Reboiler/Condenser Heat Exchanger

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NOTE: This feature is distributed with SysCAD but is currently in BETA. Please contact us ([email protected]) if you run into any issues using this model. This page is currently under development and details may change. Use with caution - we do not guarantee compatibility between different BETA versions.

Navigation: Models ➔ Energy Transfer Models ➔ Reboiler/Condenser Heat Exchanger

Related Links: Reboiler Condenser Heat Exchanger Example


General Description

The Reboiler/Condenser Heat Exchanger (RCX) is used to transfer heat from one stream to another. The RCX Heater unit model is based on patented technology[1] and was developed with in association with the patent holder[2]. In principle, an internally-recirculating heat transfer fluid (HTF) transports heat from the "hot" stream to the "cold" stream, similar to a Heat Pipe (see Wikipedia). The HTF is in the shell side, while the hot and cold streams are in separate tube sides. The RCX Heater unit can be used in a single- or dual-shell arrangement as shown below.

  1. In the Hot Side (Reboiler), heat is transferred from the hot stream to the HTF, cooling the hot stream and boiling the HTF.
  2. The HTF vapour then reports to the Cold Side (Condenser) where energy is transferred to the cold stream, heating the cold stream and condensing the HTF.
  3. The condensed HTF is then returned to the Hot Side, completing the recycle.

KX Diagram.png

KX Flowsheet.png

In a single-shell arrangement the HTF pressure is constant. However, in a dual-shell arrangement as shown to the right, the HTF vapour between Hot Side and Cold Side can be subject to mechanical vapor recompression (MVR) to increase pressure, temperature and boiling point. In this arrangement, the Hot and Cold shell sides operate at different pressures. It is then possible for the "hot" stream (the heat source used to boil the HTF) to be colder than the "cold" stream (the heat sink used to condense the HTF), thereby transferring energy from a colder stream to a hotter stream!

In dual-shell arrangement, the HTF may also be subcooled by partially flooding the Condenser, in order to minimise any flashing which would occur through the Reducer valve separating the Cold and Hot Sides. Due to adiabatic inefficiency, energy is added to the system via MVR. Energy is also lost from the system due to ambient heat loss from the shell(s).

Diagram

KX Heater.png

Inputs and Outputs

Label Required
Optional
Input
Output
Number of Connections Description
Min Max
Hot In 1 Required In 1 10 The stream inlet to the Hot Side (Reboiler).
Cold In 1 Required In 1 10 The stream inlet to the Cold Side (Condenser).
Hot Out 1 Required Out 1 1 The stream outlet from the Hot Side (Reboiler).
Cold Out 1 Required Out 1 1 The stream outlet from the Cold Side (Condenser).

Note that the recirculating HTF (Heat Transfer Fluid) is an internal flow and has no connections.

Behaviour when Model is OFF

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

  • All streams connected to the 'Hot In' inlet will flow out of the 'Hot Out' outlet with no temperature or phase change
  • All streams connected to the 'Cold In' inlet will flow out of the 'Cold Out' outlet with no temperature or phase change
  • Any sub-models, such as Reaction Block, will not occur
  • No energy exchange will occur

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

Model Theory

The KCX Heater is a composite model comprised of multiple embedded unit models. While this unit model could be built in SysCAD from its component parts and the HTF recirculation loop, such a model would be complex to implement, difficult to robustly control, and cumbersome to use within project flowsheets. As such, the implementation as a composite model with internal iterative solver and control provides a robust standalone unit model.

The user specifies the following:

  • Hot and cold streams (by Pipe connection)
  • HTF component (internally recirculating shell-side species, no SysCAD stream connection required)
  • Hot and Cold Side tube details:
    • HTC
    • Area
    • Reactions
    • Pressure drop
  • Hot and Cold Side heat loss from shell
  • Physical layout:
    • Single- or dual-shell
    • MVR (if dual-shell):
      • Temperature or pressure change
      • Mechanical and adiabatic efficiencies
    • Subcooling (if dual-shell):
      • HTC
      • Flooded fraction of Cold Side area

The RCX Heater unit model uses internal iterative calculations to determine the required HTF temperature(s), pressure(s) and mass flowrate to satisfy the energy balance. This uses the nominated HTF component thermodynamic data from the Species Database. Note that both the liquid and gas phases of the HTF component must be defined.

