OVERVIEW OF DISTILLATION
Distillation is simply defined as a process in which a liquid or vapor mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat. The process is based on the fact that the vapor of a boiling mixture will be richer in the components that have lower boiling points. Hence, when this vapor is cooled and condensed, the condensate will contain more volatile components. At the same time, the original mixture will contain more of the less volatile material.
The primary equipment employed in the process of distillation are distillation columns, which are designed to achieve this separation efficiently. Although the layman has a fair idea as to what “distillation” means, the important aspects that seem to be missed from the manufacturing point of view are: (1) distillation is the most common separation technique; (2) it consumes enormous amounts of energy, both in terms of cooling and heating requirements; (3) it can contribute to more than 50% of plant operating costs. The best way to reduce operating costs of existing units, is to improve their efficiency and operation via process optimization and control. To achieve this improvement, a thorough understanding of distillation principles and how distillation systems are designed is essential.
As stated, distillation is the process of heating a liquid until some of its ingredients pass into the vapor phase, and then cooling the vapor to recover it in liquid form by condensation. The main purpose of distillation is to separate a mixture by taking advantage of different substances’ readiness to become a vapor. If the difference in boiling points between two substances is great, complete separation may be easily accomplished by a single-stage distillation. If the boiling points differ only slightly, many redistillations may be required. In the simplest mixture of two mutually soluble liquids with similar chemical structures, the readiness to vaporize of each is undisturbed by the presence of the other. The boiling point of a 50-50 mixture, for example, would be halfway between the boiling points of the pure substances, and the degree of separation achieved by a single distillation would depend only on each substance’s readiness to vaporize at this temperature. This simple law was first stated by 19th- century by the French chemist Frangois Marie Raoult (known as Raoult ’s law).
The term “still” is applied only to the vessel in which liquids are boiled during distillation, but the term is sometimes applied to the entire apparatus, including the fractionating column, the condenser, and the receiver in which the distillate is collected. If a water and alcohol distillate is returned from the condenser and made to drip down through a long column onto a series of plates, and if the vapor, as it rises to the condenser, is made to bubble through this liquid at each plate, the vapor and liquid will interact so that some of the water in the vapor condenses and some of the alcohol in the liquid vaporizes. The interaction at each plate is equivalent to a redistillation. This process is referred to by several names in the industry; namely rectification, fractionation, or fractional distillation.
If two insoluble liquids are heated, each is unaffected by the presence of the other and vaporizes to an extent determined only by its own nature. Such a mixture always boils at a temperature lower than is true for either substance alone. This effect may be applied to substances that would be damaged by overheating if distilled in the usual fashion. Substances can also be distilled at temperatures below their normal boiling points by partially evacuating the still. The greater the vacuum, the lower the distillation temperature.
Basic Components of Distillation Columns
There are a variety of configurations for distillation columns, each designed to perform specific types of separations. A simplified way of classifying distillation columns is to look at how they are operated. In this manner, the two major types are batch and continuous columns. In a batch operation, the feed to the column is introduced batch-wise. That is, the column is charged with a ‘batch’ and then the distillation process is conducted. When the desired separation is achieved, a next batch of feed is introduced. In contrast, continuous columns process a continuous feed stream. No interruptions occur unless there is a problem or upsets with the column or surrounding process units. They are capable of handling high throughputs and are the most common of the two types. The following discussions focus on continuous columns.
Continuous columns can be further classified according to: (1) the nature of the feed that they are processing (binary column - feed contains only two components, and multi-component column - feed contains more than two components); (2) the number of product streams they have (multi - product column - column has more than two product streams); (3) where the extra feed exits when it is used to help with the separation (extractive distillation – where the extra feed appears in the bottom product stream , and azeotropic distillation - where the extra feed appears at the top product stream ); (4) the type of column internals (tray column - where trays of various designs are used to hold up the liquid to provide better contact between vapor and liquid, and hence achieve better separation, and the packed column - where instead of trays, packings are employed to effect contact between vapor and liquid).
