Downtime, batch reactor

Note that in this case the batch reactor is operating in the production mode only two thirds of the time. This situation is not extraordinary and allowances must be made for downtime in order to meet design requirements properly. 

Initially, Ca0 = Cb0 = 0.17 lbmol/cuft. Required conversion of NaPr is 60%. Reactor sizes are to be found for three modes of operation (a) A batch reactor with downtime of 30 minutes/batch (b) A two stage CSTR to which the feed contains 0.17 lbmol/cuft of each reactant (c) A single stage CSTR. 

Batch reactors are tanks, usually provided with agitation and some mode of heat transfer to maintain temperature within a desirable range. They are primarily employed for relatively slow reactions of several hours duration, since tlie downtime for filling and emptying large equipment may be an hour or so. Agitation maintains uniformity and improves heat transfer. Modes of heat transfer are illustrated in Figs. 23-1 and 23-2. 

In a flow reactor, reactants are loaded continuously at a steady pace, react while flowing through the reactor, and the ensuing products of the reaction are taken off continuously. This enables a steady-state operation of the reactor and eliminates the downtime associated with a stoj>-and-go, or batch, process. To the contrary, a batch reactor is loaded and unloaded for each separate reaction run. Generally, the shorter the reaction time of the process is, the more advantageous is a flow reactor compared with its opposite, the batch reactor. 

The main reason why batch reactors are often preferred in the industiy is the simple operation and lower contamination risk. Still, batch fermentation has a low productivity due to the lag phase of the culture product inhibition downtime for cleaning, sterilising and filling the reactor, low butanol concentration and low initial substrate concentration. This is why fed-batch and continuous fermentatiOTi modes are intensively examined (Lee et al. 2008). 

Daily Yield Say the downtime for filhng and emptying a reactor is and no reaction occurs during these periods. The reaction time of a first-order reaction, for instance, is given by = —In (1 — x). The daily yield with n batches per day will be... 

If the downtime is 45 min per batch, what size reactor is needed for 90% conversion ... 

In case of the standard multipurpose batch plant the downtime of the reactors for the product transfer into temporary storage vessels, the transportation of the vessels to the storage and the cleaning of the reactors before product changeover have to be included. This was modelled by an adjustment of the CIP times of the three stations to 60 min. 

Continuous operation provides high rates of production with more constant product quality. There are no downtimes during normal operation. Reactant preparation and product treatment also have to run continuously. This requires careful flow control. Continuous operation can involve a single stirred tank, a series of stirred tanks or a tubular-type of reactor. The latter two instances give concentration profiles similar to those of batch operation, whereas in a single stirred tank, the reaction conditions are at the lowest reactant concentration, corresponding to effluent conditions. 

All of the reactor systems described so far have shortcomings of one sort or another. They may have excellent throughput, but introduce too many particles into the process. They may be awkward to disassemble and clean, leading to substantial downtime. Uniformity on individual wafers has historically been adequate at 5%. Today, uniformities of 1 to 2% are being required because of more stringent process requirements, and this is hard to achieve over all the wafers in a large batch load. Uniformity of dopant on a wafer is also a concern,... 

A plant operability analysis helps to establish the correct cycle time. In certain situations, the batch cycle of the entire plant is not determined by the units around the main reactor. Additional time may be needed downstream for product finishing in equipment that operates in series with the main reactor. The batch cycle time T is a composite of three contributions (1) the sum of the batch residence times t(i) in the M true batch units of the plant that operate in series (2) the sum of the residence times t(j) in the N semicontinuous trains of the plant that operate in series and (3) the sum of the downtimes t(k) in series encountered in the total batch cycle ... 

It may be desirable in small batch systems to switch a reactor rapidly from one process to another, with minimal cleaning and downtime ... 

---