1. LPG PRODUCTION FROM NATURAL GAS

2. LPG RECOVERY FROM NATURAL GAS
INTRODUCTION
What is LPG or LP Gas?
LPG OR LP Gas is the abbrievation of liquified petroleum gas. This group of products includes saturated hydrocarbons-propane(C3 H8) & butane(C4H10), which can be stored/transported seperately or as a mixture.

3. They exist as gases at normal temperature and atmospheric pressure.
They liquefy at moderate pressures, readily vaporising upon release of pressure, the property which permits transportation and storage of LP Gas in concentrated liquid form.
60% of LPG in the world today is produced from natural gas while 40% is extracted from refining of crude oil.

4. PHYSICAL PROPERTIES OF LPG.
It is colourless and cannot be seen.
Since it is odourless LPG is odorized by adding an odorant to detect any leak.
It is slightly heavier than air and hence upon leaking, will flow to lower lying areas.
In liquid form, its density is half that of water and hence it floats initially before it is vaporised.

5. It is non-toxic but can cause asphyxiation in very high concentrations in air.
Note:
LPG expands upon release and 1 litre of liquid will form approx. 250 litres of vapor.

6. COMBINATION OF PROPANE AND BUTANE
Properties of propane and butane are similar enough to be in useful mixtures.
Both are saturated hydrocarbons. they do not react with each other.
Butane is less volatile and boils between
-9.00C to 0.60C propane is morevolatile and boils at –42 0C.

7. Both products are liquids at atm
Both products are liquids at atm.pressure when cooled to temperatures lower than their boiling points.
Calorific values of both are almost equal.( 12000kcal/sm3)

8. LPG RECOVERY FROM NATURAL GAS-CRYOGENIC PROCESS.
Cryogenic gas plants are those which operate at very low temperatures upto –70 to –1000 C. When gas is chilled to this temperature, most of the ethane and practically all of the propane and heavier hydrocarbons will liquefy.

9. The liquids formed can then be seperated by a series of fractionation towers into ethane, propane, butanes and natural gasoline.
Refrigeration absorption plants are another means of recovering ethane and heavier components from gas.

10. NATURAL GAS COMPOSITION
COMPONENT VOL.%
METHANE C
ETHANE C
PROPANE C
BUTANES C
GASOLINE C
CO2 & N
H2S

11. COMPARISION OF RECOVERY FROM
CRYOGRNIC REFRIGERATION
Recovery.
60-90%
90-98%
100%
ABSORPTION REFRIGERATION
Recovery.
ETHANE %
PROPANE 70-90%
BUTANE 100%
NATURAL
GASOLINE 100%

12. CONCLUSION
You can see from the comparision that the main difference is in the ethane recovery.
Ethane is a useful feedstock in petrochemical plants.
With methane,it releases 75% more heat than methane when used as fuel gas. But ethane is worth more as petrochemical feed stock.

13. FUNCTIONAL SECTIONS OF A CRYOGENIC PLANT
Cryogenic plant has four functional sections.
Dehydration to remove moisture from gas.
Chilling and expansion to liquefy part of the gas.
Demethanizing to remove methane which liquefies.

14. CRYOGENIC OPERATIONS
Compression to boost pressure to sales gas pipeline pressure.
Root the liquid to light end fractionating tower t1 to remove C1 and C2.
Bottoms of t1 to LPG tower to separate LPG and natural gasoline(NGL).
LPG to spheres and NGL to tanks for storage.

15. DEHYDRATION UNIT
Gas contains moisture and all of it is to be removed prior to its temperature reduction to sub zero. Moisture is removed in a dry desiccant adsorption system which is a fixed bed tower with desiccant such as silica-gel, molecular sieves, etc.

16. DEHYDRATION UNIT
Conventionally, in all gas processing industries world wide molecular sieves are being extensively used today for its higher selectivity in the order of H2O, H2S and other heavy hydrocarbons but not CO2.

17. MOLECULAR SIEVES ADSORPTION(FLUID-SOLID)
Gas flows from top to bottom in the fixed bed vertical dryer where moisture gets adsorbed by the molecular sieves which are nothing but alumina silicates in pellets form and reduces the moisture conc. to less than 4ppm.This in turn, suppresses the moisture dew point and maintains it around –700C.

18. ADSORPTION CYCLE
After an optimum time cycle, a reversal in dew point is observed, i.e. as the partial pressure of the moisture content in the gas increases due to saturation, the dew point starts increasing towards positive side which means the dew formation will take place at much higher temperature when gas is chilled to –500C leading to ice formation choking the pipe line.

19. REGENERATION OF THE DESICCANT
After an optimum time period of about 10 hrs the tower is changed to stand by one and the former will be regenerated by hot fuel gas and cooled to normal temp. This cycle takes about 10 hrs.

20. EXPANSION-COMPRESSION PROCESS
The turbo expander-compressor is a devise for chilling the inlet gas and re-compressing the outlet gas. The expander is simply a gas turbine. Gas enters at a high pressure and drives the expander with its own kinetic energy, expands to lower pressure and thus gets chilled to as low as –700C resulting in condensation of the gas partially.

21. The compressor is a conventional centrifugal type that is on the same shaft as the expander. The energy required to drive the compressor is the same as the energy given up by the gas passing through the expander.

22. THEORY OF GAS EXPANSION
Gas at a temperature above absolute 0 and a pressure about absolute 0 contains internal energy. We cant see the energy but we know it exists because it produces work. Energy is contained in the gas in the form of heat, pressure ,or velocity.

23. One of the basic laws of science(first law of thermodynamics) is that energy contained in a fluid cannot be either created or destroyed. Thus, if energy is added to or removed from a gas, there must be a change in heat content(temperature) or pressure or velocity.

