bic called Structure I. Larger molecules (C3H8, i-C4H10, n-C4H10) form a diamond-lattice called Structure II.

Normal paraffin molecules larger than n-C4H10 do not formStructure I and II hydrates as they are too large to stabilizethe lattice. However, some isoparaffins and cycloalkaneslarger than pentane are known to form Structure H hy-drates.13

Gas composition determines structure type. Mixed gaseswill typically form Structure II. Limiting hydrate numbers (ra-tio of water molecules to molecules of included gaseous com-ponent) are calculated using the size of the gas molecules andthe size of the cavities in H2O lattice.

From a practical viewpoint, the structure type does not af-fect the appearance, properties, or problems caused by the hy-drate. It does, however, have a significant effect on thepressure and temperature at which hydrates form. StructureII hydrates are more stable than Structure I. This is why gasescontaining C3H8 and i-C4H10 will form hydrates at higher tem-peratures than similar gas mixtures which do not containthese components. The effect of C3H8 and i-C4H10 on hydrateformation conditions can be seen in Fig. 20-19. At 1000 psia, a

0.6 sp. gr. gas (composition is shown in Fig. 20-22) has a hy-drate formation temperature which is 12-13F higher thanpure methane.

The presence of H2S in natural gas mixtures results in asubstantially warmer hydrate formation temperature at agiven pressure. CO2, in general, has a much smaller impactand often reduces the hydrate formation temperature at fixedpressure for a hydrocarbon gas mixture.

The conditions which affect hydrate formation are:

Primary Considerations

Gas or liquid must be at or below its water dew point orsaturation condition (NOTE: liquid water does not haveto be present for hydrates to form)

Temperature Pressure Composition

Secondary Considerations

Mixing Kinetics Physical site for crystal formation and agglomeration

such as a pipe elbow, orifice, thermowell, or line scale Salinity

In general, hydrate formation will occur as pressure in-creases and/or temperature decreases to the formation condi-tion.

Prediction of Sweet Natural Gas HydrateConditions

Fig. 20-18, based on experimental data, presents the hydratepressure-temperature equilibrium curves for pure methane,ethane, propane, and for a nominal 70% ethane 30% propanemix.

Fig. 20-19 through 20-21, based on gas gravity, may be usedfor first approximations of hydrate formation conditions andfor estimating permissible expansion of sweet natural gaseswithout the formation of hydrates.

The conditions at which hydrates can form are strongly af-fected by gas composition. Compositions used for the construc-tion of Fig. 20-19 through Fig. 20-21 are given in Fig. 20-22.The gases are saturated with water.

Example 20-3 Find the pressure at which hydrate forms fora gas with the following composition. T = 50F.

Component MoleFraction Mole Wtlb per lb-molof Mixture

C1 0.784 16.043 12.58C2 0.060 30.070 1.80C3 0.036 44.097 1.59iC4 0.005 58.124 0.29nC4 0.019 58.124 1.10N2 0.094 28.013 2.63CO2 0.002 44.010 0.09

Total 1.000 20.08

Mole wt. of gas mixture = 20.08

= MWgas

MWair =

20.08 28.964

= 0.693

FIG. 20-14Calculated Water Content of Acid Gas Mixtures

to 10,000 psia

20-10

From Fig. 20-19 at 50F

P = 320 psia for 0.7 gravity gas

Example 20-4 The gas in Example 20-3 is to be expanded from1,500 psia to 500 psia. What is the minimum initial temperaturethat will permit the expansion without hydrate formation?

The 1,500 psia initial pressure line and the 500 psia final pres-sure line intersect just above the 110F curve on Fig. 20-21. Ap-proximately 112F is the minimum initial temperature.

Example 20-5 How far may a 0.6 gravity gas at 2,000 psia and100F be expanded without hydrate formation?

On Fig. 20-20 find the intersection of 2,000 initial pressure linewith the 100F initial temperature curve. Read on the x-axis thepermissible final pressure of 1100 psia.

Example 20-6 How far may a 0.6 gravity gas at 2,000 psia and140F be expanded without hydrate formation?

