480 Operability OperWindow 2011

Process Operability Class MaterialsOperating Window

Copyright © Thomas Marlin 2013The copyright holder provides a royalty-free license for use of this material at non-profit

educational institutions

FC1

LC1

FC1

TC1

TC2

T10

T12

T11

T13

fuel

LC1

L2

LAHLAL

F4

Basic flowsheet Design with Operability

PROCESS OPERABILITY : THE OPERATING WINDOW

Key Operability issues

1. Operating window

2. Flexibility/ controllability

3. Reliability

4. Safety & equipment protection

5. Efficiency & profitability

6. Operation during transitions

7. Dynamic Performance

8. Monitoring & diagnosis

In this Lesson, we will learn

• What is an Operating Window? - Flash Drum, CSTR

• What defines the “Frame”?- Distillation

• How can we set equipment capacity (the operating window) to achieve desired operation?- Equipment capacity: Heat exchanger, pump- Alternative Equipment: Pump, flash

• How do we determine if operation is possible within the window?- Pump, distillation

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

F1

F2 F3

T1 T2

T3

T5

T4

T6 P1

L1

AC

L. Key

OPERATING WINDOW

The range of achievable steady-state operations. This is affected by manipulated and disturbance variables. The limitations can be due to equipment (e.g., maximum flow), safety, product quality, etc.

Flash Drum Example

-20

0

20

40

60

80

feed

tem

pera

ture

(C)

50 70 90 110 130 150 170 190

feed flow

Design

Minimum heating

Maximum liquid productvalveopening

Maximum heatingvalve opening

feasible


1. Operating window


3. Reliability






Define Oper. Window

OPERATING WINDOW

The variables in the plot can be

• Set points of controlled variables

• Disturbance variables

The frames (boundaries) of the window can be

• “hard” constraints that cannot be violated

• “soft” constraints than can be violated at a (usually large) economic penalty

Class Workshop: Determine the category for each of the constraints for the flash drum.


1. Operating window


3. Reliability






Define Oper. Window

-20

0

20

40

60

80

feed

tem

pera

ture

(C)

50 70 90 110 130 150 170 190

feed flow

Design

Minimum heating

Maximum liquid productvalveopening

Maximum heatingvalve opening

feasible

OPERATING WINDOW

Minimum heating valve opening is “hard”

Maximum feed valve opening is “hard”

Minimum feed valve opening is “soft”(The valve can be fully closed)


1. Operating window


3. Reliability






Define Oper. Window

OPERATING WINDOW

A B

-rA = k0 e -E/RT CA

feasible

infeasible

infeasible

TA

Reactant

Solvent

Coolant

Note:

This shows a range of set points that can be achieved (without disturbances).

Class Workshop: Discuss the operating window for this non-isothermal CSTR.

What do you note about the shapeof the operating window?


1. Operating window


3. Reliability






Define Oper. Window

OPERATING WINDOW



1. Operating window


3. Reliability






• We can determine the operating window using modelling (flowsheeting)

• If the plant exists, we could determine the operating window empirically (but maybe make off-specification products)

• The operating window is not always a polygon

• The operating window is not always 2-dimensional (can be much higher dimension)

• Operation can occur outside the window during transients (or when assumptions are violated)

Define Oper. Window

OPERATING WINDOW


Design Procedure

• Set goals and design specifications

• Select process technology

• Define process structure (sequence)

• Simulate the flowsheet

• Design equipment

The flowsheet typically involves basic M&E balances, equilibrium and rate processes. It does not consider practical issues for achieving the operation.

Equipment design achieves the base case flowsheet (plus other concerns). This sets the “capacity” of the plant.

The design must define the range of operations (set points and disturbances) to be achieved.

We can accept less than full production rate or top efficiency for extreme situations.

We must document specifications and range or operations and review with all stakeholders!

These “specifications” are in the

Design Basis Memorandum.

Define Oper. Window

OPERATING WINDOW

Determine the constraints (limitations) that define the frame (boundary) of the window

feasible

Process variable 1

Proc

ess

varia

ble

2


1. Operating window


3. Reliability






The frame defines the “size” of the operating window. These are typically physical bounds, equipment operation and stream specifications.

