Batch recycle mode

Most of the experiments were carried out in a pilot-scale reactor (Fig. 8.25). For each experiment, four pieces of coated monoliths were stacked carefully in line and mounted in a 2 m- high heated column of 50 mm diameter. The preheated liquid feed was distributed via a spray nozzle. After each pass, the liquid was collected in the liquid-supply vessel and was circulated continuously through the reactor (batch-recycle mode). The preheated countercurrent nitrogen stream was fed after one pass to a condenser to separate the liquid, and then vented. The condensables were collected in a phase separator, from where the water could be tapped off, whilst the... 

The reactor can be operated in a batch recycle mode. One of the advantages of SPE HDH technology is that separation and recycling of a supporting electrolyte are unnecessary, which can greatly reduce process cost. 

Fig. 19.9 Sketch of the flow undivided electrochemical reactor working in batch recycle mode designed for the hydroxylation of chlorobenzene and phenol (Hsiao et al. 1993)...

Using a batch recycle mode, oily wastewater was treated in the photoassisted advanced oxidation systems. Their results indicated that UV/H2O2 oxidized oily compounds into organic acids with the efficiency being greater at acidic pH. The oxidation rate was enhanced in the presence of Fe " ". 

There are many variants on the batch-recycle mode. A practical example is shown in Fig, 19. Semi-batch operation allows for continuous or intermittent addition of reactant. 

As an example of the technique, we may consider the reduction of penta-chlorophenol (PCP). The reactor consisted of two compartments, each 10 cm long, 2 cm wide and 0.5 cm deep, the cathode being a packed bed of carbon fibre, which was separated from the anode by a cation-exchange membrane (Nation 426). In the batch-recycle mode, constant current (10 A) electrolysis of 1 dm of 50 mg dm PCP in 0.1 mol dm " Na2S04 plus 0.1 mol dm NaOH removed PCP to a level < 0.5 mgdm within 30 min. The overall current efficiency for complete dechlorination was approximately 1 %, the energy consumption being 36 kWhm"l... 

Pagano et al. (1995) confirmed that the use of a R-UFB led to significant dechlorination of Aroclor 1248 spiked sediments. A 6 L colunm was packed with sand and operated at 30°C (Figure 14.14). Sanitary landfill leachate was used as carbon source, since it contains a vast amoimt of volatile fatty acids. The microbial community present in the sediments was used as a source of dechlorinating bacteria. After 91 days of batch recycle mode, 23% of Aroclor was dechlorinated, the highest removal was observed within 7 weeks, biphenyl (a liver toxin) was the main breakdown product. The microbial community was not identified. 

Size reduction that is achieved in a wet mill will depend in part on the residence time of the batch in the mill. Residence time can be controlled by operating in single-pass mode, where the batch is pumped through the milling device from one vessel to another, or in recycle mode. For both, it is important to measure slurry flow rate to confirm that residence times are maintained as a process is scaled up. When the batch is recirculated, one way to quantify this is to convert elapsed time to number of batch turnovers (batch turnovers = elapsed time X flow rate through the mill/total batch volume). It can take several passes or batch turnovers to achieve a steady-state particle size distribution, depending on the mill. 

Working in the overloaded mode seems more feasible in the closed-loop recycling mode compared with the elution batch mode. 

DiGiano (1996) suggested a batch-recycle membrane test as an alternative. The test operates in batch mode by recirculating both feed and permeate. The advantage of this test is the lower feed volume required (3 L versus 60 to 200 L for the RBSMT). Moulin (1993) described a dynamic membrane test, that allows determination of the suitability of a membrane to obtain the required health standards (cytotoxicity). 

The batch recycle differential reactor is used to obtain adsorption rate data to evaluate various kinetic models for cadmium ion adsorption on Sol-AD-lV. Approximately 0.25 g of the solid extractant is placed between glass beads inserted in a glass tube of 1.0 cm internal diameter. Solutions of various cadmium ion concentrations (25 to 300 mg/1) are contacted in a batch mode over 720 minutes, with intermediate sample taken for atomic absorption spectroscopic measurements of the cadmium ions. The external mass transfer resistances are minimized by operating at 16 ml/min. Results of the experiments are shown in Figure 7.23. 

The factors in Tabic 2.5 may be reviewed briefly. The mode of operation may involve a batch (simple batch or batch recycle) or a continuous (plug flow stirred tank or cascade) reactor, with respect to electrolysis (as detailed in section 2,4). The mode of operation must also be considered with regard to product extraction from the cell, which may be batch or continuous. In the simplest case, the products are removed via the exit flow from the reactor... 

Owing to the fast catholyte flow rate necessary to avoid side reactions, the conversion of acrylonitrile to adiponitrile per pass of the cathoylte through the cell was only 0.2%. This necessitated the use of an external reservoir and determined the mode of opemtioti of the process, le. batch recycle. A fraction of the reservoir was bled continuously Into the extraction plant and further... 

Although the system could have been operated in a continuous mode, it was decided to operate it in a batch mode. In the batch high-recycle mode the reactor mass balance is that for a stlrred-tank batch reactor and thus over a period of time is analogous to the mass balance for a plug flow reactor over a distance along... 

Fig. 7.1 A possible mode of operation for metal-extraction cells. The majority of the duty is met by the primary cell which is part of a batch-recycle loop incorporating the storage tank. The secondary, or tailing , cell is used as a single-pass (or cascade) reactor to reduce the metal to a level suitable for discharge.

The ion-exclusion process for sucrose purification has been practiced commercially by Firm Sugar (104). This process operates in a cycHc-batch mode and provides a sucrose product that does not contain the highly molassogenic salt impurities and thus can be recycled to the crystallizers for additional sucrose recovery. 

Another common situation is batch hydrogenation, in which pure hydrogen is introduced to a relatively high pressure reactor and a decision must be made to recycle the unabsorbed gas stream from the top of the reactor or use a vortexing mode for an upper impeller to incorporate the gas from the surface. 

Continuous Stirred Tanks Without Biomass Recycle. The chemostat without biomass recycle is a classic CSTR. The reactor is started in the batch mode. 

Figs. 2a and 2b show typical results obtained with the electrocatalytic YSZ recycle reactor operated in the batch mode. The initial CH partial pressure was 20 kPa, i.e. 20% CH in He at atmospheric pressure. The recirculation... 

---