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Understanding Stills: Columns

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  • Harry
    Almost everyone knows that a still has a boiler at one end, and a condenser at the other end. Joining these two together is a conduit, to transport fluids
    Message 1 of 2 , Jun 14, 2009
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      Almost everyone knows that a still has a boiler at one end, and a condenser at the other end.  Joining these two together is a conduit, to transport fluids from one end to the other for further processing.  This conduit is called a "riser" in a potstill, and a "Column" in a fractionating still.

      In potstills, the riser (if used) and the "lyne arm" (connector pipe), serve to transport vapours out of the boiler and into the next stage, which is usually a product condenser.  Sometimes the next stage is an intermediary stage known as a "thumper", which is really a second "plate" that affords another distillation.

      But in a fractioning still, the column serves a more complicated purpose.  Yes it transports vapours, but it also transports liquids, and separates and enriches the substances (and others)  many times over.  How is this achieved?  Read on...

      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.

      The flow 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. 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.

      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. 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.

      Packed beds provide far more surface contact area than do plates, given similar heights.  Thus packed bed columns of significantly shorter design can perform the same or better separation duties than the plate or tray columns.

      The secret to good separation and enrichment is the amount of contact surface between liquids and vapours.  The more surface there is, the better the mass transfer capability for a given height of column.

      This is the reason we choose packed bed internals over tray internals for our small hobby stills.  Increased performance, smaller size, lower construction and operation costs, mean the packed column is far superior to the plate column for small (below six inch diameter) stills.

      [Parts of this essay are extracts from the book "HANDBOOK OF CHEMICAL PROCESSING EQUIPMENT"
      by Nicholas P. Cheremisinoff, Ph.D.
      Copyright © 2000 by Butterworth-Heinemann
      ISBN 0-7506-7126-2 ]


      regards Harry

    • Harry
      Micro-Distillery Hardware - Condensers by Harry Jackson Condensation of vapour occurs in a variety of engineering applications. For example, when a
      Message 2 of 2 , Jul 9, 2009
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        Micro-Distillery Hardware - Condensers

        by Harry Jackson


        Condensation of vapour occurs in a variety of engineering applications. For example, when a vapour is cooled below its saturation temperature, or when a vapour-gas mixture is cooled below its dew point, homogeneous condensation occurs as a fog or cloud of microscopic droplets.

        Condensation also occurs when vapour comes in direct contact with a subcooled liquid, such as spraying a fine mist of subcooled liquid droplets into a vapour space, or injecting vapour bubbles into a pool of subcooled liquid. The most common type of condensation occurs when a cooled surface, at a temperature less than the local saturation temperature of the vapour, is placed in contact with the vapour.  Vapour molecules that strike this cooled surface may stick to it and condense into liquid. [1]

        The above description of condensation infers that some exchange of heat is taking place in this process.  Of course this is correct, and the equipment tused for such actions is called a "heat exchanger".

        To understand condensation and condensers in distilling, it is necessary to first understand exactly what a vapour is. A vapour is a visible suspension in the air of particles of some substance.  It is a substance in the gas phase at a temperature lower than its critical temperature.  This means that the vapour can be condensed to a liquid or to a solid by increasing its pressure, without reducing the temperature [2].  At first this may seem confusing, but read on...

        Latent Heat (enthalpy) is the "hidden" heat when a substance absorbs or releases heat without producing a change in the temperature of the substance, eg, during a change of state.[3]
        To bring a liquid mixture to boil, energy in the form of heat must be transferred into the mixture.  Once the liquid is at boiling point, still more heat must be applied to bring the now boiling mixture to a rapidly vapourizing condition (a "phase change").  Here is a graphic description of the process, showing the two phase changes that occur when taking a solid to liquid, and then to the gaseous state or phase:

        Heating Curves [3]

        A substance is heated at a uniform rate:

        • Temperature of the solid rises uniformly until the melting point is reached.

        • At the melting point heat is absorbed and used to melt the solid without any temperature change (latent heat of fusion), all the energy is going into weakening the intermolecular forces between the particles in the solid.

        • When all the solid has melted to a liquid, the temperature starts to increase uniformly again until the boiling point is reached.

        • At the boiling point heat is absorbed without any change in temperature (latent heat of vaporization), all the energy absorbed is being used to overcome the intermolecular forces between the particles in the liquid.

        • When all the liquid has been vaporized to gas the temperature will once again increase.

        Condensers are a specific form of heat exchanger. Condensers convert the alcohol vapours that were produced by heating the wash, into liquid form.  They do this conversion by removing an amount of heat called latent heat, from the vapours.  Note there's no mention of removing any more heat than necessary to achieve the phase change. If more heat is removed, then the condensate becomes subcooled.  In heat transfer terminology, this heat is often referred to as the Latent Heat of vapourization, but in this instance because we are talking about condensers and condensation, it would be more correct to call it the Latent Heat of Condensation.  The term vapourization implies putting heat in, whereas condensation is the reverse, i.e. taking heat out.  However both terms are acceptable and are used interchangeably.


        The Condensing Process

         The condensing process starts with heat transfer, because heat is transferred from the hot vapours, through a dividing barrier (usually a copper tube wall) and absorbed by a coolant fluid on the other side of the barrier.  Heat energy always flows directionally from the hotter to the cooler substance, and temperature will always try to equalise between adjacent items of different temperature..

        The heat transfer reduces the temperature of the vapours to the dew point and sometimes below (subcooling; depends on operator and/or processing efficiency).  Thus the vapour collapses back to a liquid, the primary product of the process.  The newly warmed coolant is continuously removed from the heat exchanger, making room for fresh coolant to absorb heat and continue the process.  The primary product, liquid ethanol is  recovered, but may be further split into two streams or fractions;  one for product and another to be sent back to the system for re-vapourization and re-condensation.  This second stream is known as Reflux, and this return process is called refluxing (similar concept to transistor amplification feedback in electronics).    This has the benefit of further increasing the strength and purity of the primary product in a single processing run.

        During condensation, the liquid collects in one of two ways, depending on whether it wets the cold surface or not. If the liquid condensate wets the surface, a continuous film will collect, and this is referred to as filmwise condensation. If the liquid does not wet the surface, it will form into numerous discrete droplets, referred to as dropwise condensation. All surface condensers today are designed to operate in the filmwise mode, since long-term dropwise conditions have not been successfully sustained. [1]

        Dropwise condensation is a complex phenomenon that has been studied for over sixty years. It involves a series of randomly occurring subprocesses as droplets grow, coalesce, and depart from a cold surface. The sequence of these subprocesses forms a dynamic "life cycle." The cycle begins with the formation of microscopic droplets that grow very rapidly due to condensation of vapor on them and merge with neighboring droplets. Therefore, they are constantly shifting in position. As a result, rapid surface temperature fluctuations under these droplets occur. This active growth and coalescence continues until larger drops are formed. Although inactive due to condensation, these drops continue to grow due to coalescence with neighboring smaller droplets. Eventually, these large, so-called "dead" drops merge to form a drop that is large enough so that adhesive forces due to surface tension are overcome either by gravity or vapor shear. This very large drop then departs from the surface, sweeping away all condensate in its path, allowing fresh microscopic droplets to begin to grow again and start another cycle. [1]




        Handbook of heat transfer / editors, W.M. Rohsenow, J.P. Hartnett, Y.I. Cho. m 3rd ed.
        Ch.14 "CONDENSATION" by P. J. Marto, Department of Mechanical Engineering, Naval Postgraduate School, Monterey, California











        regards Harry

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