High-purity solvent recycling on an industrial scale
David Roth - Process EngineerAlzenau, )
OFRU’s tried-and-tested pointed cone design is used for the distillation vessel; this vessel form enables a larger heat transfer area than flat, bottom-heating designs, and makes it easy to drain highly viscous distillation residue from the sump with the assistance of gravity by opening the sluice gate at the deepest point of the vessel. In order to maintain the excellent heat transfer, spring-loaded conductive PTFE scraper blades run across all heat surfaces to continually remove deposits. All the parts of the plant that come into contact with solvents were manufactured in stainless steel or using solvent-resistant sealants in order to prevent corrosion.
In order to minimise the load on the thermally sensitive solvent as far as possible and to invest less time in heating up the feed material, distillation is carried out under vacuum. In keeping with the vapour pressure curve for the mixture, the boiling point is roughly halved at operating pressure. As the separation performance of the reflux column is dependent on a precisely maintained head pressure, a vacuum control system was installed to automatically feed small volumes of air into the column head. The differential pressure measurement between the distillation vessel and the column head means that the plant operator can be warned about hydraulic flooding of the column.
A two-stage liquid ring pump was used to create the vacuum with the solvent distillate being used as the operating medium. In order to optimize the plant’s final pressure, the pump’s operating equipment circuit is also cooled.
As water is ejected as a low boiling substance and the solvent is ejected as a high boiling substance during the required separation activity, and as the pump medium was to be kept as clean as possible, a vacuum-proof header tank and an atmospheric distillate tank were provided.
During reflux distillation operation, the distillate mass flow is measured in a Coriolis mass flow meter after the condenser and piped into a buffer tank. The process control system calculates the current reflux volume fully automatically based on several factors. This reflux volume is then spread across the column head and drips down through the tower packing; a mixture then rises in vapour form from here. The separation precision is increased by balancing the heat and mass transfer.
As a complex mixture of several impurities had to be separated from the valuable product, the reflux ratio is increased in several stages based on the head temperature measured until the final switching temperature is reached. Varying the reflux ratio in this way helps to improve the yield of the target product.
Once a preset changeover temperature has been reached, the plant is transferred to production operation. After the Coriolis measuring point, the distillate passes through the vacuum stage to the product tank. All the measurement data that is accrued, such as pressure, temperature, throughput volume and machine settings, is continuously recorded and is available to the plant user for statistical analysis and optimisation work. If the plant software is provided with a function upgrade at the customer’s request, a remote software maintenance interface is available and this can be used to process the majority of these enquiries directly from the OFRU site in Alzenau, without incurring any expensive and lengthy travel.
To ensure that a safe and productive plant was handed over to the customer, all the components were certified according to ATEX. Other international authorisations for potentially explosive atmospheres (UL, IECEx, etc.) are available.
To sum up, an efficient, safe and reliable distillation plant for large-scale recycling of a valuable solvent together with the associated distillation process was developed, built and put into operation. The high degree of process automation means that the plant can be kept in operation 24/7 with virtually no user intervention. The high requirements in terms of purity, yield and throughput were all met; the amortisation period is significantly less than twelve months despite the equipment costs.