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Liquid Stabilization

Stabilization.Removal of Gaseous Components of Liquefied Process Fluid.

Done by Stripping or Heating.

Produces Stable Liquid To Satisfy Gas Line TransportSpecification

To Meet Storage TemperatureRequirement

To Obtain Additional Revenue.

Product Vapour Pressure Must NOT beGreater than the Storage Pressure atthe Maximum Storage Temperature

Liquid TVP = C . RVPTVP is a Function of Composition soBelow is Approximation

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FractionationRecovery of Max. HC Liquid Stable UnderStorage Condition with Minimum Vol. Of Solution Vapour RemovedThis is Achieved by Fractionation

Separation of Raw HC Liquid intoits Components in Series of Columns or Towers.Bottom Component is C5+(Natural Gasoline)

Stabilizer

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Liquid Stabilization Unit with LTS

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Refrigeration Applications

Refrigeration Applications. Propane Liquefaction.

NGL.

LNG.

Recovery of Liquid from Stock TankVapour.

Low-Temperature Separation.

Well Stream Must be Rich ofHydrocarbons.

Note that a Btu of Heat SubtractedFrom a System by RefrigerationRequires More Work to Achieve thana Btu Supplied To System byHeating.

Refrigeration Systems Must beTotally and Carefully Insulated.

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Dehydration

. HYDRATESDefinition. Hydrates Are Crystalline(ice-like)

Compounds Formed by Combination of Water and Hydrocarbons Under Pressure at Considerable Higher Temperature Than Water Freezing Point.

Hydrates OccurrenceIn Pipeline.In EquipmentValves.Regulators.ChokesIn Formations as Hydrate

Rock.

DefinitionRemoval of Water or Water Vapor

Reasons for Water RemovalWater and Natural Gas(Hydrocarbons) Form Hydrates.(Large Quantity of Free Water Is Needed for This Formation )

Natural Gas Containing H2O Combined With CO2 /H2S is Corrosive.

Condensed H2O From Natural Gas Causes Slugging Flow Conditions.

H2O Increases Fluid Flow Volume and Pipeline Capacity.

H2O Decreases Natural Gas Heating Value.

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Hydrates Crystal . The Water Framework Though is

Ice-like, It Has Void Space and It is Weak.

The Hydrocarbon or “Guest” Molecules are Held Together by Weak Bonds Within the Void of the Crystalline Network or Structure of the Water to Stabilize the Water Structure.

The Water Framework Holds the Hydrocarbon Molecules in a Void Space or Network.

Hydrocarbons in Hydrates. Methane CH4 . 7H2O

Ethane C2H6 . 8H2O

Propane C3H8 . 18H2O

Butane C4H10 . 24H2O

Non-Hydrocarbons in Hydrates.CO2 CO2 . 7H2O

H2S H2S .6H2O

The Hydrate CrystalThe Water or “Host” Molecules are Linked Together by Hydrogen Bonds Into Cage-like Structures Called Clathrates.

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Hydrate Crystalline Structure Two Basic Structures:

Structure II Diamond

Structure I Cubic or Body-Centered

Smaller Hydrocarbon Molecules (C1,C2,CO2, & H2S) Form More Stable and Cubic Structures.

Larger Hydrocarbon Molecules (C3 & iC4 ) Form Less-stable and Diamond Structures.

Molecules Larger Than C4 Cannot Form Hydrates Because They Cannot Fit Into the Cavity in the Water Molecule Structure.

Hydrate Crystalline Structures. .

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Properties of Hydrates Physical Properties of Hydrate Structures STRUCTURE I STRUCTURE II Lattice Shape Body-Centered Diamond

Cubic Stability More Stable Less Stable

Water Molecules per Unit Cell 46 136

Cavities per Unit Cell Small 2 16 Large 6 8 Typical Gases That Form in Each Cavity Methane* Propane** of this Structure Ethane* I-Butane** H2S n-Butane** CO2 neo-Pentane** * Small **Large

They Have Fixed Chemical Composition BUT No Chemical Bond

They Behave Like Chemical Compounds.

They Are Physically Like Ice or Wet Snow Crystals but Do Not Have Solid Structure of Ice.

They Have Less Density Than Ice.(SG 0.96 – 0.98)

They Sink in Liquid Hydrocarbons and Float in Water.

