Process Simulation

PROCESS SIMULATION

REFINERY PROCESSES

SAMPLER

Modelling and Optimization

John E. Edwards

Process Simulation Engineer, P & I Design Ltd

First Edition, June 2013

P&I Design Ltd
Released by

P & I Design Ltd

2 Reed Street, Thornaby TS17 7AF

Private distribution only

Printed by Billingham Press Ltd, Billingham TS23 1LF

Process Simulation Refinery Processes

Contents

Section 1Refinery Processes5

Section 2Thermodynamics9

Section 3Crude Column13

Section 4Vacuum Still23

Section 5Splitting and Product Purification27

Section 6Hydrotreater43

Section 7Catalytic Reformer47

Section 8Amine Treatment53

Section 9Miscellaneous Applications57

Section 10General Engineering Data59

Section End70

Preface

The process industry covers a broad spectrum of activities that involve the handling and treatment of gases, liquids and solids over a wide range of physical and processing conditions. This manual provides a comprehensive review of the fundamentals, definitions and engineering principles for the study of processesencountered in hydrocarbon processing using steady state simulation techniques.Applications are presented for a wide range of processing units involving design and operations.

Process simulations are carried out using CHEMCAD™ software by Chemstations, Inc. of Houston. This manual has been developed with the full support of Chemstations simulation engineers based in Houston.

The simulation of crude distillation at atmospheric pressure, vacuum distillation and sour gas amine treatment is covered in Section 13 Process Measurement and Control of the book “Chemical Engineering in Practice” by J.E.Edwards. This manual includes these topics and extends the study to other refinery processes including splitters, stabilizers,hydrotreaters and reformers.

Thermodynamics are reviewed with special reference to the application of pseudocomponent curves and crude oil databases

Each topic is in the form of a condensed refresher and provides useful practical information and data.Each section is numbered uniquely for contents and references, with the nomenclature being section specific. The references are not a comprehensive list and apologies for unintended omissions.

Reference is made to many classic texts, industry standards and manufacturers’ data. Information has been mined from individual project reports and technical papers and contributions by specialists working in the field.

The Author

John E.Edwards is the Process Simulation Specialist at P&I Design Ltd based in Teesside, UK.

In 1978 he formed P&I Design Ltd to provide a service to the Process and Instrumentation fields. He has over fifty years’ experience gained whilst working in the process, instrumentation and control system fields.

Acknowledgements

A special thanks to my colleagues at Chemstations, Houston, who have always given support in my process simulation work and the preparation of the articles that make up this book:

N.Massey, Ming der Lu, S.Brown, D.Hill, A.Herrick, F.Justice and W.Schmidt of Germany

Also thanks to my associate P.Baines of Tekna Ltd for help with the organic chemistry topics.

Section 1

Refinery Processes

References

Shrieve, “Chemical Process Industries”, Chapter 37, 5th Edition, McGraw Hill, 1984.
J.A.Moulijn, M.Makkee, A.Van Diepen, “Chemical Process Technology”, Wiley, 2001.
G.L.Kaes, “Refinery Process Modelling”, Athens Printing Company, 1st Edition, March 2000.
W.L.Nelson, “Petroleum Refinery Engineering”, 4th Edition, McGraw Hill, 1958.

Overview

Most refinery products are mixtures separated on the basis of boiling point ranges. The block diagram, by API, shows overall relationship between the refining processes and refined products.

Refining is a mature, complex and highly integrated operation. Columns with a wide variety of internals are used in many stages of the process. Fractional distillation under vacuum and pressure conditions is used to separate components. Light ends are steam stripped and the heavy ends are vacuum distilled at reduced the temperatures. Stabilizers are used to remove light ends, including LPG, to reduce the vapor pressure for storage and subsequent processes. Absorbers and strippers are used to remove unwanted components such as sulphur.

Simple distillation processes do not produce sufficient gasoline above the minimum required octane number. This is achieved by converting heavy to light hydrocarbons using catalytic processes including fluidic catalytic cracking (FCC), hydrotreating, hydrocracking, catalytic reforming and alkylation.

