process operability class materials operation during transitions copyright © thomas marlin 2013 the...

Process Operability Class MaterialsOperation during Transitions

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

educational institutions

FC1

LC1

FC

1

TC

1

TC

2

T

10

T

12

T

11

T

13

fuel

LC

1

L2

LAHLAL

F

4

Basic flowsheet Design with Operability

OPERATION DURING TRANSITIONS

We will learn about operation during transitions

Transitions involve planned transient behavior 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

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






SU & Shutdown

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

FC

1

Cold feed

Hot effluent

Cold product



1. Operating window


3. Reliability






SU & Shutdown

FC

1

Cold feed

Hot effluent

Cold product

Is the designcomplete?

Heating fluid

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



1. Operating window


3. Reliability






Need heating

when the reactor

effluent is cold.

SU & Shutdown

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

Bottom tray

Bottoms product

Thermsiphon reboiler



1. Operating window


3. Reliability






SU & Shutdown

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

Bottom tray

Bottoms product

Thermsiphon reboiler

Valve is normally closed, opened only during startup

Before reboiler is functioning, no vapor flows, liquid weeps through trays

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



1. Operating window


3. Reliability






SU & Shutdown

Process flow: Occasionally, equipment must be shutdown for preventative maintenance and modifications. What is needed?

Unit A

Unit B



1. Operating window


3. Reliability






SU & Shutdown


Unit A

Unit B

• Cooling

• Storage

• Pumping

• Heating

Discuss advantages and disadvantages. How big is the tank?



1. Operating window


3. Reliability






SU & Shutdown


Unit A

Unit B

• Cooling

• Storage

• Pumping

• Heating

Cooling and subsequentheating and coolingis inefficient.

Let’s by-pass the tankwhen possible.

The tank must have a holdup time (V/F) equal to at least the unit shutdown time.

The inventory can be adjusted just before the shutdown.



1. Operating window


3. Reliability






SU & Shutdown

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






SU & Shutdown

OPERATION DURING TRANSITIONSREGENERATION

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)

• Filtration that must be backflushed


1. Operating window


3. Reliability






Regeneration


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

• Different feeds

• Exothermic reactions

• Different operating conditions

• Different downstream processing

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

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


1. Operating window


3. Reliability






Regeneration


What is an important factor in defining the structure for continuous processes with regeneration?

How can we best maintain the continuous process operation?

Parallel equipment with isolation valves

Single equipment with sufficient storage

Or, must we shutdown the entire plant when regenerating an individual unit?


1. Operating window


3. Reliability






Regeneration

C5+

C2H6

C2H4

C3H6

C4H10

Ethane

Propane

Gas oil

Feed stocks Fired heaters Separation system


Example of Olefins plant

Coke buildup: Requires periodic shutdown/decoke

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


1. Operating window


3. Reliability






Regeneration

C5+

C2H6

C2H4

C3H6

C4H10

Ethane

Propane

Gas oil

Feed stocks Fired heaters 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)?

• Might significant hazards occur during decoking?


1. Operating window


3. Reliability






XAir and steam

Regeneration

X

Exhaust safely

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

per

atu

re (

C)


Example trend for one furnace

• What affects the slope of temperature vs. time?

• What happened at days 45 and 97?

• How do we select the best operations?

= decoke


1. Operating window


3. Reliability






Regeneration



1. Operating window


3. Reliability






Regeneration

TC

Example – heat exchanger is fouled and must be cleaned.

Process fluids

Pro

cess

flu

id

What equipment is required to be able to take this exchanger out of service for cleaning or repair?



1. Operating window


3. Reliability






Regeneration

Example – heat exchanger is fouled and must be cleaned.

Process fluids

Pro

cess

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

OPERATION DURING TRANSITIONSBLOCKED OPERATION

“Blocked operation” involves short periods of continuous, steady-state operation with frequent switches. This policy is required when many products are produced using the same equipment.

Usually, the material produced during the transition has lower value, or in some cases, zero value. Thus, transitions should be expedited.

To satisfy customer demands, product must be stored since no one product is produced continuously at all times.


1. Operating window


3. Reliability






Blocked operation

OPERATION DURING TRANSITIONSBLOCKED OPERATION- Lube Oil Manufacture


1. Operating window


3. Reliability






Blocked operation

Solvent recovery and by-product processing

Solvent

Only one “base stock” manufactured at a time

Only one “base stock” processed at a time; switch about every two days

Subsequent processing

OPERATION DURING TRANSITIONSBLOCKED OPERATION

Process issues related to blocked operation.

• Reduce production rate and mixing during transition

• Recycle “mixed” material during transition or store for later re-processing

• Transitions from/to some operations are not possible (two phase become miscible, hazardous intermediate state, intermediate product has very low or negative value, etc.)

• Desire very fast transition – fast process dynamics with “strong” manipulated variables


1. Operating window


3. Reliability






Blocked operation

OPERATION DURING TRANSITIONSLOAD FOLLOWING

Some parts of a plant produce material that is required for production elsewhere, and the other section(s) of the plant determine their needs independently. The supplier must satisfy the demands.