Define the frame

OPERATING WINDOW

Class Workshop: Determine typical constraints that affect the operating window for a distillation tower.

FR

FV

xB

xD


1. Operating window


3. Reliability






Define the frame

OPERATING WINDOW

Class Workshop: Distillation Constraints

FR

FV

xB

xD

Pumping, pipe, valve capacity

Maximum cooling capacity

Maximum and minimum liquid and vapor flow rates

Maximum and minimum liquid and vapor flow rates

Flow pipe, valve capacity

Maximum heating

Minimum natural circulation to reboiler

Product composition

Product composition


1. Operating window


3. Reliability






Define the frame

OPERATING WINDOW

The design specification will define a boundary of the operating window.

Heat exchanger Q = U AY (T)lm

What are the “worst case” operating conditions we would use to design the heat exchanger?

Hot process fluid into shell

Cooling water into tubes


1. Operating window


3. Reliability






The exchanger exists to cool this stream

Size of Oper. Window

OPERATING WINDOW

The design specification will define a boundary of the operating window – The Worst Case gives the largest area for heat exchange.

Hot process fluid into shell

Cooling water into tubes

Highest flow rate,Highest temperature

Lowest temperature

Lowest flow rate,Highest temperature

Greatest fouling,Lowest U

How do wedetermine

values?


1. Operating window


3. Reliability







OPERATING WINDOW

The design specification will define a boundary of the operating window.

Consider the flow system. What variables must we determine? What is the “worst case” we would use to design the system, specifically the required pump outlet pressure?


1. Operating window


3. Reliability







OPERATING WINDOW

The design will define a boundary of the operating window - Worst case gives the largest pump.

What variables must we determine?

- Pipe diameter - by guideline (Liq: 1 m/s, Gas: 30 m/s)- Pump horsepower - from highest flow rate and PP and

the lowest suction pressure

P

Highest vesselpressure

Highest pressure drop

Highest pressure drop

Highest flow,largest friction

factorLowest level (lowest head)


1. Operating window


3. Reliability







OPERATING WINDOW

In general, we want a large operating window. Why not always design and construct equipment with very large capacities?

Class Workshop: Complete the following table.

Advantages

Disadvantages

Small equipment*

Large equipment

Just satisfies base case


1. Operating window


3. Reliability





8. Monitoring & diagnosis * = small equipment just satisfies base case design point


OPERATING WINDOW


Advantages

Disadvantages

Small equipment

Low capital cost

Most efficient at base case

Achieve “precise” operation (smaller equipment to adjust)

Cannot achieve higher capacity

Cannot compensate for large range of disturbances

Cannot achieve fast transition (no overshoot in manipulated variable)


1. Operating window


3. Reliability







OPERATING WINDOW


Advantages

Disadvantages

Large equipment

High capital cost

Likely lower efficiency at base case and lower production rates

Might not achieve “precise” operation at base case

Can achieve higher capacity

Can compensate for likely range of disturbances

Can achieve faster transition (allows overshoot in manipulated variable)


1. Operating window


3. Reliability







OPERATING WINDOW

In general, we want a large operating window. Why not design and construct equipment with very large capacities?

So, we design plants that have “just the right” capacity in “the right places”. We have to consider the Boundaries and the Internal Points of the operating window.

The following class workshops demonstrate examples of equipment designs that achieve operability with acceptable cost through modest modifications to the process structure.


1. Operating window


3. Reliability







OPERATING WINDOW

Some designs increase the operating window

Centrifugal pumps - Configurations to increase the operating window

Series

Parallel

Pumps provide “pressure (head)” and “flow”. How do we select the correct option, if needed?

Flow rate

Hea

d

Typical pump head curve


1. Operating window


3. Reliability







OPERATING WINDOW

Some designs that increase the operating window

Centrifugal pumps - Configurations to increase the operating window

Series

Parallel

Series: This configuration provides higher pressure at (approximately) the same flow rate.

Parallel: This configuration provides higher flow rate at (approximately) the same pump exit pressure.