Crude petroleum consists of thousands of chemical species. The main species are hydrocarbons but there can be significant amounts of compounds containing sulphur (0-6%), oxygen (0-3.5%) and nitrogen (0-0.6%). The main groups are:

Aliphatic or open chain hydrocarbonsas detailed in the table:

Aliphatic or open chain hydrocarbons (hc)
Descriptor / Properties / Class / Formula / Member
-ane / saturated hc, unreactive / paraffins / CnH2n+2 / C2H6 ethane
-ene / unsaturated hc, forms additive compounds / olefines / CnH2n / C2H4 ethylene
acetylenes / CnH2n-2 / C2H2 acetylene
-ol / reactive, OH replaced / alcohols, phenols / RCH3OH / C2H5OH ethyl alcohol
-one / additive compounds / ketones / RR1.CO / (CH3)2.CO acetone

n-paraffin series or alkanes (CnH2n+2)

This series has the highest concentration of isomers in any carbon number range but only occupy 20-25% of that range and make low octane gasoline. Most straight run (distilled directly from the crude) gasolines are predominately n-paraffins. The light ends primarily consist of propane(C3H8), n-butane (C4H10) together with water which are defined as pure components.

iso-paraffin series or iso-alkanes (CnH2n+2)

i-butane (C4H10) is present in the light ends but these compounds are mainly formed by catalytic reforming, alkylation or polymerization.

olefine or alkene series (CnH2n)

This series is generally absent from crudes and are formed by cracking (making smaller molecules from larger molecules). They tend to polymerize and oxidize making them useful in forming ethylene, propylene and butylene.

Ring compounds

Naphthene series or cycloalkanes (CnH2n)

These compounds are the second most abundant series of compounds in most crudes. The lower members of this group are good fuels and the higher members are predominant in gas oil and lubricating oils separated from all types of crude.

Aromatic series

Only small amounts of this series occur in most common crudes but have high antiknock value and stability. Many aromatics are formed by refining processes including benzene, toluene, ethyl benzene and xylene.

Lesser Components

Sulfur has several undesirable effects including its poisonous properties, objectionable odour, corrosion, and air pollution. Sulfur compounds are removed and frequently recovered as elemental sulfur in the Klaus process.

Nitrogen compounds cause fewer problems and are frequently ignored.

Trace metals including Fe, Mo, Na, Ni and V are strong catalyst poisons and cause problems with the catalytic cracking and finishing processes and methods are used to eliminate them.

Salt, which is present normally as an emulsion in most crudes, is removed to prevent corrosion. Mechanical or electrical desalting is preliminary to most crude processing.

Crude oil is classified on the basis of density as follows:

Lightless than 870 kg/m3 31.1° API

Medium870 to 920 kg/m3 31.1° API to 22.3° API

Heavy 920 to 1000 kg/m3 22.3° API to 10° API

Extra-heavygreater than 1000 kg/m3 <10° API

Bitumen

Heavy or extra-heavy crude oils, as defined by the density ranges given, but with viscosities greater than 10000 mPa.s measured at original temperature in the reservoir and atmospheric pressure, on a gas-free basis

Natural Gas

Light hydrocarbon mixture that exists in the gaseous phase or in solution in crude oil in reservoirs but are gaseous at atmospheric conditions. Natural gas may contain sulphur or other non-hydrocarbon compounds.

Natural Gas Liquids

Hydrocarbon components recovered from natural gas as liquids including ethane, propane, butanes, pentanes plus, condensate and small quantities of non- hydrocarbons.

Atmospheric and vacuum distillations produce the different fractions as detailed in the table below.

