PTQ Q1 2023 Issue

0 0.2 0.6 0.4 0.8 1 1.2 1.4

0 0.5 1.5 1 2 2.5 3

0.1

0.2

0.3

0.4

0.5

0.6

0.02

0.04

0.06

0.08

0.1

0.12

Water, mole fraction

Hydrogen, mole fraction

Power

Hins h elwood

Hybrid

Data

Slt data

Power

Absorb

Hybrid

Slt data

Figure 3 Water gas shift reaction – resistance from water

Figure 4 Water gas shift reaction – resistance from hydrogen

rate-limiting, restricting access to new reactants. The effect of hydrogen can be represented as a fractional order com- positional dependency or with the adsorption mechanism:

by the sum of mercaptan and methane, which then allows fitting for the conversion of mercaptan to methane. SO 2 has multiple roles in TGU reaction kinetics. A known effect of SO 2 is the moderation of hydrolysis of COS and CS 2. 3 The more subtle role of SO 2 , revealed with discre- tised finite element bed modelling, is providing an alternate conversion pathway for CS 2 , moderating the production of mercaptan and COS. Additionally, SO 2 provides a direct conversion of CO (versus water gas shift/hydrogenation), acknowledging a substantial enhancement influence on CO conversion when present. Figure 5 compares reactor outlet concentrations mea- sured from experimental data versus model predictions for CO, COS, mercaptan, and methane. The parity plots show a good fit of data across the operation range of temperature from 260ºC to 335ºC and actual gas hourly space veloc- ity from 1,500 to 6,000 hr -1 . Composition of the feed was: H 2 = 2%; CO₂ = 7%, 14%, 21%; H2 S = 0.8%; CO = 1%; COS = 0.025%, 0.05%; SO 2 = 0.4%, 0.2%, 0%; CS2 = 0%, 0.025%; H 2 O = 13%, 26%, 39%; N 2 = balance. This rigorous reaction kinetics model was ultimately implemented as a hydrogenation reactor model in the proprietary SulphurPro, providing an essential cog in rate- based, plant-wide simulation of SRUs and TGUs. The reac- tion scheme comprised 11 individual reactions, and the

rate= –k eff [CO] [H 2 ]

a [H

2 O]

b [CO

2 ]

The hydrolysis reaction of COS is reversible and equilib- rium limited, as are the shift reactions of CO and COS. All the other reactions have diminishingly small equilibrium values and are considered irreversible. When reversible, kinetics must respect equilibrium, so forward and reverse comply with:

k forward /k reverse = K p and k reverse = k forward / K p . where K p = [R]

r [S] s / ([A] a [B] b )

The hydrolysis reaction of COS and shift reactions of CO and COS share several components, so these are parallel reaction pathways. Since the reaction system is not at equi- librium, kinetic parameters for all paths (including reverse reactions) must be defined. Additionally, COS is a product of CS₂ hydrolysis. An interesting aspect of CS₂ hydrogenation is the forma - tion of methyl mercaptan, which is further hydrogenated to methane and hydrogen sulphide. The extent of the first reaction, the production of methyl mercaptan, is determined

0 0.00005 0.0015 0.001 0.002 0.0025

0.00015

COS

0.0001

0.00005

0.0005

0.001

0.0015

0.002

0.0025

0.00005

0.0001

0.00015

0.00008

CHSH, CH

0.00006

0.00004

0.00002

0.00004

0.00006

0.00008

0.00002

0.00004

0.00006

Experimental data (Outlet mol%)

Figure 5 Parity plots comparing outlet gas concentration from experimental data to model predictions

PTQ Q1 2023