Internally, both the Hot and Cold Sides use the same LMTD heat transfer logic as any heater. Tube-side reactions (including heat of reaction) and pressure drop are evaluated before heat exchange. The RCX Heater iterative solver is bound by the following requirements:

  1. Energy transfer from Hot Stream by LMTD = Energy absorbed in vaporising HTF at Hot Side saturation temperature
  2. Energy transfer to Cold Slurry by LMTD = Energy released by condensing HTF at Cold Side saturation temperature
  3. Energy into the system = Energy out of the system (this is complicated by MVR adiabatic inefficiency, tube-side heat of reaction, and ambient heat loss)

This iterative calculation also includes checks and stops for various forbidden/unsolvable cases, such as Shell-Tube temperature crossover, HTF pressure below minimum allowance, and HTF temperature approaching the component's critical temperature.

Data Sections

The default access window consists of several sections:

  1. HeaterKX tab - This first tab contains general information relating to the unit.
  2. Results tab - This tab contains unit results and intermediate values used for calculation of energy balance.
  3. HotRB - Optional tab, only visible if the Reactions are enabled on the Hot side. Please note that the RCX Heater unit will IGNORE any Source, Sink or HX in the reaction file.
  4. ColdRB - Optional tab, only visible if the Reactions are enabled on the Cold side. Please note that the RCX Heater unit will IGNORE any Source, Sink or HX in the reaction file.
  5. QHotFeed - Optional tab, only visible if ShowQHotFeed is enabled. This and subsequent tab pages, e.g. QHotFeed.. and Sp, shows the properties of the mixed stream as the feed to the Hot side.
  6. QColdFeed - Optional tab, only visible if ShowQColdFeed is enabled. This and subsequent tab pages, e.g. QColdFeed.. and Sp, shows the properties of the mixed stream as the feed to the Cold side.
  7. QHotProd - Optional tab, only visible if ShowQHotProd is enabled. This and subsequent tab pages, e.g. QHotProd.. and Sp, shows the properties of the product stream from the Hot side.
  8. QColdProd - Optional tab, only visible if ShowQColdProd is enabled. This and subsequent tab pages, e.g. QColdProd.. and Sp, shows the properties of the product stream from the Cold side.
  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.

