Author of case history:
Dr. Mary A. Kaiser
DuPont Company
Case is based on published material (Plant/Operations Progress, April 1991).

The problem:

A plant on the Gulf Coast makes adipic acid by aqueous nitric acid oxidation of cyclohexanone at elevated temperatures. The isolated adipic acid is recrystallized. The wet adipic acid is dried in a rotary dryer which feeds the powder through a heated zone at 140 C. Ambient air circulates through the dryer.

Within a three week period, the plant experienced two dust fires and explosions in the dryer. While not large enough to cause damage, the incidents halted production costing the plant $200,000 loss per day.

Beyond the cost of the shut-down, the incidents demonstrated that a problem existed which was capable of causing an explosion which could conceivably cause serious damage. For reasons of both safety and economics a solution has to be found.

This type of incident has never been previously seen in the 15 year lifetime of the plant. When the dryer was opened for cleaning discolored and charred deposits were found in the heated zone.

The plant director has called the Analytical section for assistance in preventing a further incident.

Discuss the implications of the cost of a plant shut down on your priority setting. Who should you talk to before starting laboratory work?

Background: The typical industrial scientist is always working on high priority problems. One must be able to differentiate between levels of "top priority". A useful criterion is to ask if the company is losing money because of the problem. A plant shut down, which affects the company's income, the well being of the plants employees, and the plans, and possibly financial well being of customers depending on the product, is cause to drop everything and spent long hours to solve the problem.

The plant director may, or may not, be as familiar with the process as a plant engineer, or even an operator. Before analyzing a sample, analyze the problem. In doing this talk to staff associated with the process at all levels. The more people you talk to the more likely you are to find the critical fact that may solve the problem.

A sample was delivered to Analytical. It was a coherent lump with one smooth side and one rough side.

Discuss an approach to the problem.

Physical properties of Adipic Acid are listed in Table 1. (Table from original article.)

Melting Point 152 C
Solubility in water 2.5 wt. % at 25 C
Specific Gravity 1.34 at 18 C
Boiling Point 333 C at 760 mm Hg (decomposes)
Flash Point 196 C
Autoignition Temp. 420 C

Remember that the solution is not just what happened, but also "how it happened", so that it can be prevented in the future.

Clues to what happened can often be found in the circumstances surrounding the incident. What questions should you ask those involved in the operation?

What changes in the process had occurred within the month preceding the first incident? These could include changes in reactor or dryer temperature, errors in temperature sensors, a new supplier of cyclohexanol or of nitric acid.

The technical director insists that nothing has changed in this period.

The sample submitted is a solid, the configuration of which reflects the physical structure of the reactor.

The first response to a solid should be to look at it, and this should be done with as good a magnifying glass as is available. Microscopy should be a first step. Scanning electron microscopy can frequently offer clues beyond those of appearance, because fluorescent x-rays can be used to form a surface map of elemental concentration.

Scanning electron microscopy showed semi-quantitative evidence of iron, with higher concentrations on the smooth side.

Discuss the chemical implications of this observation. (Was the reactor a ferrous metal?)

Color formation in a process can result from gross contamination or a catalytic color forming reaction. In the case of organics the latter is common. Heavy metals are suspect. Confirmation and quantitative information was obtained by analysis of the inorganic residue of the char. In this case atomic absorption spectroscopy was used. Iron, silicon, aluminum and chlorine were detected.

How would you prove or disprove iron catalyzed degradation? Is this a reasonable hypothesis in light of the insistence that nothing had changed in the process?

Thermal and thermal gravimetric methods were applied to study the behavior of adipic acid at elevated temperatures. These methods also permit the capture of degredation products for simultaneous or sequential analysis by spectrocsopic techniques.

Data obtained in this situation was as follows.

Differential scanning calorimetry of the recovered sample showed the melting endotherm at 152oC and a broad exotherm starting at 250oC and intensifying at 334oC. The sample ignited when exposed to air at this temperature.

Thermal gravimetric analysis of the char showed weight loss below 100oC and above 150oC.

ARC (Accelerating Rate Calorimetry) determines heat of reaction, kinetics of reactions and vapor pressure for solid and liquid samples. It is also used to predict the potential for self-heating of materials stored for long periods. The addition of 7% iron to a sample of adipic acid resulted in an exotherm at 135 C when heated in air slightly above atmospheric pressure.

Discuss these results, and propose the next step

.

Confirm the conclusion by duplicating the effect

The client will be best convinced that you have arrived at a correct solution if you can duplicate the plant incident in a realistic experiment.

In this example, samples of adipic acid (c. 100g) were placed in a 250 ml beaker, insulated in a looselly closed can and put into a forced circulation oven with programmable heating elements. Both the oven and sample temperatures are monitored, Air is introduced via a dip tube. Results are given in Table 2.

Results from Externally Heated Insulated Container (Table 2)

Run Sample Events
1 Adipic acid None
2 Adipic acid + Aluminum None
3 Adipic Acid +Carbon steel films Exotherm c. 185 C, Decomposition
4 residue from heater, adipic acid Exotherm c. 160 C, Decomposition
5 calcium silicate + adipic acid None

Discuss your conclusions. Remember that the goal is not to understand what happened, but to prevent a repitition.

Samples of both adipic acid and nitric acid from the plant were analyzed for iron. Analytical was able to obtain retained samples of these materials from several months in the past, before the incidents happened. These samples showed no evidence of iron contamination.

A sample of adipic acid from the supply that was used during the week of the first incident was found. It also contained no excess iron. The nitric acid used at that time was from the same supply that had been analyzed earlier.

The plant is hesitant to start up without understanding the situation. Discuss what to do next.

You have been communicating with the technical director of the plant, and with a plant engineer. You visit the plant, and get into a discussion with an operator. The operator tells you that during the period of the incidents, a mechanism that fed adipic acid to the drier was poorly maintained. Lack of lubrication led to grinding noises which persisted until the lubrication was corrected.

What do you do now?

A suggestion to the plant engineer that the screw drive be dissembled led to the observation that steel had been shaved off the drive and had occasionally been fed into the dryer.

There is no way to duplicate the conditions of the incident. To start the plant under these circumstances the lab director wants proof that the explanation is chemically reasonable. You study the reaction, and find that adipic acid forms a complex with iron which decomposes exothermally to cylclopentanone, water and carbon dioxide. Sl ow oxidation of adipic acid in contact with the iron caused adiabatic self-heating with eventually led to ignition and explosion.

Analytical problem listing

• 
  
  Table of Contents