Purification of 2,3-butanediol from fermentation broth: process development and techno-economic analysis

Background 2,3-Butanediol (2,3-BDO) is a synthetic chemical compound that also can be produced by biomass fermentation, which is gaining share in the global market as an intermediate product for numerous applications, i.e. as liquid fuel or fuel additive. Several metabolic engineering fermentation strategies to enhance the production of 2,3-BDO were developed. However, the recovery of 2,3-BDO from its fermentation broth remains a challenge due to its low concentration and its solubility in water and other components. Thus, a cost-effective recovery process is required to deliver the required purity of 2,3-BDO. This paper presents a new process development and techno-economic analysis for 2,3-BDO purification from a fermentation broth. Results Conventional distillation and hybrid extraction-distillation (HED) processes are proposed in this study with detailed optimization and economic analysis. Particularly, a systematic solvent selection method was successfully implemented to determine a good solvent for the proposed HED configuration based on numerous experimental data obtained with each solvent candidate. NRTL and UNIQUAC property methods were evaluated to obtain binary interaction parameters of 2,3-BDO through rigorous Aspen Plus regression and validated using experimental data. Total annual cost (TAC)-based optimization was performed for each proposed configuration. Even though the HED configuration required 9.5% higher capital cost than conventional distillation, placing an extraction column before the distillation column was effective in removing water from the fermentation broth and significantly improved the overall process economics. Conclusions Oleyl alcohol was found to be the most suitable solvent for the HED of 2,3-BDO due to its high distribution coefficient and high selectivity. The proposed HED drastically reduced reboiler duty consumption and TAC by up to 54.8 and 25.8%, respectively. The proposed design is expected to be used for the commercial scale of 2,3-BDO production from fermentation process. Electronic supplementary material The online version of this article (10.1186/s13068-018-1013-3) contains supplementary material, which is available to authorized users.

is the activity coefficient of molecule i, x is the mole fraction, ij (≠ ji) is the interaction parameter between groups i and j, Gij is as defined in equation 2, m is the measured data, k is Boltzmann's constant, is the nonrandomness constant for binary ij interactions. , , and are binary interactions of , , between a pair of groups i and j, respectively.

UNIQUAC equation:
Ф is the segment fraction, θ is the area fraction, r and q are respectively pure component relative volume and surface area parameters, τij (≠ τji) is the interaction parameter, and z is the lattice coordination number (equal to 10). The r and q values are obtained from the Aspen Plus databank.

Sizing the distillation column
A distillation column was determined to have a sieve tray with 0.61 m tray spacing. The column diameter was determined by the column flooding condition that fixes the upper limit of the vapor velocity. The operating velocity is usually between 70 and 90% of the flooding velocity [1,2]; in this study, 80% of the flooding velocity was used as the default. Column analysis in Aspen Plus ® V9 was used to calculate the diameter of the column and ensure the satisfaction of hydraulic flow inside of the column.

Sizing the extraction column
Due to the number of stages required for the extraction column, an agitated extraction column was used in this work. The column diameter and height were determined from the methods provided by Todd [3]. The diameter of the column will be evaluated so that the column operates at 75% of the flood point.

Capital cost (CC)
Guthrie's modular method was applied [4]. where Fm, Fp, and Fd are the construction material, pressure variation, and design variation factors, respectively.
Particularly for the HED configuration, the vacuum pump cost ( ) was calculated according to Ji et al. [5] and is presented in the following equations. where is the total feed flow (kmol/h), is the gas constant (0.0831 m 3 bar/kmol K), 0 is the temperature at the standard condition (273.15 K), and 0 is the pressure at the standard condition (1.013 bar). where CHPS is the cost of the high-pressure steam; CCW is the cost of cooling water; and CE is the cost of electricity.

Operating cost (OC):
The power consumption of the vacuum pump ( ) and the condenser in the HED configuration were calculated according to Ji et al. [5] using the following equation, which involves the total feed flow. Because the distillate stream is condensed before the vacuum pump, the power consumption ( ) is over-predicted (but the influence of this on the total cost is small).
where is in kW, is the total feed flow (kmol/h), is the absolute temperature of vapor at the intake conditions (298 K), is the heat capacity ratio (1.33), is the discharge pressure (1.013 bar), and is the condenser operating pressure in bar.
In addition, the price of oleyl alcohol (solvent) was 3,517 USD / ton, obtained from the MOLBASE database [7].

Total annual cost (TAC):
The total annual cost includes the annual capital cost (ACC) and the annual operating cost (AOC). The annual investment cost was obtained from the literature and refers to the annual payments over the life of the project [8].
where i is the interest rate per year (8%), and n is the project duration (10 years).