Dynamics Quick Start Guide

Quick Guide: Convert HYSYS Steady-State models into Dynamics v1

07/01/2008 1

Quick Guide Convert HYSYS Steady-State models into Dynamics

The purpose of this document is to provide a simple and quick advice to enable the HYSYS users

to produce HYSYS Dynamics models of real operating units, detailing the basic step to follow

and the minimum dynamic data required for each unit operation. It is assumed that the user

knows HYSYS in Steady-State.

Four basic steps will be followed as shown (percentages show typical time fraction of total time):

1. Collect Engineering Design and Process Plant Data. Firstly, apart from the PFDs and P&IDs, all equipment data of the equipments is collected:

- Columns: Number of real trays, tray type, tray spacing, diameter, weir height and total weir

length from design datasheets.

- Pumps: Head/flow and efficiency/flow curves, from Pump Test Certificates

- Compressors: Head/Vol. flow and efficiency/Vol. flow curves, from compressor vendor.

- Expanders: Power/Mass flow and efficiency/Mass flow curves, from expander vendor.

- Heat Exchangers, Heaters and Coolers: Shell and tube volumes.

- Aircoolers: Volume (of fluid tubes), number of fans, airflow per fan.

- LNG Exchangers: Layers Pattern, Geometry. Thermal conductivity, Cp and density of metal.

- Vessels: Volume, height, nozzles, level taps, from design datasheets

- Valves: Cv, Type or Characteristic curve, DeltaP, from valve datasheets.

- Relief valves: Type, setting pressures, from valve datasheets.

- Controllers: PV, OP, SP, Range and tuning gains from DCS.

- Elevations: From plant floor elevations sheet, only if contribution is significant.

For the process plant data, a representative period of time (1 to 3 days, 1 minute sample) is

selected when the process stays at steady conditions and then the average of all available

instrumentation devices (Pressures, Temperatures, Flows, Level, controllers PV, OP, SP) is

calculated. Laboratory compositions analysis from the feed and products is also taken for that

day. It was important to ensure that the process didn’t suffer major changes during the selected

period; otherwise it will be difficult to calibrate the model in the third step.


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2. Introduce Engineering Data into the model

In order to replicate the plant conditions, it is necessary to introduce all unit operations existing in

the plant with a significant impact in the simulation. Most of the data like volumes or PID

controllers will not be used in the Steady-State mode, but it is entered now in steady-state mode

to simplify the process and keep the entire data model in one Aspen HYSYS file case.

Distillation column:

The geometrical data to introduce in the column tray section (reflux drums, reboilers, pumps and

valves will be outside the column sub-flowsheet) is the following:

• Diameter

• Tray spacing and type (real number of trays, not theoretical)

• Weir height

• Total Weir length

This information needs to be introduced in the column tray section of the column sub-flowsheet

as appear below:

The bottom tray is a sump type, and it is additional to the trays of the column. For example, if a

column has 50 real trays, in HYSYS we will need to put 51 trays (50 normal trays + 1 sump).

The weir length should be the total weir length, which is the geometrical sum of all weirs of a tray

in case of multiple paths. In HYSYS the flow-paths data should be always 1.

It is important to remark that HYSYS consider all hold-up in the tray as Clear-Liquid (ie. “Clear

liquid” is the liquid to which the aerated mass would collapse in the absence of vapour flow). The

“Aeration Factor”, which represent the bubbles of the vapor going up through the liquid of the

tray, makes that the real amount of liquid in the tray is much lower than considering all the

calculated hold-up volume to be full clear liquid. To properly handle this aeration factor in the


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simulation it is necessary to artificially reduce the weir height (and/or weir length) until we reach

the right holdup volume from the column manufacturer. For example, reducing the weir height

from 50 mm to 10 mm, the right hold up volume can achieved. The below figure shows the real

height of the froth or foam on the trays and the equivalent clear liquid volume after discounting

the aeration factor.

