Sulfuric acid is an important product of the chemical industry, and is deployed in the most several of processes. It is used in many organic processes as a catalytic agent for synthesis processes or to dry gases such as chlor, bromine, or chloromethane, whereas in the fertilizing industry, sulfuric acid is a basic ingredient for the final product. Concentrated sulfuric acid is also deployed to dehydrate hydrochloric acid, nitric acid, or acetic acid.
Sulfuric acid usually exits the process in a diluted state, and is often contaminated with organic substances. A recycling is possible if the acid is re-concentrated to the initial composition, and if one achieves to remove the organic contaminations. QVF has acquired extensive know-how during the past 50 years for sulfuric acid recovery.

>> Pre concentrationing up to 70 wt%
>> High-level concentrationing up to 96 wt%
>> Chlor dehydrationing/sulfur dioxide with sulfuric acid
>> Nitric acid concentration with sulfuric acid
>> Concentration of watery HCl solution ranging up to HCl gas manufacture
>> Concentration of sulfuric acid for recycling in nitration processes
>> Sulfuric acid for deployment in the process of DNT nitration

The System Sulfuric Acid / Water
The sulfuric acid / water system displays a maximum azeotrope at 98.3 wt% sulfuric acid concentration and a temperature of 338°C at ambient pressure conditions. A large separation factor exists as may be seen in illustration 1. Furthermore, almost no sulfuric acid is contained in the vapour up to a concentration of about 70 wt%. The vapour concentration of the sulfuric acid increases very strongly only from a fluid phase concentration of about 85 wt%.

Illustr. 1: Equilibrium diagram – Separation factor	Illustr. 2: Equilibrium diagram – Pressure contingent

Basics for the Concentration Process
Nowadays, concentrationing processes are mostly performed under vacuum in order to be able to reduce the boiling temperature, especially for high concentrations. The pressure behavior due to the boiling temperature is displayed in illustration 2 for better understanding. Furthermore, the range limits for a cooling water temperature of 20°C and saturated steam temperature of 200°C (15 bar) are displayed. For example, one may extract from this illustration that a sufficient temperature differential exists at a product concentration of 80 wt% and at a pressure of 150 mbar regarding the heating and cooling agent. Contrarily, if one wants to achieve a final concentration of 92 wt%, for example, it may be derived that a temperature difference of 25°C is achieved on the heat steam side at a pressure of about 50 mbar. At the same time, the exhaust steams (in order to simplify, pure water at 0% sulfuric acid concentration is assumed) may just be condensed by cooling water of 20°C.

<<<<Illustr. 3 displays the boiling temperature for 3 different pressures over the whole concentration range

The following notes for the concentration of sulfuric acid may be derived from the equilibrium data and the vapor pressure curves:

• One cannot achieve 100% concentration by performing water dehydration due the azeotrope. Yet, it is more economical to perform concentration only up to a maximum sulfuric acid concentration of 96 wt% due to the possibility to increase the concentration by adding SO3.
  
• One may operate with simple-design evaporator facilities up to a final concentration of about 70 wt%, due to the low partial pressure of sulfuric acid in the vapor phase.
  
• In the so-called range of high-concentration displaying concentrations higher than 70 wt%, supplementary measures are necessary in order to reduce the proportion of sulfuric acid in the distillate.
  

see also: Reference list
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