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.

[Freeman]

Cooling Towers

A. With industrial cooling towers, cooling to 90% of the ambient air saturation level is possible.

B. Relative tower size is dependent on the water temperature approach to the wet bulb temperature:


C. Water circulation rates are generally 2-4 GPM/sq. ft (81-162 L/min m2) and air velocities are usually 5-7 ft/s
(1.5-2.0 m/s)

D. Countercurrent induced draft towers are the most common. These towers are capable of cooling to within 2 °F
(1.1 °C) of the wet bulb temperature. A 5-10 °F (2.8-5.5 °C) approach is more common.

E. Evaporation losses are about 1% by mass of the circulation rate for every 10 °F (5.5 °C) of cooling. Drift losses are around 0.25% of the circulation rate. A blowdown of about 3% of the circulation rate is needed to prevent salt and chemical treatment buildup.

[Freeman]

Drum Type Vessels

A. Liquid drums are usually horizontal. Gas/Liquid separators are usually vertical

B. Optimum Length/Diameter ratio is usually 3, range is 2.5 to 5

C. Holdup time is 5 minutes for half full reflux drums and gas/liquid separators

Design for a 5-10 minute holdup for drums feeding another column

D. For drums feeding a furnace, a holdup of 30 minutes is a good estimate

E. Knockout drum in front of compressors should be designed for a holdup of

10 times the liquid volume passing per minute.

F. Liquid/Liquid separators should be designed for settling velocities of 2-3 inches/min

G. Gas velocities in gas/liquid separators, velocity = k (liquid density/(vapor density-1))^0.5,

where k is 0.35 with horizontal mesh deentrainers and 0.167 with vertical mesh deentrainers. k is 0.1 without mesh deentrainers and velocity is in ft/s

H. A six inch mesh pad thickness is very popular for such vessels

I. For positive pressure separations, disengagement spaces of 6-18 inches before the mesh pad and 12 inches after the pad are generally suitable.

[Freeman]

Electric Motors and Turbines

A. Efficiencies range from 85-95% for electric motors, 42-78% for steam turbines

28-38% for gas engines and turbines

B. For services under 75 kW (100 hp), electric motors are almost always used.

They can be used for services up to about 15000 kW (20000 hp)

C. Turbines can be justified in services where they will yield several hundred

horsepowers. Otherwise, throttle valves are used to release pressure.

D. A quick estimate of the energy available to a turbine is given by:

where: Delta H = Actual available energy, Btu/lb

Cp = Heat Capacity at constant pressure, Btu/lb 0F

T1 = Inlet temperature, 0R

P1 = Inlet pressure, psia

P2 = Outlet pressure, psia

K = Cp/Cv

[Freeman]

Evaporation
A. Most popular types are long tube vertical with natural or forced circulation. Tubes range from 3/4" to 2.5"
(19-63 mm) in diameter and 12-30 ft (3.6-9.1 m) in length.
B. Forced circulation tube velocities are generally in the 15-20 ft/s (4.5-6 m/s) range.
C. Boiling Point Elevation (BPE) as a result of having dissolved solids must be accounted for in the differences between the solution temperature and the temperature of the saturated vapor.
D. BPE's greater than 7 °F (3.9 °C) usually result in 4-6 effects in series (feed-forward) as an economical solution. With smaller BPE's, more effects in series are typically more economical, depending on the cost of steam.
E. Reverse feed results in the more concentrated solution being heated with the hottest steam to minimize surface area. However, the solution must be pumped from one stage to the next.
F. Interstage steam pressures can be increased with ejectors (20-30% efficient) or mechanical compressors (70-75% efficient).


Filtration
A. Initially, processes are classified according to their cake buildup in a laboratory vacuum leaf filter :
0.10 - 10.0 cm/s (rapid), 0.10-10.0 cm/min (medium), 0.10-10.0 cm/h (slow)
B. Continuous filtration methods should not be used if 0.35 sm of cake cannot be formed in less than 5 minutes.
C. Belts, top feed drums, and pusher-type centrifuges are best for rapid filtering.
D. Vacuum drums and disk or peeler-type centrifuges are best for medium filtering.
E. Pressure filters or sedimenting centrifuges are best for slow filtering.
F. Cartridges, precoat drums, and sand filters can be used for clarification duties with negligible buildup.
G. Finely ground mineral ores can utilize rotary drum rates of 1500 lb/dat ft2 (7335 kg/day m2) at 20 rev/h and 18-25 in Hg (457-635 mm Hg) vacuum.
H. Course solids and crystals can be filtered at rates of 6000 lb/day ft2 (29,340 kg/day m2) at 20 rev/h and 2-6 in Hg (51-152 mm Hg) vacuum.

