composicon con c7+

8/12/2019 composicon con c7+

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2 SPE 81121

Lower plant inlet gas temperatures will require less water to beremoved by the glycol. Second, lean glycol temperature at thetop of the contactor will affect the water partial pressure at thetop stage. Consequently, high glycol temperatures will resultin high water content in the over head gas. However, thistemperature is normally no cooler than 10 oF above the inletgas to prevent hydrocarbons in the feed from condensing inthe solution. This limit is normally maintained by a gas/glycolexchanger that cools the lean glycol to approximately a 10 o Fapproach using the dry gas.Other parameters in the plant have limited or no effect on thedry gas water content. The number of equilibrium stages in theregenerator has only a slight effect on the lean glycol purity.Equilibrium at the reboiler temperature and pressure isapproached in the reboiler so that additional stages have noeffect. Operating temperature of the lean/rich glycol exchangeronly significantly impacts the reboiler heat duty.

Results

Figures 7,8,9 and 10 illustrate the effect of the number ofequilibrium contact trays on residual water contentusing a 400 oF reboiler. The Figures present comparing dew pointdepressions and actual water content. Increasing the numberof trays and the concentration of circulating glycol allows thegas to approach equilibrium with the lean glycol at a lowerglycol circulation rate. Considering a typical glycol circulationrate , Figures 7 and 8 illustrate that a 99.4% Glycolconcentration in different equilibrium-stages contactor. acirculation rate of 1300 bbl/day TEG would be required toapproach equilibrium. In a 99.6% Glycol concentration indifferent equilibrium stage contactor, a circulation rate of 1100

bbl/day TEG would be required to approach equilibrium.

Significantly higher flow rates would still be required whenonly 4 ideal stage is used.As stated earlier, applications requiring high dew pointdepressions will virtually always utilize stripping gas in theregenerator. Low dew points simply cannot be achieved usingthe maximum 98.8 wt % glycol obtainable with a 400 oF reboiler at atmospheric pressure. These low dew points will needup to 99.9 wt % glycol in the absorber. Increasing reboilertemperature is not an option due to the thermal decompositiontemperature of 404 oF for TEG. Even a 400 oF reboiler canresult in glycol decomposiion due to higher film temperatures.Further, stripping gas has a much greater effect than increasingefficiency, stripping gas should be introduced in a shortcolumn after the hot glycol is removed from the reboiler.Stripping gas may be place directly in there boiler, but thehigh water partial pressure in the vapor space limits the masstransfer driving force.small stripping gas rates of 1 scf/gal circulated solution have a

pronounced difference. With this stripping gas rate, the drygas will contain about half the water of the same processwithout stripping gas. Increasing the stripping gas rate beyond2 to 3scf/gal will have little impact on dew point depression.In this phase of the work, a plant with different C7+ in thefeed was modeled. The parameters in Table 1 were used as inthe previous section but different mol % of C7+ was added tothe plant feed. The amount of water in the feed was calculated

by model so that the gas remained at the dew point.Figure 4,5 and 6 illustrate the effect of this C7+ concentrationon the dehydrated gas water content and water dew point. Theresults indicate that the addition of C7+ slightly decrease thewater content and the water dew point temperature. Thesedata indicate that if c7+ molf fration is increased the purity ofglycol at the bottom of absorption tower will increase.


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SPE 81121 3

Figure 1- Schematic of Dehydration system in Gachsaran oil field

Table 1- Gachsaran gas feed to dehydration system

Component Mol%C1 69.05C2 13.2C3 8.29IC4 1.16

NC4 2.51IC5 0.65

NC5 0.68 NC6 0.4H2S 0.84

CO2 2.06H2O .82 N2 0

C7+ 0.34

C7+ Mw 113.09

C7+ Sp. Gr. 0.7483Temperature F 130

Number of trays 5Absorption tower

pressure drop (psi)5

Rate (MMSCFD) 145Pressure (psig) 625


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4 SPE 81121

Fig- 4 Water contents with respect to Inlet gas C7+ Mol percent(Absorption tower)

