Keywords

1 Introduction

With the increasing consciousness on environmental impact of the fossil-based petrochemical industries, research on biochemical production was emerging in recent years. The US Department of Energy (US DOE) listed 12 chemical building blocks which have the potential to be commercially produced through biological pathway (Werpy & Peterson, 2004). In this regard, succinic acid appeared to be among the top rank in the list. Succinic acid is also known as amber acid or butanedioic acid, which is a dicarboxylic acid with molecular formula of C4H6O4. Such acid is an intermediate in the tricarboxylic acid cycle produced during aerobic metabolism, and an end product of anaerobic metabolism (Tan et al., 2014). This chemical can be produced via various strict and facultative anaerobic microorganisms and used as a precursor in many industries, such as pharmaceutical, food, agriculture, polymer, cosmetic and textile industries. In the pharmaceutical industries, for instance, succinic acid is used in the preparation of active calcium succinate, active pharmaceutical ingredients, vitamin A, anti-inflammatory-erthyrodiol derivates, sedative, antipasmer, etc. Succinic acid is utilized in a polyester synthesis and acts as an intermediate in the manufacturing of plasticizer, resins, biodegradable solvents, engineering plastics, surfactants, detergents, etc. The major market potential of succinic acid lies as a precursor for the production of other chemicals (Allied Market Research, 2020).

Compared to the fossil-based succinic acid production, bio-succinic acid production is a benign environmental route (Xu et al., 2021). The usability of renewable raw materials and its carbon dioxide (CO2) fixation are the major advantages of bio-succinic acid production. Apart from being carbon neutral, it requires 30–40% lower energy consumption as compared to conventional fossil-based succinic acid. These factors have led to the commercialization of bio-based succinic acid in recent years (Kuenz et al., 2020). In fact, production of bio-succinic acid could serve as a downstream process for CO2-producing biofuel industries such as bio-methane, bio-ethanol and bio-hydrogen plants as it could absorb CO2 in the process, reducing CO2 concentration while purifying the biogas in the process. (Tan et al., 2018).

In this chapter, a commercial process simulation tool, i.e. SuperPro Designer (SPD) v12 (www.intelligen.com) is used to model a newly developed bio-succinic acid production process from palm biomass, i.e. oil palm frond (OPF) that is collected from the oil palm plantation. The process was developed based on laboratory-scale experimental work (Luthfi et al., 2016).

2 Process Simulation

Figure 1 shows the simulation flowsheet developed in SPD for a bio-succinic acid production process. The process is operated in batch mode, with an annual operating time of 7920 h. The feed stream of the process consisted of 1000 kg of OPF. Note that OPF is a lignocellulose material, and is modelled as a stock mixture in SPD. The properties of OPF are given in Table 1, obtained from OPF sample taken from the oil palm estate of Universiti Kebangsaan Malaysia, located at Bangi, Selangor, Malaysia (Tan et al., 2017).

Fig. 1
A model diagram depicts the process simulation for bio-succinic acid production in S P D. It includes the pretreatment section, fermentation, and downstream processing.

Simulation flowsheet for bio-succinic acid production in SuperPro Designer (SPD) v12

Full size image
Table 1 Properties for OPF feed stream
Full size table

As shown in Fig. 1, the OPF is sent to the ball mill where its juice is pressed from the fresh OPF, before being sent for centrifugation. Apart from juice removal, the ball mill also serves as physical treatment that enlarges the surface area of the bagasse, so to allow more effective pretreatment at later stage. The OPF juice from the centrifuge is transferred to the fermenter, while the separated bagasse from ball mill and centrifuge are sent to the Pretreatment Section. This follows the same process as reported by Luthfi et al. (2017).

In the Pretreatment Section, the OPF bagasse is first treated with sodium hydroxide (NaOH) of 4 wt%; the latter disrupts the crystalline structure of the lignocellulosic bagasse, so to recover its fermentable sugars. The liquid effluent from the pretreatment unit is then cooled to 40 °C before it is sent to a rotary siever. The latter removes the black liquor, which consists of hydrolyzed xylan and lignin. Effluent from the rotary siever is transferred into the enzymatic reactor. In the enzymatic reactor, hydrochloric acid (HCl) is first used to neutralize the NaOH content, while enzyme (consisting of cellulase and xylanase) is later used to break down the structural carbohydrate of pretreated bagasse into fermentable sugars (Luthfi et al., 2019). The hydrolysate from the enzymatic reactor is then sent for membrane filtration to remove all suspended solids. The filtrate (OPF sugar) is sent to the fermenter.

