Group AJaninne

LizSteveJaniceAndrea

A distillation column was being recommissioned following a routine shut down. The plant operators began to slowly, bring the unit up to normal operating conditions. At 7:00am the following morning there was a violent explosion and subsequent fire. The explosion was heard and felt up to 30km away.

Applying the principles learned throughout the course of this presentation we will discover how this explosion could have been prevented.

Source: (www.hse.gov.uk, 2002)

“OVER 40% OF THE ENERGY USED ANNUALLY IN CHEMICAL PROCESS INDUSTRIES IN THE UNITED STATES GOES TO DISTILLATION PROCESSES ”*

*Source: (Sabarathinam, 2002)

How Much Energy is that?

2,499,743,943,870,000,000 Joules

• That is enough energy to run around the world a trillion times!!!!

• In fact, distillation accounts for almost 15% of all industrial energy consumed in the United States!!!*

*Source: (Sabarathinam, 2002)

How much is Distillation Used?

• Worldwide, distillation accounts for 95% of all separation processes*

• In the United States alone how many columns do you think exist?

*Source: (Sabarathinam, 2002)

40,000+

How much money is locked How much money is locked up in distillation operations?up in distillation operations?

• In the United States:In the United States:

Over $8 billionOver $8 billion

THAT’S $12.8 BILLION CANADIAN!!

When is Distillation Used?*

• When the relative volatility is greater than 1.5

• When the product is not temperature sensitive

• When there are negligible solids in the feed

*Source: (Woods, 1995)

Two-Component Distillation:

The diagram below displays a typical 2 component distillation column we have seen so many times before……… 

Source: http://lorien.ncl.ac.uk/ming/distil/distileqp.htm  

Distillation is a combination of mass transfer, heat transfer, fluid mechanics, thermodynamics, and of course….Process Control!!!!

2A04 – Heat Exchangers

3M04 – Tray Selection/Design

3D03 – Component Equilibrium

3004 – Fluid Flow Properties

3P03 – System Operation

Interesting Equipment….

• ReboilersReboilers

• Purpose?

– Essentially act as a heat exchanger which brings the liquid at the bottom of the column to its boiling point, and returns vapour back into the column.

• Types:– Thermosyphon Reboiler– Kettle Reboiler

Kettle Reboiler (Gravity-fed)Kettle Reboiler (Gravity-fed)

1. Liquid from tower is partially vapourized2. Domed section of the reboiler separates liquid and vapour 3. Vapour flows back to the tower 4. Liquid overflows the baffle

Source: http://lorien.ncl.ac.uk/ming/distil/reboil.htm

Thermosyphon Reboiler:Thermosyphon Reboiler: 

•Driving force to promote flow through the reboiler is a density difference between the reboiler feed line and the reboiler return line  

Source: http://lorien.ncl.ac.uk/ming/distil/reboil.htm  

Operability – Operating Window In the operation of a distillation tower, narrow limits are imposed to avoid inefficiency.

Inefficiencies include:• flooding • weeping • reboiler and condenser violations Response to these violations act to:• protect the equipment• protect distillation tower against overpressure,

overtemperature and overflow• achieve desired separation

1. Locating constraints within the distillation column

2. Provide protection against violations

3. Permit operation to proceed in the presence of violations by remaining inside the operating window

The Three Objectives of Operability in Distillation are as follows:

Operability

Locating Constraints within Distillation Column:

1. Tray

2. Reboiler

3. Condenser

Operability – Operating Window

TRAYS• Liquid falls through the

downcomer

• Outlet weir ensures a set liquid height (hold up) on tray

• Trays act to provide separation of components within the distillation tower

Typical Sieve Tray Construction

Downcomer

Tray Deck

Weir

vapour

Tray EfficiencyVapour flow conditions effect tray efficiency

Tray efficiency is a measure of deviation from vapour liquid equilibrium

Tray efficiency can be inferred by comparing vapour temperature leaving a tray to liquid temperature leaving a tray

100% Tray Efficiency 0% Tray Efficiency

450ºFVapour

400ºFVapour

350ºFLiquid

400ºFVapour

400ºFVapour

350ºFLiquid

BAD

Implication of Tray Efficiency

WEEPING CAN OCCUR

• occurs at low vapour flow rates causing liquid on the tray deck to flow through the tray perforations rather than the downcomer

What could cause weeping?

