Engineering task and calculation objective
The HNO3 module calculates the phase equilibrium and the properties of the nitric acid–water system. If you need to calculate the properties of nitric acid — density, specific heat capacity, thermal conductivity, viscosity, surface tension or the partial pressures of HNO3 and H2O above the solution — you obtain them as functions of temperature, pressure and concentration for the liquid and the gaseous phase.
Nitric acid is a key chemical of the fertilizer and explosives industries (Ostwald process, ammonium nitrate) and an important oxidizing and pickling agent. In plant engineering, the engineer encounters it in the absorption towers of NOx absorption, in bleachers, concentration plants and coolers. For the thermal design of this equipment, the module supplies consistent property data based on Perry's Chemical Engineers' Handbook (5th and 6th editions), the Gmelin handbook and Landolt-Börnstein.
The vapor-liquid equilibrium with the partial pressures of both components and the saturation pressure of the mixture forms the basis for absorption, desorption and evaporation calculations — including the question of up to which concentration nitric acid can be concentrated by distillation.
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 HNO3 concentration of the solution (mass fraction), the temperature and, if applicable, the pressure. This fixes the state point of the binary system HNO3–H2O.
- Evaluate the phase equilibrium: From the stored equilibrium data, the partial pressures of HNO<sub>3</sub> and H<sub>2</sub>O above the solution and the saturation pressure (total vapor pressure) of the mixture are determined. Depending on the position relative to the azeotrope, the vapor composition deviates significantly from the liquid composition.
- Calculate the liquid-phase properties: Density, specific heat capacity, thermal conductivity, dynamic and kinematic viscosity, and surface tension of the acid are interpolated from the literature data as functions of temperature and concentration.
- Determine the vapor-phase properties: For the vapor phase, the corresponding quantities are provided as needed for condensation, absorption and heat transfer calculations in columns and condensers.
- Derive dimensionless numbers and use the results: Prandtl number, thermal diffusivity and coefficient of thermal expansion are formed. Together with the equilibrium data, the values enter the design of absorbers, acid coolers, evaporators and condensers of nitric acid plants.
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 HNO3 | pHNO3 pHNO3 | Pa |
| Partial pressure HNO3 | pHNO3 pHNO3 | Pa |
| Saturation pressure | pS,1 pS,2 | Pa |
| Saturation pressure | pS,1 pS,2 | Pa |
| liquid | - xHNO3 xHNO3 | mol-% |
| liquid | - xHNO3 xHNO3 | mol-% |
Frequently asked questions
Why can nitric acid be concentrated by distillation only to about 68 %?
The HNO3–H2O system forms an azeotrope at ambient pressure at around 68 % HNO3 by mass, with a boiling point maximum at about 122 °C. At the azeotropic point, vapor and liquid compositions are equal and simple rectification ends there. Highly concentrated acid (98 to 100 %) is therefore produced by extractive rectification with sulfuric acid or magnesium nitrate as entrainer, or by pressure processes. The equilibrium data of the module show the position of the azeotrope and the achievable vapor compositions.
What must be considered in the materials selection for nitric acid?
Unlike hydrochloric or dilute sulfuric acid, nitric acid is oxidizing and passivates stainless steels: austenitic CrNi steels such as 1.4306 or 1.4571 have good resistance up to medium concentrations and temperatures; for severe conditions, special materials are used (e.g. Si-alloyed steels, aluminum for highly concentrated acid, titanium or zirconium). Critical areas are condensate zones with changing concentration and hot, highly concentrated (fuming) acid. The temperature and concentration profiles delivered by the module are input quantities for this resistance assessment.
How strongly do density and viscosity change with concentration?
The density rises approximately monotonically from 998 kg/m³ (water, 20 °C) to about 1400 kg/m³ for the 68 % azeotropic acid and around 1510 kg/m³ at nearly 100 %. The viscosity passes through a maximum in the medium concentration range and falls markedly with increasing temperature. For the design of pumps, piping and heat exchangers, the properties must therefore be evaluated consistently at the actual operating point (temperature and concentration).
Does the module also cover red fuming nitric acid (RFNA)?
The scope is the binary system HNO3–H2O. Fuming nitric acid additionally contains dissolved NO2/N2O4 and is thus a ternary system with its own vapor pressure and corrosion behavior; the binary data must not be applied to it. Likewise, the NOx chemistry in the absorption (oxidation of NO, equilibria of the nitrogen oxides) is not part of the property calculation and must be modeled separately.