Airlift loop tanks (ALT) are used in a many industrial applications where close contact among the different phases is essential. We can find such examples in the chemical, petrochemical, biochemical and minerals processing industries. The ALT principle is based on creating a zone within the tank through which gas rises. In this way a difference in density is obtained between the gas-liquid-solid riser zone and the liquid-solid downcomer zone; this density difference induces circulation of the liquid and solid phases through both zones. When the gas velocity is increased, gas bubbles inside the liquid phase are also transported into the downcomer. This circulation creates a good mixture of all three phases and efficient suspension of solid in the liquid. A main parameter that characterizes ALT operation is the circulation velocity, which can be quantified by the riser superficial liquid velocity. This parameter is a function of the operational adjustable variables such as gas flow-rate, liquid and solid densities, fractional holdup, solid size and tank geometry. The following variables were determined by experimental work: pressure losses in ALT, superficial solid velocity in riser and downcomer, superficial liquid velocity in riser and solid holup in riser. Finally the relationship between these variables was established. (C) 2003 Society of Chemical Industry.