The document discusses the formation of hydrates, which are molecular complexes of gases and water, highlighting the types common in industrial settings, particularly their formation temperatures. It outlines the method for predicting hydrate formation temperature based on gas composition using vapor-solid equilibrium constants and detailed calculations. Additionally, it presents examples using simulation software to determine hydrate formation temperatures at various pressures, resulting in a hydrate curve that indicates regions for and against hydrate formation.

1. HYDRATE FORMATION TEMPERATURE
Mohamed A. Hassan, CEng MIchemE
Page 1 of 3
Hydrates are a form of weakly bound molecular
complex in which a guest molecule is trapped within
a cage of water molecules. The cage of water
molecules has the pentagonal or hexagonal faces
with an oxygen atom at each vertex.4
There are two types of hydrates commonly
encountered industrially: Type I and type II. A third
type H is less commonly encountered.
Type I formers in natural gas are methane, carbon
dioxide and hydrogen sulfide. Type II formers are
nitrogen, propane and isobutane.5
Hydrate formation temperature:
It is essential for any gas plants to determine the
temperature and pressure of hydrate formation to a
given gas composition. Hydrates may hinder the
plant operation.
If the composition of the stream is known, the
hydrate temperature can be predicted using vapor
solid equilibrium constants using the following
equation.1
∑
𝑦𝑛
𝑘 𝑛
= 1.0
Where, yn: mole fraction of hydrocarbon component.
N in gas on water-free basis
Kn: vapor-solid equilibrium constant for
component where it is determined experimentally.
The values of vapor-solid equilibrium constants at
various temperatures and pressures are given in
figures 1 to 5.
The amount of water vapor in the gas is assumed to
be always less than the amount required by fully
saturate the gas.
Figure 1: Vapor-solid equilibrium constant for
methane2
Figure 2: Vapor-solid equilibrium constant for
ethane2
Figure 3: Vapor-solid equilibrium constant for
propane2

2. HYDRATE FORMATION TEMPERATURE
Mohamed A. Hassan, CEng MIchemE
Page 2 of 3
Figure 4: vapor-solid equilibrium constant for
isobutane2
Figure 5: Vapor-solid equilibrium constant for CO2
& H2S2
For components heavier than butane, the equilibrium
constant is taken as infinity.1
Figure 6: Vapor-solid equilibrium constant for n-
butane2
• The steps for determining the hydrate
temperature at a given system pressure as as
follows:
1. Assume a hydrate formation temperature
2. Determine Kn for each component
3. Calculate (yn/kn) for each component
4. Sum the values of (yn/kn).
5. Repeat steps until the summation of (yn/kn) is
equal to 1.0
Example:
The following example is try handout
calculations of hydrate formation temperature
and compare the results with simulation
softwares results.
The gas is composed of 27.3 % methane, 58.7 %
ethane, 10.8 % propane and 3.2 % butane.

3. HYDRATE FORMATION TEMPERATURE
Mohamed A. Hassan, CEng MIchemE
Page 3 of 3
Determine:
1. The hydrate formation temperature at 3500
KPa using the above hand-out calculations
and simulation software
2. Draw gas hydrate curve using the handout
calculations and simulation software.
1. Determine hydrate formation temperature:
Comp. Y K10 Y/K10 K15 Y/k15
Methane 0.27 1.45 0.188 1.62 0.16
Ethane 0.58 0.34 1.72 0.95 0.617
C3 0.1 0.1 1.08 ∞ 0
C4 0.03 0.2 0.16 ∞ 0
1 3.15 0.78
By interpolation, the hydrate formation
temperature is 14.535 C where summation of
(Y/K) equals one. The Aspen Hysys hydrate
formation temperature is 14.7927o
C, while
pro/II result is 14.73o
C.
2. Drawing the hydrate curve:
Repeating the same calculations at different
pressures to draw hydrate curve, then using
Pro/II and Aspen Hysys to find the hydrate
temperature.
Pressure
(KPa)
Model T Pro/II T HYSYS T
3500 14.535 14.73 14.7927
3000 13.945 13.44 13.5001
2500 11.511 11.88 11.9643
2000 10.004 10.09 10.1664
1500 7.385 7.69 7.755
Figure 7: hydrate curve
On the left side of the curve is the hydrate
formation region. When pressure and
temperature are in this region, water and gas will
start to form hydrate. On the right side of the
curve is the non-hydrate formation region. When
pressure and temperature are in this region, water
and gas will not form hydrate.
The hydrate formation temperature can be predicted
using the specific gravity and pressure using the
figure, but this method is being used when gas
composition is not known.
Figure 8: pressure-temperature curve for predicting
hydrate formation temperature2
References:
1. Surface Production Operations, 2nd
edition,
Volume 2, Design of gas handling systems
and facilities, Maurice Stewart.
2. Gas processors suppliers association GPSA
Engineering data book, 10th
edition.
3. Gas dehydration field manual, Maurice
Stewart.
4. Carbon capture and storage, Stephan A.
Rackley.
5. Natural gas hydrates – A guide for engineers,
3rd
edition, 2014.
1400
1900
2400
2900
3400
3900
7 9 11 13 15
PressurekPa
Temperature C
Model Pro/II HYSYS