Before you enter data for a stream, you should have placed the unit oper-ations on your flowsheet and connected them together with feed, prod-uct, and recycle streams. You should also have declared the components that will be present in your simulation and specified the thermodynamic methods you want to be used.
To define a stream completely you must specify its:
■ Thermal condition
■ Composition
■ Flowrate
➤ To open the Stream Data dialog box, double-click on the stream, or right-click on the stream and select Data Entry from the menu.
Figure 17:
Stream Data Dialog Box
Thermal Condition
PRO/II requires that you provide the thermal condition for all external feed streams. You must also supply the stream thermal condition if you choose to enter a recycle estimate. To define the thermal condition, you must specify two of the following three properties:
■ Pressure
■ Temperature
■ Phase
Temperature and/or Pressure
From the First Specification drop-down list select either Temperature or Pressure. If you want to supply both, select Temperature here and Pres-sure as the Second Specification. Enter values in the fields supplied.
Phase
Phase data are supplied as the Second Specification after you have sup-plied temperature or pressure. When defining the phase, you can specify the stream as one of:
■ a saturated liquid at its bubble point
■ a saturated vapor at its dew point
■ a mixed phase stream at a liquid fraction between 0.0 and 1.0, on a molar, weight or volume basis. Enter the value in the field provided.
For example:
If you do not define the phase, PRO/II will determine it with a phase equilibrium calculation at the specified temperature and pressure.
Composition and Flowrate
PRO/II requires you to specify the composition and flowrate of all exter-nal feeds and estimated recycle streams. You can enter the composition of a stream in one of four ways:
■ Using defined components
■ Using assay or distillation data
■ By referencing to another defined stream
■ By defining a stream as containing only solids
➤ Select the stream type in the Stream Type group box. You define the flowrate after choosing the stream type.
Compositional Stream
Compositional streams are made up of pure components: library, user-defined or petroleum. You must always provide the composition.
➤ Click to open the dialog box.
Figure 18:
Flowrate and Composition Dialog Box
If you select Individual Component Flowrates, PRO/II adds the individ-ual component flowrates to get the total rate.
If you provide a total stream flowrate, the sum of individual composi-tions entered should be 1.0 (fraccomposi-tions), 100 (percentages), or the flowrate that you supply. If not, check the Normalize Component Flowrates Based on Specified Fluid Flowrate box and PRO/II will adjust them for you.
If a component does not exist in a particular stream, enter a zero value for that component, or leave blank.
Flowrate and Composition...
Both composition and flowrate can be given on a molar, weight, standard liquid volume or standard gas volume basis. You can mix bases. For example, you can enter the total flowrate on a molar basis and enter the component rates on a weight basis.
To change the basis, click in the relevant field and click at the top of the dialog box.
Standard Conditions
If you enter data on standard liquid or standard vapor volumetric bases, PRO/II will use the density of the phase you specify, regardless of the actual physical state of the stream at the specified thermal condition.
For liquid volume, PRO/II determines the molar flowrate using the liquid densities of the components at standard conditions (60°F and 1 atm). In cases where a component is a vapor at standard conditions, the estimated density value comes from the GPSA handbook. If the GPSA value is unavailable, PRO/II extrapolates from the density of the saturated liquid at atmospheric pressure.
For vapor volume, PRO/II uses the defined standard vapor conditions to determine the molar flowrate. The actual values of the standard tempera-ture and pressure (and therefore the computed flowrate) depend on the default units of measure that you are using. For the metric and SI sys-tems, STP defaults to 0°C and 1 atm of pressure. For English units, the STP default is 60°F and 1 atm of pressure. You can change the standard vapor conditions for your simulation using
in the Default Units of Measure for Problem Data Input dialog box.
After determining the molar flowrate, PRO/II performs an internal flash to bring the stream from STP to the thermal condition that you have specified.
Petroleum Assay Stream
Assay streams differ from compositional streams by the way in which their compositions are entered and referenced. When you input an assay stream, instead of explicitly stating how much of each species is present, you provide simple experimental data. PRO/II uses that data to charac-terize the stream's composition in terms of petroleum components.
Typically, a laboratory-scale batch distillation analysis, such as the ASTM D86 procedure, is performed to characterize a crude stream.
UOM
Standard Vapor Conditions...
Generating Assay Curves
The beaker is charged with the crude sample and heated. The tempera-ture increases as the lighter fractions boil out of the mixtempera-ture. While the liquid is boiling, the temperature (T) and the total condensed volume (V) are periodically recorded, as are quantities such as gravity. Gravity data are commonly reported.
The quantities measured in this batch distillation experiment constitute the assay data for the sample. PRO/II then uses correlations to translate the assay data into TBP data from which all physical and thermody-namic properties are calculated.
The assay is represented as a plot of the temperature versus the cumula-tive percent distilled. PRO/II uses this distillation curve, along with an analysis of both the light pure components (e.g., propane, butanes, and pentanes) and the gravity data, to develop a set of petroleum components for the stream. These derived components are then used within the flow-sheet simulation to model streams that are characterized by assay data.
PRO/II uses petroleum components in its internal calculations and can translate the simulation results back to assay data for output.
Thermometer Condenser
Burner V
T
Te mp er at ur e
% Distilled
Assay Data Entry
➤ In the Stream Data dialog box, Select Petroleum Assay in the Stream Type group box. Click to open the dialog box.
In addition to the thermal conditions, you must provide:
■ Flowrate
■ Distillation Data
■ Gravity Data Flowrate
➤ Enter a value in the field.
