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Tutorial UNIT 3.1

(Q and A)

1. What are the components of “Ra Natural !as "i#$

Raw natural gas typically consists primarily of methane (CH 4), the shortest and lightest hydrocarbon

molecule. It also contains varying amounts of

• Heavier gaseous hydrocarbons ethane  (C!H"),   propane  (C#H$),  normal butane  (n%C4H&'),

isobutane (i%C4H&'),   pentanes and even higher molecular weight hydrocarbons. hen processed

and purified into finished by%products, all of these are collectively referred to as *+ (atural

*as +iuids).

• -cid gases carbon dioide (C/!), hydrogen sulfide  (H!0) and 1ercaptan such as methanethiol

(CH#0H) and ethanethiol (C!H20H).

• /ther gases   (!) and helium (He).

• ater  water vapor and entrained liuid water.

• +iuid hydrocarbons perhaps some natural gas condensate (also referred to as casing head 

 gasoline or natural gasoline) and3or crude oil. 

• 1ercury very small amounts of mercury primarily in elemental form, but chlorides and other 

species are possibly present.

%. &efine the parameters of re'uired 'ualit of endproduct processed *as+

Raw natural gas must be purified to meet the uality standards specified by the maor pipeline

transmission and distribution companies. 5hose uality standards vary from pipeline to pipeline and are

usually a function of a pipeline system6s design and the mar7ets that it serves. In general, the standardsspecify that the natural gas

• 8e within a specific range of heating value (caloric value). 9or eample, in the :nited 0tates, it

should be about &,'#2 ; 2< 8tu  per  standard cubic foot  of gas at an absolute  pressure  of &

atmosphere and "' =9 (4& ; 2< 1>  per normal cubic meter  of gas at & atmosphere of absolute

 pressure and ' =C).

• 8e delivered at or above a specified hydrocarbon dew point temperature (below which some of 

the hydrocarbons in the gas might condense at pipeline pressure forming liuid slugs which could

damage the pipeline).

• 8e free of particulate solids and liuid water to prevent erosion, corrosion or other damage to the

 pipeline.

• 8e dehydrated of water vapor sufficiently to prevent the formation of methane hydrates within the

gas processing plant or subseuently within the sales gas transmission pipeline.

• Contain no more than trace amounts of components such as hydrogen sulfide, carbon dioide,

mercaptans, nitrogen, and water vapor.

• 1aintain mercury at less than detectable limits (approimately '.''& ppb  by volume) primarily to

avoid damaging euipment in the gas processing plant or the pipeline transmission system from

mercury amalgamation and embrittlement of aluminum and other metals.

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3. What e mean , “-rocessin* of natural *as$+ What are the o,ecti/es of

processin* and the steps in se'uence for processin* the *as ith process flo

dia*ram+

*as processing is an instrumental piece of the natural gas value chain. It is instrumental in ensuring that

the natural gas intended for use is as clean and pure as possible, ma7ing it the clean burning and

environmentally sound energy choice. /nce the natural gas has been fully processed, and is ready to be

consumed, it must be transported from those areas that produce natural gas, to those areas that reuire it.

 atural gas processing plants are used to purify the raw natural gas etracted from underground gas fields

and brought up to the surface by gas wells. 5he processed natural gas, used as fuel by residential,

commercial and industrial consumers, is almost pure methane and is very much different from the raw

natural gas.

5he processing of gas means removal of unwanted components from the mi so as to ma7e to a

transportable clean fuel and also separate out valuable hydrocarbon component as value added products to

 be used to etract more commercial values compared to fuel use.

5he processing includes the following seuence of operations

&. Raw natural gas is commonly collected from a group of adacent wells and is first processed at

that collection point for removal of free liuid water and natural gas condensate. 5he condensate

is usually then transported to a  petroleum refinery and the water is disposed of as waste water.

!. 5he raw gas is then pipelined to a gas processing plant where the initial purification is usually the

removal of acid gases (hydrogen sulfide and carbon dioide). 5here a many processes that are

available for that purpose but -mine gas treating is the most widely used process.

#. 5he acid gases removed by amine treating cannot be disposed off in the atmosphere so the

recovered acid gases are routed into a sulfur recovery unit which converts the hydrogen sulfide in

the acid gas into elemental sulfur. 5here are a number of processes available for that conversion,

 but the Claus process is by far the one usually selected. 5he residual gas from the Claus process is

commonly called tail gas and that gas is then processed in a tail gas treating unit (5*5:) to

recover and recycle residual sulfur%containing compounds bac7 into the Claus unit. 5he final

residual gas from the 5*5: is incinerated. 5hus, the carbon dioide in the raw natural gas ends

up in the incinerator flue gas stac7 .4. 5he net step in the gas processing plant is to remove water vapor from the gas using either the

regenerable absorption (chemistry)  in liuid  triethylene glycol (5?*), commonly referred to as

glycol dehydration, or a  @ressure 0wing -dsorption (@0-) unit which is regenerable adsorption

using a solid adsorbent.

