Natural Gas Processing I Chapter 1 Fundamentals PTRT 1317 Natural Gas Processing I Chapter 1 Fundamentals

Fundamentals Fluid Understanding fluids is critical any substance that can flow No definite shape Assumes the shape of the container Gases AND liquids are both fluids Even solids can be fluids (fluidized bed reactors, grain bins, etc) Understanding fluids is critical Solving operating problems Increasing operating efficiency Avoiding operating hazards

Important Questions When a process change is planned What is the fluid? Composition Liquid, gas, or solid What are the properties of the fluid Numerous physical and chemical properties What are we attempting Increase temperature or pressure Remove trapped moisture What are the present conditions of the gas Temperature, pressure, volume, composition

Fluid properties Properties distinguish one fluid from another Temperature Pressure Gravity (density) Miscibility Solubility Phase Vapor pressure Boiling point Freezing point Hydrate point Heat capacity Flammability Composition All result in one way or another by the structure of fluids.

Fluid Composition (Structure) Everything composed of atoms Atoms often arrange in groups (molecules) Fluid contains ~1023molecules Behavior of fluid defined by behavior of the molecules Simplest fluid theory (works in most cases quite well) – marbles in a jar Make the molecules do what you want and the fluid will do what you want.

Temperature Usually degrees Fahrenheit (F) Critical temperature (Tc) – if actual temperature is GREATER than Tc then phase of fluid (gas or liquid) depends on pressure Absolute temperature needed for calculations – degrees Rankine R = 460 + F

Pressure Force per unit area – lbs/square inch (psi) Pressure in pipe or vessel is usually read with a gauge – psig Not the true (absolute) pressure Difference between pressure and atmosphere Absolute pressure – psia MUST be used for calculations involving gases PSIA = PSIG + 14.7psi

Critical Pressure (Pc) Pressure above which the gas will NOT condense into a liquid Important in gas processing when separating gases If pressure is above Pc for either component of a mixture of two gases then they cannot be separated by condensation.

API Gravity = (141.5/specific gravity) – 131.5 Density – weight per unit volume Specific gravity – ratio of the fluid density to that of water at 60F (for liquids) or air at standard conditions (for gases) API Gravity measured in degrees, lighter the liquid the higher the API Gravity API Gravity = (141.5/specific gravity) – 131.5

Immiscible or Miscible Two liquids that do NOT generally mix Lighter liquid floats on top of heavier liquid (after enough time for them to separate) Miscible liquids DO mix together and will not separate even if they have different values of API Gravity All hydrocarbons in Gas Processing are miscible. Hydrocarbons are immiscible with other fluids used (water, MEA, or glycol)

Solubility Two immiscible fluids (gasoline and water) Some of the water (small) will dissolve in the gasoline. Some of the gasoline (small) will dissolve in the water Solubility of water in gasoline is ½ lb of water per 1000 lb gasoline This small amount can cause freezing and other processing issues. All immiscible liquids will have some solubility which must be dealt with during processing

Phase Solid, Liquid, or Gas (vapor) Gas – billions of molecules moving rapidly in all directions. Colliding with each other and the walls of the vessel holding the gas Force of the collision with the walls of the vessel gives rise to pressure To reduce the pressure: Decrease the number of particles reduce the force of the collisions (decrease temperature) spread the collisions across a larger area (increase volume)

SOLIDS FLUIDS Liquid Phase Solid Phase Gas Phase

Phase Transition Temperature at which substance changes phase solid ↔ liquid (thawing, freezing) Liquid ↔ gas (boiling, condensing) solid ↔ gas (sublimation, deposition{frost}) Transition temperature depends on the pressure. Phase Diagram plots boundaries between different phases

Kinetic Theory of Gases Review Assignment #1. Temperature increase speeds up the velocity of the molecules. Decrease slows down their velocity velocity is proportional to Volume Increase takes longer for particle to hit the wall and spread across larger area – pressure decreases Pressure = Force / Area

Vapor pressure Equilibrium between vapor and liquid in a closed vessel Some molecules escape the surface Some are trapped on the surface Changes depend on: Substance Temperature Review vapor pressure simulator at: http://www.wisc-online.com/objects/index_tj.asp?objID=GCH4304

Vapor Pressure (cont.) Pressure is usually controlled in distillation and fractionation towers Liquid boils when vapor pressure equals the pressure inside the vessel Thus to boil a liquid the temperature must be increased if pressure is fixed To stop boiling the temperature must be reduced For each liquid (at each pressure) there is a temperature at which the liquid will boil = boiling point. At each temperature there is a pressure (the vapor pressure) at which the liquid will boil

Volume Expansion Free expansion of a liquid as a result of temperature increase. Critical for light liquids Requirement of head space requirement when filling vessels. Gas Processors Association requires a 20% outage (head space volume) for T above 0 ⁰F

Freezing Pure hydrocarbons not typically an issue because freezing temperature is low Free water will often cause freezing problems at moderately low temperatures Hydrates can form at temperatures as high as 80 ⁰F Hydrates are an icy mixture of hydro-carbons and water. Can plug systems Glycol or methanol injection can inhibit formation. Avoid free water using careful operations is best

Hydrates

Hydrocarbons of Interest

Physical Properties Specific gravity relative to water at 60 ⁰F or air at 14.7 psi and 60 ⁰F Lighter than air has Specific gravity < 1.0 Methane lighter than air Boiling point depends on pressure Vapor pressure vs boiling temperature next slide

Physical Properties of other fluids

Mole Percent Mole is weight in pounds numerically equal to the molecular weight of the substance Ratio of number of moles of one gas to the total number of moles Multiplied by 100 to yield % Mole percent and volume percent are the same for gases but not for liquids Example: methane molecular weight is 16. Therefore 1 mole of methane is 16 lbs.

