5. Introduction
Many methods have been developed to separate chemical mixtures by
exploiting physical or chemical differences between the individual spe-
cies. Solvent extraction exploits solubility differences, while distillation
exploits volatility differences. Both can be simulated using the column
models in PRO/II.
In its simplest form, distillation involves the boiling of a liquid followed
by the condensation of the resulting vapor. It is an ancient technology;
Aristotle wrote of its use in converting sea water to freshwater. It is also
a very modern technology; distillation is the most widely used separation
process in the petroleum and petrochemical industries and, in the United
States it consumes approximately three percent of the total energy.
In 1893, Sorel made the first attempt to describe the distillation process
mathematically by publishing the now-standard mass and energy balance
equations for a steady-state, continuous, staged distillation column.
Because of the inherent complexity of the distillation process, an analyt-
ical solution of these equations is impossible. Graphical techniques, such
as the McCabe-Thiele and Ponchon-Savarit methods, were developed in
the 1920's to approximate their solution. These methods work well for
some binary distillation problems and are still taught in university curric-
ulum because they illustrate the fundamental principles involved. How-
ever, a true characterization of a column can come only from solving the
rigorous mass and energy balance equations. This is where process sim-
ulation and PRO/II enter the story.
Using PRO/II and its graphical user interface, PROVISION, you can
simulate entire flowsheets containing many distillation columns. Some
of the reasons for simulating distillation columns are:
s Design:Computer simulation is an integral part of distillation column
design. It provides you with the answers you need to achieve the
desired separation at minimal cost. The obvious advantage of
designing a piece of equipment on a computer is that you can "try
before you buy." It is safer to make your mistakes on the computer
than in the plant.
s For an existing column, test the effect of differ-
Operations and Retrofit:
ent feedstocks, examine internal vapor and liquid loading, experi-
ment with duties, and in general, test the effect that process changes
have on column performance. Retrofit calculations might include
moving feed and product trays to improve profitability and making
equipment modifications to meet new environmental regulations.
Hydrocarbon Distillation Workbook 1
6. s Assess the feasibility of using surplus
New Uses for Existing Equipment:
equipment in a new service. For example, one PRO/II user saved his
company significant money by demonstrating that a seldom-used
hydrocarbon fractionating column could perform de-watering tasks
in a new process.
s Maximize your profits by decreasing operating costs
Optimization:
and/or increasing your product values (e.g., by obtaining higher
purity products). Determine the feed tray location and reflux ratio
that minimize the column's energy usage, while still achieving the
necessary separation. PRO/II's OPTIMIZER will automatically find the
operating conditions that maximize profit.
s Compare plant data with simula-
Troubleshooting and Data Reconciliation:
tion results. Discrepancies may indicate problems such as damaged
trays or poorly calibrated flow meters. SIMSCI's DATACON pro-
gram is specifically designed to solve data reconciliation problems
and can perform plant-wide reconciliation calculations.
About This This workbook complements SIMSCI's PRO/II’s Simulating Refinery
Workbook Processes training course. Since much of the course time is dedicated to
hands-on examples, you will not necessarily go through the document
page by page. The workbook does, however, follow the course sequence
and you may want to jot notes in the margin. We strongly recommend
that you read this workbook from cover to cover once and then use it to
refresh your memory later on.
Objectives After completing this workbook, you will be able to:
s The appropriate distillation curves interconversion to use for your
specific simulation.
s Convert assay data into petroleum cuts and blend mutiple assays.
s Measure the quality of a stream by predicting its Refinery Inspection
Properties.
s Describe how distillation columns work and note the strengths and
limitations of PRO/II's distillation algorithms.
s Translate a column from actual trays to theoretical stages.
s Select the most appropriate algorithm, initial estimate generator
model, and level of damping for a given column.
s List the main sources of inaccuracy in column simulations.
s Model a natural gas sweetening unit and control its recycle streams.
2 Introduction
7. s Enter specifications and variables that lead to a converged solution.
s Appreciate how the choice of assay characterization method and cut-
point set impacts simulation results.
s State the differences between kettle and thermosiphon reboilers and
how to properly simulate them using PRO/II.
s Calculate feed furnace duties without creating recycle loops.
s Combine product assays to reconstitute a feed assay.
s Simulate a crude column by adding complexity to successive runs, a
vacuum column including cracking in the flash zone, or air leaks.
and an FCC main fractionator with a three phase condenser.
s Troubleshoot nonconverging columns.
Where to Documents
Find User manuals are shipped with your copy of PRO/II. A complete set of
Additional documents is provided online in the form of .PDF files that are most con-
Help veniently viewed using Adobe Acrobat Reader 3.0. If you required addi-
tional manuals, contact your sales representative.
Online Help
PRO/II comes with online Help, a comprehensive online reference tool
that accesses information quickly. In Help, commands, features, and data
fields are explained in easy steps. Answers are available instantly,
online, while you work. You can access the electronic contents for Help
by selecting Help/Contents from the menu bar. Context-sensitive help is
accessed using the <F1> key or the What's This? button by placing the
cursor in the area in question.
Technical Support
PRO/II is backed by the full resources of Simulation Sciences Inc. (SIM-
SCI), a leader in the process simulation business since 1966. SIMSCI
provides the most thorough service capabilities and advanced process
modeling technologies available to the process industries. SIMSCI's
comprehensive support around the world, allied with its training semi-
nars for every user level, is aimed solely at making your use of PRO/II
the most efficient and effective that it can be.
For North American hotline support, call 1-800-SIMSCI1.
Hydrocarbon Distillation Workbook 3
8. Laboratory Tests and Interconversions
A brief description of laboratory tests performed on petroleum streams is
given in this chapter. Tests are performed on finished products as part of
the specifications that must be met. Tests are also performed on interme-
diate materials and used as control points for the plant operations.
Terminology Before we begin, you should become familiar with some terminology.
API Gravity API gravity is an unusual means of reporting the densities for petroleum
stocks. Examination of the formula for API gravity shows that API
behaves inversely to the specific gravity of the material. Water is the base
for this system, with an API gravity of 10.0. Most refined products are
less dense than water, i.e., their API gravity is greater than 10.0.
API = 141.5/SPGR - 131.5 (1)
Examples
Water = 10.0 API
Kerosene = 45.0 API
Motor Gasoline = 58.0 API
Natural Gasoline = 75.0 API
Characterization The characterization factor is attributed to Universal Oil Products and/or
Factor Professor Watson of the University of Wisconsin. This factor is a mea-
sure of the paraffinicity of a stock, and was invented for the analysis of
crude oils. The factor is relatively constant through the entire boiling
point range for crude oils. Note that this factor is really an early attempt
to incorporate the effect of composition into the prediction of properties
for petroleum streams. It has been amazingly useful in this respect.
