Improvement of the Caustic Soda Evaporation Process at the Kemerovo Nitrogen Fertilizer Plant
Abstract Generated abstract
This paper examines the concentration of caustic soda solutions at the Kemerovo Nitrogen Fertilizer Plant from the existing commercial level of about 42 percent NaOH to a more economically desirable 55 to 60 percent. It argues that conventional steam heated evaporators face difficulties at these concentrations because of sodium chloride crystallization, tube incrustation, high viscosity, and circulation requirements, and evaluates direct electric heating as an alternative. Material and heat balance calculations are presented for evaporation at atmospheric pressure, including sodium chloride crystallization, water removal, energy use, and electricity cost. The calculations suggest that direct electric heating could intensify the process, simplify equipment, maintain high energy utilization, and provide an economically favorable route for producing more concentrated caustic soda solutions.
Full Text
IMPROVEMENT OF THE CAUSTIC SODA EVAPORATION PROCESS AT THE KEMEROVO NITROGEN FERTILIZER PLANT
Communication 1
V. P. PISHCHULIN, V. I. KOSINTSEV, G. M. IZMAILOV, N. P. KONKOV
(Presented by the scientific seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production)
To obtain commercial caustic soda, the solution from electrolytic baths containing 95–105 g/l NaOH and 190–210 g/l NaCl, i.e., an 8–10% alkali solution saturated with sodium chloride, is evaporated to an NaOH concentration of not less than 42%; in this process the sodium chloride content in the solution is reduced to 4% owing to crystallization of its excess.
At the Kemerovo Nitrogen Fertilizer Plant, evaporation of the solution from electrolytic baths to commercial specifications is carried out in two stages in evaporator units with tubular heating chambers. In the first stage, the caustic soda solution is evaporated in a three-effect evaporator unit to a concentration of 400–420 g/l NaOH, i.e., a 28–30% alkali solution saturated with NaCl is obtained. The second stage is carried out in evaporators with an external tubular heating chamber having a heating surface of 63 m² and a tube length of 5 m, with forced circulation of the solution. The solution leaving these apparatuses contains 42–46% NaOH and 3–4% NaCl.
It is economically more expedient to produce, as commercial product, caustic soda solutions with an NaOH content of 55–60%. However, evaporation of caustic soda solution to such sodium hydroxide concentrations in steam-heated evaporators is difficult because of sodium chloride crystallization, incrustation of the tube surfaces, and the need to use powerful circulating pumps to ensure a sufficient circulation velocity of the highly viscous solution.
In the laboratory of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production of Tomsk Polytechnic Institute, on the basis of investigations carried out to determine the regularities of the passage of alternating electric current through the solution being evaporated [1], a method has been proposed for concentrating caustic soda solutions in apparatuses with direct electric heating, which makes it possible to intensify substantially the process of evaporating viscous sodium hydroxide solutions. This method of carrying out the process leads to simplification of the evaporator design and to a reduction in its dimensions; the natural circulation velocity of the solution being evaporated increases to a value of about 2.5 m/sec. The coefficient of utilization of electrical energy is not less than 98%.
The present communication gives enlarged calculation data for the process of evaporating caustic soda using direct elec-
Table 1
| Description | Initial solution | Evaporated solution, 55 | Evaporated solution, 60 |
|---|---|---|---|
| NaOH content in the solution, mass % | 42 | 55 | 60 |
| NaCl content in the solution, mass % | 4 | 1 | 1 |
| Mass of solution, kg | 2381 | 1818,2 | 1666,7 |
| NaCl crystallized, kg | — | 77 | 78,5 |
| Temperature of the solution, °C | 131 | 151 | 160 |
| Heat capacity of the NaOH solution, kcal/kg·deg | 0,485 | 0,482 | 0,481 |
| Heat capacity of solid NaCl, kcal/kg·deg | — | 0,216 | 0,219 |
| Water evaporated, kg | — | 485,8 | 635,8 |
| Heat content of secondary steam, kcal/kg | 639,4 | 639,4 | 639,4 |
Table 2
| Description | Description | Evaporated solution, 55 | Evaporated solution, 60 |
|---|---|---|---|
| NaOH content in the solution, mass % | 55 | 60 | |
| Heat consumption | with the evaporated solution, kcal | 132300 | 128200 |
| Heat consumption | with crystalline NaCl, kcal | 2410 | 2751 |
| Heat consumption | with secondary steam, kcal | 310600 | 406500 |
| Total heat consumption, kcal | 445300 | 537451 | |
| Energy consumption, $\dfrac{\text{kcal}}{\text{kW·h}}$ | 294010 341,9 |
386151 449,1 |
|
| Energy consumption at a utilization coefficient of 98%, kW·h | 348,9 | 458,3 | |
| Cost of electric power, rub. | 3,283 | 4,313 |
electric heating from a concentration of 42% NaOH to a concentration of 55–60% as applied to the Kemerovo Nitrogen Fertilizer Plant.
The results of material-balance calculations for the evaporation of caustic soda over the indicated limits of variation in the concentration of sodium hydroxide, as well as the thermophysical properties of the solutions [2, 3], are given in Table 1. The pressure for evaporation using direct electric heating was taken as 1 atm, and the total content of sodium hydroxide in the solution as 1000 kg. The heat of the initial solution is \(Q = 2381 \cdot 0.485 \cdot 131 = 151300\) kcal.
The expenditure items of the heat balance, as well as the energy consumption required to carry out the process, are given in Table 2. Here the energy consumption is determined from the difference between the total heat expenditure and the heat of the initial solution.
The cost of electric power shown in Table 2 was calculated according to the selling price of electric power at the Novokemerovo Thermal Power Plant, which, according to 1971 data, was 0.941 kopecks per kWh. From the cost calculation it is evident that carrying out the evaporation process using direct electric heating in this case does not require large expenditures. Taking into account the above-mentioned advantages of direct electric heating, it may be concluded that this method, as applied to the evaporation of a caustic-soda solution for the given NaOH concentration range, is highly economical.
REFERENCES
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N. I. Kozlova, V. P. Pishchulin. Application of direct electric heating for evaporating NaOH solution. Report at the conference of young scientists of the city of Tomsk, Tomsk, 1971.
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Chemist’s Handbook. Vol. 1, Moscow–Leningrad, Goskhimizdat, 1962.
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Chemist’s Handbook. Vol. III, Moscow–Leningrad, Khimiya Publishing House, 1964.