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Column  Packings

       AceChemPack  High Performance Column Packings are well received around the world which got lots of praise from the petrochemical and chemical industry. AceChemPack  is the leading column packing manufacturer in the world that continues to develop new, advanced column packings. AceChemPack's column packings are available in all types of materials (ceramic, metal, plastic)  and are designed to meet any requirements of a column system or process designer, which has high performance, very low pressure drop, resistance to plugging, or extremely low bed height.

       AceChemPack  High Performance Column Packing combines the performance advantages of saddle and ring styles into one high performance packing. Its unique shape ensures low liquid holdup and low pressure drop. The external geometry prevents the column packings from interlocking or entangling, ensuring the randomness and optimum surface area within the packed bed, while the internal fingers, arches and vanes promote optimum interfacial gas/liquid contact with minimal drag or hold-up. Energy consumption is reduced, due to lower reflux ratios.

         Structured Column Packing is formed from vertical sheets of corrugated thin gauge with the angle of the corrugations reversed in adjacent sheets to form a very open honeycomb structure with inclined flow channels and a relatively high surface area. To simplify installation, it is delivered in preformed slabs or segments that are sized to fit through the vessel manways. A high surface area will only provide efficient mass transfer if it is effectively used to increase vapour liquid-contact and therefore a variety of surface enhancements are available to promote liquid spreading over the packing surface. The low resistance to vapour flow together with efficient use of available surface tends to give structured packings significant performance advantages over random packings in high vapour rate/low liquid rate systems. 
       AceChemPack  offers two performance-improving structured column packings: Corrugated Structured Packing, fabricated from ceramic, sheet metal or engineered plastics, and wire gauze packing.

         AceChemPack Metal Column Packing includes: metal  intalox saddles, metal super intalox saddles, Nutter ring, metal pall ring, metal Cascade mini rings, metal conjugate rings, metal rectangle saddles, metal  VSP (eight four inner radian ring).

 Applications of Column Packing:

Absorption and Stripping (where high capacity and numerous stages are required)
Distillation towers (from deep vacuum to const pressure)
Heat transfer (refinery fractionators and olefin plant quench columns)

 

Packed Column Design

     Designing a randomly packed column is a subtle blend of art and science.  Packed columns are most frequently used to remove contaminants from a gas stream (absorption).  However, packed columns can also be used to remove volatile components from a liquid stream by contacting it with an inert gas (stripping).  They are also used in distillation applications where the separation is particularly difficult due to close boiling components.  While we'll discuss all of these applications, we'll focus on absorption.  However, the design methods are similar for any of the scenarios.
     The first step in designing a packed tower is more science than art.  The equilibrium data between the contaminant and the solvent (or the distillation components) is needed for the analysis.  If tabulated data for your system is unavailable and the total amount of the contaminant is small (as it usually will be), Raoult's Law can be used to estimate the equilibrium data for absorption or stripping applications.  For distillation, equilibrium data can be predicted by selecting the appropriate thermodynamic model (see Choosing a Thermodynamic Model for Use in Simulation).  The operating line for the tower is constructed differently depending on whether you're dealing with distillation or absorption /stripping.  Since we're focusing on absorption, we'll use it as an example.  In absorption/stripping, the operating line is constructed differently depending on whether the contaminated stream can be considered "dilute" or if it must be treated as a concentrated stream.

   Usually, it is safe to treat the stream as dilute if the contaminant makes up less than 10 mole percent of the stream.  For streams that cannot be considered dilute, the mass transfer coefficients must be evaluated in terms of the gas and liquid flows.   Then, graphical evaluation of several integral relationships must be completed.   This type of evaluation is outside the scope of this article and a text should be consulted for solving these types of problems.  For this article, we will consider dilute streams which are more common for packed tower absorption and stripping.
     Dilute streams allow the column designer to assume constant mass transfer and the operating line can be constructed in terms of the simplified balance shown below:

L out  x out  + G out  y out = L in  x in  + G in   y in

This relation is used in the following manner:

Suppose you wish to remove acetone from a gas stream of 10,000 mol/h in a packed column.  The inlet gas contains 2.6 mole percent acetone and the outlet gas stream can contain no more than 0.5 mole percent acetone.  Assume a pure water stream enters the packed tower at a rate of 8,000 mol/h.

L out x out + G out y out = L in x in + G in y in
(8000) x out + (10000)(0.005) = (8000)(0)+(10000)(0.026)

x out = 0.02625

Just as in the McCabe-Thiele analysis of distillation, the equilibrium stages are stepped off between the two lines.  Note that for stripping, the operating line would be on the other side of the equilibrium line.
     Once the theoretical number of stages have been determined, you can proceed with the design of the column by following the three steps that we'll outline below.

