1. Hydrogen via Thermochemical cycles
Vapor-Liquid Equilibrium for the system SO2-O2-H2O
Moises Romero,
Prof. R. W. K. Allen Dr. G. H. Priestman
Chemical & Biological Engineering.
Introduction
Thermochemical cycles have great potential for massive scale, carbon-neutral hydrogen production[1]. Of particular interest are the Sulphur Iodine (SI) and the Hybrid Sulphur (HyS) cycles, which have in common the high temperature thermal decomposition of H2SO4 [2,5]. No known thermodynamic data exists for this particular system. In order for a design-based approach to be taken, accurate vapour-liquid data is needed for the decomposition products SO2-H2O-O2. This work experimentally investigates both the binary SO2-H2O and ternary SO2-O2-H2O systems at pressures up to 15 bar and temperatures up to 80ºC.
Fig. 4 Equilibrium
Dissociation Equations
Fig. 5 Vapour Liquid
Equilibrium Equations
Aim
The objective is to support hydrogen production with a main focus on advancing research into the oxygen separation stage of the HyS and SI cycles. These experiments determine the solubility of SO2 and O2 in water. This is a difficult task, as the components that are being worked with are at high pressure, temperature, are toxic and corrosive.
Fig. 6 Condition of electroneutrality
Results & Conclusions
Under certain conditions (>60ºC, mSO2>0.38), the oxygen fugacity coefficient in multicomponent mixtures increases further than unity. At these conditions, the combined effect of reduced salting out effect and increased fugacity is sufficient to increase the dissolved oxygen concentration above the equilibrium solubility in pure water. This is an important finding. The O2 separator design should avoid these conditions to improve efficiency.
The model gave a good correlation at moderately high pressures, as shown in figure 7, where plotted lines are multicomponent models and points are the different experiments carried out at the same conditions.
The divergence at high pressures in the binary solution is thought to be due to the formation of two liquid phases, and the underestimation of the dissolved SO2 salting-out effect. Online spectroscopic measurements are being developed to measure this phenomenon.
3D Physical Reactor Design and piping arrangement
Fig. 1. Equilibrium
reactor. a) Digital
Interface b) Computer
c) Temperature controller
d) N2 outlet e) Sample outlet
f) Sample inlet g) Mat heater
h) Liquid chamber i) PFA Valve 2
j) PFA Valve 1 k) Rotating stand l) PTFE liner m) Gas Chamber
n) Thermocouple ñ) Mat Heater o) Sample Gauges p) N2 gauges q) N2 Inlet
Fig. 7 Multicomponent experiments at 40 degrees. Points are experimental sampling derived from SO2, O2 and titration analysis from both phases. Top corner, raw experimental response.
Experimental Work
An equilibrium reactor (shown in figures 1 and 2) was developed to determine the solubilities of interest. Experiments were compared to a model based on weak electrolyte thermodynamics[3,4]. The reactor consists of two main chambers, the larger one is the gas chamber, the smaller the liquid chamber. This is rotated, ensuring gases are well mixed. The temperature is controlled by rope and mat heaters, using LabVIEW®, and monitored by heat switches and internal and external thermocouples. A PEEK pressure transducer is used, as well as PEEK valves and PTFE lining throughout the entire reactor (to avoid SO2 metal-catalysed oxidation).
Future Work
The formation of a second phase and the inability to measure concentrations online led this work into in-situ spectroscopic alternatives. Still in development, these are promising solutions because of the lack of understanding of the mixing behaviour of the aqueous solution, as well as showing information for future species addition, being H2SO4 the most likely. On figure 8 are the proposed spectroscopic probes, showing a Raman and a ATR-FTIR high pressure probes.
Fig. 2 Reactor Picture (above). Insulation is needed to preserve the reactor temperatures and avoid unnecessary loss of heat.
Fig. 3 Problem Schematic.
Fig.8 FTIR and Raman probes, for the in-situ analysis of gaseous and liquid species (right).
References
[1] Stolten (2010) Hydrogen and Fuel Cells, Wiley VCH
[2] Momirlan (2002) Current status of Hydrogen Energy, Renew. Sustain. Energy Rev. 6 (12)
[3] Elder, Romero, Shaw, Allen, Priestman (2011) Measurements of the Solubility of Sulphur Dioxide in Water for the Sulphur Family of Thermochemical Cycles. Intl. J. Hydrogen Energy (In press)
[4] Zemaitis, JF, Clark, DM, Rafal, M, Scriviner, NC (1986) Handbook of Aqueous Electrolyte Thermodynamics. AIChE DIPPR
[5] Elder, R, Allen, R, (2009) Nuclear heat for hydrogen production: Coupling a very high/high temperature reactor to a hydrogen production plant. Progress in Nuclear Energy 51
Mathematica Modelling
A comparison was made between a thermodynamics-based model for the SO2-O2-H2O interactions in solution and the experiments. The theories of phase equilibrium and electrolyte thermodynamics are required to calculate solubility. An understanding of the equations used to calculate equilibrium of the two component system, SO2 and H2O was necessary; as well as a calculation method which included oxygen. While strong electrolytes are totally dissociated, the model needed to account for the dissociation of only a portion of molecules into ions, a characteristic of weak electrolytes. To solve the two-phase problem (represented in figure 3), equilibrium dissociation equations (figure 4), vapour liquid equilibrium equations (figure 5) and the electroneutrality equation (figure 6) are needed. Other equations such as equations of state, activity and fugacity coefficient calculations are best described in the literature [3,4].
Chemical & Biological Engineering ‘Engineering from Molecules’