Problem 1
Question 1.Shortly describe the purpose of gas sweetening processes in gas processing.
Gas sweetening is the process for the removal of mainly acid gases (H2S and CO2) and, in addition, the simultaneous removal of sulphur organic species (RSH, COS, CS2) from process gas. It is an essential step of sour gas processing for natural gas treatment, , LNGs, refineries and petrochemicals in order to meet transport and market specifications, to comply with environmental regulations for emissions and to control corrosion. As a consequence, concentrations must be limited.
The removal of acid gases (H2S and CO2) can be achieved with a wide range of different technologies. The chemical absorption of acid gases on amines is the most common but molecular sieves and non-regenerative adsorbents can be also used for specific applications. Chemical absorption of acid gases can be improved by use of activators or by combined use of amines and physical solvents allowing the removal of additional sulphur compounds (RSH, COS, CS2).
Question 2. Shortly describe main methods for gas sweetening in gas processing.
Problem 2 : Laboratory analysis of a sample ( gas A) from the reservoir of natural gas is shown in table 1 below
Table 1 : Composition of the gas A
Component
C1
C2
C3
n-C4
i-C4

Volume %
86.5
3.5
2.4
2.1
1.6

Component
n-C5
n-C6
N2
H2S
CO2

Volume %
1.4
1.1
0.5
0.2
0.7

 Question 1
Classify the gas A ( sweet or sour, dry or wet, lean or fat ) ?
Volume % of H2S is very small so the gas A is sweet gas.
Volume % of C1 is between 80- 90 so the gas characterization is medium
Question 2
Calculate the specific gravity of the gas A, the net and gross heating values ( kJ/m3) of the gas A and the flammability limits of the gas A in the air mixture.
MW air = 28.96 ( or 29) ,
𝛾
𝑔
𝑀
𝑊
𝑔𝑎𝑠
𝑀
𝑊
𝑎𝑖𝑟

Table 2. MW calculation.
Component

𝑦
𝑖

MW
(𝑦
𝑖)(𝑀𝑊)

C1
0.865
16
13.84

C2
0.035
30
1.05

C3
0.024
44
1.056

i-C4
0.016
58
0.928

n-C4
0.021
58
1.218

C5
0.014
72
1.008

C6
0.011
86
0.946

N2
0.005
28
0.14

H2S
0.002
34
0.068

CO2
0.007
44
0.308

Total
1.000
𝑀
𝑊
𝑚𝑖𝑥=
20.562



𝛾
𝑔
20.562
28.96=0.710
At 15°C and 101.325 kPa :
Table 3. Gross heating value calculation.
Component

𝑦
𝑖

HHV (MJ/m3)
(𝑦
𝑖)(𝐻𝐻
𝑉
𝑖)

C1
0.865
37.707
32.617

C2
0.035
66.067
2.312

C3
0.024
93.936
2.254

i-C4
0.016
121.404
1.942

n-C4
0.021
121.792
2.558

C5
0.014
149.363
2.091

C6
0.011
177.554
1.953

N2
0.005
0.000
0.000

H2S
0.002
23.784
0.048

CO2
0.007
0.000
0.000

Total
1.000

𝐻𝐻𝑉
𝑚𝑖𝑥=
45.775


Table 4. Net heating value calculation.
Component

𝑦
𝑖

LHV (MJ/m3)
(𝑦
𝑖)(𝐿𝐻
𝑉
𝑖)

C1
0.865
33.949
29.366

C2
0.035
60.429
2.115

C3
0.024
86.419
2.074

i-C4
0.016
112.008
1.792

n-C4
0.021
112.396
2.360

C5
0.014
138.381
1.937