1. Operating window


3. Reliability







Feed

Vaporproduct


fluidSteam

F1

F2 F3

T1 T2

T3

T5

T4

T6 P1

L1

A1

L. Key

OPERATING WINDOW

Some designs that increase the operating window

The vapor flow rate is usually small. However, in some cases (e.g., start up) , it is 20 times more that its typical value. What do we do?


1. Operating window


3. Reliability







Feed

Vaporproduct


fluidSteam

F1

F2 F3

T1 T2

T3

T5

T4

T6 P1

L1

A1

L. Key

OPERATING WINDOW

The vapor flow rate is usually small. However, in some cases, it is 20 times more that its typical value. What do we do?

We provide a larger pipe and valve in parallel. The pressure control will adjust the small valve first, then the large valve.


1. Operating window


3. Reliability







OPERATING WINDOW

Determine whether the process and equipment function correctly everywhere within the window.

feasible

Process variable 1

Proc

ess

varia

ble

2


1. Operating window


3. Reliability






After the frame has been established, we check the internal points. Are there any “donut holes”?

“Holes” in Oper. Window

OPERATING WINDOW

Equipment must function correctly within the operating window

heating

FC

Velocity increases; Bernoulli says that pressure decreases

Cold (20C) liquid

Orifice meter


1. Operating window


3. Reliability







Any concerns about this design?

Porifice=P1 – P3

Distance

pres

sure

Sensors: Principles of the orifice meter

PorificeMeasure pressure drop

From: Superior Products, Inc. http://www.orificeplates.com/


Nice visual display of concept.

In practice, pressure difference is measured by a reliable and electronic sensor =

Porifice

Bernoulli’s eqn.

General meter eqn.

Installed orifice meter(requires density measurement)

0 = aver. density

C0 = constant for specific meter

Installed orifice meter(assuming constant density)

31 PPKF Most common flow calculation, does not require density measurement

v = velocity

F = volumetric flow rate

f = frictional losses

= density

A = cross sectional area

Relate the pressure drop to the flow rate

P

cooling

K

Take square root of measurement

Multiply signal by meter constant K FC

Measure pressure difference

“Measured value” to flow controller

When an orifice meter is used, the calculations in yellow are performed.

Typically, they are not shown on a process drawing.


liquid

General meter eqn.

v = velocity

F = volumetric flow rate

f = frictional losses

= density

A = cross sectional area

Relate the pressure drop to the flow rate

Cmeter

Reynolds number

We assume that the meter coefficient is constant. The flow accuracy is acceptable only for higher values of flow, typically 25-100% of the maximum for an orifice

Sensors: Are there limitations to orifices?

Porifice=P1 – P3

Distance

pres

sure

Sensors: Is there a downside to orifices?

What is a key disadvantage of the orifice meter?

Pressure loss!

When cost of pressure increase (P1) by pumping or compression is high, we want to avoid the “non-recoverable” pressure loss.

Ploss = P1 – P2

Non-recoverable pressure drop

OPERATING WINDOW


heating

FC

Velocity increases; Bernoulli says that pressure decreases

The fluid can partially vaporize. The pressure difference will

not reliability indicate the flow rate!

Cold (20C) liquid

Orifice meter


1. Operating window


3. Reliability







OPERATING WINDOW


heating

Simple solution, • Locate flow measurement where the pressure is highest and temperature lowest.

• Ensure that flashing does not occur - design calc’s

FC

Cold (20C) liquid


1. Operating window


3. Reliability







OPERATING WINDOW


Bottom tray

Bottoms product

reboilerCentrifugal pump

Any concerns about this design?

Hint: Describe the condition of the liquid in the bottom of the tower Bubble point

What happens when the pressure is reduced?


1. Operating window


3. Reliability







OPERATING WINDOW


Bottoms product

reboiler

Centrifugal pump

Pressure drop due to flow frictional losses

Pressure drop due to the velocity increase in the eye of the pump

What happensin the pump?