3 NBP
K = ---------------
- (2)
SPGR
Examples
Paraffins = 13+
Kansas Crude Oil = 11.8
Cracked Gasoline = 10.9
Condensed Aromatics = 10.0
Using the plot in Figure 1, you can predict the viscosity of a given sam-
ple from its API gravity and characterization factor.
4 Laboratory Tests and Interconversions
9. Figure 1:
Viscosity as a
Function of API
Gravity
950
Cu
bic
Av
Viscosity at 210ºF
era
600
ge
Bo
ilin
gP
oin
t,
200
Fº
10.0 12.5
"K" Factor
Ref: Watson, et.al, IEC, 27,1460,(1935).
API Gravity
Laboratory API Gravity
Tests API gravities are determined directly by floating a hydrometer of the
appropriate range in the sample. The value is supposed to be corrected to
a basis of 60°F, however, not all lab technicians apply the correction fac-
tor.
Reid Vapor Pressure
Reid vapor pressure is a measure of volatility. The test is somewhat
empirical, and was designed for streams with true vapor pressures less
than about 25 psi. A special procedure is used for crude oils.
100ºF
Reid vapor pressure (RVP) is commonly used as a control and/or specifi-
cation. In the purest sense, it is a measure of the normal butane content
of motor gasoline. It is used as a control point for crude oils in this same
sense. Historically, crude oil was valued by its API gravity, with a higher
API being a more valuable crude oil. Crude oil suppliers soon learned to
cheat this system by blending low value butane into the crude oil, raising
its API. The RVP test detects this practice.
The ASTM procedure is very explicit regarding this test. The typical lab
worker does not always follow the procedure exactly. For example, the
sample is to first be chilled to 36°F. The chilled sample is placed in a
Hydrocarbon Distillation Workbook 5
10. bomb with four parts air, the lid with the attached pressure gauge tight-
ened, and the bomb shook and placed in a constant temperature bath at
100°F. After a specified time (again not always followed by the lab), the
pressure gauge is read and reported as RVP in PSI units.
The test approximates the true vapor pressure at 100°F for gasoline
range materials (5 - 15 RVP). In fact, differences between TVP and RVP
for these materials are within the reproducibility of the test. Crude oil
exhibits a much wider deviation between RVP and TVP. The test is
reproducible within about 0.5 psi. The accuracy of the test is also
strongly affected by the techniques used in collecting and storing the
sample prior to the test, as well as the actual lab technique used by the
chemist.
An old rule of thumb for refiners is that each psi of RVP corresponds to
about one percent normal butane (isobutane and lighter materials cannot
be placed in motor gasoline). Thus, a six RVP gasoline contains approx-
imately six percent normal butane.
Distillations True Boiling Point (TBP)
The true boiling point distillation is run in a batch fractionating still with
reflux. There is some inconsistency in the number of trays used in stills,
however, all TBP devices tend to separate the components in boiling
point order. As the lighter components are removed the pressure of the
still is reduced to keep the temperatures below 650°F. Above 650°F, a
significant amount of cracking takes place, resulting in tar and light gas
products. Depending on the still, the heaviest material that can be
removed corresponds to a normal boiling point of approximately 950°F.
TBP distillations require substantial time and are costly. They are viewed
as valuable wine, to only be consumed when absolutely necessary.
ASTM D86
D86 distillations are fast and inexpensive tests. They are run at atmo-
spheric conditions and considerable cracking of the sample occurs as a
temperature of 650°F is approached. This is not a fractionating type dis-
tillation, and the temperatures do not necessarily correspond to the boil-
ing points of the material in the mixture. For example, the recorded
initial boiling point (IBP) is always substantially higher than the lightest
material in the mixture (which escapes before the first drop and is
reported as "loss"). The end point (EP) is lower than the heaviest mate-
rial in the mixture (which remains in the flask as "residue"). D86 distilla-
tions are most useful when compared to other D86 distillations.
The old style ASTM equipment is depicted in the illustration. Lab equip-
ment has been improved in recent years, which eliminates much of the
6 Laboratory Tests and Interconversions
11. inaccuracies due to of lab technique. However, the empirical nature of
the test is still the most limiting factor.
Thermometer
Conden
ser
100 CC
Burner
The nature of the test does not lend itself to great accuracy. The test
reproducibility for a given sample may be plus or minus 5 to 10°F (or
higher), depending on the temperature range. Lab and sampling tech-
niques also affect the test. For example, in drawing a hot sample into an
open container, some of the light material escapes.
The atmospheric pressure for the lab affects the results. Test results
should be corrected to a basis of 760 mm Hg. Note that for a city such as
Denver, Colorado (elevation one mile) the corrections to the ASTM at
lab conditions are substantial, as shown in Table 1.
Table 1: ASTM Correction Factors
Lab Presssure Observed Correction (°F)
(mm Hg) Temperature (°F)
600 100 + 10
300 + 15
600 + 20
ASTM D1160
This D1160 test is a bit more complex because of the vacuum device.
Moreover, the vacuum level may vary somewhat throughout the test.
Typical vacuums are in the range 2 mm Hg to 10 mm Hg. Laboratories
almost always pressure correct all D1160 distillations to 760 mm Hg.
Thermometer
C on d e To Vacuum
ns e r
Burner
Hydrocarbon Distillation Workbook 7
12. The D1160 test is run under vacuum, and designed for heavier stocks.
D1160 distillations at 2 mm of mercury are fairly common. Because of
the low pressures, the D1160 distillation has better fractionation than the
D86 and the results are much closer to a TBP.
The D1160 initial boiling point is always higher than the lightest mate-
rial in the mixture. For most stocks, the end point is never reached, with
the heaviest portion of the sample remaining in the still when the test is
completed. As a rule of thumb, D1160 distillations are able to distill
more of the mixture than a TBP device, because of the lower vacuum
that can be used.
When compared to TBP distillations (on the same pressure basis),
D1160 distillations compare well for temperatures corresponding to 50
volume percentage and more distilled. Again, the accuracy of the test in
determining the composition of a heavy petroleum stock is affected by
sampling and lab techniques, as well as the reproducibility of the test.
ASTM D2887
The D2887 procedure is relatively new, and was designed to circumvent
the high costs for TBP distillations. In this procedure, gas chromatogra-
phy is used to separate the components in volatility order. This gives a
close approximation of the TBP distillation, particularly for high boiling
stocks.
Interconversion ASTM distillations must be converted to the corresponding TBP distilla-
of ASTM tion, for use in defining the components in a petroleum stock. W. C.
and TBP Edmister and associates were the first to publish correlations in this
Distillations regard that were generally accepted and used by industry. Much of this
correlation work was done in the late 1940's and early 1950's and
involved a relatively small number of samples.
