CHAPTER 2
What is an Object?
An Object refers to the icon, which appears on the GPS-X Drawing Board when selected from the Process Table. An object is a symbolic representation of a unit process without any process model associated. There are several items associated with the object:
1. The hydraulic configuration,
2. Physical attributes,
3. Operational attributes,
4. Output Variables,
5. Stream labels; and,
6. Sources.
The hydraulic configuration refers to the number of connection points on an object and the operation of these connection points; that is, whether the connection point is incoming or outgoing. The parameters and stream labels are under the Parameters sub-menu and the Labels... sub-menu, respectively. Additional sub-menu items are found under the Parameters menu, which are process model and library specific. The Output Variablesmenu item is used to select model variables for display on a graph, while the Sources menu item displays the object number that acts as a source for some of the Parameters.
Not all menus are active. For example, until the model is chosen for an object, the Parameters and Initialization menus (if they exist for the object) are not active and appear greyed out. Once the model has been specified, these menus will become active.
Types of Objects
Common Properties
Before describing each object that appears on the GPS-X Process Table, an outline of the properties common to some objects is presented. The objects in the Process Table can be described as either having volume or not. There are some deviations from this general rule, but they can be ignored for now. For example, the control splitter object does not have any volume (called “zero volume”), while the equalization basin does have volume. The zero volume objects do not have any dilution or residence time while the objects with volume do.
Objects with volume have one or more influent connection points and one or more effluent connection points. For example, the settler objects have one influent connection point plus three effluent connection points, while the PLUG FLOW-TANK (2)object has three influent connection points and two effluent connection points. The effluent connection point(s) will have an overflow connection plus one or more pump connections. The overflow is located at the upper right-hand corner of the object (as oriented in the Process Table), while any additional output connections are located on the right-hand side or bottom of the object. The additional output connections (either called pump or underflow) simulate a constant or variable flow pump so that a flow rate can be specified (up to the maximum pump capacity).
The volume of fluid in the tank depends on the net influent - effluent flow. If this is a negative value, then the volume of the tank will decrease until it reaches zero. At this point the effluent will equal the influent, regardless of the pump flow set. If the net influent-effluent flow is positive, the tank volume will increase until the maximum (specified by the user). At this point, the tank begins to overflow, so that the effluent flow (sum of the overflow and pump flows) will be equal to the influent flow. The effluent flow over and above the effluent pumped flow rates will leave through the overflow connection point. If the net influent-effluent flow is zero, then the volume will not change.
The mass balance for variable volume tanks is shown in Equations 2.1-2.4:
Equation 2‑1
Equation 2‑2
Equation 2‑3
Equation 2‑4
where:
= influent flow rate
(m3/d)
=
effluent flow rate (m3/d)
= influent concentration (mg/L)
=
effluent concentration (mg/L)
=
rate of reaction (mg/L/d)
=
liquid volume (m3)
= time (d)
Another feature of the objects with volume is their initial volume. When a simulation begins, the user can specify what volume the tank initially has using a logical ON/OFF variable called start with full tank, located under Initial Conditions > Initial volume. If this logical switch is set to true (ON), then the tank will be full at the beginning of the simulation.
If this logical variable is false (OFF), then the user can specify the initial reactor volumeat the start of the simulation. As a result, users can specify the tank starts with a full volume in two ways:
1. Either setting the logical variable as “true” (ON); or
2. By setting the variable as “false” (OFF) and manually specifying the starting volume equal to the maximum tank volume.
From Equation 2‑1, the concentration of a conservative material in objects with volume relative to its influent concentration can be calculated as shown in the following equations:
Equation 2‑5
where:
=
conservative component concentration (g/m3)
= influent
concentration (g/m3)
=
effluent concentration (g/m3)
At steady state, the time derivatives are zero and Equation 2‑5 becomes:
Equation 2‑6
If the SRT is defined as:
Equation 2‑7
where:
=
solids retention time (d)
Combining Equation 2‑6 and Equation 2‑7 gives:
Equation 2‑8
The hydraulic residence time (HRT) is defined as:
Equation 2‑9
Combining Equation 2‑8 and Equation 2‑9 gives:
Equation 2‑10
This equation shows the ratio of the
concentration of the conservative component in the object to its
concentration in the influent. At steady state, it is directly
proportional to the
value.