Pulping Technology and Equipment

Andreas W. Krotschek

4.2.8.1 Batch Cooking versus Continuous Cooking

Batch cooking systems process wood chips into pulp on a discontinuous basis.

Today’s batch digesters are usually between 300 and 400 m3 in size, and a number

of digesters is needed to produce the total pulp capacity of a mill. In contrast,

today’s continuous digesters are several thousand cubic meters in size, and the

largest ones currently installed produce in excess of 3000 tons of pulp per day.

Advanced liquor management techniques have been developed over the past

decades for both continuous cooking and batch cooking. These techniques allow

both systems to follow the rules established from an increasing understanding of

cooking chemistry for obtaining high-yield, high-strength pulp with excellent

bleachability and low reject amounts. In addition, all of the above is made available

at reduced consumption rates of cooking chemicals and energy.

Most – if not all – of the above benefits have resulted from the implementation

of alkali and temperature profiles over the duration of the pulping process, compared

to the prior art of adding all cooking chemicals to the chips at the beginning

of the process.

The liquor management of modern cooking technologies generates liquor in various

locations of the cooking system, which is fairly corrosive to carbon steel. That is

why, today, digesters of all types are typically constructed of duplex stainless steel.

So what are the fundamental differences between batch and continuous cooking?

Continuous cooking obviously offers continuous processing of wood chips

and continuous production of pulp. At the same time, the consumption of cooking

chemicals and utilities is continuous, reducing storage requirements and load

variations imposed on other mill areas. Batch cooking excels in flexibility regarding

wood furnish and pulp quality. Softwood and hardwood digesters can be operated

side-by-side, and the pulp quality can be virtually switched between digesters.

In addition, the modular concept using a number of identical digesters often

allows an easier capacity increase.

On average, there may be certain advantages for continuous cooking with regard to

the consumption of alkali and steam. For a given application, however, advanced continuous

and batch cooking technologies will deliver similar pulp quality, bleachability,

and rejects amount. Both will also feature in-digester washing and a similar consumption

of electrical power. Any comparison in this respect must be regarded qualitative,

because significant variations exist for the above factors within both batch

and continuous cooking plants, depending upon the specific set-up.

Each cooking system is custom-designed under consideration of wood species,

target pulp quality and other aspects. There is no general optimum system. From

case to case, a specific pulping application may generate a preference for the batch

or the continuous concept. Yet, in many instances the decision for a particular

concept will boil down to a matter of philosophy.

366 4 Chemical Pulping Processes

©2006 WILEY-VCHVerlag GmbH&Co.

Handbook of Pulp

Edited by Herbert Sixta

Дата добавления: 2015-10-21