Calculating vents: DIERS methods - High vapor pressure systems

In the chemical industry, most reactors and many units are equipped with an emergency vent to prevent them bursting in the event of accidental overpressure. However, these venting devices rarely have sufficient capacity to control all possible causes of pressurization. The sizing scenario for these vents often bears little or no relation to the chemical reactions involved in the process. This is particularly the case in multi-purpose plants, where the same reactor is used for a wide variety of processes over the life of the plant.

The most frequent cases of accidental pressurization of reactors or equipment are as follows:

• overpressure resulting from pushing a liquid reaction mixture with air or nitrogen to accelerate its transfer to another device;

• fire around a reactor leading to excessive pressurization by solvent vapor pressure;

• excessive pressurization by solvent vapor pressure due to excessive heating or loss of cooling on a reactor, distillation column ;

• excessive pressurization due to accidental containment of a reactor or capacity and liquid injection by a pump;

• total filling of a reactor, enclosure or insulated pipe with a liquid and thermal expansion of this liquid;

• accidental ingress of heat exchange fluid into an enclosure following a leak on an internal heat exchanger.

These pressurized enclosures need to be protected by valves or rupture discs. The sizing methods proposed by the American Petroleum Institute (API) are generally used. These calculation methods are based on the proven fact that the discharge from these vents is solely gaseous, or exceptionally liquid in the case of pressurization by thermal expansion of liquid or by injection of liquid into an enclosure. The surface area of these vents is also relatively small compared with the volume of the protected enclosure.

Vents designed to protect reactors and other equipment against excessive pressurization resulting from runaway chemical reactions are generally much larger in area than the vents described above. This may be due to the rapid kinetics of the chemical reaction(s) involved, but also to the well-known fact that the vent release in this case is a two-phase mixture of gas or vapor and entrained liquid reaction mixture. Sometimes,...