Ballast Tray Design Manual Bulletin No 4900
Distillation is the most often used means to separate two or more components, exploiting the physical properties of different boiling points. Separation by distillation is completed in stages with chemical equilibrium at each point in the process to build a composition profile.
Ballast Tray Design Manual Bulletin No 49001. Glitsch, Inc. GUtsch Ballast Tray Design Manual, 5th Ed. Ballast Tray Design Manual, Bulletin No.
The products are taken out at the peak of the composition, usually at the top and bottom of a tower, with intermediate levels of composition between the extremes. Separating components to increase value The main reason systems are designed to separate components is that separated components have more value than the mixture of components. The raw material of hydrocarbons is a mixture, as in crude oil or natural gas condensate. Chemical intermediates and reaction products will have a mix of byproducts or unreacted components. Separating these byproducts or components permits efficient recycle or use of products as purified substances.
Another reason for separating components is to prepare a feed for further processing. An example is removing lights and heavies from an isomerization unit feed. While separations are used in virtually all chemical process systems, distillation is the most prominent. It requires a large consumption of energy and capital expenditure, but it remains the most widely used separation technique because it is well-understood and proven in a wide spectrum of applications. 1 Many academics have developed heuristics—a hands-on interactive approach to learning that enables a person to learn something for themselves—for designing separation systems, and others have proposed algorithms for finding the perfect method to design a separation system. These methods are useful for training purposes, but often get lost or overlooked within the urgent schedule of producing a process design. 2,3 Most designs are created with a series of practical constraints, such as:.
Project schedule. Available plot space. Flexibility to handle feedstock variations. Easy understanding and control of operations.
With this in mind, some simplified ideas are proposed here that build upon previous work, but which may be more practical and useful to today’s designers. Maintain component separation It seems obvious to follow the basic principle, “Once components are separated, do not remix them again.” However, numerous commercial operations do, in fact, have remixing embedded in the process.
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One classic example was a C 8 aromatics processing plant that used super-fractionation to separate the ethylbenzene and p-xylene (α EB:pX = 1.06). The feed to this unit originated from three types of processes:. Hydrogenated pyrolysis gasoline comprising approximately 60% EB. Catalytic reformate comprising approximately 18% EB.
Aromatics transalkylation comprising 1%–2% EB. Key components are remixed in a series of two major EB fractionators, with a total number of 600 trays to remove the ethylbenzene.
The source material also had benzene/toluene (B/T) fractions, combined in upstream units and operating in parallel to remove the BT cut. The material then proceeded to a series of two major EB fractionators with a total number of 600 trays to remove the ethylbenzene. The first system of three towers in series concentrated the stream to approximately 50% EB purity. The final two towers in series produced an EB product of 99.3% purity ( FIG.
Upon examination of the fundamental separations taking place in the unit, the stream value was downgraded by the upstream mixing, as the most difficult components to separate were equalized by combining the streams containing different EB content. These streams were separated again at substantial cost. The remedy was to simply segregate the streams from an upstream fractionation train, permitting complete shutdown of one of the major super-fractionation towers ( FIG. Segregating the streams from an upstream fractionation train avoids remixing. In older plants, process designs are often created out of expediency and morphed into an inefficient configuration after the original construction. Evaluating complex systems helps eliminate instances of separation that lead to remixing. Complete easy separations first One of the classic heuristics advocated by most scholars, this advice follows common sense.
4 It is natural to follow the path of least resistance. In a scientific view of multi-component distillation, the “easy first” approach is described by a pre-fractionation system followed by a main fractionation system. The pre-fractionation system removes the “clutter,” placing the extraneous components into proper zones better suited for broad separations; meanwhile, the main fractionation system will separate the more difficult components with the closest relative volatility ( FIG. A prefactionator arrangement places the extraneous components into proper zones more suited for broad separations.
The conventional approach to the three-component separation is to carry out the separations in a series operation using two fractionation columns. The obscure downside to this approach is the “back-mixing” of components that takes place in the bottom of the first column ( FIG. The middle boiling product is concentrated at an intermediate point within the column, only to be downgraded to a lower concentration at the bottom of the column. Therefore, components that were previously separated have been recombined. A convenient way to circumvent this thermodynamic inefficiency and follow the concept of prefractionation is to use a dividing wall within the column to separate the vessel into different functional zones ( FIG. A Dividing wall column (DWC) distillation is not new to the field of chemical engineering; it traces its roots to 1935.
