1. Predicting the energy efficiency of a recuperative bayonet decomposition reactor for thermochemical sulfur cycles
Dr. Max Gorensek, PE; Dr. Tommy Edwards
Computational Sciences Directorate
April 15, 2009
4th NEA Info Exchange – Nuclear H2 Production
Apr 13-16, 2009 Oakbrook, IL

2. Outline Background Bayonet Reactor Pinch Analysis Recuperation
Helium Heating
Statistical Design of Model Experiments and Analyses
Results
Conclusions
Gorensek, MB; Edwards, TB. Energy efficiency limits for a recuperative bayonet sulfuric acid decomposition reactor. submitted to I&EC Research 23 Feb 2009

3. SNL Bayonet Decomposition Reactor Concept
All-silicon carbide construction
Bayonet reactor consists of one closed ended tube co-axially aligned with an open ended tube to form two concentric flow paths
Heat applied externally
Liquid fed to annulus, vaporized, passed through catalyst bed
Product returns through center, heats feed through recuperation
Advantages include internal heat recuperation, only one connection at cool end, corrosion resistance, and low fabrication cost
Silicon carbide bayonets are an off-the-shelf item (thermocouple tubes)
H2SO4 in
SO2, O2, H2O out
catalyst
High-
Temperature
Heat
Cool base

4. Bayonet Decomposition Reactor Scale-up Concept
Multiple parallel bayonet reactors manifolded at base
Hot He applied externally with baffling to ensure counter-current flow (w.r.t. fluid being heated in annulus)
Insulation layer as needed to keep base cool, prevent cross-pinch heat transfer
Hot (~850°C) He from heat source
Feed, product manifold
Insulation
Cool (~500°C) He to heat source
He flow baffling
Multiple parallel bayonets
Catalyst beds
Graphic courtesy P.S. Pickard, SNL

5. HTGR Heat Transfer System
50°C min ΔT between primary He and secondary He at IHX, 25°C min ΔT between secondary He and process fluid at PCHX.

6. Pinch Analysis of Bayonet Reactor
Assume bayonet design achieves good heat transfer
Reaction accomplished with practical bayonet length
Reasonable temperature differences attained
25°C approach between helium heat source and annulus
10°C approach between annulus and inner tube
Other assumptions
Steady-state operation
Plug (single-phase) or homogeneous (two-phase) fluid flow
Fluid flow path cross-sections well-mixed
Local thermodynamic and phase equilibrium
Minimum heating target for decomposition reaction can be calculated from a pinch analysis.
Heating target provides a lower limit on the energy requirement – at least this much will be needed.

7. Pinch Analysis Methodology
Track sulfuric acid fluid element passing through bayonet with Aspen Plus*
Ramp temperature from inlet to Tmax and back in increments
Maintain phase and dissociation reaction equilibria
Liquid-phase electrolyte dissociation
Vapor-phase (H2SO4  SO3 + H2O) dissociation
Add SO3 decomposition equilibrium when T ≥ Tcat, remove when T reaches Tmax
Determine temperature – enthalpy relationship
Prepare heating (annulus) and cooling (center-tube) curves
Import into Aspen Energy Analyzer
* Gorensek, M.B.; Summers, W.A. Hybrid Sulfur flowsheets using PEM electrolysis and a bayonet decomposition reactor. Int. J. Hydrogen Energy 2008, doi: /j.ijhydene

8. High-temperature heating target
Bayonet Reactor Pinch Analysis – Recuperation
High-temperature heating target
320.9 kJ/mol
80.1% H2SO4 feed,
90-bar pressure,
900°C peak process temperature,
equilibrium decomposition,
10°C min ΔT
Heat rejection target
94.5 kJ/mol

9. Bayonet Reactor Pinch Analysis – He Heating
80.1% H2SO4 feed,
90-bar pressure,
900°C peak process temperature,
equilibrium decomposition,
10°C min ΔT recuperation,
25°C min ΔT He heating

10. Statistical Analysis – Control Variables
These variables “control” the process and are of interest when the output of a computer experiment is a measure of process performance
Variable
Minimum
Maximum
ΔTmin,recup, °C 10
100
ΔTeq, °C
-25
Pi, bar 90
Tcat, °C
600
740
Ti, °C 48
150
Tmax, °C
750
900
xi, mole fraction SO3
(wt% H2SO4)
0.068
(30)
0.384
(90)
Min ΔT for recuperation
Temp approach to equil.
for SO3 decomposition
Inlet pressure
Catalyst bed inlet temp
Inlet temperature
Catalyst bed outlet temp
Acid feed concentration