RCX Heat Exchanger Page

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

Tag (Long/Short) Input / Calc Description / Calculated Variables / Options
Tag Display This name tag may be modified with the change tag option.
Condition Display OK if no errors/warnings, otherwise lists errors/warnings.
ConditionCount Display The current number of errors/warnings. If condition is OK, returns 0.
GeneralDescription / GenDesc Display This is an automatically generated description for the unit. If the user has entered text in the 'EqpDesc' field on the Info tab (see below), this will be displayed here.
If this field is blank, then SysCAD will display the UnitType or SubClass.
Requirements
On Tick Box Switches the RCX Heat Exchanger on/off.
Shell ...
HTF.Component List Box Selection from list of HTF (Heat Transfer Fluid) component. Any component with species in both the gas and liquid phases found in the project species configuration will appear in this list. Water is always available as H2O(l) and H2O(g) are always required species in project configuration.
HTF.LiquidSp Calc The liquid species of the selected HTF component.
HTF.VapourSp Calc The gas species of the selected HTF component.
HTF.CriticalT Calc The critical temperature of the selected HTF component.
Layout
MVR.Option None No MVR (Mechanical Vapour Recompression) is included. This allows selection between one or two shell operation.
Increase Pressure MVR is enabled with control over the increase in HTF saturation pressure (saturation temperature change is a result).
Increase Temperature MVR is enabled with control over the increase in HTF saturation temperature (saturation pressure change is a result).
TwoShells Check Box Only available if MVR.Option = None. If this is true, Hot and Cold sides are separate shells with independent heat loss options. If this is false, the unit has a single shell and heat loss is only available for the Hot side.
Subcooling None No sub-cooling of the HTF inside the Cold side shell.
Flood Fraction Sub-cooling is enabled within the Cold side shell by partial flooding.
FloodFraction Input The fraction of Cold side heat transfer area which is flooded.
LiquorHTC Input The heat transfer coefficient of the flooded Cold side tubes (i.e. between the Cold side stream and the liquid phase HTF).
HotSide / ColdSide ... - Note that the Hot (Reboiler) and Cold (Condenser) side input fields are identical.
UserHTC Input The heat transfer coefficient of this side's heat transfer area.
Layout Overall The heat transfer area is defined by area input.
Tube Detail The heat transfer area is defined by tube sizing and layout. This is calculated by [math]\displaystyle{ \pi \times TubeOD \times Tubes \times Passes \times PassLength }[/math]
UserArea Input Available if Layout = Overall. The defined heat transfer area for this side.
TD.TubeOD Input Available if Layout = Tube Detail. The diameter of the heat transfer tubes.
TD.Tubes Input Available if Layout = Tube Detail. The number of heat transfer tubes.
TD.Passes Input Available if Layout = Tube Detail. The number of passes of heat transfer tubes.
TD.PassLength Input Available if Layout = Tube Detail. The length of each pass of heat transfer tubes.
PressDropReqd Input The required pressure drop through the tube side. Note that this is a pressure change inside the tubes and does not affect the pressure of the HTF.
HeatLoss.Method
(Not available for ColdSide if Single Shell.)
None No heat loss.
Fixed HeatFlow Heat loss is a user input power value.
Fractional Heat loss is calculated as a fraction of heat transfer within this side.
Loss to Ambient Heat loss is calculated using shell surface area, HTC and external temperature.
HeatLoss.HeatFlowReqd Input Available if HeatLoss.Method = Fixed HeatFlow. The required heat loss from the shell of this side.
HeatLoss.FracReqd Input Available if HeatLoss.Method = Fractional. The required fraction of heat transfer as heat loss from the shell of this side.
Shell.Diameter Input Available if HeatLoss.Method = Loss to Ambient. The shell diameter of this side. Used for calculating shell area.
Shell.Length Input Available if HeatLoss.Method = Loss to Ambient. The shell length of this side. Used for calculating shell area.
NOTE: This is the same value as TD.PassLength. Changing one will change the other.