Besides the geometrical data, an actual pressure profile over the trays has to be filled in steady

state. Usually only top and bottom pressures are specified and then HYSYS automatically

calculate a linear pressure profile. Once the column is solved in SS is it necessary to calculate the

“k” values of every tray for the dynamic mode, so the right pressure drop (dry hole + static head

on tray) is calculated for every tray, before bringing the column to run in dynamic mode. This is

performed by pressing the button “All stages” in the Dynamic tab section of the column tray

section as showed below. The calculated “k” values for every tray will be used by the Dynamic

simulation, but can be corrected by hand if the right pressure profile is not achieved.


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Pumps Head and efficiency curves, from Pump Test Certificates, as shown below.

Typically pumps run at fix speed, so only one curve needs to be entered. (one for head/flow and

another for efficiency/flow). These curves can be used also by the Steady-State solver, so both

Dynamic and Steady State solutions will be consistent.


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Compressors

Head/Flow and Efficiency/Flow curves for every speed from compressor map as shown below.

If the compressor load is regulated by Inlet Guide Vanes (IGV), then the curves for every IGV

position (instead of speed) needs to be entered in the same way.

Similarly to the pumps, compressor curves can be also active in Steady-State mode, so consistent

values a calculated later in Dynamics.


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Expanders

Power/Mass Flow and Efficiency/Mass Flow curves for every speed. Data is supplied from

expander vendor tests.

Similarly to the pumps and compressors, expanders’ curves can be also active in Steady-State

mode, so consistent values a calculated later in Dynamics.


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Heat Exchangers, Heaters or Coolers

In steady state heat exchangers will be solved from the thermodynamic point of view, normally

specifying inlet/outlet temperatures and then UA is calculated by HYSYS. This UA factor will be

used by the Dynamic solver.

We will need to calculate separately (from the exchanger geometrical data) the total volume of

the tube side and shell side of the heat exchanger and introduce them in the Dynamic model

parameters as shown below.

If the Shell or Tube “UA reference flow” is left empty, then the UA value will be constant for all

flow regimes. But if a UA reference flow is specified, then HYSYS will internally modify the

used UA value depending on the current flow.

It is important that the value specified in steady state for the pressure drop on the tube and on the

shell side is accurate, as in dynamic a resistance k to the flow will be calculated for both the tube

and the shell pass based on what specified in steady state as pressure drop.

If the Shell or Tube “k reference flow” is left empty, then the k value will be constant for all flow

regimes. But if a k reference flow is specified, then HYSYS will internally modify the used k

value depending on the current flow (Only for smaller flows).


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Air Coolers

Number of Fans, Design speed, Demanded speed, Design airflows and inlet/outlet conditions

should be already introduced in Steady-State mode. This data is taken from the design datasheet

and from the plant conditions and it need to reach a consistent solution. For example, we can not

expect an outlet air temperature of 200Deg C.

Total hold-up volume of the process stream needs to be entered in Dynamic tab.

As already indicated for the heat exchangers, it is important that the values of the pressure drop to

be accurate, for the same reason as specified for the heat exchangers.

The “Demanded speed” should be accessible as an OP for a controller, if not, a “Selector Block”

can be used as shown in the figure to have access to the “Demanded Speed” of the fans.


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LNG Exchangers.

The LNG exchanger has to firstly be solved in Steady-State as below:

In Dynamics, the LNG exchanger is divided in a number of Zones (typically from 5 to 10). Every

Zone is composed by a number of Sets, and every Set has a number of Layers. The user, with the

layer pattern information of the exchanger, has to enter for every Zone the configuration of the

layers in the Dynamic tab as shown below (from the 10 zones, only shown Zones 0, 1, 8 and 9):


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Introducing the DeltaP, k values are calculated in Steady-State with the button and used for

Dynamics when “Flow eqn” chekbox is checked as show below:

In Rating/Sizing the geometry (width and length) and metal properties (Thermal cond., CP and

Density) of every Zone needs to be specified as show below:


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In Rating/Layers the perforation, height, fins per meter of width (Pitch) and fin and plate

thickness needs to be specified for every layer of every zone.

In Rating/Heat transfer internal and external U value needs to be specified.


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Vessels, Separators and Tanks

Orientation, geometrical data, nozzles location, level taps as per design datasheets. In case the

separator presents a boot or a weir for phases’ separation, the geometrical data relative to these

have to be filled. The length to enter doesn´t count the heads height.

