Question

Following figure shows part of a plant in which five reactors were in parallel. There were two gas-feed lines with cross connections between them. Oxygen was also fed to the reactors, but the oxygen lines are not shown. Al the time of the incident only two reactors, Nos. 1 and 4, were on line. The operator thought valve B was open, so he shut valve A. This stopped the flow of gas to No. 1 reactor. The oxygen flow was controlled by a ratio controller (i.e. a controller that manipulated oxygen flow rate as per gas flow rate), but it had a zero error, and a small flow of oxygen continued. When the operator realized his mistake and restored the gas flow, the reactor contained excess oxygen.

To reactors Shut down Shut down Shut dowin On line Opan Open iniialy Shut Opan Gas

Would this cause any hazard? If so, describe it in detail. Comment on the design of the process in terms of inherent process safety. What modifications would you suggest to make the process safer? Write an SOP for the safe operation of the process if all five reactors are to be operated in parallel.

Answer 1

In parallel reactors, the feed is equally distributed. If we assume that both the gas feed lines have same component, in that case reactor 1 have excess oxygen. Initially some amount of gas with oxygen was present in the reactor 1. If the reaction is explosive or exothermic than excess oxygen may cause the explosion or otherwise only excess amount of the oxygen will be present in the reactor if the reaction is non explosive. The excess amount of the oxygen have to be evacuated before opening the gas feed valve.

For a safer process, gas flow meter and pressure gauge should be installed for each reactor so that the flow of the feed and pressure can be monitored. There should be a pressure vent on each reactor which released the gas when pressure increased inside the reactor due to excess gas.

If all the reactors have similar volume and feed rate then the conversion of each reactor will be equal. All the reactors are connected in parallel so, the equal feed is distributed to each reactor. In other words, reactors in parallel behaves as a single reactor.