Vent to incinerator
Amine loss makeup
TGTU recycle
Q-8
TGTU to SRU
Amine makeup
Lean amine
39
38
1
31
Q-5
Amine cooler
RCYL-2
Blowdown
Condenser
Amine absorber
32
14
37
10
RA pump
28
30
29
Rich amine
Regenerator
13
LR HEX
VLVE-100
Steam
34
Q-4
1
Reboiler
36
Tubes
27
Q-Reb
33
LA pump
Cooling tower
1
Condensate
35
Hydrogenation reactor
Q-7
SRU tail gas to TGTU
Q-6
Q-2
Q
Q-3
21
17
18
20
Quench
26
25
4
Q
Waste heat
Gas heater
Chiller
Cooler
Q-9
5
22
CW pumps
23
24
Makeup
Purge
Q-1
Figure 1 Typical tail gas treating unit (TGTU)
• Amine absorber : Selectively absorbs H 2 S using a selective solvent. • Amine regenerator : Strips H 2 S from the rich amine solution. • Sulphur recycle stream : Recycles H 2 S back to the SRU front-end. A representative process flow diagram of a TGTU is shown in Figure 1 . Evaluations criteria Central to the TGTU is the amine absorber, a column tasked with removing H2 S from the treated tail gas stream while allowing CO2 (either originally present in small amounts
in the SRU feed acid gas or generated from hydrocarbon combustion in the front-end thermal step) to slip through. Recovering and recycling CO2 in the TGTU is counterproductive, as the molecule plays no role in sulphur recovery or Claus reactions; moreover, it adds unnecessary load to the amine regenerator that strips H 2 S from the circulating solvent. Hence, the main objective of a TGTU will be to absorb, recover, and recycle H 2 S from the tail gas back to the Claus unit, while allowing CO2 and other inerts to be slipped through unabsorbed. This seemingly straightforward function is a delicate balancing act involving mass transfer, thermodynamics, and chemical reaction kinetics. A misstep in absorber design or operation can result in undesirable CO2 co-absorption or H 2 S slip, thereby resulting in SOx emissions non-compliance, increased energy costs, and, of course, lower than the target 99.9%+ SRE. Therefore, to design, optimise, or troubleshoot such systems, robust process simulation modelling is essential. The primary focus of the rest of the article will be a thorough scrutiny of a TGTU amine absorber and using process simulation models of these towers under various conditions to analyse performance and
Typical TGTU absorber feed conditions (refinery sulphur block)
Temperature
45ºC
Pressure
0.15 kgf/cm² (g)
Standard vapour flow Composition (mol%) H 2 S
10,000 m³/h or ~11,500 kg/h
2 1
CO 2
H 2 N 2
0.5 90 6.5
H 2 O
Table 1
Refining India
30
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