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Unit Operations Laboratory

Rules-of-Thumb: Chemical Process Design

This section contains engineering rules-of-thumb that may be useful for the design problem calculations.Engineering rules-of-thumb are rough estimates that represent thecollective experiences of many engineers. While the rules-of-thumbcannot be used in place of detailed economic and process-specificdesign calculations, they can be used to obtain reasonable estimatesof many process parameters. As a result, they may be useful for designparameters which must be chosen by the Planner in the designcalculation section of each report.

Chemical Process Design

Overview Chemical engineers are often responsible for the design, construction,and operation of chemical plants and processes. Design engineers areconstantly searching for information that will aid them in these tasks.Engineering publicatios, process data from existing equipment,laboratory and pilot-plant studies are just a few of the many sources of information that design engineers must use.It is important for students to learn the difference between"theoretical" designs and "practical designs". Design calculations mustoften be modified by engineers to reflect economic, safety,construction, and maintenance realities that will affect the design. Forexample:

• Design calculation for a reactor might show that the optimum pipe diameter is D= 3.43 inches. A survey of supplier catalogs will quickly show that schedule-40steel pipe is not manufactured with this diameter. The design engineer must thenchoose between either the 3.07 or 3.55 inch diameter pipe that can be easiltyobtained from the vendor.

• Design calculations for a distillation column might show that a 600 ft tower isrequired to achieve the specified product separation. The maximum height of towers are generally limited to about 175 ft, however, because of wind-loadingand construction considerations. A 600 ft tower would therefore need to be builtin several different sections if alternative designs were not available.

You should strive to achieve as much realism as possible in your design calculations.

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Chemical Process Design: Rules-of-Thumb

• Typical Specifications for Utilities in a Process Plant• Piping and Flow Rates• Pumps•

Distillation and Gas Absorption• Extraction, Liquid-Liquid• Evaporators• Mixing and Agitation• Heat Exchangers & Fouling Coefficients• Heat Transfer Coefficients

ChEn 5401 Design Tips:

• Estimating Tower Heights & Diameters• Estimating the Yearly Production Rate•

Product Recovery and Yields• Specifying Pumps for your Design Project

For students who have not yet taken the department's design course, the rules-of-thumb inthis section have been taken from two books that are currently available at Walter Library:

Chemical Process Equipment: Selection and DesignStanely M. Walas, Butterworths, Boston, MA 1988

Walter Reference: TP157.W334 1988Plant Design and Economics for Chemical Engineers, 4th Ed.

M.S. Peters and K.D. Timmerhaus, McGraw-Hill, Inc., New York, NY,

1991.Walter Reserve: TP155.5 .P4 1991Typical Specifications for Utilities in a Process Plant

1. Steam (Low Pressure): 15-30 psig,Steam (High Pressure): 100 psig.

2. Cooling Water: Supplied at 90 °F from a cooling tower, and returned at 110-120°F.

3. Cooling Air: Supplied at 80-100 °F and returned at 100-140 °F.4. Compressed Air: 45 or 150 psig.5. Instrument Air: 45 psig, 0 °F dewpoint.

6. Electricity: 1-100 hp, 220-550 V.

Go back to the top of the page. Piping and Flow Rates

1. Schedule 40 steel pipe is the most common. Higher pressures require higher schedule number (i.e. thicker walls).

2. Control valves require a 10 psi drop.

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3. Globe valves are used for control. Gate valves are for most other services.4. In large or complicated piping networks, the optimum size of the pipe depends on

many different factors (e.g., relative costs of capital investments, power,maintenance, etc.). These types of calculations are beyond the scope of thiscourse--if you need to do them, however, Perry's and/or Peters & Timmerhaus

will point you in the right direction.

