UniSim Design Clean Fuels PPkg User Guide

UniSim Clean FuelsProperty Package

Reference Guide

CopyrightJune 2005 R350 Release

The information in this help file is subject to change over time. Honeywell may make changes to the requirements described. Future revisions will incorporate changes, including corrections of typographical errors and technical inaccuracies.

For further information please contact

Honeywell300-250 York StreetLondon, OntarioN6A 6K2Telephone: (519) 679-6570Facsimile: (519) 679-3977

Copyright Honeywell 2005. All rights reserved.

Prepared in Canada.

iii

Table of Contents

1 Introduction .........................................................1-1

1.1 Meeting New Sulphur Levels in Motor Gasoline ..... 1-3

2 Gasoline Fractionation..........................................2-1

2.1 Gasoline Sulphur Species Distribution ................. 2-2

2.2 Light/Medium Gasoline Fractionation................... 2-5

2.3 Improve Fractionator Design.............................. 2-8

3 Clean Fuels Property Package............................3-1

3.1 Introduction .................................................... 3-2

3.2 Thermodynamic Model ...................................... 3-2

4 Clean Fuels Pkg Extension....................................4-1

4.1 Using the Clean Fuels Pkg Extension ................... 4-2

4.2 Clean Fuels Pkg Extension User Interface ............ 4-3

4.3 Clean Fuels Pkg View ........................................ 4-4

5 Clean Fuels Pkg Tutorial .......................................5-1

5.1 Introduction .................................................... 5-2

5.2 Flowsheet Setup .............................................. 5-3

5.3 Modeling the Gasoline Fractionator ....................5-10

5.4 Plot Utility ......................................................5-15

A References ...........................................................A-1

Index.................................................................... I-1

Introduction 1-1

1-1

1 Introduction

1.1 Meeting New Sulphur Levels in Motor Gasoline.............................. 3

1-2

1-2

The increasing environmental concern of sulphur content in petroleum products mean refiners are needing to find better ways of managing sulphur pool target levels in gasoline. The complexity of modeling these processes with the accuracy in the very low ppm region requires highly accurate thermodynamic methods for modeling and optimization. To meet the need for increased model reliability, a new property package, the Clean Fuels Pkg, has been developed specifically for systems of thiols and hydrocarbons. The new property package features new methods, estimation routines as well as extensive new databases of pure component properties and mixtures.

This user manual is a comprehensive guide that provides the steps needed to use the Clean Fuels Pkg in a UniSim Design flowsheet. To apply the Clean Fuels Extension efficiently, the manual describes the property package views as well as its capabilities. A simple flowsheet model of a gasoline fractionator is constructed using the Clean Fuels Pkg and the steps of its construction are given in the tutorial. The tutorial presents the basic steps needed to build the flowsheet model. Each view is explained on a page-by-page basis to give a complete description of the data requirements in order to use the property package efficiently. This User Guide does not detail UniSim Design procedures and assumes the user is familiar with the UniSim Design environment and its conventions. Here you will find the information required to build a UniSim Design flowsheet and work efficiently within the simulation environment.

Introduction 1-3

1-3

1.1 Meeting New Sulphur Levels in Motor Gasoline

With new strict global-wide legislation regulating undesirable emissions from internal combustion engines, refineries are facing challenging design decisions to meet lower sulphur targets in motor gasoline. With these regulations continuing to evolve, reducing sulphur to target levels will likely involve some of the highest capital costs for refiners.

During the early 1990s gasoline sulphur levels were approximately 340 ppmw [1]. With new levels set in 2000, refiners are reducing sulphur to 150 ppmw. By 2006, the US EPA proposes to reduce sulphur to 30 ppmw with phased reductions beginning in 2004. European regulations call for reductions to 50 ppmw by 2005 while Canadian regulations require 30 ppmw by 2004 [1]. Farther ahead, the US EPA has called for even lower targets of 10 ppmw. Continuously lower levels of gasoline sulphur present new challenges to develop and identify viable low cost solutions for reduced gasoline sulphur content in motor gasoline.

Effective solutions to manage gasoline sulphur content involve choosing the best technology options for sulphur removal, as well as selecting designs that best fit the operating philosophy for refiners. Important to gasoline sulphur management strategies is understanding how the various sulphur species are distributed in fractionator gasoline cuts which is critical in determining the optimum operating conditions of gasoline fractionators.

As sulphur content of gasoline is reduced, gasoline fractionation will become increasingly important. Key in the optimum design of new or existing equipment is the construction of accurate flowsheets of gasoline fractionation processes. Fundamental to the construction of flowsheet models is the accurate VLE representation of thiol containing mixtures of hydrocarbons.

1-4 Meeting New Sulphur Levels in Motor

1-4

Gasoline Fractionation 2-1

2-1

2 Gasoline Fractionation

2.1 Gasoline Sulphur Species Distribution............................................ 2

2.2 Light/Medium Gasoline Fractionation ............................................ 5

2.3 Improve Fractionator Design ......................................................... 8

2-2 Gasoline Sulphur Species Distribution

2-2

2.1 Gasoline Sulphur Species Distribution

Various sulphur compounds are distributed throughout the gasoline TBP range. The amount of sulphur species in motor gasoline depends on a number of factors including the crude source, treating methods and gasoline cut point. The boiling range of FCC gasoline does not change significantly with sulphur levels2. Therefore knowing the temperature range where the various sulphur species distil and how much of each sulphur species is present at a given TBP temperature is important in operating fractionation equipment that meet sulphur pool target levels. A list of sulphur compounds is shown in the table below together with the hydrocarbon boiling point ranges and UniSim Design component information.

