The mass transfer behavior of a single impeller and multi- impeller reactor was studied by an electrochemical technique which involves measuring the limiting current of the cathodic reduction of K3Fe(CN)6 in a large excess of NaOH as a sup- porting electrolyte (Selman and Tobias, 1978). The rotating impeller (nickel plated copper) acted as the cell cathode while a cylindrical stainless steel sheet lining the container wall acted as anode.

The performance of the reactor in removing copper ions from dilute CuSO4 solution by constant current (galvanostatic) cathodic deposition was also studied to examine the effect of interfering factors such as surface roughness and simulta- neous gas evolution on the validity of the mass transfer design equation obtained using the ferricyanide system.

In view of the fact that drag reducing polymers  can reduce mechanical energy consumption in agitated vessels (Al-Ammeri, 1987; Quraishi et al., 1976; Mashelkar et al., 1975), it would be of interest to study the effect of these polymers on the rate of mass transfer at the rotating impeller, to this end polyethylene oxide (Polyox WSR-301) a product of Union Carbide was used in the form of slurry (Little et al., 1991).

the catalyst particles with the solution in case of continuous operation of stirred tank reactor. Other stirred tank reactors have been suggested where the catalyst can be supported on the wall of the tank (Askew and Beckmann, 1965; Mizushina et al., 1969; El-Shazly et al., 2004; Sedahmed et al., 2014), or on the surface of an internal cooler (Fouad et al., 2013), such designs suffer from limited reaction area and hence low rate of reaction. To increase the reaction area stirred tank reactors with a packed bed fixed to the reactor wall (Mowena et al., 2013) or rested on the reactor bottom (El-Naggar et al., 2014) were investigated for their mass transfer behavior. Despite the advantages offered by the fixed bed  stirred  tank reac- tors they suffer from the high pressure drop and the high pumping power in case of continuous operation. Accordingly more work is needed to develop more efficient heterogeneous liquid–solid stirred tank reactors. In this respect, the aim of the present work is to study the mass transfer behavior of a new heterogeneous stirred tank reactor where the diffusion controlled reaction takes place at the rotating impeller blades. The rotating impeller consists of 8 cylindrical blades, to further increase the reaction area and the space-time yield of the reac- tor, a multi-impeller reactor was also examined for its mass transfer behavior. The suggested reactor offers the following advantages: (i) Impeller rotation not only enhances the rate

2. Experimental technique

The batch reactor (Fig. 1) consisted of a cylindrical plexiglass container of 15 cm diameter and 25 cm height. The working electrode (cathode) was 8 cylindrical blade rotating impeller made of nickel plated copper (Fig. 2). The impeller was fixed to a vertical insulated stainless steel shaft of 0.5 cm diame- ter through a nickel plated hub of 2.2 cm diameter. The shaft was connected to the stem of a 0.5 horse power variable speed motor through a plastic sleeve. Impellers with blades of diam- eter 0.6 and 1.0 cm were used, blade length ranged from 2.6 cm to 4.1 cm. The anode of the cell was a cylindrical stainless steel sheet lining the full length of the container wall. A sin- gle and three separated impellers mounted on the same shaft were used in the present study, distance between impellers in multi-impeller reactor ranged from 1 to 3 cm. Unbaffled and baffled cells were used, in the latter case four baffles made of plastic strips of the dimensions 25 × 1.25 × 0.02 cm were fitted to the cell wall at 90◦. Impeller rotation speed was con- trolled by means of a variac and was measured by an electronic tachometer. The motor was fixed firmly against a brick wall to avoid vibration and eccentric motion. To examine the per- formance of a continuous reactor, a recirculating system  was