Experienced-Based Rules of Chemical Engineering

[Freeman]

Experienced-Based Rules of Chemical Engineering

By Eng. Chris Haslego

Experience is typically what turns a good engineer into a great engineer. An engineer that can look at a pipe and a flowmeter and guess the pressure drop within 5%. Someone who
can at least estimate

the size of a vessel without doing any calculations. When I think of such rules, two authors come to my mind, Walas and Branan. Dr. Walas' book, Chemical Process Equipment: Selection and Design has been widely used in the process industry and in chemical engineering education for years. Mr. Branan has either helped write or edit numerous books concerning this topic. Perhaps his most popular is Rules of Thumb for Chemical Engineers. Here, I'll share some of these rules with you along with some of my own. Now, be aware that these rules are for estimation and are not necessary meant to replace rigorous calculations when such calculations should be performed. But at many stages of analysis and design, these rules can save you hours and hours.

Physical Properties

Viscosities of organic liquids vary widely with temperature

Liquid density varies with temperature by:

** Viscosities of organic liquids vary widely with temperature
Liquid density varies with temperature by:

Gas density can be calculated by:


Boiling Point of Water as a Function of Pressure:
Tbp (°C) = (Pressure (MPa) x (1x109))0.25

Materials of Construction

Material

Carbon Steel
Advantage : Low cost, easy to fabricate, abundant, most common material. Resists most alkaline environments well.
Disadvantage : Very poor resistance to acids and stronger alkaline streams. More brittle than other materials, especially at low temperatures.

Stainless Steel

Advantage : Relatively low cost, still easy to fabricate. Resist a wider variety of environments than carbon steel. Available is many different types.

Disadvantage : No resistance to chlorides, and resistance decreases significantly at higher temperatures.

254 SMO (Avesta)
Advantage : Moderate cost, still easy to fabricate. Resistance is better over a wider range of concentrations and temperatures compared to stainless steel.
Disadvantage : Little resistance to chlorides, and resistance at higher temperatures could be improved.

Titanium
Advantage : Very good resistance to chlorides (widely used in seawater applications). Strength allows it to be fabricated at smaller thicknesses.
Disadvantage :While the material is moderately expensive, fabrication is difficult. Much of cost will be in welding labor.

Pd stabilized Titanium
Advantage : Superior resistance to chlorides, even at higher temperatures. Is often used on sea water application where Titanium's resistance may not be acceptable.
Disadvantage :Very expensive material and fabrication is again difficult and expensive.

Nickel
Advantage : Very good resistance to high temperature caustic streams.
Disadvantage :Moderate to high expense. Difficult to weld.

Hastelloy Alloy
Advantage : Very wide range to choose from. Some have been specifically developed for acid services where other materials have failed.

Disadvantage :Fairly expensive alloys. Their use must be justified. Most are easy to weld.

Graphite
Advantage : One of the few materials capable of withstanding weak HCl streams.
Disadvantage: Brittle, very expensive, and very difficult to fabricate. Some stream components have been know to diffusion through some types of graphites.

Tantalum
Advantage : Superior resistance to very harsh services where no other material is acceptable.
Disadvantage : Extremely expensive, must be absolutely necessary.