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THE FUTURE OF LOW SPEED SCRUBBING TECHNOLOGY 
 

 

Alexander Michael Brown  

Department of Chemical Engineering, University of California, Berkeley, CA, USA. 

 

Abstract: Low Speed Scrubbing Technology (LSST) is a new technology for the purification of industrial 

exhausts from both gaseous and solid pollutants. It does not belong to either a counter-flow or co-flow system, 

but is based on the idea of regime change. The LSST scrubber, known as the Low Speed Absorber (LSA), is free 

of any mechanical or pressure loads, has low hydraulic resistance, and has no unreliable structural units. It is a 

foolproof and serviceable construction. 

Keywords: Low Speed Scrubbing Technology (LSST), Low Speed Absorber (LSA), gas-liquid interaction, 

sorption, chemisorption, wet scrubber, absorber, counter-flow, co-flow, regime change 

 

Introduction  

Non-catalytic sorption/chemisorptions as a whole, including gas-liquid interaction in particular, is conceptually 

a phenomenon of a physico-chemical nature. It consists of the physical process of gas dilution in the liquid phase 

followed by a chemical interaction. These processes are well understood1-5. Theory and design principles permit 

rate (efficiency) and limits of the sorption depending on both physico-chemical parameters of the process and 

engineering design to be quickly and reliably estimated. In engineering, these processes are usually done in units 

known as wet-scrubbers (or absorbers), the scrubbing process (i.e., gas/liquid interaction) proceeding as either a 

counter-flow or co-flow regime.  

In addition to the conventional co/counter-flow technologies, Low Speed Scrubbing Technology (LSST) is 

intended for the purification of industrial exhausts from both gaseous and solid (dust) pollutants6,7. It does not 

belong to either a counter-flow or co-flow system, but is based on the idea of regime change8-12. For practical 

embodiment, a scrubber of special design known as Low Speed Absorber (LSA) is required. To gain an insight 

into both the theoretical basis and to design features of this technology, it is useful to follow a process in the 

simplest one-stage LSA.   

General description   

One-stage LSA conceptually consists of the contacting chamber, separation chamber, fixed (immovable) 

centrifugal separator, and various hydraulic units (Fig. 1) and functions as follows.   

The initial contaminated gas enters the contacting chamber from the bottom upward and starts to ascend. At this 

stage it meets a liquid jet descending via back-pipes from the separation chamber joined from above. Initially, 

the discharge liquid jet is compact. Then it simultaneously starts to fall under gravity and break down into smaller 

fragments (regular and irregular drops) on exposure to the ascending gas. As this takes place, the small drops 

(i.e., with weights smaller than the dynamic pressure of the ascending gas) are instantly picked up by the 

ascending gas and start to ascend jointly with the gas. The heavier drops continue to descent and split 

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concurrently, thus regenerating both small and large drops. Eventually, spontaneous self-fragmentation 

terminates.   

Aerosols result from this process and continue to fall until retarding forces become initially equal to gravity and 

then more than gravity for any liquid fragment. From this point on, the dispersed liquid phase starts to ascend.  

   
Figure. 1. Schematic diagram of one-stage Low Speed Absorber (LSA). 

This aerosol leaves the contacting chamber and enters the fixed centrifugal separator of the separation chamber 

located above. Welded curvilinear blades of the fixed centrifugal separator direct haze onto the wall of the 

separation chamber. Here actual phase separation occurs: the gas exits into the atmosphere as the cleaned exhaust, 

while the liquid flows down the wall and accumulates at the bottom of the separation chamber. Then the densest 

portion (i.e., lower layers of the liquid) is drained via a drainage outlet, while the most aerated portion (i.e., upper 

layers) is partially recycled under gravity via back-pipes to the contacting chamber.  

In this way, there is an inner automatic recycling of the liquid phase between the contacting and separation 

chambers. Its special feature lies in the fact that the times of phase contact relative to gas and liquid are dependent 

on different factors and hence can be controlled independently. Contact time on the gas side is completely 

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predetermined by the gas speed, while contact time on the liquid side depends on the gas speed and the regime 

of liquid phase renewal (i.e. ratio of the feed and drained liquid fluxes). The latter, in turn, is a function of the 

current physico-chemical properties of the liquid and dispatch state. Contact time on the liquid side is evidently 

unlimited and should be set accordingly to the efficiency of mass exchange by means of the variation of liquid 

phase renewal. Hence, control valves of feed liquid and drainage can vary the contact time from the liquid side 

independently and simply. By and large, this provides high flexibility of the practical control of LSA operation.  