Crude Petroleum Fractional Distillation
Temperature / <30ºC / 40-70 ºC / 70-120ºC / 120-150 ºC / 150-300 ºC / >350 ºC / Residue
Description / Gaseous Hydrocarbon / Gas oil / Naptha / Benzene / Kerosene / Heavy oils / Asphalt or Bitumen
Density / 0.65 / 0.72 / 0.76 / 0.8
Composition / C3H8, C4H10 / C5H12, C6H14 / C6H14, C7H16 C8H18 / C8H18, C9H20 / C10H22, C11H24 C12H26 to C18H36 / C18H38 to C28H58
Applications / Gas fuel or enrichment / General solvent, aviation spirit / Solvent for oils, fats & varnishes / Solvent for oils, fats & varnishes / Home heating
Jet fuel / Diesel, fuel oils / Roads, Wax paper
Gasoline, contains C6H14, C7H16, C8H18 40-180 ºC

Further fractionation of the 70 to 150ºC cut is required to obtain the naptha and benzene cuts.

Vacuum distillation of the topped crude is required to obtain Light Vacuum Gas Oil (LGVO) and Heavy Vacuum Gas Oil (HVGO)

When the difference in volatility between components is small a solvent of low volatility is added to depress the volatility of one of the components. This process is known as extractive distillation. Butenes are separated from butanes using this method with furfural as the extractant.

When a high volatility entrainer is used the process is known as azeotropic distillation. Anhydrous alcohol is formed from 95% aqueous solution using benzene to free the azeotrope and high purity toluene is separated using methyl ethyl ketone as the entrainer.

Typical Crude Oil Products Profile Ref EIA March 2004 Data
Product / Refined gallons/barrel (gal/bbl)
Gasoline / 19.3
Distillate Fuel Oil (Inc. Home Heating and Diesel Fuel) / 9.83
Kerosene Type Jet Fuel / 4.24
Residual Fuel Oil / 2.10
Petroleum Coke / 2.10
Liquified Refinery Gases / 1.89
Still Gas / 1.81
Asphalt and Road Oil / 1.13
Petrochemical Feed Supplies / 0.97
Lubricants / 0.46
Kerosene / 0.21
Waxes / 0.04
Aviation Fuel / 0.04
Other Products / 0.34

Refinery Process Summary:

Section 2

Thermodynamics

References

G.L.Kaes, “Refinery Process Modelling”, Athens Printing Company, 1st Edition, March 2000
American Society for Testing and Materials (ASTM International), Standards Library

Global K and H Models

The following table gives a summary of suitable K and H models for common refinery processes.

Refinery Processing Thermodynamic Models Summary
Process / K Model / H Model (Forced)
Crude Atmospheric Distillation / Grayson Streed / Lee Kessler
Vacuum Distillation / ESSO / Lee Kessler
Hydrotreater / SRK / SRK
Sour Gas Treatment / Amine / Amine
FCC Gas Treatment / Peng Robinson / Peng Robinson
Propylene Splitter / Peng Robinson / Peng Robinson
Compression / BWRS / BWRS

Grayson Streed K model is primarily applicable to systems of non-polar hydrocarbons. It is good for modelling hydrocarbon units, depropanizers, debutanizers, or reformer systems.The approximate range of applicability is as follows:

Temperature Range Pressure Range

0 to 800°F 3000 psia

-18 to 430°C < 20000 kPa

ESSO K model predicts K-values for heavy hydrocarbon materials at pressures below 100 psia. The average error for pure hydrocarbons is 8% for p* > 1 mmHg, and 30% for p* between 10E-06 and 1 mmHg according to API Technical Data Book Vol 1. It is good for modelling vacuum towers.

Lee Kessler H model is good for hydrocarbon systems.

AMINE K model is based on the Kent Eisenberg method to model the reactions with diethanolamine (DEA), monoethanolamine (MEA), methyl diethanolamine (MDEA) being included.

The chemical reactions in the CO2-Amine system are described by the following reactions:

RR'NH2+↔H+ + RR'NH

RR'NCOO + H2O↔RR'NH + HCO3

CO2 + H2O ↔HCO3- + H+

HCO3- ↔CO3- - + H+

H2O ↔H+ + OH-

Where R and R' represent alcohol groups. The reaction equations are solved simultaneously to obtain the free concentration of CO2. The partial pressure of CO2 is calculated by the Henry's constants and free concentration in the liquid phase.

