Controlled depressurization

This measure is different from emergency pressure relief. A controlled depressurization is activated in the early stages of the runaway, while the temperature increase rate and the heat release rate are slow. 

If a runaway is detected at such an early stage, a controlled depressurization of the reactor may be considered. As an example, during an amination reaction, a 4 m3 reactor could be cooled from 200 °C to 100 °C within 10 minutes without external cooling, just by using a controlled depressurization allowing evaporative cooling. Obviously, the scrubber and the reflux condenser must be designed to work with independent utilities. 

This is the continuation of Worked Example 3.1. If there is loss of control of an amination reaction, the temperature could reach 323 °C (MTSR), but the maximum allowed working pressure of 100 bar g would be reached at 249 °C (MTT). Thus, the question is If the reaction can be controlled by depressurizing the reactor before the safety valve opens, that is, before 240 °C is reached, what would the vapor release rate be To answer this question, information about the reaction kinetics is required. The only information is that at 180°C, a conversion of 90% is reached after 8 hours. If we consider the reaction to follow a first-order rate equation, justified by the fact that ammonia is in large excess, we can calculate the rate constant at 180 °C  

This is calculated at 180 °C for the charge of 2kmol and for a conversion of zero, which is conservative. It would be reasonable to interrupt the runaway at its very beginning, for example, at 190 °C. If we consider that the reaction rate doubles for a temperature increase of 10 K, the heat release rate would be 56kW at 190 °C. The latent heat of evaporation can be estimated from the given Clausius-Clapeyron expression  

If a runaway is detected in an early stage, where the temperature and the pressure increase is slow, a controlled depressurization of the reactor can be considered. This is only suitable for cases where a volatile solvent is present the reactor is slowly depressurized until evaporative cooling occurs. Obviously, a scrubber and/or a reflux condenser must be installed and designed to work with independent utilities. 

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Gas was immediately produced from hydrates via controlled depressurization and thermal stimulation tests, without question regarding the gas source. 

A final controlled depressurization occurs in which the vessel is brought to its lowest pressure. Impurities are desorbed and removed to some extent. 

In case of cooling failure, could the temperature be stabilized by controlled depressurization (Latent heat of evaporation of water A H v = 2200 kj kg"1). 

In the examples given above, we see how important an early intervention is in case of runaway. Whatever the measure considered, the sooner it becomes active the better. An exothermal reaction is obviously easier to control at its beginning, before the heat release rate becomes too great. This is true for emergency cooling as well as for controlled depressurization. Thus, the idea arose to detect a runaway situation by an alarm system. The first attempt in this direction stems from Hub [15, 16], who proposed evaluating the second time derivatives of the reactor temperature and the first derivative of the temperature difference between reactor and jacket, giving a criteria for a mnaway ... 

Then the stabilization of the temperature at MTT can be assessed in a similar way as for class 3, by using the determined thermal activity, following the procedure represented in Figure 10.10. Here the thermal activity of the secondary reaction may be ignored, but it should be checked whether the gas production rate by the secondary reaction remains uncritical. This assesses the controllability of the runaway by using controlled depressurization or hot cooling (cooling by evapora-... 

Tightness of the containment over a longer period of time is an important factor in the retention of radionuclides in support of natural deposition processes and engineered safety features. To this end, facilities for controlled depressurization of the containment have been installed in many plants (see Section 7.3.4.4.). 

The retention of fission products by scrubbing the steam flow in a BWR pressure suppression pool is based on the exchange of matter between gas and liquid phases. Similar scrubbing processes are also at work in other accident situations, such as in a PWR steam generator tube rupture event (see Section 6.2.3.), as well as in the Venturi scrubbing process during controlled depressurization of the containment after a core melt accident (see Section 7.3.4.4.). 

The heat transport from the core melt into the sump water, as well as the production of permanent gases in the core melt - concrete interaction, will result in a pressure increase inside the closed containment. Provided that no measures for controlled depressurization are undertaken (see next section), then, according to the results of thermodynamic calculations, the steam-gas pressure within the containment would reach the postulated failure value of the containment steel shell of about 0.9 MPa after 5 to 10 days, depending on the accident sequence. Such an overpressure failure will not be a catastrophic burst of the shell, but rather the enlargement of the operational leaks to a size permitting the escape of gas and steam at a rate high enough to keep the pressure inside the containment at a... 

The late overpressure failure of the containment steel shell can be prevented by a controlled depressurization, in the course of which the gas-steam mixture escaping from the containment is directed to an additional system in which the radionuclides are retained. The purpose here is to further reduce the possibility of release of radionuclides to the environment as a consequence of a severe reactor accident. Practical application of this idea can be based on various principles an overview of the design of different systems was given by Schlueter and Schmitz (1990). 

Automatic depressurization system (ADS). The ADS provides a controlled depressurization of the plant. It is actuated passively by the pressure difference between the reactor vessel and an accumulator filled with water and nitrogen ... 

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