The ratio of the equilibrium constants is called the “relative volatility” of the
keys. Setting fractionation requirements for light ends towers is usually done to
meet a product specification. More often than not, this specification is the vapor
pressure of the heavier fraction or the tolerable amount of a heavy key allowed in
the lighter fraction.
Sometimes, however, a specification for the separation may not be given. Under
this circumstance, some judgment must be made in determining the most reasonable separation that can be achieved with the equipment. This item of the handbook
addresses calculation techniques that satisfy either premise.
It is common to use process simulation software (HYSYS, Pro/II, and others) to
perform the design calculations for light ends units. This allows case studies to
optimize the process design. The details of the computer simulations are beyond the
scope of this handbook. The manual procedure for the calculations is described
here. It is helpful to understand the manual approach to establish the background
and limitations behind the computerized calculations.
Light Ends Specifications Cases
Case 1: Setting separation requirements to meet a specification
In this case it is required to determine the amount of butanes that can
be retained by a light naphtha cut to meet a RVP specification. The steps are
as follows:
Step 1. Calculate the properties of the C 5 + naphtha from a component breakdown. These properties should give weight and mole rates per hour.
Step 2. Carry out a bubble point calculation of the C 5 + fraction and inserting
butane as the unknown quantity x.
Step 3. The equilibrium constant used (K) in the calculation will be at the
temperature and pressure conditions of the RVP (i.e., normally at 100
F
which is the test temperature).
Step 4. Either the equilibrium constants given in the charts in Appendix 3 of the
chapter entitled “Process equipment for petroleum processing” in this handbook or the relationship of vapor pressure divided by the total system pressure
may be used.
Step 5. By definition, the total moles of liquid given is equal to the total moles of
vapor (calculated) in equilibrium at the bubble point. Thus equate and solve
for x as the quantity of butanes tolerable to meet RVP.
Case 2: Setting fractionation where no specification is given
Step 1. Determine the composition of the feed in terms of real components,
pseudo-components, or both. Calculate this in moles/h and mole fractions.
Step 2. Select the key components, and decide the minimum distribution of one
or other key. For example, in a debutanizer C 5 s allowed into LPG must not be
more than 2 % of the total C 4 s and lighter. This is to protect the Butane LPG
“Weathering” Test Specification.
Distillation of the ‘‘Light Ends´´ from Crude Oil in Petroleum Processing
203
keys. Setting fractionation requirements for light ends towers is usually done to
meet a product specification. More often than not, this specification is the vapor
pressure of the heavier fraction or the tolerable amount of a heavy key allowed in
the lighter fraction.
Sometimes, however, a specification for the separation may not be given. Under
this circumstance, some judgment must be made in determining the most reasonable separation that can be achieved with the equipment. This item of the handbook
addresses calculation techniques that satisfy either premise.
It is common to use process simulation software (HYSYS, Pro/II, and others) to
perform the design calculations for light ends units. This allows case studies to
optimize the process design. The details of the computer simulations are beyond the
scope of this handbook. The manual procedure for the calculations is described
here. It is helpful to understand the manual approach to establish the background
and limitations behind the computerized calculations.
Light Ends Specifications Cases
Case 1: Setting separation requirements to meet a specification
In this case it is required to determine the amount of butanes that can
be retained by a light naphtha cut to meet a RVP specification. The steps are
as follows:
Step 1. Calculate the properties of the C 5 + naphtha from a component breakdown. These properties should give weight and mole rates per hour.
Step 2. Carry out a bubble point calculation of the C 5 + fraction and inserting
butane as the unknown quantity x.
Step 3. The equilibrium constant used (K) in the calculation will be at the
temperature and pressure conditions of the RVP (i.e., normally at 100
F
which is the test temperature).
Step 4. Either the equilibrium constants given in the charts in Appendix 3 of the
chapter entitled “Process equipment for petroleum processing” in this handbook or the relationship of vapor pressure divided by the total system pressure
may be used.
Step 5. By definition, the total moles of liquid given is equal to the total moles of
vapor (calculated) in equilibrium at the bubble point. Thus equate and solve
for x as the quantity of butanes tolerable to meet RVP.
Case 2: Setting fractionation where no specification is given
Step 1. Determine the composition of the feed in terms of real components,
pseudo-components, or both. Calculate this in moles/h and mole fractions.
Step 2. Select the key components, and decide the minimum distribution of one
or other key. For example, in a debutanizer C 5 s allowed into LPG must not be
more than 2 % of the total C 4 s and lighter. This is to protect the Butane LPG
“Weathering” Test Specification.
Distillation of the ‘‘Light Ends´´ from Crude Oil in Petroleum Processing
203