The AMINE K Model in CHEMCAD treats the absorption of CO2 in aqueous MEA as a fast chemical reaction, in other words, gas film controlled implying a very low stripping factor. However it is known that this process is liquid film controlled since Henry’s Law controls the diffusion of CO2 into the liquid prior to chemical reaction taking place.

Section 3

Crude Column

Crude Column Simulations
Case/File Name / Description
R3.01 / Crude Column Feed

References

H.Kister, “Distillation Design”, McGraw-Hill, ISBN 0-07-034909-6
G.L.Kaes, “Refinery Process Modelling”, Athens Printing Company, 1st Edition, March 2000
W.L.Nelson, “Petroleum Refinery Engineering”, 4th Edition, McGraw Hill, 1958

Process Description

The simplified process flow diagram shows the basic layout for the crude and vacuum distillation units.

Desalted crude is preheated with the pump around and topped crude heat exchangers prior to being heated to ~620ºF in the direct fired furnace. Above this temperature thermal decomposition (cracking) will take place resulting in increased light ends and fouling of heat exchange surfaces due to carbon based deposits. The following initial guidelines are suggested:

1.For paraffin based crudes at moderate furnace temperatures, an estimated cracked gas rate of 5.0 SCF/bbl (42 gal/bbl) crude oil is reasonable.

2.For asphalt based crude oil a cracked gas production of 2.5 SCF/bbl crude oil is suggested.

3.The cracked gas may be given an arbitrary composition as follows:

50 mol% methane, 40 mole% ethane, and 10 mole% propane.

The feed to the atmospheric crude tower is a mixed vapor-liquid phase of ~0.4 vapor fraction. The vapours flow upwards and are fractionated to yield the products.

Crude towers are typically 4m diameter, 20–30m in height with 15–30 trays.

A typical Process Flow Diagram for a crude unit, including pump-around circuits and side strippers, is shown. The column is modelled on the basis of theoretical stages, as opposed to actual trays, so it is necessary to apply tray efficiency ηto translate the actual trays NA to theoretical trays NT where η=NT/NA. Note that commercial simulators provide various tray efficiency models, which are not suitable for crude distillation columns. Tray efficiency η should be based on experience. The relationships between NA and NT are indicated in the diagram.

The liquid product sidestreams contain light hydrocarbons which must be removed to meet the initial boiling point specification for the products. The liquid sidestreams are fed to strippers that use either a reboiler or steam to strip out these light components which are returned to the crude tower. Current preference is to use reboiled side strippers for the lower boiling products to reduce the heat load on the crude tower condenser and the sour water stripper.

Side strippers are typically 1-2m diameter, 3m in height with 4–8 trays representing 2–3 theoretical stages. Height limitations can be met by using structured packing which has high capacity and low HETP values as compared to trays.

Pumparound cooling circuits provide reflux to remove the latent heat from hot flash zone vapors and condense the side products.A pump-around zone may be considered equivalent to an equilibrium flash where equilibrium liquid is recirculated. The large flow of pump-around liquid creates a region of constant liquid composition that eliminates fractionation. The heat removed preheats the crude feed.

Section 4

Vacuum Still

Vacuum Still Simulations
Case/File Name / Description
R4.01 / Vacuum Unit

Vacuum distillation is used to separate the high boiling bottoms from the crude column. The Vacuum Unit process flow diagram is shown with distillation UnitOp 1 selected as Tower+.

The thermodynamic selection is K Model ESSO and H Model Lee Kessler.

