A dual-purpose tank container is provided, designed for transporting unrefined or refined fluid within the same internal space, eliminating unproductive deadheading. The tank container includes an inner tank in a frame, and outer insulation shell, the tank being equipped with at least one diaphragm so that a fluid, for example crude oil or diesel fuel, can occupy inner space without risk of contamination from a residual fluid. The high viscosity unrefined fluid can be almost completely drained using fluid draining channels present on the diaphragm surface, increasing transportation economy and eliminating any residual fluid pockets. At least one inlet present on the upper tank body supplies a solvent vapor/fluid to condition the space between the diaphragm and the tank wall, thereby preventing sticking of the diaphragm to the tank inner surface.