in process of dialysis there are two openings for water in and out.One opening is at bottom for water inlet and one is at top for water outlet .Why is outlet kept on upper side?

Dialysis is the process of  separating the dissolved ions or molecules of small dimensions by means of their rate of diffusion. When the outlet is down due to gravitational force all particles may come out and the separation may not be effective. When its upward the heavier molecules cannt move against the gravitational pull, so that they wont pass out. Only small dimensional particle moves away.