(465b) Study of Flare Minimization Opportunities during an Ethylene Plant Shutdown

It has always been beneficial for an ethylene plant to reduce flaring during its shutdown because the reduction of burned-off gases reduces pollutant generation, saves raw materials, and also lowers the utility/energy costs.  An ethylene plant shutdown generally includes three stages of operations: feed and inventory reduction, facility shutdown, and unit decommissioning (involving liquid discharge, vapor discharge, and nitrogen (N₂) purge).  In this paper, a dynamic simulation model has been built to study flare minimization opportunities of a front-end de-ethanizer ethylene plant shutdown operations.  In its normal shutdown procedure, three of the five furnaces will be firstly cut off, and the charge gas compressor (CGC) will be reduced to a small operating load while internal recycle lines in CGC section will be kept open for safety operation.  Secondly, the liquid levels of various vessels are reduced to their minimum operating values.  Thirdly, the left furnaces will be brought off-line when CGC shutdown is started.  Finally, the residual liquid and vapor inventories will be sent to the flare system and nitrogen will then be injected into each vessel for purge operations.

To minimize the shutdown flaring and smooth the plant shutdown operation, various operational improvements have been initiated and examined in this paper via the developed dynamic simulation model. For example, such an idea has been tested that shutting down all furnaces first and then recycling vapors from downstream columns to the CGC inlet to necessitate CGC running meanwhile enable flare source recovery.  After that, there are much less hydrocarbons left in downstream vessels after CGC is shut down and before the nitrogen purge operation.  Compared with base case, significant flare emissions have been saved.  With the help of plant-wide dynamic simulations, other flare minimization initiatives have also been tested and quantified.