Module 4: Liquid Characteristics - Gas Absorption

Features

Gas Absorption
Henry's Law
Practice Problems

Objectives

Using solubility data at a specific temperature, determine the Henry's law constant for a substance that is slightly soluble in a liquid.
Explain the factors that affect the absorption of gases into the liquid phase.

Gas absorption is one of the main techniques used in pollution control devices such as wet scrubbers and spray-dryer-type dry scrubbers for the removal of acid gas compounds and water-soluble organic compounds. The acid gas is absorbed as it comes into contact with the liquid. The rate of pollutant capture increases as the contact between the liquid and the pollutant-laden gas increases. Therefore, factors such as (1) turbulent mixing of the pollutant-containing gas stream and the liquid and (2) increased surface area of droplets promote absorption. Absorption is also used in emission sampling trains to capture gaseous pollutants by "bubbling" the gas through liquid in the impingers.

In absorption processes (see Figure 1), the gas-phase pollutant is brought into close contact with the liquid to facilitate diffusion across the gas-liquid interface. The gaseous pollutant must diffuse through a thin boundary layer on the gas side of the interface (gas film) and a thin boundary layer on the liquid side of the interface (liquid film.). Once the pollutant enters the liquid phase, it can simply dissolve, or it can react with other chemicals also in the liquid. In the case of simple dissolution, there may be a definite limit to mass transfer. Once the pollutant in the liquid phase has reached its solubility limit, there is no net transfer of pollutant across the gas-liquid interface. At this point equilibrium has been reached whereby the amount of gaseous pollutant that continues to dissolve equals the amount coming out of solution and reentering the gas phase. Henry's law (discussed in the next section) can be used to determine the solubility limit of absorption.

Each of the four steps depicted in the Figure takes time. Usually one step takes relatively longer than the others and is called the rate-limiting step. During absorption the passage of the gas molecule through the gas film or the liquid film almost always defines the rate-limiting step.

#1: Study Graphs A and B below. (Note that the Gas Film and Liquid Film have been drawn larger than the normal scale.)

Which graph depicts a process that is gas-film controlled?
Which graph depicts a process that is liquid-film controlled?

Answer to Question

If the liquid phase contains chemicals with which the dissolved gaseous pollutant can react, equilibrium may never be established. By chemical reaction, the pollutant can change form and in effect reduce its concentration in the liquid phase, thereby allowing more pollutant molecules to enter the liquid through mass transfer. As long as there is a continual conversion of the pollutant species in the liquid phase to another substance, equilibrium cannot be reached (a condition that is usually beneficial for pollutant absorption).

Henry's Law

Henry's law is a function of the type, temperature, and constituents of a liquid. Therefore, Henry's law can be used to determine the condition and type of liquid that will maximize the removal efficiency of pollutants.

Henry's law basically states that the amount (i.e. mole fraction) of a slightly soluble gas dissolved in a liquid is proportional to the partial pressure of the gas. The units for Henry's law constant are expressed in several forms using slightly different formulas. The idea and usefulness of Henry's law is the same. The most common form of the equation for Henry's law in the air pollution control field is given below.

For dilute solutions, the mass transfer equilibrium level for absorption is defined by Henry's law, which is shown in Equation 1.

Where:

Note: Using Equation 1, the units for H are (mole fraction in gas/mole fraction in liquid). Sometimes Henry's law constant is also expressed in units of atm/mole fraction, 1/mole fraction, or m³ atm/gm mole.

Equation 1 is the equation of a straight line that starts at the origin and has a slope of H. Henry's law can be used to predict solubility only when the equilibrium line is straight, which is the case when solute concentrations are very dilute. Most air pollution applications have relatively low pollutant (solute) concentrations; therefore, Henry's law is often applicable. If more than one gas is reacting with a liquid, Henry's law is applied to each gas individually.

Using the case of the pollutant, SO₂, being absorbed into water, Figure 4 illustrates the following points:

Henry's law applies at these concentrations of SO₂ in water (i.e. both lines are straight).
The Henry's law constants (H₁ and H₂) equal the values for the slopes of the two respective lines.
Liquid temperature affects the solubility of gases in liquids and therefore affects the value of Henry's law constant.

