Tank
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General Description
The general purpose tank serves as a storage unit for material. It is commonly used to store liquids, slurries and gases and can either be open or closed. Material can enter and leave from various positions and vents. When materials are mixed in the tank, a homogenous product is produced.
The tank behaviour differs for ProBal and Dynamic. In ProBal the tank model is very similar to the tie model. If the General Model (GM) is enabled in the tank, the user can control the split between the outgoing streams.
The Reaction Block (RB) allows the user to configure any number of reactions within the tank.
If the Heat Exchanger (HX) is enabled, then the user must connect links to the HX input and the HX output. The Heat Exchange page needs to be configured with the relevant heat exchange area and overall heat transfer coefficient. The model combines all of the incoming streams, performs any reactions, and then carries out the heat exchange operation. Please note that the fluid in the heat exchanger will transfer energy via sensible heat ONLY. There is NO condensing or boiling in the heat exchanger.
In dynamic mode, the tank will include a surge which will allow the tank level to go up and down depending on the flow conditions. A number of extra options related to managing the dynamic behaviour of the model are available.
Diagram
The diagram shows a drawing of the tank, with a number of connecting streams. When the user inserts the tank into a flowsheet, a different graphics symbol can be selected from a pull down menu. The streams may be connected to any area of the drawing without affecting the actual connection point. For example, the user may draw the gas vent connection to the bottom of the tank, but it will still only vent gases from the tank. SysCAD ignores the physical position of the connection. In dynamic mode, you can set the connection height of the links.
Inputs and Outputs
| Label | Input / Output | No. of Connections | Description | |
|---|---|---|---|---|
| Min | Max | |||
| Input | In | 0 | 20 | General purpose inlet |
| Output | Out | 0 | 20 | General purpose outlet |
| Overflow | Out | 0 | 10 | Outlet only. (Dynamic mode only) |
| GasVent | Out | 0 | 10 | Outlet for vapour species. (Dynamic mode only) |
| HX_in | In | 0/1* | 1 | Heat exchanger inlet. *This is required only if the HX block is selected. |
| HX_Out | Out | 0/1* | 1 | Heat exchanger outlet. *This is required only if the HX block is selected. |
Model Theory - ProBal
In ProBal, all the input streams (excluding the HX streams) are perfectly mixed, then any sub-model actions occur on the mixture. This is very similar to the Tie model.
Model Theory - Dynamic
In Dynamic mode the tank has content (surge), and the sub-models are specified as follows:
- the Reaction Block (RB) can be specified to act on EITHER the combined feed to the tank, or on the contents of the tank, not both. If the user chooses to use the RB in the tank contents, then the reactions must have Rate constants.
- the Heat Exchanger (HX) sub-model acts on the content of the tank
- the Vapour Liquid Equilibrium (VLE) sub-model acts on the content of the tank
- the sub-models Makeup Block (MU) and Environmental Heat Exchanger (EHX) always act on the combined feed to the tank, NOT on the Contents of the tank. Therefore, they will behave in exactly the same way as in ProBal mode.
Vapour Control in Inline Tanks
In an Inline tank, the vapours in the tank can be controlled by two methods.
- The user can choose to specify the (maximum) volume of vapours retained in the tank using the FixedVolume method. This volume can be zero (ie. no vapours are retained in the tank) and can also be larger than the volume specified for the tank itself. This is not related to the volume of solids and liquids in the tank or the volume specified for the tank itself.
- The user can choose to allow the vapours to fill up the available space in the tank using the AvailVolume method. The available volume is the total volume of the tank minus the volume occupied by liquids and solids retained in the tank. Thus the allowable volume of vapours could change from one iteration to the next as the amount of liquids and solids in the tank changes.
No vapour will exit the tank until the vapour volume has been filled. If a gas vent is connected, then any vapours exiting will be sent to this connection. Otherwise the vapours will be sent to a vent direct link, similar to spills.
