Unisim Design Clean Fuels Ppkg User Guide

UniSim Clean Fuels Property Package Reference Guide

Copyright June 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 Honeywell 300-250 York Street London, Ontario N6A 6K2 Telephone: (519) 679-6570 Facsimile: (519) 679-3977 Copyright Honeywell 2005. All rights reserved.

Prepared in Canada.

Table of Contents 1

Introduction ......................................................... 1-1 1.1

2

3

4

5

A

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

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

Clean Fuels Property

Package............................ 3-1

3.1

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

3.2

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

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

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

References ...........................................................A-1 Index.................................................................... I-1

iii

iv

Introduction

1-1

1 Introduction

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

1-1

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.

1-2

Introduction

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. 1-3

1-4

Meeting New Sulphur Levels in Motor

1-4

Gasoline Fractionation

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

2-1

2-2

Gasoline Sulphur Species Distribution

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.

Component name

UniSim Design Sim Name

NPB °F

BPT 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

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

2-2

Gasoline Fractionation

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. DIPPR ID

UniSim Design ID

METHYL MERCAPTAN

1801

353

ETHYL MERCAPTAN

1802

354

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

Formula

Component Name

CH4S C2H6S C3H8S

2-3

2-4

Gasoline Sulphur Species Distribution

Formula

Component Name

DIPPR ID

UniSim Design ID

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

C5H6S

2-METHYLTHIOPHENE

2844

733

C5H6S

3-METHYLTHIOPHENE

2845

3217

C5H6S

3-METHYLTHIOPHENE

2845

734

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

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 140390°F, and benzothiophenes and substituted benzothiophenes in the 390-430°F heavy gasoline. Above 390°F the total sulphur increases significantly with temperature.

2-4

Gasoline Fractionation

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.

2-5

2-6

Light/Medium Gasoline Fractionation

Figure 2.1: Simulated Thiophene Peak of FCC Gasoline

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

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.

2-6

Gasoline Fractionation

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.

2-7

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.

2-7

2-8

Improve Fractionator Design

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 thiolhydrocarbon 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 thiolhydrocarbon binary pairs. To fill in missing parameters for systems of binaries forming azeotropes, a newly developed

2-8

Gasoline Fractionation

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. Figure 2.2 VLE Diagram for nPropylMercapatn and Hexane at 1 atm

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

2-9

2-10

Improve Fractionator Design

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. Figure 2.3: Effect of Fractionator Design on Gasoline End Point

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

2-10

Clean Fuels Property Package

3-1

3 Clean Fuels Property Package 3.1 Introduction................................................................................... 2 3.2 Thermodynamic Model ................................................................... 2 3.2.1 Estimation Methods .................................................................. 7

3-1

3-2

Introduction

3.1 Introduction The Clean Fuels Property Package is a specially designed property package for the accurate VLE representation of thiolhydrocarbon 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 Model Selecting 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.

3-2

Clean Fuels Property Package

3-3

The Clean Fuels Property Package uses an optimal twoparameter cubic equation of state TST (Twu-Sim-Tassone)3 to represent Clean Fuels Processes. The TST cubic equation is represented as follows: RT a P = -----------– -----------------------------------------------------v – b v 2 + 2.5bv – 1.5b 2

(3.1)

and can be rewritten in the form, RT - – -------------------------------------------------a P = -----------v – b ( v + 3b ) ( v – 0.5b )

(3.2)

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: 2 2

a ( T c ) = 0.427481R T c ⁄ P c

(3.3)

b = 0.086641RT c ⁄ P c

(3.4)

Z c = 0.296296

(3.5)

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 thiolhydrocarbon systems in the entire composition range is the accurate calculation of pure component vapour pressures.

3-3

3-4

Thermodynamic Model

You can use the Twu alpha correlation4. NM

N ( M – 1 ) L ( 1 – Tr )

α = Tr

e

(3.6)

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:

α = α

(0)

+ ω(α

(1)

–α

(0)

)

(3.7)

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:

a

*

= b

*

*

E

E

b vdw ⎞ A 0 A 0vdw a vdw 1 - ⎛ ------------------- + --------⎜ - – ----------------- – ln ⎛⎝ --------------⎞⎠ ⎟ * b ⎠ RT b vdw C v0 ⎝ RT

b =

∑ ∑ xi xj i

j

1 --- ( b i + b j ) 2

(3.8)

(3.9)

bvdw is used for b.

