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NGL Fractionation

Oil and Gas Processing Project

mohamed nour el din

on 11 May 2011

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Transcript of NGL Fractionation

Ahmed Said Hamza
Alaa Epead
Mina William
Mohamed Nour El Din
Rami Mohamed Introduction to gas processing From reservoir Oil wells ‘associated gas’

Gas and Condensate wells ‘non-associated gas’ Raw gas Methane is associated with
1. Oil
2. Water and water vapor
3. Other hydrocarbons (NGLs)
4. Hydrogen Sulfide Oil out Horizontal or vertical separator

Mechanism of gravitational force Water out Absorption by glycol

Adsorption by solid-desiccant NGL also out Absorption by lean absorbing oil

Refrigeration Sour is bad sweet is good Sweetening by amine solutions to remove hydrogen sulfide Finally!!! Pure Natural Gas Now the processed gas is pure natural gas, mostly methane, and ready for consumer use
Purpose of NGL separation and fractionation Avoid liquid formation

Economic Incentive NGL SEPARATION Mechanical Cryogenic Refrigeration Lean Oil absorption a process in which the NGL's are removed by contacting the natural gas with a liquid hydrocarbon solvent (oil)

the resulting rich oil is subjected to a distillation process to separate NGL's for sale and recycling the regenerated lean oil to the absorber column

Rich oil is sent to a stripper where the absorbed vapours are removed by heating the rich oil, vaporizing the absorbed vapours

The vaporized vapours are liquefied and transferred to storage

liquid recovery: 99% of the butane , 65-–75% of the propane, 15-–25% of the ethane Disadvantages Expensive because it consumes a large amounts of fuels

Difficult to predict the efficiency of the system in removing the liquids from the gas as the lean oil deteriorate with time

So complicated system to be operated

Cannot recover ethane and propane effectively, where it requires circulating large amounts of absorption oil Solid Bed adsorption This method uses adsorbents that have the capability to adsorb heavy hydrocarbons from natural gas

The adsorption process is particularly applicable to lean gas streams containing relatively low concentrations of hydrocarbons heavier than propane

This process is appropriate if the gas is at a high pressure

Silica gel adsorption plants are frequently operated with extremely short cycle times

Plant processing of gas containing 0.192 gal/MSCF of pentanes and higher hydrocarbons Operating precautions Remove all solids and free liquids from the incoming gas stream
Prevent bed agitation at the top of the tower
Gas flow through the bed should be held to about 50% of the design capacity for the first 10 or 12 cycles
Pressure reduction in particular can cause adsorbent breakage
A buffer layer equal to about 5% of the total bed volume should be used on top of adsorbents which fracture in the presence of free liquids. The buffer particle density should be similar to that of the main bed Membrane separation process The components dissolve in the polymer at one surface and are transported across the membrane as the result of a concentration gradient
Based on a high-flux membrane that selectively permeates heavy hydrocarbons and it must be relatively impermeable to methane.
These hydrocarbons permeate the membrane and are recovered as a liquid after recompression and condensation. Advantages 1. Low capital investment
2. No moving parts
3. Flexibility
4. Low environmental impact
5. Reliability
6. Ease of incorporation of new membrane developments Disadvantages 1. A clean feed is required

2. Particulates, and in most cases entrained liquids, must be removed

3. Filtration to remove particles down to one micron in size is preferred

4. There may be a considerable energy requirement for gas compression, because membranes use pressure as the driving force of the process Selection of NGL recovery Process Gas pressure, and residue gas pressure
Liquid products desired
Liquid fractionation infrastructure
Utility costs and fuel value
Plant location
Existing location infrastructure
Market stability
Gas composition
Operation mode Main guide lines for selection of the NGL recovery process 1. In case of sufficiently high pressure

2. When the feed gas pressure is close to the treated gas pressure

3. When the feed gas pressure is clearly below the required pipeline pressure

4. When the feed gas pressure is equal to or lower than the required pipeline pressure Conclusion Solid bed adsorption is much better than self-refrigeration process.

At low feed gas pressure and for strict dew point specifications, economical analysis favours the solid bed adsorption process.

