award winning technology for an industrial solvent

8/6/2019 Award Winning Technology for an Industrial Solvent

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Award Winning Technology for anIndustrial Solvent

Mike Ashley



Award Winning Technology for an Industrial Solvent

Successful development of vapour phase ester hydrogenation, prompted the concept of usingthis equilibrium reaction in reverse to manufacture esters by alcohol dehydrogenation.

Production of ethyl acetate from ethanol was an obvious candidate and the concept wasquickly demonstrated. Improved catalysts indicated an economically feasible route andfundamental chemical engineering research was conducted to investigate reaction kinetics,fluid flow and transfer processes. The process was scaled up from the laboratory via “mini-plants” without recourse to conventional pilot plant operations. Scale factors for unitoperations varied from 80,000 up to 170,000. The first commercial plant was successfullystarted-up in May 2001 since when interest has been shown by companies with access tofermentation ethanol derived from sustainable resources.

The basic flowsheet for this novel process is shown below.

The process is in three sections; reaction, product recovery and ethanol dehydration.

Reaction

Dry ethanol is heated and vaporised before entering the dehydrogenation reactor where it isconverted to ethyl acetate. The reaction is endothermic and the vapour is reheated severaltimes in the reactor to maintain reaction temperature. Crude liquid product is separated fromthe cooled reactor outlet stream and the overhead vapour stream, containing mainlyhydrogen, is scrubbed with part of the ethanol feed to recover organics before the majorportion is exported. Liquid product and a small quantity of hydrogen are fed to a trickle bedreactor, where the carbonyl impurities are hydrogenated to alcohols.

Product Recovery

The stream leaving the hydrogenation reactor contains mainly ethyl acetate, unreactedethanol and small quantities of water. This is fed to two columns using a pressure change toremove the azeotrope composition. Unreacted ethanol is separated for recycle and a crudeethyl acetate stream is then further distilled to remove trace impurities.

Ethanol Dehydration

In the reaction section, by-product reactions produce a small amount of water. This watermust be removed before unreacted ethanol can be recycled. Water removal can either be

FeedEthanol

Hydrogen

Recycle Ethanol

EthylAcetate

EthanolDehydration

Dehydrogenation SelectiveHydrogenation

Refining



integrated within the ethyl acetate unit or part of a larger external facility depending onwhether feedstock ethanol has a high water content for example; sourced from a fermentation unit. Molecular sieve processes are generally most economic, althoughconventional entrainer distillation systems may be used.

Chemical Engineering Challenges

Most novel chemical processes present chemical engineering challenges. The followingsection describes some of the challenges that were addressed during the development andcommercialization of the ethyl acetate process. Resolution of these problems required acombination of fundamental chemical engineering research and application of state-of-the-artsoftware tools for steady state and dynamic simulation and computational fluid dynamics.

Dehydrogenation Reactor Design

Ethanol dehydrogenation to ethyl acetate is asequential reaction. In the first step, ethanol is

dehydrogenated to acetaldehyde. In the secondstep the acetaldehyde reacts with ethanol and isfurther dehydrogenated to give ethyl acetate. Thefirst stage of the reaction is endothermic while thesecond stage is exothermic and the overall reactionis endothermic. Kinetics of the reactions weremeasured using a spinning basket reactor.

Langmuir-Hinchelwood expressions weredeveloped for the major series and parallelreactions and fitted to the experimental data. Thefinal kinetic model was then used to developtubular and multiple adiabatic bed reactor models

and a four bed adiabatic reactor with inter-bedreheating was the economic choice for thisapplication. In parallel a small scale reactor wasused to study catalyst performance and deactivation.

Hydrogenation Reactor Design

Small quantities of methyl ethyl ketone and n-butyraldehyde are made on thedehydrogenation catalyst. Preliminary studies indicated that these compounds are extremelydifficult to separate from ethyl acetate by fractionation. Selective hydrogenation to theirequivalent alcohols considerably simplified separations and proved to be the economicoption. Laboratory tests identified a platinum group metal/carbon catalyst operating in a

trickle bed regime as having the required selectivity.

