Location of Reboiler Return Inlet Nozzle in Column

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Location of ‘reboiler return inlet nozzle’ and its effect on Distillation Column

Reboilers are the heat exchangers used to provide stripping section vapors for Distillation columns. The location of reboiler return line nozzle to the column plays a key role in column performance. Here are probable combinations for the nozzle location with the corresponding effect on the column. A nozzle location suitable for one application may not be suitable for the other. Hence based on the column hydraulics, a proper selection has to be made for the nozzle location by the designer. Most of the Chemical Process Industries have Distillation as their major process. A distillation column can be classified into two sections –

Enriching Section: That part of the column which enriches the Lower Boiling Component

Stripping Section: The part of the column which strips out the Higher Boiling Component. The vapors required for the stripping section may be generated at the column bottom by either direct injection of steam or by the use of reboilers. The use of steam or reboiler depends on the components to be separated, degree of separation, heat loads, column design etc. The aim of reboilers is to provide the vapors required for the column. Depending on the use reboilers can be classified as Natural Circulation or Forced Circulation reboilers. The Natural Circulation reboiler is referred to as the “THERMOSIPHON REBOILER“. These reboilers have the tube side inlet as the column bottom liquid and the tube side outlet as a two phase flow of vapor and liquid (which is provided to the column bottom through the vapor inlet nozzle).

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The general assembly of a distillation column and thermo siphon reboiler is shown in figure 1.

C 1 2 3 4 4 HLL 5 4 7 NLL 6 8 LLL 11 12 9 13 10 14 Figure: 1

1 Bottom Tray 2 Downcomer 3 Seal Pot 4 Inlet Nozzle 5 High Level Indicator 6 Low Level Indicator 7 Level transmitter 8 Steam 9 Vertical Thermosiphon

Reboiler 10 Condensate 11 Outlet Nozzle (for

reboiler) 12 Outlet Nozzle (for pump)

13 Level Control Valve

14 Pump C Distillation Column

HLL High Liquid Level NLL Normal Liquid Level LLL Low Liquid Level



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This article mainly emphases on the location of the vapor inlet nozzle. Factors to be considered are as follows: Nature of inlet nozzle: The inlet nozzle may be welded “ straight “ to the column or may be welded at some suitable angle “ inclined “ to the column as shown in figure 2.

Figure: 2 Nature of entry into the column: The nozzle may enter the column in “non-tangential“ manner or in “tangential” manner. These are as shown in figure 3.

Figure : 3

Physical location on the column:

The nozzle could be located at the column bottom. The nozzle could be submerged in :

Normal Liquid Level Zone (NLL),

High Liquid Level Zone (HLL),

Low Liquid Level Zone (LLL).



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Based on These three factors several combinations are possible. Each of these combinations

and its possible effects on the column is discussed in brief. The exact selection of the

combination depends on column hydraulic, operating conditions etc.

There are 16 ways for the location of reboiler return line nozzle to the distillation column.

The designer has to select the best possible way based on column hydraulics, operating

conditions etc.

1) Straight inlet nozzle –Non-tangential entry –located in HLL zone: (Figure 4)

The nozzle is located just below the liquid surface in the HLL zone. The hydraulic head is less

in this region.

Effect : When the reboiler vapors enter the zone, slugs of liquid are formed. These slugs rise

up and can dash with the bottom plate. This may result in :

- Slugs of liquid and vapor dashing to the bottom plate to give rise to the “bumping

effect“.

- Dislocation of bottom tray,

- More liquid entrainment.

2) Straight inlet nozzle –Non-tangential entry –located in NLL zone: (Figure 5)

The nozzle is located just below the HLL zone. The hydraulic head is more as compared to

that in the HLL zone.

Effect: Here also the slugs of liquid and vapor are formed and they rise up due to density

difference. But owing to greater hydraulic head, these slugs “ break-up” disturbing the

liquid level in the HLL zone. This may result in :

- Turbulent HLL zone,

- Less liquid entrainment and hence less “bumping effect“.

- Fluctuating NLL zone.

3) Straight inlet nozzle –Non-tangential entry –located in NLL zone: (Figure 6)

The nozzle is located in the lowest liquid level zones of the column. Due to this it

experiences the largest hydraulic head as compared to the HLL and NLL zone.



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Effect: The slugs of liquid and vapor breaks and the possibility of reaching the liquid surface

is remote. This may result in:

- Turbulent NLL zone,

- Least chances of “bumping effect “,

- Short circuiting of vapors to the bottom pump suction line leading to ‘cavitation‘ in

pumps.

4) Inclined inlet nozzle – non-tangential entry – located in HLL zone: (Figure 7)

The nozzle is welded inclined to the column and is submerged in the HLL zone. The liquid

will rise in the nozzle up to the normal liquid level. The inlet line vapors have to overcome

the liquid head in the nozzle and column both.

Effect: The result of inclination in this zone will be:

- The inlet flow will impinge on the liquid surface,

- Due to the impingement, the liquid will dash to the walls of the column leading to

corrosion and erosion,

- Level transmitters may show erroneous values,

- Less possibility of short circuiting,

- Less chances of “bumping effect “.

5) Inclined inlet nozzle – non-tangential entry – located in NLL zone: (Figure 8)

The nozzle is welded inclined to the column and is submerged in the NLL zone. Due to the

liquid head in the nozzle, the slug breaks before reaching the liquid level.

