Chapter 5

Hydrocarbon Liquids RecoveryHydrocarbon Liquids Recovery

Gas Processing and LiquefactionGas Processing and Liquefaction

Chapter 5:

ByAbdul Halim Abdul Razik

FKKSA, UMP

Topic Outcomes:Topic Outcomes:

Describe the terms of Sales Gas and Natural Gas Liquid (NGL)

Describe the fractionation/distillation process and perform related calculations

Describe the functions of Low Temperature Separation Unit (LTSU) and Product Recovery Unit (PRU) in gas processing plant

Describe the processing installation that must be designed to meet natural gas transport or final specification

Describe the refrigeration process and perform related calculations (Similar to chapter 6)

Sales Gas Sales Gas Sales Gas is the treated natural gas that mainly consists of

methane, ethane and small amount of propane and butane. In GPP, this sales gas is the main product which is taken from the overhead of the de-methanizer column.

Below is the typical natural gas (sales gas) compositions:

Components Before 1995 (vol %) After 1995 (vol %)

Methane, CH4 84.75 92.74

Ethane, C2H6 10.41 4.07

Propane, C3H8 0.98 0.77

i-Butane, i-C4H10 0.07 0.08

N-Butane, n-C4H10 0.04 0.06

Nitrogen, N2 0.39 0.45

Carbon dioxide, CO2 3.36 1.83

Total 100.00 100.00

Natural Gas Compositions and Natural Gas Compositions and SeparationsSeparations If a natural gas contains a relatively large fraction of

hydrocarbons other than methane (i.e condensate gas or associated gas), separation of these heavier components are needed to avoid formation of liquid phase during transport.

This separation is usually performed by lowering gas temperature with the formation of a liquid phase (according to the component’s boiling point shown by Figure 7.15). In this regard, it is often called as fractionation process. Other than that, absorption and adsorption can be also applied in doing such separation.

From figure 7.15, it is representing normal boiling point of natural gas fractions. Separation by lowering temperature needs to get the temperature below this normal boiling point, which is at 1 atm. In this context, separation by lowering temperature known as fractionation by refrigeration.

Figure 7.15: Standard Terminology with Respective Boiling Points

Fractionation ProcessFractionation Process Fractionation is process that utilized to separate mixtures into

individual products. Fractionation involves separating components by relative volatility. The difficulty of a separation is directly related to the relative

volatility of the components and the required purity of the product streams.

All gas processing plants producing natural gas liquids require at least one fractionator to produce a liquid product which will meet sales gas specifications.

In a fractionator, heat is introduced to the reboiler to produce stripping vapors. The vapor rises through the column contacting the descending liquid.

The vapor leaving the top of the column enters the condenser where heat is removed by some type of cooling medium. Liquid is returned to the column as reflux to limit the loss of heavy components overhead.

Fractionation Schematic Diagram

Fractionation Process Cont..Fractionation Process Cont..

Internals such as trays or packing promote the contact between the liquid and vapor streams in the column.

Intimate contact of the vapor and liquid phases is required for efficient separation. Vapor entering a separation stage will be cooled which results in some condensation of heavier components.

The liquid phase will be heated which results in some vaporization of the lighter components. Thus, the heavier components are concentrated in the liquid phase and eventually become the bottom product.

The vapor phase is continually enriched in the light components which will make up the overhead product.

Fractionation Process Cont..Fractionation Process Cont.. The vapor leaving the top of the column may be totally or partially

condensed. In a total condenser, all vapor entering the condenser is

condensed to liquid and the reflux returned to the column has the same composition as the distillate or overhead product.

In a partial condenser, only portion of the vapor entering the condenser is condensed to liquid. In most partial condensers, only sufficient liquid will be condensed to serve as reflux for the tower. In some cases, however, more liquid will be condensed than is required for reflux and there will actually be two overhead products, one a liquid having the same composition as the reflux and the other a vapor product which is in equilibrium with the liquid reflux.

Example of fractionators in LTSU and PRU are de-methanizer, de-ethanizer, de-propanizer and de-butanizer.

Example of CalculationExample of Calculation

For the given feed stream, estimate the product stream compositions for 97% propane recovered in the overhead product with a maximum iso-butane content of the overhead stream is 1%.