A point of any scrubbing technology is to mix the gas and liquid phases as well as possible. Diverse conventional 

devices (e.g., packings, mechanical agitators, or mixers, as well as injectors, nozzles, etc.) are designed for 

suitable solution of the mixing problem.   

LSST has no need for any man-made mixing device because it is the counteraction between inertia forces of the 

ascending gas and the gravity forces of the descending liquid that triggers self-fragmentation and acts as a natural 

stirrer. This “stirrer” is uniformly distributed over the whole volume; it affects all spatial points of the contacting 

chamber. Therefore, the back-pipes connecting the separation and contacting chambers are standard tubes rather 

than some specific injectors. The mode of operation (continuous, periodical, etc.) of drainage and supply of feed 

liquid, as well as the amount of the recycling liquid, is dictated by both sorption features and liquid properties. 

It facilitates easy remote control through the control valves. In this way, the low speed scrubbing process begins 

as a counter-flow while it terminates as a co-flow. 

Theoretical fundamentals 

The most general equation of unsteady heat/mass exchange between the gas and dispersed liquid phase is of the 

form 1   c 1 2c 1 (c, ) 

   2 g  

                                                       Ped  S ( , )                                            ( 1 )  

 are dimensionless co-ordinates, gS={ g ,g ,g } 0 is the metric tensor. Leaving aside Here  , 

 , any details of formal transformation, Eq. (1) can be eventually reduced1,13,14 to the well-known equation 

of unsteady diffusion as follows: 

c d c  

 

                                                            x x                                                                  ( 2 )  

The solution of Eq. (2) depends on the nature of transfer processes, reciprocity of transfer phenomena and 

chemical processes, as well as initial and boundary conditions. In the approximation of the diffusion boundary 

layer, these items are usually considered with the help of similarity criteria in the following way:  

                                                           Nud = F (Re, Ped, Sc, Fr)                                                    ( 3 )  

  

Re Lu ,    Ped 
Lu ,    Prd(Sc)  ,   Fr u2 

                                           d d gL  

  

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Physically, these determining criteria consider the nature of gas flow (Reynolds number Re), the interdependence 

of convection and diffusion transfer (Peclet number Ped), the similarity of the speed and concentration fields 

(either Schmidt number Sc or Prandtl number for diffusion Prd), and scale factor (Froude number Fr). The most 

comprehensive theoretical analysis of various specific cases (e.g., Stokes regime/potential flow, 

laminar/turbulent motion, small/large Pe-criteria, surface/volume chemical reaction) can be found in Gupalo et 

al 6. Practical use of similarity theory requires knowledge of both hydro/gas-dynamics essentials and numerical 

values of parameters as initial and boundary conditions (i.e., design features). The former can be revealed by 

pure analysis, while the latter are only experimentally determined.  

Hydro/gas-dynamics  

As can be seen from above, LSST possesses specific hydro/gas-dynamics. Compared to conventional co/counter-

flow systems, the essentials of LSST hydro/gas-dynamics can be explained as follows. To provide a stable LSST 

regime, the counteraction between inertial forces of the ascending gas and gravity forces FN of the descending 

liquid takes place under prevalence of the former relative to the drop of any R-size. The fall rate of the very large 

drops R >R>Rsf (compact liquid jet R  and the boundary of self-fragmentation Rsf are the limits) is independent 

of the drop weight and size while solely is dependent on gravity 

                                                              FN ~ (mg)l ~(4/3) gR3                                                    ( 4 )  

At the same time, the fall rate of the smaller drops R<Rsf is essentially dependent on their size. Any R-drop 

maintains its integrity until consolidating forces (surface tension Fs) 

                                                                      Fs ~  R                                                                    ( 5 ) exceed 

breaking forces of resistance to the motion, the latter being a function of the motion regime and size of drop Re 

uL Recr   laminar flow                                        ( 6 )  

                                                 Recr   turbulent flow 

  

For the turbulent flow around a small sphere R~10-6 m, local Recr~10, while resistance forces Ft are proportional 

to the drag force of the gas and the drop size 

                                                                  Ft ~ ( u2)gas R
2                                                             ( 7 ) 

The comparison of Fs and Ft (Eq. 5, 7) shows that the upper limit of fragmentation is non-existent, i.e., the 

breaking forces prevail over the consolidating ones under all conditions, including the limit R  of compact liquid 

jet. Physically, this means that any free-falling compact liquid jet disintegrates inevitably sooner or later. 

