Unit Operations Laboratory
Tray Drying Experiment
Dr. Srdjan Nesic

Background - Objective - Equipment - Emphasis - Prelab - Hints

Background (Return to top)

Your company has recently been asked to put in a tender for 100 small units to be used for drying of a variety of exotic mushroom types. Your boss has asked YOU to design a simple batch tray dryer to do the job - after all you are the only chemical engineer in the division. After conducting some basic research you find out that tray drying is the most primitive and labor intensive of the various drying technologies that you could use in this case. In addition, you find virtually no information on drying of mushrooms, but there is heaps of information on potatoes! During your next meeting with your boss, you bravely put forward an alternative proposal employing a combined freeze/vacuum drying technology (which you learned is often used for drying of expensive pharmaceuticals). You argue that this modern drying technique would do a wonderful job on the mushrooms and would preserve their overall quality and flavor  (an important factor when 4 oz. can cost up to $50 in retail). In addition your proposed solution is energy efficient, environmentally friendly and easily lends itself to modular design. You admit that the down side is the higher price of the ultra modern unit, which of course requires regular maintenance and skilled personnel. Your boss is unimpressed. He impatiently explains that the units will be sold to a Third-world customer and will be spread around the countryside, where manual labor is cheap and unskilled, there is plenty of wood to burn, nobody worries about the environment and modular design is unheard of.  In addition he tells you that his tennis partner (who is a chef in one of the downtown expensive hotels) has told him that these fancy mushrooms usually look like and cook like potatoes anyway, even if the taste is somewhat more subtle.  You get two weeks to complete the preliminary design and are told to use potatoes as the model material.

Design Objective (Return to top)

Produce a preliminary design for a batch tray drying unit which can process up to 500 kg of raw mushrooms per day.  List all key operations/equipment necessary to complete the design and justify your selection. Provide basic data on fundamental operating variables such as temperature, humidity, flow rate, etc.  Specify the approximate size of the unit and determine the required capacity of the main components (e.g. heaters, fans). In order to produce a preliminary design, you will need some quantitative information about the drying process. You should decide what this information is (your experimental objective) and determine it experimentally.

Equipment and Supplies (Return to top)

Available equipment includes a small experimental tray drying unit, probes for velocity, temperature and air humidity measurements. Available supplies include tap water, distilled water, methanol, typical laboratory glassware, a balance accurate to 0.0001 g and a small oven.  You may also use any other equipment or supplies available in the unit operations laboratory if you need it.

Emphasis (Return to top)

• A thorough understanding of the main characteristics of a drying process and the influence of key parameters.
• Use of equations describing the drying operation (including their limitations).
• Careful planning of experiments to efficiently obtain both results and a quantitative measure of their accuracy.

Prelab Guidelines (Return to top)

These are in addition to the standard guidelines in the syllabus.

· Introduction
Cover all the main processes involved in the drying process, such as heat and mass transfer, air flow, phase change, etc. Write out equations that describe them. Identify all the main parameters appearing in those equations such as temperature, air humidity, moisture content of dried material, etc. Decide which of these you must know and which you can guesstimate and still produce a reliable design. Then decide which parameters you can measure in the lab experiment in order to back up your decisions. Find data on the important physical properties that you might need such as thermal conductivity, latent heat, diffusivity, sorption isotherms, etc.

· Experimental Methods
Identify what you want to measure and why. Consider how best to measure each parameter using the equipment available in order to save time and to minimize the experimental error.

· Expected Data and Results
Outline how the data which you will be measuring fit into the theoretical and computational framework you have set up as a basis of your design. Are there any checks and balances that need to be done? What will you do if they do not work out as theory suggests?

· Prelab Meeting
Prepare for a challenge.

Hints (Return to top)

• Too many to be listed here.  It is strongly recommended that the group meet with Dr. Nesic several days prior to the start of the prelab write-up to discuss many fine points of this project.  This meeting usually includes the tour of the experimental apparatus and some preliminary decision-making. A quick read through introductory chapters of any drying textbook is strongly recommended prior to the first meeting. This should naturally lead to brief review of selected chapters from your favorite fluids, heat and mass transfer and thermodynamics textbooks.

· Send email to Dr. Nesic at: nesic@bobcat.ent.ohiou.edu.

· Return to top of this tray drying handout.

(Last modified on 01/13/02)