Recovery, Concentration and Purification of Mineral Acids

Concentration of Sulfuric Acid and its Application

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 chlorine, bromine, orchloromethane, whereas in the fertilizing industry, sulfuric acid is abasic 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. DDPS has acquired extensive know-how for sulfuric acid recovery.

• Pre-concentrating up to 70 wt%
• High-level concentrationing up to 96 wt%
• Chlorine dehydrationing/sulfur dioxide with sulfuric acid
• Nitric Acid Concentration with Sulfuric Acid
• Concentration of dilute 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

Basics for the Concentration Process

Nowadays, concentrationning 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 (15bar) 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.

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.

Nitric Acid Concentration
with Sulfuric Acid

The system nitric acid / water (HNO3/H2O) displays a maximum azeotropeat about 68 wt% and at a boiling temperature of 120°C at ambient pressure. In order to manufacture high-concentrated nitric acid, it is necessary to overcome the azeotrope point by using high-concentrated sulfuric acid.

Rectification processes are used for the pre-concentration of nitric acid (HNO3) from below 55 wt%, where water is distilled to the head section and nitric acid to the bottom, up to 67 wt%. These columns are operated with minimum reflux ratio for energy saving reasons so either tray columns or glass packaging columns (DURAPACK) are used.

Sulfuric Acid Addition for Azeotrope Change

The diagrams below the impact of different sulfuric acid concentrations to the azeotrope point. Azeotrope disappears beyond a sulfuric acid concentration of 50 wt%, yet for technical rectification, an acid concentration of about 70 wt% is to be targeted.

Sulfuric Acid Dilution

Process Description

The dilution chamber illustrated in figure 1 has the capability of diluting acid of any concentration to produce sulfuric acid of any desired concentration.

For example:

98% w/w H₂SO₄ 30% H₂SO₄

20% oleum  98% H₂SO₄

Typically, the concentrated acid is fed down the inner of two concentric pipes while the dilution water is passed down the outer one. The acid and water mix at the outlet of these pipes and then slowly flow up through a packed bed -in order to ensure good mixing and the avoidance of local hot spots - and exit from the chamber via an overflow pipe.

Typically, the concentrated acid is fed down the inner of two concentric pipes while the dilution water is passed down the outer one. The acid and water mix at the outlet of these pipes and then slowly flow up through a packed bed -in order to ensure good mixing and the avoidance of local hot spots - and exit from the chamber via an overflow pipe.

During the mixing/dilution process, considerable heat can be liberated and it is normal practice for the hot dilute acid leaving the dilution chamber to be cooled. In some circumstances, the amount of heat liberated within the dilution chamber is sufficient to produce boiling of the acid and so, in order to avoid this, provision is made for the recirculation of a portion of the cooled dilute acid, thus ensuring the temperature of the acid leaving the chamber is below its boiling point.

Throughputs and sizes of the standard range of dilution chambers are shown in the table on the reverse side.

There are several distinct advantages to using glass dilution equipment:

• Product purity is guaranteed. The glass dilution equipment can be used in the production of high purity grades of sulfuric acid. For example, battery, reagent and electrolytic, product purity being dependent only on the purity ofthe acid and dilution water. For these grades, glass dilution equipment is often used in conjunction with a glass S0₃ absorber.
• The equipment is simple and easy to operate.
• The equipment is extremely versatile. Borosilicate glass is completely inert to all sulfuric acid concentrations and so can be used over wide concentration ranges.