Essential for the accurate prediction of azeotropes occurring between thiols and hydrocarbons is the accurate calculation of

Component nameUniSim Design Sim Name

NPB °FBPT Range °F

UniSim Design Comp ID

Formula

Sulphur Components in Light Gasoline

Ethyl Mercaptan E-Mercaptan 95.09 70-90 354 C2H6S

Dimethyl Sulfide diM-Sulphide 99.23 75-80 380 C2H6S

Iso-propyl Mercaptan 2C3Mercaptan 126.61 110-130 3162 C3H8S

Tert-butyl Mercaptan t-B-Mercaptan 147.59 120-150 524 C4H10S

Methyl Ethyl Sulphide M-E-Sulfide 151.97 130-140 381 C3H8S

n-Propyl Mercpatan nPMercaptan 150.89 115-130 389 C3H8S

Thiophene Thiophene 183.29 140-200 384 C4H4S

Iso-Butyl Mercaptan 2-M-1C3Thiol 191.21 180-200 732 C4H10S

n-Butyl Mercaptan nBMercaptan 209.23 185-200 390 C4H10S

Dimethyl disulfide diMdiSulphid 229.53 190-200 385 C2H6S2

2-Methyl Thiophene 2MThiophene 234.59 200-250 733 C5H6S

3-Methyl Thiophene 3MThiophene 239.81 210-270 734 C5H6S

Tetrahydrothiophene Thiolane 250.01 220-260 526 C4H8S

1-Pentyl Mercaptan 1Pentanthiol 259.95 245-255 525 C5H12S

Hexyl Mercaptan 1Hexanethiol 306.77 290-340 847 C6H14S

Benzothiopene ThioNaphtene 427.81 400+ 3116 C8H6S


2-3

pure component vapor pressures. For this, the most up to date pure component data (DIPPR) was used in the development of the Clean Fuels Property Package methods. A list of sulphur species supported in UniSim Design for the Clean Fuels Property Package is shown in the table below.

Formula Component Name DIPPR IDUniSim Design ID

CH4S METHYL MERCAPTAN 1801 353

C2H6S ETHYL MERCAPTAN 1802 354

C3H8S n-PROPYL MERCAPTAN 1803 389

C4H10S tert-BUTYL MERCAPTAN 1804 524

C4H10S ISOBUTYL MERCAPTAN 1805 732

C4H10S sec-BUTYL MERCAPTAN 1806 731

C6H14S n-HEXYL MERCAPTAN 1807 847

C9H20S n-NONYL MERCAPTAN 1808 3068

C8H18S n-OCTYL MERCAPTAN 1809 871

C3H8S ISOPROPYL MERCAPTAN 1810 3162

C3H8S ISOPROPYL MERCAPTAN 1810 695

C6H12S CYCLOHEXYL MERCAPTAN 1811 3280

C7H8S BENZYL MERCAPTAN 1812 3319

C3H8S METHYL ETHYL SULFIDE 1813 381

C4H10S METHYL n-PROPYL SULFIDE 1814 730

C6H14S DI-n-PROPYL SULFIDE 1817 846

C4H10S DIETHYL SULFIDE 1818 382

C2H6S DIMETHYL SULFIDE 1820 380

C4H4S THIOPHENE 1821 384

C8H6S BENZOTHIOPHENE 1822 3116

C4H10S2 DIETHYL DISULFIDE 1824 383

C11H24S UNDECYL MERCAPTAN 1825 958

C10H22S n-DECYL MERCAPTAN 1826 945

C5H12S n-PENTYL MERCAPTAN 1827 525

C2H6S2 DIMETHYL DISULFIDE 1828 385

C6H14S2 DI-n-PROPYL DISULFIDE 1829 848

C12H26S n-DODECYL MERCAPTAN 1837 3013

C8H18S tert-OCTYL MERCAPTAN 1838 3373

C7H16S n-HEPTYL MERCAPTAN 1839 865

C4H10S n-BUTYL MERCAPTAN 1841 390

C6H6S PHENYL MERCAPTAN 1842 391

C4H8S TETRAHYDROTHIOPHENE 1843 526

C2H6OS DIMETHYL SULFOXIDE 1844 950

2-4 Gasoline Sulphur Species Distribution

2-4

Quantifying sulphur species by hydrocarbon boiling range requires fractionating 20-30 narrow boiling range (10-20°F) using an ASTM D2892(TBP) column or TBP column with 15 theoretical stages and a 5/1 reflux ratio2. A highly fractionated gasoline sample will be discontinuous up to about 390°F due to the different sulphur species boiling point ranges. Sulphur distribution, sulphur species and hydrocarbon TBP can then be plotted using this information. Sulphur species content in gasoline change from primarily mercaptans in the low boiling range IBP-140°F material to thiophenic compounds in the 140-390°F, and benzothiophenes and substituted benzothiophenes in the 390-430°F heavy gasoline. Above 390°F the total sulphur increases significantly with temperature.

