Process development and optimisation of lactic acid purification using electrodialysis

Abstract

Cell free sodium lactate solutions were subjected to purification based on mono- and bi-polar electrodialysis. Lactate concentration in the product stream increased to a maximum of 15% during mono-polar electrodialysis. Stack energy consumption averaged 0.6 kW h kg⁻¹ lactate transported at current efficiencies in the 90% range. Under optimum feed concentration (125 g l⁻¹) and process conditions (auto-current mode with conductivity setpoints of minimum 5 and maximum 40 mS cm⁻¹), lactate flux reached 300 g m⁻² h⁻¹ and water flux were low for mono-polar electrodialysis averaging 0.3 kg H₂O per M lactate transported. Glucose in the concentrate stream solutions was reduced to <2 g l⁻¹. Acetate impurities enriched from about 0.5 g l⁻¹ in the feed stream to 1.5 g l⁻¹ in the concentrate stream solutions. After mono-polar electrodialysis, the concentrated sodium lactate solutions were further purified using bi-polar electrodialysis. Water transport during bi-polar electrodialysis reached figures of 0.070–0.222 kg H₂O per M lactate. Free lactic acid concentration reached 16% with lactate flux of up to 300 g m⁻² h⁻¹. Stack energy consumption ranged from 0.6 to 1 kW h per kg lactate. Under optimised process conditions current efficiency during bi-polar electrodialysis was consistently around 90%. Glucose was further reduced from 2 to <1 g l⁻¹ in the free lactic acid solution. Acetic acid impurity remained at around 1 g l⁻¹. Significant reduction in colour and minerals in the product streams was observed during electrodialysis purification.

Introduction

In fermentation processes the number of purification steps required, and their individual step yields determine the overall yield of the process. Currently the overall yield of typical purification processes composed of about 8–10 unit operations is in the range of 50–80%, a yield that can diminish minor process achievements in fermentation processes (Walter, 1998). The employment of new purification technologies and integrated process configurations therefore presents exciting possibilities to reduce the number of purification steps and production costs respectively.

Lactic acid production processes suffer from several drawbacks namely, productivity and process economics. To overcome the inhibitory effects of lactic acid, either the lactic acid must be continuously removed from the fermentation vessel, or it must be neutralised with alkali during the fermentation to convert lactic acid to its less inhibitory dissociated form. The resultant lactate salt however presents extra downstream costs especially when the free lactic acid is the desired product. Apart from treatment with alkali, various techniques have been proposed to continuously remove the lactic acid as it is formed and these range from stripping with organic solvent to the use of electrodialysis systems (Yabannavar and Wang, 1991, Hongo et al., 1986). Most industrial producers of lactic acid still employ the precipitation process for the purification of lactic acid, which leads to the generation of 0.36×10⁶ t a⁻¹ crude calcium sulphate, a by-product which is normally dumped in the environment as waste (US Department of Energy, 1999). Bi-polar electrodialysis has the potential to help reduce the generation of industrial salt waste streams. Although promising prospects for lactate purification by bi-polar electrodialysis have been presented before (Eibensteiner, 1998; Bar and Byszewski, 1996; Siebold et al., 1995), a reluctance to embrace the technology still persists.

Electrodialysis (ED) is an electro-membrane process in which ions are transported through ion exchange membranes from one solution to another under the influence of an electrical potential. The principle of mono-polar electrodialysis is depicted in Fig. 1. Indicated is the arrangement of the ion exchange membranes to form distinct channels namely, diluting stream (DS) and concentrating stream (CS). In most applications, multiple anion and cation exchange membranes are arranged in an alternating pattern between an anode and a cathode to form a series of concentrating and diluting cells in the stack (between 5 and 500 cell pairs, typically more than 100). A third channel, the electrode rinse stream is located on either ends of the stack. During operation an aqueous electrolyte solution is circulated in the electrode rinse stream to facilitate the transfer of the electric current and, to remove gases produced at the electrodes.

The performance of an electrodialysis process can be determined by calculating the so called current efficiency (CE) of a given run. Current efficiency gives the efficiency of current utilisation in transporting salts from the diluting stream to the concentrate stream and is usually given as a percentage. Current efficiency was calculated based on the following relation:

$$\text{Current efficiency}\text{(\%)=100}\text{×}\text{N}\text{×}\text{F/i}$$

where i is the current density (A m⁻²), F is the Faraday's constant and N is the mole flux (mol s⁻¹ m⁻²).

The parameter N was calculated from V_cA_m⁻¹dC_cdt⁻¹, where V_c is the volume of concentrate solution, A_m is the total effective membrane area installed, C_c is the salt (sodium lactate) concentration in the concentrate solution and t is the time of operation.

Current efficiency is always less than 100%. Factors known to lower current utilisation efficiency include, poor membrane selectivity, water transport by osmosis or with solvated ions, and loss of electric flows through the stack manifold (Strathmann, 1991).

Electrodialysis has found large scale application in the food and dairy industry (Andres et al., 1995; Lopez Leiva, 1988); pharmaceutical (Chen et al., 1995), chemical, textile, and water treatment industries for purposes ranging from recovery, concentration, and purification, to brackish water desalination (Strathmann, 1991), and ground water denitrification (Hell et al., 1998). Integrating a microfiltration module between an electrodialysis unit and a continuous immobilised cell fermentation, Von Eysmondt and Wandrey (1990) observed a several fold increase in acetic acid productivity and concentration. Also working on acetic acid production, Chukwu and Cheryan (1999) described a continuous integrated process where the permeate from the membrane bioreactor was directly fed to the on-line mono-polar ED unit where the acetic acid was recovered, concentrated (to 134 g l⁻¹) and purified simultaneously. The residual sugars and nutrients were retained in the diluting stream of the ED and recycled back to the bio-reactor to facilitate optimum material utilisation with concomitant reduction in waste generation.

Bi-polar membrane electrodialysis also referred to as water splitting electrodialysis, can convert aqueous salt solutions into acids and bases. A water splitting stack is similar to a conventional (mono-polar) electrodialysis stack but incorporates a third type of membrane, the bi-polar membrane, which is composed of a cation and an anion membrane layers laminated together. This membrane can split water molecules that have diffused at the interface into their hydrogen and hydroxyl ions. The combination of each bi-polar membrane with a cation and an anion exchange membrane on either side creates a stack with three separate compartments (streams) namely, base, salt, acid (Fig. 1).