Refluxing Condensation Systems (Dephlegmators)

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Process Engineering Guide: GBHE-PEG-HEA-516

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Process Engineering Guide: Refluxing Condensation Systems (Dephlegmator)

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 BACKGROUND 2 5 DESIGN CALCULATIONS 10 5.1 Flooding 10 5.2 Estimation of Thermal Performance 12 5.3 Estimation of Pressure Drop 13 6 NON-CONDENSING GASES 13

7 RECOMMENDED AREAS OF APPLICATION 13 7.1 Distillation Columns 13 7.2 Knock-back Condensers 14 7.3 Boiling Coolants 14 7.4 Economic Considerations 14



8 ADVICE ON INSTALLATION 14 8.1 Vertical ’U’-Tube Exchangers 14 9 NOMENCLATURE 15

10 BIBLIOGRAPHY 16 FIGURES 1 DIFFERENTIAL AND INTEGRAL FLASHES 3 2 VERTICAL TUBESIDE REFLUX CONDENSER 5 3 VERTICAL ‘U’-TUBE SHELLSIDE REFLUX CONDENSER 18 4 'STAB-IN' HORIZONTAL BUNDLE REFLUX CONDENSER 7 5 CROSS FLOW SPIRAL HEAT EXCHANGER CONDENSER 8 6 ALFA-LAVAL TYPE G SPIRAL CONDENSER 9 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 16



0 INTRODUCTION/PURPOSE Most condensers are designed such that the vapor and condensate flow in a co-current fashion. There is a class of condenser, known variously as ’Reflux condenser’, ’Dephlegmator’ or ’Knock-back condenser’ in which the condensate flows in a counter-current fashion to the vapor. This design offers certain advantages, but poses some design problems. 1 SCOPE This Process Engineering Guide discusses the design of condensers in which the condensate flows by gravity in the opposite direction to the vapor. It covers estimation of the heat transfer and pressure drop as well as how to estimate whether the condenser is liable to flood. Duties for which the design is appropriate are discussed. 2 FIELD OF APPLICATION This Guide is intended for process engineers and plant operating personnel in the GBH Enterprises worldwide, who may be involved in the design or operation of refluxing condensers. 3 DEFINITIONS No specific definitions apply to this Guide. 4 BACKGROUND Reflux condensers are usually proposed either for distillation columns, or to be mounted on reactors in which there is a boiling solvent which it is required to return to the reactor. Their principal advantage is that they eliminate the need for vapor pipework, reflux drums and pumps, as they are mounted directly onto the vapor generating item. They also act as fractionating devices, the vapor becoming progressively richer in the more volatile components as it passes up the condenser. If used as the partial condenser of a distillation column, this can be an advantage; one or more separation stages can be achieved.



However, if total condensation of a multi-component mixture is required, the fractionating tendency can be a real disadvantage. In the extreme case, it may prove impossible to condense totally a mixture which would condense in a normal co-current flow system. This can be understood with reference to Figure 1. If a vapor is cooled from point A, it initially starts to condense at the dew point B, at a temperature of Td. The composition of the initial condensate corresponds to point D. If the vapor and condensate flow in a co-current fashion in intimate contact, they will remain approximately in equilibrium. As the stream is cooled further, the vapor composition follows the line BF, while the condensate follows DC. When the temperature reaches the integral bubble point, Tbi, all the vapor has condensed and the liquid is at point C. This process is known as ’integral condensation’. If the condensate is continuously removed from contact with the vapor, the vapor then follows the line BFG, not completely condensing until the differential bubble point, Tbd , is reached. The liquid follows the line DE, where E, the point of total condensation, has the same composition as the starting vapor at A. This process is known as ’differential condensation’. (This is an idealized model; in practice it is not possible to remove all the condensate as it forms, and the degree of fractionation indicated will not be achieved.) The processes in a refluxing condenser are in many ways similar to the differential condensation model. However, partial mixing in the condensate film and interactions between the phases complicate the process. FIGURE 1 DIFFERENTIAL AND INTEGRAL FLASHES



4 Four types of reflux condenser geometry are common: (a) Vertical tubeside, (see Figure 2). This design is attractive if exotic

materials are necessary for the process fluid, as the shell can be fabricated from carbon steel. Design methods are most established for this geometry.