C3H6O2S 3-MERCAPTOPROPIONIC ACID 1873 3153

COS CARBONYL SULFIDE 1893 355

H2S HYDROGEN SULFIDE 1922 15

CS2 CARBON DISULFIDE 1938 364

C12H8S DIBENZOTHIOPHENE 2823 3441

C12H26S tert-DODECYL MERCAPTAN 2838 3460

C5H6S 2-METHYLTHIOPHENE 2844 3216




C2H4O2S THIOGLYCOLIC ACID 2872 3134

C5H9NS N-METHYLTHIOPYRROLIDONE 3888 3223

C4Cl4S TETRACHLOROTHIOPHENE 4877 3169

C4H10O2S THIODIGLYCOL 6855 3195

C2H6OS 2-MERCAPTOETHANOL 6858 3138

C4H10OS ETHYLTHIOETHANOL 6859 3192

C2H6S2 1,2-ETHANEDITHIOL 6860 3139

Formula Component Name DIPPR IDUniSim Design ID


2-5

2.2 Light/Medium Gasoline Fractionation

As sulphur content of motor gasoline is mandatorily reduced, gasoline fractionation will become increasingly more important. Light gasoline thiophene content determines the total sulphur content of a treated gasoline stream. The IBP-140°F hydrocarbons contain primarily C2 and C3 mercaptans and up to 90% of these mercaptans can be extracted in caustic treating processes. Thiophene however can not be extracted using these methods. The thiophene NBP is 183.29°F. Due to strong hydrocarbon-thiol molecular interactions, thiophene distils with hydrocarbons between 140°F and 200°F. Peak thiophene concentration occurs at about 165-170°F boiling range2. Thiophene content varies with each crude and the amount of hydrotreating, however it can represent up to 75% of the sulphur in the 140-180°F hydrocarbons. Therefore 140°F+ material in light gasoline increases treated stream sulphur content.

A simulated plot of an FCC naphtha and the distribution of thiophene with increasing hydrocarbon boiling point is shown in Figure 2.1. The plot was constructed using a simulation model of an Oldershaw still with 70 theoretical stages at 20/1 reflux ratio and equal narrow boiling range cuts of 5% volume distilled. Results are shown in the table below. Qualitatively, the sulphur distribution curve of FCC gasoline increases rapidly, with thiophene beginning to boil with hydrocarbons at approximately 140°F as shown in Figure 2.1. The predicted peak sulphur concentration occurs at 168°F. Sharp fractionation of the light/medium gasoline can increase yield significantly while still meeting treated product sulphur levels2.

2-6 Light/Medium Gasoline Fractionation

2-6

The table below shows the Simulated Distillation Data of Thiophene Distribution in a FCC Gasoline.

Fractionation of light/medium gasoline fractionation requires a dedicated gas plant column. The column efficiency will determine light gasoline yield and thiophene concentration in gasoline. Medium/heavy gasoline fractionation is performed in the main fractionator with heavy gasoline produced as a side cut product, to minimize energy consumption and capital costs.

Figure 2.1: Simulated Thiophene Peak of FCC Gasoline

Percent Distilled Volume Temperature °F Sulphur ppm wt

20% 95.60 0.00

25% 117.15 0.00

30% 142.28 0.11

35% 151.77 10.9

40% 168.61 1354.0

45% 182.07 36.8

50% 196.67 0.00

55% 220.88 0.00


2-7

Light/medium gasoline fractionation separates feed to the casuistic extraction process from the medium boiling range gasoline. The caustic extraction process converts mercaptans to disulfides, which are easily extracted. Caustic extraction can remove between 80-90% of the C2/C3 mercaptans.

The amount of thiophene entering the feed caustic extraction process or its equivalent leaves with the treated product stream. Thiophene begins to distil with C6 hydrocarbons boiling above 140°F. Thiophene content peaks in the 165-170°F boiling range so increasing levels of 140°F+ material increases the treated product stream sulphur level. If thiophene content and not the mercaptan extraction efficiency controls the treated product sulphur level, then the light gasoline 140-160°F boiling material must be controlled to meet product stream sulphur targets. The 140-160°F boiling range hydrocarbons make up 7-9 wt% of the total FCC gasoline2, light gasoline yield can be increased significantly with good fractionation by lowering the amount of 140-170°F boiling material in light gasoline product which allows higher light gasoline yield. Sharp fractionation is achieved through an appropriate number of column trays, controlling reflux and energy input.

The table that lists the sulphur compounds together with the hydrocarbon boiling point ranges and UniSim Design component information in Section 2.1 - Gasoline Sulphur Species Distribution, lists the sulphur species that are present in light gasoline.

2-8 Improve Fractionator Design

2-8

2.3 Improve Fractionator Design

Here the fractionation objective is to determine the optimum number of trays and reflux that will result in sharp fractionation of light and medium gasoline. The optimum values are achieved using accurate VLE models.

Understanding how sulphur is distributed in gasoline is the first step in determining the gasoline cut point to achieve the necessary sharp fractionation between light and medium gasoline. In designing a gasoline fractionation column, the design objective is to ensure that thiophene is controlled in the gasoline distillate. Even small amounts of thiophene contained in the light fraction can add significantly to gasoline sulphur levels.

