Concentration of HCl acid above the azeotropic point

Introduction

The system HCl/water forms an azeotrope at atmospheric pressure at about 108°C with an HCl concentration of 20.2wt%.
If the acid concentration is lower than in the azeotropic mixture, the acid can be concentrated only up to the azeotropic point at this pressure. Further concentration needs special procedures.

The following technological options have been realised:

• Extractive rectification technology (addition of an extractive agent)

• Dual-pressure technology (variation of system pressure)

• Combination of both technologies

All technologies can also be used for the production of HCl gas.

Extractive Rectification Technology

The mostly common technology to over come the azeotropic point is the extractive rectification as presented below. This technology suppresses the azeotropic point and increases the relative volatility of HCl. This is achieved by the addition of a third ingredient, the extractive agent having strong hygroscopic properties. In the column the extraction agent captures the water of the feed and leaves the column at the bottom. Hydrochloric acid of high concentration or even HCl gas can then be produced at the head of the extractive rectification column. Subsequently, the extractive agent is freed from water in an evaporator and recycled into the extractive rectification process.

Extractive rectification technology

Extractive agent

An appropriate extractive agent must have the following physical properties.
The agent

• must increase the relative volatility of HCl towards water
• should be soluble with the components of the feed acid throughout the whole concentration range
• should display a high boiling point relative to the components of the feed acid, and
• should, finally, lead to an economic separation with respect to energy consumption

Sulfuric acid and concentrated aqueous solutions of metal chlorides, such as MgCl₂ and CaCl₂ are for example appropriate extraction agents for the concentrationing of hydrochloric acid.

Pre-concentration and purification before the extractive rectification

The lower the water content in the feed for the extractive rectification is the smaller is the amount of extractive agent to be added and the lower is energy consumption and investment for the sulfuric acid recycling step.  Therefore a pre-concentration of the feed can be advantageuous if installed before the extractive rectification. Other steps which are often realized in combination with an HCl-concentration are the removal of fluorine and high boiling components as well as debromination processes. The below illustration shows such a HCl-preconcentration with a removal of high boiling components or precipitated salts, a subsequent debromnination with chlorine followed by an extractive rectification with sulfuric acid to produce pure concentrated hydrochloric acid.

Extractive rectification combined with pre-concentration and purification

Dual-pressure Technology

An alternative to the extractive rectification is the dual pressure technology taking advatag of the fact that the composition of the azeotropic HCl/water-mixture depends on the pressure. This means that there is more HCl in the azeotrop  at a lower pressure p1 than in the azeotrop at a higher pressure p2 - (HCl azeo p2) < (HCl azeo p1). Therefore, it is possible to overcome the azeotropic point by combining 2 rectification steps at two different pressures.  The principle of the dual-pressure technology is presented below.

At first, the aqueous HCl mixture is treated by rectification at a lower pressure than atmospheric pressure, so that the azeotropic mixture (HCl azeo.p1) for this lower pressure is produced as bottom product - water is the head product. The bottom product is fed in a second column operated at a higher pressure than atmospheric pressure. In the second column the azeotropic mixture (HCl azeo p2) is also the bottom product but here the head product is concentrated hydrochloric acid of the desired concentration (HCl>azeo p2). The bottom outlet of the second column provides an HCl concentration (HCl azeo p2) lower than the azeotropic composition of the bottom product of the first column due to the increased system pressure. Accordingly the bottom product of the second column can be recycled back to the first column.

The pressure range from 100mbar to about 3bar is technically easily feasible. But within this pressure range the change of the composition of the azeotropic point is marginal so that a lot of energy is required by the 2 evaporators for a single distillative separation due to the required high reflux ratios in both columns.

Dual-pressure technology
 

Combination of Extractive Rectification and Dual-Pressure Technology

The dual pressure process can also be combined with the extractive rectification. This can be advantageous if the extractive agent (EA) is not reducing the volatility of water and pushing the azeotrope in the same direction as a reduced pressure. Ammonium chloride is an example for such an extractive agent. In contrast to the extractive rectification mentioned above the water is hence not captured by this extractive agent but the hydrogen chloride is leaving the column at the bottom together with the extractive agent. Illustration 4 shows the flow chart of such a combined technology for the concentration of hydrochloric acid.

Description of this improved dual pressure process

The feed acid is pre-heated in W4, then mixed with the extractive agent and then released into flash vessel B1. The resulting vapor and the liquid are fed into the vacuum column K1. The mixture is rectified in the vacuum column K1 so that basically water results in the column head and a ternary mixture of the extractive agent, water and hydrochloric acid leaves the column bottom K1.

This mixture is fed by pump P1 into the pressurized column K2. In column K2 the hydrochloric acid is separated from the extractive agent which leaves the column K2 as bottom product being recycled by pump P4 and mixed again with the feed.  The over azeotropic hydrochloric acid leaves the column K1 and is condensed in heat exchanger W1 which serves as heater for column. K1

Advantage

Such a combined process becomes interesting if due to the addition of the extractive agent the operating conditions respectively the temperatures in the heat exchanger W1 permit its use as evaporator for column K1 and as condenser for K2 saving a lot of energy compared to the sole dual pressure process described above.