reactive absorption & non-equilibrium absorption

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Topics on Separation System Design and Special Problems

CHE-396 Senior Design Reactive Absorption and Estimation of Solvent Losses

Reactive Absorption & Non-equilibrium Absorption

Senior Design CHE 396

By Chemical Concepts

Clint McElroy

Jeromy Miceli

Mike Viirre

Chris Woltz

Prof. Andreas Linninger

Table of Content

Introduction____________________________________________________________1

Section I

Overview of Physical Absorption_____________________________________2

Isothermal Example Problem_________________________________________7

Heat Effects______________________________________________________9

Solvent Losses___________________________________________________11

Estimation of Solvent Losses________________________________________12

Example Problem: Absorption of Acetone with Acetic Acid_______________16

Design Decisions_________________________________________________18

Section IIIntroduction to Reactive Absorption___________________________________19

Process Operation_________________________________________________21

The Rate Equation for Mass Transfer and Reaction_______________________22

References_______________________________________________________25

Introduction

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

(1)

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

(2)

Henrys law becomes

yB = (HB / ptot)xB

(3)

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 (

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