III. SEPARATIONS
This is the third Pulse post on scale-up considerations.
Distillation is used to separate components in a feed mixture based upon their relative boiling points. A simple, continuous column can separate two mixed components into distinct product streams.
Our understanding of the performance of both plate and/or random/structured packings in distillation has improved over all column design the years. However, process engineers still cannot predict column properties using only the thermal and physical properties of the system being distilled.
Pilot plant distillation tests are usually required because critical scale-up data is missing or contradictory. Pilot tests are performed because management wants to minimize risk when building an expensive facility.
During distillation design, engineers need to determine the correct designs and material for construction: whether materials are corrosive or non-corrosive, and whether a high vacuum, or atmospheric, or high-pressure system will characterize the column. Atmospheric or pressure operation of the column is preferable to using a vacuum system.
The principal parameter upon which all column design is based depends on the relative volatility of the key components to be separated.
Once the top and bottom stream compositions are specified in the design phase, the condensation point of the top stream and the boiling point of the bottom stream can be determined at various pressures.
The temperature of the cooling water can be variable as well, changing by season or weather conditions. Cooling tower water used for condensing is typically supplied at 90 deg. F (“worst-case” summer temperature). City water, river water or chilled water may be available to provide a lower coolant temperature. River water in winter can be cooler than plant-chilled water in the summer. A facility located at Taft, Louisiana, on the Mississippi River showed this effect in the extreme: chilled river water in January developed vacuums that were superior to those of plant-chilled water in July.
A term commonly used is “Height Equivalent to a Theoretical Plate,” or HETP – the height of packing to provide an ideal single stage of separation. Typically, the pilot tower diameter should be at least 10 times the size of the packing.
A large number of pilot plant tests are performed during the various stages in process development. This is especially true if novel packings or trays are used. Also important are the chemical analytical tools used (GC, LC, boiling point).
Example I
Distillation can be used to purge impurities from an operation. In the operation of an acetone unit distillation train, production personnel realized that impurities were building up in the unit, and “burping” into the product. The question was posed to R/D as to how to prevent this.
Computer modeling of the distillation train showed that these impurities were building up in one of the distillation purification columns, where two immiscible organic components were coming into contact.
An engineering assessment, based on these simulations, suggested that a practical method of removing the concentrating impurities was to install a series of taps within the column, several inches apart. These could be opened to partially drain column accumulations. This fix was installed, and acetone quality improved markedly.
Crystallization/Fractional Crystallization
Crystallization processes are widely used for purifying solid materials. These processes are the best choices for processing solid materials needing high purity, such as pharmaceuticals. A crystallization process must be robust, reproducible and scalable. To guarantee reliable scale-up of a crystallization process, process experience and a robust pilot system are essential.
Cooling Crystallization
This is the most straightforward system – a solution is cooled until the solute crystallizes out, as illustrated below:
The simplest cooling crystallizers are tanks equipped with a mixer for internal circulation, as a temperature decrease is obtained by heat exchange with an intermediate fluid circulating in a jacket. Operations can be batch or continuous. Batch processes normally provide a relatively variable quality of product as a function of batch time.
Evaporative Crystallization
Another option is to obtain the precipitation of the crystals by increasing the solute concentration above the solubility threshold. To obtain this, the solute/solvent mass ratio is increased by evaporation. This process is not impacted by temperature changes.
Most industrial crystallizers are of the evaporative type, whose production accounts for more than 50% of the total world production of crystals.
The most common type is the forced circulation (FC) model
To achieve effective process control, it is important to control the retention time and the crystal mass, to obtain the best conditions in terms of crystal surface and the fastest growth.
Crystallization Development
The key step towards a robust crystallization process is a detailed solid-state phase diagram of the system. This provides an overview of the possible phases a solid can demonstrate, and an understanding of the temperature-dependent behavior of the crystals.
Based on the chemical compatibility of the substance with various solvents, a first set of solvents or solvent mixtures is selected to determine the temperature-dependent solubility of the solutes.
Initial seeding is important to avoid supersaturation. Temperature and crystal size are important to prevent premature dissolution or uncontrolled crystallization (“crashing”).
Optimizing the Process: Once an initial crystallization process is established, further work is needed for optimization. Small changes in a crystallization process may lead to undesirable events such as agglomeration, phase conversion, or premature nucleation.
Scaling up from Multigram to Kg Supply: The design process is repeated on a larger scale (250 g to >1kg) for proof of concept or direct delivery. All relevant information gained during the design and optimization stages, involving specialists from all disciplines, allows for a reliable piloting process.
Example I
The final processing step for Bisphenol A is a crystallization step to separate the crude Bis-A from the mother liquor, followed by centrifugation and washing to remove the remaining liquids. A plant where I was responsible for R/D wanted to expand their Bis-A unit capacity, but plant engineers were uncertain if the crystallizers could handle the additional load. I was asked to look into the issue.
Fortunately, the R/D facility where I worked had a pilot crystallization unit where I could do the study. However, it was a vacuum crystallization unit, in which crystallization was initiated by removing liquids via vacuum – which presented a unique set of challenges. To maintain atmospheric conditions in the crystallizer, the unit was 32 feet off the ground (32 feet H20 = 1 atm. static pressure). The Bis-A unit used cooling crystallization; that is, crystallization was initiated by cooling the hot solution. The vacuum design of the pilot unit was antithetical to good cooling design. Vacuum crystallizers tend to plug up during cooling operations.
After consultation with other R/D engineers, the secret to preventing plugging appeared to be a very slow cooling process. A special automatic control system was developed to cool the Bis-A solution to saturation over 24 hours, with the hope of preventing plugs from developing in the cooling heat exchanger.
After several shakedown runs the experimental run was performed, using flow parameters mimicking the Bis-A plant. We cut it close, but were able to complete the run just before plugging occurred, and demonstrated that the plant crystallizers could manage the additional load.
