Q Max Process

CHAPTER 1.6Q-MAX™ PROCESS FORCUMENE PRODUCTION

Gary A. Peterson and Robert J. SchmidtUOP LLC

Des Plaines, Illinois

INTRODUCTION

The Q-Max™ process converts benzene and propylene to high-quality cumene by using aregenerable zeolitic catalyst. The Q-Max process represents a substantial improvementover older cumene technologies and is characterized by its exceptionally high yield, supe-rior product quality, low investment and operating costs, reduction in solid waste, and cor-rosion-free environment.

Cumene is produced commercially through the alkylation of benzene with propyleneover an acid catalyst. Over the years, many different catalysts have been proposed for thisalkylation reaction, including boron trifluoride, hydrogen fluoride, aluminum chloride, andphosphoric acid. In the 1930s, UOP introduced the UOP catalytic condensation process,which used a solid phosphoric acid (SPA) catalyst to oligomerize light olefin by-productsfrom petroleum thermal cracking into heavier paraffins that could be blended into gaso-line. During World War II, this process was adapted to produce cumene from benzene andpropylene to make a high-octane blending component for military aviation gasoline.Today, cumene is no longer used as a fuel, but it has grown in importance as a feedstockfor the production of phenol.

Although SPA is a highly efficient and economical catalyst for cumene synthesis, it hastwo important limitations:

1. Cumene yield is limited to about 95 percent, because of the oligomerization of propy-lene and the formation of heavy alkylate by-products

2. The catalyst is not regenerable and must be disposed of at the end of each catalystcycle.

In recent years, producers have been under increasing pressure to improve cumene prod-uct quality so that the quality of the phenol produced downstream (as well as acetone andalpha-methylstyrene, which are coproduced with phenol) could be improved. Twenty-fiveyears ago, most phenol was used to produce phenolic resins, and acetone was used prima-rily as a solvent. Today, both phenol and acetone are used increasingly in the productionof polymers such as polycarbonates and nylon. Over the years, improvements to the SPA

1.69

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process managed to keep pace with the demand for higher cumene product quality, butproducers still sought an improved cumene process that would produce a better-qualityproduct at higher yield.

Because zeolites are known to selectively perform many acid-catalyzed reactions, UOPbegan searching for a new cumene catalyst that would overcome the limitations of SPA.UOP’s objective was to develop a regenerable catalyst that would increase the yield ofcumene and lower the cost of production. More than 100 different catalyst materials werescreened, including mordenites, MFIs, Y-zeolites, amorphous silica-aluminas, and beta-zeolite. The most promising materials were modified to improve their selectivity and thensubjected to more-rigorous testing. By 1992, UOP had selected the most promising cata-lyst based on beta-zeolite for cumene production and then began to optimize the processdesign around this new catalyst. The result of this work is the Q-Max process and the QZ-2000 catalyst system.

PROCESS CHEMISTRY

The synthesis of cumene from benzene and propylene is a modified Friedel-Craftsalkylation, which can be accomplished by many different acid catalysts. The basic alkyla-tion chemistry and reaction mechanism are shown in Fig. 1.6.1. The olefin forms a carbo-nium ion intermediate, which attacks the benzene ring in an electrophilic substitution. Theaddition to the olefin double bond is at the middle carbon of propylene, in accordance withMarkovnikov’s rule. The addition of the isopropyl group to the benzene ring weakly acti-vates the ring toward further alkylation, producing di-isopropyl-benzene (DIPB) and heav-ier alkylate by-products.

The QZ-2000 catalyst functions as strong acid. In the QZ-2000 catalyst, the active sur-face sites of the silica-alumina structure act to donate the proton to the adsorbed olefin.Because the QZ-2000 catalyst is a strong acid, it can be used at a very low temperature.

1.70 ALKYLATION AND POLYMERIZATION

Reaction Mechanism

+ CH2 = CH – CH3

Benzene Propylene

Acid

Cumene(Isopropylbenzene)

CH

CH2 = CH – CH3

CH2 = CH – CH3

CH2 – CH2 – CH3

CH2 = CH – CH3

AcidCH3 – CH – CH3

(Favored)

+ +

+ CH2 CH + H+

+

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH

Cumene

+

Propylene

AcidCH

Diisopropylbenzene

Secondary Reaction

Primary Reaction

FIGURE 1.6.1 Alkylation chemistry.

Q-MAX™ PROCESS FOR CUMENE PRODUCTION



Q-MAX™ PROCESS FOR CUMENE PRODUCTION 1.71

Low reaction temperature reduces the rate of competing olefin oligomerization reactions,resulting in higher selectivity to cumene and lower production of heavy by-products.

