B41oa oil and Gas Processing Section a flow Assurance Heriot-Watt University

1.3.1 Guest Molecules and Cages in Hydrate Structures 
As mentioned before guest molecules fill various cages in the gas hydrate 
structures. Considering the fact that each cage has a certain size, one can 
conclude that the occupancy of cages depends on the size of guest molecules. 
Thus, large molecules cannot fit in small cages, whereas small molecules can 
occupy some of the large cages. In this section the occupancy of various 
cages in the three gas hydrate structures is discussed. 
In structure-I hydrates small molecules, such as methane, CO
2
and nitrogen fill 
the 5
12 
cavities. The small molecules could fill some of the large cavities. 
However, if larger molecules such as ethane are present, the majority of large 
cavities will be occupied by ethane. 
In addition, recent studies show that C
1
-C
2
binary systems could form both 
structure-I and structure-II hydrates, depending on the relative concentrations 
and the conditions that prevail. 
In structure-II hydrates, small molecules fill the small cages but the majority of 
the large cavities are filled by intermediate-sized molecules, such as C
3
H
8
, i-
C
4
H
10
, C
6
H
6
, and n-C
4
H
10
. A small number of large cavities may be filled by 
small molecules. In general, structure-II hydrates will not form if the 
intermediate-sized molecules are not present in the fluid system. 
When structure-H is the stable structure, small and intermediate cavities are 
filled by small molecules. The large cavities are filled by large molecules, such 
as C
7
H
14
, adamantane, and smaller. 
In some cases the size of molecule (e.g., n-C
4
) could be slightly larger than the 
size of the cage. This is possible by imposing deformation on the cavities 
which is generally supported by the small cavities in the gas hydrate structure. 
A guide as to what sized gas molecule will fit into which cavity, and thus which 
hydrate structure is formed, is provided in Figure 7. 

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TOPIC 1: Gas Hydrates 
 
 
 
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Figure 7: Gas Molecules and the Cavities they Occupy (adapted from 
Sloan, E. D (1998)). 

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TOPIC 1: Gas Hydrates 
 
 
 
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NIVERSITY B41OA December 2018 v3