Spontaneous self-fragmentation terminates at the point when the velocity head of the ascending gas becomes 

equal to the forces that determine both the scale and integrity of the drop (i.e., surface tension). Thus, the lower 

limit of selffragmentation is dictated by their equality Fs ~ Ft  

Rsf u2  

                                                                                                                                     ( 8 ) 

Hence, Rsf is dependent on the gas speed. It is easy to see that for pure water ( 1000 kg/m3, 75  65 kg/s2, 

75 3.8.10-4 kg/m.s) and typical LSST regimes u~(10-20) m/s, the average size distribution lies mainly within 

the range Rsf ~(100-500).10-6 m. Redistribution of size grading in the domain located below the level of the 

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backpipes is governed by the scrubbing speed and properties of the liquid phase. Specifically, the surviving drops 

Rsf>R>R1 maintain their integrity and continue to fall in the turbulent regime as long as the condition Re>Recr 

of Eq. (6) remains valid. Finer drops R1>R>R2 fit the condition Re<Recr of Eq. (6) and fall in the laminar regime. 

They are exposed to the breaking forces of the conventional gas-dynamic (Stokes) nature 

6 Ru  6 Ru  

(1 /R) 

                                                           FSt=                                                       ( 9 )  

As follows from Eq. (4, 9), limiting R1-values fall in the range R1~ (50-100).10-6 m for typical LSST gas speeds 

u~(10-20) m/s. Finally, the finest drops R<R2 maintain their integrity while never falling. Once generated, they 

are immediately carried by the ascending gas into the separation chamber. The main feature of their motion lies 

in the fact that their absolute velocity is equal to the velocity of the gas carrier. In other words, the relative 

velocity of their motion equals zero. Thus, collisions rarely occur and this finest fraction survives up to the final 

exhaust (unless special measures are implemented).   

The upper limit R2 is dependent on scale height of the medium H=kT/mg and for the limiting case of free fall, it 

can be estimated as  
2 1/7 

kT  

  

                                                                  R2  3l g2  10-6  m. 

Thus, the hydro/gas-dynamic pattern of disintegration can be generalized as follows:  

R >R>Rsf  -free   (Newton)  fall,   progressive  self-fragmentation,   settling  rate  is independent of drop size,  

Rsf>R>R1 - turbulent motion, partial fragmentation of the coarsest portion of aerosols is admissible, settling rate 

depends on drop size as  u~R1/2,  

R1>R>R2 - laminar (Stokes) motion, no fragmentation, settling rate depends on drop size as u~R2,   

R2>R       - forced ascent (no fall), neither fragmentation nor drop growth occurs. 

The zone of self-fragmentation (domain between the level of the back-pipes and lower spatial boundary of 

aerosol) contains a very wide spectrum of drops R2<R<R . At the same time, the domain located above the level 

of the back-pipes contains finer aerosol R2<R<Rsf  (thin haze), i.e., this zone is far more uniform in the sense of 

size distribution.  

So far only destruction has been considered. Various transfer effects (e.g., gravity, diffusion, convection) lead to 

collisions followed by coagulation of drops resulting in their growth 15. Physically, the upper limit of this growth 

is evident, since any newly formed drop Rnew to be smaller than the limiting values Rsf, R1, R2 along the whole 

associated range of physical conditions. Otherwise, if such drop Rnew> Rsf, R1, R2 is generated, then it will be 

destroyed again. 

The growth of any drop R( ) along the fall distance dZ=ud  depends on both number of collisions and liquid-

water content Gl kg/m3. For free fall, the relation between current mass dm of some R-drop and motion 

parameters  

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2 g lR4 Gl  
2dR ~ Gl R2dZ ~ Gl R2ud   ~   9 d                         ( 10 )                         dm ~ (4/3) iniR 

 allows for simple estimation of both dynamics of drop growth (terms 2 and 3)  

  

 dR Gl 

~ 

                                                                  dZ 4 l                                                                  ( 11 )   

 and fall time considers this growth (terms 2 and 5 of Eq. (10))  

18  

 
gGl Rini                                                                 ( 12 )                                                                 f   

Eq. (11) shows that the drop growth is independent of the motion regime, while Eq. (12) demonstrates that the 

fall time is independent of the final drop size (since the latter does not appear in Eq. (12)), being dependent only 

on the initial size Rini. Eventually, the gravity force acting on any drop becomes less than the force of tractive 

resistance resulting in the termination of the fall.  

