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DISTILLATION COLUMN: TYPES AND DESIGNING CONSIDERATION
Several distillation columns are designed to accomplish certain separations and vary in complication.
1 TYPES OF DISTILLATION COLUMNS
1.1 BATCH COLUMN
The feed to the column is added batch by batch in this process. After charging the column with a “batch,” the distillation process begins. The next batch of feed is supplied once the required process goal is accomplished.
1.2 CONTINUOUS COLUMN
Continuous columns handle a constant feed stream. There are no interruptions unless there is an issue with the column or the associated process units that result in high throughput.
Continuous columns are further divided based on the type of feed:
Binary column – the feed has only two components
Multi-component column – the feed has more than two components
Multi-component columns are most often utilized in oil refineries.
2 DISTILLATION COLUMN MAIN COMPONENTS
Distillation columns consist of various components, each utilized to transfer heat effectively or improve material transfer. There are various important components in a typical distillation column:
A vertical shell for separating liquid components
Internals of the column, such as plates/trays and/or packings to improve separation of components
A reboiler to generate vapors
A condenser for condensing vapor at the top of the column
A reflux drum for collecting condensed vapor and transferring liquid (reflux) back into the column
The column’s internals are contained in the vertical shell, which, together with the reboiler and condenser, forms a distillation column. The diagram below represents the typical distillation unit with a single input stream and two output streams.
3 TERMINOLOGIES AND IMPORTANT OPERATION
The “feed” is the liquid mixture usually fed to a tray called “feed tray” in the middle of the column. The feed tray separates the column into sections, the bottom (stripping) and the top (rectification). The feed descends the column till it reaches the reboiler bottom, in which it is collected.
Heat is provided to the reboiler to produce vapor. Any appropriate fluid can be used as a heat source; however, steam is frequently used in process industries. In process industries, the output of a column can be utilized as the heating source for the other column to get maximum energy utilization and reduce energy waste. The vapors produced in the reboiler are resent into the unit at the bottom of the column. The liquid extracted from the reboiler is the bottom product or simply bottoms.
A condenser cools the vapors that go up the column as it leaves the top of the unit. The condensed liquid is collected in a “reflux drum.” Some fluid is recirculated to the column top, referred to as “reflux.” The “distillate,” or top product, is the condensed liquid that comes out from the column.
4 DISTILLATION COLUMN INTERNAL COMPONENTS
The important part of the distillation column internal is the trays that contribute significantly to the performance of the column. Trays are made of different designs and associated complexities, linked with the feed needed to be distillate, purity requirements, processing, and capital cost. Following are some of the important types of trays for the distillation column
4.1 BUBBLE CAP TRAYS
A riser is placed over each hole in a bubble cap and covered with a cap. The riser and cap are spaced apart to allow vapor to flow. The cap’s vapor rises are directed downward before bubbling through the tray’s contents.
4.2 SIEVE TRAYS
Sieve trays are like metal plates with holes drilled into them. Through the liquid on the plate, the vapor rises straight up. Design factors include the quantity, size and hole arrangement.
4.3 VALVE TRAYS
In valve trays, holes are covered by caps that can be lifted. Vapor flows raise the caps, producing a flow space for vapor to travel through. The lifting top sends the vapor horizontally into the liquid, allowing greater mixing than sieve trays.
4.4 DESIGN OF TRAYS AND LIQUID FLOW
The passage of vapor and liquid through a tray and a column is demonstrated in the figure below.
A downcomer is a pair of conduits on either side of each tray. Gravity causes the liquid to descend through the downcomers from one tray to the next.
A weir on the tray ensures some liquid (holdup) on the tray and that the holdup is at an appropriate height; for example, bubble caps are covered by liquid.
Because vapor is lighter than liquid, it raises the column and is pushed to pass through the liquid via the perforations in each tray. The “active tray area” refers to the space on each tray that allows vapor to travel through.
A tray functions as a small column, with each tray completing a portion of the separation operation. We may extrapolate from this that the more trays there are, the greater the degree of separation and that total separation efficiency is highly dependent on tray design. By studying the liquid and vapor distribution on the tray, trays are intended to optimize vapor-liquid interaction. Greater vapor-liquid interaction translates to better separation at each tray, which leads to improved column performance.
4.5 STRUCTURED PACKING
Structured packing refers to materials designed for distillation columns. Structured packings comprise thin corrugated metal plates or gauzes positioned so fluids run convoluted courses through the column, creating a large surface area for phase interaction.
Corrugated sheets of perforated embossing metal or wire gauze are used to make structured packaging. The result is a honeycomb structure with slanted flow channels that has a large surface area yet very low gas resistance to flow. These properties provide considerable performance improvements.
4.6 PACKINGS VS. TRAYS
By replacing a segment of trays with packings, a tray column with throughput issues can be de-bottlenecked. This is because:
Packings improve the interfacial area for liquid-vapor interaction;
Separation efficiency can be enhanced for the same column height; and
Packed columns are smaller than trayed columns.
Because of this difference, packed and tray columns are referred to as continuous-contact and staged-contact columns.
6 GENERAL DESIGN CONSIDERATIONS
The design of a distillation column is often divided into two parts: a mechanical design and a process design. The goal of the process design is to figure out the number of theoretical stages, column diameter, and tower height needed. On the other hand, the mechanical design concentrates on the internals of the tower and the arrangements and design of the heat exchanger. Several elements are required when developing a distillation column, including environmental and safety standards, column performance, design economics, and other criteria that may limit the processes.
The following are some general design guidelines to consider:
The most cost-effective way of separating liquids is distillation.
Relative volatility is the ratio of vapor pressures for ideal mixes (medium temperature, low pressure and non-polar.
The operating pressure of a tower is frequently governed by the temperature of the available cooling medium in the condenser or the maximum permitted reboiler temperature.
Sequencing in the distillation column:
Start with the simplest separation — fewer trays and reflux.
Remove components one by one as overhead products when relative volatility and feed concentrations do not fluctuate greatly.
When the concentration in the feed fluctuates greatly, the relative volatilities do not remove the components in the sequence of decreasing concentration in the feed.
The ideal reflux ratio in terms of cost is around 120% to 150% of the minimum reflux ratio.
A safety factor of at least 10% over the number of steps calculated using the best approach is recommended.
The reflux pumps should have at least a 25% safety factor during reflux operation.
Reflux drums are nearly always horizontally installed and intended for a 5-minute holdup at half the drum volume.
Due to wind load and foundation concerns, tower heights should be limited to 175 feet (53 meters).
A tower’s length/diameter ratio should be less than 30 and ideally less than 20.
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