Issues regarding the evaporation and concentration of sulfuric acid an
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The feed rate of sulfuric acid AN (10%) is 25 t/h, while the output concentration is 50%, at a rate of 5 t/h; the evaporation volume is 20 t/h. I would like to ask those with expertise: why is the output concentration set at 50% when designing the sulfuric acid AN evaporator, rather than 30% or 40%? Moreover, is a discharge concentration of 50% feasible in actual evaporation and concentration operations? Will there be any problems? I would appreciate the advice from all of you experts.
The solubility of sulfuric acid An at 0 degrees is 41.3%, and at 100 degrees it is 50.5%. Following this trend, the solubility should be higher at an evaporation temperature of 110 degrees at atmospheric pressure. So there is no issue with crystallization inside the evaporator. But is the discharge done for crystallization? Are there any special uses for discharge concentrations of 30% or 40%? This type of evaporation should be carried out using 4–6 effect evaporators, with a water production ratio between 3 and 5; steam at a temperature of around 120–130 degrees is required, which results in high heating costs. At a cost of 120 yuan per ton of steam, and with a water generation ratio of 4, the cost of steam for producing one ton of solid AN sulfate is 270 yuan. However, the price of AN sulfate is only 700–1000 yuan per ton.
2# yjqin1 Thank you. Your reply. First, the evaporation and crystallization of the AN sulfate solution is carried out to extract AN sulfate; there is no special use for 30% or 40% solutions. But the problem encountered during the design process was determining what concentration of sulfuric acid an is optimal for achieving the best crystallization results and highest efficiency – is 50% a good choice?
As a further addition: the boiling point of a saturated solution of sulfuric acid at atmospheric pressure is 108.2 degrees, corresponding to a concentration of 53.5%. The solubility of sulfuric acid An changes little with temperature; therefore, evaporation crystallization should be used instead of cooling crystallization, right? I need to ask you for advice on this. Is it to draw the evaporated and concentrated AN sulfate solution out of the evaporator, and then add seeds in the crystallizer to obtain AN sulfate crystals? In any case, the sulfuric acid AN solution must be supersaturated. For example, at a discharge concentration of 50%, the crystallization temperature must be below 60 degrees in order to obtain a significant amount of sulfuric acid an.
This post was last edited by luaf on 2012-9-11 at 12:55. A few more words. As mentioned in the first reply, this type of evaporation should be carried out using 4–6 effect evaporators, with a water production ratio between 3 and 5; steam at a temperature of around 120–130 degrees is required, which results in high heating costs. At 120 yuan per ton of steam, and with a water generation ratio of 4, the cost of steam for producing one ton of solid sulfuric acid AN is 270 yuan. Therefore, the operating cost (steam consumption) is very high. Our pseudo-multi-effect membrane evaporation can perform pre-concentration, thereby **saving steam consumption. Pseudo-multieffect evaporation can be understood as a heat-driven membrane separation process that is highly energy-efficient. For example, to concentrate 10% sulfuric acid An to 30% (by evaporating 75% of the water), the water-to-product ratio is 6–7, but atmospheric pressure steam is used. Moreover, if 30% sulfuric acid an is concentrated to about 50% using a conventional two-effect evaporator, the water production ratio is approximately 1. Furthermore, the secondary steam (at atmospheric pressure) from conventional two-effect evaporators can be used as a heat source for pseudo-multi-effect membrane evaporators. Therefore, the water generation ratio for the entire process is above 7. At a cost of 120 yuan per ton of steam, and with a water generation ratio of 7, the cost of steam for producing one ton of solid sulfuric acid AN is reduced to 170 yuan.
The discharge concentration is designed at 50% for the crystallization of sulfuric acid AN; we have designed it to reach 70%
In our plant, the triple-effect evaporation process for ammonium sulfate is used; the system reaches supersaturation at the three effect stages, with a solid content of up to 70%. After centrifugation, the mixture is sent directly to a centrifuge for separation, resulting in crystal particles of small size. I’m not sure whether the concentration you’re referring to is the amount of ammonium sulfate in the mother liquor, or the volume ratio of the crystals to the mother liquor.
Won’t 70% prevent scaling? I plan to design a double-effect evaporator that uses heat transfer oil for heating; however, at temperatures over 100 degrees, the saturated concentration is only 50%. Won’t external circulation heating cause blockages in the heat exchanger? And won’t the mixing tank suffer from scaling? Is it necessary to apply pressure?