(612e) Modeling of High Temperature Shell and Tube Heat Exchanger and Decomposer for Hydrogen Production

Hydrogen can be an attractive energy carrier if it can be produced cleanly and in a cost-effective manner. Sulfur-iodine water splitting cycle is a baseline candidate thermo-chemical process for industrial hydrogen production. The cycle consists of the following three chemical reactions which yield the dissociation of water:

I2 (l) + SO2 (g) + 2H2O = 2HI (aq) + H2SO4 (aq) (1)

2HI (g) = H2 (g) + I2 (g) (2)

H2SO4 (g) = H2O (g) + SO2 (g) + 1/2O2 (g) (3)

Reaction of the sulfuric acid decomposition (Eq. 3) is the most critical part of the cycle via very high temperature and extremely corrosive environment. The sulfuric acid decomposition is carried out in two steps. In the first step, sulfuric acid is assumed to decompose into water and sulfur trioxide (Eq. 5). The reaction is volumetric type of reaction and requires temperature 450ºC. The second step, sulfur trioxide decomposes to produce oxygen and sulfur dioxide (Eq. 6) The reaction is surface reaction which require catalyst and high temperature (700ºC and higher).

H2SO4 (g) = H2O(g) + SO3(g) (4)

SO3(g)= SO2(g)+0.5O2(g) (5)

In the literature different types of high temperature heat exchanger and chemical decomposer are used to support the sulfuric acid decomposition reaction. This paper shows numerical simulations of the sulfuric acid decomposition process in shell and tube heat exchanger and chemical decomposer with straight tube configuration. The shell and tube heat exchanger consists of a bundle of tubes enclosed within a cylindrical shell. It is assumed that sulfuric acid is completely decomposed to the sulfur trioxide (SO3) and water (H2O) before the entrance to the decomposer. In the modeled decomposer one fluid (mix of SO3 and H2O gases) flows through the tubes and second fluid (helium) flows within the space between the tubes and the shell.

The computation geometry of the heat exchanger, used in current study, is created based on the assumption that the inlet and outlet manifolds are properly designed to provide uniform flow distribution for the inner tubes and shell side. Because of the assumption the geometry includes only one tube and part of the shell zone around the tube. The created geometry is two-dimensional axisymmetric geometry.

Helium is made to pass through the shell which provides the required heat for the decomposition of sulfur trioxide. Based on the literature search the dimensions for the computation domain were chosen as follow: the tube is 4.5m in length and 40mm in diameter with an entrance length of 1.5m. The shell is 80mm in diameter. The length of the catalytic bed which is modeled using porous media assumption is 1.5m.

The chemical reaction is modeled using Arrhenius equation. The rate constants for the reaction are computed from are obtained from experiments completed in INL.

A nonstaggered grid storage scheme is adapted to define the discrete control volumes. In this scheme, the same control volume is employed for the integration of all conservation equations, and all variables were stored at the control volume's cell center. The numerical scheme used in this study is a power-law differencing scheme, and the solver used is a segregated solver. The SIMPLE algorithm is used to resolve the coupling between pressure and velocity. The governing equations, which were discrete and nonlinear, are linearized using an implicit technique with respect to set of dependent variables. The algebraic equations are solved iteratively using an additive correction multigrid method with a Gauss-Seidel relaxation procedure.

Because sulfur trioxide decomposition reaction is catalytic surface reaction, the choice of the catalyst is very important to get high decomposition percentage. In the used numerical model the different type of the catalyst are modeled by selection of appropriate activation energy value for the Arrhenius equation. Besides, the concentration of the catalyst in the catalyst carrier influence to the activation energy significantly. For example, for the 1 wt % Pt the activation energy is 32.67 kJ/mol; for the 0.1 wt % Pt the activation energy is 46.24 kJ/mol.

Fluid flow, heat transfer and chemical reaction calculations were done for the shell and tube configuration using porous media approach, with counter and parallel flow arrangements.

The percentage decomposition of sulfur trioxide is found to be 93% for the counter flow arrangement. For the parallel flow arrangement the decomposition percentage of sulfur trioxide is found to be 92%.

The parametric studies for different activation energies have been completed in the research. It was found, that sulfuric trioxide decomposition percentage increases when the activation energy decreases for the both parallel and counter flow cases.

The obtained results can be used for the optimization of the shell and tube high temperature heat exchanger and decomposer design and operating conditions to increase the decomposition performance and minimize pressure drops.