On Fig. 20-20, the 140F initial temperature curve does not in-tersect the 2,000 psia initial pressure line. Therefore, the gas may

FIG. 20-15Calculated Water Content of Acid Gas Mixtures

20-11

FIG. 20-17Water Content of 5.31% C3/94.69% C1 Gas in Equilibrium with Hydrate

Mixture T, F P, psiaWater Content lb/MMscf

Experimental Eq 20-1 Fig. 20-12& 20-13 Fig. 20-15

11% CO2/89% C1 100 2000 40.6 42 39.2 4311% CO2/89% C1 160 1000 286 277 287 285

20% CO2/80% C1 100 2000 40.6 43 44.1 4620% CO2/80% C1 160 1000 282 278 287 29020% CO2/80% C1 160 2000 172 182 172 194

8% H2S/92% C1 130 1500 111 105 112 106

27.5% H2S/72.5% C1 160 1367 247 258 273 260

17% H2S/83% C1 160 1000 292 278 290 293

C1/CO2/H2S30%/60%/10% 100 1100 81 72 NA 86

C1/CO2/H2S9%/10%/81% 100 1900 442 72 NA NA

5.31% C1/94.69% CO2 77 1500 109.2 38 NA NA5.31% C1/94.69% CO2 122 2000 164.6 105 NA NA

FIG. 20-16Comparison of Experimental vs. Calculated Water Contents for Acid Gases

20-12

be expanded to atmospheric pressure without hydrate forma-tion.

Conditions predicted by Fig. 20-19 through 20-21 may be sig-nificantly in error for compositions other than those used to derivethe charts. For more accurate determination of hydrate formationconditions, the following procedures should be followed. In addi-tion, Fig. 20-20 and 20-21 do not account for liquid water and liquidhydrocarbons present or formed during the expansion. These canhave a significant effect on the outlet temperature from the pres-sure reduction device.

Hydrate Prediction Based on Compositionfor Sweet Gases

Several correlations have proven useful for predicting hy-drate formation of sweet gases and gases containing minimalamounts of CO2 and/or H2S. The most reliable ones require agas analysis. The Katz method14,15 utilizes vapor solid equilib-rium constants defined by the Eq 20-3.

Kvs = yxs

Eq 20-3

WARNING: Not good for pure components only mixtures.The applicable K-value correlations for the hydrate forming

molecules (methane, ethane, propane, isobutane16, normal bu-tane17, carbon dioxide, and hydrogen sulfide) are shown in Fig.20-23 to 20-29. Normal butane cannot form a hydrate by itselfbut can contribute to hydrate formation in a mixture.

For calculation purposes, all molecules too large to form hy-drates have a K-value of infinity. These include all normal par-affin hydrocarbon molecules larger than normal butane.

Nitrogen is assumed to be a non-hydrate former and is alsoassigned a K-value of infinity.

The Kvs values are used in a dewpoint equation to deter-mine the hydrate temperature or pressure. The calculation isiterative and convergence is achieved when the following ob-jective function (Eq 20-4) is satisfied.

i = 1

i = n

(yi / Kvs ) = 1.0 Eq 20-4

Prudence should be exercised when some higher molecularweight isoparaffins and certain cycloalkanes are present asthey can form Structure H hydrates.

Example 20-7 Calculate the pressure for hydrateformation at 50F for a gas with the following composition.

ComponentMole

Fractionin Gas

at 300 psi at 400 psi

Kvs y/Kvs Kvs y/Kvs

Methane 0.784 2.04 0.384 1.75 0.448Ethane 0.060 0.79 0.076 0.50 0.120Propane 0.036 0.113 0.319 0.072 0.500Isobutane 0.005 0.046 0.109 0.027 0.185n-Butane 0.019 0.21 0.090 0.21 0.090Nitrogen 0.094 * 0.000 * 0.000Carbon dioxide 0.002 3.0 0.001 1.9 0.001

Total 1.000 0.979 1.344

* Infinity Interpolating linearly, y/Kvs = 1.0 at 305 psia

FIG. 20-18Conditions for Hydrate Formation for Light Gases

FIG. 20-19Pressure-Temperature Curves for

Predicting Hydrate Formation

20-13

The experimentally observed hydrate-formation pressure at50F was 325 psia.

Example 20-8 The gas with the composition below is at3500 psia and 150F. What will be the hydrate conditions whenthis gas is expanded?

Component Mole FractionC1 0.9267C2 0.0529C3 0.0138iC4 0.0018nC4 0.0034nC5 0.0014Total 1.0000

Solution Steps:

1. Make several adiabatic flash calculations at differentpressures and plot on a pressure versus temperaturegraph. (See Fig. 20-30)

InitialPressure,

psia

InitialTemperature,

F

FinalPressure,

psia

FinalTemperature,

F3500 150 300 38

3500 150 400 45

3500 150 500 52

3500 150 600 58

3500 150 700 64

2. Assume some temperature and predict the hydrate for-mation pressure for this gas using the solid-vapor K-

data. Plot the results on Fig. 20-30. Sample calculationsfor 200 and 300 psia are provided below. This calculationhas been repeated for 400, 500, 800 and 1000 psia to de-velop Fig. 20-30.

