-Suitable for more concentrated Suspension
2. Cross-flow: The fluid runs parallel to the membrane
-Minimize cake formation
-Recycle feed stream
-Require more energy
Membrane Process is essentially a separation process based on molecular properties. The advanteges of membrane process include:
Ceretainly, the disadvantges exist, includng membrane fouling, low membrane life time, low selectivity, and high capital and operating cost.
Based on the driving forces, the following processes are defined as:
Pressure driven membrane processes
-Simple screening mechanism
Pore size 0.01 μm - 10 μm
DP » 0.01 to 0.5 MPa
-Low pressure process
-Most effectively remove particles and microorganisms (bacteria)
-Colloids/Macromole ---> theoretically pass through the membrane
-Screening and Adsorption
Pore size 1 - 100 nm
DP 0.1 to 1 MPa
-Membrane is classified in terms of Molecular Weight-Cut off (MWCO) : 1000 - 100,000
-Two layers: a thin (0.1 to 0.5 µm), skin layer and a porous substructure support layer
-Separation of macromolecules
-Only surface deposition: no internal pore plugging; so, relatively easy to remove, irreversible
- NF Removes molecules in the 0.001 micron range
DP 0.5 to 6 MPa
MWCO: 0.2 to 200
-NF is essentially a lower-pressure version of reverse osmosis
-NF performance characteristics between reverse osmosis and ultrafiltration
Electrical driven membrane processes
•Tubular membrane module
- Membrane is cast inside the support tube
- Tubular membranes have a diameter of 5 - 15 mm
- High SS tolerance
- Flow is usually inside out
-Mainly MF and UF
- Low packing density, high prices per module
• Hollow fibre membrane module
- Consists of a bundle of hundreds and thousands of hallow fiber
- Entire assembly is inserted into a pressure vessel
- Feed can be applied inside of the fiber (inside-out flow) outside (outside-in flow)
- Highest packing density of all.
- Hollow fiber is used mainly for NF and RO
•Spiral wound membrane module
- Flexible permeate spacer is provided between two flat sheet membranes
- Membrane: sealed three side and open side is attached to perforated pipe
- Flow is in a spiral pattern.
- Membrane envelop is spirally wound along with a feed spacer
- Filtrate is collected within the envelop and piped out
- Packing density:high
- RO and NF
• Plate and frame
Membrane Fouling: Deposition or accumulation of solids on the membrane.
Fouling causes resistance to flow through the membrane and eventual decline in overall flux.
Three major mechanisms of resistance flow:
-Gel/cake formation due to concentration polarization
But what about the cost?
The techniques can be divided into four major categories based on the physical mechanisms:
Cell entrapment is a simple and useful method to provide a simulation of high-density cells used in small-scale bioreactors where a cell recirculation method with e.g. filter and continuous centrifuge is rather difficult. But there is a need for using the cell entrapment method in industry due to additional cost of immobilization material.
Self-aggregation of cells leading to formation of a flock and easy sinking is very useful for cell recirculation, and thus performs cell retaining. Yeast has a natural flocculation capability which is usually employed in the brewing industry. The flocculation of yeast is a reversible, asexual and calcium-dependent process. Lectin-like proteins, so-called flocculins, which stick out of the cell walls of flocculation cells, selectively bind mannose residues present in the cell walls of adjacent yeast cells. Calcium ions in the medium are needed in order to activate the flocculins. Once the yeast forms flocks, the separation of cells from broth can easily be carried out by sedimentation; thus cell recirculation is performed without any force-driven separation, i.e. filtration and centrifugation.
A large number of aliphatic acids are present in dilute-acid hydrolyzates originated from wood extractives, lignin degradation and sugar degradation.
The undissociated acids are harmful to the cells and inhibit cell growth. They are liposoluble and thus can diffuse across the plasma membrane into cytosol and may dissociate intracellularly. In order to maintain intracellular pH, protons must be transported across the membrane by the action of plasma membrane ATPase. This results in an increase of ATP consumption, and thereby causes lower biomass yield. In anaerobic conditions, ATP generation is achieved by the ethanol production pathway, resulting in higher ethanol yield at the expense of biomass formation. However, above critical extracellular concentration of undissociated acid, the diffusion rate of undissociated acid can exceed the transport capacity of the plasma membrane ATPase, and intracellular acidification occurs. It is found that the limit of extracellular pH at different acetic acid concentrations which allow yeast to grow. It was found that growth was possible at a pH not less than of 4.7 in cultivation containing 10 g/L acetic acid. Therefore, acetic acid is innocuous if it exists in low concentration or the cultivation is carried out at a pH higher than the extracellular pH limit.
There are a number of phenolic compounds recognized in lignocellulosic hydrolyzates, including
These compounds are mainly liberated from lignin degradation in addition to aromatic wood extractives. The phenol aldehydes and phenol ketones were found as the worst inhibitors. Moreover, it was also shown the low molecular weight phenolic compounds are more toxic.
Phenolic compounds are considered to be important inhibitors due to their inhibitory effect in fermentation of lignocellulosic hydrolyzates. These compounds partition biological membranes and cause loss of integrity, hence disturb their ability to serve as selective barriers and enzyme matrices. The inhibition mechanism of phenolic compounds has not been elucidated yet.
Studies in inhibitory action of phenolic compounds have been carried out using higher concentrations than are actually present in the hydrolyzates. The water solubility of phenolic compounds is limited and depends on the composition of the liquid, which is different in hydrolyzates and the defined medium; therefore it is possible that the concentration at which the microorganism suffered has been lower. In addition, S. cerevisiae assimilates vanillin, hydroxybenzaldehyde, and syringaldehyde during fermentation, while growth has been reported on cathecol, recorcinol.
Furfural and 5-hydroxymethyl furfural (HMF) have been found as further hydrolysis products of pentoses and hexoses respectively. Pentoses form furfural in high yield; but if the furfural is not removed as formed, it partially condenses into high-molecular-weight materials. By an analogous process, hexoses yield HMF which, on continued heating, yields levulinic acid and formic acid. Furfural has been reported to be a strong inhibitor for S. cerevisiae. The furfural concentration above 1 g/L was found to decrease significantly the CO2 evolution rate, the cell multiplication and the total viable cell number in the early phase of fermentation. During anaerobic fermentation, reduction of furfural to furfuryl alcohol occurs with high yields, while furoic acid is produced from oxidation of furfural during aerobic cultivation. In both cases, NADH-dependent alcohol dehydrogenase (ADH) is believed to be responsible for furfural conversion in yeasts.
HMF is chemically related to furfural and thus has similar inhibitory effects as furfural, except that it has a lower conversion rate which might be due to lower membrane permeability. It is also discovered that an addition of 4 g/L of HMF decreased the CO2 evolution rate (32%), ethanol production rate (40%), and specific growth rate (70%). However, these inhibitory effects were less than those caused by the same amount of furfural, and thus HMF cannot be considered as acutely toxic as furfural for growth and fermentation of S. cerevisiae.
The conversion rate of furfural is much faster than the conversion rate of HMF. Furthermore, HMF is converted to 5-hydroxymethyl furfuryl alcohol with a similar mechanism as it was shown in the case of furfural conversion.
The glycerol obtained from transesterification is separated from biodiesel by gravity. The concentration of this type of glycerol is ~50% (wt),which is so-called raw glycerol and contains the contaminants such as alcohol, soap, base catalyst, salts, and untreated organic materials.
Crude glycerol can be obtained through the following purification of raw glycerol:
Crude glycerol purification: