Lactic acid is a versatile organic acid with many uses. It plays an important role in the food, pharmaceutical, leather and textile industries as a valuable industrial chemical. One of the most valuable and promising applications is using it to make biodegradable and biocompatible lactic polymers, such as polylactic acid (PLA).
Lactic acid is a naturally occurring compound produced through the fermentation of carbohydrates by lactic acid bacteria, primarily Lactobacillus and Streptococcus species.
The development of lactic acid production using biotechnological methods and optimization of the processes are of great importance under several environmentally friendly conditions of low production temperature, low energy requirements, and high purity.
The ultimate goal is a high-quality production of lactic acid with higher effectiveness and economy of the separation process, and optimize lactic acid separation processes for enhanced productivity and quality.
In this article, we will delve into the intricacies of separating and purifying lactic acid.
As a process equipment supplier, I am excited to share actionable insights on how to recover and purify lactic acid effectively.
Why Is Lactic Acid So Important In The Industrial Sector?
Lactic acid is a colorless, water-soluble liquid with a mild acidic taste. It can be produced by humans, animals, plants, and microorganisms.
It is the simplest hydroxy carboxylic acid with an asymmetric carbon atom.
Lactic acid is naturally present in two optical isomers, D- and L-form. L-lactic acid is the preferred isomer in the food and pharmaceutical industries due to the harmful effects of elevated levels of the D-type isomer.
L-lactic acid is used in the manufacture of biodegradable and biocompatible polylactic acid polymers because it can be absorbed by the human body. It is an environmentally friendly alternative. It is possible to manufacture biodegradable plastics as well, which can be used in the preparation of scaffolds for biocompatible artificial organs and for self-dissolving suture material.
Due to the unique properties of PLA, lactic acid is most likely to become necessary for the production of biodegradable plastics and to become a chemical in very high demand.
Efficient separation and purification processes are therefore critical to obtaining high-quality lactic acid. The separation process involves isolating lactic acid from the fermentation broth, which typically contains impurities such as microbial cells, sugars, and organic compounds.
Without effective separation, these contaminants can adversely affect the quality of the final product, potentially rendering it unsuitable for certain applications.
Purification steps are also required to remove any residual impurities and by-products that may be carried over from the separation process. These impurities can affect the colour, taste and odour of the final product and can also reduce its stability and shelf life.
By implementing effective separation and purification processes, businesses can ensure that their lactic acid product meets the desired purity requirements and quality standards for the intended application.
This can lead to increased customer satisfaction, improved product performance, and a competitive advantage in the market.
What Influences Lactic Acid Recovery And Purification?
- Feedstock Selection And Fermentation Conditions.
Biological fermentation from renewable sources can produce only L-lactic acid. The feedstock required for industrial lactic acid production must meet the requirements of low cost, low contaminant levels, rapid fermentation rates, high lactic acid yields, and little or no by-product formation.
Lignocellulosic resources, such as wood and crop residues, are the most attractive and cost-effective feedstocks for the production of L-lactic acid as they are renewable, widespread and cheap. Therefore, it is very suitable to use lignocellulose as raw material to ferment and produce L-lactic acid in our country.
In addition, corn stover is rich in cellulose, hemicellulose, and lignin, and the sugars derived from its decomposition are economically very attractive for lactic acid fermentation. Thus, the effective use of corn stover not only reduces the burden on the environment but also provides an abundant and inexpensive source of bio-energy.
- pH Adjustment And Acidification Techniques.
pH adjustment and acidification techniques are crucial in lactic acid recovery and purification. After fermentation, the pH of the broth is typically adjusted to a suitable range for lactic acid separation.
This step is achieved by adding an acid or base, depending on the specific process requirements and the desired lactic acid form (L-lactic acid, D-lactic acid, or DL-lactic acid). The optimal pH range for lactic acid extraction is typically between 2.5 and 4.5, depending on the specific separation method employed.
Acidification conditions can influence the recovery and purification of lactic acid. High levels of impurities such as salts and organic acids can reduce the efficiency of separation methods, resulting in lower yields and increased operating costs.
Therefore, the selection of appropriate acidification techniques and optimization of process parameters are crucial in ensuring efficient lactic acid recovery and purification.
- Selection Of Separation Methods Based On Solubility And Selectivity.
When selecting separation methods based on solubility and selectivity, it is important to consider the specific needs of the production process. Liquid-liquid extraction involves using an organic solvent to selectively extract lactic acid from the aqueous phase.
The solvent choice depends on factors such as solubility, selectivity, and ease of recovery. Membrane processes, including nanofiltration or pervaporation, can selectively separate lactic acid molecules and solvents based on size and molecular weight.
