Flash Distillation

When the temperature of a material consisting of only a single volatile component such as pure Water is increased sufficiently then the boiling point will be reached where the vapor pressure of the compound is the same as the system pressure. If additional heat is applied then the compound will continue to boil at the same boiling point temperature until 100% of it has been boiled away. However, if the material contains two or more compounds then this mixture will exhibit a temperature range where the initial boiling point will be observed. As heat continues to be applied to the multi-component mixture then more and more of the mixture will be boiled away as the composition of the remaining liquid changes along with the mixture boiling point. For example, a mixture of Water and Methanol will have an initial boiling temperature and a final boiling temperature where all of the mixture will become vaporized. If a process calls for heating the mixture to an elevated temperature (above the initial boiling point) as part of a distillation process then the amount of material that will vaporize can be approximated using a flash calculation.

The purpose of the Flash Transfer model is to calculate the material split between that occurs between the vapor phase and the liquid phase that remains in the flash vessel where the mixture is being subjected to boiling conditions. As an example, a 50% wt/wt mixture of Methanol and Water at 760 mm Hg pressure has an initial boiling point of 81.3°C (where boiling first occurs) and an upper boiling point of 91.7°C where the mixture finally becomes 100% vapor. If one assumes that the vapor is being condensed in an over head heat exchanger and collected in a receiver, then the Flash Transfer model can be used to approximate the composition and mass of material that remains in the flash vessel and the distillate material that is collected in the receiver.

The Flash Transfer model uses the Rackford-Rice method for solving single stage equilibrium conditions and assumes that the remaining liquid in the flash vessel and the exiting vapor are at equilibrium and that the material balance of each component is maintained. The Flash Transfer model also assumes that the vapor leaving the flash vessel is to be condensed by an overhead heat exchanger and collected in a receiver vessel. Emissions that are calculated by the Flash Transfer model are based on the displacement emissions that occur when the distillate enters the receiver.

The Flash Transfer model requires that the process material to be processed consists of only a single phase with a minimum of two components. At least one of the components must be volatile. Process mixtures may contain non volatile components such as dissolved solids as long as there are at least one volatile component.

Examples of materials that meet this requirement:
(1) a solution of Ethanol and Water.
(2) a solution of Sodium Chloride and Water.
(3) a solution of Toluene, Tetrahydrofuran, and Xylene.

Examples of materials that do not meet this requirement are:
(4) a liquid phase of only Water – since there is only one volatile component and there is only a single boiling point temperature.
(5) a solid consisting of only sand – since there are no volatile components.
(6) a mixture consisting of an aqueous phase and a non-aqueous phase. In the case of a two phase problem there is not enough information available to solve the flash composition problem.

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.

· Lock: Process activities may be configured to be locked or not locked 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 filling process takes. For example, an entry of 1.0 hr will cause the emissions from this filling 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).

Note that the user may also enter the flow rate for charging material to the vessel. For example, entering 10.0 gpm in the time field would cause Emission Master to calculate the charging time by dividing the flow rate into the volume of the material being charged. The final resulting time calculated by the program would be used for the charging activity. Entering 10.0 lpm in the time field would prompt Emission Master to calculate the charge time using liters per minute. Specifying a flow rate in the time field is limited to gpm or lpm units.

· Pressure (mm Hg): This entry represents system pressure of the process vessel during the filling operation. 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 for use within the program.

· Purge/Sweep: Pressing this button will take you to the Purge/Sweep dialog window where the noncondensable gas and purge rate may be specified. If the rate is set to 0.0 scfh (standard cubic feet per hour) then the modeling will be performed using the gas specified as the noncondensable component in the emission calculations.

· Initial 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 .

· Transfer From: This is the main interface for configuring Emission Master with the inlet stream contents to be transferred into this vessel from a remote vessel.

[Transfer] Press Transfer to access the Vessel Source Selection interface and to configure the transfer model with the material that will be used for the flash calculation. Once the Vessel Source has been selected then Emission Master will enter into the standard flash calculation interface where the temperature and pressure conditions are established and the flash calculation implemented. Once the flash calculations have been completed then the user is welcome to change either the flash temperature or pressure and to have the flash calculations repeated. Once the flash calculation has been made and the distillate material determined then the program will return to the Flash Transfer dialog window where the distillate material that will be transferred from the still to the receiver can be seen in the Transfer from box.

· 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.

· 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 non-vented 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 entered directly in the edit box or entered using the notepad like interface.