Operating modes semi-continuous

A major problem to be solved for multiproduct plants is the occurrence of disparities in the cycle times and size requirements for the different stages. In the following it will be assumed that the size factors as well as the cycle times of all units are independent on equipment size. This assumption is usually relaxed in further stages of the design. In case of batch heating and cooling, or reactors operated in semi-continuous mode, this is necessary in order to adopt the cycle times to the capacity of equipment, which is related to batch size. 

Therefore, the conclusion from this experiment is that top-stabilisation mode, although feasible, is not suitable for our reactor. Under this mode of operation, the semi-continuous feeding becomes an inconvenience. 

The blow tank is automatically taken through repeated cycles of filling, pressurizing and discharging. Since one third of the cycle time is used for filling the blow tank, a system required to give a mean delivery rate of 20 t/h must be able to deliver a peak rate of over 30 t/h. Dense phase transport is thus a batch operation because of the high pressures involved, whereas dilute phase transport can be continuous because of the relatively low pressures and the use of rotary valves. The dense phase system can be made to operate in semi-continuous mode by the use of two blow tanks in parallel. 

In some cases we may benefit from adopting a semi-continuous mode of operation, e.g. to a batch of one reactant we continuously feed the other reactant, while removing a volatile product continuously. An example where this is advantageous is the production of ethyl-4-pentenoate, CH2=(CH2)3(CO)OEt from allyl alcohol and triethyl orthoacetate, CHs-CfOEt). Continuous addition of allyl alcohol to a batch of triethyl orthoacetate and continuous removal of the produced ethylalcohol (and. some allyl alcohol) by distillation resulted in high yields of the dersired ester ethyl-4-pentenoate. By contrast, if allyl alcohol and triethyl orthoacetate were reacted in a batch-wise manner the product consisted of a 1 1 mixture of the desired ester and the undesired ester (Anderson, 2000, p 279 Bollyn and Wright, 1998). 

The minimal cost of equipment was used as the criterion in the design of the plant, which was to be operated in a non-overlapping mode. For a plant consisting of MB true batch units (MS = 3) and MS semi-continuous units (MS = 5) which are grouped in MST semi-continuous trains (MST = 3), the cycle time is given by Eqn. (7.4-10). Combining this expression with Eqn. (7.4-22) and rearranging yields ... 

The next two steps after the development of a mathematical process model and before its implementation to "real life" applications, are to handle the numerical solution of the model s ode s and to estimate some unknown parameters. The computer program which handles the numerical solution of the present model has been written in a very general way. After inputing concentrations, flowrate data and reaction operating conditions, the user has the options to select from a variety of different modes of reactor operation (batch, semi-batch, single continuous, continuous train, CSTR-tube) or reactor startup conditions (seeded, unseeded, full or half-full of water or emulsion recipe and empty). Then, IMSL subroutine DCEAR handles the numerical integration of the ode s. Parameter estimation of the only two unknown parameters e and Dw has been described and is further discussed in (32). 

This bioreactor mode refers to a tank fermenter operated semi-continuously. The rate of the feed flow, F0, may be variable, and there is no outlet flow rate from the fermentor. As a consequence of feeding the reactor volume will change with respect to time. 

Study the system for the kinetic case nAi = nA2 and nB1semi-continuously with slow feeding of B and (ii) as a batch reactor with B charged initially to the reactor and zero feed of B. Compare the results obtained for the two differing modes of reactor operation. 

Study the semi-continuous mode of operation further, using different values of feed rate and feed concentration to the reactor. Determine the feeding conditions that are most beneficial in terms of selectivity. 

The system can be operated in the parallel mode, discontinuously (batch-wise) with each reactor as an independent unit, semi-continuously or as a reactor cascade. Both homogeneous and heterogeneous reactions as well as product and catalyst separation and catalyst recycling are possible. 

The bioreactor operation mode is normally defined at the outset of process configuration. Insect cells have been cultured in almost all known cultivation modes batch [10], repeated-batch [70], perfusion [71-74], fed-batch [75, 76], semi-continuous [77,78] and continuous [79]. In spite of this multitude of different strategies, the batch or, eventually, fed-batch mode is normally preferred due to the lytic infection cycle of the baculovirus. 

Figure 1-2 Operating parameters necessary for ozone mass balance(s) on a continuous-flow stirred tank reactor (for operation in semi-batch mode Ol - 0).

Bacterial inactivation is achieved by CO2 absorption in the liquid phase, even though the reason why it happens is still not clear. In this respect, batch- and semi-continuous operating modes are substantially different. In the batch system the residence time, i.e., the time of contact between gas- and liquid phase, must be sufficient to allow the diffusion of CO2 in the liquid, and is therefore a fundamental parameter to assure a desired efficiency. In the semi-continuous system the contact between the phases is localized in the surface of the moving micro-bubbles. In this second case, the efficiency of the process is influenced by temperature, pressure, gas flux, bubble diameter, and other parameters that modify the value of the mass-transfer coefficient. Therefore, it is not correct to use the residence time as a key parameter in the semi-continuous process. In fact, a remarkable microbial inactivation is reached even with an exposure time of 0 min (i.e., pressurizing and immediately depressurizing the system) these two steps are sufficient to allow CO2 to diffuse through the liquid phase. 

