Challenge Solved (See Answer Below)
Things were going smoothly at the Breaux Bridge Drug Co. pilot plant during the midnight to 8 a.m. shift until Therese Lagniappe, the reactor operator, let the run instruction sheet get too close to the Coleman stove that was being used to heat water to prepare her bihourly cup of Community Coffee.
What followed ended in a total loss of the run sheet, the coffee, and a substantial portion of the novel Lagniappe was writing.
Remembering the less than enthusiastic reaction she got the last time she telephoned her supervisor in the middle of the night, Lagniappe decided to rely on her memory of the required flow-rate settings.
- The two liquids being feed to a stirred-tank reactor were circulostic acid (CSA: MW = 75, SG = 0.90) and flubitol (FB: MW = 90, SG = 0.75).
- The product from the system was a popular over-the-counter drug that simultaneously cures high blood pressure and clumsiness.
- The molar ratio of the two feed streams had to be between 1.05 and 1.10 mol CSA/mol FB to keep the contents of the reactor from forming a solid plug.
- At the time of the accident, the flow rate of CSA was 45.8 L/mon.
She set the flow of flubitol to the value she thought had been in the run sheet: 55.2 L/min
Did Lagniappe set the flow of flutibol to the correct value?
If not, how would she have been likely to learn of her mistake (Note: the reactor was stainless steel, so she could not see the contents.)
Author's Response and Challenge Solution:
Lets take a look at the ChEnected challenge problem for August.
The problem involves the mixing of two chemicals in a specific molar ratio. The problem asks if the ratio of these two chemicals falls within a given range based on flow rates, specific gravitys, and molecular weights for each species.
Lets take a look at a solution submitted by Jack from Johns Hopkins University:
"The easiest way to see if Lagniappe set the flow of flubitol correctly is to convert the volumetric flow rates of both CSA and flubitol into molar flow rates. We can then compare the ratios and see if it falls within the range of operation.
Volumetric Flow rate * Specific gravity / Molecular Weight = Molar Flow Rate
45.8 [L/min] * (1000 [mL] / 1 [L]) * (.90 [g/mL]) / (75 [g/mol]) = 549.6 mol/min
55.2 [L/min] * (1000[mL]/1[L]) * (.75[g/mL]) / (90[g/mol]) = 460 mol/min
Ratio of CSA/FB:
549.6 [mol/min] / 460 [mol/min] = 1.195
We can see that she actually did not set the flow rate of flubitol correctly, since the molar ratio is over the maximum ratio of 1.10. "
Congratulations on a perfect solution Jack!
The problem mentions that if the ratio is not correct solids will form in the reactor, and then asks how an operator might first notice this problem.
Jack and many of the rest of you correctly point out that solid formation would interfere with any mixers in the tank, reducing their speed or increasing their power requirements.
A solid plug in the reactor would also cause an increase in pressure, an increase in fluid level and reduction of fluid flows. Depending on how the reactor is designed any one of these could trigger an alarm.
Thank you for your solution Jack, and everyone else who submitted an answer! Be sure to participate in next month's challenge. If you have an Idea for a ChEnected challenge problem we would love to hear it!