Ch E 452: Process Design, Analysis, and Simulation Regulating Process Conditions

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Ch E 452 Process Design,Analysis, and
SimulationRegulating Process Conditions

• David A. Rockstraw, Ph.D., P.E.
• New Mexico State University
• Chemical Engineering

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Regulating Process Conditions

• In most cases, processes are regulated, either
  directly or indirectly, by the manipulation of
  the flowrates of process and utility streams.
• Changes in flowrates are achieved by opening and
  closing valves.
• Either pressure OR flowrate (not both
  simultaneously) can be regulated by altering the
  setting of a single valve.

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Ball Valve

• versatile
• often used in slurry flows
• provide speed of operation
• up to 4 diameter line
• fully open/closed by ¼ turn
• expensive
• small pressure drop (straight through hole)

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Ball Valve
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Gate Valve

• provide almost full flow
• minimum turbulence and fluid trapping
• minimal pressure drop
• used where operation is infrequent
• 100 seal
• packing wears out, high maintenance
• atmospheric leaking
• threaded stem, more cumbersome to open
• never used for throttling

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Gate Valve
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Globe Valve

• used most often for throttling or modulation
• disc and seat material must be compatible with
  fluid in service
• inexpensive, but high pressure drop

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Butterfly Valve

• fully open/closed with ¼ turn
• best suited to low pressure, low flow service
• sediment buildup usually minimal (due to narrow
  body design)
• installed in up to 48 pipe
• no packing to wear out, no atmospheric leaking
• not a positive seal except on water or steam lines

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Butterfly Valve
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Check Valve

• prevents backflow by automatically seating when
  flow direction reverses
• three basic types swing, lift, and ball
• fluid velocities should generally be low and
  nonpulsing
• installed as far as possible from pumps

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Check Valve
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Control Valve (CV)

• A CV is operated by an automated actuator to
  maintain a process variable at a set point. A CV
  consists of the body, bonnet, packing, trim,
  actuator assembly.
• Actuators come in many types. Typical ¼? valve
  actuator
• is pneumatically operated, spring opposed
• operates in response to a 12 psi range change in
  instrument signal
• full range signal change causes the valve to
  stroke a distance of about ½
• standard spring has a deflection rate of 25 lbs
  per 1/8
• spring operates against a diaphragm with an
  effective area of 7½
• can be operated as air-to-close (ATC) or
  air-to-open (ATO)

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Control Valve (CV)

• A control valve trim refers to the inner valve
  set consisting of the plug, stem and seat. The
  trim is the device that is positioned in the flow
  path to adjust rates. The selection of valve
  trim size is a key element in obtaining a control
  valve that will provide stable operation in the
  process.

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Control Valve (CV)
bonnet
valve body
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Sizing CV Trim

• Calculate trim coefficient

FL ? liquid flow rate (gpm) FG ? gas flow rate
(SCFH) FS ? steam flow rate (PPH) s ? sp. gravity
relative to air or water DP ? pressure drop (P1 -
P2, psi) P1 ? upstream pressure (psia) P2 ?
upstream pressure (psia) V ? sp. volume of
upstream steam T? temperature
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Sizing CV Trim

• Compare calculates trim coefficient to
  manufacturers selection chart
• Shown is the trim size chart of Badger Valve
  (Tulsa, Oklahoma)

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Temperature Measurement

• Thermocouples wells
• High Temp Fiber Optics
• Infrared Thermometers
• Thermister

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Measuring Process Variables

• Pressure
• Differential pressure (dP cell)
• Strain gauge
• mV output transducers
• Load cells
• Current output transducers

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Measuring Process Variables

• Flowrate
• Orifice and venturi (dP)
• Mass
• vortex shedding
• Magnetic
• Ultrasonic (Doppler)
• turbine

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Measuring Process Variables

• Liquid Level
• Float valve
• dP Cell
• Ultrasonic

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Measuring Process Variables

• Composition/Physical Properties
• Concentration
• pH (probe)
• Chromatograph (gas, liquid)
• Spectroscopy (UV/Vis, FTIR, NIR)
• Conductivity (probe)
• Physical Properties
• Density (Berthold Technologies)
• Thermal Conductivity (Hukseflux)