Nozzle diameter and location (1 of the 3 elevations has to be specified Base, Ground or %)

Elevations of the tap for level transmitters (PV High and PV Low) are specified:


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Valves Characteristic (equal percentage, linear, quick opening or user curve), pressure drop and opening

from valve datasheets as shown below. The option to choose one of the valves manufacturers is

given to accommodate the calculation of the Cv.

The Cv will be calculated by pressing the Size Valve button. The Aspen HYSYS calculated Cv

should be close to the one provided by the valve supplier. In case this is not occurring a special

attention had to be dedicated to the valve to understand the reasons of the discrepancy. In any

case the Aspen HYSYS calculated Cv will be used for the dynamic simulation.

Relief Valves

Type, setting pressures, and other optional details from valve datasheets


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PID Controllers

Action direction, controller mode, PV Range and equivalent PID tuning parameters from DCS as

shown below.

The tuning parameters to enter for each control have to be the ones according to the standard

definition of PID controller. In case the actual control uses a different reference equation, it will

need to perform the conversion of the tuning parameters in such a way that the ones filled

correspond to the standard velocity form definition used by default by HYSYS:

On/Off controllers: This controller generates a digital output (0 or 1) to control a PV. It incorporates latch capability

with higher/lower dead band.


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Elevations It needs to be entered for all equipments that are not located at ground floor. This data needs to be

entered in the Dynamics\Holdup\Nozzles window for each equipment as shown below:

To allow accessing to these parameters, the fidelity option of the Aspen HYSYS integrator has to

be checked and the Static Head contributions has to be enabled:

It is important to remark that the Steady-State solver doesn´t take into account Static-head

contribution, but when it is passed to Dynamics mode new static head contributions are

considered (for example for reflux lines or feed lines in large columns), this affects to the flow in

certain valves which need to be revised to have the right DeltaP and flow.


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3. Calibrate the model with Process Data.

This is the most critical phase of the process since the model has to be manually tuned and

calibrated to reflect the true conditions in the real plant. The user has to manually equilibrate the

discrepancies of the model versus plant by changing and trimming certain values and parameters:

boundary conditions, other property packages, feed compositions, DeltaP and OP of valves, tray

efficiencies, heat losses, pressure drops, etc. Sometimes this task is not trivial, and frequently

other hidden equipment or devices problems are found when the model resists matching the plant

data. There is not a stepped rule to follow since every case is different, but some advices are

given to calibrate certain units operation.

Feed Streams: The first and more important task is to define properly the feed streams

compositions. Feed Lab analysis and/or trustable online analyzers for the selected period of time

are needed. Sometimes there is only lab data of the product streams; in that case the feed streams

will need to be back-calculated from the known streams using the Balance block or Mixer block:

Columns: To calibrate detailed column models it is better to begin with a simplified column

model where all the top and bottom sections are inside the column sub-flowsheet object. Then,

once the simple model is calibrated and converged, a second detailed model is produced

including all the required valves, pumps, aircoolers, heat exchangers, etc.

Once the simple column model is correctly calibrated, then the detailed column model can be

calibrated. The column Tray-Section should be identical to the simple model, so it will converge


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using the same reflux and reboiler inlet streams data from the simple column sub-flowsheet

streams. Then the remaining detailed top and bottom section will need to be converged and

calibrated.

The initial calibration is performed in a simple model because the HYSYS column object has an

internal individual solver that will allow as to quickly try with different column specifications to

reach the best fit with the process plant data.

Temperature profile can be copy/paste into Excel to monitor the matching with plant data.

Temperature Profile C3splitter

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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230

Tray

Te

mp

era

ture

HYSYS Temp

Real Temp


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If column model doesn´t match with plant data, the following points can be revised:

1. - Composition of feed stream is correctly characterized.

2. - Units of volumetric flow-meters are entered correctly (ACT_m3/hr or STD_m3/hr)

3. - Correct topology is considered inside column sub-flowsheet (ie. Two condenser drums, etc)

4. - Pressure profile is correctly specified. Often it is not known or wrong bar/barg units are used.

5. - Property package is properly selected. Use property package wizard assistant.