However, DWC technology has failed to achieve popularity due to a lack of understanding about what is happening inside the column, difficulty in simulating the designs and general inertia in adopting new technologies. With better understanding, faster computer simulators and robust mechanical designs, this hesitancy is gradually changing. Consider functional separation. Back-mixing in the bottom of the first column is one of the downsides of separations in a series operation. Many applications can benefit from DWC technology and alternative types of thermal coupling. The principle of a DW to create separation zones presents other options for operations inside a column.
Consider the case of liquefied petroleum gas (LPG) recovery from refinery fuel gas. The two conventional approaches to remove C 3 (or C 3/C 4) from a mixture of C 1 to C 5+ components are distillation with refrigeration, and absorption into a heavy oil followed by stripping of the gases from the absorbing material. Refrigerated distillation has an additional cost for the refrigeration duty, high-pressure operation and expensive equipment. Absorption followed by stripping requires two columns to make the separation. The absorption section also builds a concentration peak of the intermediate component, only to downgrade the concentration that was achieved by mixing with the heaviest components (in violation of “no remixing”). This process has introduced a new method for making the absorption and stripping in the same column by using a dividing wall.
A DW within the column is used to separate the vessel into different functional zones.a In this case, the wall separates the top of the column into two zones: absorption and stripping. The column operates at moderate pressure, with cooling water as the cold utility. The absorption oil captures the C 3+ components and allows the C 2 components to escape as a vapor without condensing. The C 3 (or C 3/C 4) components are brought to the bottom of the wall near a peak in composition, where they are passed to the stripping section of the column on the other side of the wall for removal into the overhead as a condensed liquid product. Ideally, sufficient heavy components in the feed stream that can be recycled to the top of the absorption zone are available so that no external feed material is used.
As shown in FIG. 6, this method is applied to LPG recovery. B The general concept of absorption and stripping in the same column with a TDW can be applied where a non-condensable stream is mixed with a heavy oil product, such as a hydroprocessing stabilizer. C Different variants of the concept can be used in revamp situations of existing two-column systems. Functional separation extended Continuing with the concept of keeping components separated, certain applications exist where two streams are combined into a common fractionation system out of convenience to separate some light and heavy components.
These grouped fractions may not have the same composition and would be more valuable if kept segregated. Rather than have two separate fractionation systems, a top dividing wall column (TDWC) can be used to segregate the streams in the same column into two overhead products with a common bottom product. C 3 (or C 3/C 4) components are passed to the stripping part of the column on the other side of the wall for removal into the overhead as a condensed liquid product.
This method is applied to LPG recovery. B One example is xylene fractionation within a paraxylene complex. If the paraxylene recovery section is based on selective adsorption, then a strict requirement exists to keep the C 9+ components out of the C 8 fraction. Multiple feeds enter into a common xylene tower to accomplish the C 9+ removal. However, the C 8 fractions will have different levels of EB or pX comingled together inside the tower, only to be separated again in the adsorption section of the plant at a higher cost. If the xylene column was designed with a TDW, then the streams with low EB (or high pX, for example) could be fed to a different entry point in the adsorption system to reduce the separation energy and debottleneck that part of the plant.
Still another application of functional utility within a distillation column is to exploit the potential for heat recovery at a higher temperature in multi-component distillation. Typically, a three-cut distillation tower using a conventional side-draw or DW with side-draw will have the lightest component at the top of the column. The latent heat from condensing duty will be made at the equilibrium temperature of this component at column pressure. Often, this pressure is too low for recovering useful heat. The side-draw will have only sensible heat to transfer at the intermediate temperature. Using a TDWC, the intermediate cut can also be taken as a distilled product, with latent heat available at a higher temperature. One strategy using a TDWC is to intentionally skew the temperature profile, such that the intermediate cut can also be taken as a distilled product, with latent heat available at a higher temperature ( FIG.
Here, the feed side will distill the top product at low overhead temperature, while the intermediate cut will be taken out as overhead from the opposite side of the wall at a higher temperature. The enthalpy from the mid-fraction can be used to preheat the main column feed, or to reboil a separate distillation system to save overall energy. Back to the basics Stepping back again into history, the basic principles of distillation column design must always be followed, even with advanced schemes.
These principles include:. Optimization of feed location—This avoids pinch in the stripping vs.