Components in the feed

Molar flowrate (kmol/hr)

C2 2.4

C3 162.8

iC4 31.0

nC4 76.7

C5 76.5

Total 349.4

Some Important Terms in Hydrocarbon Liquid Some Important Terms in Hydrocarbon Liquid Recovery Process (After fractionation is taking Recovery Process (After fractionation is taking place)place)

Natural gasoline or condensate which is a light gasoline representing mainly the C5+ fraction.

LPG fraction which includes propane and butanes (normal butane and iso-butane)

NGL fraction which contains C2, C3, C4 (iso and normal), natural gasoline – Process goal is not to separate between natural gasoline and LPG.

LNG- by lowering the temperature to about -160oC at 1 atm. Mainly contains methane and generally contains ethane. This product will specifically discussed in Chapter 6: Hydrocarbon Gas Liquefaction Process

LTSULTSU The low temperature separation unit (LTSU) is designed to produce

sales gas by separating heavier hydrocarbons from methane. In GPP A, this unit is designed to produce 250MMSCFD of sales gas

plus plant fuel consumption. Therefore, the total production rate is approximately 266MMSCFD.

The LTSU is accomplished in 3 steps:◦ Cool down by heat exchange (by using refrigeration process.

Refrigeration will be discussed later)◦ Expansion to reduce the temperature further (either the use of

expander or valve)◦ Demethanization to separate the condensed components from

the gas (fractionation process) Methanol injection system is necessary to avoid hydrate formation

especially at refrigeration section.

PRUPRU

The product recovery unit (PRU) consists of three fractionation systems in series. It has de-ethanizer, de-propanizer and de-butanizer. All of these are fractionation towers.

In any fractionation towers, the part below the feed tray is a stripping section, and the part above the feed tray is rectification section.

Control of any tower requires that the product take offs rates match the product quantities in the feed.

For stripping defects (bottom getting lighter), increase the bottom temperature. For rectification defects (overhead getting heavier) decrease the top temperature.

Fractionation by RefrigerationFractionation by Refrigeration

If the natural gas is transported by pipeline, the processing installation must be designed to meet either transport or final specifications.

Three main objective and main specification are summarized below:

Objective Main specification

Gas for transport Hydrocarbon dew point

Commercial /sales gas Gas heating value

Lean gas NGL recovery factor

Fractionation by Refrigeration Cont..Fractionation by Refrigeration Cont.. If the gas is to be transported, the main requirement is to

prevent the formation of liquid phase. For example, during transportation, the minimum

temperature of the gas is 0oC under 7 MPa, the dew point condition must not exceed this temperature at the same pressure.

However, the pressure generally varies considerably in the pipe, as a result of the pressure drop.

To avoid possible liquid-phase formation, one condition frequently imposed is to set the cricondentherm (in this context, the minimum temperature of the gas before dew point occurs) at a value not exceeding the minimum temperature during transport i.e 0oC in the example discussed.

Fractionation by Refrigeration Cont..Fractionation by Refrigeration Cont..

Figure 7.16: Phase Envelope for a Condensate Gas

Fractionation by Refrigeration Cont..Fractionation by Refrigeration Cont..

From figure 7.16, it illustrates the phase envelope for a condensate gas at the outlet of the high pressure separator placed at the wellhead.

The point corresponding to the pressure and temperature conditions in the separator is located on the dew-point curve.

The shape of the phase envelope of the gas after processing (curve A) is such that no liquid will appear as long as the temperature remains above the minimum value imposed.

Fractionation by Refrigeration Cont..Fractionation by Refrigeration Cont..

A second objective is to deliver a commercial/sales gas. Such gas must be distinguished by a certain range of gross heating value (GHV) lying, for example, between 39 100 and 39500 KJ/Nm3.

This specification normally imposes more thorough fractionation than the simple adjustment of the dew point fro transport. Curve B shows the phase envelope of such gas.

Finally, the ultimate goal may to maximize the production of NGLs, by producing a lean gas stripped off most of the hydrocarbons other than methane.

This happens particularly when the residual gas is re-injected of flared. The phase envelope of the gas obtained is shown by curve C.

ConclusionConclusion

In GPP, fractionation processes happen in LTSU and PRU. This fractionation process is required to avoid the

formation of liquid during transportation of sales gas, to meet specified heating value of sales gas and to produce NGLs, LPG and gasoline.

Beside sales gas, single fractions produced such as C2, C3, C4 and C5++ are valuable products that will be sold to the nearby plants.

Typically, fractionation is accomplished by refrigeration and high pressure/low temperature separators (distillation and/or flash) depending to the purity required.