Absorption Column Design

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Questions and Answers

For a dilute system, what is the most common method of expressing the rate of mass transfer per unit volume of packed bed?

Using a single driving force based on the difference in solute concentration in the gas and liquid phases.

Using a driving force based on the difference in solute molar fractions in both phases.

Using a combination of partial-pressure and concentration driving forces, one for each phase.

Using a driving force based on the difference in solute partial pressure in the gas phase and solute concentration in the liquid phase. (correct)

In the context of a packed column, what is the primary reason for determining the overall gas-phase coefficient?

The gas-phase coefficient is more stable and less influenced by variations in operating conditions.

It is easier to measure the gas-phase coefficient experimentally.

It simplifies the calculation of the number of transfer units (NTU).

The liquid phase resistance is usually negligible due to the solute's high affinity for the liquid. (correct)

What is the key assumption used to simplify the integral in Eq. (15-13) for dilute solute mixtures?

The solute concentration in both phases is negligible compared to the carrier gas concentration.

The mass transfer is dominated by the gas phase resistance.

The operating line is parallel to the equilibrium line.

The equilibrium relationship between the solute in the gas and liquid phases is linear. (correct)

How is the NTU calculated in Eq. (15-38)?

It is calculated using a logarithmic expression involving the inlet and outlet concentrations and a parameter A. Signup and view all the answers

What does the parameter A represent in Eq. (15-38)?

The ratio of the liquid-to-gas flow rate multiplied by the overall mass transfer coefficient and the equilibrium constant. Signup and view all the answers

What is the primary function of the additional height in a packed column besides the packing itself?

To allow for phase disengagement and flow distribution. Signup and view all the answers

Which of the following factors does the generalized pressure drop correlation in Fig. 15-7 primarily depend on?

The type of packing material and the size of the packing. Signup and view all the answers

When designing an absorber or stripper, what is the significance of choosing a suitable pressure drop per unit height of packing?

It determines the size and cross-sectional area of the absorber or stripper. Signup and view all the answers

What is the primary assumption when dealing with dilute mixtures in absorption or stripping processes?

The separation process is isothermal Signup and view all the answers

What is the main complication when dealing with concentrated mixtures in absorption or stripping?

The need for computer-aided design methods Signup and view all the answers

What is the primary objective of the separation process in absorption or stripping?

To remove a specific percentage of the solute from the gas stream Signup and view all the answers

What is the preferred method for manual calculations when more than a few stages are required?

The algebraic method for manual calculations Signup and view all the answers

What is the purpose of obtaining equilibrium data in the design of absorption or stripping processes?

To determine the number of equilibrium stages required Signup and view all the answers

What is the main advantage of using the Kremser method for single-section cascades?

It is applicable for absorption and stripping of dilute mixtures Signup and view all the answers

What is the purpose of identifying feed components, concentrations, and flow rates in the design of absorption or stripping processes?

To establish the objectives of the separation Signup and view all the answers

What is the main difference between the design procedures for absorption or stripping and distillation?

The design procedures are identical Signup and view all the answers

Why are mechanically agitated devices often better choices for extraction?

Because they can handle high liquid viscosities and low differences in phase density Signup and view all the answers

What is the primary purpose of evaluating the number of theoretical stages in extraction?

To select the appropriate dimensions of the equipment Signup and view all the answers

Which type of extractor is characterized by the presence of external mechanical agitation?

Continuous counterflow contactors with mechanical agitation Signup and view all the answers

What is the primary advantage of the Karr reciprocating-plate column?

High liquid capacity per unit of cross-sectional area Signup and view all the answers

Which of the following is NOT a type of extraction equipment?

Heat exchangers Signup and view all the answers

What is the primary purpose of Table 15-8 in the content?

To provide general guidelines for selecting extraction equipment Signup and view all the answers

What is the primary difference between mixer-settlers and continuous counterflow contactors?

The design and operation of the equipment Signup and view all the answers

Which of the following extractors has the lowest liquid capacity per unit of cross-sectional area?

Graesser extractor Signup and view all the answers

What feature of an absorbent is preferred to enhance mass- and heat-transfer rates in a column?