The Edmister correlations were included in the API Technical Data
Book about 1963. These correlations stood the test of time (nothing else
was generally available) until recently. An API sponsored project at
Penn State University led by Professor Daubert resulted in "improved"
conversion methods, relating ASTM and TBP distillations. Daubert, et.
al., also developed a relationship between D2887 and D86 distillations.
The work was published in 1986 and placed in the API Tech Data Book
in 1987. The correlations are still being tested by industry, and some
small revisions have already been made.
In a survey of several major refiners by SIMSCI, all refiners stated two
conflicting remarks: a) we don't think that the Edmister correlations are
accurate b) we use them as a standard for our conversions because noth-
ing else is available. Some refiners used their own correlations and
claimed higher accuracy, however, much of this is a moot point anyway
8 Laboratory Tests and Interconversions
13. when one considers the tests themselves and the effects of poor sampling
and lab techniques.
Simulation users generally accept the conversions as the precise, and fail
to remember the inherent inaccuracies in the correlation and fitting of the
experimental data behind the conversions. All of these conversion meth-
ods (both Edmister and Daubert) are approximations, based on limited
laboratory data of limited accuracy and reproducibility. Note that the
nature of the ASTM tests makes the accurate prediction of IBP's and
EP's an impossibility.
D86 Conversion: Edmister Method (API63)
The Edmister D86 conversion correlation is shown below. The procedure
is simple: convert the ASTM 50% point to a TBP 50% point and then
work up and down from that base applying the appropriate delta temper-
atures. Interestingly, the Edmister 50% correlation plots as a straight line
on log-log paper.
Figure 2: Edmister
D86 Conversion
-5 30
Correlation
3 0 10-
0
0
-7
IBP-10 50
0
100
-9
70
TBP 50% - ASTM 50%
TBP DT
90-EP
200 ASTM 50% 850 ASTM DT 70 90
Note that the Edmister curves for IBP to 10% and 90% to EP are very
limited. For wide boiling mixtures (and some not so wide) the curves
must be extrapolated. Obviously, the predicted IBP or EP for the corre-
sponding TBP curve is more fiction than truth.
D86 Conversion: Daubert Method
Daubert took a different approach and correlated each point distilled as a
separate equation.
TBP = a(D86)b (3)
where a, b are supplied for IP, 10, 30, 50, 70, 90, 95, and the uncertainty
of fits are IP = +/- 16°F and 95 = +/- 12°F. Daubert TBP IP is higher than
Edmister TBP IP.
Hydrocarbon Distillation Workbook 9
14. He avoided the problem with end points-- no curve was developed for
end point. Hence, when the Daubert method is used, the end point must
be extrapolated by PRO/II. A probability extrapolation is used for this
purpose and the results are probably no worse or better than those
obtained by the Edmister correlation. Note the reported deviation from
the experimental data in the Daubert regression fits. Again, an accurate
prediction of the IBP should not be expected.
The Edmister correlation has one advantage over the Daubert correla-
tion. The TBP curve predicted by this method is always monotonic. For
stocks with relatively flat distillations, the Daubert method can predict a
distillation curve that decreases with percentage. This, of course, is
impossible and you should correct it.
It is important to keep in mind that the correlation methods have their
imperfections. Inter conversion of ASTM and TBP distillation is hardly
an exact science, nor does it appear that it will ever be. The correlations
are still useful, and have proven themselves over the years in represent-
ing petroleum materials.
D86 Conversion: Edmister-Okamoto Method
Edmister and Okamoto (1959) developed a method which is still widely
used for converting ASTM D86 curves to TBP curves. If the Edmister-
Okamoto method is specified as the conversion method, their procedure
(converted from the original graphical form to equations by SIMSCI) is
used for conversion of D86 to TBP curves.
D86 Conversion: API94 Method
This method is detailed in the 1994 API Technical Data Book which was
developed by Daubert, T.E.. It uses an approach similar to that of the API
1963 procedure, which always produces a monotonic TBP curve.
D86 Conversion: Old API63 Method
This method, while no longer the default, is still available for users
whose flowsheets may be tuned to the results using the old method. This
method was recommended (and shown in graphical form) in older edi-
tions of the API Technical Data Book. The graphical correlation has
been converted to equation form by SIMSCI.
The old API cracking correlation is presented as:
log (D) = -1.587 + 0.00473 T (4)
where D = correlation to add, °F, and T = observed temperature, °F
It is far from elegant, but does make an attempt to correct ASTM distilla-
tion points (475°F and higher) for the effects of cracking in the flask.
10 Laboratory Tests and Interconversions
15. D1160 Conversion: Edmister Method
The D1160 to TBP correlation developed by Edmister is shown below. A
base of 10 mm Hg was chosen for the correlation. Thus, a necessary step
in applying the correlation is to first correct the D1160 distillation to 10
mm Hg with a Cox chart. The resultant distillation is converted to a 10
mm Hg TBP and the Cox chart is again applied to bring the TBP to a
basis of 760 mm Hg. Use of the Cox chart introduces some additional
error into the procedure. It may be more accurate to enter D1160 tests
directly as TBP's, and avoid the double Cox chart conversion. Moreover,
a 2 mm Hg D1160 test is very close to a TBP.
Figure 3: Edmister
D1160 Conversion 10 M M HG
Correlation
0
- 10
-5
IP
30
TBP DT
ND
A
0
-3
10
ASTM DT 120 160
D2887 Conversion: Daubert Method
The Daubert correlation to convert D2887 simulated distillation to D86's
is presented as:
D86 = a(SD)bFc (5)
where F = f(SD10, SD50) and a, b, c are supplied for IP, 10, 30, 50, 70,
90, 95. The D86 is then converted to TBP using Daubert.
Note that D2887 distillations must undergo a double conversion: D2887
to D86 and D86 to TBP. Moreover, D86 tests are not very applicable to
high boiling stocks. Therefore it is recommended that D2887 distilla-
tions be entered as TBP distillations and the double conversion be
avoided for stocks heavier than diesel fuel (about 700°F endpoint).
Hydrocarbon Distillation Workbook 11
16. Conversion of Assay Data to Petroleum Cuts
In order for assay data to be useful in a flowsheet simulation, they must
be converted to a discrete set of petroleum components. The flowchart in
Figure 4 describes the procedure that PRO/II uses to interpret and trans-
form the assay stream data into useful compositional information.
Figure 4: Assay
Processing Flowchart Distillation Data
Convert Data to Equivalent TBP
Curve @ 760mm Hg
Distribute Assay Curve
Assay Processing Steps
into Cuts
Determine Moles, Mass and
Volume for Each Cut
Process Light Ends
Light Ends
in Stream
Determine Average NBP, SPGR
and MW for Pseudocomponents
Characterize Other
Thermophysical Properties for
Pseudocomponents
Set of
Petroleum Components
This section explains the processing required to convert the assay data to
its corresponding set of petroleum components.
