Absorption of Hydrogen Chloride

The absorption of hydrogen chloride in water behaves strongly exothermic. The necessary removal of absorption energy is performed in different manners, at least for high HCl concentration levels, as further described in the following adiabatic and isothermal absorption process. Important thermodynamic data for the design of plants are described in the following:

Thermodynamic Data for Absorption

The following explanations relate only to the case of physical absorption due to solubility behavior in fluid phase. Solubility processes with regard to chemical reactions are not put into scope.

The absorption equilibrium of the system hydrogenchloride/water is presented in illustration 1. The hydrogen chloride partial pressure values within the gas phase related to the concentration values for hydrochloric acid apply only for constant temperatures. The partial pressure of hydrogen chloride is very low for low concentration levels of hydrochloric acid. It only rises significantly in the case of higher fluid concentration levels.


Illustr. 1: Absorption Equilibrium State for HCl/Water    


Illustr. 2: Boiling Temperature of Hydrochloric Acid

The absorption of hydrogen chloride in water is an exothermal process to about 2100 kJ/kg HCl. That is why the boiling state is usually achieved for high concentration levels of hydrogen chloride within the gas flow. The boiling temperature is contingent to the acid concentration, as displayed in illustration 2.  A maximum temperature of 108.6°C is achieved for approx. 21 wt% of HCl . The boiling temperature reduces in an almost linear fashion for further increasing concentration levels of hydrochloric acid.

Almost parallel-deferred boiling diagrams unfold for lower total pressures, as e.g. displayed in illustration 2. The equilibrium diagram for a 1 bar pressure system is relevant for the design of absorption under boiling conditions, as displayed in illustration 3.  Hydrogen chloride is hardly volatile for low concentration levels of hydrochloric acid, meaning that the gas phase is almost free of hydrogen chloride. The same concentration values are experienced at the azeotropic point for the steam and fluid phase. The volatility of hydrogen chloride strongly increases only beyond this point. The gas phase consists practically of only hydrogen chloride for a fluid concentration level of 40 wt% for hydrochloric acid.


Illustr. 3: Equilibrium Diagram for HCl/Water    


Illustr. 4: Absorption Equilibrium of SO2 in Water

A maximum concentration of hydrochloric acid may not be exceeded for absorption under boiling conditions due to the S-shaped equilibrium diagram. It amounts to 35 wt% at a total pressure of 1 bar and pure inflow of hydrogen chloride, whereas slightly lower values are adjusted usually.

If further components of the inflowing feed gas can be absorbed, their absorption equilibrium behaviors must also be included in the design of the process. Illustration 4 displays the absorption equilibrium of SO2 in water for different temperatures, as an example. It may be recognized that only low concentrations are achieved in the aqueous phase, in comparison to the system HCl/water. Therefore, water is not a good agent for the absorption of SO2. The absorption capability, however, is great enough that it should not be disregarded.

The concentration levels of components displaying absorption behavior are usually low for technical applications that one may assume that no counteracting affects exist. Thus, the equilibrium behaviors of single components may generally be specified for design independently from each other.

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

The sulfuric acid / water system displays a maximum azeotropeat 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 vapor up to a concentration of about 70 wt%. The vapor 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, 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.


llustr. 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.

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.


Flow Sheet of Nitric Acid Concentrationing

The flow sheet shows the acid feedpoints, and also the bleach column that may be needed, which strips free NOx with airfrom the concentrated nitric acid. The contaminated air is lead to the NOx absorption. Within the nitric acid concentration unit, sulfuric acid extracts water from the bottom of the column and must again be re-concentrated.

Overall Process of Nitric Acid Concentration

The flow chart shows the interaction of different process steps ranging from pre-concentration to high-concentration with bleach line, sulfuric acid concentration, and NOx absorption. QVF has researched this process with regard to energy saving potentials, and has applied for a patent, now pending (USA 10/296, 297).


Low energy consumption is achieved by:

  • Indirect heating of the high-concentrated nitric acid by employing a horizontal evaporator.
  • Supplementary and indirect heating performed by intermediate heaters and energy integration.
  • Optimized inflow conditions due to preliminary mixing of the feed flows.

Sulfuric Acid Dilution

Sulfuric acid isone of the world's most widely used chemicals and finds numerous applications in the fertilizer, chemicals, water, dye, and iron and steel industries.

The principle method used for its manufacture - the Contact Process - typically produces acid of 98.5% w/w concentration or stronger in the form of oleums.  However,many of the processes and applications utilizing sulfuric acid require weaker acid than this. As a result, dilution equipment isoften necessary in order to meet the requirement for this weaker acid.


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 H2SO4DroppedUriImg-3 30% H2SO4

20% oleum DroppedUriImg-4 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.

Diameter (mm)
Material Flow (gph)


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 S0absorber.
  • 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.

Request More Information

Request a Quote

Want pricing? Have Questions?
We are here to help.