dehydration and desalting - unimasr engineering program (pce) –petroleum field operations (che...

Petrochemical Engineering Program (PCE) – Petroleum Field Operations (CHE N303) – Spring 2009/2010

Dr. Fatma Ashour - Dr. Sahar El-Marsafy

DEHYDRATION

AND

DESALTING



Wet Crude Oil

Crude oil from a GOSP may contain very

small droplets of salty water

Many of these droplets are held in

suspension by a thin film of oil that

surrounds them. These droplets are tightly

bound.

Other droplets of water are not surrounded

by a film of oil, these are freely suspended.



Dehydrating & Desalting of

Wet Crude Oil

The purpose is to remove water and salts

from oil that comes out of the GOSPs.

This oil is called Wet Crude, because it

contains too much water and salt to meet

the following requirements:

Water content 0.3% wt, max.

Salt content 10 pounds of salt/1000 bbl

oil, max.



Dehydration of Crude Oil

Water content < 0.2 - 0.5% by volume

Water: Corrosion & scaling for the equipment

Water forms

Free water

Emulsified water

Dissolved water



Water Forms

Free Water

(F.W.)

Suspended

Water (SS.W.)

Soluble Water

(S.W.)

Emulsified

Water (E.W.)

W/O Emulsion

“Regular”

O/W Emulsion

“Inverse”



Free water: Settled in 5 min.

Suspended water: require heat - oil viscosity

Soluble water: lower particle size (Solubility)

Emulsified water: drops - phase

W/O emulsion: oil drops in water phase

O/W emulsion: water drops in oil phase

Water Forms



Factors Promoting Stability of

O/W Emulsion

Oil viscosity

Interfacial tension

Size of dispersed water particle

Density differences

Ratio of volumes of the two phases

Salinity of water: salinity increase the density



Basic Approaches of Handling

Wet Crude Oil



Dehydration Techniques

Heating

Chemical Treatment

Electrical Aid



Heating Techniques

Mechanism: Reduce Oil Viscosity

Types: Direct & Indirect Heating



Wet Crude Heating

Wet crude from a GOSP is heated to 60°C in two

stages before water and salts are separated.

The two stages are a preheating stage and a heating stage.

From the preheating stage, the warm wet crude oil passes

through the shells of a number of crude/diesel oil heat

exchanger. Hot diesel oil flow through the heat exchanger

and heats the wet crude to 60°C.

The diesel oil is heated in a furnace. Heating lowers the

viscosity of the wet crude. The thinner the oil is, the less able

it is to hold water droplets in suspension.



Wet Crude Heaters

Heat ExchangersHot oil

Heater Dry Crude

From Desalters

Hot Diesel

HeaterWet Crude

to

Dehydrator &

Desalter

Diesel

Oil Ret.

Wet Crude

From GOSP

Wet Crude HeatersDry Crude



Wet Crude Heaters



Scale Inhibitor Injection

Scale inhibitor is injected into the wet crude before preheating, salt water tends to form hard scaledeposits when it is heated. The scale inhibitor prevents the formation of scales in the exchanger as the wet crude is heated.



Chemical Treatment

De-emulsifiers

Mechanism:

Absorbed to the O/W interface

Rupture the film

Added at the manifold



Chemical Treatment

De-emulsifiers



Electrical Aid

Mechanism: electrostatic separation

Film is destroyed

Di-pole attraction

Surface is expanded – ellipsoids

Water droplets combine, grow in size & settled down

Attractive with large volumes of fluids

Used with chemicals



Chem-electric Dehydrator



Dehydration and Desalting

Each train consists of three main vessels, one dehydrator and two desalters. Each vessel is identical in construction and operation.

The vessels remove salt water droplets from the crude in a three stage process.

The dehydrator receives hot wet crude from the heat exchanger. The crude spreads across a tray in the vessel under the electrical grid called an electrostatic coalescer.



A high voltage, low ampere current passes through the coalescer, the current creates a strong electrostatic charge in the vessel, this charge attracts the small salty water droplets in the crude, causing them to rise and join together or coalesce to form large drops which fall out of suspension, and separate at the bottom of the vessel.

The dehydrator removes only the freely suspendedwater, the tightly bound droplets of salt water that remains in the crude stream must be removed in the first and second stage desalters.

Dehydration



Dehydrator & Desalter

Oil

Outlet

E.T. E.T. E.T.

OIL IN

Inside View of Dehyd. / Desalter

Water Out

H. VOLTAGE EI. GRID OIL

WATER



Dehydrator & Desalter



Desalting

The major components of a wet crude train are:

Wet crude heater Dehydrator Chemical injection pump Wash water system Mixing valves 1st stage desalter 2nd stage desalter Dry crude storage Storage tank



Desalting (cont.)

Wastewater and contaminants are discharged from the bottom of the settling tank to the wastewater treatment facility. The desalted crude is drawn continuously from the top of the settling tanks and sent to the crude distillation tower.