T = 40F y200 psia 300 psia

Kvs y/Kvs Kvs y/KvsC1 0.9267 2.25 0.4119 1.75 0.5295C2 0.0529 0.50 0.1058 0.205 0.2580C3 0.0138 0.055 0.2509 0.030 0.4600iC4 0.0018 0.0225 0.0800 0.0105 0.1714nC4 0.0034C5 0.0014Total 1.0000 0.8486 1.4189

y/Kvs = 1.0 @ 227 psia

3. The intersection of the lines in Fig. 20-30 will be the pointat which hydrates start to form. In this example, the re-sult is 500 psia and 52F.

Note: Fig. 20-20 would predict permissable expansion only toa pressure around 700 psia.

The Katz correlation is not recommended above 1000-1500psia, depending on composition. Prediction of hydrate forma-tion conditions at higher pressures requires the use of othermethods. Sloan, et.al.18 present an alternate set of Kvs valueswhich, in general, are valid to 4000 psia. McLeod & Campbell19present experimental hydrate data for natural gas mixtures

See Caution on Fig. 20-19.

FIG. 20-20Permissible Expansion of a 0.6-Gravity Natural Gas

Without Hydrate Formation

See Caution on Fig. 20-19.

FIG. 20-21Permissible Expansion of a 0.7-Gravity Natural Gas

Without Hydrate Formation

20-14

up to 10,000 psia as well as a correlation for estimating highpressure hydrate formation conditions. Blanc & Tournier-Las-serve20 provide experimental hydrate data to 14,500 psiaand compare prediction correlations with experimentaldata.

Hydrate Predictions for High CO2/H2SContent Gases

The Katz method of predicting hydrate formation tempera-ture gives reasonable results for sweet paraffin hydrocarbongases. The Katz method should not be used for gases contain-ing significant quantities of CO2 and/or H2S despite the factthat Kvs values are available for these components. Hydrateformation conditions for high CO2/H2S gases can vary signifi-cantly from those composed only of hydrocarbons. The additionof H2S to a sweet natural gas mixture will generally increasethe hydrate formation temperature at a fixed pressure.21

A method by Baille & Wichert for predicting the tempera-ture of high H2S content gases is shown in Fig. 20-3122. Thisis based on the principle of adjusting the propane hydrateconditions to account for the presence of H2S as illustratedin the following example.

Example 20-9 Estimate the hydrate formation tempera-ture at 610 psia of a gas with the following analysis using Fig.20-31.

Component mol %N2 0.30

CO2 6.66H2S 4.18C1 84.27C2 3.15C3 0.67iC4 0.20nC4 0.19C5+ 0.40

MW = 19.75 = 0.682

Solution Steps:1. Enter left side of Fig. 20-31 at 600 psia and proceed to

the H2S concentration line (4.18 mol%)2. Proceed down vertically to the specific gravity of the gas

( = 0.682)3. Follow the diagonal guide line to the temperature at the

bottom of the graph (T = 63.5F).4. Apply the C3 correction using the insert at the upper left.

Enter the left hand side at the H2S concentration andproceed to the C3 concentration line (0.67%). Proceeddown vertically to the system pressure and read the cor-rection on the left hand scale (2.7F)

Note: The C3 temperature correction is negative when on theleft hand side of the graph and positive on the right hand side.

TH = 63.5 2.7 = 60.8F

Fig. 20-31 was developed based on calculated hydrate con-ditions using the Peng-Robinson EOS. It has proven quite ac-curate when compared to the limited amount of experimental

Mole FractionC1 0.9267 0.8605 0.7350C2 0.0529 0.0606 0.1340C3 0.0138 0.0339 0.0690iC4 0.0018 0.0084 0.0080nC4 0.0034 0.0136 0.0240nC5 0.0014 0.0230 0.0300

Sp. Gr. 0.603 0.692 0.796

FIG. 20-22Gas Compositions Used for Fig. 20-19 through 20-21

FIG. 20-23Vapor-Solid Equilibrium Constants for Methane

20-15

GPSA Engineering Data Book [Gas Processing] 12th ed_2004FMPreface to the Twelfth EditionAcknowledgmentsTable of Contents01 - General Information02 - Product Specifications03 - Measurement04 - Instrumentation05 - Relief Systems06 - Storage07 - Separation Equipment08 - Fired Equipment09 - Heat Exchangers10 - Air-Cooled Exchangers11 - Cooling Towers12 - Pumps & Hydraulic Turbines13 - Compressors and Expanders14 - Refrigeration15 - Prime Movers For Mechanical Drives16 - Hydrocarbon Recovery17 - Fluid Flow and Piping18 - Utilities19 - Fractionation and Absorption20 - Dehydration21 - Hydrocarbon Treating22 - Sulfur Recovery23 - Physical Properties24 - Thermodynamic Properties25 - Equilibrium Ratio (K) DataMembers