Esters to Alcohols and back again

Don McKinley

ESTERS TO ALCOHOLS AND BACK AGAIN(first published in Hydrocarbon Engineering, April 2001)

Hydrogenation of vaporised C18+ hydrocarbons at around 220°C does notimmediately spring to mind as being the most cost-effective processingroute to alcohols. Don McKinley* describes how ten plants producing fourvery different alcohol products have proved otherwise. Recent researchinto the reverse process -dehydrogenation of alcohols into esters -has ledto a process licensed to upgrade ethanol to more valuable ethyl acetatewithout using acetic acid.

These plants supply products ranging from butanol (from C8 ester), through 1,4- butanediol(BDO) (from C6 or C8 esters) and 1,4-cyclohexanedimethanol (CHDM) (from C10 ester), toC12-C18 fatty alcohols (from C13-C19 esters). All but one of these plants have been licensedby Davy Process Technology (DPT), and capitalise on the discovery in the 1980's of thebenefits of low-pressure vapour-phase hydrogenation of esters.

With six plants licensed, Davy’s BDO technology is now the leading process available, and alicensing arrangement has recently been concluded which gives BASF non-exclusive rights touse Davy's BDO technology in their future plants. Meanwhile, a unique low-pressure vapour-phase CHDM process was developed by DPT for Eastman Chemicals, the world's leadingmanufacturer of CHDM. Eastman have since successfully commissioned a plant in Spainusing the Davy technology. Also in the Far East, two plants have been licensed to make fattyalcohols from coconut/palm kernel fatty acids using Davy's esterification and esterhydrogenation technologies. The first plant in the Philippines was fully accepted just onemonth after feed ester introduction.

Early Days

For nearly 100 years Davy Process Technology has been involved in syngas-basedprocesses -initially the processing of coke-oven off-gas followed by the production of townsgas by naphtha steam reforming. The discovery of natural gas in the North Sea gave theimpetus to develop other syngas-based processes. Principal among these is the longassociation that DPT have had with ICI (now Synetix) as their main methanol licensee. DPThas now built 85% of ICI's LP methanol technology licences, representing 42% of the world'sinstalled methanol capacity.

Next was the development and commercialisation of the rhodium-catalysed LP Oxo Processwith Union Carbide Corporation and Johnson Matthey. This enormous step change intechnology from the established HP cobalt-catalysed process gave such an economic benefitthat many existing cobalt plants were replaced under "shut-down economics". In 1977 UnionCarbide, Johnson Matthey and DPT (then Davy Powergas Ltd.) received ChemicalEngineering's prestigious Kirkpatrick Award for outstanding group effort in new chemicalengineering technology.

The LP Oxo Process has been hugely successful. Together with Union Carbide, DPTcontinues to develop the process and has now granted 25 licences to 19 operators in 14countries throughout the world. Over 90% of the world's oxo plants licensed in the last 25years use this process. Main products are butanol and 2-ethylhexanol (2EH), both producedby the hydroformylation of propylene. One of the 2EH plants is also configured to produce 2-propylheptanol (2PH) from normal butenes. Both 2EH and 2PH are plasticiser range alcoholsused to produce plasticisers controlling flexibility in PVC products. The technology has nowbeen extended to the production of detergent range alcohols (CIO to C18) following processdesign development and extended testing in a continuous mini-plant at DPT's newTechnology Centre. The first licensed application to convert a higher olefinic feed to detergentalcohols will be brought on-stream by Sasol Chemical Industries in South Africa in 2002.

Problem Solved

One of the early LP Oxo plants used a reactorconfiguration that resulted in a high level of oneester by-product during the hydrogenation ofbutyraldehyde to butanol. Hydrogenating thisbutyl butyrate ester to make more butanol wasan obvious remedy. However, there was a sitelimitation of 40 bar and 250°C for the reaction.Liquid- phase hydrogenation required highpressures. Vapour-phase hydrogenation wastherefore investigated in 1981.