Implication of Tray EfficiencyFLOODING CAN OCCUR• Due to high vapour flow rates or• Due to high liquid flow rates

In both cases this causes liquid to accumulate on the tray deck which results in the back up of liquid in the downcomer

What could cause flooding?

Distillation Column Operating Window

• Column pressure is the one of the major constraints in distillation operation

• Column pressure is typically controlled by adjusting the heat removal in the condenser

Overall distillation operation is determined by:

1. Trays which define separation efficiency

2. Condensers which define column pressure

3. Reboilers which define the vapour flow conditions and drive the distillation tower by supplying sufficient amount of vapour

Distillation Column Operating Window

• In normal operation, there are two primary “handles” that can be adjusted to manipulate the behaviour of the distillation column (assuming pressure and level control)

1. Condenser Duty (reflux flow)

2. Reboiler Duty (boilup flow)

Steering the Distillation Column

Feed

Distillate

BottomsReboiler

CondenserPC

LC

LC

Reflux Drum

TC

P

T

Flare

F

T

T

dP

Distillation Column Operating Window

Vessel Pressure limit

Weeping

Steam condensate capacity

Flooding

Con

dens

er

Maximum production

ReboilerFour Possible Violations that can Occur:1. Weeping2. Flooding3. Condensing4. Pressure Vessel

Interested in frame of window in order to design the towers and operate them efficiently and troubleshoot them accordingly

Weeping Violation

Weeping

Steam condensate capacity

FloodingC

onde

nser

Maximum production

Vessel Pressure limit

ReboilerIncreasing reboiler duty at a constant pressure.

Column Pressure

Boi

lup

Rat

e

Case 1: Weeping

Feed

Distillate

BottomsReboiler

CondenserPC

LC

LC

Reflux Drum

TC

P

T

Flare

T

T

dP

T

T

dP

Temperature indicates separation efficiency. A change in differential pressure could be an indication of weeping

Increasing the steam rate will increase the boilup and pressure rising through the column, in order to hold up the liquid on each tray

Flooding Violation

Weeping

Steam condensate capacity

FloodingC

onde

nser

Maximum production

Vessel Pressure limit

ReboilerDecreasing reboiler duty at a constant pressure.

Column Pressure

Boi

lup

Rat

e

Case 2: Flooding

Feed

Distillate

BottomsReboiler

CondenserPC

LC

LC

Reflux Drum

TC

P

T

Flare

F

T

T

dP

Temperature indicates separation efficiency. A change in differential pressure could be an indication of flooding. The flow sensor indicates a loss of liquid level in the bottom of the column.

Decrease reboiler duty in order to reduce the vapour being condensed. This allows the column to reach equilibrium.

T

T

dP

F

LC

Condensing Violation

Weeping

Steam condensate capacity

FloodingC

onde

nser

Maximum production

Vessel Pressure limit

Reboiler

DO NOTHING!

Column Pressure

Boi

lup

Rat

e

Case 3: Not Enough Condensing

Feed

Distillate

BottomsReboiler

CondenserPC

LC

LC

Reflux Drum

TC

P

T

Flare

dP

LC

The level control will indicate a high level in the reflux drum and condenser.

DO NOTHING:The high level of vapour will increase the column pressure overtime. The column will correct itself using the pressure and level controls.

Vessel Pressure Violation

Weeping

Steam condensate capacity

FloodingC

onde

nser

Maximum production

Vessel Pressure limit

Reboiler

Decreasing reboiler duty.

Column Pressure

Boi

lup

Rat

e

Case 4: Activated Safety Valve

Feed

Distillate

BottomsReboiler

CondenserPC

LC

LC

Reflux Drum

TC

P

T

Flare

dP

P PC

The vessel pressure limit is exceeded as indicated by the pressure sensors and the activation of the safety valve.

Decrease reboiler duty in order to produce less vapour and decrease the column pressure.