[Freeman]

Pressure and Storage Vessels
Pressure Vessels
A. Design Temperatures between -30 and 345 °C (-22 to 653 °F) is typically about
25 °C (77 °F) above maximum operating temperature, margins increase above this range
B. Design pressure is 10% or 0.69 to 1.7 bar (10 to 25 psi) above the maximum operating
pressure, whichever is greater. The maximum operating pressure is taken as 1.7 bar (25 psi)
above the normal operation pressure.
C. For vacuum operations, design pressures are 1 barg (15 psig) to full vacuum
D. Minimum thicknesses for maintaining tank structure are:
6.4 mm (0.25 in) for 1.07 m (42 in) diameter and under
8.1 mm (0.32 in) for 1.07-1.52 m (42-60 in) diameter
9.7 mm (0.38 in) for diameters over 1.52 m (60 in)
E. Allowable working stresses are taken as 1/4 of the ultimate strength of the material

G. Thickness based on pressure and radius is given by:

where pressure is in psig, radius in inches, stress in psi, corrosion allowance in inches.
**Weld Efficiency can usually be taken as 0.85 for initial design work

H. Guidelines for corrosion allowances are as follows: 0.35 in (9 mm) for known corrosive fluids, 0.15 in (4 mm) for non-corrosive fluids, and 0.06 in (1.5 mm) for steam drums and air receivers.

[Freeman]

Storage Vessels

I. For less than 3.8 m3 (1000 gallons) use vertical tanks on legs

J. Between 3.8 m3 and 38 m3 (1000 to 10,000 gallons) use horizontal tanks on concrete supports

K. Beyond 38 m3 (10,000 gallons) use vertical tanks on concrete pads

L. Liquids with low vapor pressures, use tanks with floating roofs.

M. Raw material feed tanks are often specified for 30 days feed supplies

N. Storage tank capacity should be at 1.5 times the capacity of mobile supply vessels.

For example, 28.4 m3 (7500 gallon) tanker truck, 130 m3 (34,500 gallon) rail cars

[Freeman]

Piping
A. Liquid lines should be sized for a velocity of (5+D/3) ft/s and a pressure drop of
2.0 psi/100 ft of pipe at pump discharges
At the pump suction, size for (1.3+D/6) ft/s and a pressure drop of 0.4 psi/100 ft of pipe
**D is pipe diameter in inches
B. Steam or gas lines can be sized for 20D ft/s and pressure drops of 0.5 psi/100 ft of pipe
C. Limits on superheated, dry steam or gas line should be 61 m/s (200 ft/s) and a pressure drop of 0.1 bar/100 m or 0.5 psi/100 ft of pipe. Saturated steam lines should be limited to 37 m/s (120 ft/s) to avoid erosion.
D. For turbulent flow in commercial steel pipes, use the following:

E. For two phase flow, an estimate often used is Lockhart and Martinelli:
First, the pressure drops are calculated as if each phase exist alone in the pipe, then

F. Control valves require at least 0.69 bar (10 psi) pressure drop for sufficient control
G. Flange ratings include 10, 20, 40, 103, and 175 bar (150, 300, 600, 1500, and 2500 psig)

H. Globe valves are most commonly used for gases and when tight shutoff is required. Gate valves are common for most other services.I. Screwed fitting are generally used for line sizes 2 inches and smaller. Larger connections should utilize flanges or welding to eliminate leakage.J. Pipe Schedule Number = 1000P/S (approximate) where P is the internal pressure rating in psig and S is the allowable working stress of the material is psi. Schedule 40 is the most common.