There are several important components in a distillation column, each of which is used either to transfer heat energy or enhance mass transfer. The major components in a typical distillation are: a vertical shell where the separation of liquid components is carried out, column internals such as tray plates and/or packings which are used to enhance component separations, a reboiler to provide the necessary vaporization for the distillation process, a condenser to cool and condense the vapor leaving the top of the column, a reflux drum to hold the condensed vapor from the top of the column. The liquid (reflux) is recycled back to the column.
The column internals are housed within a vertical shell, and together with the condenser and reboiler, constitute a distillation column. A schematic of a typical distillation unit with a single feed and two product streams is shown in Figure 1. The liquid mixture that is to be processed is called the feed. The feed is introduced usually somewhere near the middle of the column to a tray known as the feed tray.
The feed tray divides the column into a top (enriching or rectification) section and a bottom (stripping) section. The feed flows down the column where it is collected at the bottom in the reboiler. Heat is supplied to the reboiler to generate vapor. The source of heat input can be any suitable fluid, although in most chemical plants this is normally steam. In refineries, the heating source may be the output streams of other columns. The vapor raised in the reboiler is re-introduced into the unit at the bottom of the column. The liquid removed from the reboiler is known as the bottoms product or simply, the bottoms. Refer to Figure 2 for a simplified view.
The vapor travels up the column, and as it exits the top of the unit, it is cooled by a condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is recycled back to the top of the column and this is called the reflux. The condensed liquid that is removed from the system is known as the distillate or top product. Refer to Figure 3 for a simplified view.
The terms "trays" and "plates" are used interchangeably. There are many types of tray designs, but the most common ones are bubble cap trays, valve trays, and sieve trays. A bubble cap tray has a riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapor. Vapor rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray. Bubble-tray or plate towers typically consist of a number of shallow plates or trays over each of which the liquid flows in turn on its way down the tower. The gas enters at the bottom of the tower and is made to flow through a number of bubble caps on each plate. These caps may be of various shapes, though they usually take the form of inverted cups, and their edges are slotted so that the gas escapes from them into the liquid in the form of bubbles.
The layout of a typical plate is illustrated Figure 4. The illustration shows the arrangement of bubble caps on a plate together with the risers through which the gas enters the bubble caps and the downcomers which carry the liquid from plate to plate. The inlet weir assists in distributing the liquid over the plate, while the outlet weir maintains the desired depth of liquid. Bubble-plate towers may be preferred to packed towers when: (a) the liquid rate is so low that a packed tower could not be used effectively since the packing would not be adequately wetted; (b) when a difficult distillation duty is required; (c) there is a risk that solid matter may be deposited. Bubble-plate towers, which can be fitted with manholes, are more easily cleaned than packed tower configurations.
In valve trays, perforations are covered by liftable caps. Vapor flows lifts the caps, thus self creating a flow area for the passage of vapor. The lifting cap directs the vapor to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays.
Sieve trays are simply metal plates with holes in them. Vapor passes straight upward through the liquid on the plate. The arrangement, number and size of the holes are design parameters. Because of their efficiency, wide operating range, ease of maintenance and cost factors, sieve and valve trays have replaced the once highly thought of bubble cap trays in many applications.
The Bow of liquid and vapor through a tray column is complex. Liquid falls through the downcomers by gravity from one tray to the one below it (refer to Figure 5). A weir on the tray ensures that there is always some liquid (holdup) on the tray and is designed such that the holdup is at a suitable height, e.g., such that the bubble caps are covered by liquid. The vapor flows up the column and is forced to pass through the liquid, via the openings on each tray. The area allowed for the passage of vapor on each tray is called the active tray area. The hotter vapor flows through the liquid on the tray above, and transfers heat to the liquid. During this process some of the vapor condenses adding to the liquid on the tray. The condensate, however, is richer in the less volatile components that is in the vapor.
