Condensation of carbonyl and furan impurities: Purification of phenol

Condensation of carbonyl & furan impurities
Purification of phenol

The cumene peroxidation route is by far the most important commercial processes to produce phenol. The cumene hydroperoxide is decomposed in the presence of dilute sulfuric acid or hydrochloric acid to form phenol and acetone. The reaction mixture is subsequently deacidified with either a strong or a weak base resin to remove the residual acid.

This neutralization is necessary as to prevent corrosion problems in the downstream distillation and rectifying columns. After neutralization, acetone is removed by distillation to leave the crude phenol. The impurities in phenol are numerous. The carbonyls and especially acetol and the furans are the most troublesome because they cannot be easily removed by distillation and can form color bodies in the phenol during downstream processing. The presence of color bodies makes the phenol unacceptable for certain uses such as caprolactam and optical grade bisphenol A production.

Phenol impurities

Cumene

Dimethylbenzyl alcohol
or Dimethyl phenyl carbinol

Alpha-methyl styrene

Cumyl phenol
Acetophenone
Alpha-hydroxy acetone (Acetol)
Mesityl Oxide and Phorone

2-Methylbenzofuran

Several phenol plants around the world have successfully used a resin-catalyzed process to purify their phenol. In this process, the phenol with impurities is passed through a bed of a porous strong acid resin catalyst which alkylates these impurities with phenol to form higher molecular weight compounds which are more readily separated by the final phenol distillation.

Typical Commercial Phenol Specifications

Property	Value
Freezing points	40.85 °C
Color, ASTM Pt-Co	5
Phenol concentration, wt %, min	99.99
Water concentration, wt %, max	0.01
Impurities, ppm
• Carbonyls	40
• Hydroxyacetone	10
• -methylstyrene + cumene	5
• Acetophenone	3
• 2-methylbenzofuran	2
Total (dry)	100
Source: Kirk Othmer technology, Phenol

At one phenol plant, the use of a resin catalyzed purification process has reduced the concentration of impurities as shown in the following table.

Compound Influent	Influent (ppm)	Effluent (ppm)
Carbonyl compounds	150 - 900	3 -38
Mesityl oxide	130-700	0-24
Cumene and AMS	250-1000	0-6

Process Conditions

Typical operating conditions for the purification process are shown in the following table.

Influent temperature	100 - 120 °C
Flow rate	2 - 4 BV/h
Minimum bed depth	1 m
Catalyst lifetime (typical)	1- 1.5 year

The catalyst used in this process is a strong acid polymeric catalyst with good thermal stability. Because the catalyst slowly desulfonates and accumulates polymeric tars over time, the activity of the catalyst deteriorates and it is necessary to increase the reactor temperature during the run in order to maintain the desired reduction of impurities. Eventually an upper temperature limit is reached beyond which other side-reactions begin to become important and the catalyst conversion is too low. For these reasons, use of thermally-stable catalysts such as Amberlyst™ 16 and 36 is recommended.

In most installations, the catalyst is supplied in water-wet form. As part of the startup process, therefore, the catalyst therefore needs to be dehydrated by passing dry phenol through the catalyst and sending the wet phenol to distillation for redrying.

Recommended catalysts

Amberlyst™ 16Wet	Standard phenol purification catalyst. Good activity and resistance to polymer fouling.
Amberlyst™ 36Wet	Higher activity catalyst with improved thermal stability, allowing longer run lengths and lower impurity levels.

For sampling, pricing and availability of AMBERLYST™ catalysts please contact your Rohm and Haas sales representative.