PWC Basics: A Simple Chemical Process Recycle A Fresh A REACTOR C - PowerPoint PPT Presentation

PWC Basics: A Simple Chemical Process Recycle A Fresh A REACTOR C A  B Cooling O Duty L MATERIAL RECYCLE FEHE U M N ENERGY RECYCLE PROCESS INTEGRATION Minimizes A consumed per kg B product Steam consumed per kg B product Product B ENHANCES PROCESS PROFITABILITY EPWC Workshop, Bangkok, Jan 13, 2019

PWC Basics: Chemical Process Operation Key Production Objectives Safety Stability Economics Production Product Effluent Rate Quality Specs Operate plant to meet production objectives 24X7 Production Objective Itself Can Change Process Disturbances Ambient Sensor Equipment Raw material Conditions Noise Characteristics Quality EPWC Workshop, Bangkok, Jan 13, 2019

PWC Basics Safety Operate Process at ≡ Steady State Stability 0 Accumulation Rate Rate Generation − + = Rate In Out Rate Need PWC to drive accumulation of all independent inventories to zero EPWC Workshop, Bangkok, Jan 13, 2019

PWC Basics • Regulatory Control System – Drives all inventory accumulation terms to zero – Ensures plant operation around a steady state • What steady state to operate at – Economic Optimum • Minimize expensive utility consumption • Maximize production EPWC Workshop, Bangkok, Jan 13, 2019

Plantwide Control Hierarchy PLANTWIDE CONTROL SYSTEM Optimization Layer (updates every few hrs) Economic SETPOINT Operation M Supervisory Control Layer e (updates every few mins) a s SETPOINT u Safe & Stable r Regulatory Control Layer e Operation (updates every few secs) m e n t s SIGNAL TO VALVE PLANT EPWC Workshop, Bangkok, Jan 13, 2019

Regulatory PWCS Design • What to Control – All independent inventories (DOF) • Material – Liquid level or gas pressure • Energy – Temperature or vapor pressure • Component – Composition, tray temperature (inferential) – Throughput or Production Rate • Degree of tightness of control – Should energy inventories be tightly controlled? – Should surge level inventories be tightly controlled? • What to manipulate – Pair close • Fast dynamics • Tight closed loop control • Location of through-put manipulator a key decision for inventory management and economics EPWC Workshop, Bangkok, Jan 13, 2019

The Transformation of Variability Perspective HEAT EXCHANGER EXAMPLE TC Control Steam in Valve TT Transmitter HEAT EXCHANGER Process Stream in Process Stream out Condensate out Steam Flow Steam Flow Temperature Temperature CONTROL Agent for transformation / SYSTEM management of process variability EPWC Workshop, Bangkok, Jan 13, 2019

Where to Transform Variability • Surge level – Does not affect steady state – Regulate loosely for filtering out flow transients • Energy Inventories – Regulate tightly to guarantee safety (rxn runaway?) • Product quality – Regulate tightly – Minimize “free” product give -away • Production rate – Often “loose” is OK ( eg meet the monthly target) • Recycle loop circulation rates – Regulate to avoid large drifts – All equipment inside recycle loop see acceptable variability EPWC Workshop, Bangkok, Jan 13, 2019

Nonlinearity in Material Recycle Loops STEADY STATES NO FEASIBLE Recycle Rate Snowballing Fresh Feed Rate Fixing the fresh feed rate of a recycled component is NOT a good idea Possibility of overfeeding induced instability EPWC Workshop, Bangkok, Jan 13, 2019

Material Recycle Snowball Effect TPM FC Recycle A Fresh A PC RC LC TC REACTOR C A  B LC Cooling O Water L U M Recycle loop shows large swings N R Large Throughput De-rating TC F A % LC Product B Time EPWC Workshop, Bangkok, Jan 13, 2019

Material Recycle Snowball Effect TPM FC Recycle A Fresh A PC RC LC TC REACTOR C A  B LC Cooling O Water L U M N No large swings in recycle rate Lower Throughput De-rating TC LC Product B EPWC Workshop, Bangkok, Jan 13, 2019

Alternative Material Balance Control Schemes IC IC Fixed Feed FC Recycle Floats P F Recycle FC IC Fixed Recycle IC P F Feed Floats Recycle Configure control structure to transform recycle rate variability out of the recycle loop EPWC Workshop, Bangkok, Jan 13, 2019

PWC Basics: Throughput Manipulation THROUGHOUT MANIPULATOR (TPM) The setpoint adjusted to effect a change in production/processing rate TPM * FC LC LC LC Unit 2 Unit 3 Unit 1 Lost production TPM * FC LC LC LC Unit 2 Unit 3 Unit 1 No (min) production loss EPWC Workshop, Bangkok, Jan 13, 2019

PWC Basics: TPM Selection • When is TPM choice flexible – Large storage tanks supply the fresh feed(s) – Variability in storage tank level is acceptable • Allows structures that bring in fresh feed(s) as make-up • Usually plant designs have large recycle rates – Design in the snowballing region – Capacity bottleneck then is likely inside the loop • Where to locate the TPM – Inside the recycle loop – If multiple recycle loops, on a common branch – If bottleneck is known, AT the bottleneck EPWC Workshop, Bangkok, Jan 13, 2019