For small piping installations, however, the following rules-of-thumb are sufficientlyaccurate:

Type of fluid Type of flow Velocity (ft/sec)

Nonviscous Inlet to pump 2-3

Process Line or Pump Discharge 5-8

Viscous Liquid Inlet to pump 0.2-0.8

Process Line or Pump Discharge 0.5-2

Gas 30-120

Steam 30-75

Go back to the top of the page. Pumps As chemical engineers, you will most likely encounter two differenttypes of pumps:

1. Centrifugal pumps, and2. Positive-displacement pumps.

Centrifugal pumps are more common, but positive-displacement are used to achieve high pressures.I. Centrifugal Pumps A centrifugal pump, in its simplest form, consists of an impellerrotating inside a casing. The impeller imparts kinetic energy to thefluid. The velocity head, which is created by moving fluid from the low-velocity center to the high-velocity edge of the impeller, is convertedinto pressure head when the fluid leaves the pump.

Centrifugal pumps are simple to construct, low cost, and deliver thefluid at a uniform pressure without shocks or pulsations. In addition,they can handle liquids which contain large amounts of suspendedmaterials.Centrifugal pumps CAN be throttled (partly shut off) on the dischargeside to control the flow rate of the material being pumped.the pump.For more information about centrifugal pumps, go to the pump tutorial.

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II. Positive-Displacement Pumps There are two types of positive-displacement pumps:

1. Reciprocating PumpsThe chamber is a stationary cylinder. The liquid is drawn into the cylinder by the

withdrawal of a piston. The liquid is forced out of the cylinder on the returnstroke.2. Rotary Pumps

The chamber moves from the inlet to the discharge and back again. In a gear rotary pump, for example, two intermeshing gears rotate; the liquid is trapped inthe spaces between the teeth and forced out at the discharge.

Reciprocating and rotary pumps can be used to very high pressures, whereas centrifugal pumps are limited in their head and are used for lower pressures.Reciprocating and rotary pumps CANNOT be throttled (partly shut off)on the discharge side--pumping against a closed valve will damage the

pump! To change the flow rate, the speed of the motor must beadjusted--this is difficult to do using an electric motor (it gets veryexpensive!).Go back to the top of the page. Distillation and Gas Absorption

1. The optimum reflux ratio is generally 1.2-1.5 times the minimum reflux ratio.2. The optimum number of trays is about twice the minimum number of trays, (N m).3. It is advisable to choose a safety factor of about 10% the number of trays

calculated by the best available means.4. Commercial towers are limited to a height of about 175 ft (maximum) because of

wind loading and construction considerations.5. Typical tray efficiencies for distillations are 60-90% for light hydrocarbons andaqueous solutions.

6. Typical tray efficiencies for gas absorption and stripping are 10-20%.7. For vapor-liquid contacting: the height-equivalent-to-a-theoretical-plate (HETP)

is 1.3-1.8 ft for 1-inch pall rings and 2.5-3.0 ft for 2-inch pall rings.8. Packed towers are typically designed to operate near 70% of the flooding rate and

below the loading rate.

Go back to the top of the page. Liquid-Liquid Extraction

1. The dispersed phase in a liquid-liquid extraction column should be the phase thathas the higher volumetric flow rate. In equipment that is subject to backmixing,however, the dispersed phase should be the material which has the smaller volumetric flow rate.

2. The most significant cost in a liquid-liquid extraction process is the cost of thesolvent. As a result, the solvent will generally be recycled after use--as a result,the solvent entering the column will generally contain some of the product (it is

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very rarely "pure"). NOTE: This is why you have to measure the concentration of acetic acid in thetoluene feed for the liquid-liquid extraction lab.

3. You will not recover 100% of the product from a liquid-liquid extraction column!Columns are typically designed to recover 90-99% of the product. High-value

products would be closer to 99%.

Go back to the top of the page. Evaporators

1. Dissolved solids cause the boiling point of a solution to increase. The boiling point elevation can produce differences of 3-10 °F between the solution and thesaturated vapor.

2. For appreciable boiling point elevations, the optimum number of effects in seriesfor a forward-feed evaporator is 4-6 effects. Mechanical vapor recompression can

be used to reduce the number of stages.