They Contain 90% Water by Weight

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Conditions Necessary for Hydrates Formation

Hydrate Formation Temperature Temperature Below Which Hydrates

Will Form at a Particular Pressure.

They Form at Hydrate Temperature of the Gas and NOT That of the Component Gases.

The Hydrates Formed Are Mixtures of the Hydrates of the Component Gases Rather Than Hydrate of the Natural Gas.

Presence of Free Water.

Natural Gas at or Below its Water Dew Point.

Operating Temperature Below Hydrate Formation Temperature for That Pressure And Fluid Composition.

Operating at Higher Pressure That Increases the Hydrate Formation Temperature.

Presence of Small Hydrate Crystal.

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Conditions Necessary for Hydrates Formation

Operating at High Velocity or Agitation Through Equipment and Pipe Network.

Turbulence Encourages Hydrate Formation; Hence Their Presence Mostly Downstream of Valves, Regulators, Orifice Plates, Chokes Sharp Bends, Elbows, etc. and Upstream of these Devices if Flow is Turbulent and Temperature is Low.

Crystal Formation Sites Pipe-Elbows, Orifice Plates, etc.(Where Full Force of Stream Flow Cannot Prevent Hydrate Build-up).

Presence of Scales and Solid Corrosion Products.

Hydrates Form at Gas-water Boundary With the Forming Molecules Coming From the Solution.

Parameters Such as High Temperature That Encourages High Solubility Enhances Hydrate Formation.

Contaminants Such as H2S and CO2 are More Soluble in Water Than Hydrocarbon and as Such, More Conducive for Hydrate Formation.

Very High Solution GOR Encourages Hydrates Formation Due to High Gas Molecules Presence

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Effect of GOR on Hydrates Formation

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Hydrates Formation Prediction Parameters Predicted

Temperature OR Pressure

at Which Hydrates Will Form.

Katz’ Gas Gravity Method. Uses Gas Gravity, Pressure and

Temperature.

It is Simple but Only an Approximation.

Values Excellent for Methane and 0.7 or Less SG Natural Gas.

Not Good for Pipeline Gases.

Less Accurate for Natural Gas With SG Between 0.9&1.0 Useless for Streams With Sulfur Compounds and/or Larger Molecules.

Procedure Given Gas Gravity and

Temperature or Pressure

Hydrate Formation Pressure or Temperature is Got From Katz Graph

If Gas Composition Fractions are Given, SG is then Calculated B4 Going to Graph

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Katz Pressure-Temperature Curves for Hydrates Formation Prediction

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Katz Hydrate Formation Condition Estimation Method

Example 4-3 Estimate Hydrate Formation

Temperature of Natural Gas With the Composition Shown Below at 1000 psia.

Component Mole %

N 10.1 C1 77.7 C2 6.1 C3 3.5

i-C4 0.7 n-C4 1.1 C5+ 0.8 (Assume C6)

Step 1 Compute the Specific Gravity Component Mole % MW Z.MW N 10.1 28 2.83 C1 77.7 16 12.43 C2 6.1 30 1.83 C3 3.5 44 1.54 i-C4 0.7 58 0.41 n-C4 1.1 58 0.64 C5+ 0.8 86 0.69 100.0 20.38 SG = 20.38/28.9625 = 0.7 . Step 2 Read Hydrate Formation

Temperature From Katz Curve Hydrate Formation Temperature = 65 0F

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Equilibrium Constant(K-Value)Hydrate Formation Estimation

Method Parameters Employed

Vapor-Solid Equilibrium Constant (K-Values)

Pressure Temperature.

Employs Calculations Similar to Vapor-Liquid Dew-Point Calculations with K-Values.

Has Vapor-Solid K-Values for Methane C1, Ethane C2, Propane C3, Iso-Butane I-C4 Neo-Butane n-C4, Carbon Dioxide CO2 and Hydrogen Sulfide H2S

Estimation Procedure Given Gas Composition,

Estimate Hydrate Formation Temperature.

Determine K-values for Components at Estimated Hydrate Formation Temperature.

Compute Zi/kv

Repeat Above Steps Until Zi/kv = 1

Temperature at Which Zi/kv = 1 is Hydrate Formation Temperature

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Equilibrium Constant(K-Value) Curves

Vapor-Solid K-Value for Methane

Vapor-Solid K-Value for Ethane

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Equilibrium Constant(K-Value) Method Calculation

Example 4-4 Determine the Hydrate Temperature of Example 4-3 Using Equilibrium-

Constant Method.

