August 2024
Petrochemical Technologies
The purification of benzoic acid using dividing wall columns
In one recent project for a customer based in Asia, the authors’ company suggested the combination of distillation and crystallization techniques to obtain high-quality purified benzoic acid. Encouraged by the successful startup of this processing train, the customer began to schedule a second project. The authors’ company was asked to optimize the purification process to improve cost and operational efficiencies. The two existing columns, which remove lights and heavies, respectively, from the crude benzoic acid feedstock were investigated to identify key opportunities. The feasibility study based on the feed composition confirmed that the existing columns could be converted into a single dividing wall column (DWC).
Q. YANG, P. FAESSLER and L-F. NI, Sulzer Chemtech, Winterthur, Switzerland
Benzoic acid—the simplest aromatic carboxylic acid—is widely used in various industries and applications, including in chemical, food and pharmaceutical manufacturing, among others. Due to its numerous uses, the production of benzoic acid has consistently grown, and its global production capacity is currently estimated between 900,000 tpy and 1.1 MMtpy. A common method of producing benzoic acid consists of oxidizing toluene with oxygen. This process involves the conversion of the methyl side chain of toluene into a carboxylic acid group using a transition metal catalyst in the presence of air as the necessary oxygen source. However, this process also leads to the formation of various byproducts. As a result, the crude benzoic acid must undergo purification to meet the required specifications for its intended applications.1
In one recent project for a customer based in Asia, the authors’ company suggested the combination of distillation and crystallization techniques to obtain high-quality purified benzoic acid (FIG. 1). Encouraged by the successful startup of this processing train, the customer began to schedule a second project. The authors’ company was asked to optimize the purification process to improve cost and operational efficiencies. The two existing columns, which remove lights and heavies, respectively, from the crude benzoic acid feedstock were investigated to identify key opportunities. The feasibility study based on the feed composition confirmed that the existing columns could be converted into a single dividing wall column (DWC).2
FIG. 1. A simplified block flow diagram.
DWC technology is an innovative approach in distillation processes that combines multiple separation tasks into a single unit. It involves the installation of a dividing wall that separates the column into different sections. The key advantages of DWC technology are its ability to achieve higher separation efficiency and energy savings compared to traditional distillation columns installed in a series. By integrating multiple separation tasks, the need for separate columns is also eliminated, reducing capital expenditure (CAPEX).3,4 Implementing DWC technology requires special know-how in the design of process and column internals, as well as expertise in risk mitigation.
The DWC proposed by the authors’ company was adopted, and the data collected during commissioning confirmed that the predicted savings in CAPEX, space and energy were achieved.
Purification of benzoic acid products. The first phase of the project utilized a direct sequence for benzoic acid separation. In the first distillation column (C1 in FIG. 2), the crude benzoic acid obtained from the oxidation of toluene is fed to the system after the removal of bulk toluene. The column separates and removes the low-boiling components (toluene, benzaldehyde, benzylalcohol, etc.) in the column head. Simultaneously, a benzoic acid-enriched stream is extracted as the bottom product of the first column and fed into the second distillation column (C2 in FIG. 2). There, the benzoic acid is further separated from the high-boiling components (benzoylbenzoate, phthalicanhydride, etc.). The purified benzoic acid is collected from the column head, while the high-boiling components leave the bottom section of the column.
FIG. 2. Separation columns for the removal of light and heavy components from the first project.
In the second project, purified benzoic acid leaves the column from the middle section, while light- and heavy-boiling components are removed from the top and the bottom of the column, respectively. As shown in FIG. 3, a wall divides the middle portion of the column into four sections (Sections 2–5). Above and below the wall, there are two common sections, Sections 1 and 6.
FIG. 3. The DWC column used in the second project.
It is obvious that using a DWC instead of two columns immediately eliminates one condenser and one reboiler while the number of product pumps could be lowered from 4 to 3. The footprint for the two columns could be reduced in the plant, as well. These results translate into savings in capital costs.
The unique expertise linked to DWC. As previously mentioned, special know-how is required in the design of a DWC. This know-how will be detailed in this section.
Process optimization. Presently, there is no DWC model in commercial flowsheet simulation software, and DWC must be set up manually. Up to 10 variables are to be determined in simulations.
FIGS. 3 and 4 demonstrate the complexity of DWC setups. In the simulation, the liquid stream running down from Section 1 must be split before it enters Sections 2 and 3. Likewise, the vapor stream from Section 6 splits when it rises into Sections 4 and 5. FIG. 4 shows the concentration profile in the DWC. On the feed side of the dividing wall, Sections 2 and 4 separate light and heavy boiling components while the middle boiling component (i.e., benzoic acid) is not concentrated either upwards or downwards. Rather, it splits, and some of the benzoic acid goes up with light boiling components while the rest goes down with heavy boiling components. On the other side of the dividing wall, a side draw is taken at the location where the concentration of benzoic acid peaks.