Because of the strong molecular interactions between hydrocarbons and sulphur containing compounds these mixtures are non-ideal and can form azeotropes that are difficult to model accurately. Typically an activity coefficient model would best represent a non-ideal system. However because of the presence of alkanes, olefins and oils as well as non-condensable components in systems of gasoline, an equation of state is always preferred for calculation of hydrocarbon binaries. An equation of state however is not suitable for thiol-hydrocarbon binary pairs. By combining the equation of state with an activity model through a new Helmholtz Excess Energy AE mixing rule and using an accurate vapor pressure model, the VLE representation of hydrocarbon-thiol systems is possible, representing both ideal and non-ideal binaries equally well. The new mixing rule model is able to predict accurately thiol-hydrocarbon azeotropes as well as the azeotrope temperature and composition.

The new Clean Fuels property package methods also include a binary interaction parameter database regressed for 101 thiol-hydrocarbon binary pairs. To fill in missing parameters for systems of binaries forming azeotropes, a newly developed


2-9

thiol-hydrocarbon binary estimation method is available which will predict the azeotrope composition and temperature. All the new methods developed are based on experimental data. Figure 2.2 compares the Clean Fuels property package results for the system nPropylMercapatn-Hexane with other methods. As can be seen, the conventional equation of state (EOS) methods fail while the effect of vapour pressure on the calculation of the azeotrope for the activity model is highlighted clearly. Although, the activity model performs fairly well in this instance, its performance deteriorates with increasing temperature and pressure. Selecting the correct thermodynamic model for modeling gasoline fractionation is important.

With a highly accurate VLE thermodynamic model, up to date binary and pure component databases as well as reliable estimation routines, the simulation of gasoline fractionation towers can be used to better optimize new designs. For existing equipment, towers can be rated accurately for performance

Figure 2.2 VLE Diagram for nPropylMercapatn and Hexane at 1 atm

2-10 Improve Fractionator Design

2-10

changes where ultra low sulphur levels are required.

In the optimization of a gasoline fractionator, two design variables are considered. Increasing the column number of trays2 and the amount of reflux. Both have the same affect of reducing the gasoline end point, however as Figure 2.3 illustrates, the effect of increasing the reflux is more dramatic in controlling the end point temperature of gasoline.

For existing gasoline fractionation towers, increasing reflux may increase column tray traffic, so tower internals need to be considered to handle the added capacity.

Figure 2.3: Effect of Fractionator Design on Gasoline End Point

Clean Fuels Property Package 3-1

3-1

3 Clean Fuels Property Package

3.1 Introduction................................................................................... 2

3.2 Thermodynamic Model ................................................................... 2

3.2.1 Estimation Methods .................................................................. 7

3-2 Introduction

3-2

3.1 IntroductionThe Clean Fuels Property Package is a specially designed property package for the accurate VLE representation of thiol-hydrocarbon containing systems. The Clean Fuels Pkg contains the latest advances made in the development of cubic equations of state and mixing rules. A new vapour pressure alpha function is available that is correlated against DIPPR vapour pressure data as well as DIPPR pure component properties for 1454 UniSim Design components. New databases are available containing regressed coefficients for 101 thiol-hydrocarbon binary pairs, and a new proprietary thiol-hydrocarbon estimation method is able to predict the formation of azeotropes and calculate the binary parameters from infinite dilution activity coefficient data. The Clean Fuels Pkg allows User Data to be supplied for azeotropes and infinite dilution activity coefficient data as well as supporting 49 DIPPR thiol containing components listed in the table of the sulphur species supported in UniSim Design for the Clean Fuels Property Package in Section 2.1 - Gasoline Sulphur Species Distribution.

3.2 Thermodynamic ModelSelecting an appropriate thermodynamic model to represent Clean Fuels processes requires the selection of an appropriate cubic equation of state that will allow better prediction of liquid densities of mid-range to heavy hydrocarbons and polar components. Also a highly accurate vapour pressure alpha function is needed that extrapolates correctly beyond the critical point. A suitable mixing rule is necessary that can allow hydrocarbon-hydrocarbon binary pairs to be modelled with the accuracy of an equation state while able to represent non-ideal thiol-hydrocarbons as well as an activity model. Finally, the selection of a suitable thermodynamic model involves choosing an appropriate activity model that would allow the new mixing rules to transition the van der Waals one-fluid mixing rules for hydrocarbon binaries.


3-3

The Clean Fuels Property Package uses an optimal two-parameter cubic equation of state TST (Twu-Sim-Tassone)3 to represent Clean Fuels Processes. The TST cubic equation is represented as follows:

and can be rewritten in the form,

The values of a and b are at the critical temperature and are found by setting the first and second derivatives of pressure with respect to volume to zero at the critical point:

where:

c = critical point

The value of Zc from the SRK and PR equations are both larger than 0.3 while Zc from the TST equation is slightly below it, closest to the real one for many substances.

A prerequisite for the accurate VLE representation of thiol-hydrocarbon systems in the entire composition range is the accurate calculation of pure component vapour pressures.

(3.1)

(3.2)

(3.3)

(3.4)

(3.5)

P RTv b–------------- a

v2 2.5bv 1.5b2–+------------------------------------------------------–=

P RTv b–------------- a

v 3b+( ) v 0.5b–( )---------------------------------------------------–=

a Tc( ) 0.427481R2Tc2 Pc⁄=

b 0.086641RTc Pc⁄=

Zc 0.296296=

3-4 Thermodynamic Model

3-4

You can use the Twu alpha correlation4.