The feed is defined by the following specification:

Feed rate360 m3/day

Bulk gravity0.9168 specific gravity

Feed temperature150ºF

Feed pressure58 psia

Distillation curve volume % based on TBP at 1 atm

The column specifications are:

Vacuum Column Data
Description / Specification
Number of strippers / 0
Number of pumparounds / 2
Number of exchangers / 1
Number of side products / 2
Stages / Theoretical 8 Feed 8
Column pressures / Top 30 mmHg dP 35 mmHg
Stripping Steam condition / 335ºF and 115 psia
Bottom steam flow / 166.67 lb mol/h
Condenser / Total
Reboiler / None
Pumparound 1
Stages / Draw-3 Return-1
Flow / 276218 kg/h Phase liquid
Duty / 0 MJ/h
Pumparound 2
Stages / Draw-5 Return-4
Flow / 538139 kg/h Phase liquid
Duty / 0 MJ/h
Side Product Draw 1
Stage / 3
Flow / 72 m3/h Phase liquid
Side Product Draw 2
Stage / 5
Flow / 213 m3/h Phase liquid
Side Heat Exchanger
Stage / 8 No duty (Feed stage)
Stage Specifications
Stage / 3 1 kmol/h Liquid flow
Stage / 5 85 m3/h Liquid Flow
Stage / 8 69 m3/h Liquid Flow

Pseudocomponent Curves allow group plots to be generated for the streams:

Section 5

Splitting and Product Purification

Splitting and Product Purification Simulations
Case/File Name / Description
R5.01 / Deethanizer
R5.02 / Debutanizer Depropanizer
R5.03 / Debutanizer Reflux Depropanizer
R5.04 / C3 Splitter
R5.05 / C4 Splitter
R5.06 / C4 Splitter Tray Column
R5.07 / Kerosene Splitter

References

H.Kister, “Distillation Design”, McGraw-Hill, ISBN 0-07-034909-6
G.L.Kaes, “Refinery Process Modelling”, Athens Printing Company, 1st Edition, March 2000
G.L.Kaes, “Practical Guide to Steady State Modelling of Petroleum Processes
H.Kazemi Esfeh and I.Aalipour mohammadi, “Simulation and Optimization of Deethanizer Tower”, 2011 International Conference on Chemistry and Chemical Process, Singapore

Introduction

A primary activity in hydrocarbon processing involves the fractionation and purification of light ends using columns, the most common being stabilizers, deethanizers, debutanizers and depropanizers. A typical purification plant schematic is shown:

Deethanizer

The deethanizer removes ethane (C2H6) and lighter components which may be fed to the olefines unit for production of ethylene (C2H4) or polyethylene or polypropylene products. Bottoms are fed to the debutanizer.Design for C2 mole fraction or C2/C3 mole ratio in the bottoms.

Debutanizer

The debutanizer separates mixed LPG product (mostly C3’s and C4’s) and a stabilized condensate (C5+). Design for RVP in bottoms with 12 psia being typical and reflux ratio 0.5 – 1.0

Depropanizer

The depropanizer separates propane (C3’s) as overheads from the butane (C4) to the bottoms.

Stabilizer

Stabilizers are used to remove light ends (mainly C4’s) from condensate to meet Reed Vapour Pressure (RVP) specification for future processing or to allow storage in floating roof tanks. Design for RVP in bottoms with 12 psia being a typical maximum value

All purification units use the bottom tray or reboiler temperature and reflux for control. The stabilizer uses bottom tray or reboiler temperature alone as there is no condenser for reflux control. Using these parameters in process simulation allows predicted product properties to be compared against actual process conditions. Simulation parameters can be adjusted to match current behaviour to provide a powerful troubleshooting tool.

Splitters are used extensively in hydrocarbon processing, including C2’s, C3’s, C4’sand Naphtha. The process simulation methods used are similar to those for the purification process with the CHEMCAD SCDS UnitOp being used.

Tray Column Industry Practice and Efficiencies (1)
Process / Actual Trays / Overall Efficiency / Theoretical Trays
Naphtha Splitter / 25 - 35 / 70 - 75 / 18 - 25
C2 Splitter / 110 - 130 / 95 - 100 / 105 - 125
C3 Splitter / 200 - 250 / 95 - 100 / 190 - 240
C4 Splitter / 70 - 80 / 85 - 90 / 60 - 68

C2 Splitter (C2H6 – C2H4)

This involves the separation of ethylene from ethane using low temperature distillation. The splitter is normally operated at high-pressure,utilizing closed-cycle propylene refrigeration. The objective is to obtain a high % recovery of high purity ethylene. This process is a high energy user and costly.