A range of highly active non-precious metalcatalysts were developed and tested in thedesired operating range at DPT's own TechnicalCentre in Stockton-on- Tees. Reproducibleresults were obtained in a range of bench-scaletests and this information, together with tests onan 8 inch diameter pilot plant reactor, gaveconfidence to go directly to full scale. Thereaction fluid, being a single vapour phase,meant that hydraulic scale-up problems wereminor compared to liquid phase (trickle-bed)scale-up. Ester Hydrogenation Column, Shinwha, Korea

The first commercial application came on-stream in October 1983. The initial catalyst chargenot only performed as predicted but exhibited long life. Only one plant was actually built but anew catalyst and reaction system had been proved commercially - paving the way for furtherdevelopments.

The Next Step

DPT then looked for other applications where these highly active catalysts could be used,especially where low-pressure vapour-phase ester hydrogenation would offer significantadvantages.

Several basic classifications of esters were examined:

• primary esters, e.g., laurates, stearates• saturated di-esters, e.g., oxalates, succinates• unsaturated di-esters, e.g.,maleates• aromatic esters, e.g., benzoates.

In each case, hydrogenation of the esters produces the corresponding alcohols forming theester. For example, alkyl maleates and succinates produce 1,4-butanediol while methyllaurateproduces lauryl (C12) alcohol.

C11H23 CO.OCH3 + 2 H2 = C12H250H + CH30H

DPT has developed commercial capability for each of these four types of ester.

Butanediol

The first application to be developed after butyl butyrate was for 1,4-butanediol. This has alsobeen the most successful so far, with six plants currently licensed. Combined licensedcapacity confirms the DPT process as the most cost-effective BDO process available.

All the plants use maleic anhydride feedstock (produced from cheap n-butane) which is firstesterified to make di-alkyl maleates. The first two plants use ethanol to make diethyl maleate

with ethanol being recycled, while all subsequentplants use methanol to make dimethyl maleate.At the start of the development a review of sixcommercially available esterification processesrevealed that none of them provided the desiredhigh processing efficiency combined with lowsulphur contamination of the esters and lowenvironmental impact from catalyst disposal. Theuse of solid esterification catalysts was thereforeinvestigated and developed as a separatetechnology area.

The first two licensed plants were started up inKorea and Japan in 1992. Both have a capacityof 22,500mtpa of BDO equivalent (includes co-product tetrahydrofuran, THF). A smaller11,250mtpa plant was successfullycommissioned in China in early 2000. This wasthe first to use methyl esters produced in animproved esterification process based on a resin-catalysed reaction column.

A similar larger plant for 33,750mtpa BDO wascommissioned mid-2000 in Taiwan for TCC, anda 100,000mtpa plant for BASF Petronas is due tostart up next year, 2002. A sixth licence for50,000mtpa has been granted in Saudi Arabiaand this is now proceeding through design,engineering and procurement.

Butanediol Plant, Chengli, Cina

For all of these licences, DPT has provided all the catalysts, a comprehensive Front EndEngineering Package (FEEP) for the detailed engineer, FOB supply of certain critical items ofequipment such as reactors, compressors, etc., comprehensive operator training, and fullcommissioning and laboratory advice and guidance. DPT stands alone as an independenttechnology company and is free to supply basic engineering packages to any competentcontractor.

Natural Detergent Alcohols

In 1988, during BDO development, DPT actively started a full development programme toconvert fatty acids into fatty alcohols. Traditionally this was a liquid- phase hydrogenationprocess using methyl esters (usually made by trans-esterification of fats and oils) or waxesters (made by reacting feed fatty acids with recycled product alcohol). All these processesused very high pressures (200 to 300 bar), and temperatures as high as 300°C.

Tests showed that DPT's vapour-phase hydrogenation could be operated at much lowerpressures and temperatures. An optimum operating pressure of 40 bar was selected withreaction temperatures well below 250°C. At these conditions, the hydrogenation section canbe constructed in carbon steel, avoiding the alloy/stainless steels normally needed tominimise hydrogen embrittlement.

Obviously, a large molar excess of hydrogen is recycled to ensure the C12-C18 esters in thehydrogenation reactor feed are fully vaporised. However, this has the advantage of a lowtemperature rise across the catalyst bed, allowing high selectivities to be achieved whenoperating at temperatures appropriate to throughput and catalyst activity.