24. JOULE-THOMPSON EFFECT
Gas in a cryogenic plant has been cooled to a lower temp. of about –400C by reducing the pressure from say 60kg/cm2 to 30kg/cm2 by using a choke or a pressure reduction valve.
The internal energy of the gas does not change when its pressured is lowered because no heat is entering or leaving the

25. Pressure reduction valve, and no work is being done to turn a turbine or other devise, so the internal energy of the gas has to remain the same.
The temperature drop which occurs when the pressure is reduced is called the joulethompson (j-t) effect.
Note:
In a typical expansion-compression process, of the total heat removed from the gas, 10-15% occurs in the expander and for the balance removal takes place in the heat exchangers.

26. If the latent heat to be further removed we have to go for additional refrigeration in addition to exp-comp.
The liquid which condenses when the inlet gas is chilled is a mixture of methane, ethane, propane, butane and gasoline(C5+). The methane whose volume is maximum, is to be removed from the others for them to be of commercial value.

27. LIGHT END FRACTIONATION TOWER.
In this fractionation tower methane and ethane are boiled out and goes out as top product at a set temp. and pressure while the remaining goes out as bottom product or feed stream to next fractionation tower. Efficient operation ensures minimum loss of propane from the top. Heat duty to boil out the top streams is supplied by medium pressure steam in the reboiler.

28. PARAMETERS AND CONDITIONS PREVAILING IN LEF TOWER:
TOP PRESSURE: 27.0 KG/CM2
TOP TEMP : C
BOTTOM TEMP : C
NO. OF TRAYS : 35 BUBBLE CAP
TOP PRODUCT ANALYSIS (BY VOL.)
METHANE 65%
ETHANE %
PROPANE %
REMAINING: % CO2.

29. Over head condenser is a shell and tube partial con-denser to condense only heaviers and use propane as the chilling medium.
Reboiler is of kettle type which maintains liquid level on a level control valve allowing the excess liquid to the next tower which is called the LPG fractionator. M. P. steam is used as the heating medium.

30. LPG FRACTIONATION TOWER.
In this section by setting appropriate temp. and pressure at the top and bottom of the tower recovery of maximum propane and butane mixture is facilitated. The by-product from the bottom is called natural gasoline or NGL.
The heating media in the reboiler is M. P. steam and the reboiler type is thermo syphon where all the liquid is vaporised and put back into the column.

31. PARAMETERS IN THE LPG TOWER
TOP PRESSURE: KG/CM2
TOP TEMP : C
BOTTOM TEMP.: C
NO.OF TRAYS : 35(BUBBLE CAP)
ANALYSIS OF THE PRODUCTS
LPG PRODUCT(TOP): C %
C %
BOTTOM PRODUCT(NGL): C5+
C6,C7,…

32. PRODUCT STORAGE
LPG storage: spheres since it is stored in vapor-liquid form the stresses exerted by the fluid will be distributed uniformly radially all along the spherical surface.
storage pressure: 10.5kg/cm2
temperature : C
Storage of NGL: floating roof tanks at 1.0 kg/cm2 and 30.00C

33. ADDITIVES: MERCAPTANS
Since LPG is odourless some additive is to be administered for any leakage.
Any sulphurous compound in controlled quantities is added such as: RSH where ‘R’ is any methyl group, ‘S’ is sulphur and ‘H’ is hydrogen. qty: approx 30 ppm

34. CRITICAL COMMERCIAL PROPERTIES OF C3-C4
PROPERTY PROPANE BUTANE LPG
(50%MIX)
SP.GR-LIQ
AT 15o C
SP.GR-VAP
VAP.PR(38DEG)
BOILING.PT(DEG)
(AT ATM.PR)
AUTO IGN.TEMP
LAT.HEAT.VAP(BTU/LB)

35. REID VAPOR PRESSURE OF LPG
Vap.pr. of LPG is important and is found by Reid’s VAP. pressure apparatus.
RVP(reid vap.pr.) of LPG:
at 650C :10-26 kg/cm2
at 380C : kg/cm2
Volatility :this is also called weathering and done at 95% evaporation of a sample of LPG and it should fall in the range of –2.0 to +2.00C for optimum evaporation.
Calorific value of LPG:12000kcal/SM3.

36. LPG FROM NATURAL GAS- ITS CHALLENGES
The production and consumption of natural gas is on the rise world wide as a result of its wide availability, ease of transportation and use and clean burning charecteristics. The emerging commodity nature of natural gas has created tighter competition among the natural gas processors .

37. Processing rights, resulting in increasingly narrow operating margins between the processing costs and the market price for which the recovered liquids can be sold.
Carbon dioxide(CO2 ) is a particularly trouble some contaminant found in natural gas. Many NGL recovery processes require CO2 removal to avoid solid ice formation in the cold sections of the processing plant.

38. Instead of going for the CO2 removal equipment, which will increase the equipment cost and the operating cost, CO2 tolerant process are actively envisaged in the NGL and LPG recovery facility.

39. Within the liquids recovery facility, there are both operating costs and operating flexibility issues that directly impact the processing costs.
While it is easily recognized that the efficiency of the selected liquids recovery process is an important factor in the processing cost.

40. Process to either recover or reject ethane without sacrificing the efficiency or propane recovery is often the critical factor in determning the profitability of a gas processing plant.
With the historically cyclic nature of ethane values as a petrochemical feed stock, it is absolutely vital for the gas processors to be able to quickly respond to the changing market conditions.

41. To maximise profits. When the value of the ethane as a liquid is high, maximum ethane recovery for the gas processor gives more income. When the value of ethane as a liquid is low, selling the ethane in the residue gas for its BTU value gives higher income, so efficient rejection of ethane with out sacrificing propane recovery is the key to plant profitability when the liquids market is depressed.

44. Thank you