Mixtures of gases – Mole % Avagadro’s Number is 6.022 x 1022

Heat Heat = Transfer of energy Three phases of matter (solid, liquid, gas) depend on the amount of heat energy transferred to the substance British Thermal Unit (Btu) 1.0 BTU = amount of heat required to raise 1 lb of WATER by 1 ⁰F

Heat Sensible Heat = Heat energy flowing in (out) that causes temperature to rise (fall) NO change of state (phase) occurs Latent Heat = Heat energy that causes a phase transition NO temperature change Latent heat values are usually MUCH larger than sensible heat values

Sensible and Latent Heat

Heat Transfer Temperature difference CAUSES heat to transfer (analogous to falling down a hill) Also depends on how much substance you have. e.g. 1 pail of water takes less heat to cause a 1 ⁰F increase than 1 swimming pool of water. Must have temperature difference AND sufficient quantity of cooling fluid

Combustion Process of burning (oxidizing) Helpful combustion Heaters Boilers Gas turbines and internal combustion engines Understanding combustion helps avoid the harmful effects of unwanted fires Three essential requirements Supply of oxygen Combustible fuel Heat energy (ignition)

Combustion control Heaters and boilers – air supply Engines – fuel supply Efficiency Incomplete combustion Dry-gas loss (stack loss) Analysis of off gases (flue gas) to look for products formed by combustion Oxygen content less than 2.5% Increase amount of excess air

Combustion control (cont.) Analysis of off gases (flue gas) to look for products formed by combustion Dry gas loss Oxygen content greater than 5% Excessive heat lost to stack Excess fuel consumption for no additional Btu value Reduce excess air supply

Combustion control (cont.) Extinguishing Unwanted Fires Cut off fuel supply Cut off supply of oxygen (smother) Cut off supply of heat (cooling) Fuel Properties Flammable range (typically between 1% and 15%) Increasing pressure with normally widen the flammable range Flash point – minimum temperature that produces minimal vapors to “flash” but not burn continuously Fire point – minimum temperature that produces enough vapors to support combustion

Applications Light liquids Heavy Liquids Low boiling temperatures High vapor pressures Vaporize easily Heavy Liquids High boiling temperatures Low vapor pressures Do not vaporize easily

Volatility Describes the ease with which you can vaporize a fluid Plant operations dictated by relative volatility of the fluids being processed Most processes involve mixtures of fluids composed of individual hydrocarbons Mixture in liquid AND vapor phase Amount present in each dictated by volatility

Fluid Mixture Equilibrium Mixture of several fluids has a range of boiling points Initial boiling point = temperature at which the fraction with the lowest boiling temperature boils (vaporizes) Final boiling point = temperature at which the fraction with the highest boiling temperature boils. At any given temperature between these two extremes some of the fluid will be in the vapor phase and some will be in the liquid phase

Fluid Mixture Equilibrium Vapor Light Molecules Vaporize Liquid Heavy Molecules Concentrated Heavy Fluid Molecules Light Fluid Molecules

Describing Gas Processes Many pieces of equipment working as a unit. Connection of this equipment normally illustrated with a Process Flow Diagram (PFD) Various symbols are used to describe pieces of equipment. We will use the more common items in this class

PFD Symbols

PFD Symbols

PFD Symbols

PFD Symbols

Simple Separation System Cooling Water Injection Fire Separation Required

Analysis Improvements needed Open flame is a hazard Replace flame with a shell and tube heat exchanger (reboiler) Separation of water is a problem Replace injection with a shell and tube heat exchanger (condenser) Heavy component contaminates light component Use staged separation

Simple Separation System Required Cooling Water Injection Reboiler Steam Heavy Liquid

Simple Separation System Hydrocarbon Separation Required Steam Heavy Liquid Cooling Water Condenser

Separation System Steam Heavy Liquid Cooling Water Condenser

Separation System System shown has too many pieces of equipment and is excessively complex Tower with trays will reduce complexity Counter-flowing gas with liquid through bubble caps improves heat exchange and separation efficiency

Rising Vapor Rising Vapor Boiling Liquid Downcomer

Operating Guidelines Operation is a compromise between temperature control and flow control Difficulties arise when bottom (liquid) and top products (vapor) are both on temperature control. Most systems: Reboiler heat input on bottom temperature control Reflux on flow control Bottom product is usually worth more than the top product so to maximize the volume of bottom product some top product is allowed up to the maximum allowable by specification Pressure control will be covered later