Assignment on Method for removal of impurities from natural gas

1. 1
Rafay Arfin (CH-15011)
METHODS FOR REMOVAL
OF IMPURITIES FROM
NATURAL GAS

2. Table Of Contents
METHOD FOR REMOVAL OF OXYGEN............................................................................................................... 1
Apparatus and Method Of CATALYTIC OXIDATION ...................................................................................... 1
ADVANTAGES ................................................................................................................................................ 1
LIMITATIONS ................................................................................................................................................. 1
REFERENCE.................................................................................................................................................... 1
METHOD FOR REMOVAL OF CO2....................................................................................................................... 2
Apparatus and Methods Of DDR-Type Zeolite Membrane Process.............................................................. 2
Advantages.................................................................................................................................................... 3
Reference ...................................................................................................................................................... 3
METHOD FOR REMOVAL OF NITROGEN ........................................................................................................... 3
Apparatus and Method Of Cryogenic Distillation Process............................................................................ 3
Advantages:................................................................................................................................................... 4
Limitations..................................................................................................................................................... 4
References..................................................................................................................................................... 5
METHOD FOR REMOVAL OF H2S....................................................................................................................... 5
Apparatus and Method Of Iron Sponge Process........................................................................................... 5
Advantages.................................................................................................................................................... 5
Limitations..................................................................................................................................................... 5
Reference ...................................................................................................................................................... 5

3. 1METHODS FOR REMOVAL IF IMPURITIES FROM NATURAL GAS
METHOD FOR REMOVAL OF OXYGEN
CATALYTIC OXIDATION Is used for removal of Oxygen from natural gas
Apparatus and Method Of CATALYTIC OXIDATION
The catalytic removal of oxygen from a natural gas stream is achieved by passing the gas at an elevated
temperature over a catalyst bed where the oxygen reacts with a portion of the natural gas to form CO2 and
water. The gas stream enters an inlet separator to remove entrained liquids, followed by an inlet filter to
remove particulate matter that may foul the solid catalyst bed. The gas stream is then pre-heated through
cross exchange with the hot gas leaving the catalytic reactor. The reaction temperature is a function of the
gas composition, gas oxygen content, treated gas oxygen specification and catalyst media. A minimum
concentration of oxygen in the inlet gas is required in order to maintain the proper reaction temperature. If
there is insufficient oxygen, additional heat may be added by a supplemental process heater. The gas
stream then enters the reactor where the oxygen in the stream reacts with some of the hydrocarbons
(usually the C3+ fractions) in the natural gas stream and produces water and carbon dioxide. The hot gas
stream is then cooled via cross exchange with the inlet gas stream and after-cooler heat exchangers. After
cooling, the product gas enters an outlet separator where condensed water is removed from the treated
gas. The outlet separator is optional and is required only if there is significant oxygen being removed from
the gas. For typical cases where there is a low concentration of oxygen, water is made at low ppm levels
and the after-cooler may not condense any water.
PSB Industries - Deoxo
PSB Industries offers the Deoxo process for oxygen removal from natural gas streams that do not contain
or have removed H2S and other sulfur compounds upstream. The natural gas enters a coalescing pre-filter
where aerosol mists and particulate matter are removed. The stream is first heated in the cross exchange
and then, if the oxygen content in the gas results in too low a temperature rise in the catalytic reactor,
heated by an electric pre-heater to 500 °F or higher. Depending on the volume of gas and available utilities,
other types of heaters, e.g. direct-fired or a heat exchanger using heat medium fluid, might be more
suitable than electric heating. The heated gas then enters the stainless steel vessel containing palladium
catalyst where the oxygen reacts with hydrocarbons. Sulfur contaminants such as hydrogen sulfide,
mercaptans and or other organic sulfides can lead to catalyst poisoning. If sulfur- containing contaminants
are present in the stream, then they must be removed prior to the Deoxo, e.g., via a guard bed.
ADVANTAGES
• Comparatively cheaper
• Commercially available.
LIMITATIONS
• Dust must be removed before afterburning
• Deactivated catalysts cannot be regenerated and are disposed of.
REFERENCE
http://trimeric.com/assets/oxygen-removal-in-natural-gas-systems-lrgcc-paper.pdf

4. 2METHODS FOR REMOVAL IF IMPURITIES FROM NATURAL GAS
METHOD FOR REMOVAL OF CO2
DDR-TYPE ZEOLITE MEMBRANE IS used for removal of CO2 from natural gas
Apparatus and Methods Of DDR-Type Zeolite Membrane Process
The general technologies used for CO2 removal are the absorption process and membrane process. The
membrane process is strongly required for bulk CO2 removal from such low quality natural gas because the
chemical absorption process requires large amounts of energy, and thus a hybrid process consisting of the
chemical absorption process and the membrane process is commonly used for the low quality natural gas.
The binary mixed gas in the feed vessel (V01) was pressurized by the Feed Gas Compressor (K01) up to 9.0
MPa(g) and the lube oil mist was eliminated by the Carbon Bed (F01). The operating pressure was adjusted
by the Pressure Regulator (PR01). After PR01, the gas was branched to the main line and the minimum flow
line. The minimum flow rate for K-101 in the return line was maintained by PR02. The main line was heated
to the specified temperature and fed to the Membrane. The retentate gas was depressurized by PR03. On
the other hand, the permeate gas was kept at the specified pressure by PR04 and mixed with the
depressurized retentate gas and then returned to V01. Lines upstream of the pressure regulators (PR01,
PR02, PR03 and PR04), were heated by the Heating Tapes (E01 and E02) in order to prevent condensation
by cooling due to the Joule-Thomson effect.

