Fall 2008

ChE 545: Mass Transfer Operations I

Lecture 1: Fundamental Concepts

Chemical processes can be batch, continuous or semi-continuous; and consist of

o Key Operations

Chemical reactions

Physical separations

o Auxiliary Operations

Phase separations

Heat transfer

Size reduction, etc.

Processes can be represented by block flow diagrams at a basic level.

Separations are non-spontaneous processes requiring energy expenditure.

Separations are characterized by a decrease in entropy and increase in availability

(exergy).

Separations can be equilibrium-driven or rate-driven.

Separations can be effected through the use of

o Energy Separating Agents (ESA), or

o Mass Separating Agents (MSA)

ESA are usually preferred over MSA due to:

o Possible MSA contamination in product streams

o Make up requirements for MSA

o Need to separate MSA for reuse

The extent of separation is limited by the equilibrium, the rate of separation

dependent upon the mass transfer rate.

The material balance for the component i in a separation step is:

Where, n is the number of moles, F is the feed stream, N the number of phases

into which the feed is separated.

The split fraction (SF) and split ratio (SR) for a component i in the kth separator

are defined as:

;

where 1 and 2 represent two outlet phases. Split fraction is based on the feed,

while split ratio is based on the product (or outlet) stream.

Separation power (or separation factor) - SP - is based on concentrations in two

outlet phases (or streams). SP is always defined for any component i with respect

to some other component j.

SPs can be related to SFs and SRs.

Thermodynamic Considerations:

The minimum work needed to achieve a certain separation can be calculated from

the stream availabilities.

The actual work needed to effect the separations within a finite time requires an

additional amount equal to T0 Sirr to compensate for the lost work (LW). The

second law efficiency for the separation is based on the Wmin and LW.

Equilibrium

Chemical potentials ( ), also known as the partial molar Gibbs free energy, for a

component are equal in all phases at equilibrium.