How to Design Overhead Condensing Systems

T h e state of  in pipelines, pipelines, a n d t he physical arrangement of  equiprnent around th e distillation tower, establish th e design for Ineeting Ineeting hydraulic a n d piping conditions o f  the th e system. parameters for

  Robert Kern, Hoffmann

I

,

Gravity-flow

systems of  dis an d thermal conditions in condensing systems tillation columns, are the reve revers rsee of  those in reboiler circuits. The inlet line to condensers can carry superheated or saturated vapor, or dispersed vapor-liquid mixtures. Fluid is cooled cooled in the exchanger, exchanger, an d partial or full condensation takes takes place. place. The condenser's condenser's outlet line ca n have stratified and dispersed two-phase flow, satu subcoolcd liquid. liquid. In addition, the th e rated liquid, or subcoolcd ing fluid can ca n be a mixture of two substances. Thus, this type of condensa tion offer offerss a wide rang e of classification of classification from a thermodynamic standpoint [1]. In contrast, contrast, saturated saturated liquid liquid normally normally flow flowss in the Th e liquid is vaporized while downcomer of  reboilcrs. The passing through the exchanger. exchanger. The reboiler' reboiler'ss outlet line carries a dispersed vapor-liquid mixture having a vapor content of  30 to of  total flow flow.. (See Part 9 1975, of  this series for more details, Chern. Eng., Aug. 4, 1975, p. 107.) examine the th e hydraulics for In this article, we the following: 1. Condensers with gravity-flow return lines. Condensers with pumped-reflux pumped-reflux lines lines.. 2. Condensers 3. Two-stage condensation. these groups, groups, hydraulic-design hydraulic-design an d plpmg Within these th e state of  fluid design conditions vary, depending on the th e lines an d the physical th e instal in the physical arrangement of  the lation. Vacuum technology has its own systems,equipment systems,equipment an d terminology. Piping design of  vacuum-condensing systems are outside the Euler-Bernoulli-Darcy theories an d are no t included in this article. "For biography of  author,

 Eng., Aug.

Roche Inc.* .,

Th e state of  fluid in the pipelines, an d the th e hydraulic o The

I

 La

ar ar    Horizontal condensers-A condenser in rangements is located above the th e level of th of  thee terminating point of  the condenser's outlet line, as shown in F I I  12.. For the horizontal condenser in F lIb, vapor and an d F 12 th e exchanger at the top, an d subcooled liquid enters the leaves at th e bottom. The looped-outlet pipe ensures ensures a permanent liquid level in the condenser. This liquid through gh the reflux branch an d level is controlled throu through the takeoff line to storage. The Th e static-head pressure difference, between th e th e condenser's outlet line vertical vertical overhead line line and an d the for the arrangements in F II  can be written as: (1)

must be equal to or greater than the sum of  (1) the th e pipe-syste pipe-system m resistan resistance, ce, between between refere reference nce points A an d B; (a) exchanger pressure drop, an d (3) required pressure difference across the th e control valve, (2)

Th e required distance, H, between between fractionator fractionator inlet an d exchanger centerline can be calculated from Eg. (1) as: (3) an b e neglected by assuming Th e vapor column c an 0 in Eq. (1) (1) and an d (3). (3). All pressures pressures are in psi; P2 densities, P, in Ib/ft 3 ; and an d dimensions, H, in ft. As Eq. (3) shows, for a minimum of  elevation differ th e exchanger ence between the top to p of  the column an d the centerline, centerline, the piping an d components resistances must also be minimal.

=

1975. p. 113.

129 CHEMICAL ENGINEERING SEPTEMBER 15. 1975

Vapor

,

1

Drain

I

)

Horizontal condensers with gravity-flow reflux

Generally, in condensing systems, the unit loss in the piping is low-about a tenth or a hundredth of  1 psi/lOO ft. Inlet an d outlet resistances to process equip ment usually take a considerable portion of  th e pipeline resistance an d should no t be ignored in the calculations. (Using three decimal places in the calculations is not unusual.) In horizontal condensers, condensation takes place in the sheIl. This gives lower resistance than th e tubeside. A baffle (or baffles) in the exchanger is in the horizontal plane through the exchanger's centerline. If  necessary, two inlet an d two nozzles can halv.e the total flow, an d reduce entrance an d exit resistances considerably. In this case, th e inlet an d outlet piping should be sym metrical. Th e subcooled liquid for the arrangement in F /1 b can be drained or pumped directly to storage. Th e F /la is usually directed through a product stream cooler before storage.

