Henry's Law

When we use liquid activity coefficient (LACT) methods, the standard-state fugacity for a component is the fugacity of the component as a pure liquid. This basis is not very useful for dissolved gases, especially when they are above their critical temperature. Therefore, it is more convenient to use a standard state defined at infinite dilution. We can use Henry’s Law to accomplish this. The Henry’s Law approach is also useful for representing trace solutes such as organic pollutants in water.

In AVEVA Process Simulation, you can use Henry's Law in conjunction with the following LACT methods:

• Non-random two-liquid (NRTL)

• Electrolyte NRTL

• Universal Quasi-Chemical (UNIQUAC)

• Dortmund UNIQUAC Functional-group Activity Coefficient (UNIFAC)

• Wilson

When you configure a compositional Fluid Type that uses one of the preceding thermodynamic methods, you can specify whether AVEVA Process Simulation uses Henry's Law to calculate the fugacity for super-critical gasses. If you chose to use Henry's Law, you must specify the components in the Fluid Type that AVEVA Process Simulation should treat as solute components. AVEVA Process Simulation automatically designates components with critical temperatures less than 400 K as solute components. Please see Configuring a Fluid Type for more information.

Thermodynamically, the Henry's constant of a light gas (solute) i in a solvent j is defined as the infinite-dilution limit of the ratio of fugacity (fi) to mole fraction (xi):

Unless the pressure is high or there is vapor phase association, we can replace the fugacity with the component partial pressure, yiP, where yi is the component vapor mole fraction and P is the system pressure.

AVEVA Process Simulation correlates Henry's constants to the following functional form:

where

T is the temperature, in Kelvin

P is the pressure, in kPa

Hij is Henry’s constant for component i in pure solvent j, in kPa/mole fraction

C1, C2, C3, C4, and C5 are correlation coefficients that AVEVA Process Simulation can read from the Henry data banks

The correlation coefficients from the Henry data banks have the following specified validity limits:

• Minimum temperature (Tmin) – If the system temperature is less than the minimum temperature (T < Tmin), we clip the calculation temperature and evaluate the Henry's constant at Tmin instead of T.

• Maximum temperature (Tmax) – If the system temperature is greater than the maximum temperature (T > Tmax), we clip the calculation temperature and evaluate the Henry's constant at Tmax instead of T.

• Minimum pressure (Pmin) – If the system pressure is less than the minimum pressure (P < Pmin), we clip the calculation pressure and evaluate the Henry' constant at Pmin instead of P.

• Maximum pressure (Pmax) – If the system pressure is greater than the maximum pressure (P > Pmax), we clip the calculation pressure and evaluate the Henry's constant at Pmax instead of P.

If there is no data available for a validity limit, we don't apply any clipping for that limit.

Mixing Rule

When the system contains a mixture of Henry's solvents, we use logarithmic averaging per mole to calculate the Henry's constant of a solute for the solvent mixture:

where

A is the set of solvent components in the solvent mixture

Hi is the Henry's constant for component i in the solvent mixture

HiA is the Henry's constant for component i in pure solvent A

XA is the mole fraction for solvent A normalized over only the solvent components

Units of measure considerations for Henry's Law

It is important that the correlation coefficients, C1, C2, C3, and C4, that are stored in the Henry data banks be consistent with the assumption that T is in Kelvin, P is in kPa, and Hij represents kPa/mole fraction. However, these coefficients have been regressed by using different units of measure (UOMs). Typically, we can convert the coefficients from their original UOM system to the Kelvin-kPa basis. To be more specific, the Henry correlation is available in the following form:

Here, TUOM and PUOM represent temperature and pressure expressed in UOMs that may be different from Kelvin and kPa. Moreover, we assume that Hij,UOM has the same pressure UOMs as PUOM. Further, suppose that we can convert TUOM and PUOM to Kelvin and kPa, respectively, with the following formulas:

where

aT and aP represent conversion factors

Then, we can rewrite the Henry correlation in the following form:

The coefficients C1,Bank, ..., C5,Bank represent the values that are stored in the Henry bank. We obtain them from the original coefficients as follows:

The following tables give the conversion factors aT and aP for various UOMs:

Example

In this example, TUOM is given in Rankine and PUOM is given in psia. Therefore, aT = 0.55555556 and aP = 6.89476, as summarized in the following table.

The following table shows the resulting Henry correlation coefficients:

Reference

1. Prausnitz, J.M.; Lichtenthaler, R.N.; Gomes de Azevedo, E., Chapter 8, Molecular Thermodynamics of Fluid Phase Equilibria, 2nd ed.; Prentice Hall: Englewood Cliffs, NJ, 1986.