If the system pressure is changed to a lower pressure, volatile components in the vessel’s gas space will be swept from the tank through the process vent. If the event takes place in a short period of time, the amount of air that leaks into the vessel (at connections) will likely be insignificant. Assuming that the batch remains at a constant temperature, the depressurization model can be used for estimating the VOC losses from the vessel.
A brief description of data fields contained on the Depressurization data entry window is provided below:
· Activity Title: A single line of text which provides a brief reference to the process step being modeled and which will be used by the program as a Title Page entry.
· Vent I.D. Field: If the vessel, condenser, or other control device vent contains a vent identification number or code, then this information may be entered in this edit field or selected from the drop down list from previous entries.
· Equipment: This button is used to access the Equipment Database to select a vessel or similar equipment item for use with this processing model. If this activity step was pre-existing and is only being edited, the program will prompt the user to determine whether it should simply use the selected vessel (with or without) the previous contents. The user may also select to have the program exchange the newly selected vessel in every step involving the original vessel.
· Link: Process activities may be configured to be linked or not linked when the process is recalculated. If this box is checked then the current activity will be re-evaluated during process recalculations. If this box left unchecked then the mixture definitions will remain unchanged during process recalculations. Occasionally, the process may contain a step that involves a chemical reaction or a liquid-liquid extraction. Under these conditions, the user may wish for the mixture composition to remain constant during the process recalculation.
· Duration (HR): Data entered in this field represents the amount of time that the heating process requires. For example, an entry of 1.0 hr will cause the emissions from this purging model to take place over a one hour period. The emission rate will be calculated by dividing the total emissions from this step by one hour. The expression may be entered in minutes (15 min) or hours (0.25 hr). A raw number (0.25) will be assumed by the program to be an entry in hours (0.25 will be converted to 0.25 hr).
· Initial Pressure (mm Hg): Enter into this field the system pressure of the process vessel prior to depressurization . The default unit of measure for the system pressure is millimeters of mercury or mm. Hg. The program will allow input based on other units such as atmosphere (atm), inches of water (in. H2O), or kiloPascal (kPA) but these units will be converted to mm Hg.
· Final Pressure (mm Hg): Enter into this field the final pressure for the depressurization model.
· Free Space (Gal): The free space for a vessel is the amount of volume in the vessel that does not contain process liquid. This value is calculated by subtracting the volume of the liquid contents of the vessel from the volume of the vessel when it is empty.
· Contents: If this is the first time that a vessel is being used in the process then it will be shown as empty. However, if the vessel has been featured during an earlier step of the current process then the initial contents box will display the resulting mixture from its last operation. Press the Edit button to define a new mixture or to modify the existing one. Press the Clear button to reset the initial contents to be empty. Press the Mix DB button to copy a mixture from the currently connected Process Database . The mixture temperature will be copied to the Initial Temperature entry field on the main dialog when the mixture description is accepted.
· Control Devices: A control device is an equipment item which, when installed in the process vent line, reduces the level of certain compounds in the final gas stream. Control devices are selected for the Filling model from the active Controls Database. The Filling Model allows for multiple control devices to become part of the venting scheme for the source vessel. Press the Controls button access the main Controls Database interface.
(1) A process condenser may not be preceded by a control condenser or other control device.
(2) A control condenser may not be preceded by a scrubber or other control device.
· Non-Emitting: Check the Non-Emitting box if this activity will not have any vent emissions. This condition can exist under certain processing operations. For example, filling a vessel using residual vacuum in the vessel or filling a vessel when the process vent is tied directly back to the original charge vessel. Other operations may involve charging material to an unvented vessel and allowing the pressure to build during the process.
· Comments: Comments may help to document specific details that the user would like to make for this model. Comments may be ented directly in the edit box or entered using the notepad like interface.