In additionally, because of the heat input from the vapor, the liquid on the tray boils, generating more vapor. This vapor, which moves up to the next tray in the column, is richer in the more volatile components. This continuous and intimate contacting between vapor and liquid occurs on each tray in the column and brings about the separation between low boiling point components and those with higher boiling points. In essence, a tray serves as a mini-column, with each one contributing its share to the overall separation. As such, the more trays there are in a column, the better the degree of separation. Hence, the overall separation efficiency depends significantly on the design of the tray. Trays are designed to maximize vapor-liquid contacting, and hence focus is given to the extent of liquid distribution and vapor distribution achieved by the design.
The more intimate the contacting between vapor and liquid, the better the separation that each tray achieves. This means that fewer trays will be needed in order to achieve the same degree of separation. This will result in lower construction costs and energy consumption. Trays alone do not always provide the intimate contact sought. As such, tray designs are sometimes assisted by the addition of packing configurations. Packings are simply passive objects that are designed to increase the interfacial area available for vapor-liquid contacting. Figure 6 illustrates some common geometries of inert packing materials often used in distillation applications. Their role is simply to provide additional surface contact between the vapor and liquid in the column, and to do so without introducing excessive pressure drop across the column.
High pressure drop means that more energy is needed to drive the vapor up through a distillation column, and as such there would be higher operating costs. Another very important reason why inert packing materials are considered is in debottlenecking a column. A tray column that is facing throughput problems can be debottlenecked by replacing a section of trays with packing. The packing will provide additional interfacial contact area for the liquid-vapor contact, thereby increasing the efficiency of the separation for the same column height. In addition, packed columns tend to be shorter than tray-type columns. The packed column is often referred to as a continuous-contact column, whereas a trayed column is called a staged-contact column because of the manner in which the vapor and liquid come into contact.
The function of reboilers has already been discussed. These components are essentially heat exchangers that are used to transfer heat to bring the liquid at the bottom of the column to its boiling point (refer also to discussions in Chapter 1). The principle types employed are jacketted kettles, simple kettle type reboilers, internal reboilers, and thermo-syphon reboilers. Examples of each type are illustrated in Figure 7.
The process of distillation is aimed at the separation of components from a liquid mixture. This process depends on the differences in boiling points of the individual components. Also, depending on the concentrations of the components present, the liquid mixture will have different boiling point characteristics. This means that distillation processes depends on the vapor pressure characteristics of liquid mixtures.
The vapor pressure of a liquid at a particular temperature is the equilibrium pressure exerted by molecules leaving and entering the liquid surface. Some general concepts to recognize about regarding vapor pressure are first, energy input raises the vapor pressure. Also, vapor pressure is related to boiling. A liquid boils when its vapor pressure equals the surrounding pressure. The ease with which a liquid boils depends on its volatility. Liquids with high vapor pressures (i.e., volatile liquids) will boil at lower temperatures. We should also recognize that the vapor pressure and hence the boiling point of a liquid mixture depends on the relative amounts of the components in the mixture. Distillation is accomplished because of the differences in the volatility of the components in a liquid mixture.
It is the boiling point diagram that provides an understanding of the process. The boiling point diagram shows how the equilibrium compositions of the components in a liquid mixture vary with temperature at a fixed pressure. Consider an example of a liquid mixture containing 2 components: A and B. Figure 8 shows the boiling point diagram for this binary mixture. The boiling point of A is that at which the mole fraction of A is unity. The boiling point of B is that at which the mole fraction of A is zero. In this example, A is the more volatile component and therefore has a lower boiling point than B. The upper curve in the diagram is called the dew-point curve while the lower one is called the bubble-point curve. The region above the dew-point curve shows the equilibrium composition of the superheated vapor while the region below the bubble-point curve shows the equilibrium composition of the subcooled liquid. For example, when a subcooled liquid with mole fraction of A=0.4 (point A) is heated, its concentration remains constant until it reaches the bubble-point (point B), when it starts to boil. The vapors evolved during the boiling has the equilibrium composition given by point C in Figure 8, approximately 0.8 mole fraction A. This is approximately 50% richer in A than the original liquid.
