Process optimization for partial oxidation of bacterial sludge in a sonochemical reactor

Beyers, Analene (2014-04)

Thesis (MEng)--Stellenbosch University, 2014.


ENGLISH ABSTRACT: It was found that bacterial sludge from anaerobic water treatment systems is produced internationally at a rate of 60 grams per person per day and the accumulation of the potentially hazardous by-product has become of increasing concern. The produced bacterial sludge is frequently pumped into dams, dried out and used as agricultural fertilizer. This bacterial sludge is expected to have a relatively high heating value and as such, has the potential to produce energy from the biomass. It is, therefore, advisable to utilize this energy potential as an alternative to conventional sludge disposal. This project aimed to improve the yield of syngas by optimizing the reactor design to partially oxidize bacterial sludge using a sonochemical reactor that is operated at bulk atmospheric conditions. The effect of different conditions was investigated and the optimum settings for syngas production were found by investigating temperature, pressure and the effect of the amplitude of operation that regulates the energy input by the ultrasonic equipment. The optimum conditions were used to investigate the kinetics involved in this process as well as to determine the energy consumption by the process. It was also required to study the feasibility of partially oxidizing bacterial sludge using a sonochemical reactor instead of conventional steam gasification and also as an alternative means of sludge disposal. By eliminating this pollutant source, the future environmental threat posed by an increasing population size will be minimized and energy will be utilized from a thus-far wasted energy source. The syngas that is produced is used as a green alternative to fossil fuels in the Gas-to-Liquids (GTL) process to produce liquids fuels. A thus-far wasted energy source will be consumed and fossil fuels can be saved in the process. It was found that the maximum hydrogen mole percentage produced is 0.141 mole % of the vapour phase with the maximum carbon monoxide mole percentage in the vapour phase at 1.896 mole %. This shows an improvement on work conducted by Beyers (2011) of 59 % for hydrogen, 92% for carbon monoxide and a reduction of 49 % for carbon dioxide. A kinetic study of the process indicated that the rate equations that describe the hydrogen and carbon monoxide production are zero order and, therefore, independent of initial concentration of the sludge. The rate constants were 0.0146 (mol % hydrogen/s) and 0.0183 (mol % hydrogen/s) for hydrogen and carbon monoxide, respectively. It was found that the most severe change to the higher heating value of the feed was a mere 0.27 mJ/kg from an original value of 9.81 mJ/kg. This therefore confirms that the reaction has not proceeded to completion. The statistical model predicted a maximum value for hydrogen production at 0.151 mole % in the product gas, 0.01 mole % from the measured maximum. It was also found that hydrogen is produced during the sonolysis of distilled water and that this confirms that the hydrogen production during partial oxidation of the sludge sample comes mainly from the water present in the sludge. The hydrogen produced when only using water, was found to be 0.127 mole % and when using the active sludge, the value was 0.116 mole % hydrogen in the vapour phase. The thermal decomposition of calcium carbonate in the lime that is used to treat the pH of the unit where the sludge originates from, followed by the formation of carbon monoxide during the Boudouard reaction, led to an increased amount of carbon monoxide present in the product gas. Ultrasonic intensity is defined as the amount of energy that is transferred to the sample per cubic meter of the internal surface area of the reactor vessel. It was found that the intensity that was delivered to the reactant was lower than expected as the reactor was operating at an efficiency of only 36%. The design intensity was 1.44 W/m2 and the actual delivered intensity was 0.52 W/m2. Based on a maximum yield of 0.00012 Nm3/kg, the cost of syngas production under the conditions described by this study, would amount to R 19.98/Nm3. This cost only implicates the operational expenses and does not take further downstream processing and initial capital investment repayments into account. Conventional steam gasification at a yield of 0.67 Nm3/kg has an operational syngas production cost of R 1.48/Nm3. This process was therefore found to not be economically feasible as the cost of utilizing ultrasound as opposed to normal steam gasification is more than ten times more expensive. It was concluded that the process was successfully optimized by the redesigning of the reactor and that carbon dioxide production was limited by excluding oxygen from the feed gas. It was also concluded that the sonolysis of water and the thermal decomposition of calcium carbonate, followed by the conversion of carbon dioxide to carbon monoxide, supplements the syngas production under the current operational conditions. Based on the production of no methane during the course of this study, the sonochemical process can be tied into the GTL process after the steam reforming unit. Due to the relatively high carbon dioxide content, the process will need to join the main feed gas stream that is fed into the carbon dioxide removal unit before it enters the GTL process to correct the desired feed gas ratio. Based on the very low syngas yields, the low hydrogen to carbon monoxide ratio in comparison to the required ratio of 2 as well as the high energy intensity required for this process, it can be concluded that the partial oxidation of biomass sludge in a sonochemical reactor is not feasible as an alternative technology to conventional steam gasification. The operating costs of the sonochemical unit would be nearly ten times that of steam gasification and is therefore concluded to not be a competitive technology to conventional steam gasification. It is recommended that the reactor design is reinvestigated to improve the delivered ultrasound intensity as well as the surface area where the ultrasonic waves are intensified. This would eliminate dead-zones. It was also recommended that the argon gas is continuously bubbled through the reactant mixture during experiments to eliminate the degassing effect caused when the ultrasound is initially emitted. The gas outlet of the process can then be connected to an online gas chromatograph (GC) with a thermal conductivity detector (TCD) and flame ionization detector (FID) methanizer in series as the TCD does not destroy the sample and this setup would improve the analytical process. The production of carbon monoxide from lime as well as the production of hydrogen from water during sonolysis needs to be investigated. The effect of radicals can also be studied by the addition of a radical scavenger to the process. It is recommended that the experimental design is reinvestigated and a design that will deliver similar information utilizing fewer data points should be chosen. Based on this model as well as further kinetic testing, it is recommended that a complete ASPEN model is developed to simulate the energy requirements to tie the ultrasonic process into the commercial plant. Based on this model, a complete feasibility study can then be conducted to determine the capital costs involved, the operating costs, the repayment period as well as taking the current costs of sludge disposal into account.

