Address Distillation Process Control during Design Phase ......Oct 01, 2019 · 11th AIChE...
Transcript of Address Distillation Process Control during Design Phase ......Oct 01, 2019 · 11th AIChE...
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Address Distillation Process Control during Design Phase
to Save Energy and Increase CapacityCharles D. Herzog P.E.
HERZOG PROCESS SERVICES, PLLC
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
“How can we improve distillation efficiency before the plant is even built?”
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
“How can we improve distillation efficiency before the plant is even built?”
Design for Reduced Process Variation
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Typical Distillation System CW
STM
Vent to Flare NNF
REFLUX
AI
AI
COND
DISTILLATE
BOTTOMS
FEED
AI
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
C3 Splitter – Overhead Primary ProductCW
STM
Vent to Flare NNF
REFLUX
AI
AI
COND
PROPYLENE – 95%
PROPANE
FEED
AI
Primary
Secondary
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
FCC Debutanizer – Bottom Primary ProductCW
STM
Vent to Flare NNF
REFLUX
AI
AI
COND
LIGHT ENDS
GASOLINE -- RVP
FEED
AI
Secondary
Primary
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Reduce Energy Use and Increase Capacity
• Move target closer to spec • Reduce reflux and heat per unit feed• Reduce process variation to stay on-spec
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Take These Steps to Reduce Variation • Select target using data from existing columns • Identify capital upgrades to reduce variation• Develop inferential model for ‘analyzer down’• Prioritize primary product via basic controls
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
TIME
SPEC
TIME
IMPU
RITY
INPR
IMAR
Y PR
OD
TARGET
Select Target based on Predicted Variation
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
TIME
SPEC
TIME
SPEC
Initial Target
Final Target
IMPU
RITY
INPR
IMAR
Y PR
OD
IMPU
RITY
INPR
IMAR
Y PR
OD
TARGET
Reduce Variation to Operate Near Product Spec
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Address Normal and Maximum Variation
• Estimate standard deviation from data• Estimate max variation from disturbances• Evaluate off-spec incidents at existing unit
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Consequences of Off-Spec Operation• Product may have to be diverted or flared• Off-spec tank may be required• Off-spec material may need reprocessing• Customer’s operation is adversely affected• Supplier’s reputation is tarnished
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
On-Spec Operation is a Requirement
• Must be on-spec to achieve efficiency gain• Prevent off-spec incidents through design
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Most Disturbances are Predictable
• Feed rate change• Feed composition change • Rainstorm
Design the plant to stay on-spec in these scenarios
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Capital Upgrades to Reduce Variation• Insulate overhead equipment and pipe• Increase primary product residence time• Increase size of feed drum• Add instruments to support inferential model
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Insulate Overhead Equipment and PipeMany Overhead Systems are NOT Insulated
CW
STM
Vent to Flare NNF
REFLUX
AI
AI
COND
FEED
AI
BOTTOMS
DISTILLATE
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Benefits of Insulation
• Reduce losses from light-ends vent• Reduce solar heating of reflux liquid• Stay on-spec during rainstorm
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
What Happens During a Rainstorm?