Equation (3.7) has three parameters L, M, and N. These parameters are unique to each component and are determined from the regression of DIPPR pure component vapour pressure data for 1454 components.

The generalized alpha function is used for non-library and petroleum fractions:

where:

α(0) is for ω=0

α(1) is for ω=1

Each alpha is a function of reduced temperature only.

To model both van der Waals fluids and highly non-ideal mixtures using the same Gibbs excess energy model we use the TST Zero-Pressure Mixing Rules3. The zero-pressure mixing rules for the cubic equation of state mixture a and b parameters are:

(3.6)

(3.7)

(3.8)

(3.9)

bvdw is used for b.

α TrN M 1–( )e

L 1 TrNM–( )

=

α α 0( ) ω α 1( ) α 0( )–( )+=

a* b* avdw*

bvdw*

-------------- 1Cv0----------+

A0E

RT--------

A0vdwE

RT-----------------– ln–

bvdwb

--------------⎝ ⎠⎛ ⎞

⎝ ⎠⎜ ⎟⎛ ⎞

=

b xixj12--- bi bj+( )

j∑

i∑=


3-5

avdw and bvdw are the equation of state a and b parameters which are evaluated from the van der Waals mixing rules. The Twu mixing rule given by Equation (3.8) is volume-dependent through Cv0. Cv0 is a function of the reduced liquid volume at zero pressure v0*=v0/b:

Since the excess Helmholtz energy is a weak function of pressure [5] we assume that the excess Helmholtz energy of the van der Waals fluid at zero pressure can be approximated by the excess Helmholtz energy of van der Waals fluid at infinite pressure:

A new versatile activity model NRTLTST 6 is used to describe both a van der Waals fluid and a highly non-ideal mixture:

When τij and Gij are calculated using the parameters in Equation (3.13) and Equation (3.14), the NRTL equation is obtained.

(3.10)

(3.11)

(3.12)

(3.13)

(3.14)

Cv01

w u–( )-------------------ln

v0* w+

v0* u+

------------------⎝ ⎠⎜ ⎟⎛ ⎞

vdw

–=

A0vdwE

RT-----------------

A∞vdwE

RT------------------ Cv0

avdw*

bvdw*

-------------- xiai

*

bi*

------i∑–= =

GE

RT-------- xi

i

n

∑

xjτjiGji

j

n

∑

xkGki

k

n

∑

----------------------------=

τjiAjiT

-------=

Gji exp αjiτji–( )=


3-6

However, Equation (3.12) can also recover the conventional van der Waals mixing rules when the following expressions are used for τij and Gij instead:

where:

The TST mixing rules in Equation (3.8) are density dependent through the function Cv0. Because of this density function, the mixing rule is able to reproduce almost exactly the incorporated GE model. Cv0 as defined by Equation (3.10) is calculated from v0

*vdw by solving the equation of state in Equation (3.1)at zero

pressure. This step can cause problems if there is no real root, which may occur when non-condensable components are present, for example. When this occurs, some sort of extrapolation for v0

* must be made. To omit the need for the calculation of v0

* from the equation of state, the zero-pressure liquid volume of the van der Waals fluid, v0

*vdw, is a constant, r:

Substituting Equation (3.18)into Equation (3.10), Equation (3.10)becomes:

A universal value of r=1.18 has been determined from information on the incorporated GE model and is recommended

(3.15)

(3.16)

(3.17)

(3.18)

(3.19)

τji12---δijbi=

Gjibjbi-----=

δijCv0RT----------––

aibi

---------aj

bj---------–

⎝ ⎠⎜ ⎟⎛ ⎞2

2kijai

bi---------

ajbj

---------+=

v0vdw* r=

Cr1

w u–( )-------------------ln r w+

r u+-------------⎝ ⎠⎛ ⎞–=


3-7

by Twu et al.7 for use in the phase equilibrium prediction for all systems.

3.2.1 Estimation MethodsFor systems containing thiols and hydrocarbons, some hydrocarbons and petroleum fractions form azeotropes with thiols. In cases were VLE data is not available for these systems, reliable estimation methods are necessary to predict the azeotrope and to calculate the binary interaction parameters. The Clean Fuels Pkg contains an internal proprietary estimation routine used to estimate the binary interaction parameters of thiol and hydrocarbons that form azeotropes. Binary estimation methods have been developed specifically for the thiols, enthanethiol, 1-propanethiol, 2-propanethiol, 1-butanethiol, 2-butanethiol, 2-methyl 1-propanethiol and 2-methyl 2-propanethiol in mixtures of paraffins and naphthenes, while a generalized estimation method is available to calculate the binary parameters for all other thiols. The user is also allowed to enter User applied azeotrope data or infinite dilution activity coefficient data for calculation of binary parameters.