Shell.SurfaceArea Calc Available if HeatLoss.Method = Loss to Ambient. The shell surface area of this side. Calculated as [math]\displaystyle{ /pi /times Shell.Diameter /times / Shell.Length }[/math]. Note that the "end caps" are not considered.
Shell.HTC Input Available if HeatLoss.Method = Loss to Ambient. The heat transfer coefficient of the shell (i.e. between the HTF and the external conditions).
AmbTOverride Input Available if HeatLoss.Method = Loss to Ambient. Optional override of the ambient temperature. To use the project environment ambient temperature set this to * (undefined).
Ambient_T Calc Available if HeatLoss.Method = Loss to Ambient. The external temperature used for heat loss calculation from the shell. This is equal to the project environment ambient temperature if AmbTOverride is undefined.
Reactions On Enables Reaction Block (RB) for this side. Reactions are applied inside the tubes and do not affect the HTF. Note that the RCX Heater unit will IGNORE any Source, Sink or HX in the reaction file.
Off No reaction for this side.
MVR ...
Option None No MVR (Mechanical Vapour Recompression) is included. This allows selection between one or two shell operation.
Increase Pressure MVR is enabled with control over the increase in HTF saturation pressure.
Increase Temperature MVR is enabled with control over the increase in HTF saturation temperature.
dT_Reqd Input Available if Option = Increase Temperature. The required change in HTF saturation temperature across the MVR. Pressure change is a result.
dP_Reqd Input Available if Option = Increase Pressure. The required change in HTF saturation pressure across the MVR. Temperature change is a result.
AdiabaticEfficiency / AdiabaticEff Input The adiabatic efficiency of the MVR. Inefficiency is added as heat to the HTF.
MechanicalEfficiency / MechanicalEff Input The mechanical efficiency of the MVR. This is used for calculation of the overall MVR compressor power requirement and has no effect on the HTF.
Specified Configuration - This will show a text description of the physical configuration. E.g. Two Shells, Mechanical Vapour Recompression.
Options
ShowQHotFeed Tick Box QHotFeed and associated tab pages (e.g. Sp) will become visible, showing the properties of the combined Hot stream feed flow. See Material Flow Section. This will be prior to any sub-model actions (e.g. Reactions).
ShowQColdFeed Tick Box QColdFeed and associated tab pages (e.g. Sp) will become visible, showing the properties of the combined Cold stream feed flow. See Material Flow Section. This will be prior to any sub-model actions (e.g. Reactions).
ShowQHotProd Tick Box QHotProd and associated tab pages (e.g. Sp) will become visible, showing the properties of the combined Hot stream outlet flow. See Material Flow Section.
ShowQColdProd Tick Box QColdProd and associated tab pages (e.g. Sp) will become visible, showing the properties of the combined Cold stream outlet flow. See Material Flow Section.
User Warning Limits
TrackHTF.MinP Tick Box Generate a warning message if the calculated HTF pressure falls below a specified value. E.g. to prevent ingress of ambient air.
TrackHTF.CriticalT Tick Box Generate a warning message if the HTF temperature approaches the critical temperature of the component.
HTF.MinP Input Available if TrackHTF.MinP is ticked. The minimum pressure allowed for the HTF. Note that the unit will still solve, but a warning will be generated.
HTF.ApproachCriticalT Input Available if TrackHTF.CriticalT is ticked. The dT approach to the critical temperature. Note that in this range the unit will still solve, but a warning will be generated. Above the critical temperature, the unit will not solve and will always give an error.
Convergence ...
RelTol Input The relative tolerance for the internal iterations to find HTF saturation temperature (internal iterations will stop when the step change is less than this value).
MaxIterCount Input The maximum number of internal iterations on a single Solver step for the internal convergence loop.
IterCount Calc The actual number of internal iterations used on the previous Solver step.
RelError Calc The relative error of the last internal iteration at end of the previous Solver step. (This must be lower than RelTol for the model to be "solved".)

