Multi-Stage Counter Flow Tie

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Related Links: Tie

This model is only available in SysCAD 9.3.


General Description

This model is designed to emulate equipment that contacts a Gas stream with either a Liquid or Solid stream in counter-current flow.

The Gas enters the at Stage 1 of the unit, while the Liquid (or Solid) stream enters at the Final stage of the unit (if there is only a single stage, then both feeds enter Stage 1). The user may specify any number of stages from 1 to 5 in the unit. The Liquids or Solids flow from the Final stage to the Stage 1, while Gas flows in the opposite direction from the Stage 1 to the Final stage. The user may set a Gas bypass at each stage of the unit and the Gas flow may also have a nominal pressure gradient.

This model can be used to emulate a number of unit operations that involve contacting gas with liquids or solids, including Absorbers, Kilns, Scrubbers, etc.

CCTie.png

Each stage in the model has an Evaluation Blocks (EB), and hence the user may enable Reactions (RB), Vapour-Liquid Equilibrium (VLE), Solubility (PC) and Environmental Heat Loss (EHX) in each stage. These may be evaluated in any order.

At each stage, the Liquid and Gas streams from previous and succeeding stage are combined and the Evaluation Block is applied to the resulting stream. The contents are then phase split, with the Gas being sent towards the Final stage and the Liquor sent to Stage 1. The evaluations are done in parallel for a number of iterations. At least as many iterations as there are stages are needed to propagate changes in the feeds to the products.

At each stage entrainment of Solids or Liquids into the Gas, as well as Gas into the Liquor, may be specified.

CCReactor02.png

Available Sub-Models

The following sub-models may be enabled in any, or all, of the stages:

  1. Solubility may be enabled in any stage, if this is NOT enabled globally.
  2. The Reaction Block (RB) allows the user to configure any number of reactions within a stage.
  3. The Environmental Heat Exchanger Block allows the user to add or remove energy from a stage using a number of different methods.
  4. The Vapour Liquid Equilibrium Block may be enabled in any stage.

Diagram

Counterflow Tie.png

  • The diagram shows default drawings of the Multi-Stage Counter Flow Tie with connecting streams.
  • The physical location of the streams connecting to the Multi-Stage Counter Flow Tie is unimportant. The user may connect the streams to any position on the unit.
  • When inserting a Multi-Stage Counter Flow Tie into a flowsheet, the user may choose a different symbol from the menu.

Inputs and Outputs

Label Required
Optional
Input
Output
Number of Connections Description
Min Max.
Liquid Feed Required In 1 10 Inlet liquid or slurry stream to the Final Stage of the unit.
Gas Feed Required In 1 10 Inlet Gas stream to the Stage 1 of the unit.
Liquid Product Required Out 1 1 Product Liquid or Slurry outlet from the Stage 1 of the unit.
Gas Product Required Out 1 1 Product Gas outlet from the Final Stage of the unit.

Behaviour when Model is OFF

If the user disables the unit, by un-ticking the On tick box, then the following actions occur:

  • All material in streams connected to the Gas Feed inlets will flow straight out of the Gas Product outlet, with no exchange with material entering through the Liquid Feed;
  • All material in streams connected to the Liquid Feed inlets will flow straight out of the Liquid Product outlet, with no exchange with material entering through the Gas Feed;
  • No sub-models will be called.

So basically, the unit will be 'bypassed' without the user having to change any connections.

Model Theory

In Steady State, all the Gas Feed streams are perfectly mixed, the Bypass stream is removed and the remaining Gas is added to Stage 1. Similarly, all of the Liquid Feed streams (which may also contain Solids) are perfectly mixed and added to the Final Stage (which may also be Stage 1 if the user has only selected a single stage).

Any sub-models that have been enabled in any of the Stages are carried out on the mixture.

The Liquids and Gases are split after each stage and the Gases are sent to Stage n+1, while the Liquids (and Solids) are sent to Stage n-1. The Liquid and Gas streams may contain some entrainment of the other phase, if defined by the user.

