Technological advances in the major process components — fermentation, primary purification, and secondary purification and polymerization/chemical conversion of lactic acid and its derivatives — have recently occurred. These and other advances would enable low-cost, large-volume, and environmentally sound production of lactic acid and its derivative products.

In fermentation, high (> 90%) yield from carbohydrate, such as starch, is feasible, together with high product concentration (90 g/L, 1 M). Stable strains with good productivity (> 2 g/L h) that utilize low-cost nutrients (such as com steep liquor) are available. Furthermore, the fermentation is anaerobic and thus has low power and cooling needs. All of these make the fermentation step very facile and inexpensive.

Recent advances in membrane-based separation and purification technologies, particularly in micro — and ultrafiltration and electrodialysis, have led to the inception of new processes for lactic acid production. These processes would, when developed and commercialized, lead to low-cost production of lactic acid, with a reduction of nutrient needs and without creating the problem of by-product gypsum (18-21). Desalting electrodialysis has been shown to need low amounts of energy to recover, purify, and concentrate lactate salts from crude fermentation broths (19). The recent advances in water-splitting electrodialysis membranes enable the efficient production of protons and hydroxyl ions from water and can thus produce acid and base from a salt solution (20-21). These advances have led to the development of proprietary technologies for lactic acid production from carbohydrates without producing salt or gypsum by-products (19-20). In recently issued patents to Datta and Glassner (19-20), an efficient and potentially economical process for lactic acid production and purification is described. By using an osmotolerant strain of lactic acid bacteria and a configuration of desalting electrodialysis, water-splitting electrodialysis, and ion-exchange purification steps, a concentrated lactic acid product containing less than 0.1% of proteinaceous impurities could be produced from a carbohydrate fermentation. The electric power requirement for the electrodialysis steps was approximately 0.5 kWh/lb (~1 kWh/kg) lactic acid. The process produces no by-product gypsum, only a small amount of by-product salt from the ion-exchange regeneration. Such a process can be operated in a continuous manner, can be scaled up for large-volume production, and forms the basis for commercial developments for several companies that have announced intention to be commercial producers of lactic acid and its derivative products (3-4).

Another recent entrant, Ecochem, a DuPont-ConAgra partnership, had developed a recovery and purification process that produces a by-product ammonium salt instead of insoluble gypsum cake and attempted to commercialize

the process and sell this by product as a low-cost fertilizer. A 10 — ton/yr demonstration-scale plant was recently completed to prove the process and develop products and markets for polymers and derivatives. However, due to poor choice of feedstock (whey) and purification process technology, this attempt failed. The polymer patent portfolio of DuPont was acquired by Chronopol, Inc. which is building a demonstration-scale plant for lactic acid polymer production.

The utilization of the purified lactic acid to produce polymers and other chemical intermediates requires the development of secondary purification and integration of catalytic chemical conversion process steps with the lactic acid production processes. Examples of such process steps would be dilactide production for polymerization to make high-molecular-weight polymers or copolymers and hydrogenolysis to make propylene glycol — a large-volume intermediate chemical. In the past, very little effort was devoted to develop efficient and potentially economical processes for such integrations, because only small-volume, high-margin specialty polymers for biomedical applications or specialty chemicals were the target products.

Recently, several advances in catalysts and process improvements have occurred and proprietary technologies have been developed that may enable the commercialization of integrated processes for large-scale production in the future. In a recent patent issued to Gruber et al. of Cargill, Inc. (22), the development of a continuous process for manufacture of lactide polymers with controlled optical purity from purified lactic acid is described. The process uses a configuration of multistage evaporation followed by polymerization to a low-molecular-weight prepolymer, which is then catalytically converted to dilactide, and the purified dilactide is recovered in a distillation system with partial condensation and recycle. The dilactide can be used to make high-molecular-weight polymers and copolymers. The process has been able to use fermentation-derived lactic acid, and the claimed ability to recycle and reuse the acid and prepolymers could make such a process very efficient and economical (22). In recent patents issued to Bhatia et al. of DuPont, Inc. (23-30), processes to make cyclic esters, dilactide, and glycolide from their corresponding acid or prepolymer are described. This process uses an inert gas, such as nitrogen, to sweep away the cyclic esters from the reaction mass and then recovers and purifies the volatilized cyclic ester by scrubbing with an appropriate organic liquid and finally separates the cyclic ester from the liquid by precipitation or crystallization and filtration of the solids. Very high purity lactide with minimal losses due to racemization have been claimed to be produced by this process. Recycle and reuse of the lactic moiety in the various process streams have been claimed to be feasible (30). DuPont’s patents have been acquired by Chronopol and both Cargill and Chronopol are developing their processes to commercial scale; their goal is large-scale production of biodegradable polymers in the future.

