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Home , Continuous distillation, Distillation Design

BACK TO BASICS Reprinted with permission from Progress (CEP), Design and Operation Considerations for Experiments Glenn Graham ■ Don Bunning, P.E. ■ Jeremy Rader, P.E. ■ MATRIC

Despite great advances in process simulation, the need remains for real-life distillation testing. This practical guide gives a broad overview of how to set up experimental distillation projects.

istillation is the most common separation technique - used in the chemical industry. Most of the world’s Dcommodity chemicals and nearly all liquid fuels are produced by processes that include distillation steps. The pilot-scale distillation experiments, including equipment Engineers use process simulation to model separations the discussion. in order to conceptualize, design, and optimize distillation Why are experiments necessary? experiments with laboratory- or pilot-scale distillation col- Distillation experiments may be necessary at any point umns. Experiments may be required to demonstrate proof- in the lifecycle of a process, from conceptualization to ongoing operation. After a new separation has been pro- address issues that cannot be predicted by simulations, or posed, and possibly simulated by computer, demonstrating generate samples. it experimentally may be desirable. If adequate physical - not be possible, and experiments become the only means of ciples apply and they all require some of the same types of demonstrating the separation. In other cases, samples may contactors. The concepts presented here apply to most types customer’s request. of experimental distillation columns. In some processes, large numbers of compounds are

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initially before progressing to a more complex continuous - An experimental batch setup is usually simpler than an and reliable computer simulation of the main separations experimental continuous setup. A continuous column has the additional requirements of feed and tails systems, as well as a means of controlling its operation at steady-state condi- in a laboratory or pilot plant, and if a new installation is proceeding with costly construction (1, 2). Phenomena such as foaming and fouling cannot be set up than a continuous column. - In addition, experimental time requirements tend to be - on the order of days for a batch column, whereas it may tion, which cannot generally be simulated, may occur in the column, especially in the reboiler due to high column. This time difference is due to two main distinc- and/or long residence times. If the impurities can be identi- their rate of generation. an experiment. Second, a continuous column requires a - of operating conditions. - mined with a batch column, such as: - pressure Batch vs. continuous main components • whether impurities are generated at distillation - conditions • whether foaming or fouling is a concern. feed, safety precautions, and the performance metrics that It may also be possible, in some cases, to generate customer samples from a column. If the - project is whether to use a batch or a continuous column ously an experimental batch column is appropriate. Batch distillation is commonly used for small operations in the pharmaceuticals or specialty chemicals industries, whereas (a) (b) the commodity chemicals and fuels sectors generally use Condenser Condenser continuous columns. Sometimes a batch computer simulation program can be used to model the batch separations. If the results from the Distillate Distillate Reflux simulations agree well with the experimental batch data, this Section Rectifying Feed predictions in the simulation model are accurate and the the simulation results, the system can then be modeled as a

Rectifying Section Reboil Section Stripping , which may negate the need for an experimental continuous distillation setup. Bottom Product Reboiler Reboiler an experimental program is to operate a continuous column Figure 1. One of the first considerations in designing a distillation experiment is to obtain experimental data that would better represent the whether to use a batch (a) or a continuous (b) column. proposed commercial column.

40 aiche.org/cep November 2020 CEP A continuous laboratory- or pilot-scale column is able to: Figure 2. For general-purpose laboratory testing, a column with a diameter of 25 mm is often used, such as this distillation column with a • test control schemes vacuum jacket. • generate scale-up data • produce customer samples. Often, continuous separation schemes require two or diameter columns. Therefore, it is light components and the second column may generate the product as a distillate stream. In these situations, one physi- or larger to generate more-realistic cal experimental continuous column can sometimes be used scale-up data. for the entire separation scheme by processing the mate- Synthetic or real feed correct pressure capability, number of stages, feed location, - reboiler heat source , and condenser coolant ments, synthetic feed material is temperature for both passes. If these requirements cannot be often used so that the separation of the main components can be studied to use two physical columns in series. without any interference from Sampling considerations may differ between batch and realistic representation of the process and should be studied at a later point in the experimental program. The impurities in the authentic feed may require addi- tional analytical equipment or techniques. They may also - Safety concerns Scale of equipment The scale of the equipment required for the experiments equipment design and the location in which the equipment will depend on the purpose of the experiments. If the pur- pose is to collect scale-up data, there will be a minimum col- equipment. will require a different minimum diameter. - ting or other means of shielding must be used to mitigate the danger of an implosion. Pressurized are normally conducted in metal equipment rated for the design the crimp size of the corrugated sheets. size of the required samples may dictate the scale. The deci- sion regarding equipment size, in this case, will be based on Certain operating conditions will necessitate further a trade-off between labor and capital costs, depending on the safety precautions: quantity of sample material that must be produced. • High-temperature operations will require burn protec- tion in the form of insulation or shielding. and the scale of the experimental column can be dictated by required, especially for unattended operation. the decision regarding the size of the distillation column to reacts with the process chemicals. be used. • Reactions with the potential to run away and generate e.g., • Hazardous chemicals, or noxious chemicals with

