Cleaning batch reactors

Time must be included for emptying, cleaning, and filling the batch reactor (=30 min). 

Inhibitors can be present in most reaction ingredients. They are deliberately added to monomers to prevent premature polymerization. Ingredient streams such as monomers are cleaned and handled carefully to avoid inhibition in fundamental studies, especially in most academic laboratories. Commercial processes, however, are usually operated with inhibitors present in the feed streams, particularly in the monomer. When such ingredients are used in a batch reactor, a dead time is observed before the reaction starts. 

Batch reactors are often used for liquid phase reactions, particularly when the required production is small. They are seldom employed on a commercial scale for gas-phase reactions because the quantity of product that can be produced in reasonably sized reactors is small. Batch reactors are well suited for producing small quantities of material or for producing several different products from one piece of equipment. Consequently they find extensive use in the pharmaceutical and dyestuff industries and in the production of certain specialty chemicals where such flexibility is desired. When rapid fouling is encountered or contamination of fermentation cultures is to be avoided, batch operation is preferable to continuous processing because it facilitates the necessary cleaning and sanitation procedures. 

The Diels-Alder liquid-phase reaction between 1,4-benzoquinone (A, C6H4O2) and cy-clopentadiene (B, C5H6) to form the adduct CnHm02 is second-order with a rate constant kx = 9.92 X 10 6 m3 mol 1 s 1 at 25°C (Wassermann, 1936). Calculate the size (m3) of a batch reactor required to produce adduct at the rate of 125 mol h 1, if cAo = cBo = 100 mol m 3, the reactants are 90% converted at the end of each batch (cycle), the reactor operates isothermally at 25°C, and the reactor down-time (for discharging, cleaning, charging)... 

The capital cost of a batch reactor and its auxiliary equipment is generally low, but operating costs tend to be high because of the need to have a plant operator in attendance, especially when the reactor is being emptied, cleaned and recharged with a fresh batch of material. The product from a batch reactor is also subject to unpredictable variations in quality from one batch to another. 

First of all, before we compare flow reactors, let us mention the batch reactor briefly. The batch reactor has the advantage of small instrumentation cost and flexibility of operation (may be shut down easily and quickly). It has the disadvantage of high labor and handling cost, often considerable shutdown time to empty, clean out, and refill, and poorer quality control of the product. Hence we may generalize to state that the batch reactor is well suited to produce small amounts of material and to produce many different products from one piece of equipment. On the other hand, for the chemical treatment of materials in large amounts the continuous process is nearly always found to be more economical. 

Commercial-scale batch reactors are generally used for small-lot or specialty items. This includes chemicals such as paints, dyes, and pharmaceuticals. Batch reactors are very simple and flexible. Vessels used to make one compound can be washed and reused to make other products. The ease of cleaning and maintaining batch reactors along with low capital investment and low instrumental costs... 

It is important to recognize that all surfaces that contact with the luminous gas phase participate and influence LCVD operation. Therefore, in principle, in a batch operation, the first run with clean reactor wall could not be replicated in the second run with contaminated reactor wall. Thus, it is necessary to include the step for cleaning the reactor. If only hydrocarbons were used in an LCVD, the cleaning could be done by O2 discharge prior to the normal LCVD operation. (The influence of wall contamination was described in Chapter 10.) In this respect, the effort to minimize the deposition on nonsubstrate surfaces is important even in batch operation of LCVD. Magnetron discharge is quite effective in this respect, as described in Chapter 14. 

However, each set of factors entering in to the rate expression is also a potential source of scaleup error. For this, and other reasons, a fundamental requirement when scaling a process is that the model and prototype be similar to each other with respect to reactor type and design. For example, a cleaning process model of a continuous-stirred tank reactor (CSTR) cannot be scaled to a prototype with a tubular reactor design. Process conditions such as fluid flow and heat and mass transfer are totally different for the two types of reactors. However, results from rate-of-reaction experiments using a batch reactor can be used to design either a CSTR or a tubular reactor based solely on a function of conversion, -r ... 

The total cycle time in any batch operation is considerably longer than the reaction time, as one must account for the time necessary to fill (t ) and empty (f,) the reactor together with the time necessaty to clean the reactor between batches, In some cases the reaction time calculated from Equation (4-5) may be only a small fraction of the total cycle time, t,. 

In fine chemicals industries the batch reactor is used almost exclusively. For catalytic hydrogenations the batch reactor has a number of inconveniences in case a slurry catalyst is used. The complicated cooling coils, dead spaces behind baffles, etc. are difficult to clean if the catalyst has to be removed from the vessel. After each batch the vessel has to be emptied, which also implies that the catalyst falls dry. It is known that specially in this period all reactants and products, still contained in the pores of the catalyst, rapidly may deteriorate via unwanted side reactions and so deactivate the catalyst. Usually deactivation starts as soon as the catalyst is no more protected by the solvent. As a production series of one product consists of a number of batches, the exposure of the catalyst to deactivation conditions is frequent. 

In this study protonated large pore zeolites of different structures (HY, HBeta and HMordenite) and framework Si-to-Al ratios were used in liquid phase in a batch reactor. The zeolites were calcined at 500°C and the hydrolysis was conducted at 75°C. The procedure was optimised in terms of solvent, activation, type and amount of catalyst for the hydrolysis of nitroacetanilides, currently carried out with 10 % sulphuric acid [14], and then extended to other substituted amides. The reaction, followed by GC with nitrobenzene as internal standard, was clean and no by-products or degradation were detected. 

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