Air Stripping of Volatiles Sometimes volatile compounds (generally

Air Stripping of Volatiles Sometimes volatile compounds (generally

Air Stripping of Volatiles Sometimes volatile compounds (generally VOCs ) can be removed from the aqueous phase by stripping the VOCs with an air flow. This air stripping is usually accomplished in air stripping towers. The towers serve to breakup the water into droplets and allow contact between the water and air with subsequent transfer of VOC to air phase. The premise is that the atmosphere can handle the

VOCs (though sometimes AC adsorbers are Packed tower air stripping is commonly used to remove volatile contaminates ( e.g., tetrachloroethylene, trichloroethylene, benzene, ammonia) from contaminated waters. Increasingly, it is being applied to the renovation of groundwaters that have become contaminated

with volatile organic carbon (VOC) compounds from leaking underground storage tanks, spills, or improper disposal practices. Two approaches to designing a stripping tower are; equilibrium and dynamic. First consider the equilibrium approach. This will only give an estimate of performance but it is fairly easy

to perform. Consider an air stripping column schematically shown here: G = gas flow rate (m3/sec) L = water flow rate (m3/sec) Cg1,2 = gas phase concentration of volatile

substance at top and bottom of column, respectively (moles/m3) Cl1,2 = water phase concentration of volatile substance at top and bottom of column, respectively (moles/m3) If equilibrium is attained at every point in the tower then Henry's law predicts:

Cg = HcCl A mass balance on the volatile material in the column gives: L(C l C l2 ) G(C g C g ) 1 1 2

Cg2 is generally zero (use clean gas for stripping). Again at equilibrium: Cg H c Cl 1 1 therefore:

L(C l C l2 ) G(H c C l ) 1 1 then: G C l1 Cl2 1

L Cl Hc 1 Since Cg2 Cl2 0 (by Henry's law )

G 1 L Hc This G/L ratio predicts gas to liquid flows required to get complete removal of the volatile

material. However, equilibrium (Henry's law) is never attained in a typical stripping column. This ratio is only an estimate of actual gas/liquid flow rates required. Dynamic Analysis of Stripping column In its most efficient form, the packed tower is operated in a counter-current manner: the contaminated water is loaded to the top of the

tower, while fresh air is blown in from the bottom. The packing generally consists of plastic pieces, approximately 1 to 2 inches in nominal size, which are randomly dumped into the column structure (hence the term, "dumped packing"). Common varieties include "rings" and "saddles". There are

also some unusual, honey-combed, "structured packings" being sold (at rather high cost) by manufacturers for VOC applications. Whatever its type, the packing is supported by a retention structure; the liquid is distributed over the packing using either nozzles or trays. Derivation of performance equation for this type of

stripping column follows. First define the following terms. L = liquid flow rate ( m3/min); G = air flow rate ( m3/min); C = concentration of volatile solute in the liquid ( mol/m3); P = partial pressure of the volatile solute in the gas phase (atm);

Zt = total height of packing (m); Ax = cross-sectional area of tower ( m 2); a = true interfacial area per unit bulk volume of packing ( m-1) (affects Kla) ; R = universal gas constant = 8.2056 x 10-5 m3atm/mol-oK. Performing a mass balance on the volatile solute

entering and leaving the tower yields: G Pf Pi L Ci Cf R Tg (1)

Note that Pi is approximately zero if clean air enters the column. Consider a differential height element, dz, as shown in the above diagram. The differential transfer rate, dN( mol/min) through the element is given by:

dN K l a C C* A x dz (2) C = liquid phase concentration of the volatile compound in the element dz (mol/m 3); C* = hypothetical liquid phase concentration which would be at equilibrium with the gas-phase partial pressure, P in the element ( mols/m 3);

C* P / H. Integration over the total packing height, Z t, yields: L Ci Cf K l a C C* A x Z t L

C C* L (3) log-mean driving force for mass transfer across the tower Ci C*i (Cf C*f )

C C* i i ln * (Cf Cf )

Substituting eq (3) into equation (1) with rearranging, yields: Z A LR T LR T t x g

g exp 1 K a

GH L G H C

l i LR T

C g f 1 GH The above performance equation allows for estimation of the required packed volume necessary to achieve a

desired removal, given Ci and L for different choices of G. We can define a "stripping factor" G GH F

H s LR T c L g The inverse of the stripping factor appears in three places in the performance equation. Fs characterizes the capacity for transfer, relative to

equilibrium conditions. If Fs <1, there exists a limited degree of removal, for a fixed G/L, which cannot be surpassed, regardless of how large a tower is employed (i.e., even if the exit gas stream achieves equilibrium with the entering liquid stream, removal will be limited by insufficient gas).

A typical application scheme for a stripping column is demonstrated in the following schematic: Some typical stripping columns:

Recently Viewed Presentations

  • Using the Data Acceptance White Paper and Checklist

    Using the Data Acceptance White Paper and Checklist

    Chain-of-Custody Definition. The movement and location of physical evidence from the time it is obtained until the time it is presented in court. ... Complete and unbroken records of chain of custody from sample collection to discard.
  • Modelling eye tracking dynamics with quantum theory

    Modelling eye tracking dynamics with quantum theory

    Do eye tracking dynamics allow insight into decisions? Two examples of this: In the Attentional Drift-Diffusion Model (aDDM; Krajbich et al., 2012) diffusion rates are affected by fixations, longer fixations for an option increase preference for this option, last fixation...
  • www3.imperial.ac.uk

    www3.imperial.ac.uk

    Siân Owen Science Communication Unit Communicating Science with the Creative and Performing Arts Overview Communicating Science with the Creative and Performing Arts ...
  • Prezentace aplikace PowerPoint - Boskovice

    Prezentace aplikace PowerPoint - Boskovice

    Vybudován v letech 1806 - 1836 (nechal postavit Napoleon Bonaparte). - Po stranách má 2 metry vysoké sochy. Po celém oblouku jsou vytesané vítězné bitvy Francouzů, a tak od té doby slouží jako památník padlým vojákům (navrhl Jean Francois Chalgrin)....
  • Girl Bullying: Helping mean girls make nice

    Girl Bullying: Helping mean girls make nice

    Mary Pipher and The Ophelia Project "Mean Girls" movement Tina Fey Rosalind Wiseman Academic research and the popular media Tomorrow: Recognizing and Intervening in Girl Bullying Meeting the needs of Bullies, Targets, and Girls in the Middle Girl Bullying: Helping...
  • Population and Housing Census Data Processing Activities

    Population and Housing Census Data Processing Activities

    Manual Data Processing of Census Data 2004 Population and Housing Census ... Data Entry / Verification / Validation / Back up / Tabulations Primary Data Processing Flow Secondary Data Processing Flow Creation of Census Databases Population Housing Ownership Agriculture Deaths...
  • Philosophy 103 Linguistics 103 Introductory Logic: Critical ...

    Philosophy 103 Linguistics 103 Introductory Logic: Critical ...

    Historical Answer - Logic is one of the main branches of philosophy. It is the study of rational argument and inference. Philosophical Answer - Logic is the study of arguments and their properties. It is the methodological core of all...
  • Main objectives of our research at the University of Hyderabad

    Main objectives of our research at the University of Hyderabad

    Recent Advances in Green Catalysis Chun-Chen Liao Department of Chemistry, Chung Yuan Christian University Chungli 32023, TAIWAN 主辦單位 中國化學會環境與化學委員會