Techno-economic Analysis on Bioconversion of Glycerol into 1,3-PropanediolMU Xiaojia, TENG Hu, SUN Yaqin, XIU Zhilong

Department of Bioscience & Biotechnology, School of Environment & Biological Science & Technology,

Dalian University of Technology, Dalian, Liaoning 116024, ChinaAbstract: The continuous bioconversion of glycerol to 1,3-propanediol was analyzed via modeling and optimizing the fermentation and separation process. The highest concentrations, productivities and yields of 1,3-propanediol under various operational conditions were calculated, coupling with the economic evaluation of the cost of energy and materials. The results indicated that the increase in the productivity would not simultaneously bring more profits, but the opposite, the increase in the yield of 1,3-propanediol to glycerol would bring growth to the profit, but this growth became slow once the yield was higher than 0.68 mol/mol. The economic sensitivity showed that the cost of glycerol was the key factor influencing the profit. Through applying the integrated production of biodiesel and 1,3-propanediol, the cost of glycerol could be saved while the retained profit was increased by 31%.Key words: 1,3-propanediol; techno-economics; kinetic modeling; process simulation

1. Introduction

Currently, with the increasing price of the crude oil and the shortage of petrochemical feedstock, the researches on renewable biomass resources have attracted people’s attention. Although the large-scale industrial production using bioconversion technology is rare, the approach of using renewable resources to produce bio-energy and bio-chemicals is widely recognized [1]. Among the bioconversion technologies, the bioconversion of glycerol to 1,3-propanediol (1,3-PD) is a typical example. 1,3-PD is high valued, which is widely used as the raw material of many household and personal care chemicals [2]. Compared with the conventional chemical conversion process, the bioconversion is more environmental friendly and energy saving [1]. However, there is little industrial production of 1,3-PD using bioconversion technology for due to the limitation of technology and economy.

The raw material and the operation costs are the main factors the that affect the economy of the bioprocessing, especially the raw material costs occupy 60~70% of the total cost. From the view of raw materials, the main approach of microbial fermentation production of 1,3-PD can divide into two categories: using genetic engineering bacteria with glucose as raw material [3] and using such intestinal bacteria as Klebsiella pneumonia with glycerol as raw material [4]. The research in China mainly focuses on the second method. Due to the high and fluctuating price of

refined glycerol, the industrial bioconversion for the production of 1,3-PD is limited. With the development and utilization of biodiesel, crude glycerol will be sufficient in the market [5,6]. Although the indicators of the production of 1,3-PD using crude glycerol was slightly were slighter than that using refined glycerol, the utilization of crude glycerol can dramatically reducing the cost of raw materials [7]. Therefore, the rational use of glycerol from biodiesel production to produce 1,3-PD and other high value-added products gradually attracted people's attention [5,8,9]. Currently, China has completed the pilot test of crude glycerol fermentation to produce 1,3-PD [7, 9,10].

Due to high hydrophilic, high-boiling of 1,3-PD, and the complicated composition and the low 1,3-PD concentration in the broth, there is no mature technology to separate and purify 1,3-PD simply and efficiently from the broth [11]. Currently, the mainly separation methods for 1,3-PD is: using high-speed centrifugal separation, microfiltration, ultrafiltration, nanofiltration [12] and zeolite membrane separation [13-16] to remove such biological macromolecules as bacterium, bacteria, proteins and nucleic acids, or using electrodialysis [17,18], ethanol precipitation [19] method to remove salt, proteins, polysaccharides and other substances in the broth. Then, purifying 1,3-PD product by vacuum distillation [20] (or distillation). However, these separation methods are still lack of accurate economic cost analysis. Therefore, based on the continuous fermentation of glycerol to produce 1,3-PD coupling the alcohol precipitation separation process [19 20], this study calculated the energy and material consumption by design optimization of the product concentration, productivity and the yield of 1,3-PD, as well as the simulation of downstream separation process, then conducting the economic costing and analysis.

2. Glycerol fermentation process optimization

Based on the kinetics model of glycerol fermentation to produce 1,3-PD , using the maximum product concentration, productivity and yield of 1,3-PD as optimization goal, the operating conditions of continuous fermentation reaction are simulated and optimized.

2.1 Fermentation kinetics model and optimization

Wherein the specific growth rate (μ), substrate consumption rate (qS), specific production rate for the various product (qPi), the productivity (P) and the yield of 1,3-PD (YPD / S) [21-23] were:

Significance and values of the parameters in equations (4) to (8) were see also in the literature [21-25].