From this point on, the ascending gas selects and includes all liquid drops. The bottom spatial boundary of the 

fall distance represents the separation line between the domain of the incoming gas and the self-fragmentation 

zone, while the top boundary (i.e., back-pipes level) is the separation line between the self-fragmentation zone 

and the fine aerosols. Above the level of the back-pipes, any small object R2 R Rsf (no matter whether liquid 

or solid) acquires the velocity of the gas carrier.  

Thus, the consideration of coagulation has a marginal effect on the general pattern. At the same time, it is relevant 

in relation to the current value of the interfacial area and intensity of heat/mass exchange between the gas carrier 

and the dispersed liquid phase. The collision of drops results in size redistribution during counter-way traffic of 

large (falling) drops and small (ascending) droplets, i.e., along the entire length of fall distance Z. Interaction of 

the ascending and descending dispersed objects is extremely favourable. In gas dynamics terms15, it increases 

residence time life. But more significantly, it enforces the sorption because of the continuous renewal of the 

interfacial area. Both of these factors considerably improve the mass exchange coefficient , the latter being the 

most important generalised index of the sorption efficiency4,13,16.  

As is evident, self-fragmentation time, fall time, residence time and fall distance lie in averaged ranges 

sf~[(2/104) lRiniRf)]
1/2~(0.05-0.20) s, f~(0.3-1.0) s, life~ (1.0-2.5) s and Z~(0.5-1.2) m, respectively. The 

specific interfacial surface S is the interfacial area per unit volume [m2/m3]. It is one of the most important indices 

among others. With the assumptions of normal (Gauss) size distribution and spherical shape of the drops, it can 

be approximated as S~3.103/R~(400-800) m2/m3 depending on gas speed u~(10-20) m/s and volumetric flux of 

water ~(0.02-0.10) m3/m2s. These estimates are in good agreement with the direct experimental data 17 (Fig. 

2). 

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Figure. 2. Specific interfacial surface S versus volumetric flux of water . 1- u= 10 m/s, 2- u= 15 m/s, 3-u= 

20 m/s.  

To generalise, it should be stated that in deciding on scrubbing speed, the dominance of gas momentum over 

liquid momentum   

                                                                   ( u)g>( u)l                                                                ( 13 )   

is the single crucial constraint of LSST applicability.  

Experiment  

To suit the similarity theory, the experimental apparatus for LSST investigation was designed as a transparent 

1:10 reduced scale model of a typical cylinder-shape full-scale scrubber of 50000 nm3/h gas load (Fig. 1). Values 

of the main criteria Remodel= Renature, Ped, model=Ped, nature, Scmodel=Scnature were satisfied. Other criteria, including 

Fr and thermal, were held constant whenever possible. Measurements of the contacting and separation chambers 

were equal to D=0.4 m, L=1.1 m and D=1.0 m, L=0.7 m, respectively. Total gas load, gas spead and volumetric 

flux of water varied within the limits Vg~(1.0-3.0) m3/s, u~(8-25) m/s, and ~(0.02-0.10) m3/m2s, respectively. 

Polluted primary gas contains various admixtures (e.g., phenol, ammonia, hydrochloric acid). Below are given 

data regarding primary gas polluted with the mixture {(2-8)% SO2+(1-4)%SO3}. With the control system, high-

speed (up to 5000 shots/s) photography and on-line measurement of the main process characteristics were 

possible. Other details can be found elsewhere 10,17, 18.   

The prime object lies in the direct experimental determination of the main technological (residence time, fall 

time and distance, interfacial area, size distribution and zoning of the dispersed phase) and sorption indices 

(heat/mass-exchange coefficients ), which are dependent on LSST operating parameters (gas load, scrubbing 

rate and volumetric flux of water) and LSA design features (geometric and aspect ratios, positional relationship 

of various units and hydraulic resistance). Experimental data combined with the essentials of dimensional 

analysis provide adequate LSST description to yield simple analytical quantities for practical LSA design. 