AFRIKAANSE OPSOMMING: Daar is gevind dat bakteriele slik internasionaal geproduseer word deur anaerobiese waterbehandelingseenhede teen ‘n tempo van 60 gram per person per dag en dat die opberging van hierdie gevaarlike byproduk ‘n groeiende probleem word. Die geproduseerde bakteriele slik word in damme gestoor, uitgedroog of gebruik as kunsmis in die landbou bedryf. Daar word vermoed dat hierdie baketriele slik oor ‘n hoe verwarmings waarde beskik en het daarom die potensiaal om energie te produseer uit die biomassa. Daarom is dit voorgestel om alternatiewe prosesse te ondersoek om van hierdie slik ontslae te raak en moontlik die energie wat beskikbaar is te gebruik. Die projek is daarop gefokus om die produksie van syngas te verbeter deur die reaktorontwerp te optimeer deur gebruik te maak van parsiele oksidasie van slik onder atmosferiese kondisies deur klankgolwe te gebruik. Die effek van verskillende operasionele kondisies is ondersoek en die optimale vlakke van syngas produksie is gevind deur temperatuur, druk en amplitude wat die hoeveelheid energie wat oorgedra word aan die reaktor reguleer, te ondersoek. Die optimale kondisies is ook gebruik om die kinetiese aspekte van die proses te ondersoek en ook om te kyk wat die sisteem se energie benodighede behels. Die haalbaarheid om baketriele slik parsieel te oksideer in ‘n sonochemiese reaktor is vergelyk met dit van konvensionele stoom vergassing van die biomassa en is ook ondersoek as ‘n alternatief om van die slik ontslae te raak. Deur die slik te verwyder as ‘n potensiele bron van besoedeling, kan die toekomstige omgewing’s risiko wat deur die toename in die bevolkkingsgroote tot gevolg is, verwyder word deur ‘n energie bron te gebruik wat tot dusver geignoreer is. Die syngas wat geproduseer word kan dan gebruik word in die “Gas-to-Liquids” (GTL) process om vloeistof brandstowwe te produseer. Dus sal ‘n omgewingsrisiko verminder word, ‘n energiebron word benuttig wat nooit van tevore benuttig is nie en fosiel brandstowwe kan gespaar word. Die maksimum waterstof wat geproduseer is, was 0.141 mol % in die gas fase met ‘n maksimum waarde vir koosltof monoksied van 1.896 mol % in die gas fase. Dit toon ‘n verbetering van 59 % vir waterstof, 92 % vir koolstof monoksied en ‘n vermindering van 49% in die koolstof dioksied wat deur Beyers (2011) geproduseer is. Die kinetiese studie het ondervind dat die “rate equation” van waterstof en koolstofmonoksied beskryf word deur nul-orde kinetika. Hierdie konstantes was 0.0146 (mol % waterstof/s) en 0.0183 (mol % waterstof/s) vir waterstof en koolstofmonoksied. Daar is ook gevind dat die grootste moontlik verandering in die hoe verwarmings waarde van die biomassa is ‘n skamele 0.27 mJ/kg van die oorspronklike waarde van 9.81 mJ/kg. Hierdie waarneming staaf dus die uitkoms dat die reaksie dus nie tot die einde verloop het nie. Die statistiese model het ‘n maksimum van 0.151 mol % voorspel wat 0.01 mol % meer was as die waarde wat gemeet is. Dit is ook gevind dat waterstof geproduseer word deur die sonoliese van water en dat hierdie bykomende waterstof deel uitmaak van die produkgas aangesien die slik grootliks uit water bestaan.Die hoveelheid waterstof in die gas fase wat geproduseer is tydens sonoliese van ‘n suiwer water monster, was 0.127 mol %. Die hoeveelheid waterstof in die gas fase wanneer die slik behandel is ten optimal kondisies, was 0.116 mol % gemiddeld. Die hitte degradering van kalsium karbonaat wat teenwoordig is in die kalk wat gebruik word om die pH van die produksie eenheid te reguleer, gevolg deur die Boudouard reaksie, het tot gevolg dat addisionele koolstof monoksied ook gevorm word. Ultrasoniese intensiteit kan gedefineer word as die hoeveelheid energy wat oorgedra word aan ‘n reaktant gebasseer op die oppervlak area aan die binnekant van die reaktor. Die intensiteit waarteen die