3-4

Clean Fuels Property Package

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: *

⎛ v 0 + w⎞ 1 -⎟ C v0 = – ------------------- ln ⎜ ----------------( w – u ) ⎝ v* + u ⎠ 0 vdw

(3.10)

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:

E

E

*

*

ai A ∞vdw A 0vdw a vdw ----------------- = C v0 ------------= ----------------- – ∑ x i ------* * RT RT b vdw i b i

(3.11)

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

G- = ------RT

n

∑ xj τjiGji j

∑ xi --------------------------n i ∑ xk Gki

(3.12)

k

When τij and Gij are calculated using the parameters in Equation (3.13) and Equation (3.14), the NRTL equation is obtained. A τ ji = ------jiT

(3.13)

G ji = exp ( – α ji τ ji )

(3.14)

3-5

3-6

Thermodynamic Model

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: 1 τ ji = --- δ ij b i 2

(3.15)

b G ji = -----j bi

(3.16)

where: 2

ai aj a⎞ C v0 ⎛ a i δ ij = – – --------- ⎜ --------- – ---------j⎟ + 2k ij --------- --------b RT ⎝ b i bj ⎠ i bj

(3.17)

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: *

v 0vdw = r

(3.18)

Substituting Equation (3.18)into Equation (3.10), Equation (3.10)becomes: 1 r+w C r = – ------------------- ln ⎛ -------------⎞ (w – u) ⎝ r + u⎠

(3.19)

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

3-6

Clean Fuels Property Package

3-7

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

3.2.1 Estimation Methods For 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, 2butanethiol, 2-methyl 1-propanethiol and 2-methyl 2propanethiol 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.

3-7

3-8

Thermodynamic Model

3-8

Clean Fuels Pkg Extension

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 ........................................................................... 4 4.3.2 TST CEOS Tab.......................................................................... 5

4-1

4-2

Using the Clean Fuels Pkg Extension

4.1 Using the Clean Fuels Pkg Extension Refer to Chapter 2 Fluid Package of the UniSim Design Simulation Basis manual for more information on the UniSim Design Property Package.

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.

4-2

Clean Fuels Pkg Extension

4-3

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

4.2 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.

4-3

4-4

Clean Fuels Pkg View

4.3 Clean Fuels Pkg View Like 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. Figure 4.2

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 Tab The NRTLTSTS tab as shown in Figure 4.2 contains the binary parameters for the activity coefficient model NRTLTST (NRTLTwu-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.

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Clean Fuels Pkg Extension

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

User Data The 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 Tab The TST CEOS tab contains the binary parameters for the TST (Twu-Sim-Tassone) cubic equation of state (CEOS). Figure 4.3

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Clean Fuels Pkg View

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

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Clean Fuels Pkg Tutorial

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

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5-2

Introduction

5.1 Introduction The 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

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Clean Fuels Pkg Tutorial

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

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. Property Package

Components

Clean Fuels Pkg

2C3Mercaptan, nPMercaptan, Thiophene

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

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).

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Flowsheet Setup

3. Click the Enter Oil Environment button. The Oil Characterization view appears. Figure 5.2

4. Click the Add button. The Assay view appears. Figure 5.3

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.

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Clean Fuels Pkg Tutorial

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7. In the Input Data group, click on the Edit Assay button. The Assay Input Table view appears. Figure 5.4

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

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5-6

Flowsheet Setup

9. After you have entered the assay input data, click the OK button to return to the Assay view. Figure 5.5

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. Figure 5.6

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

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Clean Fuels Pkg Tutorial

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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. Figure 5.7

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

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

Flow Rate

Mass

364008 lb/hr

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. Figure 5.8

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.

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5-8

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

Flowsheet Setup

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. Figure 5.9

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. 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.

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. 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 5-8

Clean Fuels Pkg Tutorial

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Conditions Stream Name

Light Ends

Temperature [F]

100

Pressure [psia]

114.6

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

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

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.

Temperature [F]

100

Pressure [psia]

114.6

Mass Flow [lb/hr]

364008.7

Liq. Vol Flow [barrel/day]

32784.7

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.

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Modeling the Gasoline Fractionator

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

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.

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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. Figure 5.10

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 5.11

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Modeling the Gasoline Fractionator

The figure below shows the Monitor page after adding a TBP End Point Volume Percent column. Figure 5.12

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. 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.

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Clean Fuels Pkg Tutorial

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Figure 5.13

Figure 5.14

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Modeling the Gasoline Fractionator

5.3.1 Exercises 1. 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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Clean Fuels Pkg Tutorial

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Figure 5.16

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 Utility 1. Begin a new UniSim Design case, add a Fluid Package using the Clean Fuels Pkg and add the two components 1Propanethiol 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.

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Plot Utility

Figure 5.17

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References

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.

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Index C Clean Fuels Pkg adding 4-2 tutorial 5-1–5-15 Clean Fuels Pkg Extension user interface 4-3 using 4-2 Clean Fuels Pkg View 4-4 NRTLTST tab 4-4 TST CEOS tab 4-5 E Estimation Methods 3-7 F Fractionator design through 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 4-4 User Data 4-5 P Plot Utility 5-15 R Requirements system 4-2 T Thermodynamic Model 3-2–3-7 estimation methods 3-7 TST CEOS tab 4-5 U User Data 4-5 User Interface Clean Fuels Pkg Extension 4-3

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Index

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