8.2

8.4

8.6

8.8

9

9.2

9.4

9.6

9.8

10

10.2

0 2 4 6 8 10 12

Inlet gas C7+ Mol %

A b s o r b e r g a s w a t e r c o n t e n t s ( l b / M M S C F

Fig 3 water Contents vs Glycol circulation rate

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000 3500

Glycol circulation rate bbl/day

W a t e r

C o n

t e n

t l b / M M S C F

98.8% Glycol 99% glycol 99.4% Glycol 99.6 % Glycol

Figure 2- Water dew point vs glycol concentration and circulation rate

0

20

40

60

80

100

120

0 500 1000 1500 2000 2500 3000 3500

Glycol circulation rate (bbl/day)

W a t e r d e w p o i n t

98.8 % Glycol 99% Glycol 99.4% Glycol 99.6% Glycol


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SPE 81121 5

Fig. 6 Absorber tower glycol concentration withrespect to Inlet gas C7+ Mol percent (Absorption

tower)

63.5

64

64.5

65

65.5

66

66.5

67

67.5

68

0 2 4 6 8 10 12

Inlet gas C7+ Mol %

A b s o r b e r

t o w e r o u

t l e t g l y c o l

c o n c e n

t r a t

i o n m o l

%

Fig-5 Water dew point with respect to Inlet gas C7+ Mol percent(Absorption tower)

34

35

36

37

38

39

40

0 2 4 6 8 10 12

Inlet gas C7+ Mol %

A b s o r b e r g a s w a t e r d e w

p o i n

t


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6 SPE 81121

Fig 8 Water Content for different Glycol Rtaes (Glycol 99.4%)

0

20

4060

80

100

120

0 500 1000 1500 2000 2500 3000 3500

Glycol circulation Rtae (bbl/day)

W a t e r C

o n t e n t

( l b / M M

S C F )

4 trays 5 trays 6 trays

Fig 7-Water Dew point for different GlYcol Rtaes (Glycol 99.4%)

0

20

40

6080

100

120

0 500 1000 1500 2000 2500 3000 3500


W a t e r

D e w p o

i n t F



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SPE 81121 7

REFERENCES

1. Kohl, A. and Riesenfeld, F.,"Gas Purification", GulfPublishing Co., Houston, 1985.2. Fitz, C. W., and Hubbard, R.A., "Quick, ManualCalculation Estimates Amount of Benzene Absorbed in GlycolDehydrator," Oil & Gas J., p. 72,Nov. 8, 1987.3. Gas Processors Suppliers Association, Engineering DataBook, 1987.4. Jou, F. Y., Deshmukh, R. D.,Otto, F. D., and Mather, A. E.,"Vapor-Liquid Equilibria for Acid Gases and Lower Alkanesin Triethylene Glycol," Fluid Phase Equilibria, 36, p. 11, 1987.5. Takahashi, S., and Kobayashi,R., "The Water Content andthe Solubility of CO 2in Equilibrium with DEG-Water andTEG-Water Solutions at Feasible Absorption Conditions,"

Technical Publication TP-9, GPA,1982.

6. "The Solubility of Selected Aromatic Hydrocarbons inTriethylene Glycol," D. B.Robinson Research LTD., API/GPAProgress Report, March 1991.

Fig 10 Water Content for different Glycol Rtaes (Glycol 99.6%)

0

20

40

60

80

100

120

0 500 1000 1500 2000 2500 3000 3500


W a t e r

C o n

t e n t

( l b / M M S C F )


Fig 9 - Water Dew point for different GlYcol Rtaes (Glycol 99.6%)

0

20

40

60

80

100

120

0 500 1000 1500 2000 2500 3000 3500


W a t e r

D e w p o

i n t F


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composicon con c7+