     Specify the packing type and column dimensions for a column that will be used to remove chlorine from a gas stream using an organic solvent.  Assume the separation requires 20 theoretical stages.  The vapor flow is 7000 kg/h, the average vapor density is 4.8 kg/m3.  The liquid flow is 5000 kg/h, the average liquid density is 833 kg/m3.  The liquid's kinematic viscosity is 0.48 centistokes (4.8 x 10-7 m2/s)

STEP 1:  SELECTING A TYPE AND SIZE OF COLUMN PACKING
     This is where the art of designing packed columns begins.   Some people believe that there are stringent rules surrounding the choice between random and structured column packing.  You can think of random column packing as the type that comes in a sack and it is simply dumped into the column.  Structured column packing may come in bales or intricate designs that are stacked in specific patterns.  This is probably one of those areas of engineering where past experience in the application is the best guide.  Two "areas of choice" where structured column packing is used are in very low pressure drop applications and for increasing the capacity of an existing column.   Since we're considering a new design with no serious pressure drop constraint, we'll choose the more economical random column packing(for details see: ceramic column packings, metal column packings, plastic column packing).
     Below are charts showing both English and Metric unit packing factors.  The most common random packing types are shown here:


 

     Generally, the column diameter to column packing size ratio should be greater than 30 for Raschig rings, 15 for ceramic saddles, and 10 for rings or plastic saddles.  The geometry of your column packing will typically be a function of the needed surface area and/or allowable pressure drop.  If several column packings meet your requirements, you'll typically choose the least expensive so long as it has an acceptable operating life.  For our example, we'll choose Pall rings (plastic).   For columns over 24 inches in diameter, No. 2 or 2 inch packing should be examined first.  By looking at our flowrates, the chances of our column having a diameter of at least 24 inches are good, but we'll verify this later.  For now, we'll settle on 2 inch plastic Pall rings for our initial analysis.

STEP 2: DETERMINE THE COLUMN DIAMETER
     Most methods for determining the size of randomly packed towers are derived from the Sherwood correlation.  A design gas rate, G, can be determined with the help of the figure below which is based on correlation from the Sherwood equation:

     Each line on the graph is marked with an acceptable pressure drop in inches of water per foot of packing (numbers in parentheses are in mm of water per meter of packing).  Guidelines are as follows:

  • Moderate to high pressure distillation = 0.4 to 0.75 in water / ft packing
                                                            = 32 to 63 mm water / m packing

  • Vacuum Distillation = 0.1 to 0.2 in water / ft packing
                                  = 8 to 16 mm water / m packing

  • Absorbers and Strippers = 0.2 to 0.6 in water / ft packing
                                          = 16 to 48 mm water / m packing

These guidelines are designed around "flooding pressure drops" documented in literature.  In other words, for most cases, designing with these pressure drops should help you avoid flooding.  In the later stages of design, you may want to perform a thorough flooding calculation.  Perry's Chemical Engineers' Handbook covers this topic well.  Since we are designing an absorber, we will design for 42 mm water / m packing (you could design for a lower pressure drop, but the column will increase in diameter and most likely cost).  First, we'll evaluate the x-axis of the graph above:
(L/V)(vapor density/liquid density)0.5 = (5000/7000)(4.2/833)0.5 = 0.0507
Note that 4.2 kg/m3 was used for the vapor density.  The average vapor density was given as 4.8 kg/m3.  However, at the top of the column, the vapor will be less dense and at it's highest velocity.  This is what you should design for.  As a rule of thumb, I reduce the average vapor density by about 15% for design, however if you can get real data from a similar tower, certainly do so!   Reading the intersection of the 42 mm water/m packing line and 0.05 on the axis, we find a value of 1.5 for the y-axis.

    From the previous charts, we read a column packing factor of 24 for 2 inch plastic Pall rings.  All other information is know so we can solve for G as shown on the y-axis of the graph:

G = [1.5 [(4.2)(833-4.2)]/[(10.764)(24)(0.48)0.1]]0.5 = 4.66 kg/m2 s

Now, we solve for the column cross sectional area:

Ax = Vapor Flow / G = 7000 kg/h / [(4.66 kg/m2 s)(3600 s/hour)] = 0.42 m2

and the column diameter is calculated by:

Diameter = [Ax / (PI/4)]0.5 = [0.42/(PI/4)]0.5 = 0.73 m or 2.4 ft

So our assumption of at least a 24 in column diameter is accurate.   If it had not been accurate, G would be recalculated using a smaller packing which would also correspond to a larger column packing factor.

STEP 3: DETERMINE THE COLUMN HEIGHT
     Perhaps the most interesting step in designing a packed column is deciding how tall to build it.  You should first ask yourself "What stage of the design are we currently working on?"  If the design is preliminary, the general HETP (Height Equivalent to a Theoretical Plate) will work well.  If the design requires a higher degree of accuracy, please consulting the column packings manufacturer or a book entitled Distillation Design by Henry Kister (McGraw-Hill, ISBN 0-07-034909-6).  Distillation Design contains an exhaustive list of HETP values based on the components of the system and the type of packing used (Chapters 10 and 11).  As for preliminary estimates, the following HETP values should be used:

SETUP HETP expressed as ft (meters)
Method Packing Size (in)
Distillation 1.0 1.5 (0.46)
1.5 2.2 (0.67)
2.0 3.0 (0.91)
Vacuum Distillation 1.0 2.0 (0.67)
1.5 2.7 (0.82)
2.0 3.5 (1.06)
Absorption/Stripping All Sizes 6.0 (1.83)

To determine the height of the absorption tower in our example, we multiple the 20 theoretical stages by 6 ft or 1.83 m.  We estimate the height of the tower to be 120 ft or about 37 meters.

OTHER NOTES:
     While our example problem focused on absorption, packed towers are also widely used in distillation.  Perhaps the most popular of which is the well documented vacuum distillation of ethylbenzene and styrene in the Production of Styrene.  Distillation Design covers this application very well.  If you're seeking a qualified packing manufacturer to consult with, please contact usAceChemPack're very well respected in this field and our experience is unmatched. 

Related Topic:

Ceramic Column  packing (random column packing),    Plastic Column  packing ,     Metal Column  packing ,

Structured Column  Packing ,  Column Structure,   Pall Ring Random Column Packing,    Intalox Saddles

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