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

2. 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

3. 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

4. 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.

5. 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.

6. 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 = F

8. 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 psi

9. 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.

10. 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

11. 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)

12. 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

13. 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)

14. SOLIDS
FLUIDS
Liquid Phase
Solid Phase
Gas Phase

15. 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

16. 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

17. 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:

19. 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

21. 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

22. 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

23. Hydrates

24. Hydrocarbons of Interest

25. 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

27. Physical Properties of other fluids

28. 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.

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

30. 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

31. 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

32. Sensible and Latent Heat

33. 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

34. 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)

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

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

42. 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

43. PFD Symbols

44. PFD Symbols

45. PFD Symbols

46. PFD Symbols

47. Simple Separation System
Cooling Water
Injection
Fire
Separation
Required

48. 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

49. Simple Separation System
Required
Cooling Water
Injection
Reboiler Steam
Heavy Liquid

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

51. Separation System
Steam
Heavy Liquid
Cooling Water
Condenser

52. 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

54. Rising Vapor
Rising Vapor
Boiling Liquid
Downcomer

56. 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