This difference between liquid and vapor compositions is the basis for any distillation process. A term of great importance is relative volatility. Relative volatility is a measure of the differences in volatility between two components, and hence their boiling points. It indicates how easy or difficult a particular separation will be. The relative volatility of component ‘i‘ with, respect to component “j” is defined by the following relationship:
where yi is the mole fraction of component “i” in the vapor, and xi is the mole fraction of component “i” in the liquid. We can conclude that if the relative volatility between two components is very close to one, it is an indication that they have very similar vapor pressure characteristics. This means that they have very similar boiling points and therefore, it will be relatively difficult to separate the two components by means of distillation.
Since the boiling point properties of the components in the mixture being separated are so critical to the distillation process, the vapor-liquid equilibrium (VLE) relationship is of importance. Specifically, it is the VLE data for a mixture which establishes the required height of a column for a desired degree of separation. Constant pressure VLE data is derived from boiling point diagrams, from which a VLE curve can be constructed; like the one illustrated in Figure 9 for a binary mixture. The VLE plot shown expresses the bubble-point and the dew-point of a binary mixture at constant pressure. The curve is called the equilibrium line, and it describes the compositions of the liquid and vapor in equilibrium at a constant pressure condition.
Figure 9 is the VLE plot for a binary mixture that has essentially a uniformequilibrium, and therefore represents a relatively easy separation. However, there are many cases where non-ideal separations are encountered. These more difficult distillations are defined by the examples shown in Figure 10. An important system in distillation is an azeotropic mixture. An azeotrope is a liquid mixture which when vaporized, produces the same composition as the liquid. The VLE plots illustrated in Figure 11 show two different azeotropic systems: one with a minimum boiling point and one with a maximum boiling point. In both plots, the equilibrium curves cross the diagonal lines.
These are azeotropic points where the azeotropes occur. In other words, azeotropic systems give rise to VLE plots where the equilibrium curves crosses the diagonals. Both plots are however, obtained from homogenous azeotropic systems. An azeotrope that contains one liquid phase in contact with vapor is called a homogenous azeotrope. A homogenous azeotrope cannot be separated by conventional distillation. However, vacuum distillation may be used as the lower pressures can shift the azeotropic point. Alternatively, an additional substance may added to shift the azeotropic point to a more favorable position. When this additional component appears in appreciable amounts at the top of the column, the operation is referred to as an azeotropic distillation. When the additional component appears mostly at the bottom of the column, the operation is called extractive distillation.
Another distinction describing a azeotropic system is illustrated in Figure 12. This plot describes the case of a heterogenous azeotrope. Heterogenous azeotropes can be identified by the flat portion on the equilibrium diagram. They may be separated in two distillation columns since these substances usually form two liquid phases with widely differing compositions. The phases may be separated using settling tanks under appropriate conditions.
The design of a distillation column is based on information derived from the VLE diagram describing the mixtures to be separated. The vapor-liquid equilibrium characteristics are indicated by the characteristic shapes of the equilibrium curves. This is what determines the number of stages, and hence the number of trays needed for a separation. Although column designs are often proprietary, the classical method of McCabe-Thiele for binary columns is instructive on the principles of design.
McCabe-Thiele is a graphical design that uses the VLE plot to determine the theoretical number of stages required to effect the separation of a binary mixture. It assumes constant molar overflow. This implies that the molal heats of vaporization of the components are roughly the same. In addition, it is assumed that heat effects (heats of solution, heat losses to and from column, etc.) are negligible, and that for every mole of vapor condensed, 1 mole of liquid is vaporized. The design procedure is as follows. Given the VLE diagram of the binary mixture, operating lines are drawn first. The operating lines define the mass balance relationships between the liquid and vapor phases in the column. There is one operating line for the bottom (stripping) section of the column, and one for the top (rectification or enriching) section of the column. Use of the constant molar overflow assumption also ensures that the operating lines are straight lines. The operating line for the rectification section is constructed as follows: First the desired top product composition is located on the VLE diagram, and a vertical line produced until it intersects the diagonal line that splits the VLE plot in half. A line with slope R/(R+ 1) is then drawn from this intersection point. Refer to Figure 13 for illustration of the procedure.
