This Friday my partner and I concluded the lab session on a 8-inch Pilot Plant Distillation Column. This lab is particularly exciting for its level of complexity and how well it models an actual plant. I would personally rate this as probably my favorite experiment of all Unit Ops experiments. It’s a nice refresher on the material that we had learned from Mass Transfer class. The distillation column just made much more sense to me after this experiment.
In this experiment we analyzed a pilot-scale distillation column for a binary mixture separation under steady-state conditions. A binary mixture of methanol and water is to be separated by feeding it through a distillation column of 8 inches in diameter and 14 valve trays. Based on my lab partner and my calculations. The feed was to be introduced to the column at tray number 4. Steam, cooling water, feed, reflux, distillate, and bottoms product flowrates are being measured with orifice meters. The compositions in terms of mole fractions can be checked by Gas Chromatography at three stream locations (Feed, Overhed, Bottoms). Heat losses from the reboiler, the column and the condensers can be calculated from the measured compositions and flowrates at different locations. Subsequently, the overall and Murphree tray efficiencies can be calculated from the change in heat in the system. A combinations of reflux ratios and feed flow rates was supposed to be implemented and the results from that will be used to further investigate column behavior. One thing to keep in mind that the mass and energy balance will not be totally balance due to the fact that it is not an ideal system. Heat tends to get lost moving along different sections of the column.
Here’s a scheme of the Distillation Column. As you can see, the red dotted lines are the control signals for reading the levels, flow rates, temperatures, and pressure at various locations of the column. They were then used to adjust the valves to maintain steady state.
Basically, a binary mixture of methanol and water is first pumped from the feed tank through a feed preheater into the distillation column at tray #4. The feed preheater uses the overhead vapor from the column as a source of heat. Overhead vapor is then condensed and subcooled in a series of heat exchangers. One of the heat exchangers is used to preheat the feed, the other two are condensers that being cooled by water. The condensed vapor is then sent to the reflux drum, where part of the liquid methanol is pumped out as the product, and rest is sent back as reflux. The product is sent to a holding tank, the reflux is sent back into the column at the top tray. The vapor boil-up is provided by a steam from thermos-syphon reboiler. The bottoms product is withdrawn through a cooler and pumped to the bottoms hold tank
One way to determine the overall column efficiency is by using the McCabe-Thiele Diagram, under the assumption that efficiency is the same on all trays and in the partial reboiler. We have used the McCabe-Thiele method to determine the number of ideal stages and feed tray location. Parameters such as the mole fractions of methanol at the distillate, bottoms, and feed, as well as the reflux ratio were used to perform the calculations. A plot including the equilibrium curve, feed line, and operating lines for the rectifying and stripping sections are constructed. The number of stages was then found by using the stepping method starting from the either the distillate or bottoms composition on the operating line. The Murphree tray efficiency was determined similarly. However, an effective equilibrium curve was used instead of the ideal equilibrium curved used in overall efficiency calculations.