Gas Absorber

A gas absorption unit (Figure 1) uses contact with a liquid to remove a substance from a gas mixture. Mass is transferred from the gas mixture to the liquid via absorption.

Figure 1: Typical gas absorption column.

The overall mass transfer coefficient is the rate at which the concentration of one species moves from one fluid to the other (Equation 1).

(1)

In equation 1, G_s is the gas molar flow rate per cross-sectional area of the column, p_Ag is the partial pressure of CO₂, p^*_A is the pressure in equilibrium with p_Ag, a is the interfacial area/volume or “effective area” (a function of column packing), z is the height of the packing, and K_G is the overall mass transfer coefficient in mols/(pressure x interfacial area x time). Mass transfer depends on the mass transfer coefficients in each phase and the amount of interfacial area available in the absorber. Henry's Law or Raoult's Law is applied to approximate the partial pressures. They are two laws that describe the partial pressure of a component in a mixture, and are used together in order to fully describe the behavior of the mixture at the limits of the vapor-liquid equilibrium relationship. The objective of a gas absorption column is to control the effluent partial pressure of contaminant. A liquid solvent flows counter-current to the gas stream to remove the contaminant through convective mass transfer. The overall mass transfer of a water counter-flow packed column is measured in this study to determine the effects of water flow, gas flow, and CO₂ gas concentration. The coefficients will then be compared to theoretical values.

Partial pressures were taken from each trial run. Mass transfer coefficients were calculated from these and compared to predicted values (Figure 2). The predicted values arise from the calculated operating line for the absorber (see reference 2 for an in-depth discussion of the operating line). Solid lines represent the values calculated using the operating line, while triangles represent the experimental mass transfer coefficient values. Confidence intervals for the model values and the mean mass transfer coefficient were plotted with dashed lines. These values were compared to determine how the experimental parameters (liquid flow rate, gas flow rate, and CO₂ partial pressure) affected the overall mass transfer coefficient. Under these operating conditions, only liquid flow rate had a statistically significant effect on mass transfer when compared to the confidence interval. The results showed that gas flow rate and feed composition had little to no effect on the mass transfer coefficient.

Figure 2: Model of the predicted and actual values of the mass transfer coefficient.

Theoretical K_G values for a high (30 L/min) and low (20 L/min) were calculated from mass transfer coefficient correlations and are shown as blue and green lines, respectively, in Figure 3. The experimental K_G values at a variety of liquid flow rates were plotted against the theoretical values and showed similar trends, verifying the dependence of K_G on liquid flow rate. The theoretical values showed some variation from the experimental values, attributable to minor experimental error.

Figure 3: A graphical depiction of experimental value compared to theoretical values.

The goal of this experiment was to use factors of gas flow rate, water flow rate, and carbon dioxide concentration to determine the overall mass transfer coefficient in a gas absorber. The experiment used a randomly packed GUNT CE 400 water counter-flow gas absorption column. Eight runs with two different gas flow rates, liquid flow rates, and CO2 concentrations were performed. Partial pressures were taken from the bottom, middle, and top of the column unit, and these pressures were then used to find the mass transfer coefficient.

Under these operating conditions, only the liquid flow rate had a significant statistical effect on mass transfer when compared to the confidence interval for the given conditions. The process is liquid-phase mass transfer controlled. Gas-related factors such as CO₂ concentration and gas flow rate will have little to no significance.

Gas absorption is an important mechanism for safety in the production of chlorine³. During normal operation, gas absorbers treat any consistently occurring leaks. The start-up of a chlorine operation must be treated until it produces a gas-free product. In the event of a breakdown in the process, absorbers must be used to treat the gas that has been produced. Additionally, when new leaks form, the main emergency response unit is the standby gas absorbers. Treatment units are vitally important in these operating conditions, as they help create a safe environment when dealing with a dangerous product³.

When refining natural gas, absorption towers are used to remove natural gas liquids from the gas phase⁴. An absorbing oil with an affinity to natural gas liquids removes the liquid from the gas phase, purifying the product. The oil with natural gas liquids is then further purified to recover the liquids, such as butane, pentanes and other molecules. The oil can then be used again for treatment.

Absorption is also used to remove the major impurities CO₂ and H₂S from wellhead natural gas, converting it to pipeline gas. The process uses aqueous amines or glycols as solvents at low temperatures (typically <40 °C)⁵.