Heat-accumulation condition

Neither 7 is reached nor decomposition can be triggered. If the reaction mass is maintained under heat accumulation conditions Th can be reached or decomposition can start. Hazard if Th is high. At normal process conditions the process is thermally. safe. 

No special measures are required but the reaction mixture should not be held under heat accumulation conditions for longer time. 

After loss of control of the synthesis reaction, the technical limit (MTSR < MTT) cannot be reached and the decomposition reaction cannot be triggered, since the MTSR stays below TD24. Only if the reaction mass is maintained for a long time under heat accumulation conditions, can the MTT be reached. Then the evaporative cooling may serve as an additional safety barrier. The process presents a low thermal risk. 

Therefore, no special measure is required for this class of scenario, but the reaction mass should not be held for a longer time under heat accumulation conditions. The evaporative cooling or the emergency pressure relief could serve as a safety barrier as far as their design is appropriate. 

If heat accumulation can be avoided, no special measure is required. If heat accumulation conditions cannot be excluded, evaporative cooling or the emer-... 

In this class, neither the MTT is reached nor are secondary reactions triggered. Only if the reaction mass is maintained over a longer time under heat accumulation conditions at the MTSR, can the secondary reaction lead to a slow temperature increase. It is recommended to check for gas production, which could lead to a pressure increase if the reactor was closed or to a vapor or gas release if the reactor was opened. This can be done by using the procedure represented in Figure 10.8. In general, the gas release rate will be low due to the fact that MTT < TD24. 

The situation is similar to class 1, except that the MTT is above Tm4. This means that under heat accumulation conditions, the activity of secondary reactions cannot be neglected, leading to a slow but significant pressure increase, or gas or vapor release. Nevertheless, the situation may become critical only if the reaction mass is left for a longer time at the level MTT. The assessment can be made using the same procedure as for criticality class 1, represented in Figure 10.8. The gas or vapor flow rate is an important parameter for the design of the required protection measures such as condenser, scrubber, or other treatment units. 

In the chapters devoted to reactors, it was considered that a situation is thermally stable due to the relatively high heat removal capacity of reactors compensating for the high heat release rate of the reaction. We considered that in the case of a cooling failure, adiabatic conditions were a good approximation for the prediction of the temperature course of a reacting mass. This is true, in the sense that it represents the worst case scenario. Between these two extremes, the actively cooled reactor and adiabatic conditions, there are situations where a small heat removal rate may control the situation, when a slow reaction produces a small heat release rate. These situations with reduced heat removal, compared to active cooling, are called heat accumulation conditions or thermal confinement. 

Besides this, the thermal behavior of the reacting mass and the dimensions of its containment play a key role in the analysis. This is illustrated in the example given in Table 13.1. Here the ambient temperatures were chosen in such a way that the heat release rate differed by one order of magnitude in each line [1], To simplify the calculations, the containers were considered spherical. The heat accumulation conditions increase with the size of the container, that is, from left to right. 

It can be observed, in one line, that under severe heat accumulation conditions, there is no difference in the time-scale that corresponds to the time to maximum rate under adiabatic conditions (TMRld). Thus, severe heat accumulation conditions are close to adiabatic conditions. At the highest temperature, even the small container experienced a runaway situation. Even at this scale, only a small fraction of the heat release rate could be dissipated across the solid the final temperature was only 191 °C instead of 200 °C. For small masses, the heat released is only partly dissipated to the surroundings, which leads to a stable temperature profile with time. Finally, it must be noted that for large volumes, the time-scale on which the heat balance must be considered is also large. This is especially critical during storage and transport. 

A tubular reactor is to be designed in such a way that it can be stopped safely. The reaction mass is thermally instable and a decomposition reaction with a high energetic potential may be triggered if heat accumulation conditions occur. The time to maximum rate under adiabatic conditions of the decomposition is 24 hours at 200 °C. The activation energy of the decomposition is 100 kj mol-1. The operating temperature of the reactor is 120 °C. Determine the maximum diameter of the reactor tubes, resulting in a stable temperature profile, in case the reactor is suddenly stopped at 120 °C. 