(b) Vertical ’U’-tube shellside, (see Figure 3). Units can be unbaffled as

shown; it may be desirable to add baffles to enhance the performance or to prevent tube vibration. Either conventional segmental or rod baffles can be used. Design methods for predicting flooding are uncertain. It is not possible to drain the fluid from the tubeside.

(c) ’Stab-in’ horizontal bundle, (see Figure 4). This design is only suitable for

columns of a reasonable diameter. Tubeside drainage is not a problem. The use of low fin tubing to enhance the heat transfer is possible. Design methods for avoiding flooding are uncertain.



(d) Cross flow spiral heat exchanger, (see Figure 5). (Note that Alfa-Laval also offer their type G spiral condenser, which although mounted directly onto a distillation column, has a central pipe up which the vapor flows before passing back down the spirals in co-current flow with the condensate, (see Figure 6)). This latter type is not a reflux condenser; it can be treated as a conventional down-flow condenser.

FIGURE 2 VERTICAL TUBESIDE REFLUX CONDENSER



FIGURE 3 VERTICAL ‘U’-TUBE SHELLSIDE REFLUX CONDENSER



FIGURE 4 ’STAB-IN’ HORIZONTAL BUNDLE REFLUX CONDENSER



FIGURE 5 CROSS FLOW SPIRAL HEAT EXCHANGER REFLUX CONDENSER

Note: The exchanger may be mounted directly onto a distillation column, whereupon the lower end plate and nozzle may be omitted.



FIGURE 6 ALFA-LAVAL TYPE G SPIRAL CONDENSER



5 DESIGN CALCULATIONS The presence of counter-current condensate and vapor flow presents certain problems when undertaking the thermal design of a reflux condenser. These can be divided into: (a) The prevention of flooding. (b) Estimation of the heat transfer performance. 5.1 Flooding If a liquid film is flowing down the walls of a vertical tube and a vapor is flowing upwards, the interfacial shear will result in a thickening of the film. As the vapor flow is increased, further thickening of the film takes place, and waves are induced on the surface. Finally, a point will be reached where some of the liquid will be carried back up the tube by the vapor. This phenomenon is known as flooding, and represents the limits of operation of a vertical tubeside reflux condenser. Flooding is a complex phenomenon, and there are considerable uncertainties in data and disparities between the various correlations.



5.1.1 Tubeside Flooding HTFS, in the Handbook sheet TM11 (Ref. [1]), recommend the correlation due to Alekseev et al. Calculate a Froude number:

. Calculate the flooding gas superficial velocity:

In order to allow for uncertainties, HTFS recommend a maximum design gas superficial velocity of 0.625 vgf . .The flooding velocity can be increased, and hence the tendency to flooding reduced, either by inclining the condenser by 10-20° to the vertical, which may not be practical, or by cutting the tube ends at an oblique angle (see Figure 2). However, the exact magnitude of the increases due to these changes cannot be predicted at present. It is recommended that obliquely cut tubes be used to provide an additional safety factor above the flooding velocity as calculated above.



5.1.2 Flooding In Other Geometries There are no published methods known to the author for predicting flooding in shellside condensers or spiral condensers. Tentatively, the Alekseev correlation could be used in conjunction with a hydraulic mean diameter. For a spiral exchanger, the hydraulic diameter is twice the separation between the plates, 2a. For longitudinal (unbaffled) flow on the shellside of an exchanger with triangular pitch it is:

For baffled shellside flow, no recommendations can be made. An alternative approach to the flooding problem for 'U'-tube condensers, which has been successfully used in the NE, is to check that the vapor velocities at all points in the bundle never exceed the terminal velocity of the condensate drops falling onto the collector pan. The terminal velocity of a droplet of diameter d is given by:

Droplets will be unstable and shatter if the Weber number (We) exceeds 10.

Combining these equations:

This gives the maximum velocity for drops to be collected. As for the tubeside flooding, a margin of safety should be placed on this figure; 75% is suggested.