Transalkylation of DIPB

Transalkylation is the acid-catalyzed transfer of one isopropyl group from DIPB to a ben-zene molecule to form two molecules of cumene (Fig. 1.6.2). The Q-Max process isdesigned with an alkylation reactor section, which produces about 85 to 95 wt % cumeneand 5 to 15 wt % DIPB. After recovery of the cumene product by fractionation, the DIPBis reacted with recycle benzene at optimal conditions for transalkylation to produce addi-tional cumene. With the alkylation and transalkylation reactors working together to takefull advantage of the QZ-2000 catalyst, the overall yield of cumene is increased to 99.7 wt%.

Side Reactions

In addition to the principal alkylation reaction of benzene with propylene, all acid catalystspromote the following undesirable side reactions to some degree (Fig. 1.6.3):

● Oligomerization of olefins. The model for acid-catalyzed alkylation is diffusion of theolefin to an active site saturated with benzene followed by adsorption and reaction. Onepossible side reaction is the combination of the propyl carbonium ion with propylene toform a C6 olefin or even further reaction to form C9, C12, or heavier olefins.

● Alkylation of benzene with heavy olefins. Once heavy olefins have been formed througholigomerization, they may react with benzene to form hexylbenzene and heavier alky-lated benzene by-products.

● Polyalkylation. The addition of an isopropyl group to the benzene ring to producecumene weakly activates the ring toward further substitution, primarily at the meta andpara positions, to make DIPB and heavier alkylates.

● Hydride-transfer reactions. Transfer of a hydrogen to an olefin by the tertiary carbon oncumene can form a cumyl carbonium ion that may react with a second benzene mole-cule to form diphenylpropane.

Benzene

StrongAcid

StrongAcid

CH

Cumene

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH

CH2 = CH –CH3

Diisopropylbenzene

Primary Reaction 2

Potential Side Reaction CH

Cumene

+

+

HeavyBy-product

RCH

Polyalkylate Benzene

+

RR

FIGURE 1.6.2 Transalkylation chemistry.




In the Q-Max process, the reaction mechanism of the QZ-2000 catalyst and the oper-ating conditions of the unit work together to minimize the impact of these side reactions.The result is an exceptionally high yield of cumene product.

DESCRIPTION OF THE PROCESS FLOW

A representative Q-Max flow diagram is shown in Fig. 1.6.4. The alkylation reactor is typ-ically divided into four catalyst beds contained in a single reactor shell. The fresh benzeneis routed through the upper midsection of the depropanizer column to remove excess waterand then sent to the alkylation reactor via a sidedraw. The recycle benzene to both the alky-lation and transalkylation reactors comes from the overhead of the benzene column. Amixture of fresh and recycle benzene is charged downflow through the alkylation reactor.The fresh propylene feed is split between the four catalyst beds. An excess of benzene isused to avoid polyalkylation and to help minimize olefin oligomerization. Because thereaction is exothermic, the temperature rise in the reactor is controlled by recycling a por-tion of the reactor effluent to the reactor inlet, which acts as a heat sink. In addition, theinlet temperature of each downstream bed is reduced to the same temperature as that of thefirst bed inlet by injecting a portion of cooled reactor effluent between the beds.

Effluent from the alkylation reactor is sent to the depropanizer column, which removesany propane and water that may have entered with the propylene feed. The bottoms fromthe depropanizer column are sent to the benzene column, where excess benzene is col-lected overhead and recycled. Benzene column bottoms are sent to the cumene column,


Olefin Oligomerization

R – CH – CH6 +

Hydride Transfer

Heavy Alkylate

CH C + + R – CH2 – CH3

C + + C

Diphenyl Propane

+

Polyalkylation

C3 = C3 =

C3 =C3 = C3 =

C6 = C9 =

CH

CH2 – CH – CH3 CH2 – CH – CH3

CH2 – CH – CH3Diisopropylbenzene

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH=

CH

Triisopropylbenzene

FIGURE 1.6.3 Possible alkylation side reactions.




where the cumene product is recovered overhead. The cumene column bottoms, whichcontain mostly di-isopropylbenzene, are sent to the DIPB column. The DIPB stream leavesthe column by way of a sidecut and is recycled to the transalkylation reactor. The DIPBcolumn bottoms consist of heavy aromatic by-products, which are normally blended intofuel oil. Steam or hot oil provides the heat for the product fractionation section.

A portion of the recycle benzene from the top of the benzene column is combined withthe recycle DIPB from the sidecut of the DIPB column and sent to the transalkylation reac-tor. In the transalkylation reactor, DIPB and benzene are converted to additional cumene.The effluent from the transalkylation reactor is then sent to the benzene column.

