Reflux failure and overhead system

There are several reasons for loss of reflux, including misoperation of isolation valves, reflux pump failure, failure of the control valve, etc. Reflux failure will result in accumulation of more liquid in the reflux drum, eventually flooding the drum and overhead condenser. This will result in loss of cooling. The column pressure will increase and the PRV will lift. 

However, the impact of this is generally ignored for a conservative analysis. 

Loss of reboiler heat means loss of heat input to the column, and in most cases, this will not result in any relief. However, dming frachonahon, failme of reboiler duty will increase tiie light ends fed into the bottom product, and if the bottom product is further processed to a second frachonahon coliunn, the feed to the second column will be lighter. The impact of lighter feed to the second column needs to be analyzed properly. 

API Std 2000 [13] is widely used in designing the venting requirement for atmospheric- and low-pressure storage tanks. The total venting capacity is to be at least the sum of the venting requirements for liquid movement and thermal effect. Venhng requirements are eshmated for three different scenarios  

Process engineering and design using Visual Basic 

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Check valve failure Blocked discharge Control valve failure Thermal expansion of liquid Heat exchanger tube rupture Reflux failure and overhead system Loss of reboiler heat Venting of storage tank Failure of individual motor Accidental closure of valve... 

This is a very particular application where we have the potential for overpressure due to loss of overhead condensing or reflux failure. In the event the cooling medium in the condenser is lost, additional vapour may be present at the top of the column. This additional vapour may require pressure relief. In a typical distillation system, a cooling failure also results in a loss of reflux within a short period of time (typically about 15 minutes). API RP 521 states that the required relief rates before and after loss of reflux should be considered. The Berwanger audit method encompassed both of these calculations, as it was not intuitive, which case would require the larger required relief rate. 

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