5.2 Estimation Of Thermal Performance 5.2.1 Vapor-Liquid Equilibrium Calculations For normal condensers, with co-current flow of condensate and vapor, the usual design method for calculating the condensing coefficient is that of Silver which is described in Ref. [2]. This uses an integral condensation curve, where the heat load as a function of temperature is calculated assuming that the vapor and condensate are in thermal and compositional equilibrium along the exchanger. Such a curve can be readily calculated, for example by using the programs/Tasks option in the “VAULT”, as at any point along the exchanger the total composition is known. This is not the case for a reflux condenser, because of the fractionating effects; the condensate at the bottom of the condenser is the total of that which has been condensed higher up. It is not possible to determine what the liquid and vapor compositions will be at any point. HTFS (Ref. [3]) recommend the use of a differential condensation curve, in which it is assumed that the vapor is in equilibrium with the local newly forming condensate. This is equivalent to assuming there is no mixing of the condensate, and corresponds to the curves BG and DE in Figure 1. At present, there is no easy way of calculating differential condensation curves; it is possible that an option will be added to the “VAULT” for the automatic generation of the curves at some future date, but there are no immediate plans. The data can be generated with the assistance of the “VAULT”, but only with considerable user intervention. The calculation of the differential condensation curve gives some indication of the fractionation effects. However, in practice the degree of separation achieved is likely to be less than indicated by this method, due to a combination of gas phase resistance effects and liquid phase mixing. For a single condensable component with inerts (non-condensables), the vapor composition depends only on the temperature. For this case, the integral condensation curve could be used. However, this will over-estimate the heat load for a reflux condenser, as it assumes that the condensate is cooled to the vapor temperature; this is a safe assumption for design. For a pure component, ignoring the effects of pressure drop, the condensation is isothermal, and the differential and integral condensation curves are equivalent.



5.2.2 Heat Transfer Coefficients Having calculated the differential condensation curve, HTFS recommend the use of the Silver method (Ref. [2]) for the gas phase thermal resistance. . However, beware of the effects of shear on the liquid film coefficient; see below. 5.2.3 Estimation Of Condensate Film Coefficient For vertical down-flow condensation, the effects of gas shear on the liquid film are to thin the film and increase its turbulence, both of which will increase the film coefficient. For a reflux condenser, on the other hand, vapor shear will thicken the liquid film, reducing the coefficient. This will to some extent be offset by the increased turbulence, but the overall effect cannot be readily determined. It is likely to be non-conservative to ignore the effects of vapor shear. The effects of shear are likely to be minor for vapor velocities less than half the flooding velocity, but above this could be significant. 5.3 Estimation of Pressure Drop There are no reliable methods for estimating pressure drop in the counter-current flow of liquids and vapors in tubes or other geometries. However, for designs well away from the flooding point, the pressure drop is likely to be low. The pressure drop will rise sharply as the flooding point is approached. 6 NON-CONDENSING GASES The in-tube reflux condenser may not be well suited to duties where there is a significant quantity of inert gas present, as the low velocities necessary to avoid flooding in the lower part of the tube will result in high gas phase resistances in the upper parts of the tube. A more appropriate design here could be the shellside condenser with a variable baffle pitch, but, as was pointed out above, there are no reliable flooding correlations for this geometry. However, even though the high resistance in the upper part of the exchanger results in a larger unit than is desired, it could still be financially attractive because of the large savings in pipework, reflux drum and pumps.



7 RECOMMENDED AREAS OF APPLICATION 7.1 Distillation Columns The cost saving and separation effects of a reflux condenser have been exploited to our advantage in, for example, the Amines and Alkyl Phenols areas, using internal ’U’-tube condensers with shellside condensation. Experience suggests that as a general rule, 2m of tube gives approximately one separation stage. Spiral exchangers are increasingly being used in the NE for design pressures below 10 bar. Both these designs lead to a compact plant without the vapor and liquid return pipework.

