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DDS Theory

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The following information will help you to understand the concept of displacement cooking systems.

Displacement Digester Technology Concepts

What is Conventional Batch Digesting ?

Conventional batch reactors are operated in a start/stop batch scenario.  That is, the reactants are put into the reactor vessel and allowed to react to produce a product, which is then expelled to complete a cycle.  This operation is repeated ad infinitum to produce a product on a somewhat continuous basis.  With reference to the pulp industry and in general, the reactor is the digester, the reactants are fiber bearing chips and white liquor, the product is the liberated fiber and the reaction by-products are the black liquors.  The fiber is removed from the digester along with the black liquor.  This mixture (referred to as stock) contains residual heat and unused chemicals which are not collected for further reuse.

Basic Concept of Displacement Digesters

Despite the various trade names and different supply companies, the basic concepts of displacement batch technology are standard for all commercial offerings.  That is, furnish (wood, bamboo or other cellulosic bearing chips) is fed into the batch reactor (digester), the furnish stays in the digester while liquors from the tank farm are pumped through the bed of furnish (which eventually becomes fiber) and are reused by storing the exiting liquors in the tank farm, and lastly the product fiber is discharged to prepare the reactor for the next batch or 'cook'.  The stored liquors in the tank farm have different characteristics of temperatures and Kraft cooking chemicals.  The liquors are used to heat or cool the furnish, liberating and/or liberated fiber while exposing them to different concentrations of chemicals during the distinct steps of the cooking cycle.  CabTec's DDS (pat. pend.) uses the same displacement technology, but has incorporated modifications that were discovered by CabTec International llc and others to create a more flexible displacement batch cooking system.

What is the Tank Farm ?

The tank farm is a number of vessels used to store liquor with the main function being to service the digesters which are making product.  The tank farm has pressurized vessels to maintain the stored liquor temperatures above flash point as well as atmospheric tanks.  The amount of liquors diverted into each vessel from the digesters are based on temperatures and mass balances.  Unused liquors are released from the storage vessels while transferring any excess heat into incoming liquors or to cool water to generate hot water.  There are many different tank farm configurations depending on the chemical and heat demand determined from the furnish type and quality, as well as the desired finished product type and quality.

Is the Product Produced Continuously (Batch vs. Continuous) ?

The advent of the electronic control system in the 1980's, the DCS, has enabled batch reactors to produce product on a continuous basis inherent in plug flow continuous reactors, commonly referred to as a 'continuous digester' in the pulp and paper industry.

Displacement Digester System, DDS (patent pending)

The differences in the present day batch and continuous cooking systems are determined by their ability to meet product quality, energy usage, downstream chemical demand and the flexibility in 'swinging' the furnish and/or product characteristics, such as hardwood to softwood and fiber strength, respectively.  CabTec International believes that their system, DDS, offers the best combination of the these noted considerations.

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