The startup of a distillation column is the end product of the design process. It should not be considered something that just happens after design. Startup is the culmination of the theory and practice of designing the column to meet the process objectives. The column either achieves its stated objectives or does not. This applies especially to revamps. We will emphasize revamps here as most current industrial work is done in this area rather than new construction. Nevertheless, the same analysis can be applied to new columns as well.
Analyzing the startup requirements of a distillation column revamp should begin when the decision is made to revamp column, not just prior to feed being put into the unit. The most important question that must be answered prior to a startup is how will the distillation system changes affect the initial day-to-day operations, process control of the system, and initial startup? How will the operators run the system? With the widespread use of steady-state distillation column simulations, many practical aspects are being left out. By focusing on the factors required to have a successful startup from the beginning of a revamp, problems can be minimized.
Most of our discussion will be directed towards column revamps due to their inherent complexity; however, lessons shown apply to new columns as well. Revamping distillation columns in lieu of buying new ones to increase capacity, improve product quality, or debottleneck a given limitation minimizes capital investment. In recent years there has been considerable discussion in the literature of distillation column equipment design requirements and failure of distillation columns to meet their stated revamp objectives. Many of these distillation unit equipment modifications have changed the inherent operating characteristics of the system. Some of these changes, such as adding an intermediate reboiler (Figure 1), will require startup and operating procedure and control strategy changes to allow the system to meet the design objectives. Distillation internal equipment revamps such as changing from trays to packing may require additional external equipment modifications including feed and reflux strainers to avoid distributor fouling.
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