Engineering task and calculation objective
The HCL module calculates the properties and the vapor-liquid equilibrium of the hydrogen chloride–water system. If you need to calculate the properties of hydrochloric acid — density, specific heat capacity, thermal conductivity, viscosity, surface tension or the partial pressures of HCl and H2O above the solution — you obtain them as functions of temperature, pressure and concentration for the liquid and the gaseous phase.
In plant engineering, hydrochloric acid occurs above all in chlorine chemistry, in flue gas scrubbing (HCl absorption), in the pickling of metals and in the regeneration of ion exchangers. For the design of absorption columns, graphite heat exchangers, quench and cooling systems of these processes, consistent property data over the concentration range are indispensable. The data basis is Perry's Chemical Engineers' Handbook (5th and 6th editions), the Gmelin handbook and Landolt-Börnstein.
The phase equilibrium is particularly important in the HCl–H2O system: it determines how much HCl can be absorbed at a given temperature and concentration, and where the azeotropic behavior of hydrochloric acid limits the achievable concentration.
Standard and calculation basis: Robert H. Perry, Cecil H. Chilton, "Perry's Chemical Engineers' Handbook" 5th Edition,Mc Graw Hill 1973; Robert H. Perry, Don Green, "Perry's Chemical Engineers' Handbook" 6th Edition / Mc Graw Hill 1984; Gmelin 18. Auflage; Landolt-Börnstein 6. Auflage Band II
Calculation workflow
- Specify composition and state: The inputs are the HCl concentration of the solution (mass fraction), the temperature and, if applicable, the pressure. This fixes the state point of the binary system HCl–H2O.
- Evaluate the phase equilibrium: From the stored equilibrium data, the partial pressures of HCl and H<sub>2</sub>O above the solution and the saturation pressure of the mixture are determined. The HCl partial pressure rises steeply with concentration and temperature and decides the direction of absorption or desorption.
- Calculate the liquid-phase properties: Density, specific heat capacity, thermal conductivity, dynamic and kinematic viscosity, and surface tension of the hydrochloric acid are interpolated from the literature data as functions of temperature and concentration.
- Determine the gas-phase properties: For the gaseous phase (HCl-water vapor mixture), the corresponding quantities are provided as needed for condensation, absorption and heat transfer calculations.
- Derive dimensionless numbers and use them in the design: Prandtl number, thermal diffusivity and coefficient of thermal expansion are formed. Together with the equilibrium data, the results enter the design of HCl absorbers, acid coolers and quench systems.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Temperature | ϑ1 ϑ2 | °C |
| Temperature | ϑ1 ϑ2 | °C |
| Concentration | c1 c2 | Ma-% |
| Concentration | c1 c2 | Ma-% |
| 1 | c1 c2 | g/l |
| 2 | c1 c2 | g/l |
| 1 | c1 c2 | mol/l |
| 2 | c1 c2 | mol/l |
| Density | ρ1 ρ2 | kg/m³ |
| Density | ρ1 ρ2 | kg/m³ |
| Specific heat capacity | cp1 cp2 | J/(kg·K) |
| Specific heat capacity | cp1 cp2 | J/(kg·K) |
| Dynamic viscosity | η1 η2 | mPa·s |
| Dynamic viscosity | η1 η2 | mPa·s |
| Surface tension | σ1 σ2 | mN/m |
| Surface tension | σ1 σ2 | mN/m |
| Partial pressure H2O | pH2O pH2O | Pa |
| Partial pressure H2O | pH2O pH2O | Pa |
| Partial pressure HCl | pHCl pHCl | Pa |
| Partial pressure HCl | pHCl pHCl | Pa |
| Saturation pressure | pS,1 pS,2 | Pa |
| Saturation pressure | pS,1 pS,2 | Pa |
| liquid | - xHCl xHCl | mol-% |
| liquid | - xHCl xHCl | mol-% |
Frequently asked questions
Why can hydrochloric acid not be concentrated beyond about 20 % by distillation?
The HCl–H2O system forms an azeotrope at ambient pressure at around 20 % HCl by mass (boiling temperature about 109 °C). At the azeotropic point, vapor and liquid have the same composition, so simple distillation ends there. Higher concentrations require other processes, such as pressure-swing distillation (the azeotrope shifts with pressure) or extractive methods. For absorption columns, the equilibrium conversely means that HCl gas can be absorbed very effectively up to this range at moderate temperatures.
Why is the heat of absorption of HCl so important for equipment design?
The dissolution of HCl gas in water is strongly exothermic (enthalpy of solution on the order of 70 kJ/mol at high dilution). Without sufficient cooling, the scrubbing liquid heats up, the HCl partial pressure above the solution rises and the absorption capacity collapses. HCl absorbers are therefore frequently built as cooled falling-film absorbers (often of graphite); the property data of the module provide the basis for the coupled heat and mass transfer calculation required.
What materials questions arise for equipment handling hydrochloric acid?
Hydrochloric acid is strongly corrosive toward unalloyed and low-alloy steels as well as most stainless steels; even traces cause pitting and crevice corrosion in austenitic grades. Common choices are rubber-lined or plastic-coated steels, graphite (for heat exchangers), PTFE-lined components, and highly corrosion-resistant nickel alloys or tantalum for critical areas. The property calculation does not replace the materials selection — but it delivers the temperature and concentration profiles needed for the corrosion assessment.
Over what range are the stored data valid?
The data are based on the classical reference works (Perry, Gmelin, Landolt-Börnstein) for aqueous hydrochloric acid in the technically common concentration range up to about the commercial concentrated acid (approx. 37 to 38 %) and for moderate pressures. Outside this range — for instance for over-azeotropic systems under pressure or pure HCl gas at high pressures — the results should not be extrapolated; dedicated data sources should be used instead.