electrolyte modeling basics.ppt

8/10/2019 Electrolyte Modeling Basics.ppt

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Opening new do ors wi th Chemist ry

THINK SIMULATION!

Electrolyte Modeling BasicsProcess Simulation

OLI Systems, Inc.


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Agenda

Introductions

Overview of Process Simulation

The basic OLI Process (Neutral 1)

Essentials Controllers

Recycles

Sour Gas SweeteningSimple Crude Distillation

OLI Pro (Neutral 1 again)


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Introductions

OLI Staff Jim BertholdDirector of Customer Support

Robert YoungDirector of product support

Chris DepetrisDirector of product development

Hongang ZhaoOLI Engine Support

AQSim

Pat McKenzieDirector of OLI Business

DevelopmentAJ GerbinoSenior Partner

Attendees


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OLI Supports several Process SimulatorsAspen PLUS

Aspen Hysys

IDEAS

gProms

OLI

ESP

OLI Pro ProII

Unisim


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We will discuss only the OLI Simulators

Environmental Simulation Program (ESP)

OLI Pro

Analyzers


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ESP Development started in 1990

Funded by a consortium of companies

Aker Kvaerner (formerly Davy McKee)

Chevron

Dupont

ExxonMobil (formerly Exxon)

ICI

Shell

Development Continues


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OLI Pro

Created from Honeywells Unisim Design

Updated as Unisim is updated

Contains all of the OLI thermodynamics Does not contain all of OLIs specialized unit

operations


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OLI has a vast experience in simulation

Upstream flow assurance

Subsurface flow modeling

Acid gas scrubbing Organic pollutant stripping

Dynamic pH control

Biological treatment

Crude distillation

More


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The basic OLI Process (Neutral1)


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We will be using ESP

Defining the chemistry model

Create the process

Mix blockPhase separate block

pH neutralizer block

Run the process

Review the results


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Controllers

Frequently the adjustment of pH requires theNeutralizer Block to perform a difficult

calculation. The calculation is difficult because the set point

of the Neutralizer may be on the steep part of thetitration curve.

There may be significant phenomenologicalchanges that occur while the unit is adjusting thepH.


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Controllers Frequently the Neutralizer Block is not a suitable

block because:

To control the pH you must adjust anotherupstream or downstream block

You need to control something other than pH

The set point may be an impossible case.


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Controllers Frequently the Neutralizer Block is not a suitable

block because:

To control the pH you must adjust anotherupstream or downstream block

You need to control something other than pH

The set point may be an impossible case.

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Controllers Some other parameters that can be controlled

are:

pH

Temperature

Pressure

Flow

ConcentrationOxidation/Reduction Potential

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Phase change limitations to pH control

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Feed contains

H2O

Cl2

CO2

Scrubbed with

NaOH

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Cl2(vap)= Cl2(aq)Cl2(aq)+ H2O = H

++ Cl-+ HClO(aq)

HClO(aq)=H++ClO-

As the pH increases with added NaOH, all these equilibria are shifted to theright. This scrubs the chlorine

CO2(vap)=CO2(aq)

CO2(aq)+H2O=H++HCO3

-

HCO3-

=H+

+CO3-2

HCO3

-+Na+=NaHCO3(s)But these equilibria are also shifted to the right.

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Cl2(vap)= Cl2(aq)Cl2(aq)+ H2O = H

++ Cl-+ HClO(aq)

HClO(aq)=H++ClO-

CO2(vap)=CO2(aq)

CO2(aq)+H2O=H++HCO3

-

HCO3-=H++CO3

-2

HCO3-+Na+=NaHCO3(s)As a species concentration becomes fixed by the equilibrium, then the

pH remains constant.

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Controller Remove the pH neutralizer

Add a manipulate block to control NaOH addition

Add a new mixer block to mix the separatedliquid with the manipulated NaOH

Add a control block

Run the process

Review the results

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Adding Recycle Loops Frequently a process recycles part or all of certain

streams back to up-stream units.

