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

Water Forms 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

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 Exchangers Hot oil Heater Hot Diesel Dry Crude From Desalters Wet Crude From GOSP Heater Wet Crude to Dehydrator & Desalter Dry Crude Wet Crude Heaters Diesel Oil Ret.

Wet Crude Heaters

Scale Inhibitor Injection Scale inhibitor is injected into the wet crude before preheating, salt water tends to form hard scale deposits 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.

Dehydration 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 suspended water, the tightly bound droplets of salt water that remains in the crude stream must be removed in the first and second stage desalters.

Dehydrator & Desalter Oil Outlet E.T. E.T. E.T. H. VOLTAGE EI. GRID OIL WATER OIL IN Inside View of Dehyd. / Desalter Water Out

Dehydrator & Desalter

Desalting The major components of a wet crude train are: Wet crude heater Dehydrator Chemical injection pump Wash water system Mixing valves 1 st stage desalter 2 nd 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 H 2 S < 10-100 PPM, weight) Vapor pressure: ethane > H 2 S > Propane Stabilization: removal of light HC Sour crude oil: 0.05 ft H 2 S/ gallons of oil RVP: 8-12 PSI H 2 S: A POISON hazard

Stabilization using Fractionation

Crude Cooling & Storage Dry Crude Coolers The dry crude from the desalters passes through the tubes in the wet crude/dry crude heat exchanger, then it passes through the crude after being cooled to drop the temperature of the crude stream. Storage Tanks The dry crude from the second stage desalter is constantly monitored for total salt and water content.

Crude Cooling & Storage If it meets specification then it is sent to storage tank. If it does not then it must be reprocessed. 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 are open to the atmosphere, or are maintained at atmospheric pressure by a controlled vapor blanket. These tanks fall into two categories: - Floating Roof Tanks - Cone Roof Tanks

Cone Roofed Tank This tank is used for the storage of non-toxic liquids with fairly low volatility. The roof of the tank will contain a vent, open to atmosphere, which allows the tank to breathe when emptying and filling. In oil refining, this type of tanks is used for the storage of gas oils, diesel, light heating oil, and the very light lube oils. Tanks containing flammable material will be equipped with foam and fire water jets located around the base of the roof. All storage tanks containing flammable material and material that could cause environmental damage are contained 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 tanks These are dome roofed tanks and operate at a pressures of between 3 ins water gauge and 2.5 psig. Medium pressure tanks These are hemispheroids or spheroidal which operate at pressures between 2.5 to pressures up to 15 psig. High-pressure tanks These 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 with a rounded top. They are used to store high vapor pressure hydrocarbons. Liquefied Petroleum Gas (LPG) tanks are heavily insulated to prevent ambient heat from entering the tank. There is still constant vaporization (boil off) of some of the propane/butane liquid. One volume of liquid changes to about 200 volumes of vapor. Therefore, as product boils off it creates pressure in the tank.

Heated Storage Tanks Heated storage tanks are more common in the petroleum industry than most others. They are used to store material whose flowing properties are such as to restrict flow at normal ambient temperatures. In the petroleum industry products heavier than diesel oil, such as heavy gas oils, lube oil, and fuel oil are stored in heated tanks.