Relevant data17,18 are shown in Figures 2-4 and Table 1 and can be briefly summarized. 

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Table 1.  The main LSST and LSA scrubbing variables  

  

  Indices  Wet-bed  Venturi 

tube  

One-stage 

LSA  

1  Gas load (max),  Vg  nm3/h    ~180 000    <100 000    300 000  

2  Number of stages in one set         1-2           1        Any  

3  Operational regime   Co/counter  Only co-

flow  

Compound  

4  Interfacial area,  S m2/m3      max 250    max 400   400-800  

5  Gas speed,  u m/s       20-40      50-120      10-20  

6  Contact time,   s    0.05-0.30    0.02-0.10   0.20-2.50  

7  Volumetric flux of water  

             m3/m2s  

  0.04-0.10    0.40-1.10   0.02-0.10  

8  Hydraulic resistance H, Pa  2000-2500  3000-8000     <1400  

9  Coefficient of the gas mass    

exchange  v,g m
3/m3s  

       5-8        7-12      20-30  

 (see text)  

10  Cleaning efficiency*(dust)%        <85        <95       >98  

11  Cleaning efficiency (oxides)    ~(50-

90)%  

 ~(60-95)%  ~(70-99)%  

12  Dimensions  (averaged), m   ~ 5.0 / 10.0    ~ 3.0 / 6.0  ~ 3.5/10.0  

13  Dry mass  (averaged), kg    ~ 10000     ~ 6000     ~ 9000  

   Notes :  *   Strongly depends on size distribution R=R(u).  

Specific interfacial surface S was determined experimentally by a known chemical method19. S is almost 

independent of the gas speed, while it is strongly dependent on volumetric flux of water (Fig. 2). This dependence 

can be approximated with an accuracy of 12% in the form   

                                                             S 1775 u0.2 0.63                                                            ( 14 ) 

Fig. 2 and Eq. (14) conceptually confirm the validity of the above presented theoretical approximation. At the 

same time, the weakened S-u correlation seems to be the result of some liquid loss from the contacting chamber 

as the gas speed increases.  

Both mass holdup of liquid gl kg  

                                                          gl = Gl/V =0.209 u 0.79 0.92                                                ( 15 )  

 and its residence time life in the contacting chamber  

  

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                                                                life = Lcont/                                                                  ( 16 ) are strongly 

dependent on volumetric flux of water . The volumetric holdup of gas in the contacting chamber vg is defined 

as the difference between the volume of contacting chamber V and volumetric holdup of liquid vl=gl/ l   

  

                                                                 vg = V - vl                                                                    ( 17 )  

Usually, volumetric fraction of gas g= vg/vl falls in the range (0.90-0.98). The main interest was focused on the 

direct experimental determination of mass exchange efficiency and search for opportunity of its intensification 

in the context of the unsteady phase interaction. Some of the generalised results are briefly summarized as 

follows.  

Mass-exchange efficiency of LSST was estimated with the help of the following transfer coefficients. The 

coefficient of the liquid mass exchange v,l represents volumetric gas flux from the gas phase to the liquid phase 

with respect to the whole gas-liquid volume. It was determined experimentally by the example of carbon dioxide 

CO2 desorption from its aqueous solution. v,l-values are dependent on the gas speed and volumetric flux of 

water (Fig. 3); the semi-empirical average approximation is in the form 

                                                         v,l = 0.457 u0.25 0.86                                                        ( 18 ) with an accuracy 

of 10%.   

A comparison of Eq. (14) and (18) shows that the gas speed affects both v,l and S equally. As a result, the 

coefficient of the surface mass exchange v,S , i.e., volumetric gas flux from the gas phase to the liquid phase 

with reference to the specific interfacial surface S, seems to be practically independent of the gas speed. Both 

the direct experimental plot of v,S vs u (Fig. 3) and semi-empirical analytic approximation 

                v,S = v,l/S =0.0003 u0.05 0.23                                            ( 19 )  prove this feature of mass exchange in 

LSA.  

  

 
    

Fig. 3. Coefficient of the liquid mass exchange v,l versus volumetric flux of water , 1- u= 10 m/s, 2-u= 

20 m/s. 