In Figure 13, R is defined as the ratio of reflux flow (L) to distillate flow (D), and is called the reflux ratio. The reflux ratio is a measure of how much of the material is going up through the top of the column and is returned back to the column as reflux. In a similar fashion, the operating line for the stripping section is constructed. In this case the starting point is the desired bottom product composition. A vertical line is drawn from this point to the diagonal line, and a line of slope Ls/Vs is drawn as illustrated in Figure 14. In this figure, Ls is defined as the liquid rate flowing down the stripping section of the column, while Vs is the vapor rate of flow up the stripping section of the column. The slope of the operating line for the stripping section is a ratio between the liquid and vapor flows in that part of the column . The McCabe-Thiele method assumes that the liquid on a tray and the vapor above it are in equilibrium. How this is related to the VLE plot and the operating lines is depicted graphically in Figure 15. Figure 15 shows a magnified section of the operating line for the stripping section in relation to the corresponding nth stage in the column.
The L's are the liquid flows while the V's represent the vapor flows. Parameters x and y denote liquid and vapor compositions and the subscripts denote the origin of the flows or compositions. The denotation n – 1 refers to material from the stage below stage n, while n + 1 refers to material from the stage above stage n. The liquid in stage n and the vapor above it are in equilibrium, therefore, x, and yn lie on the equilibrium line. Since the vapor is carried to the tray above without changing composition, this is depicted as a horizontal line on the VLE plot. Its intersection with the operating line provides the composition of the liquid on tray n + 1 , as the operating line defies the material balance on the trays. The composition of the vapor above the n +1 tray is obtained from the intersection of the vertical line from this point to the equilibrium line. By repeatedly applying the graphical construction technique, a number of corner sections are created, with each section being equivalent to a stage of the distillation. This is the basis of sizing distillation columns using the McCabe-Thiele graphical design method.
From the operating lines for both stripping and rectification sections, the graphical construction described above is further illustrated in Figure 16, which shows that 7 theoretical stages are required to achieve a theoretical separation. The required number of trays (as opposed to stages) is one less than the number of stages since the graphical construction includes the contribution of the reboiler in completing the separation. The actual number of trays required is equal to the ratio of the number of theoretical trays to the tray efficiency. Typical values for tray efficiency ranges from 0.5 to 0.7. Tray efficiency depends on such factors as the type of trays being used, and internal liquid and vapor flow conditions. Sometimes, additional trays are added (up to 10%) to accommodate the possibility that the column may be under-designed. Figure 16 also helps to illustrate that the binary feed should be introduced at the 4th stage. However, if the feed composition is such that it does not coincide with the intersection of the operating lines, this means that the feed is not a saturated liquid. The condition of the feed can be deduced by the slope of the feed line or so-called q-line. The q-line is that line drawn between the intersection of the operating lines, and where the feed composition lies on the diagonal line.
The state of the feed established the slope of the feed line. For examples: saturated vapor exists for q = 0; q = 1 for saturated liquid; for a mix of liquid and vapor, 0 ≤ q ≤ 1; for a subcooled liquid q ≤ 1; and for a superheated vapor, q ≥ 0.
From information the feed mixture conditions, the q-line can be constructed and applied in the McCabe-Thiele design. However, excluding the equilibrium line, only two other pairs of lines can be used in the McCabe-Thiele procedure. These are the feed-line and rectification section operating lines, the feed-line and stripping section operating lines, and the stripping and rectification operating lines. The reason for this being that these pairs of lines determine the third.
Determining the number of stages required for the desired degree of separation and the location of the feed tray is only the first step in generating an overall distillation column design. Other factors that need to be considered are tray spacings; column diameter: internal configurations; heating and cooling duties. All of these can lead to conflicting design parameters and trade-offs. Thus, distillation column design is often an iterative procedure. If the conflicts are not resolved at the design stage, then the column will not perform well in practice.