Flow Chart

CC Tie Flow Diagram2.png

Data Sections

The default access window consists of several sections:

  1. CFMultiTie - The first tab contains general information relating to the unit and allows the user to set the number of stages, Gas bypass, Entrainment, etc.
  2. EB1 - This tab allows the user to enable sub-models in the First stage of the unit. If more than one stage is enabled, then the additional stages will all have a separate page, names EB2, EB3, etc. with the same fields as shown on this page.
  3. PC1 - Optional tab, only visible if the Solubility is enabled in any of the Evaluation Blocks. Each stage may have a PC tab page.
  4. RB1 - Optional tab, this tab will be visible for any stages that have Reactions enabled in the Evaluation Block.
  5. EHX1 - Optional tab, this tab will be visible for any stages that have EnvironHX enabled in the Evaluation Block.
  6. VLE1 - Optional tab, this tab will be visible for any stages that have Vapour Liquid Equilibrium (VLE) enabled in the Evaluation Block.
  7. QGx - Optional tab, only visible if a Gas flow is enabled in the field Stream2View. This page shows the properties of the selected Gas stream.
  8. QLx - Optional tab, only visible if a Liquid flow is enabled in the field Stream2View. This page shows the properties of the selected Liquid/Solid stream.
  9. Info tab - contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  10. Links tab, contains a summary table for all the input and output streams.
  11. Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.


CFMultiTie

Unit Type: CFMultiTie - The first tab page in the access window will have this name.

Symbol / Tag
Input / Calc
Description
Common First Data Section
Requirements:
On Tickbox This variable in used to turn the unit ON or OFF. If the unit is OFF, the Gas will flow out of the Gas outlet with no change in state, and the Liquids will flow out of the Liquid outlet with no changes. So, the phases will not contact each other at all.
Stages List The user may select between 1 and 5 stages in the unit.
Stream2View List The user may select to display none or any of the internal Gas or Liquid streams.
Pressure Control

This allows the user to set if the pressure is fixed or varies across the unit.
The OperatingP - Method is used as the Reference Pressure, except where StagePressure = Linear (calc dP) or Individual.

StagePressure Fixed (Gas Feed Reference) In this case there is no pressure drop across the unit and the Liquid and Gas exit the unit with the same pressure as the pressure in the First Stage (the Gas Feed stage) of the unit. (This pressure is set in the field OperatingP - Method)
Fixed (Liquid Feed Reference) In this case there is no pressure drop across the unit and the Liquid and Gas exit the unit with the same pressure as the pressure in the Final Stage (the Liquid Feed stage) of the unit. (This pressure is set in the field OperatingP - Method)
Linear (Calc dP) In this case the unit calculates the Total pressure drop across the unit based on the Gas and Liquid Feed pressures. The Total pressure drop is then divided evenly across all of the stages.
Linear (User dP Gas Feed Ref) In this case, the First Stage (the Gas feed stage) is taken as the reference pressure and the user specifies the pressure drop across each stage of the unit. So, the pressure in the Final stage (the Gas exit stage) will be calculated.
Linear (User dP Liquid Feed Ref) In this case, the Final Stage (the Liquid feed stage) is taken as the reference pressure and the user specifies the pressure drop across each stage of the unit. So, the pressure in the First stage (the Gas entry stage) will be calculated.
Individual The user may specify the pressure in each individual stage.
StageDP_Reqd Input This field is only visible if StagePressure = Linear (User dP Gas Feed Ref) or Linear (User dP Liquid Feed Ref). The user sets the pressure drop that will be applied across each stage in the unit.
StageDP Display The calculated pressure drop across each stage in the unit. This field is not visible if StagePressure = Individual.
OperatingP
  • If StagePressure = Fixed then the entire unit operates at the pressure set by Method.
  • If StagePressure = Linear with User dP, then the user sets the operating pressure in either the First or Last stage of the unit, depending on which option is chosen. The pressure then changes across the stages according the user defined pressure drop.
  • If StagePressure = Linear (calc dP) or Individual then this section is not visible.
Method Atmospheric The unit starts at Atmospheric Pressure. The atmospheric pressure is calculated by SysCAD based on the user defined elevation (default elevation is at sea level = 101.325 kPa). The elevation can be changed on the Environment Tab page of the Plant Model.
RequiredP The unit starts at the user specified pressure.
AutoDetect If there are any liquids AND no gases present in the feed, the unit will start at the highest pressure of the feeds. Else if there are Gases in any of the feed streams, the unit will start at the lowest pressure of the feeds.
Lowest Feed The unit starts at the Lowest Feed pressure.
Highest Feed The unit starts at the Highest Feed pressure.
Reqd Input This field is only visible if 'Method' = RequiredP. The user enters the required starting or operating pressure.
Result Display This field displays the operating pressure of the unit.
Stage Requirements