3. For small boiling point elevations, the optimum number of effects in series for aforward-feed evaporator is 8-10 effects.4. The steam economy of an N-stage evaporator is approximately 0.8N lb-

evaporated-liquid / lb-of-supplied-steam.5. Interstage steam pressures may be raised using with either steam jet compressors

(20-30% efficiency) or with mechanical compressors (70-75% efficiency).

Go back to the top of the page. Mixing and Agitation Many chemical engineering operations are dependent, to a greatextent, on effective agitation and/or mixing of fluids.

• Agitation--forcing a fluid (by mechanical means) to flow in a circulatory or other pattern inside a vessel.

• Mixing--causing two or more separate phases to be randomly distributed throughone another.

Some of the potential applications of mixing and agitation include:

• Blend two miscible liquids,• Dissolve a solid in a liquid,• Disperse a gas in a liquid as fine bubbles,•

Suspend fine particles in a liquid, or • Agitate a fluid to increase the heat transfer between the fluid and a surface.

Design rules-of-thumb for mixing and agitation processes:

1. Typical horsepower (hp) requirements in agitation processes:

Operation hp / 1000 gal

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Blending or mild mixing 0.2-0.5

Homogeneous chemical reaction 0.5-1.5

Liquid-Liquid Mixtures 5

Liquid-Gas Mixtures 5-10Slurries (to keep solids suspended) 10

2. Proportions of a stirred tank relative to the tank diameter, D:o Liquid-level = D;o Turbine-impeller diameter = 0.3-0.5 D;o Impeller level above bottom = D / 3;o Impeller-blade width = 0.075-0.125 D;o Four vertical baffles with width = D / 12.

See Geankoplis Table 3.4-1 and Figure 3.4-3 for more information.

Go back to the top of the page. Heat Exchangers & Fouling Coefficients

1. Heat exchangers typically use 3/4-inch OD tubes; 1-inch triangular spacing; 16 ftlength.

2. Typical pressure drops across heat exchangers are:Boiling = 1.5 psi, andOther Services = 3-9 psi.

3. The minimum temperature approach is 20 °F (normal coolants) and 10 °F or less(refrigerants).

4. Typical conditions for cooling water:Inlet temperature = 90 °F, and

Outlet temperature = 110-120 °F.

For estimating the approximate magnitudes of the overall heat transfer coefficients, U, inshell-and-tube heat exchangers, you can refer to the following values:

Mechanism U (W/m²-K)

Water to Water 1140 - 1700

Water to Organic Liquids 570 - 1140

Water to Condensing Steam 1420 - 22700

Steam to Boiling Water 1420 - 2270

Water to Air (Finned Tube) 110 - 230

Light Organics to Light Organics 230 - 425

Heavy Organics to Heavy Organics 55 - 230NOTE: Conversion: 1 btu/h-ft²-°F = 5.6873 W/m²-K

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Fouling In practice, heat transfer surfaces do not remain clean for very long.Dirt, soot, and various deposits form on one or both sides of heattransfer surfaces and/or tubing. Like any other material, these depositsadd additional resistances to the flow of heat and reduce the overall

heat transfer coefficient, U.For example:

• Coke and other substances can deposit on surfaces in petroleum processes,• Algea can grow in cooling towers and biological processes, and• Corrosion products can form on many different surfaces--seriously affecting the

resistance to heat transfer.

