22.2 Phenol production from Cumene
22.2.2 For Peroxidation
22.2.4Process Technology (Figure 22.1)
Figure 22.1 Flow sheet of Phenol production from Cumene
22.2.5 Technical questions
1. Why unsaturates are saturated before entering the oxidization reactor?
Ans: Unsaturated compounds will produce compounds other than cumenehydroperoxide. Since cumenehydroperoxide 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, cumenehydroperoxide 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 oC and 1 atm pressure.
b) Alpha methyl styrene-phenol forms an azeotrope with 93 % of alpha methyl styrene at 162 oC 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.
22.3 Phenol from Toluene Oxidation
Oxidation to Benzoic acid
Oxidation of Benzoic acid to phenol
22.3.2Process technology (Figure 22.2)
Figure 22.2 Flow sheet of Phenol manufacture from Toluene Oxidation
22.3.3 Technical questions
1. Why are benzoic acid crystals again subjected to water wash?
Ans: The benzoic acid crystals could have some water soluble impurities despite having water wash. This is because of the fact that in real life, infinite separation factors don’t exist and components do get distributed in both phases. Therefore, an additional water wash facilitates the removal of these impurities.
2. Why phenol enters both bottom and top product?
Ans: The trick in the operation of the second oxidation reactor is to maintain temperature and pressure such that benzoic acid leaves the unit as vapour. This way, benzoic acid can be easily fractionated and sent back to the reactor. However, this has a limit as well i.e., phenol gets evenly distributed between the vapour and liquid phases.
3. Is there any opportunity for energy integration in the process?
Ans: Yes, the vapours from the reactor can be heat integrated with the reboilers in the toluene column or any other reboilers in the other two columns.
4. What exactly happens in the column that is fed with phenol + water mixture, given the fact that phenol forms an azeotrope with water?
Ans: At atmospheric pressure, phenol forms an azeotrope with water at about 9.4 wt % phenol and 90.6 wt % water. Therefore, crude phenol column shall produce a water rich stream at the top which is the azeotrope and the pure phenol product as the bottom product. From the VLE data (not shown here), it is also apparent that from about 10 wt % phenol to about 90 % phenol, the relative volatility value is pretty low and after 90 % phenol in the feed solution, the relative volatility increases significantly. Therefore, it makes sense now to understand that crude phenol fed to the first tower splits into azeotrope at the top (with 9.4 wt % phenol) and a purer phenol product at the bottom.
Eventually, the phenol + water stream entering the second unit along with the azeotrope composition is improved substantially in its composition to move away from the azeotropic composition (with more phenol in water) and hence, it should again tend to produce a purer phenol product and a waste water product at the top. The waste water product could be again an azeotrope of a different class.
Again, if we alter the pressures of these columns, the VLE data (not shown here) will reduce the phenol composition in the wastewater stream and that’s what would be probably considered to extract maximum phenol using the distillation technology.
Alternatively, extractive distillation process can be used in which toluene can be used as a solvent to alter the relative volatility. Probably, this process is more common presently in the industry than the process outlined in this lecture.