22.2.5 Technical questions
1. Why unsaturates are saturated before entering the oxidization reactor ?
Ans: Unsaturated compounds will produce compounds other than cumene hydroperoxide. Since cumene hydroperoxide will only convert to phenol but not others, the product quality will drastically reduce if unsaturates are available in the feed stock.
2. What is the basic advantage of emulsification ?
Ans: A gas liquid reaction requires maximum interfacial area to enhance reaction rates. Usually gas is sparged through liquid. But in this case, the liquid itself is emulsified so that the interfacial area can be even further enhanced.
3. Are there any safety issues related to the oxidation reactor ?
Ans: Yes, cumene hydroperoxide is explosive after a certain minimum concentration. Therefore, all designs shall ensure that this component should not get accumulated beyond a specified concentration in any section of the units.
4. Why are the vent gases recycled back ?
Ans: The vent gases in addition to N2, unreacted O2 will consist of hydrocarbons as the hydrocarbons could escape to the vapour phase due to their low boiling points even at room temperature. Therefore, these hydrocarbons including cumene are recycled back by using a condenser at the top.
5. Suggest a technology for processing acidified wash water from environmental perspective ?
Ans: The acidified wash water can be neutralized using NaOH to obtain NaCl in these waters. Eventually, the NaCl rich solution can be concentrated using multiple effect evaporators to reduce the huge water problems. Near zero discharge is difficult for such cases.
6. Why is alpha methyl styrene also recycled back to the reactor via hydrogenator ?
Ans: Alpha methyl styrene can be hydrogenated to convert to cumene. This is because alpha methyl styrene consists of a double bond in the alkyl group attached to the benzene ring. Therefore, by hydrogenation we convert unsaturated compound to cumene and this way it is recycled back to the reactor.
7. Why there is sulphuric acid even after gravity settling in the organic phase ?
Ans: This is a basic problem of equilibrium separation factors for the sulphuric acid, as the acid has propensity to remain maximum in the aqueous phase but also to a little extent in the organic phase.
8. Why vacuum distillation is required for all three distillation columns ?
Ans: To the best of the instructors knowledge, all organic compounds form azeotropes with others at atmospheric pressure when fractionated. Therefore, to bypass the formation of azeotrope, the pressure of the system is reduced which enhanced the relative volatility and eliminates the formation of the azeotrope. Further justification of this is available in the following azeotrope data bank:
- a. Phenol-acetophenone forms an azeotrope with 7.8 wt % of phenol at 202°C and 1 atm pressure.
b. Alpha methyl styrene-phenol forms an azeotrope with 93% of alpha methyl styrene at 162°C and 1 atm.
When vacuum distillation is considered, relative volatility of the mixture is improved and this improvement bypasses the azeotrope formation. Hence, purer products can be obtained.
9. Don't you think water will also enter crude phenol ?
Ans: Yes, water also enters crude phenol as solvent loss in the wash tower will provide some water into the crude phenol stream. But its concentration is not significant when compared to the concentration of the organics.