There are many reasons for using a recyclestream.

minimization of waste

increase of residence time

purification of product.

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Recycle Loops Modify chemistry model

Add mix block for halite addition

Add a split block

Connect recycle stream to original mix block

Run process

Review results

How much Caustic Reagent was used?More than in no-recycle case?

Less?

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Sour Gas Sweetening

DEA Absorber

Feed Gas Rich Amine

Flash Drum

Clean Gas

Flash Vapor

Flash Liquid

DEA Regenerator

Recycle

CO2-H2S

Water MixWater Make-UpWater

Water In

Recycle 1

DEA Mix

DEA Make-UpDEA

DEA InWater

Control

DEA

Control

Recycled DEA

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Sour Gas Sweetening

This application brief presents the case of sweetening(purifying) a sour gas from a natural gas well.

Several unit operations are employed to simulate atypical gas sweetening process configuration.

Once the sour gas components have been removed, thescrubbing liquor is regenerated to remove captured sourcomponents.

These components are corrosive and metal selection can

be an issue.

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Sour Gas Sweetening

For this example we will take a natural gas stream is approximatelytwo mole percent (mol%) sour. This means that for every 100 moles of gas there are 2 moles of

hydrogen sulfide (H2S). In addition to H2S, it is desirable to remove carbon dioxide

(CO2) since this constituent lowers the heating value of the gas

and increases the volume of gas that must be transported. Most all alkanolamine plants are designed to maximize the

removal of both of these acid gases. In a typical gas cleaning plant, natural gas is fed to an absorber

operating at high pressure. The gas is scrubbed using an approximately 58 weight percent

(wt%) diethanolamine (DEA) solution. The scrubbed sweet gas is sent on for further processing or

drying and transport via pipeline.

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Sour Gas Sweetening

The rich DEA solution exiting the absorber is sent to aflash drum operating at a much lower pressure. This step removes any light-end hydrocarbons that were

captured in the absorber. The light-end gases are sent on for further processing.

Next, the hydrocarbon-free DEA solution is fed to aregeneration column. Here heat is applied to strip the acid gas components out

of the DEA solution. Make-up water and DEA are added to maintain the lean 58

wt% DEA solution. This solution is then recycled to the absorber.

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Sour Gas Sweetening

Why does adding DEA remove CO2and H2S? The absorption of hydrogen sulfide gas follows these

equilibria:

H2S (vap) = H2S (aq) (1)

H2S (aq) = H+

+ HS-

(2) HS-(aq) = H+ + S-2 (3)

Adding a basic reagent such as DEA increases the pH ofthe solution. pH is defined as:

pH = - log aH+ (4)

where aH+is the activity of the hydrogen ion. The activityof the hydrogen ion is defined as:

aH+ = H+[H+] (5)

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Sour Gas Sweetening

Carbon dioxide follows a similar equation path: CO2(vap) = CO2(aq) (6)

CO2(aq) + H2O = H++ HCO3

- (7)

HCO3-

= H+

+ CO32-

(8)

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Sour Gas Sweetening

Where does the basic reagent come from? Adding DEA ((C2H5O)2NH) to a solution will make it more

basic:

(C2H5O)2NH + H2O = (C2H5O)2NH2+ + OH- (9)

H2O = H+ + OH- (10)

Adding DEA to the solution forces water to dissociate (Eq.10).

The hydrogen ion is complexed with the DEA molecule tocreate a protonated species and leaving free hydroxideions.

This increases the pH and all of the vapor-liquid equilibriadescribed above (by Equations 1, 2, 3, 5, 6 and 7) willshift to the right.

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Sour Gas Sweetening

There is a secondary equilibrium involving DEAcarbamate ((C2H5O)2NCO2-):

(C2H5O)2NH + HCO3-= (C2H5O)2NCO2

-+ H2O (11)

This species is stable at low temperatures andhelps to remove carbon dioxide from the naturalgas.

