Operation during transitions

We will learn about operation during transitions

Transitions involve planned transient behaviour between different initial and final conditions.

Continuous processes• Startup and shutdown• Regeneration• Blocked operation • Load following

Batch Processes• No steady-state operation

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

1

Operation during transitions

Operation during transitionsStartup and shutdown

Special equipment and procedures are required for starting and stopping process operations.

• These are when most accidents and serious hazards occur - be very thorough in planning and training

• Need to load material during startup and drain material for shutdown.

• Need to heat and/or cool to approach normal process conditions or return to ambient.


1. Operating window


3. Reliability






2

FC1

Cold feed

Hot effluent

Cold product


1. Operating window


3. Reliability






3


Equipment and process structure: Identify extra equipment needed for startup of the exothermic reactor.

What mustbe added

to start up?How will wecontrol it?

Fast dynamicsHigh gainLinearSymmetricSafe

Equipment: Identify extra equipment needed for startup of the distillation column reboiler.

Bottom tray

Bottoms product

Thermsiphon reboiler


1. Operating window


3. Reliability






5


This is what the column operation looks like at full operation. But what about startup, when there is no vapour flow back up the column?

Bottom tray

Bottoms product

Thermsiphon reboiler

Valve is normally closed, opened only during startup

Before reboiler is functioning, no vapour flows occur, and the liquid weeps through trays

From: Lieberman, N.L., Process Design for Reliable Operations, Gulf Publishing, Houston, 1983


1. Operating window


3. Reliability






6


Equipment: Identify extra equipment needed for startup of the distillation column reboiler.


1. Operating window


3. Reliability






7


http://en.wikipedia.org/wiki/File:Thermosyphon_Reboiler.png http://www.thermopedia.com/content/1078/

Is the bypass valve required for these configurations?

Occasionally, equipment must be shutdown for maintenance and modifications. What is required?

Unit A

Unit B


1. Operating window


3. Reliability






8


Unit A

Unit B


1. Operating window


3. Reliability






9


Occasionally, equipment must be shutdown for maintenance and modifications. What is required?

Industrial practice• The first step is to prepare a detailed startup

(shutdown) procedure.

• Then, we check the availability of the appropriate equipment to perform the procedure

• This is very detailed work and requires considerable experience in plant operation and plant equipment

(Talk with operators and shift supervisors.)


1. Operating window


3. Reliability






13


Catalyst, adsorbents, fouled surfaces and some other equipment require periodic regeneration. This can involve different materials flowing through the process and even in opposite directions. The switching period varies from hours to months.

Example of regeneration include the following.

• Catalyst that loses activity

• Adsorbent that has active sites filled

• Equipment that has surface coated due to coke (reactor) or polymer (e.g. reboiler)

• Filter medium that must be backflushed (e.g. membrane)


1. Operating window


3. Reliability






14

Operation during transitionsRegeneration

Regeneration involves different operating conditions that can challenge equipment or be hazardous.

• Different feeds are required. Must have extra valves/ports.

• Hazards, such as introducing oxygen into an environment that normally contains hydrocarbons.

• Different operating conditions (e.g. high temperature steam is used)

• Special transition issues can involve corrosion, contamination, hygiene, toxicology, etc.


1. Operating window


3. Reliability






15


We don’t want to shut down the entire plant when regenerating an individual unit.

How can we best maintain the continuous process operation?

Parallel equipment with isolation valves

Single equipment with sufficient storage on both sides


1. Operating window


3. Reliability






16


Recall the one-way valve discussionfrom the prior section

C5+

C2H6

C2H4

C3H6

C4H10

Ethane

Propane

Gas oil

Feed stocks Fired heaters crack the hydrocarbons

Separation system

Example of olefins plant


1. Operating window


3. Reliability






17


The operation of the heaters (reactor feed type, feed rate, temp, steam) influence the rate of coke and need for regeneration.

Coke buildup: Requires periodic shutdown/decoke

Quench

http://www.tubetech.com/images/case-studies/images/581-3.jpg

C5+

C2H6

C2H4

C3H6

C4H10

Ethane

Propane

Gas oil

Feed stocks Separation system

Example of olefins plant

• What happens when one furnace temporarily stops production for decoking (with air and steam)?

• Which of previous strategies is employed (parallel or storage)?

• Significant hazards can occur during decoking


1. Operating window


3. Reliability






XAir and steam

X

Exhaust safely

18


Quench

Fired heaters crack the hydrocarbons

Tube wall temperature : reactor 2

860880900920940960980

100010201040

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

Time slots (3 days each)

Tem

pera

ture

(C)

Example trend for one furnace

• The operators monitor the tube wall temperature.

• Roughly every 45 days the furnace is decoked

= decoke


1. Operating window


3. Reliability






19



1. Operating window


3. Reliability






Example – heat exchanger is fouled and must be cleaned.

Hot process fluid (not a utility)

Col

d pr

oces

s flu

id

Since both streams are process fluids, by-passes on each are required. If one were a utility (e.g., water or steam), no by-pass on the utility stream would be required.

TC

21


Batch operation• Often the most economical method for manufacturing

small quantities and very high purities. Is generally too expensive for producing very large quantities of material.

