Imagine taking a snowball and rolling it down the side of Mount Everest. As the ball rolls down the mountain, it gets bigger and faster. If we can stop it early in its travel down the mountain, BEFORE it gets to BIG and FAST, we can save the town at the base of the moutain. But there comes a time and place in this snowball’s journey down the side of the mountain where it has gotten TOO BIG and TOO FAST that we can no longer stop it. The only option is to evacuate the town at the bottom of the mountain BEFORE the massive snowball reaches it. This is how I explan the “P” Hazard in my HAZMAT and Process Safety Training and PHAs.
But in a well designed process, we have an “emergency system” called a “quenching system” that can STOP the exothermic reaction and avoid the catastrophic event. An emergency quench system is an automated or manual safety system designed to “quench” runaway polymerization. Because the polymerization generates intense heat, runaway reactions can trigger catastrophic over-pressurization. Quench systems deploy chemical inhibitors and/or cooling diluents to safely terminate the “chain reaction”.
NOTE: We call the old manual systems the “short straw” design, as someone has to go to the reactor and load the “kill agent” and depress it into the reactor BEFORE the process reaches it critical point. It can be done safely “sometimes”, but sometimes the rate of rise in temp and pressure makes approaching the vessel risky; hence why we dont see many manual systems these days.
Quenching can either be used to
1) directly control a chemical reaction or
2) in the treatment of an emergency vent stream.
For the direct quenching of a chemical reaction when loss of control has occurred, the quench material BOTH COOLS AND DILUTES the reactants, thereby slowing down the reaction rate and the rate of heat generation to a controllable level. Quenching may be carried out by adding the quench liquid to the reactor or by discharging the reactants to a dedicated “dump tank” that contains the quench liquid.
Quench systems are used in vent disposal systems for the treatment of streams that can not be discharged directly to atmosphere or where continuing reaction is taking place. The process involves the mixing of a solvent with the relief stream. This results in the condensation and removal of volatile components from the relief stream and / or the cooling of the vent stream which prevents further reaction from taking place.
The facility should be able to demonstrate that it has evaluated the options for emergency protection of an uncontrolled exothermic reaction. Where venting has been chosen as the means of protection, the facility should demonstrate that it has considered the consequences of a vent emission directly to atmosphere and installed appropriate vent treatment measures where necessary.
Where a quench system for the control of a reaction is installed the following should have been considered:
- compatibility of the quench material with the reactants;
- reliability of the quench delivery system;
- availability of space in the reactor / dump tank to accommodate quench material in addition to the reactor contents;
- the effects of level swell and foaming on quench addition;
- rate of mixing of the quench material with reaction fluids;
- the effectiveness of quenching for all conditions leading to loss of control.
The facility should demonstrate the validity of the approach used by appropriate test work. Consideration of the conditions leading to loss of control of the reaction are particularly important. For instance if loss of agitation can initiate this, the use of quenching to control the exotherm is questionable as the cooling and dilution effects are reliant on effective mixing. The pressure relief arrangements will need careful consideration where a dump tank is employed.
For the treatment of a vent stream there are a variety of arrangements possible. The duty of the system to either condense volatiles or control further reaction in the vent system will dictate the arrangement. A commonly used arrangement for handling two phase discharge from a reactor is a simple knockout drum containing an inventory of an appropriate quenching agent. The vent stream is sparged into the knockout drum below the liquid surface. The knockout drum itself then vents to atmosphere, a scrubber or a flare stack. Alternatively the quenching agent can be sprayed into the vessel from which the discharge arises, or it can be injected into the vent stream in a quench nozzle to effect partial condensation of the stream. Where a quenching agent is used in a vent disposal system the following should be considered:
- the duty – condensation or control of reaction;
- compatibility of the quench material with the vent stream;
- reliability of the quench delivery system;
- the effects of level swell and foaming in the reactor and/or knockout drum
As an emergency protective measure the operation of a quench system needs to be reliable. The instrument and control features of a quench delivery system should demonstrate this and features such as voting systems and hard wiring of trips should be included for critical duties. Other support systems, such as utilities, that are critical to delivery of the quench system need to be in the MI program for assured reliability.
For the design of systems for the quenching of vent streams, limited guidance is available in API RP 521 Guide for pressure-relieving and depressuring systems with more detailed guidance being available in the DIERS Technology Manual.
Maintenance requirements
Quenching systems require to be well maintained to ensure reliability. Procedures should be in place to ensure regular maintenance and testing of relevant instrument and control systems. The inspection and maintenance of pipework and vessels should be carried out to written procedures on a regular basis. Where the quality or specification of quenching agents may deteriorate over time so as to reduce effectiveness, the inventory should be replenished at set frequencies.
Effluent disposal
Control of a reaction with a quenching agent may cause irreversible contamination of the reactants. Where a quenching agent has been used in a vent disposal system, this will need to be removed before plant operations can recommence. Plans should be in place for the safe handling and disposal of effluents generated from the use of a quench system.
Source: UK’s HSE