11. Statistical Analysis – Response Variables
These variables are a measure of process performance and respond predictably to changes in the input or control variables in a computer experiment
QH,min – minimum heating target, kJ/mol SO2
Normalized to 1 mol SO2 product
Primary measure of performance
Should be <450 kJ/mol SO2 (including any pre-concentration)
X – fractional conversion of SO3 to SO2
Secondary performance measure
Tp,He – helium pinch temperature, °C
Impacts utilization of nuclear heat for decomposition
QC,min – minimum cooling target, kJ/mol SO2
Measure of waste heat available for acid concentration

12. Statistical Analysis – Experimental Design
JMP used to generate 80-point Latin Hypercube Design (LHD)
Space-filling design
80 levels for each factor
Minimum distance between design points maximized subject to even spacing of levels constraint for each factor

13. Statistical Analysis – Results
Most interesting result at left
QH,min strongly dependent on xi
QH,min<400 kJ/mol SO2 iff xi>0.228
Lowest value of QH,min calculated was kJ/mol SO2 at xi = (81.4 wt% H2SO4)
Attempts made to apply response surface modeling
Random data subsets excluded from model, used for validation
No success
Neural network modeling also failed
Heuristic approach adopted instead

14. Heuristic Analysis
LHD supplemented with additional runs seeking to minimize QH,min
Over 80 more combinations of control variables simulated
Definitive operating envelope was established
Effect of individual control variables on QH,min (within the given control variable domain)
Pi and Tmax should be as high as possible (90 bar, 900°C)
ΔTmin,recup and –ΔTeq should be as small as possible (10°C and 0°C)
Ti and Tcat have no effect in this domain
Optimal xi between 0.28 and 0.3, depending on other variables
Lowest value of QH,min found within given control variable domain is kJ/mol SO2
xi = (80.1 wt% H2SO4)

15. Effect of Peak Temperature on QH,min at Various Pressures
xi = 0.298, Ti = 25°C, Tcat = 675°C, –ΔTeq = 0, and ΔTmin,recup = 10°C

16. Effect of Pressure on QH,min at Various Peak Temperatures
xi = 0.298, Ti = 25°C, Tcat = 675°C, –ΔTeq = 0, and ΔTmin,recup = 10°C

17. Lowest QH,min at High and Low Temperatures
Ti = 25°C, Tcat = 675°C, –ΔTeq = 0, and ΔTmin,recup = 10°C

18. Helium Pinch at High and Low Temperatures
Ti = 25°C, Tcat = 675°C, –ΔTeq = 0, and ΔTmin,recup = 10°C

19. He Heating Pinch Analysis – Low Temperature
77.6% H2SO4 feed,
15-bar pressure,
700°C peak process temperature,
equilibrium decomposition,
10°C min ΔT recuperation,
25°C min ΔT He heating

20. Acid Concentration Trade-off
90-bar pressure,
900°C peak process temperature,
equilibrium decomposition,
10°C min ΔT recuperation,
25°C feed temperature,
675°C catalyst bed inlet temperature
Heating Target
Cooling Target

21. Conclusions (1 of 2)
Pinch analysis used in conjunction with statistical methods to establish limiting operating envelope of bayonet reactor without detailed heat transfer design
To achieve lowest heat target, operate bayonet at highest possible temperature and pressure and around 80% H2SO4 feed concentration
For Tmax = 900°C (ROT = 975°C), Pi = 90 bar, and 80.1% H2SO4 feed
QH,min = kJ/mol, lowest value observed
Tp.He = 539.5°C, allowing 385.5°C coolant temperature change
There is disincentive for feeding > 80% H2SO4, but effect of feeding < 80% H2SO4 partially offset by increase in cooling target, displacing some of heat needed for acid concentration – potential trade-off
Feed and catalyst bed inlet temperatures have no effect on QH,min, but catalyst bed inlet temperature can determine helium pinch at lower peak process temperatures

22. Conclusions (2 of 2)
For peak process temperatures above 800°C, increasing pressure lowers the heat target; below 800°C it raises the target
Reactor operating temperature may be lowered 150°C (to 825°C, or Tmax = 750°C) to alleviate materials concerns, but potential efficiency advantage of sulfur cycle over direct electrolysis impacted
Dropping ROT below 825°C (Tmax below 750°C) does not lead to an efficient process
For Tmax = 700°C (ROT = 775°C), Pi = 15 bar, and 77.6% H2SO4 feed
QH,min = 409 kJ/mol, 25% more than at 900°C
QH,min increases to 461 kJ/mol at 60 bar, 469 kJ/mol at 90 bar, 46% more than at 900°C
Tp.He = 631.4°C, allowing <100°C coolant temperature change