The separation of alcohols

Nieuwoudt, Traute (2002-03)

Thesis (MScEng)--Stellenbosch University, 2002.

Thesis

ENGLISH ABSTRACT: Pure primary alcohols are very valuable as raw materials and solvents. Close-boiling alcohol mixtures are produced as byproducts from the Fischer Tropsch synthesis. These byproducts include the mixtures 1-butanol+2- penta noI and 1-pentanol+2-hexanol. Due to the small difference in boiling points these alcohols cannot be separated from one another by using conventional distillation. This study has been undertaken to determine whether primary and secondary alcohols may be separated by exploitation of their chemical properties. Esterification of the alcohols followed by distillation of the esters into cuts and hydrolyses of the esters, has been attempted to separate the alcohols. This however, was unsuccessful. In this study the difference in dehydration rate of secondary and primary alcohols in acidic media has also been investigated. Several acidic resins and liquid catalysts have been used. The acidic resins gave no dehydration or extremely low dehydration rates in the liquid phase. The liquid catalysts H2S04, Oxalic Acid, NaHS04 and H3P04 were investigated. H3P04 gave excellent results. Laboratory experiments were conducted at the boiling point of the reaction mixture at atmospheric pressure. The reaction mixture was sampled at varying time intervals and analysed. The secondary alcohol dehydrated rapidly to the corresponding alkene. The primary alcohol formed symmetrical ethers at a very low rate. The primary and secondary alcohol also combined to form small amounts of unsymmetrical ethers. After the dehydration reaction the organic products can be separated from the acid with a'short path distillation unit. The primary alcohol can further be purified by conventional distillation. Conceptual process designs were done for the separation and purification of the reactor product streams of the alcohol mixtures 1-butanol+2-pentanol and 1-pentanol+2-hexanol. n laboratory scale it was found that for the separation of 85% 1-butanol and 15% 2-pentanol (mass %), 90 % H3P04 (mass %) at an acid:alcohol ratio of 1,5: 1 results in suffcient dehydration of 2-pentanol. A reaction time of 70 minutes is required. A conceptual design on the purification of the 1-butanol predicted a product quality of 99,5 % 1-butanol (mass %) and a 1-butanol recovery of 75 %. The 1-butanol recovery is low, because a major part of the 1-butanol is lost in the purification as part of the ternary azeotrope with water and n-butylether. On laboratory scale it was also found that for the separation of 85 % 1- pentanol+15 % 2-hexanol (mass %),90 % H3P04 (mass %) at an acid:alcohol ratio of 1,5:1 gives sufficient dehydration of 2-hexanol. A reaction time of only 35 minutes is required. A conceptual design on the purification of the 1- pentanol predicted a product quality of 99,9 % 1-pentanol and a 1-pentanol recovery of > 98 %. The 1-pentanol recovery is excellent, only the 1- pentanol that is converted to ethers is lost. In this study it has been proven that a dehydration separation process can be applied successfully to remove secondary alcohols from a primary+secondary alcohol mixture. Especially the removal of 2-hexanol from a 1-pentanol+2- hexanol mixture gave promising results. In order to assess the economic viability of this dehydration process an economic evaluation should be done. This could be part of subsequent studies. The dehydration separation process should be investigated further. It is believed that this dehydration separation process can be expanded to higher alcohols, e.g. 1-hexanol+2-heptanol. It would be extremely advantageous if a solid catalyst could be found for the separation. In this case the recovery of the organics from the reaction mixture would be very much easier. If a solid catatyst is not found, a continuous process using H3P04 as liquid catalyst should be developed.