The state of the feed mixture and feed composition not only affects the operating lines and hence the number of stages required for separation, but also the location of the feed tray. During operation, if the deviations from design specifications are excessive, then the column may no longer be able handle the separation task. To overcome the problems associated with the feed, some column are designed to have multiple feed points when the feed is expected to contain varying amounts of components. It is important to note that as the reflux ratio increases, the gradient of operating line for the rectification section moves towards a maximum value of unity. Physically, what this means is that more and more liquid that is rich in the more volatile components are being recycled back into the column. Separation then improves and thus less trays are required to achieve the same degree of separation. Minimum trays are required under total reflux conditions, i.e. there is no withdrawal of distillate. On the other hand, as the reflux decreases, the operating line for the rectification section moves towards the equilibrium line. The 'pinch' between operating and equilibrium lines becomes more pronounced and more and more trays are needed. The McCabe-Thiele method easily verifies this.
The limiting condition occurs at minimum reflux ration, when an infinite number of trays will be required to effect separation. Most columns are designed to operate between 1.2 to 1.5 times the minimum reflux ratio because this is approximately the region of minimum operating costs (more reflux means higher reboiler duty).
A critical consideration in the design of a distillation column is the vapor flow condition. Improper conditions such as foaming, entrainment, weeping/dumping, and flooding can cause significant inefficiencies in the separation. These conditions are often avoided based upon experienced operating design criteria established. Foaming refers to the expansion of liquid due to passage of vapor, or gas. Although it provides high interfacial liquid-vapor contact, excessive foaming often leads to liquid buildup on trays. In some cases, foaming may be so excessive that the foam mixes with liquid on the tray above. Whether foaming will occur depends primarily on the physical properties of the liquid mixtures, but is sometimes due to tray designs and conditions. Whatever the cause, separation efficiency is always reduced. Entrainment refers to the liquid carried by vapor up to the tray above and is again caused by high vapor flow rates. It is detrimental because tray efficiency is reduced: lower volatile material is carried to a plate holding liquid of higher volatility. It could also contaminate high purity distillate. Excessive entrainment can lead to flooding. Weeping is a phenomenon caused by low vapor flow. The pressure exerted by the vapor is insufficient to hold up the liquid on the tray.
Therefore, liquid starts to leak through perforations on the tray. Excessive weeping will lead to dumping. That is the liquid on all trays will crash (dump) through to the base of the column (via a domino effect) and the column will have to be re-started. Weeping is indicated by a sharp pressure drop in the column and reduced separation efficiency. Flooding occurs due to excessive vapor flow, causing liquid to be entrained in the vapor up the column. The increased pressure from excessive vapor also backs up the liquid in the downcomer, causing an increase in liquid holdup on the plate above. Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by sharp increases in column differential pressure and significant decrease in separation efficiency.
Many of the above factors that affect column operation are due to vapor flow conditions being either excessive or too low. Vapor flow velocity is dependent on column diameter. Weeping determines the minimum vapor flow required while flooding determines the maximum vapor flow allowed, hence column capacity. Thus, if the column diameter is not sized properly, the column will not perform well. Not only will operational problems occur, the desired separation duties may not be achieved.
The actual number of trays needed for a particular separation duty depends on the efficiency of the plate, and the packings if they are used. Thus, any factors that cause a decrease in tray efficiency will also change the performance of the column. Tray efficiencies are affected by such factors as fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus the proper materials of construction must be selected for tray construction.
A final consideration is weather conditions. Most distillation columns are open to the atmosphere. Although many of the columns are insulated, varying weather conditions can affect column operation. As such, the reboiler must be appropriately sized to ensure that enough vapor can be generated during cold and windy spells and that it can be turned down sufficiently during hot seasons. The same guideline applies to condensors. Other factors to consider include changing operating conditions and throughputs, brought about by changes in upstream conditions and changes in the demand for the products. These factors, including the associated control system, should be considered at the design stages because once a column is built and installed, nothing much can be done to rectify the situation without incurring additional significant costs.
With the above as an elementary background to the subject of distillation, we will turn our attention to refinery operations and the equipment typically used. Before doing so, a discussion of the properties of hydrocarbons is provided.