The initial glycerol concentration and dilution rate were chosen as variables to optimize to model, the variables ranges were: initial glycerol concentration CS0 = 0, 50,100, ..., 2000 mmol / L, the dilution rate D = 0.01, 0.02, 0.03, ..., 0.35 h-1. The model is calculated by the assistance Matlab software.

2.2 The calculated resultsThe effect of the dilution rate and initial glycerol concentration on the

concentration of 1,3-PD in broth was presented in Figure 1 (a). As can be seen in the figure, the concentration of 1,3-PD increased with the decrease of dilution rate, and approaching the maximum tolerable value of the cell (939.5 mmol / L) [24]. This was because that the residence time of glycerol in the reactor increased with the decrease of the dilution rate. Therefore, the conversion of glycerol increased and more 1,3-PD was produced. The operation conditions of the highest 1,3-PD concentration with different dilution rates (point A~F) was shown in Table 1.

Fig.1 Computational results of the concentrations, productivities of 1,3-PD and yields of 1,3-PD to

glycerol at different dilution rates and initial glycerol concentrations in feed

The effect of the initial glycerol concentration and dilution rate on the productivity was shown in Figure 1 (b). As can be seen in the figure, the productivity increased with the increase of dilution rate. Compared with Figure 1 (a), the trend of the highest productivity was on the contrary of that of the concentrations of 1,3-PD. , The maximum productivity was obtained when the dilution rate was 0.28 h-1 [26]. While the concentration of 1,3-PD was relatively lower (point D in the figure) .In addition, the yield of 1,3-PD decreased with the increase of dilution rate, as was shown in Figure 1 (c).

Table 1 The operational conditions and product concentrations of points A to F in Fig.1

3. Simulation of Separation Process

3.1 Overview of Process and general process indexesA steady simulation program PROII was applied to investigate the energy and

material consumption of the separation process of 1,3-PD. The specifications of this process were as following: 1500 t 1,3-PD per year, purity of 1,3-PD was above 99.9%, yield of 1,3-PD was 80%, production time was 300 d per year.

Fig.2 The flow chart of 1,3-PD fermentation and separation from glycerol

The separation used alcohol precipitation technology [19], which was showed in Figure2. It included the following steps: First, remove most water of the broth by falling film evaporator, and keep the water content of the concentrated broth around 70%(wt./wt.). Second, add ethanol (95%) into the concentrated with a volume ratio of 2:1 in the alcohol precipitation tank. the impurities as proteins, polysaccharides, salts was removed by precipitation. Third, recover the ethanol by ethanol recovery column (recovery of 90%). Forth, remove light component as water, acetic acid with light component fractional column. Last, refine the production with 1,3-PD distillation column. The composition of the broth for separation was chosen by point A~F in Table 1, the stream of it was calculated by the specifications mentioned above, along with the concentration of the broth, dilution rate and other parameters, which was showed in table 2. The effects of other by-productions (as 2,3-butanediol and succinic acid) to the separation were ignored for their very low mass ratio.

3.2 Results of separation simulationThe simulated results of cold-hot utility energy consumption under each

operation condition of point A~F was showed in table 2. As the dilution rate increased, the throughput of broth rise, 1,3-PD concentration of the broth was decreased, and the total energy consumption increased. The increased low pressure steam consumption was mainly used in the falling film evaporation process, while the medium pressure steam consumption was cost by the 1,3-PD distillation process, which might be caused by the reason that larger dilution rate caused more residual glycerol then rise the energy consumption of 1,3-PD distillation column. Table 3 and table 4 gave the stream parameters and utility consumptions at operational point B.

4. Economic Costing

4.1 Cost calculationTable 5 showed the cost of each operational point of Table 2. The calculation

based on the following data: glycerol was 5000 Yuan/t, ethanol was 4000 Yuan/t, low pressure steam was 77.29 Yuan/t, medium pressure steam was 100 Yuan/t, cooling water was 0.31/t, sales management cost 993 Yuan/t [27].

