APC Heuristics for plantwide control.ppt

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Advanced Process Control

Heuristics for plant wide control – Ir Prof Law Chung Lim

12/15/2014 1Event Name and Venue



Learning outcomes1. A, PS – apply process control heuristics in design

2. A, PS – able to do overall plantwide control design with heuristics



1. Plumbing• Steady state simulation is flow-driven in which the effects of pressures on

flow are not considered

• It is essential to consider realistic fluid mechanics since changing

manipulated variables is vital in the evaluation of dynamic performance and

rangeability

• If a CV goes wide open during a disturbance, control in the loop is lost

• Designing with larger pressure drop over the valve increases the maximum

achievable flow rate – rangeability increases – able to handle larger

disturbances

1. Do not use 2 CVs in a liquid filled line2. Do not install a CV in the suction of a centrifugal pump

3. All lines connecting units operating at different pressures must have CVs

4. Control gas compressor with speed, bypassing or suction throttling

5. If HE bypassing is used, bypass the stream which temperature is to be

controlled



1. Plumbing



1. Plumbing



1. Plumbing



2. Recycle• The most challenging in process design as well as control design

• Recycle can slow down the dynamics of the overall process

• Process time constant can increase by an order of magnitude; can also

result in “snowballing”

• For gas recycle, run the recycle compressor at maximum (6.2.2.3) – thehigher the recycle the higher the selectivity

1. Effect on time constant

k F = k R = 0.9

If

τ = 2.8



2. Recycle2. Snowball effects

• A 20% increase in feed flow rate produces an increase in recycle flow rate

• Flow controlling the reactor effluent can prevent the snowballing

Simple process with recycle Flow controller in recycle loop



3. Fresh feed introduction• Due to the inability to measure the flow rate accurately, a slight inaccuracy

in flow rate may result in accumulation of certain reactant in the vessel,

causing accumulation and eventually shutdown

• Example A + B → C

• assume α A >α

C >α

B



3. Fresh feed introduction• Control structure for α A > αC > αB



3. Fresh feed introduction• Poor Control structure for α A > αC > αB

• Neither of these structures uses internal information to adjust the fresh feed

flow rates



3. Fresh feed introduction• Control structure for α A > αB > αC



4. Energy management and integration

• FEHE – feed effluent heat exchanger

• Note that a high temperature heat source would be required to start up the

system

• The furnace inlet temperature, Tmix is maintained by bypassing

• The reactor inlet temperature, Tin maintained by furnace fuel

• High temperature runaways is prevented by bypassing

• Low temperature quenching is handled by furnace duty



5. Controller tuning – flow control• Most flow controllers use orifice-plate pressure drop to infer flow rate and a

CV to change the flow

• All elements have fairly fast dynamics, so very tight flow control can be

achieved

• Integral time = 0.3 min

• Since pressure drop signals are sometime noisy, the controller gain should

not be large so that these fluctuations are not amplified in the CV position

• Controller gain Kc = 0.5



5. Controller tuning – pressure control• Most pressure controllers are not tuned for extremely tight control

• Objective is to provide smooth and gradual adjustment so that rapid swing

in manipulated variables (e.g. gas flow, cooling water flow) do not upset the

process

• Default setting in Aspen dynamics Integral time = 20 min, gain Kc = 5 work

quite well in most applications



5. Controller tuning – level control• Liquid surge capacity is used in drums and column sumps to smooth out

disturbances

• There is seldom a reason to hold these liquid levels precisely at a constant

value. As long as the level is not too close to the top of the tank or too close

to the bottom, operation is not affected

• However, for reactor the precise level is required as the reactor holdup

affects the reaction rate

• Heuristics for level control – use proportional control

• Recommended controller gain for surge tanks, Kc = 2

• For reactor liquid level, recommended gain is 5-10; depending on howsensitive conversion or yield are to holdup

