Some time ago, I published a Post about the importance of Pilot Planting in scaling-up of chemical processes. I am in the process of developing an Overview monograph on this subject. As I write-up self contained sections, I will pre-post them here. To make the sections more meaningful, I will include subsections describing my own personal experiences on the specific topic.
This section is on reactor considerations.
REACTOR CONSIDERATIONS
Several considerations go into the development of pilot plant reactor designs:
A. Reaction Kinetics
In many cases, the chemist who had original responsibility for product sample development will have made only a preliminary foray into developing the process reaction kinetics that control reactor design.
There are three types of reactor system used in the Chemical Process Industries:
1. Constant Volume, Stirred Tank Reactor (CSTR): This is simply a well-mixed tank (perhaps 10-25 gallons in a first-pass pilot unit) in which reactants are fed in, and a product stream withdrawn, at such rates that the volume of reacting materials is kept constant. This is the simplest reactor to run and analyze.
2. Batch Reactors: A batch reactor is a discontinuous reactor. It is a stirred tank that is filled with the reactants before the reaction starts and emptied after it has run to completion. An example is baking a cake: the ingredients (raw materials) are added to a baking pan (the reactor), and the contents are heated to the proper temperature for an appropriate amount of time. At the end of this time, the pan (reactor) is removed and the cake is ready (reaction completed).
3. Plug Flow/Tubular/Continuous Reactors: In a Plug Flow Reactor, one or more fluid reactants are pumped through a pipe or tube. The chemical reaction proceeds as the materials travel through the PFR. At the inlet to the PFR the rate is very high, but as the concentrations of the reagents decrease and the concentration of the product(s) increases the reaction rate slows.
The equations describing a plug flow tubular reactor are the same as those which describe a batch reactor.
Personal Example I
One of the units for which I had Process Research & Development (R/D) responsibility was designed to produce styrene oxide from the reaction of styrene and peracetic acid. The reaction train consisted of five tubular reactors in series, each at a constant temperature; the temperature increased with each reactor.
The unit had long been plagued by poor process efficiency – I was asked to improve the process. As a first step, I developed the reaction kinetics of styrene and peracetic acid reacting to form styrene oxide and byproducts. I then created a computer program to model each reactor and looked at temperature profile effects on efficiency.
I concluded that the profile was the exact reverse of what it should have been: a decreasing temperature profile, rather than an increasing profile. When this new profile was applied, reaction efficiency dramatically improved.
Personal Example II
When designing a pilot unit and pilot run, the possible build-up of impurities via secondary side-reactions needs to be considered. This often necessitates running the pilot unit longer than what would be considered “typical or economical.”
The Federal government asked my company to design a manufacturing unit that would make a specific diene precursor for an epoxide needed for a special application. The chemists and engineers collaborated and developed a process that would achieve this. A small pilot unit was built both to test the process and to produce sample material for evaluation. Because of manpower and cost constraints, the unit was only operated for six hours per day, not the 24 hour/5 day manufacturing period that would normally be anticipated.
All went well with pilot operations, and a manufacturing facility was designed and built based on the pilot unit. Start-up was uneventful. However, after a week of operation, plant engineers noted an alarming build-up of a yellow, granular polymer in the diene product. The amount of this polymer appeared to increase as time went on. After much frantic investigation, R/D personnel found that the yellow polymer was the result of a side-reaction that took time to build up to a point of significance. Because the pilot unit was only operated for limited time periods, the polymer had not been detected earlier.
B. Energy Considerations:
Energy plays a key role in chemical processes. Energy is absorbed to break bonds, and is released as bonds are created. In some reactions the energy required to break bonds is larger than the energy evolved making new bonds. A reaction is exothermic if energy is released. A reaction is endothermic if energy is absorbed.
