In a typical distillation system, fluid components are separated by maintaining a temperature profile across a pressure vessel filled with contacting equipment such as trays or packing. A typical distillation system employs a reboiler and a condenser to maintain a hot to cold temperature profile from the bottom to the top of the column. In addition, liquids condensed in the condenser are usually added back to the top of the column as liquid reflux. Loss of heat removal from a distillation column can result in an additional vapor flow rate out the top of the distillation column, which may require pressure relief. Loss of heat removal can result from any of the following:
a. A loss of coolant flow to the overhead condenser, which in turn can produce a loss of reflux.
b. A loss of reflux due to failure of a reflux pump or inadvertent valve closure.
c. A loss of a cold feed stream to the top of the column.
d. A loss of pump-around flow.
Because of the complexity of distillation systems, it is difficult to predict whether loss of condensing or reflux will require pressure relief without performing the detailed required relief rate analysis. For this reason, if the possibility exists for any of the above four contingencies to occur, then the loss of cooling/loss of reflux contingencies should be identified as applicable.
The relief flow rate required in these contingencies is sometimes evaluated by simulating the loss of cooling or loss of reflux flow using a process simulator. For cases in which this approach is not feasible, API Recommended Practice 521 provides descriptions of simplified approaches to this calculation.
For complex distillation systems in which cooling is provided in several places (condenser, multiple pump-arounds, etc.), consideration should be given to the possibility of a simultaneous loss of multiple cooling sources. A review of the coolant distribution and electrical distribution systems is often required to formulate the worst case.
The topic of distillation systems would not be complete without a brief discussion of the impact that a cause of overpressure in one equipment item can have on other, connected items. This point is not unique to fractionation systems, but they do provide a very clear example of the principle. Note that a distillation system usually consists of a column, reboiler, condenser, and accumulator, which often cannot be isolated from each other by block valves. In these cases, all causes of overpressure that affect the column also affect the other pieces of equipment in the distillation system and vice-versa. For example, a ruptured tube in the reboiler could overpressure the distillation column and a fire around the reboiler, accumulator and distillation column could overpressure the associated condenser. The causes of overpressure documentation should reflect this fact, but the required relief rates need only be calculated once.