Optimisation of the heat exchangers and other equipment in the loop means that a very lowhead compressor is used to circulate hydrogen. A simple centrifugal compressor design canbe used which combines easy operation and maintenance with reduced capital cost

compared to other types of recyclecompressor. In addition, the lowoperating pressure of 40 bar meansthat make-up compression isminimised, or not required as hydrogencan be made from methanol at thispressure in an integrated plant. Totalpower requirement (make-up andrecycle) for the DPT NDA process isactually less than competing high-pressure, liquid-phase processeswhich have relatively low hydrogenrecycle flows, but high make-uphydrogen compression and feed pumppower requirements.

Two NDA plants have been licensed inthe Far East, both based on fatty acidsfrom coconut/palm kernel oil. A30,000mtpa plant for Primofina in thePhilippines was successfullycommissioned in early 1998 and asimilar plant for 50,000mtpa is beingconstructed for Dahin in Taiwan.

Natural Detergent Alcohol Plant , Primechem,Philippines

Both licences were sold with a full design package and supply of all catalysts, FOB supply ofcritical items of equipment (mainly from the UK), operator and analyst training at DPT'sLondon office and Technology Centre, plus on-site pre- commissioning and start-up/analyticaladvice and guidance.

For both projects, DPT worked closely with the Singapore office of the specialist oleochemicalcontractor, De Smet, who supplied the upstream fatty acid plant and acted as overallcontractor.

As part of the development DPT designed, fabricated and shipped a skid-mounteddemonstration unit, which Unichema operated in Holland to produce around 200 tonnes offatty alcohols, for downstream processing to surfactants for product application tests.Although Unichema did not take up their option to licence, the highly successful results of thisdemonstration programme assisted in gaining development agency funding for the first plantin the Philippines.

Cvclohexanedimethanol

This aromatic diol was also traditionally made by high-pressure (200 bar) liquid-phasehydrogenation of the corresponding di-ester. Tests showed that DPT's low-pressure vapour-phase technology could be applied. Apart from reduced capital (low pressure, carbon steelloop) and improved economics, there was much more of the desired trans isomer in theproduct. Furthermore, the trans/cis ratio could be readily controlled to meet market demands.

Eastman, the world's leading producer of CHDM, approached DPT to develop a vapourphase route. They subsequently signed a Development Agreement and thereafter wereclosely involved in monitoring DPT's test programme and process design developments. Atthe conclusion of the Development Programme, DPT granted Eastman an exclusive right toDavy's vapour-phase ester hydrogenation technology for the production of CHDM.

With input from DPT, Eastman has since built a 30,000mtpa plant in Spain, which DPTengineers and chemists assisted in commissioning in 1999. High purity CHDM is now beingproduced.

Spin-off Technologies -Esterification

Several technologies have evolved in the development of the different flow-sheets basedaround vapour-phase ester hydrogenation.

As previously mentioned, DPT uses solid catalysed esterification in the NDA and BDOflowsheets. A number of different reactor types are used to shift the reaction equilibrium to thedesired conversion. The heterogeneous resin catalyst is retained in all the reactors used andconsequently esters can be fed directly to the hydrogenation vaporiser without furthertreatment. Other homogeneous esterification catalysts would require neutralisation -givingeffluents and loss of yield -and/or separation by distillation to reduce contaminants such assulphur from reaching the hydrogenation catalyst.

However, it was found that available resins promoted significant etherification of the alcohol todi-alkyl ethers such as di-methyl ether and water. This by-product water influences theesterification equilibrium making it difficult to approach full conversion to esters. Because ofthis, DPT's catalyst manufacturer developed a new range of polymeric resin catalysts thatfeature a low make of by-product di-alkyl ethers. This means that near 100% conversion toesters can be achieved in a reactive distillation column where the resin remains on each trayand alcohol vapour passes counter-current to the feed acid. Water is stripped out overheadand esters accumulate in the sump.

DPT developed the esterification reaction column design through many laboratory tests, multi-stage pilot plant trials and full-scale hydraulic modelling tests. A total of five of thesecommercial scale reactive distillation columns have been designed and supplied by DPT. Allsupply highly converted, low sulphur esters from the column swnp with no further processingdirectly to the hydrogenation section of either the NDA or BDO processes. There is essentially100% utilisation of the feed acids/anhydride making this a highly efficient processing step withvery low environmental impact.

The reactive distillation column can be licensed for other reactions apart from esterification,some of which are under development in a multi-stage pilot unit at DPT's new TechnologyCentre.