Convert Data to Equivalent TBP Curve
Although ASTM assay data are much easier to obtain than TBP data,
they are less valuable and must first be converted to 760 mm Hg true
boiling point (TBP) curves. The next step is to fit the TBP data to a con-
tinuous curve. This step is necessary because the supplied data points
will not necessarily correspond to the desired cutpoints.
12 Conversion of Assay Data to Petroleum Cuts
17. PRO/II offers three methods for interpolating distillation curves:
s The default is the cubic spline method (known as the SPLINE
option). Cubic spline interpolation usually provides an excellent fit,
however, instabilities can arise if the input data contain a large jump.
Such jumps are usually the result of an error in your distillation data.
s In the rare cases where a spline fit is unstable, PRO/II can interpolate
the data using piecewise quadratic approximations (known as the
QUADRATIC option).
s The Probability Density Function (PDF) method is recommended
when you suspect significant errors or random noise in your assay
data. It differs from the SPLINE and QUADRATIC methods in that the
curve is not required to pass through all of the supplied points. You
can force the curve to pass through the initial and/or end points by
using the Include in PDF option. This option has a strong effect on
how incomplete distillations are extrapolated, and you are encour-
aged to refer to the PRO/II Reference Manual before using it.
For incomplete distillations (i.e., distillations that do not range from 0 to
100% distilled), PRO/II uses the first two data points to extrapolate the
TBP curve back to 0.01% volume and will similarly use the last two data
points to extrapolate the TBP curve out to 99.99%. The extrapolation
feature is particularly valuable for heavy ends distillations, which can
terminate with over 50 volume percent of the initial charge not distilled.
Distribute Assay Curve into Cuts
As an option, you can define how to partition the TBP curve into discrete
pseudocomponents, or cuts, by setting the desired number of compo-
nents within a given temperature range. Table 2 lists the default cutpoints
used by PRO/II, when user-supplied cutpoints are not provided.
Table 2: Defining Cutpoints
Temperature Range Number of Components
100-800°F (38-427°C) 28
800-1200°F (427-649°C) 8
1200-1600°F ( 649-871°C) 4
Here, 28 pseudocomponents should exist in the temperature range 100-
800°F; thus, these components each have a boiling range of 25°F. Note
that the defaults in Table 2 were originally designed for partitioning
crude oils. Material that boils below the first cut is combined with the
first cut and material that boils above the last cut is combined with the
last cut.
Hydrocarbon Distillation Workbook 13
18. Determine Moles, Mass, and Volume for each Cut
Based on the sample's average gravity (or gravity curve, if you provided
it), PRO/II calculates the number of moles, the mass, and the volume
contained in each cut.
Process Light Ends
Hydrocarbon streams often contain significant amounts of light hydro-
carbons. While there is no universal definition of light, hexane is a com-
mon upper limit. Simulation of such systems is more accurate if these
components are considered individually rather than lumped into
pseudocomponents.
It is easy to spot mismatches of the light ends to the TBP curve. When
pseudocomponents are generated with boiling points less than the high-
est boiling light end there is obviously a mismatch. Similarly, if there is a
large gap between the NBP of the highest boiling light end and the first
pseudo component, this also indicates a mismatch.
Material on the TBP curve that extends above the highest defined cut
temperature or below the lowest defined temperature is averaged into the
highest or lowest numbered cut, respectively. When this occurs, the user
may desire to extend the temperature ranges for the cuts.
PRO/II offers several techniques for processing your light ends:
s Match to TBP Curve: By default, PRO/II will match your light ends
data to the TBP curve. The rates for the light end components are
adjusted up or down, all in the same proportion, until the NBP of the
highest-boiling light end component intersects the TBP curve. PRO/
II then discards all of the cuts from the TBP curve that fall into the
region covered by the light ends data and uses the light end compo-
nents in subsequent calculations.
s Fraction of Assay: This method allows you to specify that the total
light ends flowrate be a prescribed fraction (or percent) of the overall
stream flowrate.
s Use Compositions as Actual Rates: Here the compositional entries are
used as the actual component flowrates. The total light ends flowrate
is the sum of the individual components. The flowrates are not
scaled to match the TBP curve.
s Light Ends Rate: Here you provide the total light ends flowrate and
the individual light ends components are given as fractions or per-
cents. If your individual component values do not sum to 1 or 100,
you can use the normalize component flowrates option.
Figure 5 shows graphically how the petroleum components are gener-
ated and how the light ends data are matched to the assay curve.
14 Conversion of Assay Data to Petroleum Cuts
19. Figure 5: Light Ends
Matching
Match the light ends
data to the TBP curve
at this point
Determine Average NBP, SPGR, and MW for each Pseudocomponent
Once PRO/II has defined the cuts in terms of moles, mass, and volume,
and incorporated the light ends, it determines the normal boiling point
(NBP), specific gravity (SPGR), and molecular weight (MW) for each
cut.
Computing the Normal Boiling Point
PRO/II determines the NBP for each pseudocomponent as a volume or
weight fraction average by integrating across the cut range:
x ma x
∫x T ( x ) dx
min
NBPj = ---------------------------
- (6)
x max – x min
x represents the percent liquid volume or weight distilled in cut j. PRO/II
uses these average boiling points as correlating parameters when calcu-
lating other thermophysical properties for each pseudocomponent.
Computing Average Gravity
If, in addition to the required stream average gravity value, you have
entered a gravity curve, PRO/II will calculate the average gravity for
each cut. If you supply only the average gravity for the stream, then
PRO/II uses the Watson-K factor to calculate the average gravity for
each cut. As you may recall, the Watson-K factor is a function of NBP
and specific gravity:
1⁄3
NBP
K = ------------------
- (7)
spgr
Hydrocarbon Distillation Workbook 15
20. Using the average NBP and average gravity for the stream, PRO/II com-
putes a Watson-K factor for the entire stream. The Watson-K factor is a
measure of the paraffinicity of a stock. The factor is relatively constant
through the entire boiling range of crude oils, so computing one factor
for the entire stream is a valid assumption. PRO/II then uses the Watson-
K factor and the NBP for each cut to back-calculate each cut's average
gravity as illustrated in Figure 6, where:
1⁄3
NBPj
spgr j = --------------------
- (8)
K
Computing Molecular Weights
As the next step in characterizing the pseudocomponents, PRO/II deter-
mines the molecular weight using a correlation that relates it to NBP and
gravity. Keep in mind that PRO/II's molecular weight correlations tend
to be biased toward crude oils. Whenever possible, you should supply
molecular weights to obtain a more accurate set of components. You can
supply a molecular weight curve and, if available, an average value. If
you supply both a curve and an average value, the average takes priority
and the curve will be adjusted and extrapolated to match the average.