Crystallization techniques exploit the differences in solubility between lactic acid and impurities to isolate lactic acid crystals. The crystals are then separated from the liquid phase through filtration or centrifugation.
Each method has its own advantages and disadvantages, and the selection of the most appropriate method will depend on factors such as the composition of the mixture, the desired purity level, the type and concentration of impurities present, the scale of production, economic considerations, and the available equipment.
A combination of different separation techniques is often employed in multi-step purification processes to achieve high-purity lactic acid.
- Optimization Of Purification Techniques To Remove Impurities.
Conventional lactic acid recovery methods from fermentation broth are very complex because the fermentation medium contains many components and is difficult to separate in one way.
For the production of lactic acid, waste biomass can be reused for the production of L-lactic acid using the fermentation method, which reduces the cost of raw materials, but brings its downstream separation costs to 50%-70% of the overall cost.
Therefore, the optimization and advancement of the lactic acid separation process can achieve reduced energy consumption, reduced environmental pollution, and improved stability of lactic acid on the basis of ensuring product quality.
Separation Techniques for Lactic Acid Recovery
- Liquid-liquid extraction:
Due to the product inhibition present in the biorefinery process, the lower tank fermentation broth usually has a very low concentration, around 10% content.
The separation of lactic acid from fermentation broth is often accompanied by economic problems: as lactic acid is non-volatile, it cannot be separated directly by distillation, and the evaporation of large quantities of water is very energy intensive.
The liquid-liquid solvent extraction technique allows for the selective extraction of lactic acid directly from the fermentation broth by taking advantage of the difference in partition coefficients between water and extractant and provides a higher equilibrium partition coefficient for the extraction of carboxylic acids with organic bases or amine extractants than with conventional solvents.
The solute extraction technique is usually combined with the in-situ separation of lactic acid during fermentation, i.e. the extractant selectively separates the lactic acid produced in the fermentation broth at the same time as it is produced, both to somewhat alleviate product inhibition during lactic acid fermentation and to extract lactic acid directly from the fermentation broth.
- Membrane separation:
Membranes are a new type of material with a selective separation function. The process of purifying and concentrating the different components of a feed liquid by selective membrane separation is called membrane separation.
Unlike normal filtration, membrane separation is more precise, down to the molecular level, and has the great advantage that no additives are added to the membrane separation process.
The membranes are divided into microfiltration, ultrafiltration, nanofiltration and reverse osmosis membranes depending on their pore size.
Based on the membrane material they can be divided into inorganic and organic membranes.
The selection of the correct membrane material for the different components to be separated or concentrated enables the efficient purification or concentration of the substance.
Ultrafiltration is a common means of protein removal and concentration. For proteins produced in calcium L-lactate fermentation broth, the molecular weight of the proteins is first determined using polyacrylamide gel electrophoresis, and then the proteins in the fermentation broth are selected for membrane filtration using ultrafiltration membranes of different materials. The proteins are then filtered using ultrafiltration membranes of different materials.
For calcium lactate fermentations, crystallization is the earliest and most traditional method of separation.
The crystallization of lactic acid is a simple process that uses the difference in solubility of a solution at different temperatures for a particular solute.
The pH of the lower tank fermentation solution is adjusted to allow the bacteria to settle and then the supernatant is concentrated to saturation and then cooled and crystallized to allow the calcium lactate to precipitate and wash out, removing some of the impurities and then using sulphuric acid for the three purposes of acidification, removal of calcium salts and decolorization. The solution is then filtered and concentrated before being subjected to ion exchange to remove salt ions from the solution.
The current problem with this method is that the overall yield of lactic acid is not very high, with the most traditional crystallization process giving a yield of only 45-50%.
And after decolorization using activated carbon, it is difficult to regenerate the activated carbon, resulting in a large amount of solid waste being generated in the process, and the activated carbon adsorbs the pigment as well as the lactic acid product, resulting in a lower lactic acid yield.
Purification Methods for High-Quality Lactic Acid
- Distillation:
Short Path Molecular distillation is a non-equilibrium, continuous refining process in which the separation operation is carried out under a very high vacuum.
Using the difference in the free range of the gas after vaporization of the mixed solution, it is particularly suitable for substances with low volatility, high molecular weight or high boiling point, which are difficult to separate by conventional distillation and are subject to thermal deterioration.
Molecular distillation is an efficient separation method with a high potential to recover and concentrate thermally unstable molecules such as lactic acid.
It is considered to be one of the most promising methods for lactic acid separation due to its low evaporation temperature and short residence time which minimises the problem of thermal decomposition.
Resin adsorption technology can be used for lactic acid purification applications, mainly in the adsorption of salt ions from lactic acid or direct adsorption of lactic acid.
