Use the RunbackSpecs form to specify the flow rate of liquid runback from any stage. When you enter a liquid runback specification, you must allow PetroFrac to adjust one of the following:
• Flow rate of a pumparound
• Duty of an interstage heater/cooler
Pumparounds
Use the following sheets to enter specifications for pumparounds. Use this sheet To enter
Pumparounds Specifications
Pumparound connectivity and cooler/heater specifications Report PseudoStreams Pseudostream assignment for the pumparound Hcurves Specifications Heating/cooling curve specifications
Pumparounds are associated with the maincolumn. They can be total or partial drawoffs of the stage liquid flow. You must specify the draw and return stage locations for each pumparound. For partial drawoffs, you must specify two of the following:
• Flow rate • Temperature
• Temperature change • Heat Duty
For total drawoffs, you must specify one of the following: • Temperature
• Temperature change • Heat Duty
Side Strippers
Use the Stripper forms and sheets to enter specifications for side strippers. Side strippers may be either steam-stripped or reboiled. For steam strippers, you must enter a steam stream. You can override its flow rate by specifying a steam- to-product ratio. For reboiled strippers, you must specify a reboiler duty.
PetroFrac assumes:
• A liquid draw goes from the main column to the top of the stripper. • The stripper overhead is returned to the main column.
You must specify the draw and return stage locations. You can also: • Return a fraction of the stripper bottoms to the main column
• Specify additional liquid draws from other stages of the main column as feeds to the strippers
Efficiencies
You can specify one of two types of efficiencies: • Vaporization
• Murphree
Vaporization efficiency is defined as:
Eff y K x i v i j i j i j = , , ,
Murphree efficiency is defined as:
y y
Where:
K = Equilibrium K value x = Liquid mole fraction y = Vapor mole fraction
Effv = Vaporization efficiency EffM = Murphree efficiency
i = Component index j = Stage index
To specify vaporization or Murphree efficiencies, enter the number of actual stages on the Setup Configuration sheet and Strippers Setup Configuration sheet as Number of stages. Then use the Efficiencies and Strippers Efficiencies forms to enter the efficiencies.
You can use any of these efficiencies to account for departure from equilibrium. But you cannot convert from one efficiency to the other. Magnitudes of the efficiencies can be quite different. Details on using and estimating these efficiencies are described by Holland, Fundamentals of Multi-Component Distillation, McGraw-Hill Book Company, 1981.
Convergence
For convergence PetroFrac uses:
• The sum-rates variant of the inside-out algorithm
• A special initialization procedure designed for petroleum refining applications PetroFrac generally does not need initial estimates. For ethylene plant primary fractionator/quench tower combinations, you should provide temperature estimates.
To enhance convergence, you may enter profile estimates on the following PetroFrac forms:
• Estimates
• Strippers Estimates
Temperature estimates are usually adequate. You can increase convergence stability by selecting varying degrees of damping on the Convergence Basic sheet.
Rating Mode
In rating mode, PetroFrac calculates the column profiles and product compositions based on specified values of column parameters. Examples of column parameters are:
• Reflux ratio • Reboiler duties • Feed flow rates • Furnace temperature • Pumparound loads
Design Mode
In design mode you can manipulate subsets of the column parameters to achieve certain specifications on column performance.
You can specify For any
Purity Stream, including internal streams† Recovery of any components group Set of product streams††
Flow rate of any components group Internal stream or set of product streams Flow rates of any components groups to any
other component groups
Internal streams to any other internal streams, or set of feed or product streams
Temperature Stage
Heat duty Stage
Fractional overflash Stage
TBP and D86 temperature gaps Pair of product streams TBP temperature Product stream D86 temperature Product stream D1160 temperature Product stream Vacuum distillation temperature Product stream API gravity Product stream Standard liquid density Product stream Specific gravity Product stream Flash point Product stream Pour point Product stream Refractive index Product stream
You can specify For any Reid vapor pressure Product stream
Value of any Prop-Set property Internal or product stream††† Difference of any pair of Prop-Set properties Pair of product streams Watson UOP K factor Product stream †
Express the purity as the sum of mole, mass, or standard liquid volume fraction of any group of components relative to any other group of components.
††
Express recovery as a fraction of the same components in a subset of feed streams.
†††
See ASPEN PLUS User Guide, Chapter 28.
You can also specify overflash for a furnace feed stream.
Physical Properties
Use the BlockOptions form to override the global physical property method. You can specify one method on this form, which PetroFrac uses for all stages in the main column and strippers.
You can also split the main column or a stripper into any number of segments, each using a different property method.
Use this sheet When you use different properties for Properties Property Sections The main column
Strippers Properties Property Sections A stripper
Free Water Handling
PetroFrac can perform free-water calculations in the main column and side strippers. By default, PetroFrac performs free-water calculations for the main column condenser. The free-water phase, if present, is decanted.
To do this Use these sheets
Request free-water calculations for additional stages in the main columns and strippers
Properties Freewater Stages Strippers Properties Freewater Stages Define additional water decant product streams for the main
column
Solids Handling
PetroFrac handles solids by:
• Temporarily removing all solids from inlet streams • Performing calculations without solids
• Adiabatically mixing solids removed from inlet streams with main column liquid bottoms
This calculation approach maintains an overall mass and energy balance around the PetroFrac block. But the bottom stage liquid product will not be in exact thermal or phase equilibrium with other bottom stage flows (for example, the bottom stage vapor flow).
Sizing and Rating of Trays and Packings
PetroFrac has extensive capabilities to size, rate, and perform pressure drop calculations for trayed and packed columns. Use the following PetroFrac forms to enter specifications:
• TraySizing, TrayRating, PackSizing, PackRating
• Strippers TraySizing, Strippers TrayRating, Strippers PackSizing, Strippers PackRating