Figure 6: Computing
the Component NBP's
and Gravities
28. Characterize Other Thermophysical Properties for the Pseudocomponents
All other physical and thermodynamic properties (e.g., critical properties
and enthalpy curves) required by PRO/II can be calculated from the
molecular weight, the NBP, and the gravity data by using correlations.
To change methods for property estimation, curve fitting and intercon-
versions, click Characterization Options...
and make the appropriate
selections in this dialog box.
If the default correlations do not adequately match your specific assay
data, you can try other calculation options to improve the fit.
Set of Petroleum Components
You now have a set of petroleum components, which define the assay
stream's composition and can be used in the simulation.
In this discussion we have only considered deriving a set of petroleum
components from one assay stream. In reality, multiple streams are often
used to generate a component set. The blend option allows you to gener-
ate more than one set of petroleum components from multiple streams
within a given run. This powerful feature, used for modeling a process
that has different feedstocks, particularly one that uses both virgin and
cracked feedstocks, is discussed in the next chapter.
Generation PRO/II generates the simulated distillations as follows. The components
of Simulated are boiled out of the stream in volatility order. Note that this corre-
sponds to a perfect TBP; there is no overlap of components. The 1.0 and
Distillations
98.0 LV percentage distilled points are arbitrarily defined as the initial
in PRO/II boiling point and end point. The resultant TBP undergoes some smooth-
ing before conversion to an ASTM distillation.
Many factors affect the accuracy of the simulated distillation. The widths
of the TBP cuts tend to give the simulated TBP a flat look compared to
the true TBP that corresponds to thousands of components. The arbitrary
definition of the initial and end point affects the accuracy of these points.
Moreover, the true initial and end points are determined by trace compo-
nents that may not be present in the simulated stream. Finally, the accu-
racy of the conversion routines between the simulated TBP and the
simulated ASTM must also be considered.
The IBP and EP may be inaccurate because of:
s Errors in the assay data
s Errors in the TBP conversion
s Width of pseudocomponents
Hydrocarbon Distillation Workbook 17
29. s Arbitrary definition for TBP procedure above
For these reasons, the simulated IBP and EP are questionable, and may
not behave as expected. For this reason, it is recommended that the 5 and
95 percent distilled points be used for simulation specifications.
Special There are several considerations when representing a petroleum based
Considerations stream as pseudocomponents, based on assay data. The single most diffi-
for Supplied cult property to characterize is the molecular weight, because only two
Assay Data correlating parameters are used (NBP and gravity). The characterization
methods tend to have a paraffin bias, and the molecular weights pre-
dicted from NBP and gravity will not be high enough for condensed ring
structures.
You can improve the molecular weight characterization (and hence the
pseudocomponent property generation) by providing a molecular weight
curve when possible. For example, most crude oil assays divide the
crude oil into several products, with defined weight or volume ranges on
the total crude oil and average properties such as gravity and molecular
weight. These data can be entered as a curve for the entire stream, where
the average cut gravity or molecular weight is entered at the cumulative
mid point for the product on the total crude stream. These same data can
be easily estimated from the properties and yields for the products from
the crude still.
s It is important that the TBP cut ranges include several cuts common
to overlapping products, where fractionation between the products is
being considered. For the example, several cuts are needed in the
common area where the light and heavy products have common
components.
Heavy Product
Light Product
Temperature
Overlapping
Components
Needed
TBP Percent Distilled
18 Conversion of Assay Data to Petroleum Cuts
30. s Defined light ends must match the TBP. Stabilization of a reforming
naphtha is probably not a meaningful calculation, unless the butane
and pentane components are entered as light ends. When the distilla-
tion for the petroleum stream is for a weathered sample, the light
ends should be entered as a separate stream and mixed with the
pseudocomponent stream generated from the weathered distillation.
Components
Defined
Temperature
TBP Percent Distilled
s For high boiling refinery streams such as crude oil and FCC slurry
oil, the upper end of the distillation curve is not known. For crude
oil, this can be more than 25% of the total crude charge. For FCC
units, this is a much smaller portion of the reactor effluent (about
5%). It is best to extrapolate the data to a contrived end point using
probability paper, and supply PRO/II with the extrapolated data.
While PRO/II will extrapolate the data, it has no way to know what a
reasonable endpoint for the stream might be. This leaves you at the
mercy of mathematical techniques.
Crude Oil E.P. = 1600ºF
FCC Slurry E.P. = 1200ºF
Percent Distilled
1500
Crude Oil Data
ºF
900
800
700
20 50 80 99.9
s A more accurate representation of the feed to multi-draw refinery
columns can be generated by supplying the product streams and
blending them to produce the total feed. Product data are generally
known more accurately than feed data. Moreover, light gases pro-
duced by cracking appear in the products.
Hydrocarbon Distillation Workbook 19
31. s Often, you can supply a gravity curve for a composite feed. If the
samples were cut into fractions by the laboratory, the gravities of the
individual fractions can be supplied at the mid-volume percents of
the fractions on the TBP curve. Such a gravity curve can also be con-
structed when the yields and gravities of the distilled products are
available.
s Make certain that laboratory data for ASTM D86 distillations have
been corrected for atmospheric pressure. This is important for high
altitude locations to generating good pseudocomponents. D1160 dis-
tillations are almost always corrected by the reporting lab to a 760
mm Hg basis. It is always good practice to ascertain this, however.
s Cracking is important for laboratory distillations. It can be easily
detected by plotting the distillation on probability paper. The slope
of the distillation curve changes slope and becomes very flat as
cracking occurs. The uncracked results can be estimated by continu-
ing the slope prior to the cracking as a straight line on probability
paper.
s For some problems it is necessary to use multiple sets of
pseudocomponents, i.e., multiple components with the same boiling
point, but differing gravities and molecular weights. The standard
PRO/II procedure is to simulate each assay stream separately, and
then weight average the resultant pseudocomponents into one com-
mon set for the simulation. This is not adequate for problems in
which some streams are virgin stocks and some are cracked stocks.
20 Conversion of Assay Data to Petroleum Cuts
32. Assay Blending
When your flowsheet contains more than one user-defined assay stream
(i.e., feeds and recycle estimates), PRO/II’s default operation is to create
a single set of petrocomponents to characterize all of the assay streams.
This set of petrocomponents is called a blend. If assay blending was dis-
abled, PRO/II would generate an independent set of petrocomponents for
each assay stream. The large number of petrocomponents would lead to
great slowing of the flowsheet calculations.
To create a single petrocomponent set from a group of assay streams,
PRO/II averages, or blends, the properties of the assay streams using the
equations below. Note that the summation is over the assay streams con-
taining the cut range of interest.