Table 2 Broth flows and the calculated consumptions in separation process

Table 3 Parameters in flow chart at operational point B

Table 4 Utility consumptions at operational point B (×106 kJ/h)

4.2 Profit and lossYear profit of different operation conditions was showed in table 6, based on the

1,3-PD marketing price of 3.8w Yuan/t.From table 5 and table 6, it could be seen that production cost was increased for

operational point A to F, and the profits went down. The profit increased along with yield raising of the 1,3-PD, and decreased with the raising of productivity (as showed in figure 3). The benefit of 1,3-PD yield on profit weakened during the later period, while the negative impact of productivity increased. However, it should be illustrated that the profit mentioned in Figure 3 was considered the reusing of residual glycerol (as the process showed in figure 2), which meant that the utilization of initial glycerol was 100%.

Table 5 List of cost estimates of unit 1,3-PD (￥/t)

Table 6 Profitability analysis for points A to F in Fig.1

The glycerol annual consumption without reusing of residual glycerol and the profit with 1,3-PD yield were also analyzed to investigated the effect of the reusing of glycerol, which was showed in Figure 4. It also could be seen that the increasing rate of the profit slowed when the yield of 1,3-PD grew. When the yield of 1,3-PD was below 0.68mol/mol, the profit significantly increased, however, when it was above 0.68mol/mol, the profit was not obviously grewgrown with the increased of the yield. Meanwhile, the decreasing rate of annual consumption glycerol was flattened along with the increasing of the 1,3-PD yield.

Fig.3 Effects of productivity and yield of 1,3-PD on profit

Fig.4 Effects of yield of 1,3-PD to glycerol on the glycerol consumption and profit without the reuse of residual glycerol

4.3 Analysis of Economic sensitivityConsidering the process economy and maneuverability, the economic sensitivity

of operational point B was analyzed by the price of product, cost of glycerol and utility. The results were showed in Figure 5. From this figure, it could be seen that the profit was most more sensitive with the price volatility, then with the cost of glycerol, at last with the cost of utility. It is difficult for the producer to control the price, so lower the cost was the way to increasing the profit, e.g. using cheaper glycerol materials, directly using the crude glycerol obtained as the byproduct of biodiesel, improve the separation process, lower the energy consumption and so on.

4.4 Effect of glycerol cost on the economic benefitsIt could be seen from Figure 5 that the glycerol cost around 47% of the total cost

(under the operational point B), even when all residual glycerol could be recovered. Therefore, it could largely reduce the material cost by using the crude glycerol which was obtained as byproduct of biodiesel (price was 2000Yuan/t) to produce 1,3-PD. Table 7 showed the economic benefit under operational point B with crude and refined glycerol as raw material. The income tax was base based on [27]. From the part of 1,3-PD production, using crude glycerol could increase 31% of the net profit after tax. And from the part of biodiesel production, the profit of 5206Yuan/t could be obtained from the crude glycerol by using it to producing 1,3-PD, which was benefit for lowing the cost.

Fig.5 Sensitivity analysis of price and cost

Table 7 Profitability and consumption analysis with the cost of crude and refined glycerol

Fig.6 Effects of the variation of cost and 1,3-PD’s price on retained profits with the cost of crude

(the upper surface) and refined (the lower surface) glycerol

Effects of the variation of cost and 1,3-PD price on net profits with the cost of crude and refined glycerol was showed in Figure 6. The profit would be suffer significant fluctuations when the price of glycerol or energy resources was fluctuated or the price of production great declined. It could be seen from the figure that the difference of the net profit between two kinds glycerol increased with the growing of total cost. And the crude could ensure the profit when 1,3-PD price changed. For example, when the total cost increased 50%, with refined glycerol as material, the

profit would be drop to 0 when the price of product decreased 25%, accordingly, with crude glycerol, the profit would be drop to 0 even when the price decreased 45%. Therefore,

5. Conclusions

The effects of dilution rate and initial glycerol concentration on product concentration, productivity and yield of continuous fermentation were investigated by kinetics. The energy and material consumption was calculated by separation simulation. The techno-economic analysis was also discussed. The conclusion was as following:

1) When the dilution rate increased, 1,3-PD concentration and yield increased, and productivity decreased.

2) Increase the productivity could not ensure the growing of the profit. Under the circumstance of this research, the higher the productivity was the faster the profit dropped.

3) When 1,3-PD yield was below 0.68mol/mol, the increasing of yield could obviously increase the profit. And when the 1,3-PD yield was above 0.68mol/mol, the increasing of yield could not significantly increase the profit.

4) The economic sensitivity analysis showed that, the cost of glycerol was the main factor of the profit. By using the glycerol which obtained as the byproduct of biodiesel as material, the net profit and the ability of enterprises to respond to market fluctuations could be increased.