Ascorbic acid is produced from d-sorbitol, a sugar alcohol that can be obtained from various feedstock such as corn, cassava and wheat. Sorbitol is commonly used in the industrial manufacturing of toothpaste, toiletries, food and ascorbic acid. The global sorbitol production capacity is approximately 500 kilo tonne in the year 2013 from China as the largest producer, followed by the United States and Western Europe. The price of 70% sorbitol solution of pharmaceutical grade is approximately USD 500 per tonne. The market size of sorbitol was around 1.85 million tonne in the year 2015. It is expected to reach 2.4 million tonne by the year 2023, with a compound annual growth rate of 3.5% from the years 2016 to 2023. The major sorbitol producer companies in the market are Archer Daniels Midland, Cargill, Ingredion, Roquette and Tereos.
Raw materials and prices
The feedstocks to produce ascorbic acid include corn, wheat, molasses and d-sorbitol. Meanwhile, the current price for corn is USD 146 per tonne, wheat is USD 232 per tonne, molasses is USD 261 per tonne, and sorbitol is approximately USD 500 per tonne. Pre-treatment is required to extract glucose from corn, molasses and wheat before the conversion of glucose into d-sorbitol.
Process technology selection
The three available process technologies used to produce ascorbic acid are the Reichstein process, the two-step fermentation with a single culture, and the two-step fermentation with a mixed culture. These process technologies have a similar overall yield of production, which is 60%. However, the two-step fermentation has higher efficiency and product quality than the Reichstein process [6]. In terms of process technologies economy, the atom economy values for the Reichstein process and two-step fermentation are 0.6424 and 0.5383, respectively. Atom economy is the conversion efficiency of a chemical process in terms of all atoms involved and the desired products produced. Lower capital and operating costs of the two-step fermentation process outweigh its lower atom economy; therefore, the overall revenue of the two-step fermentation is higher than the Reichstein process. The calculation of atom economy for the two-step fermentation process is shown in Table a1 in the Appendix.
The chemical equation for the Reichstein process is shown as Eq. (1):
$$\mathrm{C}_{6}\mathrm{H}_{14}\mathrm{O}_{6} + 1/2\mathrm{O}_{2} + \mathrm{NaHCO}_{3} \to \mathrm{C}_{6}\mathrm{H}_{8}\mathrm{O}_{6} + \mathrm{NaOH} + \mathrm{CO}_{2} + 2\mathrm{H}_{2} + \mathrm{H}_{2}\mathrm{O}$$
(1)
Sorbitol + Oxygen + Sodium bicarbonate → Sorbose + Sodium hydroxide + Carbon dioxide + Hydrogen + Water.
The chemical equation for the two-step fermentation process (for a single culture and a mixed culture) is shown as Eq. (2):
$$\mathrm{C}_{6}\mathrm{H}_{14}\mathrm{O}_{6} + 3/2\mathrm{O}_{2} + \mathrm{Na}_{2}\mathrm{CO}_{3} + \mathrm{NaHCO}_{3} \to \mathrm{C}_{6}\mathrm{H}_{8}\mathrm{O}_{6} + 3\mathrm{NaOH} + 2\mathrm{CO}_{2} + 2\mathrm{H}_{2}\mathrm{O}$$
(2)
Reichstein process has a higher conversion efficiency of reactant to products as compared to the two-step fermentation process. However, the capital and operating costs of two-step fermentation are much lower than the Reichstein process, and this outweighs its lower atom economy and conversion efficiency [7]. Moreover, in the Reichstein process, sorbitol in the conventional batch fermenter inhibits the growth of bacteria and high initial concentration of sorbitol decrease the rate of oxidation [8]. The overall production cost of the two-step fermentation process is two-third of the Reichstein process. In other words, the total revenue of the two-step fermentation process is higher.
