Snowballing effect

There is a third explosion limit indicated in Figure 4.1 at still higher pressures. This limit is a thermal limit. At these pressures the reaction rate becomes so fast that conditions can no longer remain isothermal. At these pressures the energy liberated by the exothermic chain reaction cannot be transferred to the surroundings at a sufficiently fast rate, so the reaction mixture heats up. This increases the rate of the process and the rate at which energy is liberated so one has a snowballing effect until an explosion occurs. 

An increased likelihood of a fair price for non-fair trade mainstream production. Even if a fair price is paid for only a small part of production, there is often a snowball effect with higher prices paid for the rest of production. 

The time to stop the snowball effect is during the course of the inspection, long before issues mount and become a lengthy FDA-483 and potential warning letter. 

Luyben (1993a) provided valuable insights into the characteristics of recycle systems and their design, control, and economics, and illustrated the challenges caused by the feedback interactions in such systems, within a multi-loop linear control framework. Also, in the context of steady-state operation, it was shown (Luyben 1994) that the steady-state recycle flow rate is very sensitive to disturbances in feed flow rate and feed composition and that, when certain control configurations are used, the recycle flow rate increases considerably facing feed flow rate disturbances. This behavior was termed the snowball effect. ... 

Luyben, W. L. (1994). Snowball effects in reactor/separator processes with recycle. Ind. Eng. Chem. Res., 33, 299-305. 

The important advantage of this strategy is that the reactor behaves as decoupled from the rest of the plant The production is manipulated indirectly, by changing the recycle flows, which could be seen as a disadvantage. However, it handles nonlinear phenomena better, such as for example the snowball effect or state multiplicity. Additionally, this strategy guarantees the stability of the whole recycle system if the individual units are stable or stabilized by local control. 

The conclusion of this analysis is that plantwide control structures relying on self-regulation can be used, but the snowball effect is avoided only when the reactor volume is large enough. For a first-order reaction involving one reactant, the reactor could be considered sufficiently large if Da > 3. 

As the rank condition (4.17) is fulfilled, a control structure fixing the flow rates of both reactants is feasible. However, the secondary reaction has only a small contribution to the mass balance of the system. Consequently, the self-regulation strategy leads to a severe snowball effect. 

The conclusion of this analysis is that plantwide control structures that use feedback to control reactant inventory do not show the snowball effect. These structures can be applied for both large and small reactors, the difference being the variable manipulated for achieving production-rate changes. 

The second issue regards the optimal plantwide material balance. It is clear that the raw materials must be fed only in amounts required by the target production and selectivity. The control structure of fresh feeds should allow flexibility, within predefined limits, both in production rate and selectivity, but avoiding large variation of recycles that might upset some units (snowball effect). 

In CS3 (Figure 5.25), the hydrogen fresh feed is increased by about 9% from 348.5 to 380, while the recycle flow of phenol remains fixed to 220kmol/h. This control structure works well. Both the production of cyclohexanone and cyclohexa-nol is increased by about 4%, while phenol makeup increases with 8%. The purity of both products remains above 98%. A somewhat shorter transition time is obtained. The fact that hydrogen pushes the plant better than phenol is quite surprising, but it can be explained by the fact that there is no snowball effect on the gas-recycle side. 

Figure 9.3 Snowball effect. Small changes of feed rate yield huge changes of internal flow rates.

Granules are particles with a more or less spherical shape. The diameter of these particles can be between 2-30 mm. Granulation is widely used in other fields of technology, if spherical particles are needed. The principle of the shaping method is best described by the snowball effect. A round dish (see Fig. 8.10) is used, which is rotating about an inclined axis, the angle of inclination of which is variable. Small particles are fed into the dish. At the same time a cohesive slurry... 

The snowball effect within a channel can multiply the number of electrons by 10s. A plate allows an amplification of 102-104, whereas by using several plates the amplification can reach 108. This detector is characterized by a very fast response time because the secondary electron path inside the channel is very short. In consequence, it is well suited to TOF analysers, which need precise arrival times and narrow pulse widths. Furthermore, the large detection area of the microchannel plate allows the detection of large ion beams from the analyser without additional focalization. However, the microchannel plate detectors have some disadvantages. They are fragile, sensitive to air and their large microchannel plates are expensive. 

In this section we explore two basic effects of recycle (1) Recycle has an impact on the dynamics of the process. Th e overall time constant can be much different than the sum of the time constants of the individual units. (2) Recycle leads to the snowball effect. This has two manifestations. one steady state and one dynamic. A small change in throughput or feed composition can lead to a large change in steady-state recycle stream flowrates. These disturbances can lead to even larger dynamic changes in flows, w hich propagate around the recycle loop. Both effects have implications for the inventory control of components. 

We call this high sensitivity of the recycle flowrates to small disturbances the snowball effect. We illustrate its occurrence in the simple example below, It is important to note that this is not a dynamic effect it is a steady-state phenomenon. But it does have dynamic implications for disturbance propagation and for inventory control. It has nothing to do with closed-loop stability. However, this does not imply that it is independent of the plant s control structure. On the contrary, the extent of the snowball effect is very strongly dependent upon the control structure used. 

However, we see in this strategy that there is no flow controller anywhere in the recycle loop. The flows around the loop are set based upon level control in the reactor and reflux drum. Given what we said above, we expect to find that this control structure exhibits the snowball effect. By writing the various overall steady-state mass and component balances around the whole process and around the reactor and column. wre can calculate the flow of the recycle stream, at steady state, for any given fresh reactant feed flow and composition. The parameter values used in this specific numerical case are in Table 2.1. 

Notice that the total rate of recycle plus fresh feed of A is flow-controlled. There is a flow controller in the recycle loop, which prevents the snowball effect. Sometimes the fresh feed of A is added directly into the reflux drum, making the effect of its flow on reflux drum level more obvious. The piping system where it is not added directly to the drum still gives an immediate effect of makeup flow on drum level because the flowrate of the total stream (recycle plus fresh feed) is held constant. If the fresh feed flow increases, flow from the drum decreases, and this immediately begins to raise the drum level. 

A stream somewhere in all recycle loops should be flow controlled. This is to prevent the snowball effect and was discussed in Chap. 2. 

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