sulzer crystallization unit

With production capacities of 200’000 tonnes per annum [approx. 220’400 tons per annum] benzoic acid, including 60’000 tonnes per annum [approx. 66’100 tons per annum] of high purity benzoic acid and 50’000 tonnes per annum [approx. 55’100 tons per annum] benzyl alcohol, Wuhan Youji is a global leader in antisepsis, preservation and antioxidation as well as an award-winning pioneer in China’s food additive industry. The company is particularly active in the production and standardization of benzoic acid and benzoates, which are widely used as preservatives. Safety, eco-friendliness and health are key aspects that Wuhan Youji promotes in its own operations and good manufacturing practice guidelines. This commitment drives the company to regularly simplify and enhance its processes.


Recently, the company decided to improve one of its most challenging processes: the purification of benzyl benzoate. Used in flavor and fragrance industries, the chemical needs to have extremely low levels of impurities and appear colorless, i.e. any remaining foreign matter should not affect the benzyl benzoate’s appearance. Therefore, separation processes that rely on crystallization are a must, in order to achieve such a highly concentrated substance.


Wuhan Youji’s existing purification method was based on suspension crystallization. It involved several complex stages as well as a number of pieces of equipment, such as crystallizers, centrifuges and waste disposal systems. In addition, the procedure required constant human intervention and manual operations.


Repeating previous success


To succeed in this ambitious task, Wuhan Youji contacted Sulzer, the leader in separation and mixing technology. The two businesses had developed a good working relationship based on previous projects – Sulzer delivered two high-performance and efficient separation systems for the purification of benzoic acid. Based on the success of these projects, the chemical manufacturer was keen on getting an innovative solution for the purification of benzyl benzoate.


“By using our proprietary, fully-automated falling film crystallizers, Wuhan Youji was able to produce benzoic acid of the highest quality and purity, greatly increasing the business’ competitiveness in China,” explains Matthias Schäfer, Project Manager at Sulzer.


Sulzer’s specialists started looking at how to simplify the purification process for benzyl benzoate by conducting extensive small-scale and pilot plant testing to identify the most suitable and efficient way to remove impurities from the chemical. The in-depth investigation by Sulzer revealed a way to purify benzyl benzoate using a finely tuned static layer crystallization process.


The method would streamline and simplify the process, as it requires only one, fully automated piece of equipment with no moving parts, and doesn’t need solvents nor does it generate waste streams. Furthermore, the technique could handle the high viscosity of benzyl benzoate.


In addition, the analyses were fundamental to estimating the number of stages required to reach a high level of purity as well as construct an ideal temperature profile. In effect, the temperature-sensitivity of the chemical required a highly accurate temperature profile to regulate the crystallization and melting phases, which are separated by narrow temperature intervals. The knowledge and expertise of Sulzer’s engineers, the in-depth testing and the high-quality separation equipment contributed to successfully overcoming these issues.


Well-planned crystallization


Based on the process developed by Sulzer, the feed in the static crystallizer undergoes stages of cooling and heating to separate benzyl benzoate from the impurities. When the temperature within the crystallizers are below the freezing point of the melt (between 14 and 4°C), benzyl benzoate crystals build up on the outer surface of the plates. Impurities are largely rejected from the growing crystals and are concentrated in the remaining melt.


Once all the benzyl benzoate fraction has been crystallized, the liquid phase is drained from the crystallizer, while the purified crystalline layer remains attached to the plates. This is then further purified by sweating (or partial melting), which is a gentle reheating up to the melting point (18°C). The resulting impurity-rich melt is drained off. After sweating, the purified benzyl benzoate crystal layer is totally melted and collected in product storage vessels.


Having two crystallization stages allows the manufacturer to reach a product purity of at least 99.9% w/w, a quality level that is far higher than what the existing suspension crystallization unit could achieve. An additional stripping stage helps to eliminate remaining impurities with boiling temperatures close to benzyl benzoate.


Scaling up to full industrial plant


Based on the successfully developed design, Sulzer proceeded to manufacture, install and start-up the full-scale unit. In order to allow Wuhan Youji to benefit from this new system as soon as possible, the entire project was completed in a record time of 12 months, and the chemical plant can now accommodate a benzyl benzoate product capacity of 5’500 tonnes [approx. 6’000 tons] per year.


Sun Bo, Technical Director at Wuhan Youji, comments: “We are extremely happy with the solution delivered by Sulzer. Not only were they able to greatly simplify our existing process, improving efficiency and resource use, but they also helped us to enhance the quality of our product. Sulzer has been a very reliable partner and we look forward to collaborating again in a near future.”


Matthias Schäfer concludes: “We are particularly delighted with the outcome of this project. Our research and development capabilities helped us create a tailored and highly efficient solution. Customer satisfaction and the continued cooperation with Wuhan Youji clearly attest to our capabilities in separation technology and our commitment to develop complete, customized solutions.”


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