The vapour retained in the tank will be available to participate in any reactions on the content of the tank.
Currently, the outlet flows are determined prior to the reactions acting on the content of the tank. This has the following implications:
- If a required reactant is fed to the tank, it may be sent to the outlet prior to reacting. This may lead to not enough reactant being available to meet the user specified extent, even though sufficient reactant has been fed to the tank. This situation may be worse as the step size of the project increases as the amount retained in the tank (and hence available to react) is proportionally less than the feed to the tank in one iteration.
- There is a lag of one iteration between the reaction occurring and the resultant affect on the outlet compositions and temperatures. If any of the products of these reactions are vapours, then this may lead to the amount of the vapour retained in the tank being more than the maximum specified by the user.
In a future release of SysCAD, the reactions will occur on the combined content of the tank and feed, prior to the outlets being set.
Flowchart
The following shows the sequence of events if sub model options are switched on. See next heading for more information.
Summary of Data Sections
- Tank-1 tab contains general information relating to the unit.
- IOOpts tab. This tab is only available in Dynamic mode.
- IOEqns tab. This tab is only available in Dynamic mode for a Buffered Tank.
- Optional tab RB, or Reaction Block, sub-model fully described in Reaction Block (RB).
- Optional tab EHX, sub-model fully described in Environmental Heat Exchanger (EHX).
- Optional tab MU, or makeup block fully described in Makeup Block (MU)
- Optional tab HX for the heat exchange block when HX streams are connected. Described in Heat Exchanger (HX).
- Optional tab VLE, sub-model fully described in Vapour Liquid Equilibrium (VLE). This tab is only available for Dynamic mode.
- Optional tab GM, or General Model for flow split, fully described in General Model (GM). This tab is only available for ProBal mode.
- Optional tab GMT, or General Thermal Model for thermal split with GM, fully described in General Model Thermal (GMT). This tab is only available for ProBal mode.
- Optional tab LL. This tab is only available for Dynamic mode.
- Optional tab Body. This tab is only available for Dynamic mode.
- Optional tab QFeed is shown if ShowQFeed is selected. This page shows the properties of the mixed stream (see Material Flow Section) as the feed to the tank. This is before any sub-model actions (eg reactions) take place.
- Optional tab QProd is shown if ShowQProd is selected. This page shows the properties of the mixed output stream (see Material Flow Section) as the product before separation to outlet streams of the tank. This is after any sub-model actions take place but before flow splits. This tab is only available for ProBal mode.
- Content tab and associated pages. This tab is only available in Dynamic mode.
- Optional DirectIO tab.
- The Info tab contains general settings for the unit, fully described in Common Data Sections.
- The last section Audit contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.
Data Sections
The following fields appear on the access window for the tank in both ProBal and Dynamic modes except where indicated otherwise. Additional fields are shown in Dynamic mode. These are described in the Tank - Dynamic Mode.