Besides, as shown in the process flow diagram (Fig. 6) in Appendix, the two-step fermentation process involves lesser steps than the Reichstein process; thus, it reduces energy and water consumptions. Also, the two-step fermentation process operates at lower temperature and pressure conditions, which is cheaper and safer than the Reichstein process. Furthermore, from the Eq. (2), the two-step fermentation process replaces the chemical reaction of sorbose to 2-keto-gulonic acid with fermentation that reduces the use of harmful solvents and reagents such as acetone. The cost of waste disposal for the Reichstein process is also much higher compared to the two-step fermentation process since the amount of waste produced is significant [9]. In conclusion, the two-step fermentation is simpler with lesser steps, lower capital and operating cost, as well as it works at a lower temperature and pressure conditions than the Reichstein process. Hence, the two-step fermentation with a single culture or a mixed culture is more preferred than the Reichstein process.
Next, the single culture and the mixed culture of the two-step fermentation process are studied and compared. In the second fermenter, the single culture only uses one bacterium, whereas the mixed culture uses two different bacteria. However, the mixed culture is hard to be detected and controlled the contamination of the fermentation. Moreover, it is difficult to obtain an optimum balance among the microorganisms involved [10], and the cultivation of two different bacteria requires more time and space. Therefore, the two-step fermentation with a single culture is chosen since it has a lower production cost with higher efficiency, and it is easier to control and monitor the fermentation process than the mixed culture.
Process description
As shown in the simulated flow sheet attached in Fig. 1, 70% of sorbitol solution is fed to the first fermenter (FR-101) along with ammonia, water and air to undergo oxidative fermentation under 30 °C and pH 6 with an initial concentration of 200 g/L [11]. In the first fermenter, the sorbitol is converted to sorbose by Gluconobacter oxydans [12] in 14 h for 98% conversion [11]. Combination of glassy carbon electrode with molybdenum and manganese oxides makes Gluconobacter oxydans. For these bacterial strains, platinum (Pt) acts as an immobilization matrix (GCE/MnOx-MoOx/Pt) where GCE stands for Glassy carbon electrode, and MnOx-MoOx is the manganese and molybdenum mixed oxides.
Before entering the second fermenter, the biomass produced in the first fermenter is being removed by passing the fermentation broth through a microfilter. Later, this fermentation broth is fermented in the second fermenter (FR-102) by Pseudoglyconobacter Saccharoketogenes for 72 h to produce sodium keto-gluconic acid [13]. In this fermenter, the conversion of sorbose to sodium keto-gluconic acid is 76% [14]. The slurry from the fermenter is then transferred to a microfilter (MF-101) to separate biomass produced by the bacteria. Approximately 1% of the liquid solution is a loss through the removal of biomass using a microfilter. Pseudoglyconobacter Saccharoketogenes can oxidize primary alcohol, secondary alcohol, aldehydes and polysaccharides.
Then, the recovery of 2-keto-gulonic acid from sodium keto-gluconic acid is made by a bipolar membrane electrodialysis (GBX-101) through the exchanges of cation and anion with water molecules [15]. Next, the recovered 2-keto-gulonic acid is fed to an evaporator (TFE-101) to remove the water before entering a continuous stirred tank reactor, CSTR (R-101). The purpose of removing water is to increase forward reaction in R-101 since water is a product in the reaction [16]. In the CSTR (R-101), 2-keto-gulonic acid undergoes esterification process with methanol at 64 °C to produce methyl gluconate [17].
Before reacting methyl gluconate with sodium carbonate in R-102, 2-ketogulonic acid is cooled to 30 °C by a cooler (HX-103). Methyl gluconate reacts with sodium carbonate to form sodium ascorbate. Then, sodium ascorbate is fed to another bipolar membrane electrodialysis (GBX-102) to recover ascorbic acid. The water produced in the reactor is evaporated in a vacuum evaporator (TFE-102) before feeding the ascorbic acid to crystallization process (CR-101) for 54 h at 4 °C. Solid ascorbic acid is then freeze-dried in a freeze dryer (FDR-101) to − 35 °C after filtration (NFD-101) process due to the heat sensitivity properties of a solid ascorbic acid [18]. Since the optimum storage temperature for solid ascorbic acid is 4 °C, it is heated before feeding into a storage tank.
The production of ascorbic acid involves a series of the biochemical and chemical reaction. This reaction produces the intermediate products such as sorbose, sodium keto-gluconic acid, keto-gluconic acid, methyl gluconate, and sodium ascorbate. On the other hand, purification of ascorbic acid requires both chemical and physical processes, which involve heat and mass transfers. Therefore, the chemical process and thermodynamics are crucial to determine the operating conditions and the type of equipment used. The thermodynamics and chemistry data for each equipment in a sequence is listed in Table 1. Meanwhile, the thermodynamics properties of each component are presented in Table a.2 in the Appendix.
