Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1993-11-30
1995-08-08
Jones, W. Gary
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 301, 162 31, 162 50, 431 1, 431186, D21C 1104
Patent
active
054395577
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of recovering energy and chemicals from a spent liquor.
2. Description of Related Art
EP-A1-0 383 565 describes a process and apparatus for carrying out endothermic reactions by using a pulse combustor provided with resonant tubes which are immersed in a fluidized bed of solid particles in a reaction zone to provide indirect heat from the pulsating combustion gases to the bed of solid particles. Black liquor is introduced into the fluidized bed of solid particles and gasified without adding any oxygen and without any molten products being formed. The pulse combustion produces velocity oscillations of about 20 Hz in frequency and acoustic dynamic pressure levels of at least about 165 dB. An acoustic field is emitted from the resonant tubes into the bed of solid particles. However, it is not clear from EP '565 that the sound in the reaction zone, in which gasification of the black liquor occurs, is of the same low frequency and high sound level as produced by the pulse combustion inside the resonant tubes. The solid particles in the bed would have a damping effect on the sound in the reaction zone. Only a fractional part of the sound effect (the decibel number produced) will be propagaded to the surroundings where the black liquor is gasified. Furthermore, the conditions in the reaction zone are essentially different from those prevailing in a soda recovery unit in which the air is supplied in a controllable manner in order to maintain different reaction levels with reduction and oxidation as will be explained below.
The combustion of spent liquors from the cellulose industry is carried out in a soda recovery unit, this constituting the largest and most expensive unit in a sulphate pulp factory. The reason for the central role of the soda recovery unit is that the chemical content from the digesting liquor is recovered therein while at the same time the wood substances are used for the production of steam. It is often the soda recovery unit that determines the capacity of the sulphate factory as a whole since there is very little possibility of gradually increasing its capacity.
The soda recovery unit differs from a steam boiler in several respects: The spent liquor fed in contains water and inorganic substances. The reactions occur in several zones in both reducing and oxidizing environment. Inorganic constituents are recovered as molten material with most of the sulphur in reduced form. There is considerable transfer of dust due to the large content of inorganic substance in the fuel and there is the risk of hydrogen sulphide emission.
The evaporated liquor--thick liquor--is sprayed into a hearth through a number of liquor spray nozzles. A reducing zone is maintained a short distance below the liquor spray nozzles, while an oxidizing zone is maintained higher up in the recovery boiler, for example, above the liquor spray nozzles. The oxidizing and reducing zones are controlled by the addition of air at different levels, for example, primary, secondary and tertiary air. The drops of thick liquor dry and are subjected to gasification on their way down to and on the melt bed. Most of the organic substances are decomposed during gasification. At the same time a considerable amount of hydrogen sulphide is emitted, as well as some sodium and sodium hydroxide in gaseous form.
The bed consists of inorganic substances and 5-10 per cent by weight carbon. Sodium sulphate is reduced to sodium sulphide in the bed. Hydrogen sulphide is also formed and is absorbed by sodium carbonate or leaves the bed in gaseous form. Where the primary air encounters the bed surface the sulphide is very easily re-oxidized to sulphate.
More air--tertiary air--is added at the level above the liquor spray nozzles, so that the environment becomes oxidizing. The hydrogen sulphide formed from the drops of liquor and the bed is oxidized to sulphur dioxide and the organic substance is almost fully combusted to carbon dioxide and water. The degree of
REFERENCES:
patent: 3064619 (1962-11-01), Fortman
patent: 4359950 (1982-11-01), Leffler et al.
patent: 4857282 (1989-08-01), Mullen
Gotaverken Energy Aktiebolag
Jones W. Gary
Nguyen Dean T.
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