Tank-1 Tab Page
| Tag / Symbol | Input/ Calc | Description |
|---|---|---|
| Common Data Sections | ||
| Closed | Tick Box | Option to treat as a closed tank. Affects how vapours are treated in Dynamic Mode. This feature is not fully implemented and should not be used at present. |
| Networked | Tick Box | Only visible if Closed option has been chosen. |
| OperatingP (previously PBPress in SysCAD 9.1 or earlier) | ||
| Method | List | Atmospheric -- outlet streams will be 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 Species tab page of the Plant Model. |
| LowestFeed -- outlet streams will take the lowest pressure of the feeds. | ||
| HighestFeed -- outlet streams will take the highest pressure of the feeds. | ||
| RequiredP -- outlet streams will be at the user specified pressure. | ||
| Saturated (not available in SysCAD 9.2 or later) -- outlet streams will be at the saturated pressure @T of the feeds before any sub-model actions (eg EHX, RB, etc) | ||
| IgnoreLowQm | Tick Box | This option is only visible if the LowestFeed or HighestFeed methods are chosen. When calculating the outlet pressure and temperature of the tank, SysCAD will ignore the low flow feed streams should this option be selected. The low flow limit is set in the field below. |
| LowQmFrac | Input | This field is only visible if the IgnoreLowQm option is selected. This is the amount any stream contributes to the total flow. For example, if the total feed to the tank is 10 kg/s, and this field is set to 1%. Then any feed streams with less than 0.1 kg/s will be ignored in the pressure calculations. |
| Reqd | Input | This field is only visible if the RequiredP method is chosen. This is user specified pressure. |
| Result | Display | The actual pressure used for the sum of the feeds which will also be the outlet pressure (unless further model options change the pressure). |
| ... | ||
| EB... (Evaluation Blocks) | ||
| EvalSeq.Feed | Display | 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. In Dynamic, the sub-models act on the feed before they are added to the contents of the tank. |
| Makeups | Input | The number of makeup blocks required. Extra dropdown options Makeup1, Makeup2, etc will be added to allow these to be switched on and off and prioritised in relation to the other sub-models. |
| MakeupX | List | This can be used to switch the Makeup Block (MU) on or off and prioritise it in relation to the other sub-models. If this is 'On' then the associated page, MUX becomes visible and may be configured. Note: This field is only visible if the entry for 'Makeups' is greater then 0. If there is one makeup then X=1. If there are two makeups, then X=1 and X=2, etc. |
| Reactions | List | This can be used to switch on the Reaction Block (RB). If this is 'On' then the associated page, RB 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 Environmental Heat Exchanger (EHX). If this is 'On' then the associated page, EHX may become visible and may be configured. Note: The user does not have to configure an environmental heat exchange, even if this block is checked. |
| ... | ||
| HeatXChg | List | This can be used to switch on Heat Exchanger (HX). If this is 'On' then the associated page, HX becomes visible and may be configured. Note:The user should connect streams to the HX inlet and outlet for the heat exchanger to operate correctly. The streams may be connected before the HX block is checked. |
| General_Model / SplitFlows | Tick Box | This can be used to enable the General Model (GM). If this box is checked then the associated page, GM, becomes visible and may be configured. Note: This option is used to adjust the outgoing streams from the unit. (Not available while the model is solving.) This option is not available in Dynamic mode. |
| General_Model_Thermal / SplitThermal | Tick Box | This option is only visible if the SplitFlows option has been chosen. This can be used to enable the General Model Thermal (GMT). If this box is checked then the associated page, GMT, becomes visible and may be configured. Note: This option is used to adjust the temperatures of the outgoing streams from the unit. (Not available while the model is solving.) This option is not available in Dynamic mode. |
| ShowQFeed | Tick Box | QFeed and associated tab pages (eg Qm) will become visible, showing the properties of the combined feed stream. See Material Flow Section. This will be prior to any sub-model (eg reactions) actions. |
| ShowQProd | Tick Box | QProd and associated tab pages (eg Qm) will become visible, showing the properties of the products. See Material Flow Section. This will be after the actions of any sub-models and before any flow splits with the GM model. This option is not available in Dynamic mode. |
Dynamic Mode Data Sections
The fields which appear in the access window for the tank in Dynamic mode only are shown in Tank - Dynamic Mode.
Hints and Comments
- If the user configures the GM, and then at a later stage adds or removes a stream (input or output), this may have an impact on the GM configuration. It is recommended that the user check the GM every time a stream change is made.
- In ProBal, the Tie model is almost identical to the tank model, except that the Tie can have Vapour Liquid Equilibrium (VLE) and currently the tank does not have this option.
- In ProBal, the tank model can be used with Demand.
- When using the Saturated method for PBPress and there are any sub-model actions such as EHX or RB, then the tank outlet conditions may not necessarily be at Saturated conditions.
- There may be mass entering or leaving the tank 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.
