Experience-Based Rules of Chemical Engineering

A Chererources Article

You are about to read an award winning article which won the 2000 Beychok-Montemayor Award at The Chemical Engineers' Resource Page.

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 necessarily 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.

A) Materials of Construction

Carbon Steel
Advantages: Low cost, easy to fabricate, abundant, most common material. Resists most alkaline environments well.

Disadvantages: Very poor resistance to acids and stronger alkaline streams. More brittle than other materials, especially at low temperatures.

Stainless Steel
Adv: Relatively low cost, still easy to fabricate. Resists a wider variety of environments than carbon steel. Available in many different types.

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

254 SMO (Avesta)
Adv: Moderate cost, still easy to fabricate. Resistance is better over a wider range of concentrations and temperatures compared to stainless steel.

Disadv: Little resistance to chlorides, and resistance at higher temperatures could be improved.

Adv: Very good resistance to chlorides (widely used in seawater applications). Strength allows it to be fabricated at smaller thicknesses.

Disadv: While the material is moderately expensive, fabrication is difficult. Much of cost will be in welding labor.

Pd stabilized Titanium
Adv: Superior resistance to chlorides, even at higher temperatures. Is often used on sea water application where Titanium's resistance may not be acceptable.

Disadv: Very expensive material and fabrication is again difficult and expensive.

Adv: Very good resistance to high temperature caustic streams.

Disadv: Moderate to high expense. Difficult to weld.

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

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

Adv: One of the few materials capable of withstanding weak HCl streams.

Disadv: Brittle, very expensive, and very difficult to fabricate. Some stream components have been know to diffuse through some types of graphites.

Adv: Superior resistance to very harsh services where no other material is acceptable.

Disadv: Extremely expensive, must be absolutely necessary.

B) Conveyors

A. Pneumatic conveyors are best suited for high capacity applications over distances of up to about 400 ft. Pneumatic conveying is also appropriate for multiple sources and destinations. Vacuum or low pressure (6-12 psig or 0.4 to 0.8 bar) is used for generate air velocities from 35 to 120 ft/s (10.7-36.6 m/s). Air requirements are usually in the range of 1 to 7 cubic feet of air per cubic foot of solids (0.03 to 0.5 cubic meters of air per cubic meter of solids).

B. Drag-type conveyors (Redler) are completed enclosed and suited to short distances. Sizes range from 3 to 19 inches square (75 to 480 mm). Travel velocities can be from 30 to 250 ft/min (10 to 75 meters/min). The power requirements for these conveyors is higher than other types.

C. Bucket elevators are generally used for the vertical transport of sticky or abrasive materials. With a bucket measuring 20 in x 20 in (500 mm x 500 mm), capacities of 1000 cubic feet/hr (28 cubic meters/hr) can be reached at speeds of 100 ft/min (30 m/min). Speeds up to 300 ft/min (90 m/min) are possible.

D. Belt conveyors can be used for high capacity and long distance transports. Inclines up to 30° are possible. A 24 in (635 mm) belt can transport 3000 cu. ft./h (85 cu m/h) at speeds of 100 ft/min (30.5 m/min). Speeds can be as high as 600 ft/min (183 m/min). Power consumption is relatively low.

E. Screw conveyors can be used for sticky or abrasive solids for transports up to 150 ft (46 m). Inclines can be up to about 20°. A 12 in (305 mm) diameter screw conveyor can transport 1000-3000 cu. ft./h (28-85 cu. m/h) at around 40-60 rpm.

Ok this is all for now. I'll update next week. Meanwhile, there is A LOT more info on Reactors, Crystallization, Electrical Motors & Turbines, Evaporation, Mixing & Agitation, Pressure and Storage Vessels…. geek paradise, in fact. All available at http://www.cheresources.com/


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