recycle loop design with pro/ii - wonderware...

Mike Donahue

Technical Support

SimSci by Schneider Electric

Recycle Loop Design with PRO/II

Agenda

●Recycle Background

●Process Simulation Background

●Recycle Convergence

●Recycle Convergence Strategy

3

Recycle Background

Recycle Definition

●A recycle stream is a term denoting a process stream that returns material or enthalpy from a downstream process unit back to an upstream process unit

Recycle Loops

● Independent Loops

● Interconnected Loops

●Nested Loops

Tear Stream Definition

●A tear stream is any stream that “opens” a loop. It is a stream that PRO/II will update until two consecutive iterations are within the specified tolerance

●A tear stream is not a recycle stream

Tear Streams?

U3S1

U1 U6U2S2

U4S4S3

R1

R2S9

U5S6S5

U7

S8

S7

One Option

Selecting the sequence = Selecting the tear streams

Tear streams Calculation sequence

R1, R2 U1, (U2, U3, U4, U5, U6), U7

U3S1

U1 U6U2S2

U4S4S3

R1

R2S9

U5S6S5

U7

S8

S7

Minimum Tear Stream (MTS)

U3S1

U1 U6U2S2

U4S4S3

R1

R2S9

U5S6S5

U7

S8

S7

Tear streams Calculation sequence

S4 U1, (U4, U5, U6, U2, U3), U7

10

^ Process Simulation Background

Steady State Modular Approach

●Sequential-Modular

˃ Flowsheet is decomposed (sequenced)

˃ Calculations are performed one unit at a time

˃ Iterate tear streams

˃ Most commercial steady state simulators use a sequential approach

●Simultaneous-Modular

˃ Flowsheet is developed as a collection of sub-flowsheets (SFS)

˃ Each SFS and collection of streams are solved together

Steady State

PRO/II is a Sequential Modular Simulator

●Recycles automatically handled

Recycle Definition

● Intuitive

˃ Clear / understandable error messages

˃ Problems localized to individual unit operations / recycles

●Robust

˃ The resolution is divided-up into several subsets that are treated sequentially

˃ This facilitates rigorous convergence, even in presence of extremely complex modules that are treated in an autonomous way

●Good heuristics for initialization and convergence

˃ No over-specifications (inconsistencies)

˃ Recycles estimates not required

˃ Recycle blocks not required

Sequential Solver Challenges

●The main challenge with a SMA

˃ Inefficient sequencing

˃ Inefficient recycles

●The combination of these issues lead to potentially long calculation times

Sequential Modular Approach (SMA)

Sequential Solvers

●Computation Time – Multiple passes are typically required to solve the flowsheet

~ 1,000

4 hours / 500

~ 30 seconds

~ 50% increase in speed

~ 500 increase in computing power

Sequencing Challenges

●How many tear streams?

●What order should we converge the units (partitioning)?

●Convergence Method?

●Many publications address tear stream determination and partitioning:

Sargent and Westerberg 1964

Forder and Hutchison 1969

Barkley and Motard 1972

Motard and Westerberg 1979

Gunderson and Hertzberg 1982

Lakshminarayanan and Rao 1991

PRO/II Sequencing Algorithms

●Minimum Tear Stream Algorithm (SimSci Method)

˃ Default

˃ Uses improved algorithms based on Motard and Westerberg

˃ Improved by SimSci

●Alternate Method (Process Method)

˃ Determines the sequence based partially on the input order

●Explicitly Defined by User


● If there are no recycle streams in your flowsheet, the SimSci sequence algorithm will determine the correct calculation sequence automatically

● If the flowsheet has recycle streams:

˃ First, the sequence algorithm is driven by reducing the number of tear streams in your flowsheet

˃ Secondly, the sequence algorithm is driven by the recycle stream estimates provided by you

●We recommend that you always provide a recycle stream estimate for each recycle loop

●These estimates will cause the calculation sequence algorithm to select the recycle streams as tear streams


●Controllers / Optimizers

˃ Single variable controllers which affect units within loops will be included in the loops

˃ Multi-variable controllers and optimizers will not be included in the affected loop

Be careful here – you may want to employ a user defined sequence

22

Recycle Convergence

Convergence

●A flowsheet has converged when the stream values (material, temperature, and pressure) stop changing within the degree of tolerance for two successive loop iterations

Convergence

●Default is to converge all streams in the loop

●Optionally can choose to converge only tear streams

Tear Stream Identification

● Identify the tear streams

Recycle Tolerance

Tolerance

●Default Criteria

��−��

��

��

�� 0.01

�� ! �� "��#1°�, 0.55°�)

'��'��

'�� (��))"��#0.01)

All 3 must be met

Acceleration Methods


●Direct-substitution

˃ Default method for recycle convergence in PRO/II. Last computed values for the tear streams are used for the next trial solution of the recycle loop.

˃ Generally stable

˃ Potentially slow

●Wegstein

˃ Provides rapid solution of recycles using an acceleration factor based on the convergence characteristics shown by previous iterations

˃ Note: This method does not work well when multiple recycle streams are interacting

●Broyden acceleration

˃ Based on the "Householder" method, which allows for rapid, stable solution of recycle loops

User-Specified Recycle Loops

32

Recycle Convergence Strategy

Recycle Challenges

● In closed loops (no purge), the recycle stream has the potential to do one of two things:

a) Build up

b) Deplete to zero

Unless the makeup rate is exactly equal to consumption + the other losses

●Consumption is known (typically) – what are the other losses?

●Major Challenges:

˃ Losses from the recycle components are usually insensitive to the recycle amount

˃ Small variations in makeup result in large recycle changes

˃ A large number of iterations are required to adapt to a small change in makeup

Recycle Strategy

Solution

1) Reference Stream to “Break the Recycle”

2) Use a Calculator / Controller / Splitter Specification

Recycle Strategy

Recycle Strategy


●Avoid complex operations when possible (move to outside loop with stream references)

Recycle Strategy



●Always have tighter tolerances on internal loop operations than on recycle

Recycle Strategy

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Unit Tolerances Recycle Tolerances

Recycle Strategy

Recycle Strategy




●Avoid thermal recycles

Recycle Strategy





●Set appropriate tolerance (mass balance)

Recycle Strategy

Iterations vs. Tolerance Recycle Methane

Simulation time for 318 iterations – 38 seconds

Recycle Strategy





●Set appropriate tolerance (mass balance)

●Use acceleration


●Minimize controllers (recycle streams) with stream splitters

Recycle Strategy


Recycle Strategy


●Examine message history

Recycle Strategy



●Supply recycle estimates

Recycle Strategy




● Investigate Sequencing

Recycle Strategy





●When using Controllers (use control)

Recycle Strategy





●When using Controllers (use control)

●Use reference streams to redefine the tearing process and eliminate thermal recycles

Recycle Strategy

●Try not to spend too much effort (time) initially designing recycle loops

●PRO/II is extremely robust at solving complex loops (out-of-the-box)

recycle loop design with pro/ii - wonderware...

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