In the fermenter, OPF juice (extracted in ball mill) and OPF sugar (from pretreatment section) are first transferred in from their operations. Mineral mixture (consisting 50 wt% of potassium dihydrogen phosphate (KH2PO4) and sodium chloride) are next added into the fermenter (Tan et al., 2016), which is then followed by sterilization-in-place (SIP). The SIP ensures no other microbe presence in the fermenter prior to the charging of the necessary microbes, i.e. Actinobacillus succinogenes. Fermentation process is next initiated upon the charging of inoculum Acinobacillus succinonogenes. Throughout the fermentation process which takes a total duration of 24 h, CO2 is charged to fermenter, while its temperature is maintained at 37 °C (Luthfi et al., 2018). Upon the completion of fermentation process, product from the fermenter is sent to the Downstream Processing Section.

In the Downstream Processing Section, several processing units are used for the purification of bio-succinic acid. Two units of membrane filtration are used to remove the suspended solids and liquid content from the fermentation broth. The suspended solid are mainly due to microbe and other traces of impurities resulted from enzymatic hydrolysis and OPF juice. Filtrate from the membrane unit is sent to a flash evaporator, where the product is further concentrated in the medium to allow a more efficient crystallization process (Luthfi et al., 2020). Effluent from the evaporator is then mixed with concentrated HCl in order to reduce its pH value, before being sent to the crystallization, where succinic acid solids are formed. Concentrated HCl is used to lower the pH for the optimum succinic acid crystallization. Products from the crystalizer are dried in a drum dryer. The latter removes remaining moisture of the crystals. Dried crystals from the drum dryer are ready for sale. Detailed setting for all units are shown in Table 2.

Table 2 Important setting and scheduling data for all units (set-up time is assumed as zero)
Full size table

3 Process Scheduling

Details of scheduling for each process units, such as duration and start time are given in the last two column of Table 2 (all units are assume to have zero set-up time). The operation Gantt chart of the process is shown in Fig. 2. SPD determines that the overall process has a batch time of 62 h, while its minimum cycle time is identified as 40 h. With the annual operation time of 7920 h, the minimum cycle time determines that the process has a yearly production of 197 batches, which is equivalent to an annual production of 6878 kg of bio-succinic acid products.

Fig. 2
The screenshot represents the algorithm for the bio-succinic acid production process on a Gantt chart. It includes details of scheduling for each process unit, such as duration and starts time.

Operational Gantt chart for bio-succinic acid production

Full size image

4 Options for Debottlenecking

SPD next identifies that the fermenter is the time bottleneck of this process (see Fig. 2 for its longest cycle time among all units, i.e. 40 h). To improve the overall annual productivity, the minimum cycle time is to be reduced. One potential solution is to make use of multiple units of fermenter. This may be done using the staggered mode function of SPD. Results of the latter are shown in Table 3. As shown, with five fermenters, the annual production is increased to 30,303 kg/y, i.e. 340% of the base case process. Note however that the use of multiple fermenters will increase the capital expenditure of the process. However, during the course of this process simulation work, economic feasibility study was not carried out due to the lack of process economic data.

Table 3 Summary of production with different units of fermenters
Full size table

5 Conclusion

In this chapter, a production process of bio-succinic acid crystal is modelled using SuperPro Designer v12. Scheduling is also performed to identify the time bottleneck of the process, which limits the annual production. The fermenter which has 40 h cycle time is determined as the time bottleneck. More fermenters are hence added to reduce the cycle time, and hence increase the number of batches and annual production of the bio-succinic acid product. Doing this leads to 340% increase of production, as compared to that of the base case process. Detailed economic evaluation should be carried out next in order to identify economic hotspots of the process.