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Sour Gas Sweetening

Steps to create the process New Process

New Chemistry

Build the process Run the process

Select Tear(s)

Evaulate results

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Simple Crude Distillation

Overview In this demonstration we will distill a typical crude using a

simple distillation scheme with a single side stripper.

The OLI approach to modeling distillation is to rigorouslyaccount for the effects of water in the oil and also considerthe effects of salts in both the water and oil phases.

Most other simulators only consider the water phase as apure phase.

Our approach will allow us to model such species as

chlorides and amine salts entrained and dissolved in theprocess streams.

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Back Story Our example considers a crude oil after it has left

the production field.

In our case we have a relatively young well that

has produced 100,000 barrels of oil per day.10,000 barrels of this oil are produce water. In aReal sample, this produced water will consist ofmany different cations and anions as well asdissolved gases.

These dissolved species can cause a host ofproblems such as fouling, scaling and corrosion.

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Back Story Continued In our example, the formation from which the oil was produced is

essentially just a salt dome (NaCl). Our oil and our produced waterwill be saturated with halite. The chloride ion can be a problemdownstream.

In normal processing this oil will be sent to an electrostatic desalterwhere the oil is washed and most of the salt is dissolved into thewater phase.

The problem with the wash water is that it also may containsignificant amounts of salt which are the introduced to the refinery.

The crude is usually maintained at moderate temperatures (150 oFto 250 oF) and at pressures sufficient to prevent boil-off (usually 75PSI above saturation pressure).

The pH of the desalted crude is maintained at pHs near neutral toprevent emulsion formation.

The desired salt content of the crude is usually near 3.5 mg/L (1pound per thousand barrels, PTB)

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Desalter simulationor a funny thing happened on the way to the CDU

FLOW ADJUSTERNACL FORMATION A-NACL FRMSALT

SATFORMATION

BRINE

FORMATION

SOLID

C-BRINE

FLOW

OIL

WATER

MIX

OIL

FORMATION

CRUDE

SIMPLE

DESALTER

WASH

WATER

A-CAUSTIC

SALT WATER

DESALTED

CRUDE

M-CAUSTIC

CAUSTIC

C-

CRUDE

PH

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Salt Composition

Stream: NACL FORMATION

Temperature 75 oF

Pressure 75 PSIA

Flow 128200 Lb/hr[1]

H2O 0.83 Mole fraction

NACL 0.17 Mole fraction

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Crude Feed

Stream: OIL

Temperature 75 oF

Pressure 75 PSIA

Flow 1.1538E+06 Lb/hr[1]

CRUDE 0.9658 Mole fraction

CH4 0.0003 Mole fraction

C2H6 0.0006 Mole fraction


n-C4H10 0.0193 Mole fraction

i-C4H10 0.0054 Mole fraction

[1]This is approximately 90,000 bbd

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Wash water is added to theseparator SIMPLE DESALTER at arate that is 6 % of the volume ofthe mixed oil and water streamFORMATION CRUDE. This isapproximately 6,000 bbd. Caustic

is added to keep the pH in the 7.0range.

The stream DESALTED CRUDE isthe stream that we will use in thedistillation simulation. Thecomposition of the stream isshown in the table to the right.

Stream: DESALTED CRUDE (a/k/a RAW CRUDE)

Temperature 250 oF

Pressure 110 PSIA

Flow 1.16773E+06 Lb/h (100,000 bbd)

H2O 0.0087 Mole fraction

CRUDE 0.9097 Mole fraction

CH4 0.0028 Mole fraction



n-C4H10 0.0181 Mole fraction

i-C4H10 0.0051 Mole fraction

NA2O 0.0165 Mole fraction

HCL 0.0330 Mole fraction

Density 1914.6 Lb/m3

Enthalpy -4.25003E+06 Cal/lmol

electrolyte modeling basics.ppt

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