AFRIKAANSE OPSOMMING: Suiwer primêre alkohole is baie waardevolle rou materiale en oplosmiddels. Alkohol mengsels, wat uit naby-kokende alkohole bestaan, word as neweprodukte in die Fischer Tropsch Sintese gevorm. Hierdie newe-produkte sluit alkohol mengsels soos 1-butanol+2-pentanol en 1-pentanol+2-hexanol in. Weens die klein verskil in kookpunte van hierdie alkohole kan die alkohole nie met konvensionele distillasie van mekaar geskei word nie. Hierdie studie is onderneem om te bepaal of die chemiese eienskappe van alkohole benut kan word om primêre en sekondêre alkohole van mekaar te skei. 'n Poging is aangewend om die alkohole met behulp van esterifikasie te skei. Die alkohole is eers ge-esterifiseer, daarna met behulp van distillasie in verskeie snitte verdeel en die alkohol is vrygestel deur hidrolise van die esters. Dit was egter onsuksesvol. Die verskil in dehidrasie tempo van sekondêre en primêre alkohole in suur mediums is ook ondersoek. Verskeie suur harse en vloeibare kataliste is ondersoek. Die suur .harse het of geen dehidrasie of baie lae dehidrasie tempo's in die vloeistoffase gegee. Die vloeistof kataliste H2S04, Oksaalsuur, NaHS04 en H3P04 is ondersoek. H3P04 het uitstekende resultate gelewer. Eksperimente is op laboratoriumskaal en onder atmosferiese druk uitgevoer. Monsters is van die reaksiemengsels by verskillende tydsintervalle geneem en geanaliseer. Die sekondêre alkohol het vinnig na die ooreenstemmende alkeen gedehidreer. Die primêre alkohole het simmetriese eters teen 'n lae tempo gevorm. Die primêre en sekondêre alkohole het ook gekombineer om gemengde eters te vorm. Kort-pad-distillasie kan gebruik word om na die dehidrase reaksie die organiese produkte van die suur te verwyder. Die primêre alkohole kan verder met konvensionele distillasie gesuiwer word. Konseptueie prosesontwerpe is uitgevoer vir die skeiding en suiwering van die alkohol mengsels 1-butanol+2-pentanol en 1-pentanol+2-hexanol nadat dehidrasie van die mengsels uitgevoer is. Op laboratoriumskaal is dit gevind dat vir die skeiding van 85% 1-butanol en 15% 2-pentanol (massa %), 90 % H3P04 (massa %) met 'n suur:alkohol verhouding van 1,5:1 effektiewe dehidrase van 2-pentanol lewer. fn Reaksietyd van 70 minute word benodig. fn Konseptueie ontwerp vir die suiwering van die 1-butanol het fn produkkwaliteit van 99,5 % 1-butanol (massa %) en fn 1-butanol opbrengs van 75 % voorspel. Die 1-butanol opbrengs is laag aangesien fn groot deel van die 1-butanol verlore gaan as deel van die ternêre azeotroop wat 1-butanol met n-butieleter en water vorm. Dit is ook op laboratoriumskaal vasgestel dat vir die skeiding van 85 % 1- pentanol+15 % 2-hexanol (massa %), 90 % H3P04 (massa %) met fn suur:alkohol verhouding van 1,5:1 effektiewe dehidrase van 2-hexanollewer. fn Reaksietyd van slegs 35 minute word benodig. fn Konseptueie ontwerp vir die suiwering van die 1-pentanol het fn produkkwaliteit van 99,9 % 1-pentanol en fn 1-pentanol opbrengs van > 98 % voorspel. Die 1-pentanol opbrengs is uitstekend, en slegs die 1-pentanol wat omgeskakel word na eters gaan verlore. In hierdie studie is dit bewys dat fn dehidrasie skeidingsproses suksevol aangewend kan word om sekondêre alkohole uit fn primêre+sekondêre alkohol mengsel te verwyder. Veral die verwydering van 2-hexanol uit fn 1- pentanol+2-hexanol mengsel het belowende resultate gelewer. Om die ekonomiese lewensvatbaarheid van so fn skeidingsproses te bepaal moet fn ekonomiese evaluasie van die proses gedoen word. Dit behoort deel van verdere studies te vorm. Die dehidrasie skeidingsproses behoort verder ondersoek te word. Dit word verwag dat die proses na hoër alkohol mengsels, bv. 1-hexanol+2-heptanol uitgebrei kan word. Dit sou baie voordelig wees indien fn geskikte soliede katalis vir die skeiding gevind word. In so fn geval sou die herwinning van die organiese produkte van die reaksiemengsel baie makliker wees. Indien fn soliede katalis nie gevind word nie, behoort fn kontinu proses waarin H3P04 as vloeistof katalis gebruik word, ontwikkel te word.

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