Studies on industrial digesters and cooking performance – Methods and applications (Helsinki University of Technology)


Industrial pulp cooking reactors, digesters, were introduced in the early stages of chemical pulping technology for chemical liberation of fibres from wood chips and other lignocellulosic materials. With cooking temperatures of 20 – 80 °C above the boiling point of water and strong chemicals such as sodium hydroxide, suiphide or suiphite, digesters had to be sturdy and big. When the first rotating digesters were replaced by stationary batch digesters or continuous chip plug digesters, the digester content was no longer mixed during the cooking process. Because of the harsh process conditions and stationary chip material it has been very difficult to make observations of the physical and chemical conditions prevailing inside industrial digesters. As a result, very little attention has been paid to gaining a more thorough understanding of the performance of industrial digesters. For example, compared to small laboratory digesters, industrial digesters are generally known to produce pulp with significantly poorer pulp strength, uniformity, reject pulp percentage etc.

The objective of the present work was to develop new techniques and to improve old ones for examining the industrial cooking process in order to obtain more knowledge about digester performance, and to make digesters more attractive as objects of scientific research. In the first part of the work, the main emphasis was on examining the cooking liquors which flow out from digesters for recirculation or to other processes. The liquors were assumed to represent the situation in the reacting chip bed with a minimum delay. A new carrier-flow method for cooking liquor analysers was invented, developed and taken into use. In this method, a small liquor sample is placed in a pure carrier flow, which carries it through simple chromatographic separations or just dilutes it due to the dispersion dilution within the carrier flow itself. The diluted and conditioned sample zone then travels through a number of flow-through detectors which measure the cooking chemicals and dissolved wood material. As a result of this work, a family of patented analyser inventions were developed for different cooking processes and pulp end product applications. Most of the analysers were introduced into cooking control system applications installed in various digester systems around the world. Besides providing commercial benefits in process control, the new analysers uncovered many interesting findings inside the digesters and thus opened a new window into the abysses of the huge digester pressure vessels. A lot of variability was discovered which prompted the idea that the processes are not uniform and that changes in cooking liquor concentrations must reflect significant variability within the chip bed being cooked. The conventional view of pulp variability occurring between individual cooking batches or as a result of time-related changes in continuous cooking evolved into a modern concept of inside digester uniformity. According to this new concept, variability occurs in space and location in terms of mass and heat transfer, in other words cooking degree and pulp quality.

In the second part of the work, the main emphasis was on the wood fibre material itself in an effort to examine the digester process in terms of input versus output performance, relying on direct observations inside the digester by means of the basket hanging technique. Systematic input-output performance studies uncovered performance parameters called “delivery”, such as pulp strength delivery; input chips cooked in the laboratory versus mill scale output pulp from the same chips. The basket hanging technique, which has been known for a long time, was developed to include three vertically hanging baskets instead of the one basket used in earlier studies. Studies by means of triple hanging baskets uncovered the vertical inside digester noniformity and corresponding pulp properties of never-discharged pulp. In addition, with this method, industrial pulp yield could be determined exactly. The studies produced a lot of interesting data on the performance of digesters operated by different cooking process modes, such as recirculation and displacement. The experiments revealed the fundamental tendency of contemporary digesters to generate non-uniformity in the reactor itself, in addition to the well known but less measured outside variability sources such as wood and chip quality and chemical charge conditions. This discovery most likely explains many of the problems associated with the applicability of cooking liquor analyser data: Inside digester variability distorts liquor measurement-based process control and should always be evaluated and taken into account before adopting on-line closed-loop control systems.

Chip-to-fibre performance studies combined with triple hanging basket experiments revealed fundamental operational differences between conventional and displacement batch cooking. The uniformity of conventional batch cooking is rather poor even at fairly high kappa number levels, whereas displacement batch cooking maintains good uniformity even in highly extended cooking. Pulp strength delivery in displacement batch cooking was best and close to loo % over the full range of kappa numbers studied. Pulp strength seems to be lost as a result of mechanical damage rather than chemical destruction, as clearly shown by the superior basket pulp strength. Because of its chemistry and uniformity, displacement kraft batch cooking seems to produce a pulp which is less vulnerable to damage, while it provides the most gentle cold discharge by pumping.

The results of the present work clearly show that successful kraft cooking is much more than a certain type of cooking chemistry, whether modified or not: It is absolutely crucial to maintain good uniformity and high pulp strength delivery, i.e. to avoid fibre damage and extra strength losses. During the course of the present work and thereafter, strength delivery has been accepted as a general way for evaluating digester performance and for setting and testing purchase guarantees. In the same way, triple hanging baskets have found new uses in process guarantee test runs related to cooking uniformity and routine evaluation of pulp yield and in other cooking tests with isolated samples.