Reactor Separator Recycle Process F A Control DOFs 9 F B Surge Levels -2 C O Given Column Pr -1 L A + B → C U --------------------------------- M Steady State DOF 6 N --------------------------------- Product C EPWC Workshop, Bangkok, Jan 13, 2019

PWCS Design: TPM at Fresh Feed CC X PC FC F A TPM LC FC FC F B C LC O L A + B → C U TC M TC N LC Product C EPWC Workshop, Bangkok, Jan 13, 2019

PWCS Design: Recycle Drifts Beware of subtle plantwide recycle loop inventory drifts Stoichiometric feed balancing Plantwide balances close slowly due to recycle Always examine process input-output structure Every component must find a way out or get consumed (DOWNS’ DRILL) FC F A IC Recycle A P C IC Recycle B F B For (near) pure C product, F A = F B EPWC Workshop, Bangkok, Jan 13, 2019

PWCS Design: TPM at Column Boilup CC X PC FC F A LC LC FC F B C O TC L A + B → C U TPM M TC N FC LC Product C EPWC Workshop, Bangkok, Jan 13, 2019

PWCS Design Steps • DOF analysis and control objectives – Production rate, Product quality – Safety limits (eg UFL < gas loop composition < LFL) – Inventories – Economic • Choose TPM – Feed set by an upstream process – On demand operation (utility plants) – Flexible • Inside the recycle loop at the feed of the most non-linear/fragile unit • If bottleneck is known, at the bottleneck inside the recycle loop • Design “local” loops for closing all independent material and energy balances around the TPM – Radiate outwards from the TPM – Check consistency of material / energy balance closure ( Downs’ Drill ) • Design economic control loops – Active constraint control & SOCV control EPWC Workshop, Bangkok, Jan 13, 2019

Mode I Optimum Operation OBJECTIVE MIN J = Boilup F A at given throughput subject to process constraints F B C O ACTIVE CONSTRAINTS L A + B → C U MAX T rxr Max reactor temperature M MAX MAX reactor level LVL rxr N prdMIN x C MIN product purity EQUALITY CONSTRAINT P Given throughput Product C UNCONSTRAINED DOFs SOCV1 L/F Reflux to feed ratio SOCV2 [A/B] rxr Reactor A/B ratio EPWC Workshop, Bangkok, Jan 13, 2019

Mode II Optimum Operation OBJECTIVE MAX J = Throughput (P) F A subject to process constraints F B C ACTIVE CONSTRAINTS O MAX T rxr Max reactor temperature L MAX MAX reactor level A + B → C LVL rxr U M prdMIN x C MIN product purity N ∆P MAX Capacity bottleneck UNCONSTRAINED DOFs SOCV1 L/F Reflux to feed ratio SOCV2 [A/B] rxr Reactor A/B ratio Product C EPWC Workshop, Bangkok, Jan 13, 2019

PWCS Design: TPM at Fresh Feed MODE II CONSTRAINTS [A/B] rxr CRC MAX , LVL rxr MAX T rxr L/F prdMIN , Δ P MAX x C CC X PC X SOCVs FC F A L/F, [A/B] rxr TPM LC FC FC F B C MAX LVL rxr LC O L A + B → C U MAX T rxr TC M TC N MAX Long Loop → Large δ MAX - δ ∆ PC LC prdMIN x C ∆P MAX - δ CC Product C EPWC Workshop, Bangkok, Jan 13, 2019

PWCS Design: TPM at Fresh Feed [A/B] rxr CRC L/F CC X PC X FC TPM F A LC FC LS FC F B C MAX LVL rxr LC O L A + B → C U MAX T rxr TC M TC N Long Loop → Large δ ∆ PC LC prdMIN x C ∆P MAX - δ CC Product C EPWC Workshop, Bangkok, Jan 13, 2019

PWCS Design: TPM at Bottleneck MAX MAX - δ [A/B] rxr CRC Negligible back-off L/F CC X PC X FC F A MAX LVL rxr LC LC FC F B C O TC L A + B → C U TPM T rxr MAX M TC N FC LC ∆ PC ∆P MAX - δ prdMIN x C CC Product C EPWC Workshop, Bangkok, Jan 13, 2019

PWCS Design: TPM at Bottleneck [A/B] rxr CRC L/F CC X PC X FC F A MAX LVL rxr LC LC FC F B C O TC TPM L A + B → C U T rxr MAX M TC LS N FC LC ∆ PC prdMIN x C CC ∆P MAX Product C EPWC Workshop, Bangkok, Jan 13, 2019

Switching Regulatory Control Structure [A/B] rxr CRC L/F CC X PC X FC TPM F A LC FC LS FC −5% F B Σ C HLC LC LTC LS O L MAX LVL rxr −2 °C A + B → C Σ U TC TC M N LS ∆P MAX ∆ PC MAX T rxr LC prdMIN x C CC Product C EPWC Workshop, Bangkok, Jan 13, 2019

Summary • Locate TPM at bottleneck inside recycle loop • Economic considerations play a major role in regulatory control layer design • COMMON SENSE MUST PREVAIL – Everything must be carefully thought through – It pays to be systematic EPWC Workshop, Bangkok, Jan 13, 2019