The QZ-2000 catalyst utilized in both the alkylation and transalkylation reactors is regen-erable. At the end of each cycle, the catalyst is typically regenerated ex-situ via a simple car-bon burn by a certified regeneration contractor. However, the unit can also be designed forin-situ catalyst regeneration. Mild operating conditions and a corrosion-free process envi-ronment permit the use of carbon-steel construction and conventional process equipment.

FEEDSTOCK CONSIDERATIONS

Impact of Feedstock Contaminants on Cumene Purity

In the Q-Max process, the impact of undesirable side reactions is minimal, and impuritiesin the cumene product are governed primarily by trace contaminants in the feeds. Becauseof the high activity of the QZ-2000 catalyst, it can be operated at very low temperature,which dramatically reduces the rate of competing olefin oligomerization reactions anddecreases the formation of heavy by-products. Thus, with the Q-Max process, cumene prod-uct impurities are primarily a result of impurities in the feedstocks. Table 1.6.1 lists thecommon cumene impurities of concern to phenol producers, and Fig. 1.6.5 graphicallyshows the reactions of some common feedstock contaminants that produce these impurities.

● Cymene and ethylbenzene. Cymene is formed by the alkylation of toluene with propy-lene. The toluene may already be present as an impurity in the benzene feed, or it may


Cumene

Drag

Heavies

Propane

Recycle Benzene

Benzene

Propylene

AlkylationReactors

Depropanizer

TransalkylationReactor

DIPB

BenzeneColumn

CumeneColumn

DIPBColumn

FIGURE 1.6.4 Process flow diagram.




be formed in the alkylation reactor from methanol and benzene. Ethylbenzene is prima-rily formed from ethylene impurities in the propylene feed. However, as with cymene,ethylbenzene can also be formed from ethanol. Small quantities of methanol and ethanolare sometimes added to the C3’s in a pipeline to protect against hydrate freezing.Although the Q-Max catalyst is tolerant of these alcohols, removing them from the feedby a water wash may be desirable to achieve the lowest possible levels of ethylbenzeneor cymene in the cumene product.

● Butylbenzene. Although butylbenzene is produced primarily from traces of butylene inthe propylene feed, it may also be created through the oligomerization of olefins.However, the very low reaction temperature of the Q-Max process reduces oligomer-ization, resulting in minimal overall butylbenzene formation.


TABLE 1.6.1 Common Cumene Impurities

Trace contaminant Concern in downstream phenol unit

Nonaromatics Form acids and other by-products in phenol unit, yield lossEthylbenzene Forms acetaldehyde, an acetone contaminantn-Propylbenzene Forms propionaldehyde, an acetone contaminantButylbenzenes Resist oxidation, an alpha-methylstyrene contaminantCymenes Form cresols, phenol contaminantsPolyalkylates Form alkylphenols, yield loss

CH + CH2 = CH – CH3

Toluene

CH

Propylene Cymene

CH3CH3

CH3

CH3CH3

CH3

CH3

CH3

Benzene Ethanol Ethylbenzene

CH2CH3

+ CH3CH = CH – CH3

+ CH2 = CH2 or CH3CH2OH

Ethylene

CH3CHCH2CH3

Benzene ButylbenzeneButylene

CH +

Cyclopropane CumeneBenzene

+ CH2CH2CH3

n-Propylbenzene

FIGURE 1.6.5 Reactions of feed impurities.




● n-Propylbenzene. The n-propylbenzene (NPB) is produced from trace levels of cyclo-propane in the propylene feed. The chemical behavior of cyclopropane is similar to thatof an olefin: It reacts with benzene to form either cumene or NPB. The tendency to formNPB rather than cumene decreases as the reaction temperature is lowered.Unfortunately, the catalyst deactivation rate increases with lower reaction temperature(Fig. 1.6.6). Because of the exceptional stability of the QZ-2000 catalyst system, a Q-Max unit can be operated for extended cycle lengths and still maintain an acceptable lev-el of NPB in the cumene product. For example, with a typical FCC-grade propylene feedcontaining normal amounts of cyclopropane, the Q-Max process can produce a cumeneproduct containing less than 250 wt ppm NPB and maintaining an acceptable catalystcycle length.

Impact of Catalyst Poisons on Catalyst Performance

A list of potential Q-Max catalyst poisons is found in Table 1.6.2. All the listed compoundsare known to neutralize the acid sites of zeolites. Good feedstock treating practice orproven guard-bed technology easily handles these potential poisons.

Water in an alkylation environment can act as a Brønsted base to neutralize some of thestronger zeolite acid sites first. However, as a result of the inherently high activity of theQ-Max catalyst, water does not have a detrimental effect at the typical feedstock moisturelevels and normal alkylation and transalkylation conditions. The Q-Max catalyst canprocess feedstocks up to the normal water saturation conditions, typically 500 to 1000ppm, without any loss of catalyst stability or activity.

