Time-averaged pressure drops have been measured in a 15 cm internal diameter reciprocating plate column, using the wateriair system under conditions of cocurrent, countercurrent and single phase (water) flow. The reciprocation frequency was varied from 1 to 4 Hz, while the stroke (peak to trough amplitude) was set at 2.54 cm. Two types of perforated plate were used, having perforation diameters 14.3 and 6.35 mm.Under single phase flow conditions, the pressure drops were in agreement with an earlier model due to Noh and Baird ( I 984). Under cocurrent conditions the gas flow had the effect of increasing the pressure drop, particularly at low reciprocation rates; this was interpreted in terms of has been developed for industrial applications in diameters up to 1.7 m. Other industrially important types of RPC are the Prochazka column (PRPC) featuring smaller perforations and downcomers for the continuous phase, and the KRIMZ and other types developed in the U.S.S.R. Detailed reviews of the extraction characteristics of RPCs are available (Lo et al., 1992;Baird et al., 1994).Research activity on gas-liquid contact in W C s has been relatively slow to develop. In the 197Os, Miyanami and coworkers operated a vibrating disk contactor (the MVDC) containing a series of reciprocating disks driven from a vertical central shaft, interspersed with donut-shaped stator plates. Hydrodynamic data (Tojo et al., 1974a) and mass transfer data (Tojo et al., 1974b) were reported. Brauer and Sucker (1979) operated a gas-liquid 30 cm diameter Karr column as a fermentor. This led to the development of a reciprocating-jet gas-liquid reactor which has been successfully applied to waste water treatment (Brauer and Hennig, 1986) and the production of antibiotics (Brauer, 1989).The pressure drop in an oscillatory flow system comprises a fluctuating component and a time-average value. The fluctuations in pressure are of obvious practical importance. The maximum (peak) pressure affects the safe mechanical design of the column, including seals. The minimum 'Author to whom correspondence should be addressed. E-mail address:bairdmhi@mcmaster.ca (trough) pressure may lead to local cavitation in cases of intense oscillation (Baird and Stonestreet, 1995). In reciprocating-plate columns, the pressure fluctuations can be measured with a transducer and recorder. When these variations are integrated with the displacement, values of the energy dissipation rate in the column can be calculated (Hafez and Baird, 1978; Baird and Rama Rao, 1995). Attention must also be given to the time-average pressure drop in a reciprocating plate column. This is usually different under oscillatory flow conditions than under steady conditions at the same time-average flows. Noh and Baird ( I 984) showed that in single-phase flow conditions the time-average pressure drop was significantly increased by reciprocation, so that additional power would be needed to pump the feeds through the system. This additional energy input is different from that needed to reciprocate the p...