Evaluation of tubular ceramic ultrafiltration membranes for the recovery of vanillin from Kraft black liquor
Advancing the chemical engineering fundamentals
Filtration - II (T2-11b)
Keywords: vanillin; tubular ceramic membrane; ultrafiltration; Kraft black liquor
The treatability of Kraft pulp liquors using ultrafiltration process has been reported in some recent publications. The Kraft pulp liquor coming from paper industry is a waste which must be treated before disposal. On the other hand, organic compounds present in liquors, especially lignin, are very useful raw material from which many valuable substances or materials can be produced, such as: vanillin, vanillic acid, dispersing agents, synthetic tannins, etc. The main purpose of lignin removal from the liquor is to separate the cooking chemicals, NaOH and Na2S to avoid losses of these chemicals from the mill. The processes which can be used for lignin removal are precipitation and ultrafiltration. Fractionation of the cooking liquor from Kraft and sulphite pulps by ultrafiltration has been studied for thirty years. In most of the applications the objective of ultrafiltration process was to purify the lignin fraction in order to use it as a fuel or valuable product.
In the process of Kraft lignin oxidation to produce vanillin the reaction media contains degraded molecules of lignin and sodium salt of vanillin (vanillate) and other species. The isolation of vanillate from the oxidized solution is an important stage in the vanillin production. During the ultrafiltration process vanillin goes to the permeate stream because of its low molecular weight. The lignin as a macromolecule stays in the retentante and its concentration significantly increased during the batch ultrafiltration process. Therefore, the ultrafiltration technique can be used as a first step in the process of recovery of vanillin from Kraft lignin oxidation.
The objective of this work is to evaluate the membrane ultrafiltration process focused on the recovery of vanillin from Kraft lignin oxidation media. For this purpose, ultrafiltration using tubular ceramic membranes with molecular weight cut-off of 1 kDa, 5 kDa, 15 kDa and 50 kDa are investigated. The influence of the lignin and vanillin concentration and pH of the mixture on the ultrafiltration process is studied as well.
The experiments are carried out in the batch concentration mode, in which retentante is recirculated to the feed tank and permeate is withdrawn to a separated reservoir. This work assumes that the formation of a gel layer takes place on the membrane surface and thus influences the separation process because it is responsible for significant flux decline. A mathematical model is used to describe experimental permeate fluxes for the different conditions assuming that the variation of resistance to permeate flow is caused by the increase in the gel layer thickness.
The highest value of lignin rejection was obtained with the membrane 1 kDa cut-off. Membranes with small cut-off show significant decline of permeate flux comparing with the membranes with larger cut-off. Increasing pH of the solution leads to a decrease of lignin rejection using membranes with 15 kDa cut-off while using membrane 1 kDa leads to opposite effect. This behaviour seems to be related to the different mechanisms of macromolecules association dependent on molecular weight and pH. The phenomena of lignin molecule association and aggregation to different degrees in various solvents lead to different molecular weight distributions. The association of the molecules at high pH lead to particle size increase mostly due to polyelectrolyte swelling and dissociation of ionisable functional groups while the pH decrease the association is due to formation of hydrogen bonds.
Presented Tuesday 18, 09:45 to 10:05, in session Filtration - II (T2-11b).
