reactive absorption & non-equilibrium absorption
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CHE-396 Senior Design Reactive Absorption and Estimation of Solvent Losses
Reactive Absorption & Non-equilibrium Absorption
Senior Design CHE 396
By Chemical Concepts
Prof. Andreas Linninger
Table of Content
Overview of Physical Absorption_____________________________________2
Isothermal Example Problem_________________________________________7
Estimation of Solvent Losses________________________________________12
Example Problem: Absorption of Acetone with Acetic Acid_______________16
Section IIIntroduction to Reactive Absorption___________________________________19
The Rate Equation for Mass Transfer and Reaction_______________________22
Absorption is the unit operation where one or more components of a gas stream are removed by being taken up (absorbed) in a nonvolatile liquid (solvent). Physical absorption and reactive absorption are the both readily used in industry today. Physical absorption, the most common of the two, can be modeled in a number of ways. The 1st being the isothermal case, the 2nd being the non-isothermal case, and finally the 3rd being the case of an non-isothermal absorber including solvent losses. Each of these cases, including a detailed sample calculation of the isothermal case, will be looked at in greater detail.
Reactive absorption involves a liquid phase reaction that effects the liquid mass transfer coefficient of the solvent. There are several advantages and disadvantages when considering a reactive absorption unit. These advantages and disadvantages along with a brief introduction on mass transfer will be included following physical absorption.
Overview of Physical Absorption
Absorption is the unit operation where one or more components of a gas stream are removed by being taken up (absorbed) in a liquid solvent. Absorption can be either physical or chemical. In physical absorption the gas is removed because it has a greater solubility in the solvent than in other gases. In other words, the gas solute has a greater affinity to be in the liquid phase. In chemical absorption the solute reacts with the solvent and remains in solution. The reaction can either be reversible or irreversible. Reversible reactions are often favored because the solvent can be regenerated, unlike irreversible reactions, where the resulting liquid must be disposed of. A simple absorption system is shown below in Figure 1.
Figure 1: Absorption Process
A brief overview of physical absorption will now be given to introduce the basic principles and assumptions used when considering such a process.
In three component systems it is often assumed that
1. Carrier gas is insoluble.
2. Solvent is nonvolatile.
3. The system is isothermal and isobaric.
The Gibbs phase rule yields three degrees of freedom for the following three component vapor/liquid system. Setting the temperature and pressure as constant, one degree of freedom remains. Equilibrium data is usually represented by plotting solute compositions in the vapor versus solute compositions in the liquid phase or by using Henrys law.
pB = HBxB
Henrys law relates the partial pressure of B in the vapor (pB) to the mole fraction of B in the liquid (xB) by using Henrys constant (HB) for the particular system. Since partial pressure is defined as
yB = pB / ptot
Henrys law becomes
yB = (HB / ptot)xB
Since H is roughly independent of the total pressure, as pressure is increased, the mole fraction in the vapor phase decreases. In other words, at greater pressures the gas solute is absorbed more into the liquid phase. Henrys law constants depend on temperature and are only valid for low concentrations of B or very dilute solutes.
The derivation of the operating lines for absorption are accomplished by material balances around the top of the column. Again, these calculations are developed assuming isothermal/isobaric operation, negligible heat of absorption, insoluble carrier gas, and a nonvolatile solvent. Figure 2 is a representation of an absorber where L and G are the solvent and carrier gas flow rates, respectively.
Figure 2: Absorber
For very dilute solutions (