1. Operating window


3. Reliability







http://www.britannica.com/EBchecked/topic-art/632655/7035/Volute-centrifugal-pump

http://www.sprayingequipmentsupply.com/pumps/centrifugal-pumps.html

Basic concept of a centrifugal pump



http://bankofamerica.com/


Towler, G. and R. Sinnott (2008) Chemical Engineering Design, Elsevier-Butterworth-Heinemann, page 254

Constant speedImpeller

diameter


http://hiramada.wordpress.com/2009/07/07/introduction-to-centrifugal-pump-technical-selection/

OPERATING WINDOW


Bottoms product

reboiler

Centrifugal pump

Cavitation: The liquid partially vaporizes. As the pressure increases in the pump, the vapor is subsequently condensed. This collapsing of bubbles (cavitation ) causes noise, vibration and erosion - all of which are bad.


1. Operating window


3. Reliability







Let’s preventbubbles from

forming.

OPERATING WINDOW


Bottoms product

reboiler

Centrifugal pump This liquid head increases the pressure at the inlet to the pump and prevents cavitation.

NPSHR: The manufacturer must define the minimum net positive suction head required.

The process engineer must design to provide it. NPSHA>NPSHR


1. Operating window


3. Reliability







NPSHA

OPERATING WINDOW


NPSHR: The manufacturer must define the minimum net positive suction head required.

From: Woods, D.R., Process Design and Engineering Practice, Prentice -Hall, 1995

The process engineer must design to provide it. How?

This is issue when liquid is at (near) its bubble point. Give examples when this is the situation in chemical processes.

• Elevate the liquid above the pump (two ways)

• Reduce friction losses

• Subcool the liquid (careful of added pressure drop)


1. Operating window


3. Reliability







OPERATING WINDOW


From: Woods, D.R., Process Design and Engineering Practice, Prentice -Hall, 1995

This is issue when liquid is at (near) its bubble point. Give examples when this is the situation in chemical processes.

We deal with liquids at their bubble points often, for example,

• Distillation/stripper bottoms• Distillation/absorber condensers and

OH drums• Flash drums• Concentration by boiling• Vapor compression refrigeration • Reactor cooling by solvent vaporization


1. Operating window


3. Reliability







OPERATING WINDOW

Regrettably, no systematic method is used in practice

First, define the range over which the plant must operate. Consider most demanding conditions.

Second, solve flowsheet for the limiting cases

Third, design equipment to function for each of the limiting cases; may have to change structure.

Fourth, ensure that interior is operable.

Fifth, add features to achieve other operability features (on list at left), as needed

Fortunately, engineers have lots of relevant experience!

INDUSTRIAL PRACTICE


1. Operating window


3. Reliability






OPERATING WINDOW

INDUSTRIAL PRACTICE

SAFETY FACTORS: Couldn’t we just design for the base case and multiply every capacity by a safety factor, (1+ X/100) ? (X = 25%, 35%, 50%, …).

This is not engineering! Any single factor would be too small for some equipment and too large for others.

After applying the proper procedure, a small safety factor can be employed for modelling uncertainty, based on experience. Typical values are 10-15%.

“For well tested process, safety factors can approach 0%” ** Valle-Riestra, J.F. (Dow Chemical Co.), Project Evaluation in the Process Industries, McGraw-Hill, New York, 1983 (pg 209)


1. Operating window


3. Reliability



6. Operation during transients



OPERATING WINDOW

INDUSTRIAL PRACTICE

SAFETY FACTORS: Some “safety factor” is built into the design procedure. After we have calculated the required pipe diameter, valve diameter, vessel size, motor power etc., we purchase the closest available size.

Since the manufactured sizes are discrete, we select the next largest size.

This provides some safety margin.


1. Operating window


3. Reliability



6. Operation during transients



OPERABILITY : THE OPERATING WINDOW


1. Operating window


3. Reliability






In this Lesson, we will learn

• What is an Operating Window? - Flash Drum, CSTR

• What defines the “Frame”?- Distillation

• How can we set equipment capacity (the operating window) to achieve desired operation?- Equipment capacity: Heat exchanger, pump- Alternative Equipment: Pump, flash

• How do we determine if operation is possible within the window?- Pump, distillation

Documents

480 Operability OperWindow 2011