3-8

Clean Fuels Pkg Extension 4-1

4-1

4 Clean Fuels Pkg Extension

4.1 Using the Clean Fuels Pkg Extension.............................................. 2

4.1.1 Adding a Clean Fuels Pkg .......................................................... 2

4.2 Clean Fuels Pkg Extension User Interface ...................................... 3

4.3 Clean Fuels Pkg View ..................................................................... 4

4.3.1 NRTLTST Tab ........................................................................... 44.3.2 TST CEOS Tab.......................................................................... 5

4-2 Using the Clean Fuels Pkg Extension

4-2

4.1 Using the Clean Fuels Pkg Extension

You can add a Clean Fuels Pkg Extension only if it exists as part of a UniSim Design case. A Property Package Extension that is part of an existing case can be accessed in the UniSim Design Basis Environment. In the Basis Environment, you can view and adjust the extension variables as you would any UniSim Design Property Package. Before creating a new Clean Fuels Pkg, the user is required to be working within a UniSim Design case that has a Fluid Package installed. The Fluid Package must consist of a property package and associated flowsheet components.

4.1.1 Adding a Clean Fuels Pkg To add a Clean Fuels Pkg to an existing UniSim Design case:

1. From the Simulation Basis Manager, click on the Fluid Pkgs tab.

2. Click the Add button to add a Clean Fuels Pkg. The Fluid Package view appears.

3. In the Property Pkg Filter group, click the Miscellaneous Types radio button.

Refer to Chapter 2 - Fluid Package of the UniSim Design Simulation Basis manual for more information on the UniSim Design Property Package.


4-3

4. From the available property packages list select Clean Fuels Pkg.

4.2 Clean Fuels Pkg Extension User Interface

The Clean Fuels Pkg Extension user interface is completely integrated into the UniSim Design working environment and conforms to all UniSim Design usage conventions for operations and data entry. If you are an experienced user of UniSim Design, you will already be familiar with all of the features of the Property Package user interface. If you are a new user, begin by reviewing the UniSim Design User Guide to familiarize yourself with UniSim Design before using the Clean Fuels Pkg Extension.

Figure 4.1

The View Property Package button allows you to view the Clean Fuels Pkg parameters.

The Clean Fuels Pkg parameters are shown on the Clean Fuels Pkg property view.

4-4 Clean Fuels Pkg View

4-4

4.3 Clean Fuels Pkg ViewLike all UniSim Design property views, the Clean Fuels Pkg view allows you access to all information associated with a particular item, such as the interaction parameter view pages. You can specify the binary interaction parameters or regress User data on the Clean Fuels Pkg view.

The Clean Fuels Pkg view has two tabs (NRTLTST and TST CEOS), and on each tab are groups of related parameters.

4.3.1 NRTLTST TabThe NRTLTSTS tab as shown in Figure 4.2 contains the binary parameters for the activity coefficient model NRTLTST (NRTL-Twu-Sim-Tassone) used in the TST (Twu-Sim-Tassone) AE Mixing Rules. This tab allows the user to view the binary parameters for the activity model and to fill-in binary parameters not present in the database or not calculated from the internal estimation methods.

Figure 4.2


4-5

User DataThe User Data button allows the user to provide either infinite dilution activity coefficient data or azeotrope data per binary in the calculation of interaction parameters for azeotrope prediction of thiol-hydrocarbon binaries.

4.3.2 TST CEOS TabThe TST CEOS tab contains the binary parameters for the TST (Twu-Sim-Tassone) cubic equation of state (CEOS).

It is recommended that unknown parameters be filled-in at all times using the UNIFAC VLE fill-in method.

Figure 4.3

4-6 Clean Fuels Pkg View

4-6

The Twu Alpha Params button allows the user access to the Twu vapor pressure alpha function parameters L, M and N, as well as access to the DIPPR pure component properties Tc and Pc.

Figure 4.4

Clean Fuels Pkg Tutorial 5-1

5-1

5 Clean Fuels Pkg Tutorial

5.1 Introduction................................................................................... 2

5.2 Flowsheet Setup ............................................................................ 3

5.3 Modeling the Gasoline Fractionator.............................................. 10

5.3.1 Exercises .............................................................................. 14

5.4 Plot Utility.................................................................................... 15

5-2 Introduction

5-2

5.1 IntroductionThe following example demonstrates how to use the Clean Fuels Pkg to model a gasoline fractionator. In this example, a light/medium gasoline is fractionated in a gas plant column. The amount of sulphur is calculated in the light gasoline and the gasoline endpoint is set to 150°F for design. The case will consist of a FCC Gasoline feed stream to the tower and two outlet streams, a light gasoline product stream and an intermediate naphtha which is sent to an upstream hydrotreater for further treating. The design objective is to maximize the yield of light gasoline since hydrotreating of gasoline results in severe octane loss.

Figure 5.1


5-3

5.2 Flowsheet SetupBefore working with the Clean Fuels Pkg Extension, you must first create a UniSim Design case.

1. In the Simulation Basis Manager, create a fluid package using the Clean Fuels Pkg. Add the UniSim Design Thiol library components 2C3Mercaptan, nPMercaptan and Thiophene.

Add the paraffins and olefins as shown in the table below, and then close the Component List view.

2. Click on the Oil Manager tab of the Simulation Basis Manager to install an oil with the TBP curve (light ends are added in the main flowsheet).

Property Package Components

Clean Fuels Pkg 2C3Mercaptan, nPMercaptan, Thiophene

Component Name

i-Butane

i-Butene

n-Butane

i-Pentane

1-Pentene

2M-13-C4==

Cyclopentene

3M1C5=

Cyclopentane

23-Mbutane

2-Mpentane

2M1C5=

1-Hexene

n-Hexane

For more information on adding library components, refer to Chapter 1 - Components in the UniSim Design Simulation Basis manual.