• A batch plant usually produces numerous products

• Each product is manufactured in a separate “campaign” involving unique feed materials and processing conditions and shared equipment.

• The process followed is called a “recipe”

• Food, pharmaceuticals, “fine chemicals”, …


1. Operating window


3. Reliability






33

Operation during transitionsBatch operations


1. Operating window


3. Reliability






34


Batch operation: All materials provided at start of the process.

Semi-batch operation: Some materials introduced after the start of the process

E-1

P-1

P-2

V-1

V-3

P-3

P-4

feed

prod

uct

Rea

ctio

nin

itiat

or

E-1

P-1

P-2

V-1

V-3

P-3

P-4

feed

prod

uct

time

time

timetime

time


1. Operating window


3. Reliability






35


prod

uct

E-1

P-1

P-2

V-1

V-3

P-3

P-4

feed

Batch can have very different operating conditions. For example, it can be necessary to heat a reactor in the beginning of the batch and cool it thereafter.

time

time

time

Flow of heat transfer medium

= heating

= cooling


1. Operating window


3. Reliability






36


Reactor productflow

E-1

P-1

P-2

V-1

V-3

P-3

P-4

time

E-2

Some plants integrate batch and continuous units. The plant must contain storage capacity to allow the continuous parts to operate without frequent shutdowns.

Distillation feed

time


1. Operating window


3. Reliability






37


Feed must be constant!Batchwise operation

Industrial Practice

The operating conditions change during transients. For what operation do we “size” the equipment?

We must size equipment for the most demanding condition of all operations experienced by the process.

Never use the average operation, especially for these extreme transient operations.

It may be required to have parallel equipment with different capacities when the normal and maximum operations are very different.


1. Operating window


3. Reliability






38

Operation during transitionsAll situations considered


1. Operating window

2. Reliability



5. Efficiency

6. Startup & Shut down



DYNAMIC PERFORMANCE

Let’s recall that the process dynamics introduce the fundamental limit to feedback control performance.

Dead time in feedback,

Time constant in feedback, P

Time constant in disturbance, D

A

Steady-state Gain, KP

How does each parameter affect feedback performance?


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCE

Let’s recall that the process dynamics introduce the fundamental limit to feedback control performance.

0 5 10 15 20 25 30 35 40 45 500

0.5

1

1.5S-LOOP plots deviation variables (IAE = 9.6759)

Time

Cont

rolle

d Va

riabl

e

0 5 10 15 20 25 30 35 40 45 500

0.5

1

1.5

Time

Man

ipul

ated

Var

iabl

e

CV limited set point overshoot, fast damping, and return to the set point

CV does not change because of dead time

Set point response


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCE

Let’s recall that the process dynamics introduce the fundamental limit to feedback control performance.Can not prevent this deviation with feedback

Disturbance response


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCE

CLASS WORKSHOP: The feedback controller is performing well but the dynamic performance is poor. What improvements can we make?

AC1


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCE

CLASS WORKSHOP CV selection: Which variables must be controlled? Why?


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCE

CLASS WORKSHOP CV selection: Which variables must be controlled? Why?

Control unstablevariables. For this process, the levels are non-self-regulating.Control variables that “drift” rapidly to unsafe or highly undesirable values. The pressure is stable but changes quickly for changes in the reboiler and condenser duties.


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCE

CLASS WORKSHOP CV selection: Which variable(s) should be controlled? Why?

The bottoms product purity is important, but the cost of the on-stream analyzer is very high.

Is there an alternative CV?


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCE

CLASS WORKSHOP CV selection:

TC

Inferential/Dominant Variables - Easily measured and strongly influence product quality and/or profit.

In many (not all) distillation towers, tray temperatures are a good inference of product composition. (The profile moves up/down the column)


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCEClass Workshop on Control structure : These two controllers will interact strongly, What can we do?

Fuel gas distribution

PC TC

Fuel oil

Fired heater

Fluid to be heated by flowing in pipe through the fire box


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCESelecting Control Structure: For multivariable systems, options exist when determining the connection of CVs and MVs. These simple guidelines can be used in selecting loop pairing.Pair CV with an MV that gives

• Fast feedback response

• Strong effect (large gain)

• The most “direct” effect. This tends to reduce interaction among loops

• Large MV range of adjustment

• Least cost for adjustment

Caution: Theseguidelines are a good starting

point. But, theycan be violatedin real designs.


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCEWorkshop on Selecting Control Structure: Determine the loop pairings and sketch them on the drawing.

v1

Hot Oil

v2

v3

L1

v7

v5 v6

Hot Oil

F1 T1 T3

T2

F2

T4T5

F3 T6

T8

F4

L2

v8

T7

P1F5

F6T9

v4

CSTRFlashFeed

tank


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCEWorkshop on Selecting Control Structure: See Marlin (2000) for explanation of this and other designs.


1. Operating window

2. Reliability



5. Efficiency




DYNAMIC PERFORMANCE

INDUSTRIAL PRACTICE

• Good performance through process structure is more reliable and preferred if the cost is acceptable.

• We are guided by control objectives (safety, protection, smooth operation, product quality, and profit)

• We seek to provide good control with simple “loop pairing” control strategies, if possible.

• We control inferential/dominant variables, which can be reset in a cascade by higher levels.

Documents

Operation during transitions