voermateriaal blootgestel word aan die klankgolwe was laer as verwag met ‘n 36 % effektiwiteit. Die ontwerp spesifiseer ‘n intensiteit van 1.44 W/m2 en die intensiteit wat fisies gelewer is, was 0.521 W/m2. Die maksimum produksie van syngas was 0.00012 Nm3/kg, wat lei tot ‘n operasionele koste van R 19.98/Nm3 onder die kondisies van hierdie studie. Hierdie koste neem nie die oorsponkilke kapitaal vir die konstruksie, of die koste van verdere behandelik van die gas, in ag nie. Konvensionele stoom vergassing teen ‘n opbrengs van 0.67 Nm3/kg het ‘n operasionele koste van R 1.48/Nm3 tot gevolg. Die proses is dus ekonomies nie ‘n aantreklike opsie nie aangesien die kostes van syngas produksie met ultraklank meer as tien keer meer is as konvensionele stoom vergassing. Daar is tot die gevolgtrekking gekom dat die reaktor optimering suksesvol was en deur geen stuurstof te voer nie, die koolstofdioksied persentasie verminder is. Daar is ook tot die gevolgtrekking gekom dat die sonoliese van water, en die hitte degradering van kalsium karbonaat, gevolg deur die Boudouard reaksie, die syngas produksie supplementeer. Aangesien geen metaan gedurende hierdie studie geproduseer is nie, kan die sonochemiese proses inskakel by die GTL aanleg na die stoom hervormingseenhed. As gevolg van die hoe koolstofdioksied konsentrasie, sal die prosesstroom gemeng moet word met die produk stroom uit die stoom hervormings proses, wat gevoer word na die koolstofdioksied verwyderings eenheid. Hierdie eenheid is daarvoor verantwoordelik om die korrekte verhouding van gasse vir die GTL voer stroom te reguleer. Gebasseer op die baie lae syngas opbrengs, die lae waterstof tot koolstofmonoksied verhouding en die hoe energie behoeftes, is daar tot die gevolgtrekking gekom dat die parsiele oksidasdie van die biomassa in ‘n sonochemiese reaktor nie ‘n haalbare alternatief is vir konvensionele stoom vergassing nie. Die operasionele koste van die sonochemiese eenheid is ongeveer tien keer meer as die van stoom vergassing en daarom is die proses nie kompeterend nie. Daar word voorgestel dat die reaktor ontwerp hersien word om die gelewerde intensiteit te verbeter, sowel as om die kontak area waar die klankgolwe gekonsentreer is, te vergroot. Dit sal dooie sones uitskakel. Daar word ook voorgestel dat argon gas gedurende die eksperiment aanhoudende geborrel word deur die reaktant vloeistof in die reaktor om die ontgassingseffek uit te skakel sodra die klankgolwe aangeskakel word. Die gas uitlaat kan dan inlyn gekoppel word aan ‘n gas chromatograaf met ‘n termiese geleidings detektor (TCD) en ‘n vlam ionisasie detektor (FID) met metaan omskakeling, aangesien die TCD nie die monster vernietig nie. Hierdie opstelling behoort analitiese methodes te verbeter. Die produksie van koolstofmonoksied uit kalk sowel as die produksie van waterstof uit water gedurende sonoliese, moet verder ondersoek word. Die effek van radikale kan ook verder bestudeer word deur die gebruik van ‘n radikaal rower gedurende die proses. Daar word ook voorgestel dat die statistiese ontwerp herondersoek word sodat minder eksperimente gebruik kan word om soortgelyke resultate te bekom met minder data punte. Gebasseer op hierdie nuwe model en ‘n kinetiese studie, word dit aangeraai dat ‘n volledige ASPEN model gebou word om te simuleer hoe hierdie sonochemiese eenheid sal inskakel met die kommersiele eenheid. Dit sal dan moontlik wees om die energie benodighede van die proses te verstaan en gebasseer daarop, kan ‘n volledige haalbaarheid studie gedoen word wat kyk na oorspronklike installasie kostes, onderhouskostes, operasionele kostes sowel as die terugbetaling van die konstruksie kostes. Dan kan ‘n vergelyking getref word met die huidige kostes om van hierdie slik ontslae te raak en om die slik as ‘n brandstof te benut.

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