The effect of these deposits (i.e. "fouling") is generally taken into account by addingresistances, 1/h di and 1/h do, to account for the fouling inside the tube, h di, and for thefouling outside the tube, h do, in the equation for the overall heat transfer coeficient, U. See

Eq. 4.3-17 in Geankoplis' text. Typical values for the fouling coefficients are shown below:Deposit h d (W/m²-K)

Distilled and Seawater 11350

City Water 5680

Muddy Water 1990-2840

Gases 2840

Vaporizing Liquids 2840

Gas Oils and Vegetable Oils 1990

NOTE: Conversion: 1 btu/h-ft²-°F = 5.6873 W/m²-K Go back to the top of the page. Heat Transfer Coefficients

The following table contains approximate magnitudes for the heattransfer coefficient for various mechanisms:

Mechanism h (W/m²-K)

Still Air 2.8 - 23

Moving Air 11.3 - 55

Moving Water 280 - 17000

Moving Hydrocarbons 55 - 1700Boiling Liquids 1700 - 28000

Condensing Steam 5700 - 28000

Condensing Organics 1100 - 2800NOTE: Conversion: 1 btu/h-ft²-°F = 5.6873 W/m²-K

Go back to the top of the page. Estimating Tower Heights and Diameters

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The basic question you need to ask is this: If the flow rate of myexperiment is X (gal/hr), but the flow rate for my design is Y (gal/hr),what tower diameter (distillation or extraction) will give me the sameconditions in each column?"

The second question you need to ask is: What tower height do I need

to achieve the specified product concentrations?BEWARE: Many students just grab the first number that they calculatewith little or no thought to what it actually implies. For example, if yourcalculations yield a 9-mile high tower, with a 10-inch diameter, thedesign is clearly not feasible! You need to rethink the problem or yourassumptions or both.As a general rule: Towers are limited to a maximum height of 175 ft(see the distillation rules-of-thumb). If your design needs to be tallerthan that, the tower would be built in several sections--which would, of course, affect how many pumps you might need, etc.Go back to the top of the page.

Estimating the Yearly Production Rate Many of the design problems ask for an estimate of the yearlyproduction rate.It goes without saying that the absolute maximum production ratewould be achieved if the process operated 24 hours/day, 365days/year...but this is not a realistic choice.Process equipment needs to be repaired, replaced, serviced, factoriesshut down for holidays, etc. In addition, it may not be wise to run theprocess at 100% capacity because it may lead to excessive wear onthe equipment.A typical value is 90% or 328 days/year.

While this type of information is sometimes unknown (and probablybeyond the scope of this course)--it is something you should start tothink about. Capital equipment costs, maintenance, and economics arean important part of engineering.Go back to the top of the page. Product Recovery and Yields In some of the design problems, you are asked to create a process thatwill recover some sort of product (e.g., pharmeacutical products,copper, etc.). It is up to you to decide how much of the product will berecovered.It is impossible to recover 100% of the product!

Typically, processes will be designed to recover 90-99% of the desiredproduct. For low-value products, the yield would probably be around90%, but for high-value products, the yield be much closer to 99%.As an additional caution, you may discover that the outlet productconcentration in a separation process (e.g., distillation, extraction, etc)is limited by the equilibrium conditions which might exist in the outletstream. For example, in distillation processes, the formation of a binaryazeotrope may make it impossible to produce a nearly pure product.

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Go back to the top of the page. Specifying Pumps for your Design Project In some of the design problems, you are asked to specify the pumpsthat will be required to move fluids within your process.Many students calculate pump sizes which are much too small for their

design project. This is because students rarely account for the pressurelosses which occur because of valves. A control valve, for example,requires a pressure drop of at least 10 psi to maintain good control.

These types of losses will affect your design calculations!" The pump tutorial has been written to help lead you through thesetypes of calculations.As a rough estimate, look at the pumps that you are using in thelaboratory. If the pumps on the experiments are much larger than yourcalculations, you have probably neglected something important (valvelosses? friction losses?).Go back to the top of the page.

Return to Unit Ops Lab Home Page

Copyright 1997 by the Regents of the University of Minnesota.Department of Chemical Engineering & Materials Science.

All Rights Reserved.

Author: Christopher J. DudaRevised: October 2, 2001

by: Raul A. Caretta URL: http://www3.cems.umn.edu/courses/chen4402/

The University of Minnesota is an equal opportunity educator and employer.