6. - Mass balance per component (from plant data) is feasible.

7. - Number of real tray is introduced (not theoretical) with their tray efficiency (table below).

8. - If present, condenser sub-cooling is considered.

9. - Instrumentation is well calibrated. Field Pressure and Temperature surveys can be made.

Column Typical number

of real Trays Typical

Efficiency

Deethanizer 25 - 35 65 - 70

Depropanizer 35 - 40 70 - 80

Debutanizer 38 - 45 85 - 90

Alky DeiC4 (reflux) 75 - 90 85 - 90

Alky DeiC4(no reflux) 55 - 70 55 - 65

Naphtha Splitter 25 - 35 70 - 75

C2 Splitter 110 - 130 95 - 100

C3 Splitter 200 - 250 95 - 100

C4 Splitter 70 - 80 85 - 90

Amine Stripper 20 - 24 45 - 55

Crude Distillation. 35 - 50 50 - 60

Stripping Zone 5 - 7 30

Flash Zone- 1st draw 3 - 7 30

I st Draw - 2nd Draw 7 - 10 45 - 50

2nd Draw - 3rd Draw 8 - 10 50 - 55

Top Draw to Reflux 10 - 12 60 - 70

FCC Main Fractionator 24 - 35 50 - 60

If after all the model still doesn´t match with plant data, then some hidden problems are occurring

in your plant that try to transgress the sacred thermodynamics laws.

Valves: Sometimes the Cv calculated by HYSYS when pressing the button “Size Valve” doesn´t

match with the Cv supplied by the valve manufacturer. There are four things to determine the Cv

of a valve: Characteristic Type (Linear, Iso%, etc), % Opening, Flow and DeltaP. The Type is

clearly indicated in the valve datasheet, %Opening and Flow is normally well known from

process plant data, and the DeltaP is usually indicated in the valve datasheet for certain valve

opening. Independently of the Cv given by the valve manufacturer datasheet, the Cv calculated by

HYSYS should be the one to use since it will be the only one who will produce the right

DeltaP/Flow relation for the given %Opening.

Thermosyphon: Frequently, the reboiler of the columns uses a thermosyphon type reboiler with

an utility hot stream. In that case, the reboiler is simulated in Steady-State with a normal

Shell&Tube heat exchanger and using a recycle if the reboiler is outside the column sub-


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flowsheet (this is normally more convenient in Dynamics to see all units operations and

controllers drawn in the main HYSYS PFD. To do this right-click in the column an use “Show

sub-flowsheet Objects” and put all objects outside the column sub-flowsheet environment).

The true thermosyphon effect (liquid self-circulate when heated due to density differences) is not

simulated rigorously by HYSYS, so it needs to be artificially forced to circulate. In dynamics it

will be forced with a flow specified stream (in yellow below), the value of the flow can be

determined from the Steady-State simulation to achieve the desired vapour fraction (usually from

0.2 to 0.8) at the outlet stream directed to the column.


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4. Switch to Dynamic and Stabilize.

Once the model is calibrated in Steady-State with all parameters calculated (UA of heat

exchangers, k values of heat exchangers or column trays, Cv of Valves, etc) and all pressure/flow

specifications are correctly configured (helped by the Dynamic Assistant) then the model is ready

to be switched to Dynamic mode. It is important to note that the warning messages given by the

Dynamic Assistant are only warnings, that is they can be perfectly ignored if the reasons to do it

are understood. For example, we can perfectly specify a “flow spec” in the previously described

thermosyphon reboiler.

The solver is switched to Dynamic Mode and the integrator started. The unit should then be

stabilized (note that static heads are not considered in Steady-State) and all the controllers can be

put in Auto. The model is then ready to be used for the operating changes and control studies.

Additional advices are given to properly run dynamic models:

Integrator time step selection: HYSYS Dynamics use a fix-size time-step solver with

parameterized execution rates for the 4 sub-layers of equations categories:

1.- Pressure/Flow Solver: Pressure-Node network and associated flows is solved. (Default rate: 1)

2.- Control and logical Ops: PID Controllers and logical blocks are calculated. (Default rate: 2)

3.- Energy Calculations: Energy balance for every unit operations is calculated. (Default rate: 2)

4.- Composition and Flash Calculation: Compositional mass balance and flash for every unit

operation is calculated. (Default rate: 2)

For most of the simulations a time step of 0.5 seconds is god enough, although it is always better

to start the simulations with smaller values (like 0.1 or 0.05) and increase the step-size

progressively until 0.5 (or higher values) taking always into account the dynamic nature of the

process to simulate.