The table shown here allows the user to set the Gas bypass and the entrainment values. The user may set a single value that is applied to each stage by ticking the 'All' box and then setting the required value in the first column. Otherwise, do not tick the 'ALL' box and set the values individually for each stage.

PressureReqd Input This is only visible if StagePressure = Individual. This allows the user to set the pressure individually for each stage, or in the entire unit if the 'All' tickbox is enabled.
GasByPass Input This allows the user to set the Gas bypass, as a single value applied to each stage, or individually for each stage.
Entrainment
VapToLiq Input This allows the user to set the Vapour entrainment in the Liquid stream, as a single value applied to each stage, or individually for each stage.
SolToGas Input This allows the user to set the Solid to Gas entrainment, as a single value applied to each stage, or individually for each stage.
LiqToGas Input This allows the user to set the Liquid to Gas entrainment, as a single value applied to each stage, or individually for each stage.
Stage Results Summary

The table shown here gives a summary of the conditions in each of the individual stages.

P Display The exit pressure from each individual stage.
VQm Display The Gas flow from each individual stage.
LQm Display The Liquid flow from each individual stage.
SQm Display The Solids flow from each individual stage.
Ti Display The Temperature into each individual stage (after the Gas and Liquid/Solid steams are combined, but before any sub-models are evaluated.)
To Display The Temperature out of each individual stage.
Internal Flow Summary

The table shown here gives a summary of variables in each of the internal Gas and Liquid/Solid streams between the stages.

Qm Display The mass flow in the individual internal streams.
Vf Display The Vapour fraction in the individual internal streams.
T Display The Temperature of the individual internal streams.

EB1

Each Stage will have an individual tab page. The user may enable the required sub-models in each stage.

Symbol / Tag
Input / Calc
Description
Stage Evaluation Block
Solubility.On Tickbox The user may enable the solubility in the Stage only if Global solubility is disabled.
EvalSequence Calc The sequence in which the sub models (which are part of the evaluation blocks) will be calculated. The sequence is determined by the priority selection for the individual sub-models. If none are selected, the default evaluation sequence is RB,EHX,VLE.
Reactions List This can be used to switch on the Reaction Block (RB). If this is 'On' then the associated page, RB1 becomes visible and may be configured.
Note: The user does not have to configure a reaction file, even if this block is checked.
EnvironHX List This can be used to switch on the Environmental Heat Exchanger (EHX). If this is 'On' then the associated page, EHX1 will become visible and may be configured.
Note: The user does not have to configure an environmental heat exchange, even if this block is checked.
VLEquilibrium List This can be used to switch on Vapour Liquid Equilibrium (VLE). If this is 'On' then the associated page, VLE1 becomes visible and may be configured.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

  DLL: Basic2.dll Units/Links Process: Tie - Multistage Counter Flow
or Group: Energy Transfer Units/Links Process: Tie - Multistage Counter Flow

See Project Configuration for more information on adding models to the configuration file.


Insert into Project

  Insert Unit Process Tie - Multistage Counter Flow

See Insert Unit for general information on inserting units.

Hints and Comments

  1. There may be mass entering or leaving the unit via the Reaction Block source or sink, so if the outgoing mass is not the same as the incoming mass, this may be a place to check first.