Effect: The result of inclination in this zone may be:

- More chances of short circuiting and hence cavitation in bottom line,

- Fluctuating NLL zone,

- Rare chances of bumping effect.

6) Inclined inlet nozzle – non-tangential entry – located in LLL zone: (Figure 9)

The nozzle is welded inclined to the column and is submerged in the LLL zone.

Effect: The result of inclination in this zone may be:



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- Due to high liquid head, less slugs reach the column and this may exert backpressure

on the inlet line,

- Maximum chances of short-circuiting and hence cavitation,

- Liquid surface in the HLL is almost stable.

7) Straight inlet nozzle –tangential entry-located in HLL zone: (Figure 10)

The nozzle is welded in such a way so as to give a tangential entry to the column in HLL

zone.

Effect: The result of tangential entry in this zone may be:

- Swirling action to the liquid,

- Fluctuating liquid surface,

- Less chances of vortex penetrating the bottom discharge line,

- More chances of liquid entrainment to the bottom plate of column.

8) Straight inlet nozzle –tangential entry – located in NLL zone: (Figure 11)

The nozzle is welded in such a way so as to give a tangential entry to the column in NLL

zone.

Effect: The result of tangential entry in this zone may be:

- Comparatively less swirling action due to liquid head,

- Fluctuations in HLL and NLL zone,

- Less chances of liquid entrainment to the bottom plate of column,

- More chances of vortex penetrating in the bottom discharge line.

9) Straight inlet nozzle –tangential entry – located in LLL zone: (Figure 12)

The nozzle is welded in such a way so as to give a tangential entry to the column in LLL zone.

Effect : The result of tangential entry in this zone may be :

- Very less swirling action due to larger liquid head,

- Fluctuations in LLL and NLL zone,

- Liquid surface almost stable in the HLL zone,

- Least chances of liquid entrainment to the bottom plate of the column,

- More chances of vortex penetrating in the bottom discharge line.



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10) Inclined inlet nozzle-tangential entry-located in HLL zone: (Figure 13)

The nozzle is welded inclined to the column for tangential entry into the HLL zone.

Effect: The effect of inclined tangential entry may be:

- Due to the inclination of nozzle, the inlet stream impinges the zone but off centered

with respect to the column,

- Formation of swirls and vortex is reduced,

- Can lead to liquid entrainment at higher velocities.

11) Inclined inlet nozzle-tangential entry-located in NLL zone: (Figure 14)

The nozzle is welded inclined to the column for tangential entry into the NLL zone.


- Fluctuating NLL and HLL zone,

- More chances of short circuiting and hence cavitation.

12) Inclined inlet nozzle-tangential entry-located in LLL zone: (Figure 15)

The nozzle is welded inclined to the column for tangential entry into the LLL zone.


- Comparatively stable liquid level in the HLL zone,

- Maximum chances of short circuiting and cavitation.

As far as inlet nozzle location in the free zone between liquid surface and column bottom

plate is considered, the location must be decided with respect to the location of seal pot

and downcomer.

The combinations could be further extended as follows (assuming that proper

disengagement space is provided by the designer):

13) Straight inlet nozzle –non-tangential entry – normal to the seal and downcomer:

(Figure 16)

The nozzle location is perpendicular to the location of seal pot and downcomer.

Effect: This combination may result in:

- Liquid entrainment to bottom tray,

- Liquid dashing against the level sensors & instruments giving erroneous results,

- Premature downcomer flooding if feed is superheated.



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14) Straight inlet nozzle –non-tangential entry –parallel to the seal pot and downcomer:

(Figure 17)

The nozzle location is parallel to the location of seal pot and downcomer. This is the most

preferred combination.


- Erosion and Corrosion of the column due to direct impact,

- Prolonged erosion and corrosion may puncture the column.

Both of these effects can be minimized by shielding the column with an impingement plate.

15) Straight inlet nozzle – tangential entry –located in free space: (Figure 18)

The inlet nozzle gives the tangential entry for the feed.

Effect: This combination may results in:

- Swirling of the liquid at the surface,

- Vortex formation,

- Heavy liquid entrainment at the column and hence plate center.

These effects can be avoided by well design of vapor horn nozzle.

16) Inclined inlet nozzle – tangential entry –located in free space. (Figure 19)

The nozzle is welded inclined to the column and admits feed tangentially.


- Off-centered impingement of feed on the liquid surface,

- Erosion and Corrosion around the column periphery.



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Fig : 4 Fig : 5 Fig : 6 Fig : 7

Fig : 8 Fig : 9 Fig : 10 Fig : 11



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Fig : 12

Fig : 13 Fig : 14 Fig : 15

Fig : 16 Fig : 17 Fig : 18 Fig : 19

Conclusion: These are the possible combinations to be looked for by the designer before deciding the actual location of the inlet nozzle. For vertical thermosiphon reboiler the combination of

– straight inlet nozzle – non tangential entry

parallel to the seal pan and down-comer is most commonly used. But the selection also depends on the type of reboiler. However the ultimate selection lies with the designer to select the most optimum location.

*First published at “Chemical Engineering World” as a cover story, in October 2004.



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Location of Reboiler Return Inlet Nozzle in Column