2. 1ercury is then removed by using adsorption processes such as activated carbon or regenerable

molecular sieves.

". itrogen is net removed and reected using one of the three processes indicated on the flow

diagram

a. Cryogenic process using low temperature distillation. 5his process can be modified to also

recover helium, if desired.

 b. -bsorption process using lean oil or a special solvent as the absorbent.

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c. -dsorption process using activated carbon or molecular sieves as the adsorbent. 5his process

may have limited applicability because it is said to incur the loss of butanes and heaver 

hydrocarbons.

A. 5he net step is to recover the natural gas liuids (*+) for which most large, modern gas processing plants use another cryogenic low temperature distillation process involving epansion

of the gas through a turbo%epander   followed by distillation in a demethaniBing fractionatingcolumn. 0ome gas processing plants use a lean oil absorption process rather than the cryogenic

turbo%epander process.

5he residue gas from the *+ recovery section is the final, purified sales gas which is pipelined to the

end%user mar7ets.

0. What are N! and define % (To) principle techni'ues for remo/in* N!s from the

natural *as stream ith a correspondin* process flo dia*ram+

A. The a,sorption method

2. The cro*enic e#pander process.

N!s The N!s are the li*ht components of natural *as in *as mi# hich can ,ee#tracted from *as phase to li'uid phase usin* cro*enic process usin* their differential

de point.

These components can ,e e#tracted separatel from li'uid phase to *as phase usin* their

differential ,oilin* point.

The mi#ture of N!s can ,e separated , fractionation process.

1. The A,sorption "ethod

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5he absorption method of *+ etraction is very similar to using absorption for dehydration.

5he main difference is that, in *+ absorption, absorbing oil is used as opposed to glycol. 5his

absorbing oil has an affinity for *+s in much the same manner as glycol has an affinity for 

water. 8efore the oil has pic7ed up any *+s, it is termed lean absorption oil. -s the natural gas

is passed through an absorption tower, it is brought into contact with the absorption oil which

soa7s up a high proportion of the *+s. 5he rich absorption oil, now containing *+s, eits

the absorption tower through the bottom. It is now a miture of absorption oil, propane, butanes,

 pentanes, and other heavier hydrocarbons. 5he rich oil is fed into lean oil stills, where the

miture is heated to a temperature above the boiling point of the *+s, but below that of the oil.

5his process allows for the recovery of around A2 percent of butanes, and $2 % D' percent of 

 pentanes and heavier molecules from the natural gas stream.

5he basic absorption process above can be modified to improve its effectiveness, or to target the

etraction of specific *+s. In the refrigerated oil absorption method, where the lean oil is

cooled through refrigeration, propane recovery can be upwards of D' percent, and around 4'

 percent of ethane can be etracted from the natural gas stream. ?traction of the other, heavier 

 *+s can be close to &'' percent using this process.

%. The 4ro*enic 5#pansion -rocess

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Cryogenic processes are also used to etract *+s from natural gas. ?ssentially, cryogenic

 processes consist of dropping the temperature of the gas stream to around %&!' degrees

9ahrenheit.

5here are a number of different ways of chilling the gas to these temperatures, but one of the

most effective is 7nown as the turbo epander process. In this process, eternal refrigerants are

used to cool the natural gas stream. 5hen, an epansion turbine is used to rapidly epand the

chilled gases, which causes the temperature to drop significantly. 5his rapid temperature drop

condenses ethane and other hydrocarbons in the gas stream, while maintaining methane in

gaseous form. 5he epansion turbine is able to convert some of the energy released when the

natural gas stream is epanded into recompressing the gaseous methane effluent, thus saving

energy costs associated with etracting ethane.

2. &efine Natural !as i'uid 6ractionation process alon*ith process flodia*ram+

/nce *+s have been removed from the natural gas stream, they must be bro7en down into

their base components to be useful. 5he process used to accomplish this tas7 is called

fractionation. 9ractionation wor7s based on the different boiling points of the different

hydrocarbons in the *+ stream.

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?ssentially, fractionation occurs in stages consisting of the boiling off of hydrocarbons one by

one. 5he name of particular fractionators gives an idea as to its purpose, as it is conventionally

named for the hydrocarbon that is boiled off. 5he entire fractionation process is bro7en down

into steps, starting with the removal of the lighter *+s from the stream. 5he particular 

fractionators are used in the following order

 

&e ethani7er

5he overhead product from the de ethaniBer is ethane and the bottoms are fed to thedepropaniBer.

&e propani7er

5he overhead product from the depropaniBer is propane and the bottoms are fed to thedebutaniBer.

&e ,utani7er

5he overhead product from the debutaniBer is a miture of normal and iso%butane, and the

 bottoms product is a C2E miture. 5he recovered streams of propane, butanes and C 2E are each

FsweetenedF in a 1ero process unit to convert undesirable mercaptans into disulfides and, alongwith the recovered ethane, are the final *+ by%products from the gas processing plant.

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