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Chapter 19, Problem 4
Explain the operation of the reactor feed heat
exchanger in the DME Process of Figure B.1.1
If T5 increases, the TIC responds by opening the
control valve, reducing the amount of Stream 6
diverted to E-202, and thus T5 decreases. For a
decrease in temperature, the reverse process
occurs.
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Chapter 19, Problem 4
How would this system respond to fouling in the
heat exchanger or to a loss in catalytic activity
in R-201?
Fouling The overall heat transfer coefficient
(U) is reduced. Since the amount of heat is Q
UAFDTlm, the only way to affect Q is to increase
DTlm. This can be accomplished by increasing the
flow of Stream 6 (closing the valve). If fouling
becomes excessive, the valve will completely
close, and further corrective action by the valve
will be lost.
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Chapter 19, Problem 4
How would this system respond to fouling in the
heat exchanger or to a loss in catalytic activity
in R-201?
Catalyst Decay loss in catalyst activity
results in a decrease in conversion. Exit
temperature will slowly drop (Stream 6). This
results in a drop in T5, and will initiate the
TIC to slowly close the CV. The net effect is
increased flow through E-202, and an increase in
U. This has a further effect downstream since
the concentration of DME changes, affecting
operation of columns T-201 and T-202.
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Chapter 19, Problem 4
What type of control strategy is being used in
this system?
Feedback Control because the control action is
initiated by changing an input variable (amount
of flow of Stream 6 through E-202) upon measuring
a deviation in the output variable (T5 leaving
E-202).
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Chapter 19, Problem 4
to R-201
from R-201
5
6
TIC
Design a control system that would regulate the
exit temperature T6 rather than the inlet
temperature T5.
E-202
Direct Method move bypass to inlet.
7
4
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Chapter 19, Problem 4
to R-201
5
from R-201
Design a control system that would regulate the
exit temperature T6 rather than the inlet
temperature T5.
6
E-202
TIC
Direct Method move bypass to inlet. This
method gives us the further advantage of being
able to directly control the conversion in the
reactor since T6 is a direct indication of
conversion.
7
4
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Chapter 19, Problem 8

• R-901
• liquid phase, exothermic reaction
• feed in stream 1 is known to vary
• heat of reaction removed by E-901

Regulate outlet temperature
1
Regulatereactor inventory
R-901
E-901
Compensate for a change in flowrate
P-901 A/B
2
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Chapter 19, Problem 8

• R-901
• liquid phase, exothermic reaction
• DHrxn partially vaporizes contents
• Vapor condensed in E-901 and returned to R-901
• R-901 operates at boiling point of contents

Regulate reactor temperature
E-901
1
R-901
Regulatereactor inventory
Compensate for a change in flowrate
2
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Distillation Control

• Distillation column control systems -
• designed to produce a product stream which has a
  specified composition and neither exceeds nor
  falls short of this mark (maintain quality
  specifications)
• assure column operation remains within the
  operating limits
• prevent flooding, slugging, excessive weeping,
  and dumping
• adjust conditions to protect components (e.g.,
  limit outlet temp of coolant to minimize fouling
  in condenser tubes)
• Manipulated Variables
• rate of (1) distillate, (2) reflux, (3) bottoms,
  (4) reboiler duty
• one selected as primary manipulated variable to
  achieve quality objective
• others required maintain operating constraints of
  liquid levels
• Controlled variables
• selected temperature, accumulator level, reboiler
  level

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Column Pressure Control

• Column pressure intimately tied to column energy
  balance
• if more liquid is vaporized in the reboiler than
  is condensed in the overhead condenser,
  accumulation of vapor causes increase in column
  pressure
• pressure control achieved by manipulating vapor
  holdup
• for columns with vapor products, pressure is
  controlled by throttling

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Column Pressure Control

• columns w/ total condensers, rate of vapor
  condensation adjusted by
• 1. changing condenser cooling flow (fouling when
  water temps exceed 50C)

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Column Pressure Control

• columns w/ total condensers, rate of vapor
  condensation adjusted by
• 2. injecting noncondensable gas into overhead
  (can effect temperature of top trays may be
  necessary to vent results in containment issues
  and product loss)

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Column Pressure Control

• columns w/ total condensers, rate of vapor
  condensation adjusted by
• 3. bypassing vapor around the condenser

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Column Pressure Control

• columns w/ total condensers, rate of vapor
  condensation adjusted by
• 4. using a flooded condenser

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Conventional Control Configurations

• 24 possible configurations of the 3 loops and 1
  free variable
• many dismissed quickly (i.e., loop includes
  entire column between controlled variable and
  manipulated variable - large time delay and large
  ? in transfer function ? sluggish operation)
• 8 possible acceptable configurations (2
  non-conventional)

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direct material balance control
composition controlled by manipulating flow rate
of a product stream
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indirect material balance control
initial action taken to control composition
manipulates the internal recycle flow by changing
the reflux rate or the reboiler duty, which
changes the boil-up rate. Subsequent actions by
other loops affect overall material balance of
column.
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internal recycle control
one product stream is set as the free variable,
the other is placed on level control to maintain
the column material balance. Deviations from
quality specs are corrected by manipulating the
internal bulk flow rates within the column.