Figure 19:
Flowrate and Assay Dialog Box
Distillation Data
➤ Click to open the Assay Definition dialog box.
➤ Select a distillation type from the list:
● True Boiling Point (TBP)
● ASTM D86
● ASTM D1160
● ASTM D2887
Note that the D86 and D1160 data are almost always reported on a liquid volume basis while the D2887 data are always reported on a weight basis. Your flowsheet can include different types of assay streams (e.g., one stream on a D86 basis and another on a TBP basis).
If your distillations data has been collected at a pressure other than atmo-spheric (760 mm Hg), you must supply that pressure.
➤ Enter the data in the Percent Distilled vs. Temperature table.
Flowrate and Assay...
Define/Edit Assay...
Gravity Data
You must supply at least the average gravity for an assay stream, expressed as API gravity, Specific Gravity or Watson K-factor. If, in addition, you have a gravity versus percent distilled data curve, you should enter it for greater accuracy. Click to enter the data.
Figure 20:
Assay Definition Dialog Box
Optional Data
The following data are optional:
■ Light Ends Analysis
■ Molecular Weight Data
■ Refinery Inspection Properties and User-defined Properties
Light Ends Analysis
Often you can identify and accurately measure the quantity of a few of the lighter components that are present in the petroleum stream. You can supply their rate and composition in terms of library components. Such precisely measured data naturally improves the accuracy of the charac-terization. If the light ends are included in the average gravity of the stream, enter them here by clicking and entering the data.
If the light ends are not included in the average gravity of the stream, enter them as a separate compositional stream and mix with the assay stream.
Gravity Curve...
Lightends...
Molecular Weight Data
If possible, you should provide measured molecular weight data because the molecular weight correlations are traditionally the least accurate of those used in hydrocarbon characterization. You can supply a molecular weight curve without supplying an average value by clicking
.
Refinery Inspection Properties and User-defined Properties
If available, you can include refinery inspection properties, such as cloud point, pour point, sulfur content, and kinematic viscosity, in assay form
by clicking . To include custom-defined
special properties, click .
Assay Processing Methodology
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 21 describes the procedure that PRO/II uses to interpret and trans-form the assay stream data into useful compositional intrans-formation.
Figure 21:
Assay Processing Flowchart
This section explains the processing required to convert the assay data to its corresponding set of petroleum components.
Molecular Weight...
Refinery Inspection Properties...
User-defined Special Properties...
Characterize Other Convert Data to Equivalent TBP
Curve @ 760mm Hg
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.
PRO/II offers three methods for interpolating distillation curves:
■ 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.
■ 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).
■ 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 15 lists the default cut-points used by PRO/II, when user-supplied cutcut-points are not provided.
Table 15: 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 15 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.
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. PRO/II offers several techniques for processing your light ends:
■ 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 com-ponents in subsequent calculations.
■ 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.
■ 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.
■ 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 22 shows graphically how the petroleum components are gener-ated and how the light ends data are matched to the assay curve.
Figure 22:
Light Ends Matching
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:
(1)
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:
(2)
NBPj
T x( ) xd
xmin xmax
∫
xmax–xmin
---=
K NBP1 3⁄ ---spgr
=
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 23, where:
(3)
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 23:
Computing the Component NBP's and Gravities
spgrj NBPj1 3⁄ ---K
=
First, Integrate to Get NBP’s . . . .
Volume % Distilled
TemperatureAPI Gravity Light Ends 1st Cut 2nd Cut 3rd Cut
. . . .Then Use Watson K Factor to Compute Midpoint Gravity for Each Cut.
Percent Distilled
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 and make the appropriate
selections in this dialog box (Figure 24).
If the default correlations do not adequately match your specific assay data, you can try other calculation options to improve the fit. For more information on characterization options, refer to the PRO/II Keyword Manual and the PRO/II Reference Manual.
Figure 24:
Assay Characterization Options
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. You may be wondering if there is any relationship between the petroleum compo-nents discussed earlier in this chapter and the petroleum compocompo-nents generated from an assay. The answer is a definite Yes. A petroleum com-ponent, whether defined as part of a compositional stream, or generated by PRO/II's assay processing machinery, is treated the same by PRO/II.
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 is a powerful feature for modeling a process that has different feedstocks, particularly one that uses both virgin and cracked feedstocks.
Characterization Options...
Controlling Assay Processing
You can control how PRO/II processes your assay data by clicking the Assay Data button on the toolbar. In this dialog box (Figure 25), you can:
■ Modify the primary cutpoint set.
■ Add, modify and delete a secondary cutpoint set.
■ Nominate a cutpoint set to be used as the default for stream assay processing. To do this you must have defined at least one secondary cutpoint set.
Figure 25:
Assay Cutpoints and Characterization Dialog Box
Then, in the Flowrate and Assay dialog box, entered from the Stream Data dialog box, you can select the cutpoint set to use with this stream.
Click on the default set of TBP cutpoints in the linked text and select the Secondary Cutpoint Set of your choice.
Reference Stream
A reference stream is a feed stream whose attributes are defined in terms of another stream (the referenced stream). The two streams have the same composition and can have the same rate (molar), temperature, and/
or pressure.
➤ Select Referenced to Stream in the Stream Type group box and click
the to open the dialog box.
Figure 26:
Reference Stream Dialog Box
Flowrate and Stream...
Typically, when using this option, you transfer the composition of one
Typically, when using this option, you transfer the composition of one