3/7/2015 1

M.A.A. Shoukat Choudhury, PhDProfessor

Department of Chemical Engineering, BUET

ChE 111 – Chapter 3Processes and Process Variables

February 2015

Process

• Process: A process is any operation by which a particular objective is accomplished.

• Materials that enter a process is referred to as the input or feed and that which leaves the process is called output or product.

Process and Input/Output

Chemical Engineering, BUET Dr. M. A. A. Shoukat Choudhury 4

Mass and Volume

density: mass per unit volume, kg/m3

Specific volume: volume per unit mass, m3/kg

Specific gravity:

SG = /ref

The most commonly used reference material is water at 4 oC

Chemical Engineering, BUET Dr. M. A. A. Shoukat Choudhury 7

Mass and Volumetric Flow Rates

flow

Chemical Engineering, BUET Dr. M. A. A. Shoukat Choudhury 8

Mass and Volumetric Flow Rates

Chemical Engineering, BUET Dr. M. A. A. Shoukat Choudhury 9

Flow Rate Measurement

Rotameter Orifice meter

A flow meter is a device mounted in a process pipe that provides

a continuous reading of the flow rate in the pipe line

Flow Rate Measurement

Chemical Composition

Mole and Molecular Weight

- Atomic Weight

- Molecular weight

- g-mole or mole

- lb-mole

- Kmole

Mass Fraction and Mole Fraction

Conversion using Mass and Mole Fractions

Mass composition to Molar Composition

Example 3.3-3

Example 3.3-3 (solution)

Average Molecular Weight

Mass Concentration and Molar Concentration

• Mass Concentration = density

• Molar concentration = number of moles per unit volume

• Molarity = molar concentration expressed in gmoles per Litre, e.g.

ppm and ppb

• ppm – Parts per million

• ppb – parts per billion

Example 3.3-5

Solution to example 3.3-5

Figure 3.4-1: Fluid pressure in a tank and a pipe.

PressureFluid Pressure and Hydrostatic Head

Figure 3.4-2: Pressure at the base of a fluid column.

PressureHydrostatic Head

Hydrostatic Pressure/Head :

The pressure exerted by the fluid at

the base of the tank. Here, P.

Show that (by force balance):

Since A does not appear in this

equation, the formula is a

applicable to any column of fluid

Equivalence betn Head and Pressure

Pressure Equation as Head

Equation 3.4-1 as fluid head (e.g., mercury):

Example 3.4-2:

Atmospheric Pressure

• Atmospheric Pressure: The pressure of the atmosphere can be thought of as the pressure at the base of a column of fluid (air) located at the point of measurement (sea level). Equation 3.4-1 can be used for calculation with Po= 0 and average values of air density and gravitational acceleration.

• Typical Values of atmospheric pressure at sea level are 760 mm Hg, 14.7 psi or 1 atm

Atmospheric Pressure, Gauge Pressure,Absolute Pressure

• Relationship between absolute and Gauge pressure

• Conventionally, psia and psig are used to denote absolute and gauge pressure in psi units

Fluid Pressure Measurement

Figure 3.4-3: Bourdon gauge.

Pressure Measuring Device

• A manometer is a U-shaped tube partially filled with a fluid of known density

Manometer

Elementary Principles of Chemical Processs, 3/E by Richard M. Felder and Ronald W. RousseauCopyright © 2005 by John Wiley & Sons, Inc. All rights reserved.

Figure 3.4-5 (p. 58)Manometer variables.

Manometer Equation Derivation

Manometer Equation Derivation

• The pressure at the height of the lower surface of the manometer fluid is same at the both arms of the manometer

• General Manometer Equation

Elementary Principles of Chemical Processs, 3/E by Richard M. Felder and Ronald W. RousseauCopyright © 2005 by John Wiley & Sons, Inc. All rights reserved.

Example 3.4-3(1) (p. 59)

Differential Manometer Equation

Special Cases of Manometer Equation

Example 3.4-3

Temperature

Temperature (cont’d)

Relation Between Temperature Scales