AIR POLLUTION CONTROL TECHNOLOGY HANDBOOK - CHAPTER 14 docx

Control of VOC and HAP by Condensation 14.1 INTRODUCTION Condensation of a vapor from an air stream can take place as a film of the condensed material on the wall of the condenser tube o

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Control of VOC and HAP by Condensation

14.1 INTRODUCTION

Condensation of a vapor from an air stream can take place as a film of the condensed material on the wall of the condenser tube or as a series of drops that form at various points on the surface Film-type condensation is the more common mechanism encountered in a condenser The film uniformly coats the surface, and the thickness

of the film increases with the extent of the surface In dropwise condensation, the surface is not uniformly covered The individual drops form and grow on the surface and tend to coalesce with neighboring drops Adhesion of the drops is then overcome

by gravitational forces, and the coalesced drops run off the surface Impurities in the vapor stream promote dropwise condensation which results in higher heat transfer coefficients Unfortunately there is not much information available on dropwise condensation Therefore, design methods are limited to the film-type case

Condensers are best applied for removal of VOC and HAP from emission streams when the concentration is greater than 5000 ppmv Removal efficiencies range from

50 to 90% The upper end of efficiencies are practically achievable for concentrations

in the range of 10,000 ppmv or greater With high concentrations of pollutant, condensers are frequently employed as preliminary air-pollution-control devices prior to other devices such as incinerators, absorbers, or adsorbers Flows up to 2000 scfm can be handled in condensers

In condensation, one or more volatile components of a vapor mixture are sepa-rated from the remaining vapors through saturation followed by a phase change The phase change from gas to liquid can be achieved by increasing the system pressure at a given temperature, or by lowering the temperature at a constant pressure The lower the normal boiling point, the more volatile the compound, the more difficult to condense, and the lower the temperature required for condensation Refrigeration must often be employed to obtain the low temperatures required for acceptable removal efficiencies

14.2 VOC CONDENSERS

The two most common types of condensers used are surface and contact condensers

In surface condensers, the coolant does not contact the gas stream Most surface condensers are the shell and tube type as shown in Figure 14.1 Shell and tube condensers circulate the coolant through tubes The VOCs condense on the outside

of the tubes within the shell Plate and frame type heat exchangers are also used as condensers in refrigerated systems In these condensers, the coolant and the vapor 14

9588ch14 frame Page 209 Wednesday, September 5, 2001 9:57 PM

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flow separately over thin plates In either design, the condensed vapor forms a film

on the cooled surface and drains away to a collection tank for storage, reuse, or disposal

In contrast to surface condensers where the coolant does not contact either the vapors or the condensate, contact condensers cool the volatile vapor stream by spraying either a liquid at ambient temperature or a chilled liquid directly into the gas stream Spent coolant containing the VOCs from contact condensers usually cannot be reused directly and can be a waste-disposal problem Furthermore, spent coolant is then contaminated with the VOC, and therefore, must undergo further treatment before disposal

14.2.1 C ONTACT C ONDENSERS

In contact condensers, a coolant, frequently water, is sprayed into the gas stream Condensation proceeds as a heat-exchange process where the air stream containing the condensable materials is first cooled to its condensation temperature, then loses its heat of condensation The coolant first gives up its sensible heat, then its heat of vaporization The balancing of the heat exchange between the two streams will determine the amount of coolant needed

Design of contact condensers is based on the gas–liquid stage concept However, spray systems operate with a high degree of back mixing of the phases This practically limits spray chamber performance to a single equilibrium stage For a direct contact device, this means that the temperatures of the exiting gas and liquid would be the same Backmixing results because the chief resistance to flow is only the liquid drops There is no degree of stabilization of the flow such as would happen

in a packed tower Anything less than perfect liquid distribution will induce large eddies and bypass streams Thus, special care must be taken to obtain a uniform spray pattern

FIGURE 14.1 Shell and tube type surface condenser schematic.