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for the safe operation of equipment. degradation reactions and/or generate impurities. Tempera- ture limitations on column joints, sealants, and associated cases, limitations on the maximum temperature of the heat- Performance metrics When designing the equipment and setting the conditions - - These metrics might include: tion in the condenser. pressure may be necessary to raise the operating tempera- ture enough to be in the correct range for the reaction. • column pressure drop and capacity as well as heat duty usually eliminates glass as a possible material of construc- • foaming or fouling problems. tion for the distillation column; the experiment, therefore, Process simulation to aid in design After the major considerations are addressed, many that operate at higher pressures may need to be housed in a design details will need to be determined, especially for a special location to allow safe operation. continuous column, which include: • number of stages • thermocouple locations - • sample point locations which allows for the use of pressure swing distillation as a • heat duties • condenser and reboiler temperatures. - Results of these simulations can also be used to determine column diameter and height, as well as the sizes of feed - experimental conditions and reduce the number of experi- ments that are required.

Pressure and temperature considerations Pressure and temperature need to be considered jointly, impact. The column should operate at atmospheric pressure, if possible. Atmospheric pressure operation eliminates the (a) (b) Figure 3. Glass columns with Oldershaw trays (4) are commonly used in - laboratories. Here, an unjacketed 4-in.-dia. column is shown (a) without and tion of the column. (b) with liquid and vapor tra!ic. These column sections were custom-made with a smaller number of holes for high liquid-to-vapor ratio and with side ports for a thermocouple or sample apparatus. Glass allows visual observation of the tray operation may be required to reduce the column or reboiler activity (froth), foaming, and fouling issues.

42 aiche.org/cep November 2020 CEP Column internals prediction for scaleup (5). contactors in a distillation column (3)- i.e., we will discuss some of the major considerations for each of these options. (4) are commonly The general inertness of glass allows for its use with most loading, foaming, fouling, and color formation. diameters, as well as other sizes by special order. Tray of this range depending on the chemical system. The - tray for the same operating conditions, so laboratory data is important and is normally accomplished with a single • corrugated wire mesh that is rolled to the diameter of the column Figure 4. Random packing allows for a lower column pressure drop than columns with trays and is available in a variety of materials, including metal, glass, ceramic, and plastic.

Figure 5. Structured packing is less commonly used in laboratory distillation columns than random packing. The packing on the left is made of corrugated wire mesh and is rolled up to the diameter of the column. The packing on the right is constructed of flat corrugated gauze sheets.

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Water is commonly used as the coolant, but a chiller may be required for compounds with low boiling points. A down- Proper liquid distribution is also required, as for random draft condenser may be required if a decanter is needed for e.g., aqueous sized correctly for the inside diameter of the column so that allow liquid channeling down the wall, which could affect Reflux control the separation. Reboiler and condenser - used as the reboiler for laboratory columns with diameters of - - - around loops with heat exchangers, thermosiphon reboilers, a column with a sidestream line. Condensers for laboratory-scale columns are usually of the updraft style and should be constructed with coils for increased surface area, especially for chemical systems with

Condensate Tapered Joint Heat losses Collection Cup (Connects to Condenser) Heat loss is a major concern for laboratory- and pilot- - Vacuum Jacket Rising Vapor Tube cant problems. Heat losses create internal , in the column to be different than what would be expected Cup Drain Tube Thermowell Port Swinger Hinge Distillate Product operating range of the column. In extreme cases, the column Swinger Line - Electromagnet umns occurs at the joints. Tall columns, or columns with Contact Location high process temperatures, are especially susceptible to Rising Vapor Tube Distillate Tube column sections. Tapered Joint Reflux Tube (Connects to Tray or Heat tape and/or insulation can be an option for small Packing Section) glass columns, but are often a requirement for larger glass - Figure 6. A liquid dividing head, shown here with a vacuum jacket, can be used to control the reflux ratio for laboratory distillation columns. The rising vapor (6): The passes through the dividing head and is condensed in a condenser located above the dividing head (not shown). The resulting liquid condensate runs back into the dividing head, where it is alternately channeled to the distillate tube and the heat tape. A thermocouple is inserted between the insula- reflux tube by the swinger (sometimes referred to as a swing arm, swing bucket, or tilting funnel). The position of the swinger is driven by an electromagnet that is tion wraps, and another thermocouple is installed inside controlled by a timer. the column to measure the process temperature. A process