• Uncontrolled increase in condenser duty • Pressure decreases rapidly—as much as 20%• Tray liquid vaporizes, goes to reflux drum• Heavy ends contaminate distillate product
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Pressure Loss Contaminates Distillate
Equilibrium Liquids Vaporize during Rainstorm
Off-Spec Materialfrom Trays
Contaminates Distillate
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Increase Primary Product Residence Time • Increase size of reflux drum or column bottom• Increase level control setting on existing drum• Dampens all disturbances automatically!!• Gives operator time to react and stay on-spec
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Example for Increased Residence TimeCW
STM
Vent
REFLUX
AIAI
COND
PROPYLENESPEC = 95%
TARGET = 95.5%
PROPANE
ORIGINAL70/30 FEEDMIXTURE
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Un-Measured Change in Feed CompositionCW
STM
Vent
REFLUX
AIAI
COND
PROPYLENESPEC = 95%
TARGET = 95.5%
PROPANE
ORIGINAL70 / 30 FEED
MIXTURE
CHANGES TO68 / 32 MIX
OVHD= 94.5%AFTER CHANGE
AT T = 0
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Example for Increased Residence TimeCW
STM
Vent
REFLUX
AIAI
COND
IMPURITYSPEC = 5.0 %
TARGET = 4.5 %
PROPANE
ORIGINAL70/30 FEEDMIXTURE
CHANGES TO68 / 32 MIX
IMPURITY = 5.5 %AFTER FEED
CHANGELONGER RESIDENCE
TIME REDUCESPRODUCT VARIATION
45 MIN v 18 MIN
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
4
4.2
4.4
4.6
4.8
5
5.2
5.4
5.6
0 5 10 15 20 25 30 35 40
Prop
ane
in D
istil
late
%
Minutes
Propane in Distillate after Feed Composition Change
Target 4.5 % Propane
Product Spec 5.0% Propane
Residence Time = 18 min
Off-Spec in 12 minutes
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Target 4.5 % Propane
Product Spec 5.0% Propane
Residence Time = 45 min
Residence Time = 18 min
Off-Spec in 31 minutes
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Increase Size of Feed Drum
• Larger drum leads to steadier feed• Steadier feed leads to steadier product• Feed composition changes also dampened
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
What Happens During Analyzer Calibration?
• Advanced control loops (APC) turn OFF• Operator makes large increase in reflux• Column operation is shifted to inferior place• Inefficiency lasts for hours following calibration
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Typical Calibration Time Line
Time hrs
Impurity %
0 4 8 12 1612 2420
Spec
Target
Inefficient Operation
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Maintain Efficiency during Calibration Period
• Consider adding redundant analyzer• Develop inferential composition model• Maintain operation at normal target
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Designing a Practical Inferential Model• Use press / temp for wide-boiling mixtures• Use component balance for pure product• Validate the model using a ‘bias’ term• Operator must have confidence in the model
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
FCC Debutanizer – Bottom Primary ProductCW
STM
Vent to Flare NNF
REFLUX
AI
AI
COND
LIGHT ENDSGASOLINE -- RVP
FEED
Secondary
Primary
TIPI
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Pressure—Temperature ModelsUse Tower Design Simulation to Generate Model
Pressure, psig
Temp, °F
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Validate the Model with Analyzer in Service• Use model to estimate RVP from P-T graph
BIAS = ANALYZER RVP – MODEL RVP
• Unchanging bias indicates good model • Highly variable bias indicates model error
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Use Model during Analyzer Calibration• Application monitors Analyzer Status via DCS• Predicted RVP = Model RVP + BIAS • Model can reset temp as pressure varies
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
C3 Splitter – Propane Balance ModelAdd Feed Analyzer during Design
CW
STM
Vent to Flare NNF
REFLUX
AI
AI
COND
PROPYLENE – 95%
PROPANE
FEED
AI
Primary
Secondary
SET PROPANE PRODUCT FLOW IN RATIO TO FEED
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Propane Balance Model• Use feed analyzer to calculate propane in feed• Subtract propane in bottom product• Difference is propane in distillate• Operator uses bottoms-to-feed to stay on spec
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Perform Basic Control System Design Early • Primary product determines the design • No longer can we defer until after startup• Operator training based on P&IDs• Difficult to change controls after HAZOP
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Control Strategy Prioritizes Wrong Product
FEEDMax Rate
CW
STM
VENTNNF
PRIMARY95% SPEC
RECYCLE
REFLUX
FC
PC
TCAIAI LC
LC
PC
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Control Purity via Material Balance
Draw more Heavies to Control Top Prod
CW
STM
VENT
PRIMARY
RECY
REFLUX
LCFC
PC
FC
LCFC AIHVY
FEED
Manage Constraints with Reflux
PC
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019
Thank You
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11th AIChE Southwest Process Technology Conference | October 1-2, 2019