FIG. 4. The concentration profile in the DWC.
Unfortunately, finding the three optimal splitting ratios mentioned above are only part of process optimization. The duty of the condenser and reboiler, along with the number of theoretical stages (NTSs) in each section must be optimized to minimize energy consumption. Without a special know-how in process optimization for DWC technology, a large-scale trial-and-error approach is time consuming and does not guarantee that the best option will be determined.
Hydraulic tools with a DWC feature. As the DWC for benzoic acid purification is a vacuum column, the alpha value (an indicator of the difficulty in separation) is influenced by the pressure drop caused by the structured packings and internals installed in the tower. Therefore, a few iterations had to be made between the process simulation tool and the hydraulic tool. Once designs were finalized, hardware designs started.
Column internals. In a DWC, the dividing wall separates the column into four compartments with chordal shapes (FIG. 5), to which the structured packings and internals must be adapted.
FIG. 5. Chordal shaped column internal
Risk mitigation. During the process design and optimization phase, some margin is typically added to the NTS and reboiler’s duty, based on process sensitivity analysis. It should be mentioned that currently, the split of vapor in the bottom section of the column is solely controlled by the pressure drop in the parallel sections along the dividing wall because none of the vapor splitting devices proposed in literature is realistic as of today.5 Conversely, the practicality of the authors’ company’s patented liquid splitter6 is evident and has been used for the second project. Plant operators can adjust the liquid flowrate to the two sides of the packed compartments along the dividing wall. Such a solution can provide some flexibility, helping to mitigate risks associated with possible fluctuations in feed compositions and discrepancies between hydraulic predictions for column internals and real-world performance.
Commissioning data. Among various successful indicators from commissioning, the requirements on recovery rate and purity of benzoic acid product were met. In addition, using a DWC resulted in lower energy consumption, with a 27% energy saving that matches the estimation conducted during the design phase. TABLE 1 summarizes the duties around the columns.
It is worthwhile mentioning that the condenser for the DWC is operated at 15°C lower than the ones in the first project. This is a general inevitability as temperature is closely linked to pressure in a distillation column.
For two conventional columns in a series, different column head pressure can be set, while in a single DWC column, there is only one head pressure. In the case of benzoic acid purification, the pressure at the bottom of the DWC is set first to ensure that the temperature minimizes the formation of byproducts. The pressure at the top of the DWC is subsequently determined by the pressure drop of the column, which is associated with NTS in the DWC, type of packing and the column diameter chosen. Despite that, the temperature of the cooling medium exiting the condenser of the DWC is sufficiently high to allow for heat conservation.
Takeaways. The authors’ company implemented a state-of-the-art DWC column for the purification of benzoic acid. The data from commissioning operations indicated that, compared to an earlier project which used two conventional columns in series, the DWC implementation resulted in a 27% reduction in energy consumption. In this article, key challenges and considerations related to DWC design were briefly discussed. The successful implementation of a DWC is expected to motivate further collaborations.
LITERATURE CITED
1 Xu, J., et al., “Development on the technique of total recovery of benzoic acid residue,” Chinese Journal of Chemical Engineering, May 2009.
2 Faessler, P. and M. Stepanski, European patent no. 3 148 661 B1, “Method for purification of benzoic acid,” March 27, 2019, online: https://data.epo.org/publication-server/rest/v1.0/publication-dates/20190327/patents/EP3148661NWB1/document.pdf
3 Schultz, M. A., et al., “Reduce costs with dividing-wall columns,” AIChE, CEP, May 2002.
4 Dejanovic, I., L. Matijasevic and Z. Olujic, “Dividing wall column—A breakthrough towards sustainable distilling,” Chemical Engineering and Processing: Process Intensification, June 2010.
5 Sun, J., et al., “CFD simulation and experimental study on vapor splitter in packed divided wall column,” The Canadian Journal of Chemical Engineering, September 2015.
6 Yuan, S.-L. and D. W. Gruszcyznski, European patent 1 980 303 B1, “Impinging jet fluid distributor,” August 26, 1998, online: https://data.epo.org/publication-server/rest/v1.0/publication-dates/19980826/patents/EP0569303NWB1/document.pdf
The Authors
Quan Yang is a Sulzer Senior Technical Expert, specializing in mass transfer components. He is in charge of air separation knowledge management within Sulzer Chemtech. Recently, his field expanded to process optimization for DWC. He holds a PhD in environmental engineering from the National University of Singapore.
Peter Faessler is a Senior Technical Consultant in the field of chemical engineering. He has worked at Sulzer Chemtech for 38 yr, holding various positions at different locations including Switzerland, Brazil, Mexico, Singapore and China.
Comments