Hydrogenolysis reaction technology to produce alcohol from organic acids or esters has also advanced recently — new catalysts and processes yield high selectivity and rates and operate at moderate pressures (31-33). This technology has been commercialized to produce 1,4 butanediol, tetrahydrofuran, and other four-carbon chemical intermediates from maleic anhydride. In the future, such technologies could be integrated with low-cost processes for the production of lactic acid to make propylene glycol and other intermediate chemicals (34).

Technical Accomplishments and Future Directions at Argonne

In the past two years, under a U. S. Department of Energy-sponsored project at ANL, several important technical advances have been achieved and demonstrated at the laboratory scale. Notably, these advances have occurred in fermentation, primary purification, and polymer synthesis. In fermentation, high product yield (95%) from starch by means of an enzymatic saccharification/fermentation process with high lactate concentration (100 g/L) and good productivity (3 g/L*h) have been achieved. The ED-based primary purification process has been operated in the laboratory in short-term feasibility experiments to obtain flux and power data for design and economics. A proprietary method to produce a high-molecular-weight copolymer of polylactic acid with other copolymers has been developed at the laboratory scale. Methods to modify and test the degradability of polylactic acid have been developed. Furthermore, the development of secondary purification processes and specialty products derived from lactic acid with targeted properties have been initiated.

The ANL program of oxychemicals and polymer feedstock production from carbohydrate-derived lactic acid is schematically shown in Figure 1. The fermentation and primary purification process to make purified lactic acid has been developed and demonstrated at ANL and elsewhere. The program is now focusing on developing efficient and economical secondary purification processes to make esters that can serve as the key intermediate for the production of a host of other chemicals, polymers, and specialty derivatives. The products and the processes to be developed or integrated are shown in Figure 1. This matches several of the target products listed in Table I that can be derived from lactic acid.

Thus, a rational program targeted at development of economical processes for key intermediates of lactic acid and its derivative products has been the primary focus at Argonne.

Conclusions

9

A wide range of products with U. S. market size exceeding 6×10 lb/yr and product

# 9

values exceeding $4×10 /yr could be potentially manufactured from lactic acid.

Degradable and environmentally sound products will provide the initial impetus for development and deployment of new lactic acid technologies and products.

Several major U. S. agriprocessing/chemical companies have built demonstration-scale plants and have identified the trends in the environmentally sound products and processes; consequently, they have plans for major large-scale plants in the future.

Novel separations processes that have recently emerged can enable large-scale and economical production of purified lactic acid without waste gypsum or salt by-product.

В

Figure 1. ANL Program of Oxychemicals and Polymer Feedstock Production from Carbohydrate Derived Lactic Acid

Several novel processes are being deployed for facile production of lactic polymer feedstock from lactic acid.

A wide variety of polymers/copolymers with many potential consumer uses could be derived as these products and processes are brought on-stream.

With the new technologies, the manufacturing costs and economics of lactic acid and its derivatives have an attractive potential in large-scale systems.

The lactic acid program at ANL has achieved several important milestones mainly in fermentation and methods of copolymer development.

The technical strategy of the program is to develop novel and economical technologies for key intermediates and products (beyond the degradable polymers) that have a wide range of potential applications.

Acknowledgments

Work supported by the U. S. Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, under Contract W-31-109-Eng-38..