Spin-off Technologies -Alcohol Dehvdrogenation

Critical understanding of the chemistry underlying the ester hydrogenation technologyportfolio, specific experience of catalyst applications and design experience of pilot andcommercial scale plants enabled DPT to explore the limits of flexibility. This has resulted in aprocess to manufacture ethyl acetate directly and solely from ethanol. This is a majordevelopment step as it adds value to ethanol co-produced in Fischer- Tropsch synthesis orsurplus fermentation alcohol. It is obviously of particular interest where there is no localsupply of acetic acid.

For these reasons DPT and Sasol entered intodiscussions which led to the signing of anagreement at the end of 1996, in which Sasol wouldsupport the development of a new direct ethylacetate technology. A very rapid development tookplace with parallel test-work and processdevelopment in Stockton and London such thatwithin two years a commercial design was in place.

Development work included generation of VLE datato support optimisation of a novel two columnpressure-swing refining system. The process uses acatalyst similar to that used in the BDO process andoperates at similar temperatures. Some of theevolved hydrogen is used in a selectivehydrogenation step ahead of refining to remove

close boiling aldehydes and ketones. The balance of the hydrogen is exported.

The first EA plant with a capacity of 50,000mtpa has been completed in Sasol's Secundacomplex in South Africa.

Spin-off Technologies -Diol Dehydration

Although high purity tetrahydrofuran (THF) forms part of the product slate in BDO plants, theamount of THF relative to BDO can only be varied within limits. Resin catalyseddehydration/etherification of BDO to THF was therefore developed to meet additional demandfor THF, which is used in elastomer production. This add-on reaction system has beenlicensed to two BDO licensees.

The Future

Davy has maintained its technology position by continuing to improve and develop the BDOand NDA processes as well as develop new applications.

Other applications of the low-pressure vapour-phase ester hydrogenation technology arebeing progressed. Most of these are directed towards other diols, which would be used incondensation polymerisation reactions to yield polyesters. These await market growth of end-use applications.

Reactive distillation applications, including autocatalytic as well as solid and liquid catalysedreactions, are being investigated. Some show promise in the fine chemicals manufacturingsector where multi-stage batch operations can be replaced by a single reaction column.

Finally, the application of Davy's alcohol dehydration technology may open some newopportunities such as methyl formate from methanol, and in ester solvent manufacture, forexample, propyl propionate from propanol feeds. Mixed alcohol feedstocks are also beingstudied. Other possibilities include transesterification of ethyl acetate with butanol to producebutyl acetate. The liberated ethanol would be recycled to the EA plant.

For proponents of renewable resources, this allows fermentation ethanol to be used as abuilding block to step off to other products, such as acetaldehyde, as well as being upgradedat low cost to a useful high-value solvent.

Licensing and Development

All these activities lead to licence agreements reflecting the history, aims and ambitions of thetechnology provider and user. Central to this is intellectual property and the reassuranceprovided by appropriate patents. As would be expected, DPT have been much more active inrecent years in extending its patent portfolio.

The income from licensing allows DPT to invest for the future as reflected in the newTechnology Centre in Stockton-on-Tees, opened in January 1999, and from which moreexciting technology developments are expected.

*Don McKinley is a Development Manager with Davy Process Technologywhere he has been employed for the last fourteen years. A graduate of theUniversity of Newcastle-upon-Tyne and a Fellow of the Institution ofChemical Engineers, he was responsible for process development of DPT'sNatural Detergent Alcohols process and the associated EsterificationReaction Column design. He subsequently led the team that successfullycommissioned the first commercial applications of these developments inthe Philippines.

For further details please contact:

Davy Process Technology Limited20 Eastbourne terraceLondon W2 6LEUK

Tel: +44 (0)20 7957 4120Fax: +44 (0)20 7957 3922Mail: dpt@davyprotech.comWeb: www.davyprotech.com

Davy Process Technology LimitedTechnology CentrePrinceton DriveStockton-on-TeesTS17 8PYUK

Tel: +44 (0) 1642 853 800Fax: +44 (0) 1642 853 801Mail: tc@davyprotech.comWeb: www.davyprotech.com

Davy Process Technology is a Johnson Matthey company