In the first step, we model a reactor as an empty pipe of 10 mm diameter and 18 mm height.

1. Select Model → GadGeo in the menu bar.
   
2. In the GadGeo module section, choose Create / Add GAD Objects from the pull-down menu and click Edit ….
   
3. In the Create / Add GAD Options dialog, set the result file name to Reactor.gdr. By default, a sphere and a general short circular fiber are selected in the list of objects to be generated. We do not need them for this tutorial, so unfold them by clicking on their names and then click the Remove button for both objects.
   
4. From the Object Type pull-down menu, choose Short Circular Fiber and click Add.
   
5. At the top right, change the length unit to mm. In the right panel, under General, enter the following values:
   1. Point1 / (mm): (5.25, 5.25, 0)
   2. Point2 / (mm): (5.25, 5.25, 18)
   3. Diameter / (mm): 10
      

6. Go to the General Options tab and define the domain settings:
   1. Set NX, NY to 105, respectively,
   2. Set NZ to 180,
   3. The Voxel Length / (mm) is set to 0.1.
   4. Make sure, that the boxes for Periodic are unchecked.
      

7. Then click OK in the dialog box and Run in the GadGeo module section.
   
8. After the creation of the structure, go to the main GeoDict GUI, e.g. by clicking the button in the opening Result Viewer. Change to the 3D View by  clicking on the corresponding button in the toolbar. In the project status section, right-click on Structure and choose Invert Structure. Confirm this by clicking Yes in the opening dialog.
   
   In this simple case, the invert command switches the Material IDs of each voxel. So all previously solid parts are now pores and vice versa
   
9. Then change the Material of ID 01 from Glass to a general Solid.
   1. Click on the Selet Constituent Materials button in the toolbar.
   2. Click on the name of Material ID 01 Glass to open the Material Selector dialog.
   3. Select the Material Manual in the Solid section (scroll up to the top).
   4. Confirm with OK to close the Material Selector dialog and observe that the material for ID 01 has changed.
   5. Click Run to apply the changes to the structure.
      
10. In the menu bar, click File → Save Structure as... and save it as Reactor.gdt to the project folder.