Pumps
A. Power estimates for pumping liquids:
kW=(1.67)[Flow (m3/min)][Pressure drop (bar)]/Efficiency
hp=[Flow (gpm)][Pressure drop (psi)]/1714 (Efficiency)
**Efficiency expressed as a fraction in these relations
B. NPSH=(pressure at impeller eye-vapor pressure)/(density*gravitational constant)
Common range is 1.2 to 6.1 m (4-20 ft) of liquid
C. An equation developed for efficiency based on the GPSA Engineering Data Book is:
Efficiency = 80-0.2855F+.000378FG-.000000238FG^2+.000539F^2-.000000639(F^2)G+
.0000000004(F^2)(G^2)
where Efficiency is in fraction form, F is developed head in feet, G is flow in GPM
Ranges of applicability are F=50-300 ft and G=100-1000 GPM
Error documented at 3.5%
D. Centrifugal pumps: Single stage for 0.057-18.9 m3/min (15-5000 GPM), 152 m (500 ft)
maximum head; For flow of 0.076-41.6 m3/min (20-11,000 GPM) use multistage, 1675 m (5500 ft)
maximum head; Efficiencies of 45% at 0.378 m3/min (100 GPM), 70% at 1.89 m3/min (500 GPM),
80% at 37.8 m3/min (10,000 GPM).
E. Axial pumps can be used for flows of 0.076-378 m3/min (20-100,000 GPM)
Expect heads up to 12 m (40 ft) and efficiencies of about 65-85%
F. Rotary pumps can be used for flows of 0.00378-18.9 m3/min (1-5000 GPM)
Expect heads up to 15,200 m (50,000 ft) and efficiencies of about 50-80%
G. Reciporating pumps can be used for 0.0378-37.8 m3/min (10-100,000 GPM)
Expect heads up to 300,000 m (1,000,000 ft).
Efficiencies: 70% at 7.46 kW (10 hp), 85% at 37.3 kW (50 hp), and 90% at 373 kW (500 hp)

[Freeman]

Compressors and Vacuum Equipment

A. The following chart is used to determine what type of compressor is to be used:

B. Fans should be used to raise pressure about 3% (12 in water), blowers to raise to less than 2.75 barg (40 psig),

and compressors to higher pressures.

C. The theoretical reversible adiabatic power is estimated by:

Power = m z1 R T1 [({P2 / P1}a - 1)] / a

where:

T1 is the inlet temperature, R is the gas constant, z1 is the compressibility, m is the molar flow rate,

a = (k-1)/k , and k = Cp/Cv

D. The outlet for the adiabatic reversible flow, T2 = T1 (P2 / P1)a

E. Exit temperatures should not exceed 204 0C (400 0F).

F. For diatomic gases (Cp/Cv = 1.4) this corresponds to a compression ratio of about 4

G. Compression ratios should be about the same in each stage for a multistage unit,

the ratio = (Pn / P1) 1/n, with n stages.

H. Efficiencies for reciprocating compressors are as follows:

65% at compression ratios of 1.5

75% at compression ratios of 2.0

80-85% at compression ratios between 3 and 6

I. Efficiencies of large centrifugal compressors handling 2.8 to 47 m3/s (6000-100,000 acfm) at suction is about 76-78%

J. Reciprocating piston vacuum pumps are generally capable of vacuum to 1 torr absolute, rotary piston types can achieve vacuums of 0.001 torr.K. Single stage jet ejectors are capable of vacuums to 100 torr absolute, two stage to 10 torr, three stage to 1 torr, and five stage to 0.05 torr.L. A three stage ejector requires about 100 lb steam/lb air to maintain a pressure of 1 torr.M. Air leakage into vacuum equipment can be approximated as follows:
Leakage = k V(2/3)
where k =0.20 for P >90 torr, 0.08 for 3 < P < 20 torr, and 0.025 for P < 1 torr
V = equipment volume in cubic feet
Leakage = air leakage into equipment in lb/h

[Freeman]

Heat Exchangers

A. For the heat exchanger equation, Q = UAF (LMTD), use F = 0.9 when charts for the LMTD correction

factor are not available

B. Most commonly used tubes are 3/4 in. (1.9 cm) in outer diameter on a 1 in triangular spacing at 16 ft (4.9 m) long.

C. A 1 ft (30 cm) shell will contains about 100 ft2 (9.3 m2)

A 2 ft (60 cm) shell will contain about 400 ft2 (37.2 m2)

A 3 ft (90 cm) shell will contain about 1100 ft2 (102 m2)

D. Typical velocities in the tubes should be 3-10 ft/s (1-3 m/s) for liquids and30-100 ft/s (9-30 m/s) for gases

E. Flows that are corrosive, fouling, scaling, or under high pressure are usually placed in the tubes

F. Viscous and condensing fluids are typically placed on the shell side.

G. Pressure drops are about 1.5 psi (0.1 bar) for vaporization and 3-10 psi (0.2-0.68 bar) for other services

H. The minimum approach temperature for shell and tube exchangers is about 20 °F (10 °C) for fluids and

10 °F (5 °C) for refrigerants.