Main Safety Concerns

• Fire Risk– Dealing with flammable liquids– Need to isolate system in case of fire

• Over Pressuring– Occurs when the condenser is restricted– Must provide pressure relief

Safety Equipment• Relief Valves

• Corrosion Monitoring– X-Ray Techniques– Radioactivity– Ultrasonic Testing– Corrosion Coupons– Corrosion Probes

• Alarms and Trips– Fail position of valves

Points of Control

• Feed Rate and Production Rates

• Temperature

• Pressure

• Level in Reflux Drum and Column Bottom

• Coolant Flow and Steam Flow

• Reflux Rate

7 Control Objectives

1. Safety

2. Environmental Protection

3. Equipment Protection

4. Smooth Operation and Production Rates

5. Product Quality

6. Profit

7. Monitoring and DiagnosisSource: Process Control: Designing Processes and Control Systems for Dynamic Performance, Marlin 2002.

1. Safety

Feed

Distillate

BottomsReboiler

Condenser

FCFC

PC

LC

LC

Reflux Drum

TC

P

PAH

LAH LAL

T

Flare

dP

Pressure control and relief in case of condenser restriction. Alarms warn of potential hazards.

PC

Flare

LAH LAL

PAH

2. Environmental Protection

Feed

Distillate

BottomsReboiler

Condenser

FCFC

PC

LC

LC

Reflux Drum

P

PAH

LAH LAL

T

Flare

dP

Avoiding flaring by it in emergency situations protects the environment for undue contamination

3. Equipment Protection

Feed

Distillate

BottomsReboiler

Condenser

FCFC

PC

LC

LC

Reflux Drum

P

PAH

LAH LAL

T

Flare

dPdP

Uses pressure drop to determine if corrosion is present. Tray temperatures can also be used.

4. Smooth Operation

Feed

Distillate

BottomsReboiler

Condenser

FCFC

PC

LC

LC

Reflux Drum

P

PAH

LAH LAL

T

Flare

dP

LC

LC

Levels are non-self regulatory and therefore need to be controlled. These level controllers will allow for stable operation.

How do we Control Product Quality?

• What variables do we want to control?

• What knobs do we have to control quality?

• Are there any limits to this control?

5. Product Quality

Feed

Distillate

BottomsReboiler

Condenser

FCFC

PC

LC

LC

Reflux Drum

TC

P

PAH

LAH LAL

T

Flare

dP

Controls distillate composition using distillate rate or reflux rate. Controls composition of the bottoms product using the temperature as an inferential control variable

TC

FC

Inferential Control• Use a measurable variable to infer a needed

variable

• Design Criteria– Measurement of a true controlled variable

– Inferential variable available

– Relationship between inferential and measured variable insensitive to operational changes

– Dynamics favourable for feedback control(Process Control, Marlin 2002)

• In this case using Temperature to infer Composition and therefore required heating.

TRAY TEMPERATURE VERSUS ANALYZERS

Tray Temperature Analyzers

• Product quality not stringent •Product quality important

•Less costly •Costly (requires more equipment)

•Allows for quick analysis •Dead time encountered in transporting a sample to chromatograph detector

•Cannot directly measure concentration

•Directly measures concentration of several components in a mixture to achieve accuracy and control

6. Profit

7. Monitoring and Diagnosis

• This depends on plant wide operations– Energy Use– Production Rates and Requirements

• Covered in the Operability Section

Troubleshooting: Class Exercise• During re-commissioning of plant PRV was

removed for re-rating.

• Plant operators began to slowly bring unit up to normal operating conditions

• At 5:00am you receive a page that the High Pressure Alarm has been activated.

We have 2 hours to prevent the explosion

Troubleshooting Symptoms

• Pressure is High on all sensors

• High level in reflux drum

• Reflux valve fully open

• Feed Flow rate at half of normal operations

• Higher than normal temperature within column

Let’s Find the Solution

Troubleshooting Diagram

Feed

Distillate

BottomsReboiler

Condenser

FCFC

P

LC

LC

Reflux Drum

TC

P

PAH

LAH LAL

T

dP

Steam What is the Solution?

Troubleshooting: Solution

• Pressure Relief Valve was removed• Flow Rate too low for the amount of heat transfer

occurring• Too much vapour is being produced in the

column, leading to condenser restriction• Reduce Boilup• Long term: A bypass on the steam flow is

required to avoid overheating/excessive vapourization