In ICEM CFD, you can connect or merge different fluid zones using the "Build Topology" and "Match Control" features. Here's a general workflow:

1. Build Topology:

- Use the "Build Topology" tool to ensure that the mesh vertices on the interface of the two zones are coincident.

2. Match Control:

- Apply "Match Control" to define the interface and set the appropriate tolerance values. This helps in ensuring smooth connectivity.

3. Edge Split:

- If needed, you can use "Edge Split" to refine the mesh along the interface, especially if there are significant differences in mesh densities between the Preheat and Reactor Zones.

4. Boundary Conditions:

- Ensure that you correctly define boundary conditions on both sides of the interface. This includes specifying the type of connection (e.g., coupled, shared, etc.).

5. Quality Check:

- After merging, perform a thorough mesh quality check to identify and address any issues. Pay attention to skewness and aspect ratio.

6. Tweaking Tolerance:

- If merging issues persist, consider adjusting the tolerances in the "Match Control" settings. Sometimes, a slight tweak in tolerances can resolve problems.

7. Solver Compatibility:

- Verify that the interface setup is compatible with the solver you intend to use. Check solver documentation for any specific requirements.

8. Debugging:

- If errors persist, use the ICEM CFD error messages as a guide. They often provide information on what aspect of the mesh is causing the problem.

or else if you want more suggestions please ask help in ICEM CFD community website

Actually, it depends on the kind of mesh you want to create: unstructured or structured. Because of the simplicity of the geometry I recommend the structured type. At the beginning, suppose that there are just one zone and create an O-Grid type for the whole geometry and do all the association processes. In the end, just split the block in the separation point and then associate the faces of blocks to the surfaces of the geometry. Note that you have just one surface between the two zones and you can name it interior or whatever you want. Because of all mesh elements are coincident and the mesh in conformal and both zones are fluid, you can give it the interior boundary condition and so the precision of your simulation will be higher than the interface boundary type. You can see some useful tutorial videos in my youtube channel "infinitycfd" with this link:

https://www.youtube.com/@infinitycfd32