L9 Gas Sweetining

Gas sweetening of natural gas

Natural Gas Processing Monday, April 17, 2023 1/21

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Sweetening Sweetening

Gas Sweetening of Natural Gas Sweetening of Natural GasGas



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Reason for sweetening Reason for sweetening Natural gasNatural gas

To meet sales specificationTo meet sales specification To prevent corrosion To prevent corrosion To allow less costly metallurgyTo allow less costly metallurgy To remove toxicity hazards (H2S)To remove toxicity hazards (H2S) To prevent freeze-ups in expanders To prevent freeze-ups in expanders

(CO2) (CO2)



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Iron sponge unit designIron sponge unit design



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Chemsweet towerChemsweet tower



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Equilibrium solvent Equilibrium solvent loadingloading



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Gas sweetening processesGas sweetening processes

ClassificationClassification of sweetening processes:of sweetening processes:– Batch processes: Iron sponge, Chemsweet, Sulfa-check and Batch processes: Iron sponge, Chemsweet, Sulfa-check and

Caustic Soda. Caustic Soda. Uses: Low gas flow rates, and small concentration of HUses: Low gas flow rates, and small concentration of H22S.S.

– Aqueous amine solutions: MEA, DEA, DGA, MDEA..etcAqueous amine solutions: MEA, DEA, DGA, MDEA..etcUses: to remove large amount of sulfur, and COUses: to remove large amount of sulfur, and CO22 when needed. when needed.

– Mixed solutions: Mixture of amines, physical solvents and waterMixed solutions: Mixture of amines, physical solvents and waterUses: absorb organic sulfur and are capable of high acid gas Uses: absorb organic sulfur and are capable of high acid gas loading.loading.

– Physical Solvents: Selexol, Rectisol, Purisol, FluorPhysical Solvents: Selexol, Rectisol, Purisol, FluorUses: Bulk removal of CO2 frequently offshore.Uses: Bulk removal of CO2 frequently offshore.

– Hot potassium carbonate solutions, analogs of the physical Hot potassium carbonate solutions, analogs of the physical solventssolvents

– Direct oxidation to sulfur, eliminate H2S emissionsDirect oxidation to sulfur, eliminate H2S emissions– Adsorption, the use is limited to low acid gas concentrationAdsorption, the use is limited to low acid gas concentration– Membranes, suitable for bulk CO2 separations, when feed-Membranes, suitable for bulk CO2 separations, when feed-

concentration is very high.concentration is very high.



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Sulfur contents in feed Sulfur contents in feed gasgas

Sulfur content in the feed gas Sulfur content in the feed gas

S < 20 lb/day S < 20 lb/day

Batch processes are economicalBatch processes are economical

S > 100 lb/day S > 100 lb/day Amine processes are preferredAmine processes are preferred

lb/day= 1.34 (MMscfd)(gr H2S/100 lb/day= 1.34 (MMscfd)(gr H2S/100 scf)scf)



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Gas Sweetening, Batch ProcessesGas Sweetening, Batch Processes

The Advantages:The Advantages:

(1)(1) Complete removal of low to medium HComplete removal of low to medium H22S concentrations S concentrations with consumption of the reactant by COwith consumption of the reactant by CO22..

(2)(2) Relatively low capital investment when compared to Relatively low capital investment when compared to regenerative processes.regenerative processes.

(3)(3) The affinity for sulfur-containing gases is largely The affinity for sulfur-containing gases is largely independent of the operating pressure.independent of the operating pressure.

(4)(4) Often the removal of organic sulfur contaminants such as Often the removal of organic sulfur contaminants such as the lower molecular weight mercaptans is adequate.the lower molecular weight mercaptans is adequate.



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Gas Sweetening,Gas Sweetening,Batch Processes, Cont.Batch Processes, Cont.

The disadvantages are:The disadvantages are:– Uninterrupted operation requires two Uninterrupted operation requires two

or more contact towers.or more contact towers.– The presence of liquids – poor The presence of liquids – poor

upstream separation or condensation upstream separation or condensation in the tower due temperature changes in the tower due temperature changes – ruins iron sponge chips and makes – ruins iron sponge chips and makes liquid sweeteners foam.liquid sweeteners foam.