Table 1 Thermodynamics and chemistry data for equipment
Process optimization and heat integration
Heat integration is performed to minimize the consumption of energy and the total operating cost. Since the production of ascorbic acid is a batch process, the period where process streams are available for heat exchange is different. Therefore, heat exchanger network only performs on process stream that is available for heat exchange at the same time.
Plant economic analysis
Economic analysis is done on the plant design of ascorbic acid to determine its total investment, total capital and total operating costs. So, the profitability of the ascorbic acid plant and the percentage return on investment (ROI) can be calculated. Breakeven analysis is also done to determine the payback period of the plant designed. The economic analysis is then assessed based on three scenarios, which are the base case, the best case, and the worst case to increase the preparedness for future emergencies. These cases are compared, and then the most feasible case is chosen.
Since the Vitamin C chemical plant is built to make profits, the cost estimation is essential to be done before the cost-effectiveness of the plant can be evaluated. Method of Guthrie is used for preliminary estimation of the cost estimation for individual equipment. The chemical engineering plant cost index for the year 2017, which is 567.5 is used to determine the purchased equipment and bare module costs. The accuracy of the cost estimation for equipment methods are affected by the project complexity, accuracy of cost information, availability of materials and performance of equipment [20]. The cost estimation for each equipment is done before the designation of each equipment, hence the dimension of the equipment such as height, length and diameter are assumed and pre-defined. Through SuperPro Designer, most of the equipment specifications such as heat transfer area, volume and area of the equipment can be obtained. The cost for the equipment where its size exceeds the size boundary is calculated as multiple units in series when the specified diameter is too large.
There are three main categories for the calculation of total capital investment, namely direct cost, indirect cost and Fixed Capital Investment (FCI). Direct cost includes cost of purchased equipment and transportation charges for all instrumentation and controls equipment, piping, yard improvements and service facilities. The cost of instrumentation and controls are calculated based on the Piping and Instrumentation Diagram (P&ID) drawn. For the second category, indirect costs include the expenses from construction, legal expenses, contractor’s fee, contingency, engineering, and supervision. FCI is the total summation of direct and indirect cost. As for the working capital, it is approximately 15% of the Total Capital Investment (TCI). Therefore, the working capital is calculated by using the “Goal Seek” function in Microsoft Excel. Finally, TCI is obtained by adding the FCI and working capital. Equations (3) and (4) below show the methodology of TCI calculation.
$$\mathrm{FCI} = \mathrm{Total}\,\mathrm{direct}\,\mathrm{cost}+\mathrm{Total}\,\mathrm{indirect}\, \mathrm{cost}$$
(3)
where working capital is 15% of TCI. Equation (4) is taken from Towler and Sinnott [21].
$$TCI=\frac{FCI}{0.85}$$
(4)
At first, for the estimation of the operating cost, raw materials cost, utilities cost and operating labors cost need to be calculated. Then, these costs are used to calculate Plant Overhead Cost (POC), Total Manufacturing Cost (TMC) and Total Production Cost (TPC). POC includes the costs for raw materials, operating labor, utilities, operating supervision, maintenance, repair cost, operating supplies, laboratory charges, and royalties [22]. Each cost of raw material is obtained from a Sigma-Aldrich manufacturer in an online website and the total cost of equipment is calculated using the CAPCOST software in Microsoft Excel [23].
For the utilities, heat duty consumption of utilities is obtained from the optimized simulation in SuperPro Designer software. Moreover, the cost of utilities for chilled water, cooling water, low-pressure steam, liquid nitrogen, and glycol are also obtained from SuperPro Designer software. Meanwhile, the pricing and tariffs of electricity are obtained from the local electricity company’s official website. The electricity usage in the plant is assumed to be medium industrial voltage usage tariff. TMC includes taxes for the property, insurance, financing and depreciation cost. TPC is the summation of POC, TMC and total general expenses such as the administrative expenses, distribution and marketing insurance as well as research and development expenses.