1. Boreskov Institute of Catalysis,
Reverse-flow reactor concept for combined SO2 and CO oxidation in smelter off-gases
A.N.Zagoruiko, S.V.Vanag
 Boreskov Institute of Catalysis,
Novosibirsk, Russia

2. Reverse-flow operation of chemical reactors – basic principles
Periodical reversals of the gaseous reaction mixture movement direction in the packed catalyst bed to the opposite and back
Use of regenerative heat exchange instead of recuperative one  capital and operation cost savings
Extremely high operation stability under fluctuations of gas inlet temperature, flow rate and composition
Ideal concept for performance of moderately exothermic reactions with gaseous reactants especially under oscillation of external parameters  purification of waste gases

3. Reverse-flow process for SO2 oxidation
Developed in Boreskov Institute of Catalysis in the late 1970-s
First commercial installation in 1982
Optimal application area – waste gases of non-ferrous smelters
relatively low SO2 concentrations – 1÷4% vol.
expressed oscillations of SO2 concentrations, temperature and flow rate typical for waste gases
More than 20 industrial units with capacities up to 110’000 st.m3/hour in Russia, Kazakhstan, Armenia, China, Bulgaria, Japan, Australia with total sulfuric acid production > 1 mln.tonnes/year
Proven advantages (compared to steady-state process):
capital and operation cost saving
extremely stable operation under oscillations of external parameters

4. Reverse-flow process for SO2 oxidation
New challenge – presence of CO in smelter off-gases
Transfer to novel highly-efficient smelter technologies leads to both the rise of SO2 content and appearance of CO (up to 1-2% vol.) in smelter waste gases
CO oxidizes at V2O5 catalyst at relatively high temperature (above 500C) which is much higher that the temperature of SO2 oxidation light-off ( C)
CO oxidation is strongly exothermic (adiabatic heat rise of ~100C per 1% vol. of oxidized CO) and leads to the increase of the catalyst bed temperature 
Risk of catalyst overheating above the thermal stability limit
Decrease of SO2 conversion due to worse equilibrium constraints
Worsening of the process operation stability and reliability under fluctuations of CO content

5. Process concept idea
Target of the study – development of the reverse-flow reactor concept for SO2 oxidation which may effectively and stably work in presence
While averaged outlet temperature Tout is connected with inlet one Tin by the value of adiabatic heat rise, the maximum temperature in the catalyst bed Tmax does not depend upon Tin when Tin is lower that the temperature of reaction ignition 
SO2 oxidation reverse-process efficiency and stability may be improved by CO separate oxidation at temperatures lower than SO2 oxidation ignition point (< ~400C for V2O5 catalyst)

6. Process concept idea
Additional inserts of CO oxidation catalyst may help to perform CO oxidation outside the high-temperature area of the bed
Problems:
V2O5 catalyst is not suitable for CO oxidation (SO2 oxidation starts earlier than CO oxidation)
conventional Pt/Pd or oxide catalyst are not stable in presence of significant amounts of SO2

7. Pt-containing catalysts on the base of fiber-glass supports (GFC)
Platinum supported at fiber-glass fabric made of high silica glass, modified by ZrO2
High catalytic activity in numerous oxidation reactions under extra-low content of noble metals ( % mass)
High resistance to deactivation in aggressive reaction media
Balzhinimaev et al., Catalysis Today, 2010
Experimentally proven both high activity in CO oxidation and high stability in gases, containing significant amounts of SO2 and SO3
No loss of Pt/GFC activity was found after more than 1000 hours of continuous pilot operation in the conditions of the real sulfuric acid plant (SO2 concentration – 7-9 % vol.)
Zagoruiko et al., Journal of the Air & Waste Management Association, 2010

8. Mathematical modeling of the reverse-flow process with GFC inserts
Two-phase nonstationary one-dimensional model of adiabatic packed beds, taking into account oxidation of both CO and SO2 at both catalysts, heat and mass transfer between gas flow and solid phase
i = SO2, SO3, CO, O2
l=0 (bed inlet) 
t=0 (process start)   = init
Ci, – concentrations of i-th component in the gas phase and at the catalyst surface respectively, u – superficial linear gas velocity in the catalyst bed, l – coordinate along the bed axis,  - stoichiometric coefficients, W – reaction rates, Q – reaction heat effect, T, - gas and solid phase temperature respectively, CP, - gas and solid phase heat capacity, , - heat and mass transfer coefficients, S – unit external surface area of solid phase

9. Mathematical modeling of the reverse-flow process with GFC inserts
Reaction kinetics
SO2 oxidation at V2O5: standard kinetics (Boreskov-Ivanov equation) for commercial catalyst
CO oxidation at V2O5:
SO2 oxidation at Pt-GFC:
CO oxidation at Pt-GFC:
Vorlow et al. Applied Catalysis, 1985
Zagoruiko et al. Chem. Eng. J., 2009
Catalyst, preliminary aged during 1000 hours of pilot plant testing, was used in kinetic experiments
Catalyst sample – 0.02% Pt on Zr-Si glass-fiber support.
Catalyst loading – 0.18 gm.
Gas mixture: CO – 1%, SO2 – 3%, O2 – 8%, He – balance.
Gas flow rate – 51 ml/min. Contact time – 0.5 sec.
Pressure – 1atm. Temperature range 200С – 350С.
Vanag S.V., PhD Thesis, 2012

10. Mathematical modeling of the reverse-flow process

11. Mathematical modeling of the reverse-flow process (without GFC inserts)
Gas composition (%vol.):
SO2 – 3.5÷4.0%,
SO3 – 0.4%,
CO – 0÷1%,
O2 – 8%
Cycle duration: 900 sec (15 minutes)

12. Mathematical modeling of the reverse-flow process (with and without GFC inserts)
Gas composition (%vol.):
SO2 – 3.5%,
SO3 – 0.4%,
CO – 0.4%,
O2 – 8%

13. Mathematical modeling of the reverse-flow process
GFC inserts are efficient in reduction of maximum catalyst temperature

14. Summary
CO admixtures in the smelter waste gases may significantly complicate the operation of SO2 oxidation reverse-flow reactor and lead to catalyst overheating, decrease of SO2 conversion and worsening of operation stability.
Reverse-flow process with a separate CO oxidation at temperatures lower than the SO2 ignition point may help to overcome negative CO influence.
The catalyst with Pt supported in small amount ( % mass) at SiO2- ZrO2 glass fabric (GFC) has the sufficient activity in CO oxidation at moderate temperatures combined with experimentally proven high resistance to deactivation by SOx.
The concept of the reverse-flow reactor with additional inserts of GFC for separate CO oxidation between the beds of vanadia catalyst and inert heat regenerative material was proposed.
According to mathematical modeling data, the proposed approach provides efficient elimination of the reverse-flow reactor overheating and subsequent problems, caused by CO appearance in smelter waste gases.

15. Thank you for your attention!