Reactor
Quench
Absorber
Stripper
H S to Claus unit
Lean amine
Optional H reducing gas
Claus unit O -gas
Rich amine
RGG (substoic burner) or steam heater
Incinerator - catalytic or thermal
Partially loaded amine to another absorber
Waste heat Hx
Condensate to sour water
Figure 1 Tail gas treating unit (TGTU)
incinerated and vented to atmosphere. Amine regeneration recycles H₂S to the SRU reaction furnace. A high degree of sulphur recovery is achieved by substantial conversion of all species to H₂S. The TGU admits a low sulphur slip with an overall SRU/TGU recovery performance of 99.9% or better. The primary performance characteristic of a TGU cata - lyst is that SO₂ should be fully converted. If SO₂ enters the quench circuit, it will foul, corrode, potentially deactivate, and subsequently degrade the amine. Secondly, a high degree of conversion is required for COS, CS 2 , and mercap - tan; otherwise, these components pass through the amine system, are incinerated and discharged to atmosphere as SO₂. Finally, any elemental sulphur not converted will plug and corrode the quench circuit. Historically, TGU design is based on fresh catalyst. The designer selects the temperature and catalyst quantity needed to achieve high conversion of non-H2S sulphur compounds and meet environmental performance require - ments. The importance of compliance with permitted envi - ronmental emissions from SOR to EOR means sufficient catalyst inventory must be provided such that even in an aged condition, the needed sulphur recovery is achieved. A temperature profile of the TGU reactor is commonly used to provide insight into catalyst performance and health.
The adiabatic reactor experiences a temperature rise from the exothermic reactions associated with the conversion of various sulphur species to H₂S and shift of carbon monox - ide to hydrogen. Figure 2 shows three approximately equal segments of the bed, with the percentage contribution of each bed to the overall temperature rise across the entire bed: top zone is green and shows 70% of DT, middle zone is blue with 30% DT, and bottom zone is red with negligible DT. The fresh catalyst is achieving almost complete conver - sion in the first two zones. The overall temperature rise across the bed is virtually constant across time (not shown), reflecting that even with sulphur slip as outlet concentrations of non-H₂S species increase, there is still a high percentage conversion. The magnitude of temperature rise is related primarily to the concentration of sulphur dioxide, carbon monoxide, and elemental sulphur in the feed. The relative amount of tem - perature rise in each zone reflects the degree of conversion in each zone. Mid-life activity distribution shifts to a 20% rise in the top zone, 70% in the middle zone, and 5% in the bottom, eventually moving lower into the middle and bottom zones as the top and middle zones deactivate. Incidentally, the temperature profile chart reflects declining activity, which results from deactivation by ageing and poisoning. The sig -
nificance of shifting reactor bed tem - perature profile and its implications for ageing, poisoning and potential bed life and the new modelling tool is the subject of the commentary here and in Part 2 of this article. Kinetic framework and model development The fundamental TGU reaction matrix is (1) hydrolysis on alumina, (2) hydro - genation, and (3) water gas shift on CoS/MoS₂. In the basic frame in Table 1 , an expanded reaction set is invoked to quantify the multiple pathways
Reaction pathways
Reactant(s)
Products Reverse
Hydrolysis H2S + CO2 H 2S + COS
Hydrogenation
Shift / Exchange
COS CS2 SO2
X
--
H2S + CO
H2S + CH3SH
-
- - -
H 2O + S (or H2S)
S
H2S
CO
X
-
H2 + CO2 H2S + CH4
CH3SH
SO2 + CS2 SO2 + CO
CO2 + S CO2 + S S + H2O
Claus H2S + SO2
Table 1
46
PTQ Q1 2023
www.digitalrefining.com
Powered by FlippingBook