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The coefficient of the gas mass exchange v,g, i.e., volumetric gas flux from the gas phase to the liquid phase 

with reference to the gas volume, was determined experimentally through the rate of adiabatic evaporation of 

water. The weaker the solution, the more valid the method 20. Therefore its use is justified considering that the 

solution strength does not exceed ~ 20%. The correlation of v,g with the length of the conacting chamber for a 

different gas speed u and volumetric flux of water  is displayed in Fig. 4. The coefficient of the gas mass 

exchange v,g represents (by definition) the average product of some local index ( v,g)loc and local interfacial 

surface Sloc. Therefore, its decrease along the contacting chamber length is attributed to the decrease of either 

( v,g)loc or Sloc. The former could occur if the diffusion on the gas side through the boundary layer quickly decays 

for any reason (e.g., fast equalisation of phase potentials). Otherwise, the governing factor is the duration of 

boundary layer formation. As to the possible decrease of Sloc, it may occur due to either coagulation (see above) 

or the effect of initial and boundary conditions.  

Any other admissible reason for v,g variation is dismissed because of the high values of Re and Pe-criteria (Re~ 

105, i.e., it is turbulent regime, while Pe~ 103 , i.e., convection is the dominant process). The elbow (saturation) 

of the curve in Fig. 4 marks the termination of the transitional unsteady period of two-phase flow formation and 

transition to the steady flow regime. 

   
Fig. 4. Coefficient of the gas mass exchange v,g versus height of the contacting chamber Lcont.  

A specific feature of the transfer coefficient  lies in the fact that it is the most generalised index of scrubbing 

efficiency and hence it is a function of both the operating conditions =  (P, T, u, ) and design features =  

(L, D, , S). However, the former is more or less restricted and cannot be widely varied, while the latter is 

completely dependent on hydro/gas-dynamics. This means that the mass exchange can be restrictively varied 

only by optimisation of the LSA design. Thus, from the specific point of view of the LSA designer, data in Fig. 

4 should be read as a practical recommendation for initial approximation of the contacting chamber dimensions.   

Depending on the given flow sheet and customer’s demands, this quantity will be specified later in the project 

development. The achieved values of index  can be found in Figures 3 and 4 and Table 1. The simplified entry 

of the overall purification efficiency can be presented in the form5:  

                                                               G ~ .   S                                                                   ( 20 ) 

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 Academic Journal of Science, Engineering and Technology 

Vol.6, Issue 1; January - Febuary 2021; 

1252 Columbia Rd NW, Washington DC, United States 

https://topjournals.org/index.php/AJSET/index; mail: topacademicjournals@gmail.com 

  

 

 
 

23 | A c a d e m i c  J o u r n a l  o f  S c i e n c e ,  E n g i n e e r i n g  a n d  T e c h n o l o g y  

|  https://topjournals.org/index.php/AJSET 

Here G is total amount of captured and fixed impurity (whether it be gas, liquid or solid),   is volumetric  

flux of liquid,  is residence time, S is specific interfacial surface, while   represents mass exchange coefficient.   

The average values of these quantities for LSST typical regimes fall within the following ranges. Volumetric flux 

of water is predetermined by gas momentum ( u)g and lies in the range ~(0.02-0.10) m3/m2s. The residence 

time is the sum of the fall time and one of the ascending co-flow along the contacting chamber 1=L/u, being 

equal to life= f+ 1~(0.2-2.5) s. It should be stressed that increase of the gas speed from u1 to u2=nu1, n>1 

implies not only the growth of the interfacial area, but also quadratic increase in the hydraulic resistance 

H=H2/H1~ (u2)
2/ (u1)

2~ (nu1)
2/ (u1)

2 n2, enforced carrying away of the dispersed phase from the 

contacting chamber and non-linear decrease of contact time ~( u)-1. In doing so, the product (S ) is correlated 

in such a way that it is a non-monotonous extremal function. Location of its extremum is dependent on the 

sorption efficiency and cannot be estimated a priori.  

Design and engineering features  

LSA units must provide both the main and some subsidiary technological operations. The inlet section is the 

domain of the incoming gas flux reorganization (Fig. 1). It is designed for gas input and (should the need arise) 

the cooling of the gas by water injection. These operations can be separated; however, the combined design offers 

definite advantages. A contacting chamber of typical length/diameter ratio (L/D) cont~(1.0-1.5) serves for mixing 

and sorption, as well as regime stabilisation in cases of wide gas load variation or frequent (spontaneous) 

input/output operations.  