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Columns with a small residence time (ie, high reflux rates) need special attention. The residence

time needs to be calculated for every tray as the “bulk liquid volume” on the tray

(TraySection\Dynamic\Holdup) divided by the internal reflux (TraySection\Performance\Flow\

Actual “LtoBelow”) and converted to seconds. This number will be important to know it, since it

will determine our maximum step size.

In order to reproduce with enough rigor the dynamic transitions inside the column, 4 flash

calculations per tray residence time needs to be assured. This can be written with the formula:

Step_Size * CFC_Rate * 4 =< Residence_Time

where CFC_Rate is the execution rate for the “Composition and Flash Calculation” solver layer.

For example, a distillation column with a residence time of 8 seconds can use a step size of 0.1

seconds and a CFC_Rate of 10 ( 0.1*10*4= 4, which is lower than 8) or can use a step size of 1

second and a CFC_Rate of 1 ( 1*1*4= 4, which is lower than 8). But can not use a step size of 0.5

seconds and a CFC_Rate of 10 ( 0.5*10*4= 20, which is higher than 8) because it will produce

not realistic dynamic responses.

Dead Time for Analyzers: On-line analyzers introduce a pure dead-time due to the time it takes

to the fluid to travel from the gathering point to the analyzers cabinet. This dead-time can be

simulated by the Delay option of the Transfer Function block of the HYSYS object palette. The

Outlet of the Transfer Function block can be then connected to the corresponding controller as the

PV value. The transfer function block is shown below:

Lag time for Temperature controllers: The temperature is an intrinsic property of the fluid that

can not be read instantaneously by the temperature transmitters. The temperature transmitters

introduce a lag time that can affect to the process control dynamics. This effect is normally

neglected since the thermal inertia of the whole system is much larger than the lag time of the

instrument, but it can be simulated if needed. When a PID controller is controlling a temperature,

there is a section in the Parameter/PV Conditioning/Stream_Temperature_Filter that allows

introducing a first order time constant to the real stream temperature:


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Level Tap for Sump: Column bottom level controllers will need to use a level tap to realistically

control the level in the bottom of the column. One option is to include a vessel with level taps to

simulate the bottom tray, but it also simpler to use a “Sump” tray type for the bottom tray and use

a spreadsheet to calculate the right level measured by the level tap, and use that value as the PV

for the controller.


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Flash efficiencies in vessels: Streams entering in a vessel are mixed with the existing holdup

(liquid and vapour) and a new flash equilibrium state is reached for every integrator time step. In

the real world, the mix is not totally perfect since all the inlet mass is not instantaneously mixed

with the vessel holdup (in HYSYS is call “Recycle”). This effect affects more to large vessels

with a large holdup. In order to take into account this effect in HYSYS Dynamic the “flash

efficiencies” needs to be specified in the Dynamics\Holdup\Advanced\efficiencies window:

The efficiency is a % value: 0 for no mixing, 100 for perfect mixing. This value is used internally

by HYSYS as the amount of mass that will be used for the flash calculation, all the other fraction

(ie: 1-Eff) will “by-pass” the flash calculation.


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Other control oriented objects: Besides basic PID controllers, HYSYS Dynamics incorporates a

series of simulation objects oriented to cover the advanced PID regulatory controllers, transfer

functions, interlock logics and emergency systems, automatic sequencers and advanced

multivariable controllers:

Boolean Gates: And, Or, delay, latch, counter, …

Split Range

Ratio

PID Controller and Feedforward

Generic MPC Controller

DMCplus Controller

Cause&Effect Matrix

Transfer Function: Lag, Lead, ramp,

Integrator, 2nd order,

sine wave, …

Override Selector

On/Off Controller Spreadsheet for

custom Anti-surge Controller

Documents

Dynamics Quick Start Guide