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14.2.2 S URFACE C ONDENSERS

In the shell and tube heat exchangers, the coolant typically flows through the tubes and the vapors condenser on the outside of the tubes In these units the pollutant gas stream must be cooled to the saturation temperature on the material being removed The problem of design is complicated by the fact that most pollutant gas streams are essentially air with a small amount of VOC or HAP included Therefore, condensation takes place from a gas in which the major component is noncondens-able In the case of a simple air stream where the other component is condensable, condensation occurs at the dew point when the partial pressure of the condensable equals its vapor pressure at the temperature of the system Since the coolant from surface condensers does not contact the vapor stream, it is not contaminated and can be recycled in a closed loop Surface condensers also allow for direct recovery

of VOCs from the volatile gas stream This chapter addresses the design of surface condenser systems only

Figure 14.2 shows some typical vapor pressure curves The more volatile the component, i.e., the lower the normal boiling point, the larger the amount that will remain uncondensed at a given temperature, hence the lower the temperature that is required to reach saturation Condensation for this type of system typically occurs nonisothermally The assumption of constant temperature conditions in the design

of surface condensers does not introduce large errors into the calculations

FIGURE 14.2 Typical vapor pressure curves.

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14.2.2.1 An Example — Condensation Temperature

Consider an air stream flowing at 771 scfm containing 13.00 mol% benzene in which 90% removal of the benzene is required The air flow entering the condenser is at 1.000 atm What temperature is necessary to achieve this percent removal? The condenser is depicted in Figure 14.3 which is Figure 14.1 labeled with the conditions

of operation specific to this example The partial pressure of the benzene at the outlet

of the condenser can be calculated as follows:

Basis: 1.000 moles of air stream including the benzene

Assumption: Condenser operates at 1.000 atmosphere or 760 mm of Hg Moles of benzene entering = 0.1300

Moles of benzene leaving = (1 – 90) × 0.1300 = 0.0130 Moles of air + benzene leaving = 1.000 – (0.1300 – 0.0130) = 0.8830

Partial pressure of benzene leaving = (0.0130/0.8830) × 760 = 11.19 mm of Hg Refer to Figure 14.2, the vapor pressure curve

For benzene the value of the abscissa Solving for tCON = 6.80°F

Therefore the condenser temperature must be below 6.80°F

FIGURE 14.3 Shell and tube type surface condenser schematic, example calculation.

1

459 67

0 00214

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14.3 COOLANT AND HEAT EXCHANGER TYPE

The next step is to select the coolant based on the condensation temperature required

Table 14.1 summarizes some possible coolants In the case of water, chilled water, and brine solutions, all remain in the liquid phase as they condense the VOC Refrigerants usually condense the VOC by absorbing the heat as they change phase from liquid to vapor The usual refrigeration cycle is used for these refrigerants The vapor is compressed and condensed at a higher pressure and a higher temperature

by a fluid at a temperature lower than the condensation temperature Frequently water can be used to condense the high pressure refrigerant Most thermodynamics textbooks contain a description of a refrigeration cycle

The problem now is to determine the size and design of the particular type of heat exchanger that is required to carry out the heat transfer needed Figure 14.1

illustrates a horizontal shell and tube heat exchanger with the coolant inside the tubes and the condensing vapor outside the tubes Vertical shell and tube heat exchanger arrangements are shown in Figure 14.4 Advantages and disadvantages

of each type are listed in Table 14.2

The design of this type of equipment requires the knowledge of suitable heat transfer coefficients These coefficients are highly dependent on the condensing material, the coolant used, and the particular arrangement of the heat exchanger They range from 10 to 300 BTU/(h-ft2-°F) Finally the design procedure would include determining the amount of coolant needed

14.3.1 A N E XAMPLE — H EAT E XCHANGER A REA AND C OOLANT F LOW R ATE

For the heat exchanger discussed previously where the flow is 771.0 scfm, the number of moles would be 2.0 moles/min or 120 moles/h Data:

TABLE 14.1 Coolant Selection

Required Condensation

80 to 100 Water

45 to 60 Chilled water –30 to 45 Brine solutions –90 to –30 Refrigerants

Heat of condensation of benzene - at 1.0 atm, 176 F Specific heats; at 77 F

Heat transfer medium CPM

° = °

13 230 25

6 96

19 65

0 65

,

.

BTU lb mole C

BTU lb F

PA

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FIGURE 14.4 Vertical shell and tube heat exchangers arrangements: (a) condensation inside tubes, downflow vapor; (b) condensation inside tubes, upflow vapor; (c)

condensation outside tubes, downflow vapor.