Vertical vapors on

130 CHEMICAL ENGINEERING

reflux condense or

of 

_----------------------------------,

Seal loops prevent a

flow of vapor in gravity-flow condensing systems

leg (Zz dimension in F /3), th e piping design should be such as to prevent siphoning that can empty the seal. If dimension ZI is smaller than Z2'  an d th e pressure  just before th e seal loop an d at th e terminating point after th e seal loop is identical (for example, with greatly reduced flow), liquid ca n be siphoned ou t of  th e seal; an d intermittently, th e condenser will no t operate well. This can be prevented if  ZI is designed to be longer than Zz (see F /3a). Fo r th e arrangement in F /3b, th e final vertical leg has a larger diameter than th e gravity-flow reflux line. Again, this ca n prevent th e siphoning of  liquid from th e seal loop. Another arrangement (F /3c) ha s a closed vent line. This ca n be opened at reduced condensate flow to keep th e seal loop filled with liquid. With this type of  vent ing, th e pressure difference across the vent valve should be zero. Therefore, it is essential to connect th e end points of  the vent line to locations where pressures ar e expected to be about equal.

Control valves in svstems should be located at a low point of  the line an d product stream. Sufficient static head the valve inlet will prevent vaporization across A product cooler should no t receive a liquid-vapor mixture. Vertical condensers-Arrangements for these condensers with gravity-flow are shown in F /2. Conden sation ca n take in the shell (F /2a), or tubes (F /2b). A single-pass vertical condenser is more suitable for liquid subcooling than a horizontal one. Th e seal loop height ca n be adjusted within a greater range than with horizontal condensers (F/2c). Th e required liquid level in th e exchanger shell is determined by the ex changer's designer. Th e hydraulic balance for the shown in F /2 is: (1/144)(H  I PI 

+

(1/144)H 3P3

+

(4)

exchanger, here is th e sum of  piping, control-valve (if any), resistances:

=

+

+

(5)

Pumped-reflux arrangements

Th e elevation difference, as expressed from Eq. (4), between th e condenser's outlet an d th e reflux inlet noz zle will be:  HI 

 H 2P2

+

Typical overhead lines for hydrocarbon distillation columns are shown in F /4. Fluid circulation in the piping is th e result of  th e thermosiphon effect in grav ity-reflux condensation. Fo r th e systems shown in F /4, there is (and most of  the time must be ) a pressure difference between th e to p of  th e tower an d reflux drum. Th e reflux drum ha s a pump, which returns th e liquid to th e to p of  th e tower or sends it to storage. In _these arrangements (besides the sum of  the static heads), actual pressure differences, also enter into th e calculations:

(6)

where PI  is th e density of  condensate in the reflux line, P3 is the vapor density in the overhead line, an d P2 is th e average density in the vertical exchanger:

(7)

Seal loop _preve_nts flow reversal

(8)

In gravity-flow condensing systems, a seal loop is -provided t o prevent a flow of  vapor in the condenser's outlet line. This loop ca n be used for hold ing a liquid in the condenser, as shown in F /la an d F/2c. If  th e gravity-flow reflux line terminates in a vertical

Fo r th e dimensions given in F /5, the static-head difference will be: (9) where

PI  is

usually vapor density, and

131  jj. 197j

P2

is vapor-liquid

I!

I

Froth flow

PI

LI

Elevated condensers and details of  reflux drum

sure of  th e overhead vapor line' can be neglected. Consider: P1H 1 = o.

Expressing H 2 from Eq. (12) in feet, we get:  H 2

(13)

In layout design, usually th e reflux drum is elevated first in accordance with the required NPSH (net posi tive suction head) of  th e reflux pump. Th e dimensions shown in Detail A of  F /5 will establish the condenser elevation from grade.

3

Slug flow is undesirable

. Liquid and vapor velocities

Slug flow ca n develop in th e pocketed condenser outlet line shown in F /5, depending on vapor-liquid proportion an d fluid velocity. Slug flow should be avoided because it ca n cause undesirable pressure surges. An empirical relation can be used to estimate th e slug-flow region. If th e velocity (calculate d with two phase density) in the pipeline is smaller than (5Pl/  P v )1/2, slug flow is possible. Th e type of  flow ca n also

type of  flow

133 CHEMICAL

SEPTEMBER IS, 1975