It is important to maintain adequate catalyst wetting to maintain catalyst effectiveness and toavoid hot-spot formation. The system favoured uses low liquid velocities and very low gasvelocities (a flow regime outside the range of published correlations). An experimentalprogramme was initiated to study pressure drop, liquid hold-up and wetting characteristics ofthis catalyst at the chosen flow conditions. These experiments proved that the commercialreactor can be operated safely and effectively provided liquid distribution and re-distributionare maintained. This work highlighted the importance of the inert fill on top of the catalyst intrickle bed reactors, to provide secondary distribution.



Flow/ kmol/m²s

0.1-0.12

0.08-0.1

0.06-0.08

0.04-0.06

0.02-0.04

0-0.02

-0.02-0

500

Distance

down bed, h

Highest 0.123

Average 0.097

Pressure = 760mmHg

Ethanol

Ethyl Acetate

Water

Pressure = 9308mmHg

Ethanol

Ethyl Acetate

Water

This diagram illustrates the predicted liquid flow distribution 0.5m below the top of the catalystbed using the commercially installed distributor.

Product recovery

The separation of ethyl acetate product from unreacted ethanol and byproducts iscomplicated by low boiling, binary and ternary azeotropes of ethanol, ethyl acetate and water.It was found that the composition of these azeotropes varies significantly with pressure andso a pressure swing distillation scheme was adopted to separate the products. An extensiveprogramme of vapour-liquid-liquid equilibrium data measurement and model regression wastherefore initiated. VLE data for all important binaries in the system were measured and

regressed to liquid activity models. The model predictions were then checked against furtherexperimental measurements of ternary azeotrope composition and multi-component flashes.

In pressure swing distillation systems, there is an optimum between the column refluxes andthe recycle stream, which gives minimum heat input to the system. In the ethyl acetatesystem extensive optimisation studies showed that minimum heat input could be achieved bylimiting the accumulation of water in the column overhead system.

The diagrams above show distillation lines for the ternary system at two pressures and showsignificant change in azeotrope composition with pressure. An interesting feature of thesystem is that because of the shape of the distillation curves, by operating with low levels of

water, it is possible to operate the high pressure column overhead composition at lowerconcentrations of ethyl acetate than present in the ternary azeotrope.



Environmental Aspects

Consumption of ethyl acetate as an industrial solvent has increased in recent years, due inthe main to it being preferred to Hazardous Atmospheric Pollutants such as MEK and MIBK.As a growth product, it was a legitimate target for process development. There were anumber of production routes available, all of which ultimately relied upon the use of non-

renewable feedstocks, primarily natural gas or ethylene. In addition these routes requireseveral steps, each with its own inefficiencies and by-product issues and were not especiallyatom efficient. The chart opposite illustrates these various routes

The major benefit of this new process is that it relies only upon the availability of ethanol, themajority of which is produced by fermentation. As fermentation ethanol is derived frombiomass and this relies on atmospheric carbon dioxide, there is no net carbon dioxidecontribution.

Logically, plants based upon this process would be located close to sources of low costethanol. Additional benefits can be realised by integration of cane sugar, ethanol and ethylacetate units in one location. Of particular benefit in this respect is the use of waste bagasseas a fuel to support the units. For the cane grower, the use of bagasse in-plant often enablesa better rate of return than can be obtained from export of power to a local grid.

Wacker

Wacker Davy

Dehydrogenation

Esterification

DirectAddition

Tischenko Ethyl

Acetate

Acetaldehyde

AceticAcid

Ethanol

CropStarch

Fermentation

Ethylene

NaturalGas



For further details please contact:

Davy Process Technology Limited20 Eastbourne terrace

London W2 6LEUK

Tel: +44 (0)20 7957 4120Fax: +44 (0)20 7957 3922Mail: [email protected]: www.davyprotech.com

Davy Process Technology Limited

Technology CentrePrinceton DriveStockton-on-TeesTS17 8PYUK

Tel: +44 (0) 1642 853 800Fax: +44 (0) 1642 853 801Mail: [email protected]

Web: www.davyprotech.com

Davy Process Technology is a Johnson Matthey company

award winning technology for an industrial solvent

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