5. 3METHODS FOR REMOVAL IF IMPURITIES FROM NATURAL GAS
Advantages
• the CO2/CH4 selectivity of the DDR Membrane is much higher than that of a conventional
polymeric membrane
• it can achieve high hydrocarbon yield with low CH4 loss
• Applicable for gas with high CO2 partial pressure
• to CO2-induced plasticization, The DDR Membrane, however, shows no CO2-induced plasticization
Limitations
• It is however still challenging to make defect-free zeolite membranes.
• Most of the reported membranes possess quite low CO2 permeance
• Expansive method
Reference
www.sciencedirect.com
METHOD FOR REMOVAL OF NITROGEN
CRYOGENIC DISTILLATION Is used for removal of Nitrogen from natural gas
Apparatus and Method Of Cryogenic Distillation Process
Inlet receiving/compression
Depending on the source of feed gas to the NRU, the removal system will require some types of inlet
receiving equipment, such as a slug catcher, a vapor/liquid separator and/or a compression.
Pretreatments
The feed to the nitrogen rejection unit is pre-treated to remove components that could freeze in
downstream cryogenic equipment. The CO2 is typically removed to 50e100 PPMV levels using amine
treating. Acid gas such as H2S is also removed to less than 4 PPMV to meet sales gas specification. After
amine treating in the acid gas removal unit (AGRU), the sweet gas is most often dehydrated with a

6. 4METHODS FOR REMOVAL IF IMPURITIES FROM NATURAL GAS
solid desiccant. Molecular sieves are generally specified because of their ability to dry the gas stream
to a water dew point well below the required specification.
downstream of the dehydration unit.
Depending on the feed composition, heavy hydrocarbons can be extracted at an intermediate
temperature level during the chilling step. TEG, which is also commonly used for dehydration, has also
been commercially demonstrated for bulk removal of aromatic hydrocarbons prior to the
chilling/condensation step.
J-T or expander chilling
Following pre-treatment, the dry, clean gas is successively chilled to condensation temperature by heat
exchange with the product streams in brazed aluminum heat exchangers. The chilling process is
accomplished by using any or all of the following:
Adiabatic expansion (JT valve)
Isentropic expansion (turbo expander).
Cryogenic fractionation
The cryogenic fractionation section of the NRU is normally located inside a cold box and is the heart of
the NRU because it controls the nitrogen in the methane-rich product stream, the hydrocarbon losses
in the rejected nitrogen stream, and the overall thermal efficiency of the process.
Recompression
For typical applications, recompression of the sales gas (or fuel gas) is usually required unless the gas
can be marketed at 20 bar(g) or less. The main force for NRU separation is provided by the pressure
differential between the feed gas and the product streams. The product streams, such as sales or fuel
gas and N2, when exit the unit at a pressure lower than the feed gas, possibly require recompression
Advantages:
• Wide range feed gas inlet pressure and flow rate.
• No design pressure limitation.
Limitations
• May not be competitive for low gas throughput (<25 MMSCFD per Finn’s (Finn, 2007) paper).
• Very low methane concentration. (100 PPM to 1.5%) in N2 vent stream

7. 5METHODS FOR REMOVAL IF IMPURITIES FROM NATURAL GAS
References
Journal of Natural Gas Science and Engineering
www.elsevier.com/locate/jngse
METHOD FOR REMOVAL OF H2S
Many processes can be used to remove H2S from natural gas. Discussed here is the BATCH PROCESS for
removal of H2S
Apparatus and Method Of Iron Sponge Process
Iron sponge fixed-bed chemical absorption is the most widely used batch process. This process is applied to
sour gases with low H2S concentrations (300 ppm) operating at low to moderate pressures (50–500 psi(g)).
Carbon dioxide is not removed by this treatment. The inlet gas is fed at the top of the fixed-bed reactor
filled with hydrated iron oxide and wood chips. The basic reaction is the formation of ferric sulfide when
H2S reacts with ferric oxide:
2Fe2O3 + 6H2S → 2Fe2S3 + 6H2O
The reaction requires an alkalinity pH level 8–10 with controlled injection of water. The bed is regenerated
by controlled oxidation
Advantages
• the large savings in labor cost for loading and unloading of the batch process.
• In this case, higher sulfur recovery per pound of iron oxide is obtained also.
Limitations
• Repeated cycling of the process will deactivate the iron oxide and the bed should be changed after
10 cycles.
• Some of the sulfur produced might cake in the bed and oxygen should be introduced slowly to
oxide this sulfur
Reference
Petroleum and gas field processing by H.K Abdel-Aal