NBPblend = ∑ NBP ⋅ Volume Fractioni (9)
i
Specific Gravity blend = ∑ cut weighti ⁄ ∑ cut volume i (10)
i i
Molecular Weight blend = ∑ cut weight i ⁄ ∑ cut molesi (11)
i i
The blend properties are used everywhere in the simulation for that com-
ponent.
Consider a flowsheet that has several user-defined assay streams, as
shown in Figure 7. It shows how PRO/II generates the properties for a
petrocomponent whose TBP ranges between 205 and 215ºC. Only three
of assay streams contain material boiling in this range so they will be the
only ones participating in the blending procedure for this cut range.
As previously discussed, PRO/II can generate all of a petrocomponent’s
thermodynamic properties from its NBP, specific gravity, and molecular
weight. Unfortunately, in most instances, PRO/II will report different
values for this cut in each of the assay streams. In stream 1, the 205-
215ºC cut range may have an NBP of 212, while in stream 2 the same cut
range may have an NBP of 211. Likewise the gravities and molecular
weights for this cut range will probably differ in each of the assay
streams. The differences may be due to errors in lab data or calculational
approximations. Or they may indicate genuine physical differences in
the assay streams.
Hydrocarbon Distillation Workbook 21
33. Figure 7: Blending
Assays TBP Feed 1, Rate=150
eed Feed 2, Rate=400
eed Feed 3, Rate=100
eed
215º
205º
% Distilled
NBP = 212º NBP = 211º NBP = 213º
SPGR = 0.85 SPGR = 0.83 SPGR = 0.85
Sulfur wt% = 0.012 Sulfur wt% = 0.010 Sulfur wt% = 0.011
Rate = 30 Rate = 60 Rate = 10
Blending
NBP = 211.5º
SPGR = 0.838
Sulfur wt% = 0.0107
Since a given petrocomponent can have only a single value for each of
its properties, PRO/II must reconcile the conflicting values to arrive at a
single set of properties for each petrocomponent. The blending proce-
dure is really a form of data reconciliation.
In this example, the three streams’ contribution to the 205-215ºC cut
range is blended to form a single petrocomponent whose NBP is
211.5ºC. Its properties are closest to those of feed 2, because the greatest
contribution of material in this cut range comes from feed 2. From this
point on, all three of these streams will be characterized by this petro-
component in this boiling range.
In PRO/II it is possible to use multiple blends (petrocomponent sets) in
the same flowsheet. The blends are named after the cutpoint sets.
Assume you have defined the cutpoint sets VIRGIN and CRACKED. The
VIRGIN blend uses 15 petrocomponents between 180 and 400ºC while
the CRACKED blend uses 10. By default, any stream characterized with
the VIRGIN cutpoint set will automatically be included in the blend called
VIRGIN. Likewise, streams you choose to characterize with the
CRACKED cutpoint set will be included in the CRACKED blend. The fol-
lowing slide discusses why you would want to use multiple blends.
In some instances, you will want to exclude a stream’s properties from
its blend (keyword XBLEND), even though that stream will use the prop-
erties of the blend. This is commonly done for assay streams that are
used as initial estimates for a recycle. Since the initial estimate can be
significantly in error, you would not want the initial estimate to influence
the blend properties.
22 Assay Blending
34. When to Use The main reason to use multiple blends is for property differentiation.
Multiple For example, if your flowsheet contains both virgin and cracked feed-
stocks, then you would want to use two different blends to account for
Blends
the different properties of the virgin and cracked streams. Although a
petrocomponent in the VIRGIN blend may have the same NBP as a petro-
component in the CRACKED blend, their gravities and molecular weight
(and K-values) can be very different. Using multiple blends preserves
these differences in the simulation and gives a more realistic representa-
tion of their processing requirements. The price of using multiple blends
is that flowsheet calculations must be performed on a larger number of
components. This always leads to longer execution times.
Property Differentiation
In the flowsheet shown below, imagine you are designing processes to
treat low and high sulfur feeds. Naturally the processing requirements
differ for each feed because of their differing sulfur content. If you used
a single blend to represent both streams, PRO/II would average their
properties to create a medium sulfur petrocomponent set, and you would
not be able to determine the processing differences for the two feed
stocks. Using a single blend for this problem would be a mistake.
Overhead
Light Oil
High Sulfur
Low Sulfur ssa
Assay
ssa
Assay
LSFO HSF
HSFO
Economical Analysis
In the flowsheet shown below, suppose you want to determine the value
of each feed stream based on the value of the products it produces. If a
single blend was used for both feed streams, then PRO/II could not tell
how much each feed contributed to the products. Using different blends
for each feed provides you with a means to track the fate of each stream.
This is useful for refinery planning and for checking designs for their tol-
erance to feedstock changes.
Assay 1
ssa Product A (X$/BBL)
BBL
Processing
Assay 2
ssa Product B (Y$/BBL)
BBL
Hydrocarbon Distillation Workbook 23
35. Crude Oil Crude oil is a very complex mixture of hydrocarbons. An API project
begun in 1931 has isolated more than 16,000 distinct compounds in one
barrel of Oklahoma crude oil. The huge number of components occur-
ring in crude oil gives it a very continuous TBP distillation. Obviously,
representing crude oil with 50 components (or even 150) does not allow
perfect matching of the tray temperatures and product compositions in a
crude column simulation.
Crude oil varies widely in composition, both by location, and with time.
Moreover, a given crude oil mix, for example, West Texas Sweet, may
vary in composition from day to day because of the individual wells in
production. Because of allocations, not all wells are produced each day.
Different crude oils are best for making certain products. Some crude
oils have a high asphaltene content and are used to produce asphalt. Oth-
ers may be light enough that the heaviest portion may be charged
directly to the FCC unit. Kerosene yield and quality are an important
consideration since this material is sold as commercial jet fuel. Some
crude oils have light naphtha components more suited to reforming than
others, and so forth.
Crude oil is a full range boiling material, with everything from methane
to heavy components boiling at 1600°F.
Crude oil gravity varies widely, with the lighter crude oils generally
being more valuable since they can be more readily converted to higher
priced products, such as gasoline, jet fuel and distillates. Historically
crude oil price was based on gravity alone, however, it did not take long
for individuals to beat this system by spiking the crude oil with cheap
LPG gas.
Certain data are reported on laboratory assays for crude oil. A TBP or
simulated TBP is reported which typically covers from 70 to 80 volume
percentage of the mixture. The boiling points for the remainder of the
mixture are unknown and you should use probability paper to estimate a
typical tail for the mixture.
A chromatographic analysis of the light ends is usually given. This is
rarely an accounting of the light ends in the crude column, which include
light gases created in the furnace by cracking.