#2: The gas stream entering a wet scrubber contains a constant concentration of SO₂ gas. Suppose that the temperature of the gas stream and consequently the temperature of water in the scrubber increases. What effect does this temperature increase have on the solubility of SO₂ in the water? See Figure 5 (Henry's Law Solubility Curve for SO₂ - H₂O).

Answer to Question

#3: What happens to the value of the Henry's law constant, H, as the temperature of the liquid increases? See Figure 5 (Henry's Law Solubility Curve for SO₂ - H₂O).

Answer to Question

Henry's law constant provides a general indication of the differences in the solubilities of gases. The Henry's law constants for a variety of common gaseous compounds are provided in Table 1. Additional information is available in standard chemical reference texts.

#4: Use the values for Henry's law constant given in Table 1. Would carbon monoxide, nitrogen oxide, or carbon dioxide, be most readily absorbed in water?

Answer to Question

The data provided in Table 1 gives a general indication of the differences in the solubility of common gases. Sulfur dioxide, hydrogen sulfide, and carbon dioxide are moderately soluble in water. The other compounds have very high Henry's law constants, which indicate that they would not be substantially absorbed into water.

Henry's law does not apply at high concentrations of most gases and at low-to-moderate concentrations of gases that react or dissociate upon entering the liquid phase. For example, the solubility plots for hydrogen chloride in water at various temperatures are shown in Figure 7. Due to the dissociation of HCl into chloride ions and hydrogen ions, the solubility relationship is curved. Accordingly, the solubilities of gases that react or dissociate must be determined empirically.

The solubility of a pollutant in a particular liquid at a given temperature limits the quantity of that pollutant that can be absorbed in a given amount of the liquid. As stated earlier, once the pollutant compound has reached its solubility limit, mass transfer backward from the liquid phase to the gas phase matches the rate of mass transfer from the gas phase to the liquid phase. However, this equilibrium limit can be removed by reacting the dissolved gas compound into a chemical form that cannot diffuse out of the liquid.

As described in the lesson on pH (in this Module), alkaline materials can be added to buffer and neutralize the H⁺ ions resulting from acid gases. Accordingly, the acid gases cannot diffuse out of solution. In these cases, the equilibrium limits indicated by the Henry's law line become effectively nonexistent. It is possible to continue dissolving the gas into the liquid phase as long as there are reactants remaining to react chemically. This is the common situation in types of air pollution control systems involving acid gases. Sufficient reactants are maintained in the liquid phase to prevent solubility equilibrium. The pH of the solution provides one indicator of the availability of reactants in the liquid phase. In these cases, mass transfer of the acid gas into the liquid is limited only by the extent of gas phase-liquid phase mixing.

#5: What adjustments can be performed to increase the solubility of a liquid?

Answer to Question

Example Problem 1.
Determine if Henry's Law Applies

Given the following data for the solubility of SO₂ in pure water at 30°C and 760 mm Hg, plot the equilibrium diagram (similar to Figure 5) and determine if Henry's law applies.

Solution:

Step 1. Convert the gas and liquid data in Table 2 to mole fractions.

This will be shown using the first set of data where:

Calculate the mole fraction of SO₂ in air, y.
Calculate the mole fraction of SO₂ in water, x. Base the calculation on 100 gm of water.

Repeat this set of calculations for each set of data. Enter the data into a table as shown in Table 3.

Step 2. Plot the data in the last two columns of Table 3.

The plot, shown in Figure 8, indicates that Henry's law does apply. The Henry's law constant in this case is 43.5.

Since Henry's law applies in this case, the solubility of sulfur dioxide in water at 30°C at any concentration within the range of the test (

273 mm Hg) can be predicted.

Practice Problems
Gas Absorption

Instructions:: Complete the Practice Problems before proceeding to the next lesson. Click on the button below.

Practice Problems

Previous Lesson
pH

Next Lesson
Review Exercises

APTI Virtual Classroom

Module 4: Liquid Characteristics - Gas Absorption

Features

Local Navigation