Desalting



Two-stage Desalter



Wash Water System

Wash water is injected into the crude upstream of the two desalters. The purpose of the wash water is to dilute the salt concentration.

The wash water is recycled between the desalting vessels.

Fresh water is used for the second stage desalting.

Salty water from the bottom of the second desalter is recycled upstream of the first desalter to act as wash water.

Wash water is then sent to a tank for disposal.



Sweetening & Stabilization

of Crude Oil

H2S < 10-100 PPM, weight)

Vapor pressure: ethane > H2S > Propane

Stabilization: removal of light HC

Sour crude oil: 0.05 ft H2S/ gallons of oil

RVP: 8-12 PSI

H2S: A POISON hazard



Stabilization using

Fractionation



Crude Cooling & Storage

Dry Crude Coolers

The dry crude from the desalters passesthrough the tubes in the wet crude/dry crudeheat exchanger, then it passes through thecrude after being cooled to drop thetemperature of the crude stream.

Storage Tanks

The dry crude from the second stage desalteris constantly monitored for total salt andwater content.



Crude Cooling & Storage

If it meets specification then it is sent to storage tank.

If it does not then it must be re-processed.

Off-specs oil is sent to wet crude storage for recycling.



Types of Tanks

There are three main types of

tank used for storing liquid

hydrocarbons.

Atmospheric storage

Pressure storage

Heated storage



Atmospheric Storage

Tanks

All atmospheric storage tanks areopen to the atmosphere, or aremaintained at atmospheric pressureby a controlled vapor blanket. Thesetanks fall into two categories:

- Floating Roof Tanks

- Cone Roof Tanks



Cone Roofed Tank

This tank is used for the storage of non-toxic liquidswith fairly low volatility.

The roof of the tank will contain a vent, open toatmosphere, which allows the tank to “breathe” whenemptying and filling.

In oil refining, this type of tanks is used for thestorage of gas oils, diesel, light heating oil, and thevery light lube oils.

Tanks containing flammable material will beequipped with foam and fire water jets locatedaround the base of the roof.

All storage tanks containing flammable material andmaterial that could cause environmental damage arecontained within a dyked area or bund.



Cone Roof Tank



Floating Roof Tanks

Light volatile liquids may be stored at atmospheric pressure by the use of “Floating Roof ”tanks where the roof of this tank literally floats on the surface of the liquid contents of the tank. In this way the air space above the liquid is reduced to almost zero, thereby minimizing the amount of liquid vaporization that can occur.

The roof is specially designed for this service and contains a top skin and a bottom skin of steel plate, held together by steel struts. These struts also provides strength and rigidity to the roof structure.

The roof moves up and down the inside of the tank wall as the liquid level rises when filling and falls when emptying. The roof movement is enhanced by guide rollers between the roof edge and the tank wall.



Floating Roof Tank

When the tank reaches the minimum practical level for the liquid contents the roof structure comes to rest on a group of pillars at the bottom of the tank.

These provide the roof support when the tank is empty and a space between the roof and the tank bottom. This space is required to house the liquid inlet and outlet nozzles for filling and emptying the tank which, of course, must always be below the roof.

The space is also adequate to enable periodic tank cleaning and maintenance.



Floating Roof Tank



Pressure Storage

Pressure storage tanks are used to prevent or minimize the loss of the tank contents due to vaporization. These types of storage tanks can range in operating pressures from a few inches of water gauge to 250 psig. There are three major types of pressure storage. These are:

Low-pressure tanksThese are dome roofed tanks and operate at a pressures of between 3 ins water gauge and 2.5 psig.

Medium pressure tanksThese are hemispheroids or spheroidal which operate atpressures between 2.5 to pressures up to 15 psig.

High-pressure tanksThese are either horizontal “bullets” with elipsoidal or hemispherical heads or spherical tanks (spheres). The working pressures for thesetypes of tanks range from 30 to 250 psig.



Dome Roof Tanks

Dome tanks are used to store high vapor pressure naphtha



Dome Roof Tanks

Pre-Stressed Concrete Tanks



Dome Roof Tanks

A dome roof tank is a closed cylinder witha rounded top. They are used to storehigh vapor pressure hydrocarbons.

Liquefied Petroleum Gas (LPG) tanks areheavily insulated to prevent ambient heatfrom entering the tank. There is stillconstant vaporization (boil off) of some ofthe propane/butane liquid. One volume ofliquid changes to about 200 volumes ofvapor. Therefore, as product boils off itcreates pressure in the tank.



Heated Storage Tanks

Heated storage tanks are more commonin the petroleum industry than mostothers.

They are used to store material whoseflowing properties are such as to restrictflow at normal ambient temperatures.

In the petroleum industry productsheavier than diesel oil, such as heavy gasoils, lube oil, and fuel oil are stored inheated tanks.

dehydration and desalting - unimasr engineering program (pce) –petroleum field operations (che...

Documents