Chemical Engineering Process Design

OPERATING PROBLEMS: SOLUTION BY DESIGN

Keith Marchildon

David Mody

Objective

To anticipate possible problems and to avoid them by good design.

There will be enough problems that you don’t foresee.

Dealing with problems that arise is called ‘trouble-shooting’.

It is an important skill for anyone in the process industries.

It is always better to design around potential problems rather than trouble-shoot them later.

Ref: Chapter 5 of the Mody/Marchildon text.

1. Build-up of extraneous substances

Coolant

Vapour

Condensate

Non-condensable gases

problem

Vapour

Condensate

Coolant

Non-condensable gases

Condensatecooler

noproblem

A, C A, B, C

A, C

B, C

A, B, C

A, C

B, C

C

BoilerProcess

Some avoidance measures

Investigate the whole chemistry of the process

Include all recycle loops if a pilot is built, and run long enough to detect any long-term problems

Provide purge points in the commercial system Be prepared to shut down and clean periodically

2. Corrosion

Some avoidance measures

Consult tables of material compatibility

Pre-test the proposed materials under actual operating conditions i.e., at high pressure if needed Install test coupons in a pilot facility

Avoid overly hot heat-transfer surfaces

Ensure the quality of construction materials: e.g., no defects, no sharp edges or crevasses, no dissimilar metals (for instance instrumentation devices)

Cover up outdoors insulation

3. Erosion, Cavitation

Examples:

Wear of pipes and other equipment due to excessive stream velocities or impact (e.g., at bends)

Pitting in valves and pumps because of vapour appearing, then collapsing

Liquidpressure

Vapourpressure

Some avoidance measures

Design piping for velocities below recommended limits ( but still high enough to avoid settling out of solids)

Ensure that the available Net Positive Suction Head (NPSHa) at the entrance to a pump is safely above the manufacturer-recommended required value (NPSHr)

Do not install liquid-flow throttling valves or control valves in situations where the throat pressure will fall below the vapour pressure of the material.

Take into account changes in elevation, which directly affect liquid pressure and boiling point

4. Flashing, Phase Separation

Some examples:

A liquid stream may split into a vapour-liquid mixture A liquid splits into two immiscible liquid phases Solid particles start to appear in a liquid or a coating starts to precipitate on solid surfaces

A vapour starts to condense into a liquid (or a solid). This is a particular problem when it happens in or before a compressor, which is designed to handle gases but is damaged by the presence of liquid

A solid starts to liquefy or to sublime

Some avoidance measures

Understand the phase equilibrium and chemistry of the process materials

Maintain conditions of pressure and temperature that prevent the undesired splits

Avoid cold spots and hot spots

If high pressure is involved, consult an academic with high-temperature viewing facility that allows observation of phase behaviour

Consider tracing or jacketing with tempered water or hot oil or using electrical tracing

5. Excessive Foaming and Entrainment

Hliquid

o

o

o

o

o

o

OO

O

o

o

o o

Hexpanded

Freeboard

Some avoidance measures

As part of the process design a determination should be made of the propensity of the materials to foam and of the means of overcoming this effect. Organic compounds possessing polar groups are especially prone to foaming.

An equation for liquid-pool expansion has already been given in Ch4_Theory_and_Experiment.

A rule-of-thumb is to keep the superficial velocity less than 0.5 feet/second or less than 0.15 meters/second and to keep the f-factor less than 0.1 (lb/ft)0.5 per second or less than 0.12 (kg/m)0.5 per second. These criteria should be kept in mind when scaling up a process.

For distillation columns the Souders-Brown criterion should also be consulted.

6. Interaction between Units

Flow A

Flow B

A

B

1 2

3

4

P P

P1 P2

P3

P2>P3>P1

Desire: P2>P3, P1>P3

SPEED

LEVEL

vessel A vessel B

Some avoidance measures

Adequately-sized common lines

Separate pumps when appropriate

Careful analysis of pressure balances – often overlooked due to preoccupation with chemistry, mass transfer, heat transfer

Provide buffer vessels to isolate units from one another

Don’t over-control one unit at the expense of another (tail wagging the dog)

or dispersed

Baker Plot -flow patterns for horizontal pipes

Low Viscosity Baker Plot

7. Liquid Hammer and Vibrations

Wake-shedding locks in to the natural vibration frequency of a heat-exchanger tube

Some avoidance measures

Consult Baker’s chart of two-phase flow regimes: avoid the slug conditions

Provide a low-point drain for vapour-line condensate

Provide the heat-exchanger vendor with flow information to allow calculation of vibration

8. Restrictions in Piping Systems

ExamplesPlant capacity increase is restricted by just-big-enough piping

Pressure drop of fittings and valves was under-estimated

Pipe got rougher because of erosion, corrosion or precipitation, causing friction factor to rise

A gravity flow is impeded by vapour entrainment

A gas flow, calculated as incompressible, is really at Mach number greater than 0.3

Onset of vaporization makes a liquid flow two-phase

Some avoidance measures

Err on the generous side in pipe sizing, unless settling is a problem; larger pipes also facilitate subsequent increase of plant capacity

Allow for fittings, valves, roughening

Consult criteria for gravity flow

Always use the compressible-flow frictional pressure-drop formula, which reduces to the incompressible case if Mach number is low

For a straight pipe in isothermal flow at any level of compressibility: P12 – P22 = (4fL/D) [(G2 RT)/(gc M)] { 1 + [2/(4fL/D)] Ln(P1/P2) }

For a straight pipe in isothermal incompressible flow:

P1 – P2 = (4fL/D) [G2 / (2 gc )]

where G is mass velocity in kg/s , M is molecular mass, f is fanning frictionfactor, L is length, D is diameter, R is the gas constant, T is temperature,gc is a conversion factor (1 for SI units), is fluid density, P is pressure

9. Scaling and Fouling

Examples

A heat transfer surface gradually becomes less effective because of scaling or fouling

A pipe experiences increasing pressure drop because of decreased diameter and increasing roughness

Some avoidance measures

Provide adequate heat-transfer area to minimize the need for high surface temperature

Provide for easy periodic clean-out of heat-exchanger tubes and of pipe runs

Consult references CEP (2001 May) p.74-77; Hyd.Proc (1999 June) p.113-117; Hyd. Proc. (1999 Jan.) p.93-95; Chem.Eng. (2002 dec.) p.46-49; CEP (2001 Nov.) p.30-37; Hyd.Proc. (2004 July) p.77-82

10. Static Build-up

Electrostatic charge can develop whenever work is done on a liquid or solid.

For a liquid the work may consist of forcing it through a pipe or a filter, agitating it, spraying it, letting it fall or settle.

For solid pellets or powders the work may consist of pneumatically transferring them through a pipe, blending them, grinding them, classifying them. Most of the hazard is eliminated by connecting vessels and piping to ground and by conductively bonding various parts of the system to one another.

Some avoidance measures

Ensure that filling systems operate at low velocity (< 3 ft/sec) if there is free fall of the fluid

Avoid mist-generating situations

Provide appropriate grounding and bonding.

For procedures where bonding is provided only periodically, allow time for the charge to ‘relax’

For particles or powders that have been in motion, provide a charge-dissipation step before loading into non-metallic containers.

Refer to API Recommended Practice 2003, “protection against ignitions arising out of static, lightning, and stray currents”