Results Page

This page provides the results of the unit model calculations, including details for the internal recirculating HTF stream.

Tag (Long/Short) Input / Calc Description / Calculated Variables / Options
Specified Configuration - This will show a text description of the physical configuration. E.g. Two Shells, Mechanical Vapour Recompression.
HotSide / ColdSide ... - Most Hot (Reboiler) and Cold (Condenser) side result fields are identical.
For the ColdSide, this applies to tubes in the condensing HTF vapour space only (i.e. excludes subcooling, if enabled).
HTC Calc The Heat Transfer Coefficient used for this side.
Area Calc The Area used for this side.
TemperatureIn / Ti Calc The temperature of the combined streams into the tubes for this side (i.e. Hot In or Cold In temperature).
TemperatureOut / To Calc The temperature of the stream out of the tubes for this side (i.e. Hot Out or Cold Out* temperature).
*For ColdSide, if subcooling is enabled, this is the temperature out of HTF vapour space only.
LMTD Calc The Log Mean Temperature Difference for this side.
DeltaT / dT Calc The change in temperature through the tubes for this side.
PressureIn / Pi Calc The pressure of the combined streams into the tubes for this side (i.e. Hot In or Cold In pressure).
PressureChange / dP Calc The change in pressure through the tubes for this side.
PressureOut / Po Calc The pressure of the stream out of the tubes for this side (i.e. Hot Out or Cold Out pressure).
MassFlow / Qm Calc The mass flow through the tubes for this side.
HeatTransfer / HX Calc The heat transfer with HTF for this side. [math]\displaystyle{ HX = HTC \times Area \times LMTD }[/math] ([math]\displaystyle{ = UA \times LMTD }[/math])
HotSide: This is the heat flow from the Hot stream to vaporise HTF. (= A)
ColdSide: This is the heat flow from condensing HTF to the Cold stream. (= D)
HeatLoss Calc Environmental heat loss from the shell of this side.
HotSide: Heat loss from Hot shell. (= B)
ColdSide: Heat loss from Cold shell. (= E) (Not available for Single Shell)
RB.HeatOfReaction / RB.HOR Calc (Only visible if Reactions are enabled) Heat of reaction within the tubes for this side.
RB.TemperatureOut / RB.To Calc (Only visible if Reactions are enabled) The temperature of the combined streams into the tubes after evaluation of reactions (RB), before any heat exchange.
HTF.CondensingPower Calc (ColdSide only, visible if Two Shell operation) Power from HTF condensation at cold side HTF conditions. (= D + E)
HTF.FlashPower Calc (ColdSide only, visible if Two Shell operation) Excess energy released flashing HTF condensate to hot side HTF conditions. (= F)
HTF.FlashMassFlow / HTF.FlashQm Calc (ColdSide only, visible if MVR is online) Mass flow of vapour generated in flash to hot side HTF conditions.
Shell ... (Hot side shell if Two Shell operation)
HTF.TransferPower / HTF.Power Calc Total power transfer to HTF. (= A - B + F)
HTF.Temperature / HTF.T Calc HTF temperature.
HTF.Pressure / HTF.P Calc HTF saturation pressure.
HTF.SatT@P Calc HTF saturation temperature at pressure. (= HTF.T)
HTF.MassFlow / HTF.Qm Calc HTF recirculating mass flowrate.
HTF.LatHtVap Calc HTF latent heat of vaporisation at shell temperature and pressure.
MVR ... (only available if MVR is online)
EntropyIn / Si Calc HTF entropy at MVR inlet temperature and pressure.
PressureIn / Pi Calc HTF pressure at MVR inlet (= HTF.P)
TemperatureIn / Ti Calc HTF temperature at MVR inlet (= HTF.T)
IdealPower Calc Required MVR power input to HTF vapour at 100% adiabatic efficiency to achieve specified dT or dP.
Power Calc Actual MVR power input to HTF vapour at defined adiabatic efficiency. (= C)
InputPower Calc Power input to MVR compressor at defined mechanical efficiency.
PressChange / dP Calc Pressure change through MVR.
PressureOut / Po Calc HTF pressure at MVR outlet.
SatT@P Calc HTF saturation temperature at MVR outlet pressure.
TemperatureOut / To Calc HTF temperature at MVR outlet.
EntropyOut / So Calc HTF entropy at MVR outlet temperature and pressure.
Subcool ...
SubcoolingPower / Power Calc Power transfer for subcooling of HTF condensate to Cold stream. (= G)
HTF.Temperature / HTF.T Calc Temperature of subcooled HTF condensate.
Flooded.MassFlow / Flooded.Qm Calc Mass flow of Cold stream used for subcooling.
Flooded.TemperatureOut / Flooded.To Calc Outlet temperature of Cold stream used for subcooling.
ColdOutlet.TemperatureOut / ColdOutlet.To Calc Outlet temperature of the combined Cold stream (i.e. Cold Out temperature).
ColdOutlet.DeltaT / ColdOutlet.dT Calc The overall Cold stream temperature change.
Results
EnergyBalance Calc The overall energy balance, this should be zero when solved. (= A - B + C - D - E - F - G)
HeatFlow Calc Total external heat flow to the RCX Heater unit. (= - B + C - E)
HTF.LiqEnthalpy Calc HTF liquor phase enthalpy at HTF.T and HTF.P (HotSide).
HTF.VapEnthalpy Calc HTF gas phase enthalpy at HTF.T and HTF.P (HotSide).
HTF.VapEnthalpy_MVR Calc HTF gas phase enthalpy at MVR.To and MVR.Po (MVR outlet / ColdSide feed).
HTF.LiqEnthalpy_Cond Calc HTF liquor phase enthalpy at MVR.SatT and MVR.Po (ColdSide).
HTF.LiqEnthalpy_Sub Calc HTF liquor phase enthalpy at SubCool.HTF.T and MVR.P (ColdSide subcooling).

Adding this Model to a Project

Add to Configuration File

Sort either by DLL or Group:

  DLL:
HeatExchange.dll
Units/Links Heat Transfer: Reboiler/Condenser Heat Exchanger
or Group:
Energy Transfer
Units/Links Heat Transfer: Reboiler/Condenser Heat Exchanger

See Model Selection for more information on adding models to the configuration file.


Insert into Project Flowsheet

  Insert Unit Heat Transfer Reboiler/Condenser Heat Exchanger

See Insert Unit for general information on inserting units.

Example Project

References

  1. Clegg, R.L. (2020) "Heat Exchanger", United States Patent Application US 2020/0284514 A1
    • Note that the technology is patented or patent-pending in various global regions, including Australia, China (ZL 2018800578696), Europe, West Africa and U.S (US20200284514A1).
  2. Clegg, R.L. (July 2020) Private Correspondence between KWA Kenwalt Australia and Refina