Falling Film Evaporator 2
From SysCAD Documentation
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Contents |
General Description
The falling film evaporator is used for liquor concentration. A schematic of the unit operation and its inlets and outlet are found in the next heading, under Diagram.
In a falling film evaporator, weak liquor is pumped to the top of the unit and distributed to the tubes where it runs down as a film. Steam is entered on the shell side, the condensation of the steam provides energy to heat and if hot enough, evaporate water from the weak liquor, thus concentrating the liquor stream.
When specifying the model requirements, the user has the option to enter in a temperature rise required for the liquor stream, this is for the case when the weak liquor entering is not close to its boiling point, thus heating of the liquor is required. If this temperature rise term is left blank, SysCAD assumes the liquor stream will enter and leave at the same temperature.
The concentrated liquor and Liquor Evap streams will leave the system at Saturated Pressure based on the Final liquor temperature.
On the shell side, a heat loss term can be specified. This accounts for any inefficiency of the unit. If this heat loss term is zero, then all the energy from steam condensation will be used to heat and evaporate the weak liquor. The condensate will leave the system at saturated temperature based on the steam pressure.
NOTES:
- To avoid any accidental incorrect feed liquor specifications, (thus left at default conditions of 20°C and atmospheric pressure), the model has a minimum pressure which the unit operation must keep to. This field is user configurable and can be used to prevent unrealistic answers from the model.
- The Falling Film Evaporation Project, which is distributed with SysCAD in the Examples folder, demonstrates the use of this model in a SysCAD project.
WARNING: Currently, the Falling Film Evaporator will NOT be included as part of the Flash Train.
Diagram
The diagram shows a drawing of the Falling Film Evaporator, with the required connecting streams. The Falling Film Evaporator expects 2 inlets: Steam In, and Weak Liquor in. Three outlets must also be connected, these are: condensate, concentrated liquor and evaporated vapour from the liquor.
Inputs and Outputs
| Label | Input /Output | No. of Connections | Description | |
| Min | Max. | |||
| Tube_In | In | 1 | 10 | Weak Liquor |
| Tube_Out | Out | 1 | 1 | Concentrated Liquor |
| Shell_In | In | 1 | 10 | Steam in |
| Shell_Out | Out | 1 | 1 | condensate out |
| Tube_Vap_Out | Out | 1 | 1 | Evaporated Vapour. |
| Shell_Vent | Out | 0 | 1 | Optional vent for non-condensible and excess steam. |
Model theory
The amount of evaporated steam from the liquor stream is calculated based on:
1) Energy from fully condensing of steam.
2) Less heat loss specified by the user
3) Less energy used to heat liquor stream to required temperature.
4) Mass of evaporated steam is = Energy available / latent heat (Final Liquor T, P)
Operating Modes Discussion
Live Steam Mode
Live Steam mode is intended as a demand mode where the actual flow is determined by the heat exchanger parameters and liquor stream temperature. At present, the steam flow is ignored and a condensate rate is determined based purely on the steam conditions. The model can either take the actual steam pressure in the feed stream, or else can select the option SetPressure to specify the steam pressure in the access window. The latter option acts like a control valve in the feed line. If the specified pressure is greater than the steam feed pressure, then a warning message will be displayed and the steam feed pressure used for the calculation.
For true mass balance, the user must set the actual steam flow to match the condensate rate determined by the model.
Condensing Mode
Falling film evaporators are a combination of flash tanks and heat exchangers so their operation has subtleties that combine issues associated with both. Unlike a normal heat exchanger, it is possible for the temperature in the liquor stream to rise or fall, depending on the evaporating pressure.
It is also possible for the unit to have condensing even if the actual feed liquor temperature is higher than the steam saturation temperature! This is because the final liquor temperature is determined by the tubeside operating pressure, and the temperature after flashing may be lower than the steam saturation temperature.
In such cases, recirculation is important to maintain a low tubeside temperature. Recirculation takes cold post flashed liquor from the outlet and mixes it with the incoming stream, reducing the effective tubeside temperature. (Recirculation is also important operationally, maintaining a high film coefficient and reducing scaling.)
A good understanding of the operating principles of evaporators is helpful in constructing and debugging models.
Flowchart
Data Sections
The default access window consists of 5 sections,
- The first tab contains general information relating to the unit.
- The second tab VLE, contains the VLE information for the tube side.
- The third tab VLESS, contains the VLE information for the shell side.
- The Info section, contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents. This is fully described in Common Data Sections.
- Links tab, only visible in SysCAD 9.2, contains a summary table for all the input and output streams.
- Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.
Class: FFEvap2 - The first tab page in the access window will have this name.
|
Tag / Symbol |
Input / Calc / Options |
Description |
Requirements | ||
| On | tick box | If this option is switched off, then the unit operation will behave like a pipe model. Thus, inlet conditions = outlet conditions and no heat transfer will take place. |
| OpMode | Condensing | This is for use in a flash train configuration where the Shell_In is supplied by a Flash tank. In this configuration, the steam input to the Falling Film Evaporator is in demand mode and the steam will be fully condensed (excluding vent amount). |
| Live Steam | This is the method that should be selected if the Shell is supplied by a Live Steam Feed. The user will specify the steam flow to the HX, the amount of steam condensed will depend on the HX configurations and flow conditions. See Operating Modes Discussion | |
| SetPressure | Tick box | Only visible with the Live Steam Method. This enables the user to set the shell pressure. |
| DemandMode | Tick box | Only visible with the Live Steam Method.
