Engineering task and calculation objective
The H2SO module calculates the phase equilibrium and the properties of the sulfuric acid–water system. If you need to calculate the properties of sulfuric acid — density, specific heat capacity, thermal conductivity, viscosity, surface tension or the partial pressures above the solution — you obtain them as functions of temperature, pressure and concentration for the liquid and the vapor phase.
Sulfuric acid is one of the most important basic chemicals; the equipment engineer encounters its aqueous solutions in contact plants, drying and absorption columns, flue gas scrubbers, and in dew point assessments of sulfur-containing flue gases. For the thermal design of the coolers, evaporators and condensers of these processes, consistent property data over the entire concentration range are needed. The data basis is Perry's Chemical Engineers' Handbook (5th and 6th editions), the Gmelin handbook and Landolt-Börnstein.
Particular emphasis is placed on the vapor-liquid equilibrium: the module delivers the partial pressures of H2O and H2SO4 above the solution as well as the saturation pressure of the mixture — the basis for evaporation and absorption calculations and for the assessment of acid dew points.
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 concentration of the sulfuric acid (mass fraction of H2SO4 in water), the temperature and, if applicable, the pressure. This fixes the state point of the binary system.
- Evaluate the phase equilibrium: From the stored equilibrium data, the partial pressures of H<sub>2</sub>O and H<sub>2</sub>SO<sub>4</sub> above the solution and the saturation pressure (total vapor pressure) are determined. Because of the strong negative deviation from Raoult's law, the water vapor partial pressure lies far below that of pure water — the basis of the drying action of concentrated sulfuric acid.
- Calculate the liquid-phase properties: Density, specific heat capacity, thermal conductivity, dynamic and kinematic viscosity, and surface tension of the solution are interpolated from the literature data as functions of temperature and concentration.
- Determine the vapor-phase properties: For the vapor phase, the corresponding quantities (density, heat capacity, viscosity, thermal conductivity) are provided as needed for condensation and absorption calculations.
- 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 acid coolers, evaporators, absorbers and dew point assessments.
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 H2SO4 | pH2SO4 pH2SO4 | Pa |
| Partial pressure H2SO4 | pH2SO4 pH2SO4 | Pa |
| Saturation pressure | pS,1 pS,2 | Pa |
| Saturation pressure | pS,1 pS,2 | Pa |
| liquid | - xH2SO4 xH2SO4 | mol-% |
| liquid | - xH2SO4 xH2SO4 | mol-% |
Frequently asked questions
Why is the water vapor partial pressure above sulfuric acid so low?
Sulfuric acid and water form a strongly non-ideal system with a pronounced negative deviation from Raoult's law: the hydration of the H2SO4 molecules binds the water in the solution. Above 96 % acid, the water vapor partial pressure at room temperature is orders of magnitude below the saturation pressure of pure water. This is exactly the basis for using concentrated sulfuric acid as a drying agent for gases — and equally for the strong heat of dilution released on mixing.
What is the sulfuric acid dew point and why is it critical for equipment?
In sulfur-containing flue and process gases, SO3 forms H2SO4 vapor with water vapor, which condenses as highly concentrated acid already at temperatures of typically 120 to 160 °C — far above the water dew point. Wall temperatures below this acid dew point lead to massive low-temperature corrosion on economizers, air preheaters and stacks. The partial pressure data of the module make it possible to assess the condensation conditions and to define minimum wall temperatures.
How strongly do the properties change with concentration?
Considerably, and in part non-monotonically: the density rises from around 1000 kg/m³ (pure water) to over 1800 kg/m³ at 96 to 98 % H2SO4. The viscosity passes through a maximum with concentration, the specific heat capacity falls markedly with increasing acid content, and the thermal conductivity decreases as well. For heat exchangers this means: a design based on properties at the wrong concentration or temperature can miss the heat transfer coefficient significantly; the values must be determined for the actual operating point.
Does the module also cover oleum (fuming sulfuric acid)?
The core scope of the module is the binary system H2SO4–H2O, i.e. concentrations up to 100 % acid. Oleum — solutions of excess SO3 in sulfuric acid — is a separate substance system with different equilibrium behavior. For designs in the oleum range, manufacturer data or specialized literature must be consulted; extrapolating the aqueous data beyond 100 % is not permissible.