The crude oil is broken into distinct product cuts which correspond to
the products from the crude still. The TBP still products are more precise
than the products from the crude still, because of the superior fraction-
ation in the TBP still.
An alternate method to simulate a crude oil mixture is to combine the
products. Product data are accurately known for most of the products,
24 Assay Blending
36. however, the topped crude distillation must be largely fabricated with
probability paper. Sometimes, the laboratory will run a low pressure
D1160 distillation for this material that can be used to develop
pseudocomponents.
Thermo Follow the application guidelines and the water decant options outlined
Methods in the following section when selecting the appropriate thermodynamic
method for your simulation.
Application Guidelines
How do you determine which method is most suitable for your problem?
You can find detailed information on this topic in the PRO/II Reference
Manual. In short, the best way to select the appropriate thermodynamic
method is to understand the assumptions, features, and limitations built
into each of the different models. A certain portion of all of our thermo-
dynamic methods is empirical. For example, the PR method is tuned
(i.e. certain parameters were selected) to accurately represent light
hydrocarbon systems below the critical point. While it can represent
heavy hydrocarbon systems, you would not expect the results to be as
accurate as light hydrocarbon systems. The basic PR method would do a
poor job at predicting equilibrium for polar systems, such as the Moon-
shiner's ethanol-water system, for the simple reason that it was not
designed for polar systems.
The kij's are binary interaction coefficients. Their presence usually indi-
cates that certain experimental data have been incorporated into the ther-
modynamic model, and you can expect an extra degree of accuracy for
these components. The absence of some binary interaction coefficients
(their values will be zero) is not necessarily a cause for alarm, it just
indicates that you might want to provide your own values or look for a
thermodynamic method that includes values.
The Grayson-Streed method usually works best for heavy ends columns
operating at low pressures (less than 50 psia or 3.5 bars). Grayson-Streed
can also be used for the downstream processing in an FCC gas plant if it
is desired to simulate the main fractionator and all downstream process-
ing in one model.
For most light ends processing, the Soave-Redlich-Kwong and Peng-
Robinson methods should be used. Both methods have numerous sup-
plied binary interaction parameters, and are capable of accurately pre-
dicting vapor liquid equilibria for sour gas systems. PRO/II has special
data to fit C2 and C3 splitters in the SRK and PR methods.
The SRKM and PRM method have special data for hydrogen and are the
best methods for predicting hydrogen solubility in liquids. Because the
Hydrocarbon Distillation Workbook 25
37. Grayson-Streed method has special liquid fugacity curves for methane
and hydrogen, it usually does an adequate job of predicting hydrogen
rich operations such as reforming and hydrocracking.
The SRKM and PRM data have special data for the solubility of light
gases in water, as does the SRKKD method. An alternative method for
calculation of light gas solubility is to use the Henry data supplied in
PRO/II.
The SOUR method is designed for the simulation of sour water strippers.
Note that the electrolytic chemistry is not considered by the calculations,
therefore, the answers must suffer some inaccuracy.
Handling of Water
For the free water or decant option, water is considered as forming an
immiscible phase with the hydrocarbon liquid.
The free water option is a convenient, efficient method to simulate the
three phase behavior exhibited by hydrocarbon- water systems when dis-
solution of hydrocarbons in the liquid water phase is small. Thus, refin-
ery columns with stripping steam and natural gas streams saturated with
water can generally be simulated adequately with this method.
The free water technology is a semi-rigorous three phase (VLLE) calcu-
lation. The vapor is first saturated with water at its vapor pressure. Water
is then dissolved in the hydrocarbon liquid up to its solubility limit, and
any remaining water is decanted as a free water phase. The solubility of
water in the hydrocarbon liquid is based on data in the component
library. For compatibility with PROCESS and earlier SIMSCI programs,
a chart in the API Data Book that relates the solubility of water in kero-
sene to temperature can alternately be selected to determine the water
content of the hydrocarbon liquid. The water solubility can also be cal-
culated with an equation of state.
The free water phase contains no dissolved hydrocarbons (or light
gases). If these were an important consideration for the problem being
analyzed, e.g., an environmental question, the free water option is not
adequate and a rigorous three phase calculation must be used.
Water K-values are computed from the water vapor pressure (PW), the
mole fraction water in the hydrocarbon liquid phase (XS), and the system
pressure (PI). For natural gas systems at pressures greater than 2000 psia
(138 bars), a chart from the GPSA Data Book that relates the partial pres-
sure of water vapor in natural gas to temperature and pressure gives more
accurate K-values for the water.
Rigorous three phase calculations must be performed for hydrocarbon-
water systems where the dissolution of hydrocarbons and light gases in
26 Assay Blending
38. the water phase are significant. All of the SRK and Peng-Robinson
options in PRO/II are capable of predicting three phase behavior, how-
ever, not all options have the necessary binary interaction parameters as
supplied data.
Figure 8: Handling
of Water Vapor
Hydrocarbon Liquid
Water
In general, the SRK-Kabadi-Danner (SRKKD) method and the SRKM
(SIMSCI method) and PRM (SIMSCI method) have large data banks of
binary interaction parameters for water with light hydrocarbons and
gases. Therefore, these methods are preferred for three phase calcula-
tions unless you have some interaction parameters to supply. It is good
practice to inspect the reprint of interaction parameters and verify that
parameters are present for components for which accurate calculations
are needed.
When the standard SRK or PR methods are selected for three phase cal-
culations, the free water (decant) option must be deactivated. This is not
necessary for the SRKKD or modified SRK and PR methods.
Hydrocarbon Distillation Workbook 27
39. Multicomponent Distillation Using PRO/II
PRO/II provides five algorithms for solving distillation problems. An
algorithm is a mathematical procedure, or strategy, for solving the col-
umn equations. Although all of these algorithms will produce identical
results, some are better suited for certain problems.
s The Inside/Out (I/O) algorithm is well suited to solving the hydro-
carbon distillation problems that are common in refineries.
s The Chemdist algorithm is capable of solving mechanically simple
columns whose components exhibit highly nonideal thermodynam-
ics.
s The Sure algorithm is very general and can solve some column con-
figurations not handled by the I/O and Chemdist algorithms. It may,
however, require more user intervention to obtain a solution than the
other algorithms. For this reason, you will usually select the Sure
algorithm only when the I/O and Chemdist algorithms do not work.
A column with total pumparounds and water draws on several trays,
for example, can only be solved with the Sure algorithm.
s The Liquid-Liquid algorithm is used to model liquid-liquid extrac-
tion columns.
s Enhanced I/O Algorithm extends the capabilities of the default
Inside-Out algorithm to support total vapor and liquid side draws,
total pumparounds, and free water phase and water decant on any
tray.