Component. Mole % KV (650F) Z/ KV. KV(630F) Z/ KV. N 10.1 0.0 0.0 C1 77.7 1.24 0.627 1.22 0.636 C2 6.1 0.90 0.068 0.77 0.079 C3 3.5 0.26 0.135 0.18 0.194 i-C4 0.7 0.007 0.065 0.08 0.087 n-C4 1.1 0.0 0.0 C5+ 0.8 (Assume C6) 0.0 0.0

100.0 0.895 0.996 Hydrate Formation Temperature = 630F

Note That K-values for n-Butane Above 55 0F and Non-hydrate Formers =

.

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Baillie and Wichert Method. Used Mostly to Predict Hydrate

Formation Temperature of Acid Gases

Range of Application Total Acid Gas Content: 1–70%

H2S Content 1 – 50%

H2S/CO2 Ratio 1:3 – 10:1

Correction Has to Be Made for C3 Content.

Chart Good for C3 Content Up to 10%

Estimation Procedure For the Above Gas at 1000 psia Compute SG As in Example 4-3

Above.(SG = 0.7)

Enter Fig. 4-35 at 1000 psia

Move Horizontally to 0% H2S

Descend Vertically to the Horizontal (SG = 0.7)

Follow Sloping Lines to the Horizontal Bottom Temperature Scale.

Read off the Hydrate Temperature = 62 0F

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Baillie and Wichert Method.• .

62

3

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Baillie and Wichert Method. Determine Correction for C3.

Interpolate For C3 = 3.5 Position on C3 Adjustment Chart.

Enter Chart at H2S = 0%

Descend Vertically to 1 X 103 psia Line

Move Horizontally to Read the Correction. = +3 0F

Add Correction. Hydrate Temperature = 62 + 3

= 65 0F

Other Methods of Hydrates Formation Temperature Estimation

Pressure -Temperature Curves by Gas Processors Suppliers Ass.

Hydrates Formation Curves for Gases Undergoing Expansion

by Gas Processors Suppliers Ass.

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Hydrates Control and Prevention High Stream Flow Rate.

Reduction of H2S and CO2 Content.

Keep Lines and Equipment Dry of Liquid Water.

If Water Must Be Present, Stream Must Flow at Above Hydrate Formation Temperature.

Application of Heat.

Dehydration

If Stream Must Have Water and Must Flow at Low Temperatures, Then Inhibitors Must Be Injected.

Inhibitors . Materials Added to Water to

Depress its Freezing and Hydrate Forming Temperature.

Inhibitor Temperature Range

Methanol Any

Di-Ethylene Glycol (DEG). -10 0F

Ethylene Glycol (EG) -10 0F

Tri-ethylene Glycol (TEG) -10 0F

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Inhibitor Concentration in Water Phase

W = D. M 100 Ki + D . M

W = Weight % Inhibitor in the Water Base M = Mol. Weight of Inhibitor

D = 0C (0F) Depression of Hydrate Point. Ki = Constant

Inhibitor Ki

Methanol, 1297 (0C) or 2335 (0F)

Glycols 2220 (0C) or 4000(0F)

Problems Caused by Hydrates Hydrates in Flow Line Reduces

Well Head Pressure.

Hydrates Can Block Flow Line and Equipment.

Hydrates Can Constrict Equipment Surface Lines and Flow Strings.

Fouling of Heat Exchangers.

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Effect of DEG on Hydrate Formation

Freezing Point of DEG Glycol

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Natural Gas Water Content NG Contains Some Degree of Water at

ALL Conditions Water Content of Oil and Gas

Expressed in STB(water)/STO(oil) for Oil

lb(water)/MM SCF(NG) for Gas

McKetta and Wehe Correlation Chart

Solubility of Water in Gas Increases with Increasing

Temperature Decreases with Increasing Pressure Dissolved Salt in Water Reduces

Solubility of Water in Gas

Dew Point Temperature at Which Natural Gas is

Saturated with Water Vapour at a Given Pressure

Water Vapour is in Equilibrium at Dew Point.

Reduction of Temperature OR Increase of Pressure Will Result Water Condensation

Dew Point Depression Difference in Dew Point Temperature

of Water Saturated Natural Gas Before Dehydration and After Dehydration

DPD = DP(Before) - DP(After)

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McKetta and Wehe Correlation Chart

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