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On the basis of 1 h of operation,

Refer to Figure 14.3 for temperatures

Condenser heat load, assuming no heat loss from the heat exchanger to the atmosphere,

TABLE 14.2

Advantages and Disadvantages of Shell and Tube Heat Exchanger Types Used

in Condensation

Horizontal Exchanger

Condensate outside tubes May be operated partially flooded Free draining

Condensate inside tubes Liquid builds up causing slugging

Vertical Exchanger

Condensate inside tubes, vertical

downflow

Positive venting of noncondensables

Wet tubes retain light-soluble components Low pressure may require large tubes

Condensate inside tubes, vertical

upflow

Used for refluxing Usually partially condensing Liquid and vapor remain in intimate contact

Condensate outside tubes, vertical

downflow

High coolant side heat transfer coefficient

Requires careful distribution of coolant Ease of cleaning

S uncon voc air cond voc uncon voc

air

0 0

,

therefore H∆

∆

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To estimate the heat of condensation at T2 = (6.8 + 460) = 466.8°R, use the Watson Equation.1

where TC = 1012°R, the critical temperature of benzene, T1 = (176 + 460) = 636°R, and ∆Hvoc at T2 = 13,236 BTU/lb-mole

Calculate the heat transfer area,

U = heat transfer coefficient

U = 40 BTU/h-ft2-°F

∆Tlog mean = log mean temperature difference

For a derivation and discussion of the log mean temperature difference see Perry and Green2 or a text on heat transfer

Calculate the coolant flow

T T

voc at T voc at T

C C

1 1

2 1

0 38

−









.

∆

H

voc at T cond voc voc to cond temp cond cond voc

cond voc

234 207

0 38

−







=

,

.

-Q=U A∆Tlogmean

mean log

,

−

− −( )

( )





















=

− −( )

( )=

80 16 8

33 51

289 640

2

cool PM in out cool

=

− −( )

(289 640 )=

,

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14.4 MIXTURES OF ORGANIC VAPORS

The condensation of mixtures of organic vapors occurs over a range of temperatures, thereby complicating the design of heat exchangers for condensing these mixtures The process for a binary mixture is illustrated in Figure 14.5 where it is presumed the pressure remains constant A vapor at point A is cooled until it reaches its dewpoint at point B Further reduction of temperature will cause the mixture to form two phases At the temperature at point C, the vapor composition is given by point D and the liquid composition by point E A constant temperature flash calculation could determine not only the compositions at this temperature but also the quantity

of vapor and liquid Continued coolant to the bubble point temperature F will produce 100% liquid with the same composition as the initial vapor Therefore, as an organic mixture cools from its dew point to its bubble point, the condensing liquid is changing composition This results in the heat of condensation varying throughout the cooling process This variation in the heat of condensation should be accounted for in the determination of the area for heat exchange and will result in a greater area than would be calculated from the log mean temperature difference method In some cases it can make a major difference in the area and, if not accounted for, can result in poor performance of the heat exchanger For a more detailed description

of the dew point, bubble point, and flash calculation methodology, refer to a thermo-dynamics textbook like Smith et al.1 The original model for sizing a condenser for mixtures of vapors was presented by Colburn and Hougen.3 This method was elab-orated upon by Silver4 and Bell and Ghaly.5

FIGURE 14.5 Equilibrium dew point–bubble point curve for a binary mixture.

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14.4.1 A N E XAMPLE — C ONDENSATION OF A B INARY M IXTURE

As an example to illustrate the methodology, consider the condensation of an iso-propyl alcohol (IPA) — water mixture in a vertical, countercurrent, upflow condenser

at 1 atm The heat exchanger is to be sized to totally condense the mixture The total flowrate is 608 lb/h at 214.4°F (101.33°C) and atmospheric pressure at a mole fraction of IPA = 0.128 Cooling water is available at 80°F with a 10°F temperature rise allowed In the vertical, countercurrent, upflow heat exchanger the vapor is condensing inside the tubes Figure 14.7 is the bubble point–dew point curve for the IPA–water system It shows this system to be an azeotrope The mixture composition

we are considering is to the left or the lower IPA composition side From Figure 14.6

for the IPA mole fraction of 0.128, the dew point is 95.8°C (204.4°F), and the bubble point is 82.6°C (180.7°F) The overall heat transfer coefficient Uo = 100 BTU/h-ft2-°F, and the gas film heat transfer coefficient hg = 7 BTU/h-ft2-°F

First calculate the heat exchange area from the log-mean temperature difference (LMTD) method with the overall heat transfer coefficient The total heat transferred can be approximated from the latent heats of vaporization and the molar composition

On a mole basis, both latent heats of vaporization for IPA and water are about equal

FIGURE 14.6 Vertical upflow total condenser, example calculation.

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