Unit Operations Laboratory

Liquid-Liquid Extraction

Dr. Ben Stuart

Background

You have recently been hired by the Big Kahuna Chemical Corporation. One of the production lines at the facility where you work yields a by-product stream which is 80 wt% n-heptane and 20 wt% isopropanol. This stream is currently considered a waste material and is therefore shipped off site to a recycling company. The recycling company sells the purified n-heptane (99.9 wt%) back to BKC Corp., of course making a nice little profit in the process.

One of BKC Corp.'s brightest engineers has recently found a way to modify the process to make it much more efficient. Unfortunately, the by-product stream is still present in the modified process. On the up-side, the n-heptane feed stock does not need to have such a high purity. In fact 98.5 wt% n-heptane with an isopropanol residual is quite sufficient. Also, during the re-plumbing the process, several pieces of equipment were removed and are available for use elsewhere in the plant.

An OU undergraduate who is working for BKC Corp. as a co-op has found an old tattered file belonging to a recently retired engineer at the plant. She notices that this engineer had recommended that BKC Corp. purify its own by-product stream when the process went on-line twenty years ago, but this notion was dismissed by the past management chain as un-economical because the required level of purity for the n-heptane feed was too high. In the file is a description of the liquid-liquid extraction process, a reference to the thermodynamics and mass transfer books describing the process, and a ternary diagram for n-heptane, isopropanol, and water. It seems that water may be an appropriate (and inexpensive) liquid for extracting the unwanted alcohol from the process stream. The co-op student immediately seeks out her supervisor and suggests that this process be evaluated for use in the new process.

Design Objective

Your task is to evaluate the possibility of treating a 20 wt% isopropanol in n-heptane solution in a liquid-liquid extractor using water as the extractant fluid. You may choose the volumetric flowrate in the laboratory, but the plant has a waste stream flowrate of 2000 gal/day and your final design must include equipment sizing for 150% of that amount. Remember that the effluent n-heptane must have a purity of 98.5 wt%. You will need to determine the appropriate flowrates of each feed to the column (paying special attention to the S:F ratio) and potential benefits of utilizing an agitator (at a specified rate, if necessary).

Equipment and Supplies

By an unbelievable stroke of luck, the unit operations laboratory in the basement of Ohio University has a scaled-down version of the equipment available at the BKC Corp. We have also seen fit to provide you with ample supplies of n-heptane and isopropanol. A tour of the apparatus, complete with a description of each of the working components, can (and should) be scheduled with Dr. Stuart or the laboratory coordinator, Mr. Jim Caeser.

Hints

Too many to be remembered and listed here. It is strongly recommended that the group meet with Dr. Stuart to discuss several fine points of this experiment several days prior to the start of the pre-lab write-up. This meeting usually includes the tour of the apparatus and distribution of the necessary ternary diagram. A quick read through Chapter 8 in Henley and Seader (and a review of ternary diagrams from your thermodynamics class - if necessary) is also strongly recommended prior to the first meeting.