Typical utilities are steam, nitrogen, oxygen, hydrogen, (processed) water.

Also, some process materials are manufactured for the process and processed without storage, often to reduce hazards.

Often, many consumers are in simultaneous operation and have time-varying demands. The produce must “follow the load” or demand in a timely manner.


1. Operating window


3. Reliability






Load following

OPERATION DURING TRANSITIONSLOAD FOLLOWING – BOILER & STEAM SYSTEM

B1 B2 B3 B4

HP

1

T2 MPproducer

HPconsumer

MP

LP

Cond

T3 T1 T4

T5MPconsumer

LPproducer

LPexport

2 3

4 5

8

9

7

10

6

11

12

1314

15

Fuelgas

Fueloil

A utilities example is shown below, with the fired boilers (B1-B4) and heat integration (producers) providing steam for power (turbines T1-T5) and heating (consumers).

High pressure

Medium pressure

Low pressure

Liquid


1. Operating window


3. Reliability






Load following

Boilers


Some issues for load following utilities

• Operating Window – Have sufficient capacity to satisfy demand. How do we know the demand?

We could measure every demand, sum them and produce that amount of steam

Bad idea: Never achieve material balance because measurement errors accumulate.

We could measure steam pressure and control it by adjusting steam generation.

Good idea: Simple and effective. No error accumulation.


1. Operating window


3. Reliability






Load following

PC

PYx

PYx

PYx

PYx

We adjust the ratios to lower fuel cost; fast pressure control is not affected.



1. Operating window


3. Reliability






Load following


B1 B2 B3 B4

HP

1

T2 MPproducer

HPconsumer

MP

LP

Cond

T3 T1 T4

T5MPconsumer

LPproducer

LPexport

2 3

4 5

8

9

7

10

6

11

12

1314

15

Fuelgas

Fueloil

An example is shown below, with the fired boilers (B1-B4) and heat integration (producers) providing steam for power (turbines T1-T5) and heating (consumers).

High pressure

Medium pressure

Low pressure

Liquid

What is the correct

response when this consumer requires

more steam?


1. Operating window


3. Reliability






Load following



• Reliability – Network to supply any demand from any supplier.

If Boiler 1 fails, are we sure that we can increase the others sufficiently?

We cannot start a “cold” boiler in time to keep plant in operation.

We may have to keep one or more boilers “warm”, even if not producing significant steam.

Must have spare capacity; all boilers in operation should not be near their maximum steam productions!


1. Operating window


3. Reliability






Load following



• Efficiency – Ability to use the most efficient producers as the demand changes

What data do we need to optimize the boiler load?

Boiler 1

0.80.820.840.860.880.9

0.920.940.96

0 50 100 150 200 250 300

Steam Load (1000 lb/h)

Eff

icie

ncy

(fra

ctio

n)For each boiler, we need a model relating efficiency to the steam generated (load).


1. Operating window


3. Reliability






Load following



• Transient - May require storage of material for startup

How do we store steam?

We don’t. We must respond rapidly!


1. Operating window


3. Reliability






Load following

OPERATION DURING TRANSITIONSBATCH OPERATION

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

• Food, pharmaceuticals, “fine chemicals”, …


1. Operating window


3. Reliability






Batch Operation


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

product

Reaction

initiator

E-1

P-1

P-2

V-1

V-3

P-3

P-4

feed

product

time

time

timetime

time


1. Operating window


3. Reliability






Batch Operation


productE-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

Flo

w o

f he

at tr

ansf

er m

ediu

m

= heating

= cooling


1. Operating window


3. Reliability






Batch Operation


Reactor product

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






Batch Operation

OPERATION DURING TRANSITIONSALL SITUATIONS CONSIDERED

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






Batch Operation

OPERATION DURING TRANSITIONSBATCH OPERATION – GASOLINE BLENDING


1. Operating window


3. Reliability






Reformate

LSR Naphtha

N-Butane

FCC Gas

Alkylate

Final BlendFT

AT

FC

FC

FC

FC

FC

Intermediate products from process

http://www.mcscbahrain.com/projects.htm

Products to customers

Manufactured continuously

One blending process

Many different customers

additives

Batch operation

Regular

Premium

Final Blend

••

OPERATION DURING TRANSITIONSALL SITUATIONS CONSIDERED

The operating conditions change during transients. The control system must follow the desired path.

E-1

P-1

P-2

V-1

V-3

P-3

P-4

time

Tem

pera

ture

Challenges remain

1. Determine the best (optimal) transient behavior

2. Achieve good process control over a wide range for a non-linear process


1. Operating window


3. Reliability






Batch Operation

OPERATION DURING TRANSITIONS

We will learn about operation during transitions

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

Continuous processes

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

Batch Processes

• No steady-state operation


1. Operating window


3. Reliability






process operability class materials operation during transitions copyright © thomas marlin 2013 the...

Documents

transitions transitions

transitions startup

extra equipment

shutdown special equipment

operation duri

safety equipment protection

operability slide

heating fluid equipment