I. Cooling tower water is typically available at a maximum temperature of 90 °F (30 °C) and should be

returned to the tower no higher than 115 °F (45 °C)

J. Shell and Tube heat transfer coefficient for estimation purposes can be found in many reference books

or an online list can be found at one of the two following addresses:

[link Point to another website Only the registered members can access]

[link Point to another website Only the registered members can access]

K. Double pipe heat exchangers may be a good choice for areas from 100 to 200 ft2 (9.3-18.6 m2)

L. Spiral heat exchangers are often used to slurry interchangers and other services containing solids

M. Plate heat exchanger with gaskets can be used up to 320 °F (160 °C) and are often used for interchanging

duties due to their high efficiencies and ability to "cross" temperatures. More about compact heat exchangers

can be found at:

[link Point to another website Only the registered members can access]

[Freeman]

Tray Towers

A. For ideal mixtures, relative volatility can be taken as the ratio of pure component vapor pressures
B. Tower operating pressure is most often determined by the cooling medium in condenser or the

maximum allowable reboiler temperature to avoid degradation of the process fluid

C. For sequencing columns:

1. Perform the easiest separation first (least trays and lowest reflux)

2. If relative volatility nor feed composition vary widely, take products off one at time

as the overhead

3. If the relative volatility of components do vary significantly, remove products in order

of decreasing volatility

4. If the concentrations of the feed vary significantly but the relative volatility do not,

remove products in order of decreasing concentration.

D. The most economic reflux ratio usually is between 1.2Rmin and 1.5Rmin

E. The most economic number of trays is usually about twice the minimum number of trays.
The minimum number of trays is determined with the Fenske-Underwood Equation.

F. Typically, 10% more trays than are calculated are specified for a tower.

G. Tray spacings should be from 18 to 24 inches, with accessibility in mind

H. Peak tray efficiencies usually occur at linear vapor velocities of 2 ft/s (0.6 m/s) at moderate pressures,
or 6 ft/s (1.8 m/s) under vacuum conditions.

I. A typical pressure drop per tray is 0.1 psi (0.007 bar)

J. Tray efficiencies for aqueous solutions are usually in the range of 60-90% while gas absorption and
stripping typically have efficiencies closer to 10-20%

K. The three most common types of trays are valve, sieve, and bubble cap. Bubble cap trays are

typically used when low-turn down is expected or a lower pressure drop than the valve or sieve

trays can provide is necessary.

L. Seive tray holes are 0.25 to 0.50 in. diameter with the total hole area being about 10% of the total
active tray area.M. Valve trays typically have 1.5 in. diameter holes each with a lifting cap. 12-14 caps/square foot
of tray is a good benchmark. Valve trays usually cost less than seive trays.

N. The most common weir heights are 2 and 3 in and the weir length is typically 75% of the tray diameter
O. Reflux pumps should be at least 25% overdesigned

P. The optimum Kremser absorption factor is usually in the range of 1.25 to 2.00

Q. Reflux drums are almost always horizontally mounted and designed for a 5 min holdup at half of the
drum's capacity.

R. For towers that are at least 3 ft (0.9 m) is diameter, 4 ft (1.2 m) should be added to the top for vapor
release and 6 ft (1.8 m) should be added to the bottom to account for the liquid level and reboiler return
S. Limit tower heights to 175 ft (53 m) due to wind load and foundation considerations.

T. The Length/Diameter ratio of a tower should be no more than 30 and preferrably below 20

U. A rough estimate of reboiler duty as a function of tower diameter is given by:

Q = 0.5 D2 for pressure distillation

Q = 0.3 D2 for atmospheric distillation

Q = 0.15 D2 for vacuum distillation

where Q is in Million Btu/hr and D is tower diameter in feet

[Freeman]

Packed Towers

A. Packed towers almost always have lower pressure drop than comparable tray towers.
B. Packing is often retrofitted into existing tray towers to increase capacity or separation.
C. For gas flowrates of 500 ft3/min (14.2 m3/min) use 1 in (2.5 cm) packing, for gas flows
of 2000 ft3/min (56.6 m3/min) or more, use 2 in (5 cm) packing

D. Ratio of tower diameter to packing diameter should usually be at least 15

E. Due to the possibility of deformation, plastic packing should be limited to an unsupported
depth of 10-15 ft (3-4 m) while metallatic packing can withstand 20-25 ft (6-7.6 m)

F. Liquid distributor should be placed every 5-10 tower diameters (along the length) for pall rings
and every 20 ft (6.5 m) for other types of random packings

G. For redistribution, there should be 8-12 streams per sq. foot of tower area for tower larger than
three feet in diameter. They should be even more numerous in smaller towers.

H. Packed columns should operate near 70% flooding.

I. Height Equivalent to Theoretical Stage (HETS) for vapor-liquid contacting is 1.3-1.8 ft

(0.4-0.56 m) for 1 in pall rings and 2.5-3.0 ft (0.76-0.90 m) for 2 in pall rings