– Hydrate formation can occur at higher Hydrate formation can occur at higher pressures and lower temperatures.pressures and lower temperatures.



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Gas Sweetening,Gas Sweetening,Batch Processes, Iron SpongeBatch Processes, Iron Sponge

Process Descriptions::Process Descriptions:: Iron sponge consists of wood chips or shavings impregnated with Iron sponge consists of wood chips or shavings impregnated with

hydrated ferric oxide, Fehydrated ferric oxide, Fe22OO33, and sodium carbonate to control pH., and sodium carbonate to control pH.

The Sweetening reactions:The Sweetening reactions:

The regeneration or revivification reactions are:The regeneration or revivification reactions are:

OHRSFeRSHOFe

OHSFeSHOFe

2332

232232

3)(6

33

SRRSOFeORSFe

SOFeOSFe

:33)(

33

323

3232



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Amine and Mixed solutionsAmine and Mixed solutions

Advantages:Advantages:– Complete removal of medium to high Complete removal of medium to high

concentrations of acid gasesconcentrations of acid gases– Relatively low operating cost per pound of Relatively low operating cost per pound of

sulfur removed compared with batch sulfur removed compared with batch processesprocesses

– The solution composition can be tailored to The solution composition can be tailored to the sour gas compositionthe sour gas composition

– Large amount of organic sulfur compounds Large amount of organic sulfur compounds can also be removed when a physical solvent can also be removed when a physical solvent is added to the amine solution.is added to the amine solution.



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Amine and Mixed solutionsAmine and Mixed solutions

Disadvantages:Disadvantages:– High capital investment compared with High capital investment compared with

batch processesbatch processes– The operational and maintenance costs The operational and maintenance costs

are significantare significant– Some of the processes, Sulfinol, and Some of the processes, Sulfinol, and

Fexsorb require license or royalty feesFexsorb require license or royalty fees



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Design of gas sweetening Design of gas sweetening processprocess

The required data:The required data:– Gas flow rate, Q, MMscfdGas flow rate, Q, MMscfd– Operating pressure, P, psigOperating pressure, P, psig– Operating temperature, T, FOperating temperature, T, F– H2S inlet concentration, ppmvH2S inlet concentration, ppmv– Compressibility factor, Z, (Fig. 3-16 to 3-Compressibility factor, Z, (Fig. 3-16 to 3-

21, if gas gravity is not known use 0.7)21, if gas gravity is not known use 0.7)– Expansion factor, f, (use 1.1 to 1.2)Expansion factor, f, (use 1.1 to 1.2)



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Calculation procedureCalculation procedure

1. Compute the gas actual flow rate, 1. Compute the gas actual flow rate, Qa, at T & PQa, at T & P

levelseaatpressurecatmospheriwhere

P

ZTQQa

714

714

4606319

.

).(

))()((.



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2. Internal vessel diameter, 2. Internal vessel diameter, IDID

Calculate the internal cross-sectional area, Calculate the internal cross-sectional area, A, and internal vessel diameter, IDA, and internal vessel diameter, ID- Assume superficial velocity, V- Assume superficial velocity, V- Cross sectional area= Q/V- Cross sectional area= Q/V- ID= internal diameter- ID= internal diameter

5.07854.0/AID



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3. Daily chemical 3. Daily chemical requirmentrequirment

Iron SpongeIron Sponge

ChemsweetChemsweet

Sulfa-CheckSulfa-Check

Caustic Caustic soda soda

dayftSHppmvQIS /))((. 3201330

dayLbSHppmvQCS /))((. 22480

daygalSHppmvQSC /))((. 204740

dayNaOHgalSHppmvQNS /%))((. 2001090 2



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4. A static bed height, L4. A static bed height, L

Typically 10 to 20 ft. The volume, RV, of Typically 10 to 20 ft. The volume, RV, of reactant is:reactant is:

3278540 ftLIDRV )()(.