Rectification zones. Balancing the NTS vs R:D ratio—Rules of thumb, such as the recommended R/D being 1.2 × minimum R/D, depend on capital costs vs.
Long-term energy costs and could change over time. Manufacturing—Despite the most clever process design, manufacturing flaws or installation errors can completely negate the benefits of the design. Trusting the party controlling the entirety of the process design, hydraulic design, manufacturing and installation is recommended.
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Tray spacing and valve type—Some of the earliest types of contacting devices were bubble cap trays, which were prominent in the 1940s and 1950s. In the 1960s, trays with moveable valves came into service, affording higher capacity and fouling resistance. Installation—Improved modern tray designs have many different features, such as optimized active area and downcomer area, flow promotion, elimination of liquid gradient and bubble promotion. The trays may be installed at closer spacing, and there may also be DWs. High-performance trays require more care during inspection and installation.
Rating methods—Given the preponderance of computer rating methods for trays and packings, it is prudent to utilize such information. One of the classic rating methods, and perhaps the best overall tool for tray design, is an equation (known as Equation 13) from the Ballast Tray Design Manual Bulletin 4900. 5 This model was developed by Francis W. Winn, who spent years in technical management at Fractionation Research Inc.
Before he worked for Fritz W. Glitsch & Sons in the 1960s (Eq. (1) Winn combined the classic entrainment flooding “C-factor” of Souders-Brown with tray spacing and liquid load variables. Details of the Souders-Brown entrainment model are thoroughly discussed by Henry Z. 6 Equation 13 is the most reliable flood correlation among the classics. Recently, Resetarits and Ogundeji tweaked Equation 13 for a slight improvement. 7 Their conclusion completely supports the fundamentals of the original F.
Glitsch model. Findings Fractional distillation is a common-sense approach to separate chemical components. Much can be learned from the experience of industry predecessors. Many of the original design principles apply, as the basic mass transfer operations do not change.
New techniques include:. The concept of an “outside-in” approach to purifying an intermediate cut in multi-component distillation, removing the clutter of light-most and heavy-most components first, to reach high-purity intermediate products by pre-fractionation. DWCs to accomplish the pre-fractionation within the same column, and advanced thermal coupling systems to move closer to ideal efficiency. Extensions of segregated columns to retain the value of compositional profiles and recover usable heat from within the distillation system.
These new techniques are based on sound engineering principles and will undoubtedly become a core part of the technology base. HP NOTES a GT-DWC and GT-TDWC, GTC Technology US LLC (patent pending). B GT-LPG MAX, GTC Technology US LLC (patent pending). C GT-Advanced Stabilizers, GTC Technology US LLC (patent pending). LITERATURE CITED.
Nadgir, V. Liu, “Studies in chemical process design and synthesis: Part V,” AIChE Journal, Vol. M., Chemical process equipment: Selection and design, Butterworth-Heinemann, 1990.
Rousseau, R. W., Handbook of separation process technology, Georgia Institute of Technology, Wiley-Interscience, New York, New York, 1987. Koch-Glitsch Inc., Ballast tray design manual bulletin 4900, 6th Ed., 1993. Z., Distillation Design, McGraw-Hill Education—Europe, New York, New York, 1992. Resetarits, M. Ogundeji, Paper No. 3a, Distillation Symposium, AIChE Spring Meeting, April 2009.
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GTC Technology US LLC, Houston, Texas Joseph C. Gentry, is vice president of technology, R&D and engineering, for GTC Technology US LLC. He previously worked for ARCO Chemical Co. And Lyondell Petrochemical Co. In the olefins and aromatics areas. Gentry earned a BS degree in chemical engineering from Auburn University and an MBA from the University of Houston.
He is the inventor of several patented separations technologies and has specialized in their applications for the petrochemical industry. GTC Process Equipment Technology, Euless, Texas Michael J. Binkley is a consultant for the GTC Process Equipment Technology (PET) group. He is a registered professional engineer in Texas. Binkley has focused his 45 years of experience in mass transfer/separations equipment development and applications with Glitsch Inc. (1969–2001) and GTC (2002–2016).
His first seven years of process engineering were with the PPG Chemicals Division in Lake Charles, Louisiana. Binkley is an inventor of several separations equipment advancement-related patents, as well as numerous product trademarks. With several GTC co-inventors, he has seven patent applications pending review by the US Patent and Trademark Office. Binkley earned his BS degree in chemical engineering from Texas Tech University. Related Articles.