Semi-batch processing

It is important to know how much heat of reaction can accumulate when assessing the hazards related to an exothermic reaction. Accumulation in a batch or semi-batch process can be the result of ... 

A way to narrow the MWD and to approach the structure of dendrimers is the addition of a small fraction of a/-functional initiator, to inimers [40,71]. In this process the obtainable degree of polymerization is limited by the ratio of inimer to initiator. It can be conducted in two ways (i) inimer molecules can be added so slowly to the initiator solution that they can only react with the initiator molecules or with the already formed macromolecules, but not with each other (semi-batch process). Thus, each macromolecule generated in such a process will contain one initiator core but no vinyl group. Then, the polydispersity index is quite low and decreases with / M /Mn l-i-l//. (ii) Alternatively, initiator and monomer molecules can be mixed instantaneously (batch process). Here, the normal SCVP process and the process shown above compete and both kinds of macromolecules will be formed. For this process the polydispersity index also decreases with/,but is higher than for the semi-batch process, M /Mn=Pn//. ... 

For SCVCP, the PDI is decreased in proportion to the comonomer ratio, y=[M]o/[I]o M, /Mn=l-I- /(/+ ) for y l [73]. The addition of a multifunctional initiator again affects the polydispersity index [72]. In the batch process it decreases with initiator functionality as M /Mn Pn/(y+l)/, similar to homo-SCVP. The effect is even more pronounced for the semi-batch process where the concentration of the inimer and the comonomer is kept infinitesimally low and M, /Mn=l-i-l//. This result is identical to the value obtained in homo-SCVP,that is, addition of comonomer does not decrease polydispersity any further. 

The presence of a polyinitiator has a small effect only in batch mode, whereas the polymer obtained in a semi-batch process is more strongly branched (DB=2/3 as compared to DB=0.465 without initiator) [40]. A similar result was found by Hanselmann et al. for ABj monomers in the presence of a core-forming molecule [79]. 

The main driver was to develop a laboratory-scale micro-channel process and transfer it to the pilot-scale, aiming at industrial fine-chemical production [48, 108]. This included fast mixing, efficient heat transfer in context with a fast exothermic reaction, prevention offouling and scale-/numbering-up considerations. By this means, an industrial semi-batch process was transferred to continuous processing. 

OS 89] [R 19] [P 69] Using a special reactor configuration with an interdigital micro-mixer array with pre-reactor, subsequent tubing and a quench, a yield of 95% at 0 °C was obtained [127]. The industrial semi-batch process had the same yield at -70 °C. 

Steam distillation is normally carried out as a semi-batch process whereby the organic mixture is charged into the still and steam is bubbled through continuously, as depicted in Fig. 3.64. 

To describe the dynamic behaviour of this semi-batch process, unsteady-state mass and energy balances are needed. Their interrelationships are depicted in Fig. 3.65. 

Block copolymer synthesis from living polymerization is typically carried out in batch or semi-batch processes. In the simplest case, one monomer is added, and polymerization is carried out to complete conversion, then the process is repeated with a second monomer. In batch copolymerizations, simultaneous polymerization of two or more monomers is often complicated by the different reactivities of the two monomers. This preferential monomer consumption can create a composition drift during chain growth and therefore a tapered copolymer composition. 

It should be noted that there are cases in which some selectivity will be lost in choosing a semi-batch mode over a simple batch reactor. If the desired product decomposes by a consecutive reaction, the yield will be higher in the batch reactor [177]. If, on the other hand, the reactants are producing by-products by a parallel reaction, the semi-batch process will give the higher yield. In any case, if the heat production rate per unit mass is very high, the reaction can then be run safely under control only in a semi-batch reactor. 

Use semi-batch processes for exothermic reactions and monitor the consumption rate of the limiting reactant. 

Designing and Operating Safe Chemical Reaction Processes (HSE 2000). Published by the U.K. Health and Safety Executive and directed to small to medium-sized chemical manufacturing companies using batch and semi-batch processes. It addresses chemical reaction hazards and inherently safer processes, hazards assessment, preventive and protective measures, and management practices. 

All polymerisations were carried out in nitrogen purged xylene solutions in a thermostatically controlled one litre glass reactor. Semi-batch processes were carried out in a similar reactor which was provided with calibrated peristaltic pumps (computer controlled when necessary) for delivering the monomer feeds. Typically, experiments were carried out at 80°C with monomer concentrations which gave solids contents in the range 10 - 60% at 100% conversion. 

