces in the evaporation plant. In a conventional batch cook with no liquor exchange the heat treatment occurs in the digester and scaling on the hot surfaces of digester heat exchangers can be found.

In SuperBatch, the displaced liquor fractions (below 100°C) from the digester collected during the displacement of the “cold” impregnation liquor by hotter liquors is used as the initial displacement liquor in the terminal displacement .

During the initial part of the terminal displacement, the digester is still at a high temperature. The initial displacement liquor, i.e., the liquor with high amount of soluble calcium, meets a high temperature and is thus heat-treated in the digester. During the heat treatment, the super-saturated calcium compounds break down to form calcium carbonate crystals. As this heat treatment occurs without hot surfaces in the digester, the crystallization of calcium is controlled without causing scaling. In the digester, the cooked chips absorb the formed calcium carbonate crystals. As the terminal displacement proceeds, the initial displacement liquor is directed to the HBL accumulator and subsequently pumped to the evaporation plant. Because of the calcium deactivation in the digester, the evaporation black liquor is not prone to calcium scaling in modern evaporation plants.