|
| HTC | Input | The user specified heat transfer coefficient. |
| Area | Input | The user specified heat transfer area. |
| AddedHeat | Input | The user specified additional heat. |
| POut | Input | The user specified output pressure of the FFE. |
| PShell | Input | Only visible with the Live Steam Method and Set Pressure tick box selected. This is the user specified Shell Pressure, the Shell Pressure must be <= steam supply pressure. Used here to act as pressure drop in the shell side. This will affect the condensate outlet temperature and heat exchange with the tube side. Can be used to control the outlet Temperature for the FFE. |
| QmVentRqd | Input | User specified amount of steam to be vented |
| ReportTolerance | Input | The required tolerance for warnings. |
| Recirculation of Liquor | ||
| Recirc | Tick Box | This enables the recirculation option. |
| RecircFraction | Input | Only visible if the Recirc option has been selected. The user specified recirculation fraction. The recirculation fraction = Recirculation Flow/(Liquor Product + Recirculation Flow) |
| User Set Limit Warnings | ||
| TubeSideDeltaT | Tick Box | This enables the tube side change in temperature warning. |
| MaxTubeSide_dT | Input | Only visible if TubeSideDeltaT is selected. The user specified maximum tube side change in temperature before a warning is generated. |
| SpecificVapLoad | Tick Box | This enables the specific vapour load warning. |
| MaxVapPerArea | Input | Only visible if SpecificVaporLoad is selected. The user specified maximum specific vapour load (flow of vapour per unit area) before a warning is generated. |
| Show Liquor Side Flows | ||
| ShowQLiqFeed | Tick Box | QLiqFeed and associated tab pages (eg Qm) will become visible, showing the properties of the combined fresh liquor feed stream. See Material Flow Section. |
| ShowQLiqProd | Tick Box | QLiqProd and associated tab pages (eg Qm) will become visible, showing the properties of the combined feed stream. This will not include any vapours which are sent to the Tube Vap Out stream. See Material Flow Section. |
| ShowQRecirc | Tick Box | Only visible if the Recirc option has been selected. QRecirc and associated tab pages (eg Qm) will become visible, showing the properties of the recirculation stream. See Material Flow Section. |
| ShowQTubeIn | Tick Box | Only visible if the Recirc option has been selected. QTubeIn and associated tab pages (eg Qm) will become visible, showing the properties of the combined liquor feed stream including the recirculation stream. This is the sum of the LiqFeed and Recirc streams. See Material Flow Section. |
Results | ||
| U*A | Calc | heat exchanger UA. |
| LMTD | Calc | heat exchanger log mean temperature difference |
| TheoDuty | Calc | Theoretical duty of the falling film evaporator heat exchanger. |
| MaxTheoDuty | Calc | The maximum possible theoretical duty of the falling film evaporator heat exchanger. |
| RqdArea | Calc | The calculated area required to perform the heat exchanger duty. |
| MaxCondDuty | Calc | The maximum possible duty available from condensing the steam. |
| Recirc.Qv | Calc | Only visible if the Recirc option has been selected. The volume flow of the recirculation stream. |
| Recirc.Qm | Calc | Only visible if the Recirc option has been selected. The mass flow of the recirculation stream. |
| Tube Side | ||
| Tube.In.T | Calc | Temperature of the stream entering the Tube side. |
| Tube.Out.T | Calc | Temperature of the stream leaving the Tube side. |
| Tube.DeltaT | Calc | The difference in temperature between the stream entering the tube side and the stream leaving the tube side. |
| Tube.In.P | Calc | Pressure of the stream entering the Tube side. |
| Tube.SatP | Calc | The saturation pressure of the stream entering the Tube side. |
| Tube.Qm | Calc | Mass flow through the tube side. |
| EnthalpyIn | Calc | Enthalpy of the stream entering the Tube side. |
| EnthalpyOut | Calc | Enthalpy of the stream leaving the Tube side. |
| VapourRate | Calc | The evaporation rate. |
| Shell Side | ||
| Shell.In.T | Calc | Temperature of the stream entering the Shell side. |
| Shell.Out.T | Calc | Temperature of the stream leaving the Shell side. |
| Shell.SatT | Calc | Shell side Saturation temperature. |
| Shell.Superheat | Calc | The degree of superheat on the shell side. |
| Shell.P | Calc | Shell pressure. |
| Shell.Qm | Calc | The mass flowrate going through the Shell side. |
| CondRate | Calc | The condensate rate. |
| CondDemand | Calc | The condensate demand. |
| DeltaH | Calc | The change of enthalpy on the Shell side. |
| SpecVapLoad | Calc | The specific vapour load (flow of vapour per unit area). |
Adding this Model to a Project
Insert into Configuration file
Sort either by DLL or Group.
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DLL: |
HeatXch1.dll |
→ |
Units/Links |
→ |
Heat Transfer: Falling Film Evaporator(2) |
|
or |
Group: |
Energy Transfer |
→ |
Units/Links |
→ |
Heat Transfer: Falling Film Evaporator(2) |
See Project Configuration for more information on adding models to the configuration file.
Insert into Project
|
|
Insert Unit |
→ |
Heat Transfer |
→ |
Falling Film Evaporator(2) |
See Insert Unit for general information on inserting units.
Hints and Comments
- Currently, the Falling Film Evaporator is NOT included in the groups of models that will form a Flash Train. Therefore, if a Flash tank and Falling film evaporator is placed together, it will NOT automatically demand steam from the flash tank. Thus, the user needs to configure the flash tank as if it is stand-alone, then the steam evaporated can be used to feed the Falling film evaporator.