Till now it is not clear for the membrane separation process whether it could control the hydrocarbon dew point properly or not. NGL FRACTIONATION After separation of NGLs from gas stream

Fractionation is based on different boiling points of the different hydrocarbons in the NGL stream. Fractionator components DE ETHANIZER PROPANIZER ISOBUTANIZER butane splitter Fractionation Operation In vertical column where liquid and vapor flow countercurrent and are brought into repeated contact

Part of the vapor condenses and part of the liquid vaporizes

Vapor becomes enriched in lighter or lower boiling point components and liquid becomes enriched with heavier or higher boiling point components schematic view of fractionating column Precautions Fractionation has to be done with minimal loss of pressure to avoid reducing the operating pressure

High pressure fractionation limitation is that it takes place at pressure below Pb at each stage of the column

Study the effect of pressure on the relative volatility of critical components in fractionating columns Types of Fractionators Packed column where instead of trays, "packings" Tray Column Packings Packings that are dumped at random into the column and they are being used in smaller columns
Packings that must be stacked by hand and they are being used in larger columns
Packing Advantages Lower pressure drop (0.2 to 0.6 inches of water per foot of packed depth)

More capacity (at a given diameter) for high liquid-to-vapor ration systems

Allow the use of smaller equipment and so lower capital cost Packing Disadvantages Column plugging

Problems with liquid misdistribution and it affects the packed columns which have a large impact on the number of the theoretical stages and channeling which is the main reason for the poor performance of large packed columns FRACTIONATOR DESIGN Overall Material Balance
In Steady State
F is feed molar flow rate
D is distillate molar flow rate
B is bottoms molar flow rate
XF is mole fraction of a component in the feed
XD is mole fraction of a component in the distillate
XB is mole fraction of a component in the bottoms Minimum theoretical trays Sm is minimum number of theoretical trays
XLK is mole fraction of the light key component
XHK is mole fraction of the heavy key component
αALPHA is relative volatility at average column temperature Feed tray N is the number of theoretical stages in the rectifying section
M is the number of theoretical stages in the stripping section
XHKF is the composition of heavy key in the feed
XLKF is the composition of light key in the feed
XLKB is the composition of light key in the bottoms
XHKD is the composition of heavy key in the distillate Column Capacity The capacity of a column is expressed in terms of the rate of either vapor or liquid flow per unit plate area

To evaluate the column hydraulics, the diameter required for vapor flow is calculated for those stages where there is a significant change in the vapor and liquid flows. Such stages include top and bottom trays, pump around stages, and the feed stage. The column diameter required to handle a given vapor load can be found by applying the following equation at the point of greatest vapor load, generally just below the top tray:
Vmax is maximum vapor velocity, ft/sec
ρL is liquid density, lbm/ft3
ρV is vapor density, lbm/ft3
C is sizing constant, ft/sec
The general method of column design is to choose a plate spacing, estimate the permissible vapor load, and calculate the column diameter as:
D is column diameter, ft
𝐐Qv is volumetric vapor flow rate, ft3/sec
Vmax is maximum vapor velocity, ft/sec If the diameter of a fractionator is 2 feet or less, it is more economical in many cases to use a packed column in place of a tray column

The height of the column is a function of the number of theoretical stages and the efficiency of the actual stages Overall heat balance The last step in the basic calculation scheme of the process design for a fractionating column is to make an overall heat balance around the fractionator
The overall heat balance around the fractionator can be written as follows: QR is reboiler heat duty,
QC is condenser heat duty,
hD is enthalpy of distillate product,
hB is enthalpy of bottom product,
hF is enthalpy of feed,
D,B,F is rate of flow of distillate and bottom and feed streams, respectively Design Procedure 1. Establish feed composition, flow rate, temperature, and pressure
2. Make product splits for the column based on feed and desired product needs
3. Establish a condenser temperature (for the overhead stream) and column pressure, which will be used for calculation of the reboiler temperature
4. Calculate minimum number of theoretical trays and the minimum reflux ratio needed to produce the desired products
5. Calculate the combination of actual theoretical trays and actual reflux ratio needed
6. Make a heat balance around condenser to determine condenser duty and around the column to determine reboiler duty
7. Size the column

New design methods are still on the conservative side THANK YOU
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