If you are unable to find the component using the default Sim Name option on the Component List view, click on the Full Name/Synonym radio button. Then type the component name in the Match field.

5-4 Flowsheet Setup

5-4

3. Click the Enter Oil Environment button. The Oil Characterization view appears.

4. Click the Add button. The Assay view appears.

5. In the Name field, type FCC Gas Oil.

6. From the Assay Data Type drop-down list on the Input Data tab, select TBP.

Figure 5.2

Figure 5.3


5-5

7. In the Input Data group, click on the Edit Assay button. The Assay Input Table view appears.

8. Add the assay input data as shown in the table below.

Figure 5.4

Assay Percent [%] Temperature [F]

0.0 108.6

5.0 167.3

15.0 190.2

20.0 201.4

25.0 213.6

30.0 226.3

35.0 239.3

40.0 252.7

45.0 266.2

50.0 279.5

55.0 292.4

60.0 305.5

75.0 348.3

90.0 407.9

95.0 425.5

98.0 458.3

100.0 490.2

5-6 Flowsheet Setup

5-6

9. After you have entered the assay input data, click the OK button to return to the Assay view.

10.Close the Assay view to return to the Oil Manager property view.

11.Click on the Cut/Blend tab to create a Blend object.

12.Click the Add button. The Blend view appears.

13. In the Name field, type FCC Gas Oil.

Figure 5.5

Figure 5.6


5-7

14. From the Cut Option Selection drop-down list of the Data tab, select Auto Cut.

15.Click the Add button to select the assay.

16.Enter the data as shown in the table below.

17.Close the Blend view to return to the Oil Manager property view.

18.Click on the Install Oil tab, and in the Stream Name column type FCC Gas Oil as shown in the figure below.

19.Click the Calculate All button to calculate the all the assays and blends. Then click the Return to Basis Environment button. The Simulation Basis Manager appears.

Figure 5.7

Flow Units Flow Rate

Mass 364008 lb/hr

Figure 5.8

The default Flow Unit is Liquid Volume ensure that you have selected Mass from the drop-down list before specifying the flow rate.

5-8 Flowsheet Setup

5-8

20.Click on the Fluid Pkgs tab, and then click the View button.

21. From Fluid Package view, click the View Property Package button. The Clean Fuels Pkg view appears.

Click the Unknowns Only button to specify the missing Binary Interaction Parameters (BIPs) using the UNIFAC VLE methods. Ensure that you have selected the UNIFAC VLE radio button.

22.Close the Clean Fuels Pkg view and the Fluid Package view.

23. From the Simulation Basis Manager, click the Enter Simulation Environment to build your flowsheet.

24.Create two streams named Sulphur Spike and Light Ends in the Simulation Environment with the following stream conditions and composition.

Figure 5.9

Conditions

Stream Name Sulphur Spike

Temperature [F] 100

Pressure [psia] 114.6

Mass Flow [lb/hr] 219.6

Composition Mass Flow [lb/hr]

2C3Mercaptan 60.1

nPMercaptan 53.5

Thiophene 106.0

Ensure that you have selected the Clean Fuels Pkg in the Current Fluid Packages list.

You can also press CTRL L to leave the Basis Environment.

For more information on adding a stream, refer to Chapter 3 - Streams in the UniSim Design Operations Guide.

Ensure that you have the Mass Flow radio button selected in the Composition Basis group of the Input Composition from Stream view before specifying the stream composition.


5-9

25.Define the FCC Gas Oil stream conditions as shown in the table below.

26. Add a Mixer with the outlet stream named FCC Gasoline, and feed streams Sulphur Spike, Light Ends and FCC Gas Oil.

27.Add a shell and tube Heat Exchanger with a 10 psi pressure drop on both shell and tube sides.

The Shell side of the heat exchanger will heat the feed to the column while the tube side cools the column bottoms product.

Conditions

Stream Name Light Ends

Temperature [F] 100


Mass Flow [lb/hr] 1.705E+005

Composition Mass Flow [lb/hr]

i-Butane 392.2

i-Butene 13543.9

n-Butane 2318.5

i-Pentane 40094.1

1-Pentene 49783.6

2M-13-C4== 1475.2

Cyclopentene 2345.5

3M1C5= 2162.2

Cyclopentane 1138.2

23-Mbutane 5138.8

2-Mpentane 30575.8

2M1C5= 3221.3

1-Hexene 12306.8

n-Hexane 6004.2

Conditions

Temperature [F] 100


Mass Flow [lb/hr] 364008.7

Liq. Vol Flow [barrel/day] 32784.7

For more information on adding a Mixer, refer to Section 5.1 - Mixer in the UniSim Design Operations Guide.

For more information on adding a Heat Exchanger, refer to Section 4.3 - Heat Exchanger in the UniSim Design Operations Guide.

5-10 Modeling the Gasoline Fractionator

5-10

28. In the Heat Exchanger property view, name the tube side feed Medium Gasoline and the outlet tube side to Hydrotreater.

29.Specify the shell side feed FCC Gasoline, and name the outlet shell side Feed to Fractionator.

30.Specify a stream temperature of 223°F for Feed to Fractionator.

31. In the Parameters page of the Heat Exchanger property view, change the Heat Exchanger Model to Exchanger Design (Weighted).

