Concentrating evaporators – Processes – Involving waste liquid
Patent
1984-03-08
1985-08-06
Smith, William
Concentrating evaporators
Processes
Involving waste liquid
162 29, 162 301, 162 3011, 162 36, D21C 1110
Patent
active
045334331
DESCRIPTION:
BRIEF SUMMARY
In many pulping processes chemical losses may be replaced by residual solutions from other processes either from the same factory or from other factories.
In the following the sulphate pulping process is discussed but the situation is similar in other processes for the production of pulp.
Examples of residual solutions which can be used to cover the chemical losses are splitting liquid from splitting of soap skimmings and residual acid from the production of chlorine dioxide.
Two processes are currently known for supplying these residual solutions to the chemical cycle. In one of the processes the residual solutions are mixed into the spent liquor to be evaporated, i.e., prior to evaporation. In the other process they are mixed into the already evaporated spent liquor, the concentrated waste liquor, immediately before the spraying into the soda recovery boiler.
When splitting soap skimmings with acid to obtain tall oil, a splitting liquid is obtained which contains sodium sulphate and lignin. In the process for production of chlorine dioxide, used as a bleaching chemical for pulp, a residual acid solution is obtained which contains considerable quantities of sodium sulphate and sulphuric acid. The residual acid solution can be used for splitting of the soap skimmings. The splitting liquid thus formed, and any excess of residual acid, can be used to cover the chemical losses in the sulphate process.
Both the splitting liquid and the residual acid solution have a very low dry matter content, usually less than 25%.
In the former process of adding residual solutions, by addition to the weak liquor, the sodium sulphate content in the liquor will increase considerably during its passage through the evaporation plant. Like sodium carbonate, sodium sulphate has what is known as a reversed solubilty, i.e. its solubility decreases with increasing temperature. In the evaporation process, which takes place within a temperature range of from about 145.degree. C. to 55.degree. C., higher temperatures at higher dry matter contents are advantageous from the point of view of thermal economy and of the viscosity of the liquor. This means that with dry matter contents of about 40-45% in a certain temperature range, the limit is reached for the solubility of inorganic salts in the solution. These will therefore crystallize on the heating surfaces in the evaporation plant and these surfaces must therefore be washed. A liquor having a considerably lower dry matter content must be used for this purpose, e.g., a mixed-base liquor or water. The washing process must be performed with reduced steam supply, resulting in loss of capacity. Furthermore, if water is used, the need of evaporation is also increased. However, the heating surfaces being contaminated due to the saturation condition for inorganic salts is not the only drawback of this method. Since the splitting liquid and the residual acid pass through the entire evaporation plant, which usually comprises 5-7 steps, the proportion of inorganic impurities increases in relation to the proportion of organic impurities in the black liquor as compared with if no addition is made to the weak liquor, whereupon the rise in boiling point at each evaporation step also increases. With an increased rise in boiling point, the effective temperature difference for the evaporation process is reduced, resulting in loss of capacity. The advantage of adding splitting liquid and residual acid in this way as compared with the other knowm method--addition to the concentrated waste liquor--is that water in the splitting liquid and residual acid can be economically evaporated in 5-7 steps.
In the second known process, the addition of residual acid and splitting liquid to the concentrated waste liquor, the heating surfaces in the evaporation plant will be less contaminated, since saturation conditions are not reached. The need of washing is thus reduced. However, since both splitting liquid and residual acid have a considerably lower dry matter content that the already evaporated black liquor, the dr
REFERENCES:
patent: 3286763 (1966-11-01), Jacoby
patent: 3289736 (1966-12-01), Rosenblad
patent: 3299942 (1967-01-01), Jacoby
patent: 4076576 (1978-02-01), Marttala
patent: 4426322 (1984-01-01), Stage
Pulp & Paper, 1979, pp. 92-95.
Inkruster II, Sulfatfabrike-SPCI-Meddelande Nr 36, pp. 28-29, (1981).
A. Ahlstrom Osakeyhtio
Smith William
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