7.2 Knock-back Condensers Because of the fractionating effects, the use of a reflux condenser configuration to return condensed vapors to a reactor is not recommended for wide boiling range multi-component mixtures where it is desired to return all of the vapors as condensate to the reactor. 7.3 Boiling Coolants If the heat is to be removed using a boiling coolant, such as a refrigerant, the normal design of condenser in the process industries would be a kettle boiler, with the refrigerant in the horizontal shell, and condensing in the tubes. If a reflux condenser with tubeside condensation is required, the boiling refrigerant will be on the vertical shellside of the exchanger. Design methods for boiling on the outside of vertical tube bundles are less sure than for kettle shells. Points to consider are the avoidance of carry-over of liquid with the vapor, and whether baffling is required for tube support. Nevertheless, this arrangement can be used with care. 7.4 Economic Considerations The reflux configuration can save significant quantities of large diameter pipework, as well as saving on reflux pumps and drums. However, in order to avoid the flooding problem, units have to be designed with relatively low inlet velocities. This can lead to larger exchangers than would be necessary for a co-current design, especially where the vapor contains significant inerts (see above), or in vacuum duties where there is a large volume of vapor. If the inerts concentration is low, the exchanger size is almost independent of condensing side velocity.



8 ADVICE ON INSTALLATION 8.1 Vertical ’U’-Tube Exchangers (see Figure 3) (a) To avoid excessive pressure drop, the collector pan diameter should be

such that the annular area is not less than 40% of the column cross-sectional area.

(b) A distance of 0.5 - 1.0 m, depending on column diameter, should be left

between the ’nose’ of the condenser bundle and the collector pan to reduce the turbulence in the vapor as it decelerates from the annulus to the space above the collector pan.

(c) A wall wiper ring, fixed between the collector pan and the bundle should

be fitted to divert condensate flowing down the wall into the collector pan. If this is not fitted there is a risk that insufficient condensate will enter the collector pan to meet the product draw-off requirements, particularly at reflux ratios below 1:1.

(d) The product draw-off nozzle should be sized correctly to avoid sucking

vapor into the liquid off-take (see GBHE-PEG-FLO-301 - Overflows and Gravity Drainage Systems).

(e) If the inerts concentration is higher than 10% v/v, it may be advantageous

to make the bundle diameter less than the column diameter and reduce the column diameter approximately one third of the way up from the base of the ’U’-tube, to increase the vapor velocity and enhance the film coefficient. It may also be advantageous to fit baffles at the top of the exchanger to improve heat transfer further. Some baffles may be required for tube support in any case. Vessels Section should be consulted for advice. Baffles should be sloped 1.5 - 2.0° to the horizontal to assist drainage For vapors with low inerts concentrations at the inlet, the coefficient will not be significantly enhanced by the above, so the additional complications can be avoided.



9 NOMENCLATURE a Gap between plates in a spiral exchanger (m) Bo Bond number, defined in equation 2 Di Tube inside diameter (m) Do Tube outside diameter (m) d Droplet diameter (m) F Viscosity correction factor, defined in equation 3 Fr Froude number, defined in equation 1 g Gravitational acceleration (m/s2) Kg Kutateladze number, defined in equation 4 n Number of tubes in the exchanger p Tube pitch (m) Q l Liquid volumetric flow (m3/s) Tbd Bubble point for differential condensation °C Tbi Bubble point for integral condensation °C Td Dew point °C U crit Maximum gas velocity for drops to be collected (m/s) U t Terminal velocity of droplet (m/s) u Relative velocity between gas and drops (m/s) v gf Flooding superficial gas velocity (m/s) We Weber number, defined in equation 9 ml Liquid viscosity (Ns/m2) rg Gas density (kg/m3) rl Liquid density (kg/m3) s Surface tension (N/m)



10 BIBLIOGRAPHY Ref Source [1] HTFS Handbook sheet TM11. ’Correlation for flooding in vertical tubes’.

P B Whalley, April 1984. [2] HTFS Handbook sheet CM 15. ’Silver method for multi-component

condensation’. D Butterworth, August 1979. [3] HTFS Handbook sheet CP15. ’Guidelines for the design of reflux

condensers’. J M McNaught, February 1986. DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: PROCESS ENGINEERING GUIDES GBHE-PEG-FLO-301 Overflows and Gravity Drainage Systems (referred to

in Clause 8.1).

Refluxing Condensation Systems (Dephlegmators)

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