5.3 Modeling the Gasoline Fractionator

The Gasoline fractionator is modeled as a distillation column in UniSim Design using a Partial Reflux Condenser.

1. Add a distillation column with a partial condenser. In the Connections page, name the liquid distillate Light Gasoline, the overhead vapor draw as Vent and the bottoms liquid as Medium Gasoline. Cond-q and Reb-q are the condenser and reboiler heat loads respectively.

2. The tower has 20 theoretical stages, and the feed to the tower enters on Stage 13.

For more information on a distillation column, refer to Chapter 8 - Column in the UniSim Design Operations Guide.


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3. The pressure in the condenser is set at 240 kPa, the pressure drop across the condenser is 55.16 kPa and the bottom reboiler pressure is at 350 kPa.

4. On the Monitor page, enter a Reflux Ratio estimate of 1.0 and turn-off this specification. Set the Ovhd Vapor Rate to 0.0 MMSCFD, the distillate rate to 1.213e+004 barrel/day (Volume).

5. Add a TBP End Point Volume Percent column specification for Liquid Distillate at 150°F (65.56°C).

Figure 5.10

Figure 5.11


5-12

The figure below shows the Monitor page after adding a TBP End Point Volume Percent column.

6. Click on the Parameters tab, and enter a top stage temperature estimate of 140°F and a Tray 1 temperature estimate of 180°F. Enter a bottoms reboiler temperature estimate of 300°F.

7. Run the column and examine the column performance.

Figure 5.12

Before running the column, ensure that the outlet streams are updated. Check the Update Outlets checkbox for the column to automatically update the outlet streams. By default the Update Outlets checkbox is checked.


5-13

Figure 5.13

Figure 5.14


5-14

5.3.1 Exercises1. Add a UniSim Design Spreadsheet (Sulphur Calculations) to

calculate the total sulphur content in ppm wt of light gasoline.

Spreadsheet Connections

Cell Object Variable

D2 Light Gasoline Comp Mass Flow, 2C3Mercaptan

D3 Light Gasoline Comp Mass Flow, nPMercaptan

D4 Light Gasoline Comp Mass Flow, Thiophene

B6 Light Gasoline Mass Flow

B7 Fractionator Spec Value TBP End Point

Figure 5.15


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2. Find the Light Naphtha TBP End Point that corresponds to less than 10 ppm wt and 1 ppm wt Thiophene Sulphur.

5.4 Plot Utility1. Begin a new UniSim Design case, add a Fluid Package using

the Clean Fuels Pkg and add the two components 1-Propanethiol and n-Hexane. Enter the Simulation Environment.

2. Open the Excel Spreadsheet Txy Plot Utility, and connect to UniSim Design.

3. Plot a Txy Diagram for system 1-Propanethiol-n-Hexane at 101.325kPa.

4. Find the azeotrope temperature and composition.

Ans. Experimental Data. (1PRSH) xazeo=0.5570, Tazeo=147.83°F.

Figure 5.16

5-16 Plot Utility

5-16

Figure 5.17

References A-1

A-1

A References 1 Halbert, T. R., Brignac, G. B., Greeley, J. P., Demmin, R. A. and

Roundtree, E. M., “Getting Sulfur on Target,” Hydrocarbon Engineering, June 2000, pp.1-5.

2 Golden, S. W., Hanson, D. W. and Fulton, S. A., “Use Better Fractionation to Manage Gasoline Sulphur Concentration,” Hydrocarbon Processing, February 2002, pp. 67-72.

3 Twu, C.H., Sim, W.D. and Tassone, V., “A versatile liquid activity model for SRK, PR and a new cubic equation-of-state TST”, Fluid Phase Equilibria 194-197, 2002, pp. 385-399.

4 Twu, C.H., Bluck, D., Cunningham, J.R. and Coon, J.E., Fluid Phase Equilibria, 69, 1991, pp. 33-50.

5 Wong, S.H. and Sandler,S.I., 1992, AIChE J., 38, 1992, pp. 671-680.

6 Twu, C.H., Wayne, D., and Tassone, V., “Liquid Activity Coefficient Model for CEOS/AE Mixing Rules” Fluid Phase Equilibria, 183-184, 2001, pp. 65-74.

7 Twu, C.H., Coon, J.E. and Bluck, D., Fluid Phase Equilibria, 150-151, 1998, pp. 181-189.

A-2

A-2

I-1

IndexC

Clean Fuels Pkgadding 4-2tutorial 5-1–5-15

Clean Fuels Pkg Extensionuser interface 4-3using 4-2

Clean Fuels Pkg View 4-4NRTLTST tab 4-4TST CEOS tab 4-5

E

Estimation Methods 3-7

F

Fractionator designthrough accurate VLE models 2-8–2-10

G

Gasoline Sulphur species distribution 2-2–2-4

L

Light/Medium Gasoline fractionation 2-5–2-7

M

Modeling the Gasoline Fractionator 5-10

N

NRTLTST tab 4-4User Data 4-5

P

Plot Utility 5-15

R

Requirementssystem 4-2

T

Thermodynamic Model 3-2–3-7estimation methods 3-7

TST CEOS tab 4-5

U

User Data 4-5User Interface


I-2 Index

I-2

UniSim Design Clean Fuels PPkg User Guide

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