Later in this workbook, you will learn some of the details of the various
algorithms. In doing so, you will gain an appreciation of each algo-
rithm's strengths and weaknesses; this information will help you select
the appropriate algorithm for your problems. First, we will focus on
PRO/II's distillation column interface.
PRO/II When you double-click on a column in a flowsheet, the distillation col-
Column umn input screen, shown in Figure 9, appears. By clicking on the appro-
priate buttons and following the prompts, you can build complex
Data Entry
distillation columns. What follows is a quick introduction to this screen's
Window features. Most will be discussed in greater detail later on.
28 Multicomponent Distillation Using PRO/II
40. Figure 9: PRO/II
Column Dialog Box
Pressure Profile
This button will initially be red bordered, indicating that you must pro-
vide some pressure data. For most applications, PRO/II performs all cal-
culations at the prescribed tray pressures. The Overall mode is the
easiest way to define a pressure profile. Simply provide the top tray pres-
sure and then specify a per-tray or total-column pressure drop. If you
want to provide pressure values on some or all stages, select By Individ-
ual Trays and enter data.
It is possible to have PRO/II's sizing and rating algorithm compute the
pressure profile from a description of the tower tray configuration and
vapor and liquid traffic. This is accomplished through the Tray Hydrau-
lics dialog box. PRO/II uses the supplied pressures as base case esti-
mates, rather than defined values.
Feeds and Products
Click this button to enter the locations, flowrates, and phases of the feed
and product streams. For multiphase feeds, you have the option of plac-
ing the vapor portion on the stage above the designated feed stage. You
can also define product pseudostreams in this dialog box. Pseudostreams
are copies of tray liquid, vapor, or pumparound streams and do not affect
column calculations. They are simply a tool that gives you access to
internal column streams. Note that it is your responsibility to maintain
the material balance for the flowsheet when you use pseudostreams.
Convergence Data
Adjust convergence parameters, tolerances, and request diagnostic infor-
mation via this button. The diagnostic information is particularly useful
for troubleshooting non-converging columns. You can also instruct
PRO/II to print complete column profiles to a file, which can be used to
initialize the column from converged solutions.
Hydrocarbon Distillation Workbook 29
41. You can also adjust the damping factor to less than one which can be
used to improve convergence when the outer loop is oscillating. Refinery
complex fractionators are given a default damping factor of 0.8. Chemi-
cals columns may require more severe damping.
The Chemdist and Liquid-liquid algorithms in PRO/II support both liq-
uid and vapor phase chemical reactions, and are suited to the same size
systems, i.e., distillation systems which have a smaller number (10 vs.
100) of chemical species. Larger systems can be simulated, but a large
number of calculations can be expected.
Thermodynamic Systems
Click this button to change the default thermodynamic model. Or, select
different models for different sections of the column. Use this option
when a single thermodynamic method cannot accurately characterize the
wide range of conditions that are possible throughout the column.
Reboiler
PRO/II provides a model for kettle reboilers, and two models for thermo-
siphon reboilers. You can use the thermosiphon models only with the I/O
distillation algorithm.
Condenser
PRO/II provides three condenser models. You can choose from partial,
bubble point, and two types of subcooled condensers. It is here that you
supply the condenser's operating conditions.
Heaters and Coolers
You can place side heaters and coolers on any tray in the column. You
can also simulate a heat leak between the column and the environment.
Use a positive duty for a heat source (heater) and a negative duty for a
heat sink (cooler).
PRO/II also allows you to supply flash zone data for an I/O column. A
flash zone may be defined for any side heater in the column and repre-
sents a single theoretical stage. This feature is especially useful when
simulating fired heaters added to a tray.
Initial Estimates
Click this button to select an Initial Estimate Generator (IEG) model and
to provide estimates of tray variables, such as temperature, composition,
and flowrate. The four different IEG models are tools that estimates the
values of all column variables from a few seed guesses that you pro-
vide. This important topic is discussed in detail later in this workbook.
30 Multicomponent Distillation Using PRO/II
42. Pumparounds
Click this button to add pumparounds to columns that use the I/O and
Sure algorithms. The pumparound configuration and specifications are
set using the linked text that appears in the pumparound dialog box.
Performance Specifications
Click this button to enter performance specifications and declare column
variables. You can specify that the column's overhead have a certain
purity, that the reboiler have a certain temperature, or that a sidestream
have a certain flowrate, for example. Specifications and Variables will be
discussed later.
Tray Hydraulics/Packing
PRO/II contains calculation methods for rating and sizing trayed distilla-
tion columns, and for modeling columns packed with random or struc-
tured packings. Trayed columns are preferable to packed columns for
applications where liquid rates are high, while packed columns are gen-
erally preferable to trayed columns for vacuum distillations, and for cor-
rosive applications. All tray rating and packed column calculations
require viscosity data, and therefore a thermodynamic method for gener-
ating viscosity data should be selected for these applications. Both types
of calculations can be applied to portions of the column and you can rate
different types of trays and/or packing within the same column.
Tray rating and sizing can be performed for new and existing columns
with valve, sieve and bubble cap trays. Valve tray calculations are done
using the methods from Glitsch. Tray hydraulics for sieve trays are cal-
culated using the methods of Fair and for bubble cap trays with the meth-
ods of Bolles. Rating and design calculations are available.
The rating option uses established correlations to calculate quantities
such as flooding factors, downcomer backup, and pressure profile. You
must provide a mechanical description of the column. For trayed col-
umns, this includes inter-tray spacing, tray diameter, and tray design. For
random packed columns, this includes the packing size, packing factor,
and column diameter. For structured packed columns, this includes the
packing type, height, and HETP, as well as the column diameter.
The sizing option calculates the column diameter. Like the tray rating
feature, the tray sizing feature has the ability to calculate the column
pressure profile.
Hydrocarbon Distillation Workbook 31
43. Tray Efficiencies
PRO/II provides three built-in tray efficiency models:
s Murphree
s Equilibrium
s Vaporization.
You can provide different values for individual trays and even different
values for each component. Most often, however, you will use overall
efficiencies described later in this workbook.
Algorithm
PRO/II provides five algorithms (computational strategies) for solving
distillation problems: Inside/Out (I/O), Sure, Chemdist, Liquid-Liquid,
and Enhanced I/O. These are described in following chapters.
Number of Trays
PRO/II assumes all trays, with the exception of subcooled condensers,
are equilibrium stages, that is, the vapor and liquid leaving the stages are
in equilibrium.
You are likely to encounter columns whose stages are numbered from
top-down as well as bottom-up. PRO/II always assumes the stages are
numbered from the top down. When a condenser is present, it is always
stage number 1. This is true even if the condenser produces a subcooled
product (i.e., no fractionation occurs). The reboiler, if present, is always
the highest numbered stage.
32 Multicomponent Distillation Using PRO/II
