Hd other


Published on

  • Be the first to comment

Hd other

  2. 2. Hydrocarbon Distillation The software described in this document is furnished under aWorkbook license agreement and may be used only in accordance with the terms of that agreement. Information in this document is subject to change without notice. Simulation Sciences Inc. assumes no liability for any damage to any hardware or software component or any loss of data that may occur as a result of the use of the information contained in this document.Copyright Notice Copyright © 1999 Simulation Sciences Inc. All Rights Reserved. No part of this publication may be copied and/or distributed with- out the express written permission of Simulation Sciences Inc., 601 Valencia Ave., Brea, CA 92823-6346.Trademarks PRO/II and SIMSCI are registered marks of Simulation Sciences Inc. Windows, Windows 95, Windows NT, and MS-DOS are regis- tered marks and/or trademarks of Microsoft Corporation. All other products are trademarks or registered trademarks of their respective companies. Printed in the United States of America, October 1999.
  3. 3. Contents Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Laboratory Tests and Interconversions . . . . . . . . . . . . . . . . 4 Conversion of Assay Data to Petroleum Cuts. . . . . . . . . . 12 Assay Blending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Multicomponent Distillation Using PRO/II . . . . . . . . . . . 28 A Closer Look at the Column Model . . . . . . . . . . . . . . . . 33 I/O Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Industrial Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Refinery Inspection Properties . . . . . . . . . . . . . . . . . . . . . 93 Tips and Tricks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103Hydrocarbon Distillation Workbook i
  4. 4. 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 1920s 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
  5. 5. 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 columns energy usage, while still achieving the necessary separation. PRO/IIs 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. SIMSCIs DATACON pro- gram is specifically designed to solve data reconciliation problems and can perform plant-wide reconciliation calculations. About This This workbook complements SIMSCIs 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/IIs 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
  6. 6. 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 Whats 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. SIMSCIs 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
  7. 7. 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 APICharacterization 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
  8. 8. 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 aHydrocarbon Distillation Workbook 5
  9. 9. 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 the6 Laboratory Tests and Interconversions
  10. 10. 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 BurnerHydrocarbon Distillation Workbook 7
  11. 11. 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 1940s and early 1950s 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 dont 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 anyway8 Laboratory Tests and Interconversions
  12. 12. 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 IBPs and EPs 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
  13. 13. 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
  14. 14. 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 TBPs, 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 D86s 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
  15. 15. 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
  16. 16. 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
  17. 17. Determine Moles, Mass, and Volume for each Cut Based on the samples 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
  18. 18. 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) spgrHydrocarbon Distillation Workbook 15
  19. 19. 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 cuts 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/IIs 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 NBPs and Gravities
  20. 20. ! # $
  21. 21. %
  22. 22. # $
  23. 23. (
  24. 24. )* (
  25. 25. + +
  26. 26. 16 Conversion of Assay Data to Petroleum Cuts
  27. 27. 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 streams 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 pseudocomponentsHydrocarbon Distillation Workbook 17
  28. 28. 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 Distilled18 Conversion of Assay Data to Petroleum Cuts
  29. 29. 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
  30. 30. 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
  31. 31. 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
  32. 32. 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
  33. 33. 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) BBLHydrocarbon Distillation Workbook 23
  34. 34. 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
  35. 35. 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- shiners ethanol-water system, for the simple reason that it was not designed for polar systems. The kijs 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 theHydrocarbon Distillation Workbook 25
  36. 36. 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 in26 Assay Blending
  37. 37. 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
  38. 38. 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- rithms strengths and weaknesses; this information will help you select the appropriate algorithm for your problems. First, we will focus on PRO/IIs 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 screens Window features. Most will be discussed in greater detail later on.28 Multicomponent Distillation Using PRO/II
  39. 39. 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/IIs 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
  40. 40. 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 condensers 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
  41. 41. 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 columns 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
  42. 42. 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