A new approach: single step bioprocessing

Thar Process may bring a change on bioethanol recovery

Currently, bioethanol producers are facing another challenging time in its history for viability due to the bad economic situation, volatile commodity markets, and high feedstock cost (if use corns). Some corn-based ethanol producers have been forced to file for bankruptcy before any cellulosic ethanol commercialization takes off.

The key to change the fate of corn-cellulosic ethanol production is still upon the product cost. Cheap feedstock is the first priority. However, any new ideas or technologies that can drop current production cost will be expected in this industry.

Recently, Thar Process receives grant from the state of Pennsylvania for distillation replacement technology, i.e.use a high-pressure propane extraction to remove the high volumes of water from fermented broth and recover bioethanol to replace an existing ethanol plant’s conventional distillation. The propane used in the extraction process can be recycled; a significant energy savings is expected to save from bypassing the conventional distillation and molecular sieve drying steps.

We are looking forward to seeing the progress and economic evaluation of the process.

Ethanol removal from reaction–separation integration

The reaction–separation integration is an attractive alternative for the intensification of ethanol fermentation processes. When ethanol is removed in-situ, the product inhibition will be reduced.The methods to remove ethanol from fermented broth are listed as follows:

· Vacuum extraction, which can be conducted by coupling of fermentor vessel with a vacuum chamber extracting the more volatile ethanol from fermentation broth which allows the partial product removal and the increase of overall process productivity.
· Gas stripping to increase the concentration of sugars in the stream feeding the fermentor and improvement of improves liquid circulation and mass transfer.
· Membrane separation. For example, ceramic membranes can be used to filter cell biomass and remove ethanol during the fermentation.The removed ethanol is then distilled and the resulted bottoms are recycled to the culture broth resulting in a drastic reduction of generated wastewater. The coupling of fermentation with the pervaporation is another case to remove produced ethanol and reduce the natural inhibition of the cell growth caused by high concentrations of ethanol product.

In membrane distillation, aqueous solution is heated for vapor formation, which go through a hydrophobic porous membrane favoring the pass of vapors of ethanol (that is more volatile) related to the vapors of water. The process driving force is the gradient of partial pressures mainly caused by the difference of temperatures across the membrane.
Liquid extraction is to use an extractive biocompatible agent (solvent) that favors the migration of ethanol to solvent phase, a process known as extractive fermentation.

Hopefully,we will have lab or trial date rather than just modeling for the evaluation, right?

Membrane operational modes

Based on operational models, membrane can be devided into two type:

1. Dead-end: The fluid flows at right angle to the membrane

-Deposited particles form “Cake Layer”
-Suitable for more concentrated Suspension

-Need more frequent cleaning

2. Cross-flow: The fluid runs parallel to the membrane

-High shear force near the membrane surface
-Minimize cake formation
-Recycle feed stream
-Require more energy
-Stable flux

Membrane separation for biorefinery

1. Detoxification

Nanofiltration (NF) and Reverse Osmosis (RO) membranes can be use to concentrate sugar and remove toxic compounds such as acetic acids, and sugar degraded products in the hydrolyzate.

2. Downstream processing and purification
Microfiltration (MF) can be used to remove particls (proteins etc) from fermented broth.

3. Desalination
ElectroDialysis (ED) can be used to remove inorganic salts

Membrane Process

The number of downstreaming steps for biobased chemical production strongly influences the quality and the price of the product. The total production costs are determined mainly by the downstreaming rather than by production of the product using fermentation.

After fermentation, a pretreatment of the fermentation broth is required to separate biomass, proteins, and cells. The multivalent salts also need to be removed for purification. he separation of inorganic salts and proteins/cells presents a special problem with the production of valuable substances from renewable raw materials. Therefore, the methods used for downstream processing will play a very important role.

Membrane Process is essentially a separation process based on molecular properties. The advanteges of membrane process include:

• It reduces the number of unit processes in treatment systems


• Potential for process automation and plant compactness


• Much smaller foot print than the conventional plants of the same capacity


• Easy scale-up, expansion and retrofication


• Less or no chemical use and provides highest quality water


• No formation of secondary chemical by-products


• Less sludge production


• Water reuse and recycling

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
Microfiltration (MF)

-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)
-High flux
-Colloids/Macromole ---> theoretically pass through the membrane

Ultrafiltration (UF)

-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

Nanofiltration (NF)

- 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

Reverse Osmosis (RO)
-Membrane: similar to UF, thin active layer; porous support layer

-Operating Pressure: 1.0 - 10 MPa

-RO has the separation range of 0.0001 to 0.001mm

Electrical driven membrane processes
-Electrodialysis (ED)

Concentration driven membrane processes
-Dialysis
-Osmosis

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Based on Modules, membrane module refers to the device which houses the membrane element:

• 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

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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:
-Pore narrowing
-Pore plugging
-Gel/cake formation due to concentration polarization

Separation processes

Separation processes based on the molecular properties are summarixed as follows:

Continuous cultivation process

Continuous cultivation process has been developed to answer the need of rapid ethanol production for fuels and chemical in the near future.

Batch cultivation is rarely applied for dilute-acid hydrolyzate cultivation since the cells is exposed directly to the high concentration level of inhibitors. This problem is commonly overcome by applying fed-batch cultivation where the concentrations of inhibitors in the media are controlled by regulation of the feeding. The continuous mode of cultivation has generally been of interest to minimize the total time of fermentation and increase the volumetric production rate of ethanol as well as decrease the investment cost. Because the bioreactor is fed continuously, it would be possible to avoid sudden inhibition to the yeast. Therefore, continuous cultivation mode is a suitable alternative for large-scale ethanol production.

Pervaporation–Membrane Process for Bioethanol Recovery

The traditional separation process, fractional distillation accounts for abou 40% of the cost for corn to ethanol production. A relatively new technology called "pervaporation" may provide considerable energy savings over traditional distillation technologies. Pervaporation is a membrane-based process used to separate and concentrate volatile compounds from a liquid mixture by selective permeation through a non-porous membrane into a vacuum permeate stream. The remaining liquid thus becomes depleted in the compound that permeates the membrane.

A US patent "Pervaporation process of separating a liquid mixture" disclosed an invention withe the abstract: " A pervaporation process for separating at least one component from a mixture of liquids, for example for separating ethanol from a fermentation mass, by a series of three separation steps: separating the mixture by a first pervaporation to form a first permeate vapor enriched in the component to be separated; fractionating the first permeate vapor, for example by temperature condensation, to form a high concentration fraction twice enriched in the component to be separated; and either distilling the high concentration fraction or a second pervaporation to form a distillate or retentate liquid thrice enriched in the component to be separated".