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5. Bed Life, BL, 5. Bed Life, BL,

days RV/SC 7.5 BL Check-Sulfa

days RV/CS 11.7 BL Chemsweet

days RV/IS BL spong Iron



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6. The charge requirements 6. The charge requirements are: are:

gallons RV 7.5 :Check-Sulfa

watergal RV 7.0

solid lb RV 11.7 :Chemsweet

bushels RV :spongIron

Bushel: A United States dry measure equal to 4 pecks or 2152.42 cubic inches, pecks = gallon



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7. Check contact time7. Check contact time

Determine the space velocity, acfh/ftDetermine the space velocity, acfh/ft33 . . This should be less than 180 acfh/ftThis should be less than 180 acfh/ft33 for for low H2S contents and 90 acfh/ftlow H2S contents and 90 acfh/ft33 for over for over

50 gr/100 scf gas 50 gr/100 scf gas

3

3

ftRV

hftacfhQvelocitySpace a

,

)/(,



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8. The heat effect:8. The heat effect:

The sulfur deposition or absorption should The sulfur deposition or absorption should be less than 15 gr/min ftbe less than 15 gr/min ft22 of bed area of bed area

100 ppmv = 100/15.9100 ppmv = 100/15.9

= 6.3 gr H2S/100 scf= 6.3 gr H2S/100 scf

)(

)/min)(/,(

areacrossbed

scfSHgrscfQdepositionS

2



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ExampleExample

An iron sponge unit; 1.5 ft diameter An iron sponge unit; 1.5 ft diameter and 5 ft height is used for and 5 ft height is used for sweetening the following gas sweetening the following gas stream: 100 Mscfd, 100 ppm H2S, stream: 100 Mscfd, 100 ppm H2S, 1% CO2, 200 psig, 80 F, SG=0.7, 1% CO2, 200 psig, 80 F, SG=0.7, elevation = 1000 ft. Verify that this elevation = 1000 ft. Verify that this design is viabledesign is viable



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Amine and Mixed Amine and Mixed SolutionsSolutions

Monoethanolamine, MEAMonoethanolamine, MEA Diethanolamine, DEADiethanolamine, DEA Diisopropanolamine, DIPEDiisopropanolamine, DIPE Diglycolamine, DGADiglycolamine, DGA Methyldiethanolamine, MDEAMethyldiethanolamine, MDEA



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Flow rates for, MEA, DEA, Flow rates for, MEA, DEA, DGA, MDEA solutionsDGA, MDEA solutions

Circulation rate, gpm=Circulation rate, gpm=K(MMscfd)(Mol pecent AG removed)K(MMscfd)(Mol pecent AG removed)

AmineAmine Soln wt%Soln wt% Mole AG/mol Mole AG/mol AmineAmine

KK

MEAMEA 2020 0.350.35 2.052.05

DEA coDEA co 3030 0.50.5 1.451.45

High loadHigh load 3535 0.70.7 0.950.95

DGADGA 6060 0.30.3 1.281.28

MDEAMDEA 5050 0.40.4 1.251.25

Mixed solventMixed solvent 7575 0.40.4 variesvaries



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Tower ID (Figure 7-9)Tower ID (Figure 7-9)

See the next slideSee the next slide



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Contact tower diameterContact tower diameter



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Reboiler Reboiler

Duty (Btu/hr)= 72,000 x gpmDuty (Btu/hr)= 72,000 x gpm Area (ftArea (ft22)= 11.3 x gpm)= 11.3 x gpm



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Circulation pumpCirculation pump

Hp=(Circulation rate)(pressure)Hp=(Circulation rate)(pressure)(0.00065)(0.00065)

HP= (gal/min) (P, psig) (0.00065)HP= (gal/min) (P, psig) (0.00065)



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OperationOperation

The keys to trouble-free operationThe keys to trouble-free operation– Check the design parametersCheck the design parameters– Anticipate condensation and hydrate Anticipate condensation and hydrate

formation during cold weatherformation during cold weather– Include an inlet separatorInclude an inlet separator– Pressurized the tower slowly Pressurized the tower slowly – Check the operation daily Check the operation daily

L9 Gas Sweetining

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