44 aiche.org/cep November 2020 CEP controller uses the difference between the temperatures necessary for unattended operation, and might include auto- - matic shutdowns based on temperature, pressure, column zones may be required, depending on the temperature pro- Analytical considerations the column to run adiabatically and eliminates internal - tant not only to arrange for the proper analytical support, but to also plan for the column sampling schedule. Sometimes and/or insulation are adequate. - Process control and instrumentation ment, frequent sampling will help to determine how much - or reboiler, or a real-time analysis, may be used to manu- determine if an intermediate compound is building up in the trays, a syringe is often used to pull a liquid sample from Continuous columns require more sophisticated control schemes (7, 8), although for laboratory experiments, these schemes are often implemented manually. - sures, then a pressure-control system will be required. The special sampling trough in the column is often required to - duced at the inlet of the pump to control the pressure in the - used for high-pressure control. PC - Cooling 12 Cooler Water Head Product - Reflux LC Drum 22 as well as for troubleshooting and matching simulation and Distillate experimental results. Heat QC Exchanger Reflux 42 Outer Insulation If the column will be Layer operated unattended, then Feed some of the controls may TC TT 82 83 Plates need to be automated ESD controls may be Steam Heat LC QT Tape Reboiler 32 51 TI Figure 7. Heat losses can be Water minimized by wrapping the column Bottom Product Thermocouple with insulation and heat tape. The In Between Column Inner heat tape and a thermocouple are Figure 8. This piping and instrumentation diagram (P&ID) shows a typical Insulation Insulation Layers Layer wrapped between two layers of control system that would be necessary for an automated continuous distillation insulation. Source: R. Nunley. column at atmospheric pressure.

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Start-up and operation. The procedure to start up a sample composition. charged prior to start-up. Commonly, a batch column is - base composition. Other concerns Chemicals with high melting points may require the purity as the last of the product is stripped out. Columns are sometimes run in a semi-batch mode. A semi-batch column is started up in the same manner as a batch column, but after distillate withdrawal has started, to be wrapped with heat tape and/or insulation. - Chemicals that tend to polymerize may require the addi- ous feed or batch-wise. tion of inhibitor at the top of the column. It is important to The start-up and operating procedures for a continuous this will affect the inhibitor concentration. Also, polymeriza- and therefore need to be carefully considered. Start-up of a continuous column is similar to that of batch column in within the column that are not exposed to inhibited liquid. onto most or all of the surfaces. desired operating conditions. Once a continuous column is operating at its control setpoints, a period of time will be required to reach steady glass columns. Metal columns may require sight glasses or rate, and analyses of the distillate and tails streams are used special cameras. to determine when the column has reached steady state. Column operation Leak testing. After a distillation column has been assem- - separation are the number of stages, feed location, feed rate, pump from the column and determining the rate of pressure and pressure. based on an estimate of the reboiler, column, condenser, and in series, the column operations need to be coordinated and implementations of a process, recycle streams may be neces- Commissioning. Before running authentic materials, Closing thoughts columns are often commissioned to ensure that they gener- - ally operate correctly, and possibly also to determine reboiler column capacity. Usually water, a minimally hazardous our capability to accurately simulate separations by distilla- pilot-scale experimental distillation testing.

46 aiche.org/cep November 2020 CEP It is reasonable to expect that the capabilities of process are also expected to continue to grow. Many cases remain in which distillation experiments are necessary and the need for experimental testing is expected to continue for the foreseeable future. CEP

Literature Cited 1. Graham, G. K., et al., “Experimental Validation of Column Simulations,” Chemical Engineering, 125 2. Graham, G. K., et al., “Experimental Methods to Verify Distilla- tion Simulations,” Chemical Engineering, 125 3. Krell, E., 4. Oldershaw, C., “Perforated Plate Columns for Analytical Batch Distillations,” Industrial and Engineering Chemistry Analytical Edition, 13 5. Kister, H. Z., 6. Nunley, R., 7. Stichlmair, J. G., and J. R. Fair, “Distillation: Principles and 8. Shinskey, F. G.,

GLENN GRAHAM is a distillation subject matter expert and senior chemical engineer at MATRIC (Mid-Atlantic Technology, Research, and Innovation Center, P.O. Box 8396, South Charleston, WV 25303; Phone: 304-552-6554; Email: [email protected]; Website: www.matricinnovates.com). Previously, he worked for Union Carbide Corp. and the Dow Chemical Co. as a distillation specialist in their R&D separations groups. He has extensive experi- ence in the conceptual design of separation processes, design and operation of laboratory- and pilot-scale distillation equipment, and chemical process modeling. Graham holds BS and MS degrees in chemical engineering from Montana State Univ.

DON BUNNING, P.E., is a senior chemical engineer at MATRIC (Phone: 304-720-1049; Email: [email protected]). He has extensive experience in process development, new catalyst develop- ment, demonstration of new technologies in pilot units, commer- cialization, on-site startup support, and technology licensing. His more than 50 years of experience in the chemical industry includes positions at Union Carbide and Dow Chemical. He has served as technology manager for acrolein and derivatives, acrylic acid, acrylic esters, vinyl acetate, and glycol ethers. Since joining MATRIC, he has been heavily involved in process development and the design and construction of pilot plants. He has a BS in chemical engineering from the Univ. of Illinois and an MS in chemical engineering from West Virginia Univ. He is also a licensed Professional Engineer.

JEREMY RADER, P.E., is the pilot plant supervisor at MATRIC (Phone: 304-720-1058; Email: [email protected]). He has worked his entire career at MATRIC and has been involved in several di!erent pilot plant technologies involving distillation and various other separation technologies. Rader’s areas of specializa- tion include pilot plant construction, pilot plant processing, control systems, and implementation of distillation control philosophies. He has a BS in chemical engineering from West Virginia Univ. Institute of Technology. He also holds a professional engineering license in the state of West Virginia.

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