A second consideration is the operating mode continuous, batch, or semi-continuous. An extensive textbook on theory, design and scale-up of multiphase reactors was published by Gianetto and Silveston in 1986 [22], supplementing "Three-phase catalytic... 

An attractive property of monolithic reactors is their flexibility of application in multiphase reactions. These can be classified according to operation in (semi)batch or continuous mode and as plug-flow or stirred-tank reactor or, according to the contacting mode, as co-, counter-, and crosscurrent. In view of the relatively high flow rates and fast responses in the monolith, transient operations also are among the possibilities. 

The SCISR can also be operated continuously, and the product so prepared has essentially the same size and size distribution as that obtained by operation in semi-batch mode under the seme conditions. 

Figure 1.3 shows a typical semi-batch (semi-continuous) distillation column. The operation of such columns is very similar to CBD columns except that a feed is introduced to the column in a continuous or semi-continuous mode. This type of column is suitable for extractive distillation, reactive distillation, etc. (Lang and coworkers, 1994, 1995 Mujtaba, 1999). Further details of semi-batch distillation in extractive mode of operation are provided in Chapter 10. 

In this mode the solvent is fed to the column in a semi-continuous fashion at some point of the column (Figure 10.3). Mujtaba (1999) noted two strategies in this mode of operation as far as charging of the initial feed mixture is concerned. 

Here, the same mixture used for example 1 is considered. Semi-continuous solvent feeding mode with full charge strategy is opted in this example. The objective is to maximise the productivity of Task 1 of the STN shown in Figure 10.6. The specification on the distillate composition is 0.95 molefraction in Heptane. The optimisation problem (OP1) is considered and both the reflux ratio and solvent rate profiles are optimised. Again two time intervals are used for the entire operation period (Task 1). In each interval, constant reflux ratio and solvent feed rate are used, the values of which are optimised. The input data are the same as those in Table 10.1 except that the maximum reboiler capacity is 25 kmol. The solvent is introduced in plate 6 (Nf). 

Before true continuous reactor trains became common, many were operated in a semi-continuous mode. Typically, there were three or four reactors in series and the styrene would be polymerized to a certain degree of conversion and transferred to the next vessel. This would allow reactants to be transferred into the vacated vessel and batch polymerization begun. This scheme was successful in normal operation, but a surge vessel was needed in case there was a problem with any of the reactors in sequence. 

The design of SMBR processes has to take into account the requirements of different types of reactions. Therefore, different types of flowsheets and operating modes can be chosen. As with a semi-continuous operation, a process without section IV (regeneration of the solvent), a five-section process or a four section process without recycle of the eluent may be advantageous. Relevant design criteria for the SMBR are discussed in Section 8.4. 

An agitated vessel may be operated in either a continuous, batch, or semi-batch mode. In continuous operations, the typical heat-transfer requirement is to maintain a set process temperature by either adding or removing heat, depending on the chemical reaction involved. In batch operations, the heat-transfer process can have a number of different functions at different stages of the operation. Examples include the ... 

Operating the reactor in a semi-continuous condition and adding substrate so as to keep its bulk concentration constant, leads to meaningful changes in reactor performance as compared to the diafiltration mode. Under either operational mode permeate flow rate continuously decreases with time. 

The concept of a circulating flow reactor was further developed in the Buss reactor technology (Figure 1.26). Large quantities of reaction gas are introduced via a mixer to create a well dispersed mixture. This mixture is rapidly circulated by a special pump at high gas/liquid ratios throughout the volume of the loop and permits the maximum possible mass transfer rates. A heal exchanger in the external loop allows for independent optimisation of heat transfer. For continuous operation, the product is separated by an in-line cross-flow filter which retains the suspended solid catalyst within the loop. Such a system can operate in batch, semi-continuous and continuous mode. 

Two quite frequently occurring questions regarding the safety technical assessment of chemical reactions cannot be placed directly into the order presented here so far. The first case is the evaluation of reactions performed imder reflux conditions. The safety assessment of such processes does not depend primarily on the general mode of operation, which in most cases is either discontinuous or semi-continuous, but on the peripheral installations to the reactor itself. In industrial practice, numerous designs can be found. Consequently the discussion here has to focus on two examples to outline the general assessment procedure. If these examples demonstrating the fimdamental approach to the assessment of processes under reflux are combined with the assessment criteria presented so far, the basis is provided to perform the safety evaluation for any other unique design as required. 

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