During oxidation of a sulfide to a sulfoxide, it was found that the peracid, in DMF, was dangerously unstable. It was shown it could convert to the dibenzoylperoxide, probably of lesser reactivity, thus permitting a build-up of potential energy during a semi-batch process. However, the explosion seems to have occurred in the reagent solution, not yet added to the sulfide. Peroxide formation might provide an explanation for the earlier incident reported below, but seems less satisfactory here. Like other formates, DMF is a reducant. Replacement of DMF by dichloromethane permitted scale-up. 

J.M. (1996) A new method for assessing the thermal stability of semi-batch processes based on Lyapunov exponents. Chemical Engineering Science, 51 (11), 3089-96. 

Alos, M.A., Nomen, R Sempere, J.M., Strozzi, F. and Zaldivar, J.M. (1998) Generalized criteria for boundary safe conditions in semi-batch processes simulated analysis and experimental results. Chemical Engineering and Processing, 37, 405-21. 

The condition for the practical implementation of such a feed control is the availability of a computer controlled feed system and of an on-line measurement of the accumulation. The later condition can be achieved either by an on-line measurement of the reactant concentration, using analytical methods or indirectly, by using a heat balance of the reactor. The amount of reactant fed to the reactor corresponds to a certain energy of reaction and can be compared to the heat removed from the reaction mass by the heat exchange system. For such a measurement, the required data are the mass flow rate of the cooling medium, its inlet temperature, and its outlet temperature. The feed profile can also be simplified into three constant feed rates, which approximate the ideal profile. This kind of semi-batch process shortens the time-cycle of the process and maintains safe conditions during the whole process time. This procedure was shown to work with different reaction schemes [16, 19, 20], as long as the fed compound B does not enter parallel reactions. 

An exothermal reaction is to be performed in a 2.5 m3 stirred tank reactor as an isothermal semi-batch process at 80 °C. The specific heat of the reaction is 180kjkg 1, the specific heat capacity of the reaction mass is 1.8 kj kg 1 K 1, and the accumulation is 30%. The reaction is to be at atmospheric pressure and boiling point is 101 °C (MTT). There is a secondary reaction (decomposition) that is uncritical below 105 °C, that is, Tm4 = 105 °C. The decomposition energy is 150kjkg 1 and this decomposition releases 5 liters of a toxic, but not flammable, gas per kg reaction mass, measured at 25 °C and atmospheric pressure. 

For a first assessment of the thermal risks linked with the performance of a batch or semi-batch process, the thermal stability of a reacting mixture is of primary... 

Since the dynamics of a batch reactor is characterized by a unitary relative order, the GMC law can be adopted [6, 14, 22, 40, 42, 65], In order to cope with model uncertainties, adaptive GMC approaches have been developed [56, 60, 62] in [27] some unknown quantities—namely, the effect of the heat released by the reaction and the heat transfer coefficient—are estimated by adopting the nonlinear adaptive observer proposed in [24] in [63], an ANN-based GMC approach is presented for semi-batch processes with relative order higher than one. 

These factors are attributed to batch and semi-batch processes rather than continuous processes. However, the use of continuous processes on fine chemical manufacturing sites is limited. It is often preferable to use the semi-batch mode as opposed to batch processes. The Appendix lists hazards of pertinent chemical reactions for toxic and reactive hazards chemicals. Information concerning the safety of various chemicals (e.g., ammonia and others) can be readily obtained from the World Wide Web. Table 12-1 shows how to access a material safety data sheet at the Vermont Safety Information (VIRI) site on the Internet. 

The parameters listed in Tab. 16.19 represent experiment V-15. The objective was to analyze the influence of absent seeds on the stability of the semi-batch process. Without external seeds, the number of particles will decrease as particles are discharged. 

Yamazaki, R., Iwamoto, S., Nabetani, H., Osakada, K., Miyawaki, O., and Sagara, Y. 2007. Non-Catalytic Alcoholysis of Oils for Biodiesel Fuel Production by a Semi-Batch Process, Jpn. J. Food Eng., 8,11-18. 

The mole sieve dehydration is a semi-batch process using a solid adsorbent to remove water from a fluid stream. The water adsorbs onto the solid. 

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