Low viscosity Signup and view all the answers

Which of the following is NOT a characteristic of an ideal absorbent?

High volatility Signup and view all the answers

What is the most common operating condition for absorbers?

Greater than ambient pressure and near ambient temperature Signup and view all the answers

Which of the following stripping agents is MOST commonly used?

Steam Signup and view all the answers

When designing an absorber, which parameter is NOT a concern during operation?

Increasing the absorbent flow rate Signup and view all the answers

What is the consequence of using high pressure and low temperature in a stripper?

Higher stage requirements Signup and view all the answers

Which characteristic of absorbents can help in the safe use of the material?

Nontoxic and nonflammable properties Signup and view all the answers

What should NOT be done when operating an absorber according to pressure and temperature guidelines?

Use conditions that condense the feed gas Signup and view all the answers

What type of adsorption is generally applicable to gases at temperatures above their critical temperature?

Unimolecular adsorption Signup and view all the answers

What is the purpose of obtaining experimental adsorption isotherms?

To provide an upper limit on the adsorption of the adsorbate Signup and view all the answers

What is the characteristic of type II isotherm?

Observed for gases at temperatures below their critical temperature Signup and view all the answers

What is the limitation of the current state of adsorption theory?

It cannot estimate adsorption equilibria accurately Signup and view all the answers

What is the purpose of the adsorption isotherm?

To express the equilibrium of the adsorbate between the fluid and the adsorbent surface Signup and view all the answers

What is the significance of type I and II isotherms?

They are desirable isotherms that exhibit strong adsorption characteristics Signup and view all the answers

What is the relationship between the partial pressure and the adsorbate loading on the adsorbent?

The partial pressure is expressed in terms of the adsorbate loading Signup and view all the answers

What is the classification of adsorption isotherms based on?

The shape of the isotherm curve Signup and view all the answers

Study Notes

Absorption and Stripping Overview

Trayed columns preferred for easier design and scale-up.
Absorbents should possess high solubility for solutes with low volatility to reduce losses and enhance recovery in strippers.
Ideal absorbents are low viscosity, non-toxic, non-flammable, and non-corrosive.
Common absorbents include water, hydrocarbon oils, and aqueous acid/base solutions; stripping agents often involve air, steam, inert gases, and hydrocarbon gases.
Operating conditions typically feature pressures above ambient for absorbers and higher temperatures for strippers to minimize equipment size.

Design Considerations

Avoid conditions that could condense or vaporize feed during absorption and stripping.
There exists a trade-off between absorbent/stripper flow rates and stage requirements, generally becoming complex for concentrated mixtures.
Designing for dilute mixtures can utilize isothermal conditions, simplifying energy balance considerations.

Design Procedures for Trayed Columns

Initiation involves setting separation objectives, identifying feed properties, selecting suitable solvents, and acquiring equilibrium data.
Separation objectives define the desired solute removal percentage from gas streams.
Graphical methods become impractical for multiple stages; algebraic methods or simulation software may be more efficient.

Determining Equilibrium Stages

Kremser method modified by Edmister is used for analyzing dilute mixtures in absorption and stripping.
Mass transfer analysis relies on mol fractions or partial pressures to identify transfer unit heights (HTU) and number of transfer units (NTU).
Total column height includes packing height and additional space for phase disengagement.

Design Implementation

Pressure drop correlations aid in determining equipment dimensions based on height and packing.
Different equipment classes include continuous counterflow contactors (spray/packed/trayed columns), mechanical agitators, mixer-settlers, and centrifugal extractors.
Selection of extraction equipment considers efficiency, dispersion enhancement, and operational advantages.

Adsorption Design Concepts

Dynamic equilibrium analysis explores adsorbate distribution between adsorbent surface and fluid based on partial pressures/concentration.
Experimental data for adsorption equilibria is often necessary due to the lack of predictive theories.
Adsorption isotherms classify into five types; types I and II are favored for strong adsorption behaviors.

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Description

This quiz covers the key considerations for designing and selecting absorbents for absorption columns, including solubility, volatility, and viscosity. Learn about the factors influencing column design and performance.