Bunimovich et al. (1995) lumped the melt and solid phases of the catalyst but still distinguished between this lumped solid phase and the gas. Accumulation of mass and heat in the gas were neglected as were dispersion and conduction in the catalyst bed. This results in the model given in Table V with the radial heat transfer, conduction, and gas phase heat accumulation terms removed. The boundary conditions are different and become identical to those given in Table IX, expanded to provide for inversion of the melt concentrations when the flow direction switches. A dimensionless form of the model is given in Table XI. Parameters used in the model will be found in Bunimovich s paper. 

Critical heat production rates (i.e., heat production rates that still do not lead to a runaway), are often determined by small scale experiments. However, the effect of scale-up on these rates, as discussed in [161], must be taken into account. An indication of the effect of scaling in an unstirred system is shown in Figure 3.2. In this figure, the heat production rate (logarithmic scale) is shown as a function of the reciprocal temperature. Point A in the figure represents critical conditions (equivalent heat generation and heat removal) obtained in a 200 cm3 Dewar vessel set-up. It can be calculated from the Frank-Kamenetskii theory on heat accumulation [157, 162] that the critical conditions are lowered by a factor of about 12 for a 200 liter insulated drum. These conditions are represented by... 

This formula is able to vigorously purge heat accumulation and promote bowel movement. It is used in a condition of severe obstruction of feces, Qi, blood and fire-toxin in the abdomen. The manifestations are severe constipation, distension, pain and fullness of the abdomen, which are increased by pressure. It also treats a condition of severe constipation where dry feces have accumulated in the intestines and a foul-smelling fluid is excreted instead of stool. The patient has a red tongue with a dry, thick, yellow coating and a wiry or tight, and rapid pulse. 

If the heat accumulates in the Large Intestine and consumes the fluid, constipation may occur. Da Huang and Mang Xiao can be used in the acute condition. 

Herbs that are bland and cold, can leach out dampness by promoting urination and purging the intestines are selected when damp-heat accumulates in the Middle- and Lower-Jiao. Herbs that tonify the Spleen, promote digestion and regulate the Qi should be selected in chronic conditions. 

This formula treats not only food accumulation, but also a severe complicated condition of food and damp-heat accumulation in the intestines. The manifestations are abdominal distension and pain, foulsmelling diarrhea, constipation and scanty urine. The tongue is red with a yellow and sticky coating. The pulse is deep and firm. 

The totality of the heat released by the reaction under study must be converted into heat accumulation, that is, into a temperature increase, which can be measured. This is obtained by eliminating the heat exchange with the surroundings, achieving adiabatic conditions ... 

A practical approach of heat balance, often used in assessment of heat accumulation situations, is the time-scale approach. The principle is as in any race the fastest wins the race. For heat production, the time frame is obviously given by the time to maximum rate under adiabatic conditions. Then the removal is also characterized by a time that is dependent of the situation and this is defined in the next sections. If the TMRld is longer than the cooling time, the situation is stable, that is, the heat removal is faster. At the opposite, when the TMRld is shorter than the characteristic cooling time, the heat release rate is stronger than cooling and so runaway results. 

Rainbow compounds from Solvay are said to overcome problems associated with compounds used to date for coloured PVC profiles. The all-PVC compounds are cost competitive and maintain colour and mechanical properties for years regardless of weather conditions. The approach involves coextruding a UV-resistant coloured PVC skin on a base profile of low-cost PVC containing no UV stabilisers. The low IR absorption rate of the skin reduces thermal deformation of the profile by hindering heat accumulation on the profile. 

Three values of the BAM test for BPO calculated each by substituting the three pairs of heat generation data, which are each calculated based on the experimental data obtained each with the three kinds of cells, which are referred to in Fig. 42, in the test, into Eq. (72) are presented in Table 4, assuming that 400 cm each of three samples of BPO are each charged in the 500 cm Dewar flask used in the BAM heat-accumulation storage test and are each placed in the atmosphere under isothermal conditions, keeping the other conditions constant. 

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