The main types of resins used for lactic acid separation are anion-exchange resins and cation-exchange resins.
Among them, cation exchange resins are usually used for the selective adsorption of metal ions in solution, while anion exchange resins are usually used for the removal of other anions from solution or for the specific adsorption of lactate ions, and both resins are usually used in combination in the process to achieve the purification and separation of lactic acid from fermentation broth.
- Hydrolysis By Esterification:
Esterification hydrolysis is the reaction of lactic acid with other alcohols in the presence of a catalyst to obtain a lipid, taking advantage of the ease of esterification of organic acids and alcohols. The selection of alcohols is based on the principle that the resulting lipids have a lower boiling point and can be easily separated, after which the lactic acid is hydrolyzed and distilled to obtain a high purity lactic acid.
It is common to combine esterification hydrolysis with other techniques to enhance the mass transfer process and remove reaction products in order to promote a positive reaction and improve the overall process yield.
Following the reaction of lactic acid with excess methanol in a reactor, the obtained mixture containing methyl lactate, water and methanol is subjected to reactive distillation and the resulting methanol is refluxed for further use in the esterification of lactic acid, while producing lactic acid.
This method can be carried out at relatively low temperatures and the stability of the resulting material is superior, the problem is the low yield of lactic acid from the process.
A Comprehensive Analysis Of The Above Methods Is Summarized In The Following Table:
| Separation and Purification Method | Yield | Purity | Pro. | Con. |
| Crystallization | 70% | 50-70% | Can be coupled with fermentation technology, proteins can be removed and the crystalline mother liquor can be reused. | There is still a large amount of uncrystallized calcium lactate in the crystallized stock solution, which requires the addition of additives to improve the crystallization yield. |
| Liquid-liquid extraction | 61.73% | 91.3% | Lactic acid can be extracted directly from the fermentation broth, the equipment is simple and easy to operate, and the purity of the lactic acid obtained is high. | Low lactic acid yields make the selection of a suitable extractant difficult. |
| Ion Exchange Chromatography | 98.9% | 99.1% | High yields and high purity of lactic acid can be guaranteed at the same time. | Large amount of waste water generated during resin treatment. |
| Hydrolysis by esterification | 77% | 90% | High purity lactic acid can be obtained. | Usually limited by the reaction equilibrium, the lactic acid yield is low. Has a phase change process and long reaction times at higher temperatures. |
| Membrane separation | 92% | 95% | No temperature rise, no phase change, low energy consumption, high selectivity, high operational flexibility. | Improperly controlled conditions can lead to membrane contamination and reduced productivity. |
| Molecular distillation | 74.63% | 91.3% | High purity lactic acid product, continuous production, suitable for high capacity industrial production. | Lower yields and more demanding operating conditions. |
For the existing lactic acid separation process, the crystallization method does not give satisfactory yields in the crystallization process alone, and the addition of additives to the crystallization target liquor to increase the crystallization yield is not economical and environmentally friendly, however, the removal of proteins in the crystallization process is an advantage that can be exploited, and the remaining fermentation residue in the crystallization target liquor can also be used.
While solvent extraction can achieve a high purity of lactic acid after extraction and reverse extraction, the selection of an extractant with a high partition coefficient, low toxicity and low volatility is currently a challenge.
The resin adsorption is satisfactory purely in terms of lactic acid yield and purity, however the resin generates a large amount of wastewater before and after use, which is environmentally unfriendly and the resin has a very limited capacity.
For the esterification hydrolysis method, the yield of the esterification hydrolysis process is low due to the limitations of the chemical reaction equilibrium, and the process requires heating and condensation with high energy consumption.
The molecular distillation method itself requires harsh distillation conditions, a high vacuum level, and a high separation effect that can be achieved through strict control of the process conditions. It requires higher operating costs and rigorous and experienced design and construction of complete sets of equipment. Our project experience is a good source of knowledge and paves the way to success in the refining steps and improve the concentration of lactic acid product.
Nevertheless, the membrane process applied to the separation of L-lactate fermentation broth has the advantages of no phase change, no additives, low energy consumption and high operational flexibility.
Conclusion:
By understanding the crucial factors influencing lactic acid recovery, selecting appropriate separation and purification methods, and optimizing process parameters will contribute to higher yields, improved product quality, and increased efficiency. By unlocking the full potential of lactic acid separation and purification, businesses can benefit from enhanced productivity and meet the stringent demands of various industries.
Implementing these effective techniques and utilizing suitable equipment will not only streamline lactic acid production but also open doors to new opportunities and applications. Embrace the knowledge shared in this guide, and witness the transformative impact it can have on your lactic acid separation processes.
So hopefully this content benefit for your bussiness consideration and project equipment selection.
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