Best Practices for Aspen HYSYS

Best Practices using Aspen HYSYS®

Wim Van Wassenhove

©2006 Aspen Technology, Inc. All rights reserved

Agenda

• Best practices

• Discuss various tips & tricks


• Values (Variables):

• Blue: User-specified

• Red: Default value

• Black: Calculated (or fixed) value

• Streams:

• Light Blue: Not Solved

• Dark Blue: Solved

• Unit Operations

• Red: Connection is missing—unable to begin solving

• Yellow: Unable to Solve or Solved with Warnings

• Black: Solved

Color Scheme Employed by HYSYS


PDF and other short cut keys

• PFD

− Shift P,T, M, F

− X, Y, 1

• General

− F12

− F11

− Ctrl D

− Ctrl U

− Ctrl W


Using Auto-connect

• Second button from the left on PFD

• Let me show you!


Adjusts / Recycles and Calc levels

• Determine what should be converged first

• Ensure the Calc level of that block has the low value


NEVER ignore an inconsistency

• Solve the issue at once; forcing Hysys to continue calculating in some instances changes the object Hysys points to as the cause. It then becomes a lot more difficult to solve the issue.

• Do not forget to look for overspecifications of composition. Go to the workbook on the compositions tab. If you do not have a compositions tab, create one! A typical scenario is that you “reuse” a stream that previously held a composition and forget to erase that composition.

• Mixing streams where one flow is positive and another is negative holds the risk of creating an inconsistency. The problem comes up when the mixing results in one component having a positive flow and another component having a negative flow. The issue usually isn’t flagged immediately with an inconsistency.

• See “Base Model” example

• See trouble shooting examples


Insure your model makes sense from the Hydraulics perspective

• Use the Equalize All setting on mixers

− You may need to add extra valves

• Model pumps if they are present

• In-process piping is probably an overkill


Make your cases robust

• Link values that need to be linked

− Use SET operations or spreadsheets

• Implement Feed-Forward for Adjusts

• Avoid specs fixing absolute flows

− Use a ratio or other spec independent of absolute flow


Spreadsheet can be used for many things

• Gathering Input & Outputs in one place

• Side calculation

− Summarizing data

− Calculate an effect not modelled by HYSYS

• Links in and out can be made with Drag&Drop

− Variables that can’t be found through variable browser can most of the time be pulled in via drag&drop


Use engineering common sense to avoid adjusts

• Iterations = slower model solution


Stream Utilities can make simulation time longer, defer them to end of solve

• Create Workbook page for utilities

• Set Calc Level to 1e5


General

• Hysys is very much oriented to interact with the user. It will provide partial results while it is running and when for some reason it failed to complete calculations. Take advantage of that information!

• Have a little faith in HYSYS! Failure to converge or solve may be due to infeasible specifications, a bad initial estimate or something else that you can change yourself.


Create the simulation with the real process constraints in mind

• Flowsheets created for engineering purposes are intended to provide information for the design of a plant. Hence, it is more logical to specify targets you want the process to achieve rather than constraints on flows, splits of Tees etc. It doesn’t really make sense to specify a UA for a heat exchanger for example, it probably does make sense to specify a minimum temperature approach.

• Realise that some constraints are really just good guesses of what values should be and not real process constraints. If you have that kind of freedom, use it to minimise the number of recycles or adjusts.


Proper analysis of the recycle location

• Preferably put a recycle in a place where you know values will be relatively stable.

• In general it is recommended to put the recycle on the stream with the largest flow in the cycle.

− Sometimes the generally recommended recycle location is not the best one.

− If you simulate a compressor with a surge control loop, in most cases this recycle flow will be zero. In this case it might be better to put the recycle in the recycle stream.


Take as many of the time consuming units as possible out of the loops

• Simulate the inside of the loop with a minimum number of operations and operations that calculate fast.

− For example, if the stream in the loop is cooled with cooling water and your specification is the streams ultimate temperature, it is probably beneficial to use a simple cooler to achieve this and calculate the complete exchanger after the loop has converged.

• Use the calculation levels to achieve the proper order.


Use column subflowsheet full potential

• The column subflowsheet provides an equation solver type solution− Usually a lot faster to converge.

− That is, once you have got a first solution!!!

• Often a good idea to simplify a system, as long as final result is the same

• Detail calculations can be done after convergence of complete flowsheet

• For example, a column with a complex condenser set-up− If you know that in the end the complete overhead vapour stream is

condensed and returns at a given temperature and pressure, there is no need to simulate the complete overhead system to solve the column. A simple condenser with a fixed temperature and pressure will do. The complete overhead system can then be modelled in the main flowsheet using the solution obtained in the column. This calculation will only have to be done once.

• More complex systems can be simulated inside the column subflowsheet using the modified Hysim inside-out method.


Column won’t converge

• A more complex is harder to converge a 1st time

• A few things to consider before abandoning the immediate convergence route:− Look at the number of iterations, not immediately the time it is taking− Bad estimates are usually worse than no estimates at all− Inactive specs are per default marked as estimates, you may want to deactivate this− Correct temperature estimates with pressure profile in wrong units = convergence failure

• If this doesn’t solve your problem− start with a simple column. Reasonable specs should pose no convergence problem− Go to the parameter page of the column and lock the flow and composition estimates. − Add extra pieces of equipment, do this in steps, converging the column between steps

• Usually better to 1st converge with flow and reflux type specifications.

• Then add the actual specifications, but don’t activate them yet

• Play with flow and reflux to see how they impact the other spec values


Column initially convergesbut fails with different feed

• Numerical problem is not impossible

• 1st investigate: is there a process reason why column doesn’t converge anymore?

• A practical example of a demethaniser

• Demethaniser with reboiler and full reflux condenser− Specifications: Methane content in btms and condenser temperature.− The initial simulation solved fine− It had been observed that with low top temperature, the column wouldn’t converge anymore− Final solution had a top temperature that was high enough not to cause problems

• New feed with slightly increased the methane content refuses to converge

• Why?− Top product of the column is a vapour stream with about 65% mole of Hydrogen.− Hydrogen can be considered an inert gas at the condenser conditions− At fixed pressure, the CH4 in the top is constrained by

• The vapour pressure of CH4 at the imposed condenser temperature• The flow of hydrogen.

− Spec on the btms CH4 fraction also defines how much CH4 needed in overhead product− Increased methane in feed has nowhere to go:

• Top temperature stops extra methane from leaving through the top• Bottom methane spec stops it from leaving through the bottom


Three-phase columns are not common in the process industry

• If HYSYS warns you about a 2nd liquid phase

− 1st think if you really want a second liquid phase

− Maybe you rather need to change a specification such that the second liquid phase disappears than struggling to converge a three-phase column.

• Typical examples that are usually not meant to be three-phase are drying columns and stabiliser columns.


Useful macros

• Enable TEE negative flow check

• Heat of Combustion of Hydrocarbons

• Install Multistage Compressor

• NameChanger

• NameSequencer

• Pipe Segment Updater

• Pump Curve Auto Generation

• Report Component Data

• Report Line Sizing

• Report Stream Cold Properties

• Report Stream Critical Properties

• SortComponentsByBoilingPoint


Useful Excel sheets

• Excel_Spreadsheet_vs4_1

• Generate TXY diagram

• HSR 1.6

• Workbook Dump of All Pages


Useful Extensions

• CmpSplitter

• ejector

• Mach

• Saturate

• Thermo selection expert

• Virtual

Best Practices for Aspen HYSYS

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