Empty Vessel Purge
Process operations frequently require that a purge be placed on a vessel or other equipment item to remove certain vapors and provide an inert atmosphere. In such cases the noncondensable gas is introduced into the vessel through an equipment nozzle at a controlled rate. If the vessel still contains process material, then the exit gas from the process vent is normally considered to be saturated with the volatile liquid components. On the other hand, if the liquid contents have been transferred to another tank and the original vessel is now empty, then the concentration of volatile organic vapors will reduce over time through dilution.
Mathematically speaking, the emission calculations for the empty vessel purge scenario is different than then partially filled vessel purge scenario. For the case of the non-empty vessel purge, the component vapor pressure in the vessel’s gas space remains constant throughout the operation and is estimated by the standard vapor pressure model. The total emissions for the vessel are calculated from the amount of purge gas used in conjunction with the constant component vapor pressures. In the case of the empty vessel purge the volatile component is assumed to be diluted in the vessel void space as it is being displaced by the incoming purge gas.
To begin an Empty Vessel Purge modeling activity, select Empty Vessel Purge from the Add_Step master pop-up menu. An explanation of the data entry fields and requirements are described as follows:
· 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 purging 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).
· Purge Compound: Select the non-condensable gas to be used from the drop down list box. If the desired compound is not in the list then the user may add it to the list through the Noncondensables Database.
· Purge Rate: Enter the purge flowrate to be use in the model in standard flow units. (scfm, scfh)
· Saturation (%): If the exiting vapors from this vessel have a saturation level which is different than 100%, then this information is entered here.
· Contents: The contents description is used as a basis for determining the vapor composition that exists in the headspace of the vessel when the purge is initiated. 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.
· 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.