Ac macpb

9. Calculate the face area of the bed, Aac:

10. Calculate the bed height, Z.

11. Estimate the pressure drop from the graph in Figure 84 or a suitable equation.

12. Set the L/D (length-to-diameter) ratio. Calculate L and D, noting that

Constraints: L < 30 ft, D < 10 ft; L/D of 3 to 4 acceptable if v < 30ft/min

13. Design (structurally) to handle if filled with water.

14. Consider designing vertically if q < 2500 actual cubic feet per minute (acfm). Consider designing horizontally if q > 7500 acfm.

ADS. 11 SIZING A CARBON ADSORBER

A printing company must reduce and recover the amount of toluene it emits from its Rotograve printing operation. The company submits some preliminary information on installing a carbon adsorption system. You, the primary consultant, are given the following information:

ADS. 11 SIZING A CARBON ADSORBER

ADS. 11 SIZING A CARBON ADSORBER

Velocity (standard air), fpm Figure 84. Activated carbon pressure drop curves.

Adsorption capacity for toluene is 0.175 lb toluene/lb activated carbon

Operation is at 10% of LEL (lower explosivity limit) for toluene in the exit air from printer

Toluene molecular weight = 92.1

Carbon bulk density (4x6 mesh) = 30 lb/ft3

Working charge is 60% of saturation capacity

Regeneration is just under one hour; assume 1.0 h

Maximum velocity through adsorber is 100 fpm

Determine the minimum size of adsorber you would recommend for a 1 x 1 system.

Calculations should include the pertinent dimensions of the adsorber, the amount of carbon, the depth of the bed and an estimate of the pressure drop. Also calculate the fan horsepower if the blower/motor efficiency is 58%.

Solution

Initially, base the calculations on 1-h regeneration time so that 1 h of adsorption is available. Key calculations and results are provided below for the toluene (TOL) and activated carbon (AC).

mjOL ~ 359

= 338 lb/h pTOL = (24/20,000)(14.7) = (0.0012)(14.7) = 0.01764 psia

WC = (0.175)(0.60) = 0.105 lbTOL/lbAC = 10.5 lbTOL/100 lbAC

mAC = (338/0.105)(1.0) = 3220 lbAC for one bed = 6440 lbAC for both beds

Suggest a horizontal 10-ft diameter by 20-ft long design. Since AP = 0.625 in. H20/in. bed,

Note: This represents a marginal design since H is slightly higher than 0.5 ft.

ADS.12 BREAKTHROUGH TIME CALCULATION

A degreaser ventilation stream contaminated with trichloroethylene (TCE) is treated through the use of a horizontal carbon bed adsorber. The adsorber is normally designed to operate at a gas flow of 8000 scfm (60° F, 1 atm), and the concentration of TCE at the adsorber inlet is 1500 ppmv. The efficiency of the adsorber is 99% under normal design conditions. Design parameters are as follows:

Actual conditions: 25 psia, 90°F

L = length of adsorber = 25 ft

D = diameter of adsorber = 8 ft

Bulk density of carbon bed = 35 lb/ft3

1. Determine the time before breakthrough occurs.

2. Recalculate the time before breakthrough occurs, based on the following transient condition. The adsorber system is on line for one hour at the above normal design conditions when the inlet concentration of TCE rises to an average value of 2500 ppmv due to a malfunction in the degreaser process; the efficiency also drops to 97.5% during this time. Assume the SAT remains the same.

436 ADSORPTION (ADS) Solution

Key calculations for part 1 are first provided:

25 / \60 + 460 = 4975 acfm qTCE = (1500 x 10~6)(4975) = 7.46 acfm

. = P(MW) qJ(:t = (25)( 131.5)(7.46) WjCE RT (10.73X90.460)

WC = BC - HEEL ± SF (safety factor) = 0.32 - 0.02 - 0 = 0.30 = 30%

ADS. 12 BREAKTHROUGH TIME CALCULATION

For transient conditions (part 2), mTCE (in carbon) = (0.30)(500)(35)

= 52501bTCE, maximum mTCE (flow, first hour) = (4.16)(60)(0.99)

= 247.1 lbXCE captured

The remaining capacity of the bed after the first hour is now mTCE (after first hour) = 5250 - 247.1 = 5003 lbXCE

'"TCE,transient (0.975)

5003

The time to breakthrough, following the transient period, is tB =60 + 740.1

Thus, the time to breakthrough has been reduced from 21 to 13.3 h.

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