address distillation process control during design phase ......oct 01, 2019 · 11th aiche...

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


“How can we improve distillation efficiency before the plant is even built?”


“How can we improve distillation efficiency before the plant is even built?”

Design for Reduced Process Variation


Typical Distillation System CW

STM

Vent to Flare NNF

REFLUX

AI

AI

COND

DISTILLATE

BOTTOMS

FEED

AI


C3 Splitter – Overhead Primary ProductCW

STM

Vent to Flare NNF

REFLUX

AI

AI

COND

PROPYLENE – 95%

PROPANE

FEED

AI

Primary

Secondary


FCC Debutanizer – Bottom Primary ProductCW

STM

Vent to Flare NNF

REFLUX

AI

AI

COND

LIGHT ENDS

GASOLINE -- RVP

FEED

AI

Secondary

Primary


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


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


TIME

SPEC

TIME

IMPU

RITY

INPR

IMAR

Y PR

OD

TARGET

Select Target based on Predicted Variation


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


Address Normal and Maximum Variation

• Estimate standard deviation from data• Estimate max variation from disturbances• Evaluate off-spec incidents at existing unit


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


On-Spec Operation is a Requirement

• Must be on-spec to achieve efficiency gain• Prevent off-spec incidents through design


Most Disturbances are Predictable

• Feed rate change• Feed composition change • Rainstorm

Design the plant to stay on-spec in these scenarios


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


Insulate Overhead Equipment and PipeMany Overhead Systems are NOT Insulated

CW

STM

Vent to Flare NNF

REFLUX

AI

AI

COND

FEED

AI

BOTTOMS

DISTILLATE


Benefits of Insulation

• Reduce losses from light-ends vent• Reduce solar heating of reflux liquid• Stay on-spec during rainstorm


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


Pressure Loss Contaminates Distillate

Equilibrium Liquids Vaporize during Rainstorm

Off-Spec Materialfrom Trays

Contaminates Distillate


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


Example for Increased Residence TimeCW

STM

Vent

REFLUX

AIAI

COND

PROPYLENESPEC = 95%

TARGET = 95.5%

PROPANE

ORIGINAL70/30 FEEDMIXTURE


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


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


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


Target 4.5 % Propane

Product Spec 5.0% Propane



Off-Spec in 31 minutes


Increase Size of Feed Drum

• Larger drum leads to steadier feed• Steadier feed leads to steadier product• Feed composition changes also dampened


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


Typical Calibration Time Line

Time hrs

Impurity %

0 4 8 12 1612 2420

Spec

Target

Inefficient Operation


Maintain Efficiency during Calibration Period

• Consider adding redundant analyzer• Develop inferential composition model• Maintain operation at normal target


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


FCC Debutanizer – Bottom Primary ProductCW

STM

Vent to Flare NNF

REFLUX

AI

AI

COND

LIGHT ENDSGASOLINE -- RVP

FEED

Secondary

Primary

TIPI


Pressure—Temperature ModelsUse Tower Design Simulation to Generate Model

Pressure, psig

Temp, °F


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


Use Model during Analyzer Calibration• Application monitors Analyzer Status via DCS• Predicted RVP = Model RVP + BIAS • Model can reset temp as pressure varies


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


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


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


Control Strategy Prioritizes Wrong Product

FEEDMax Rate

CW

STM

VENTNNF

PRIMARY95% SPEC

RECYCLE

REFLUX

FC

PC

TCAIAI LC

LC

PC


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


Thank You

address distillation process control during design phase ......oct 01, 2019 · 11th aiche...

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