• If PI is used, the integral action forces the level to be returned to its

setpoint. So, for certain period of time, outflow is greater than inflow. Thus

amplifies flowrate disturbances



5. Controller tuning – level control

If the level is kept

within a safe range

of level, p-controller

is OK for level

control

If PI level controller,

higher disturbances

is experienced inthe 2nd tank. If a

lower Kc is used,

the amplification is

even higherWhen h1 reaches max, F1 is 110 ft3/min;

thereafter, F1 increases and causes a

higher disturbance in the 2nd tank



5. Controller tuning – level control• Heuristics for level control – select the manipulated variable that has the

most effect on level

Both distillate flow and reflux flow affect

the reflux drum level

In columns with ratio greater than 3,

reflux should be used to control reflux

drum level



5. Controller tuning – composition and

temperature control• Critical for product quality and dynamic stability performance and must be

tuned for tight control

• Tuning must accurately reflect the real world in which measurement lags

always occur• So lags/dead times should be inserted for accurate/conservative tuning

• Temperature lags: 1 min; Composition lags: 3-5 min

• If we want tightest possible control and don’t mind swings and oscillations

in manipulated variable, Ziegler-Nicholas is recommended

• E.g. temperature control of exothermic reactor – tight temperature control isimportant, large changes in cooling water flow rate cause no problem

• If large and rapid changes in manipulated variable is not desired, Tyreus –

Luyben is recommended

• Tray temp control by manipulating reboiler heat input, large and rapid

changes in vapour boil-up could flood or dump the column



5. Controller tuning – interacting control loops• Tuning of multivariable interaction must account for the effects of one loop

to the other loop

• Recommendation – select the faster loop and

tune it (on automatic) first with the other loops on

manual; then tune the next fastest loop. Continue

until the slowest loop has been tuned with allother loops on automatic

• E.g. distillation column

• A temperature profile showing significant

changes at 2 locations so that dual-temperature

control structure can be implemented

• Temperature in rectifying section is controlled by

manipulating reflux

• Temperature in stripping section is controlled by

manipulating reboiler duty

• Vapour rate affect temperature throughout the column rapidly



6. Throughout handle• Conventional and straight forward way – flow control 1 of the feed streams

• Reactor

• Throughput can be set by adjusting conditions in the reactor that affects the

reaction rates

1. Temperature can be adjusted

2. Reactant concentrations can be adjusted by changing recycle flow rates

3. Setting a flow rate or heat removal or addition rate somewhere inside the

unit can establish the production rate

• Utility plant – serves as utility to a downstream customer

• Require an “on-demand” control structure

• Throughput handle is the setpoint of the flow controller on the product

stream. Therefore all inventory loops within the plant must be set up to

change flow rates from unit to unit back upstream through the process



Hydrodealkylation of TouleneProduction rate

benzene 265 lb-mole

/h

By-product 4.2 lb-

mole/h

Exothermic

Adiabatic tubular

reactor

Inlet 1150F

Outlet 1230F

Hot reactor effluent is

quenched to 1130F

Sep: vapour stream

H2 and CH4; some

purged, the rest

recycled

Liq fed to 3 columns

1st – 150 psia

Stabilizer column Benzene column Toulene column



Application of plantwide control heuristics• Throughput handle

• Maximum gas recycle

Reactor inlet temperature

has immediate and direct

effect on the reaction rate

Valve wide open, compressor

speed is maximised to keep h2-toulene ratio high to minimise

production of undesirable by-

product



Application of plantwide control heuristics• Component balances • Reactant and product

component flows enter and

leave the process are indicated

• Pressure drop in gas loop is

good indication of H2

consumed in rxn, PC adjusts

the flow rate of fresh H2

• Unconverted Toulene ends upin reflux drum of 3rd column. So

its liquid level provides good

indication of fresh toulene

needed

• All products have a way to get

out the system

• Some CH4 is removed in purge,where CC keeps CH4

concentration at some desired

level

• The rest of CH4 goes out the

top of 1st column

• Benzene from 2nd column

• Diphenyl from 3rd column



Application of plantwide control heuristics• Flow control in liquid recycle loop

• Product quality and constraint loop

• FC fixes total toulene fed into rxn

• 4 TC that maintain composition of

product streams and satisfy

process constraints

• TC in column 1 keeps CH4 from

dropping out the bottom and

affecting the purity of benzene

• The reflux-to-feed ratio in 1st

column is set to prevent losses of

benzene out the top

• TC in column 2 keeps benzene

from being lost out the bottom

• The reflux-to-feed ratio in column 2

is set to keep toulene purity

• TC in column 3 keeps toulene from

being lost out the bottom

• The reflux-to-feed ratio in column 3

is set to keep dyphenyl impurity in

the distillate below its spec

• Base level is controlled by reboiler

duty because flow rate of the

bottoms is very small

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