Sulfur does not affect Q-Max catalyst stability or activity at the levels normally pres-ent in the propylene and benzene feeds processed for cumene production. However, tracesulfur in the cumene product, for example, might be a concern in the downstream produc-tion of certain monomers (e.g., phenol hydrogenation for caprolactam). Within the Q-Maxunit, the majority of sulfur compounds associated with propylene (mercaptans) and thoseassociated with benzene (thiophenes) are converted to products outside the boiling rangeof cumene. However, the sulfur content of the cumene product does depend on the sulfurcontent of the propylene and especially benzene feeds. Sulfur at the levels normally pres-


NPB Formulation

Catalyst Deactivation Rate

Temperature

FIGURE 1.6.6 Effect of reactor temperature.




ent in propylene and benzene feeds considered for cumene production will normally resultin cumene product sulfur content that is within specifications (for example, �1 wt ppm).

Successful operation with a wide variety of propylene feedstocks from differentsources has demonstrated the flexibility of the Q-Max process. Chemical-grade, FCC-grade, and polymer-grade propylene feedstocks can all be used to make high-qualitycumene product.

PROCESS PERFORMANCE

The Q-Max unit has high raw material utilization and an overall cumene yield of at least99.7 wt % based on using typical propylene and benzene feedstock. The remaining 0.3 wt% or less of the overall yield is in the form of a heavy aromatic by-product.

The cumene product quality summarized in Table 1.6.3 is representative of a Q-Maxunit processing commercially available, high-quality feedstocks. The quality of thecumene product from any specific Q-Max unit is strongly influenced by the specific con-taminants present in the feedstocks.

Propane entering the unit with the propylene feedstock is unreactive in the process andis separated in the fractionation section as a propane product.

CASE STUDY

A summary of the investment cost and utility consumption for a new Q-Max unit produc-ing 200,000 MTA of cumene from extracted benzene and chemical-grade propylene isshown in Table 1.6.4. The estimated erected cost for the Q-Max unit assumes constructionon a U.S. Gulf Coast site in 2002. The scope of the estimate includes basic engineering,procurement, erection of equipment on the site, and the initial load of QZ-2000 catalyst.


TABLE 1.6.2 Handling Potential Catalyst Poisons

Poison Source Removal

Basic nitrogen Trace levels in feedstocks Guard bedAmmonia Common impurity in FCC propylene Water wash or guard bedArsine (AsH3) Common impurity in FCC propylene Guard bed

TABLE 1.6.3 Representative CumeneProduct Quality

Cumene purity, wt % � 99.97Bromine index � 10Sulfur, wt ppm � 0.1Specific impurities, wt ppm:

Ethylbenzene � 30n-Propylbenzene � 250Butylbenzene � 20Cymene � 5Di-isopropylbenzene � 10

Total nonaromatics � 20




The utility requirements for a Q-Max unit depend on the project environment (i.e., feed,product specifications, and utility availability). Q-Max units are often integrated with phe-nol plants where energy use can be optimized by generating low-pressure steam in the Q-Max unit for utilization in the phenol plant.

COMMERCIAL EXPERIENCE

The first Q-Max unit went on-stream in 1996. Since that time, UOP has licensed a total ofnine Q-Max units throughout the world having a total plant capacity of 2.3 million MTAof cumene. Six Q-Max units have been commissioned and three more are in various stagesof design or construction. Capacities range from 35,000 to 700,000 MTA of cumene pro-duced. Several of these units have been on-stream for more than 5 years without perform-ing a single catalyst regeneration.

BIBLIOGRAPHY

Jeanneret, J. J., D. Greer, P. Ho, J. McGeehee, and H. Shakir: “The Q-Max Process: Setting the Pacefor Cumene Production,” DeWitt Petrochemical Review, Houston, March 1997.

Schmidt, R. J., A. S. Zarchy, and G. A. Peterson: “New Developments in Cumene and EthylbenzeneAlkylation,” AIChE Spring Meeting, New Orleans, March 2002.


TABLE 1.6.4 Investment and Operating Cost for 200,000 MTA Q-Max Unit

Feedstock requirements:Extracted benzene (99.8 wt %) 132,300 MTAChemical-grade propylene (95 wt %) 74,240 MTA

Utility consumption per MT cumene produced:Electric power 12.3 kWhHigh-pressure steam 0.81 MTMedium-pressure steam 0.20 MTLow-pressure steam credit �0.31 MTCooling water 3.1 m3

Erected cost estimate $14.2 million




Q Max Process

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