For this purpose, it can be arranged with a stabilising circular collar. The latter is installed so as to divide the 

inlet and contact zones. Usually, this collar is the split ring, the details (shape, number of segments and coefficient 

of wetted passage =Sfree/Scont) being completely determined by the flow sheet. The height of the liquid bulk in 

the separation chamber hlevel is usually chosen in such a way as to provide a hydrostatic head on the recycling 

back-pipes h=hlevel-hpipes~(0.1-0.3) m. In turn, both the overall cross-section of back-pipes 

Spipes~0.785d2( h)1/2/ (2g)1/2 (here = 1.8.( h)0.92 -0.61 is the coefficient of liquid discharge) and their total 

number m~ Spipes/0.785d2 are dictated by volumetric flux of water  and gas speed u. The fixed centrifugal 

separator also has some special features, particularly the efficiency of phase separation, which is very sensitive 

with respect to the separator geometry, including number, height, and curvature of curvilinear blades. The 

dimensions of the separation chamber are dictated primarily by the demands of the total precipitable water 

(including water vapors and droplets) in the final exhaust, being dependent as well as on the dryness factor of 

the primary polluted air.  

For this demand to be fulfilled, the gas speed along the separation chamber usep=F(u, R)air should be smaller than 

the deposition rate uair(Rair) of airborne water droplets of some definite Rair-drop. Usually, the ratio (L/D)sep~(0.6-

0.8) provides a reasonable solution to this problem. In this case, the ratio between the diameters of the contacting 

and separation chambers falls in the region Dsep/Dcont~(ucont/usep)
1/2~(2-2.5).  

In its turn, any LSA can be arranged as a multi-stage unit enclosed in a common shell, any stage in a series being 

independently optimised along any parameter 9,10. As this occurs, the next stage is either an identical copy of the 

preceding stage (so-called cascade LSA) or distinctive in any respect (combined LSA).   

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Vasili I. Dimitrov                                                                                                                                                     24 

  

  

24 | A c a d e m i c  J o u r n a l  o f  S c i e n c e ,  E n g i n e e r i n g  a n d  T e c h n o l o g y  

|  https://topjournals.org/index.php/AJSET 

A cascade LSA is applied for fine cleaning, since the output of the previous stage is simultaneously the input 

reative to the following one. A combined LSA is used when each taken separately stage is optimized along 

another parameter.  

One remark of material significance 

In current engineering practice, the customer’s demands are a point in any real design. These demands are usually 

well known. They include efficiency of purification, minimisation of budget investment and/or operational cost, 

minimal water consumption, dryness of the exhaust, optimal dimensions, fool-proof design (i.e., safe 

maintenance achievable with low-skilled operator) and sometimes unforeseen demands. The order of preference 

(priority) of these demands, together with the given flow sheet, could result not only in simple changes of some 

details (e.g., dimensions) or even radical ones in design (one, two or three-stage LSA), but often predetermine 

the possibility of using wet scrubbing technology. Generally, this priority sequence is unpredictable, hence it 

should be posed and approved a priori.  

Unpredictable priority sequence of customer’s demands, specificity of LSST hydro/gas-dynamics, variable 

combination of physico-chemical parameters, character of the particular industrial site, as well as the very 

essence of scrubbing, lead to the point that LSA manufacturing is not repetitious work. There are sometimes 

apparent similarities (e.g., the same scale, impurities, physical conditions and degree of purification), which, in 

fact, are embodied into drastically different real plants. As is evident, to implement particular LSA, both standard 

design and intuitive engineering solutions (know-how) need to be used. In other words, each real case is unique 

and requires its own engineering design 8,10-12.  

Discussion and conclusions  

Comparison of relevant sorption and technology LSST ability factors with those of some conventional 

technologies, such as wet-bed3,5 and Venturi tube techniques21,22 can be found in Table 1. They show that LSST 

combines the main merits of conventional co/counter-flow technologies, while being free of their defects.   

Two key points that provide the main LSST advantages (primarily high mass-exchange efficiency) are the low 

absolute gas speed and spontaneous change of the counter-current regime to the co-current one. The decrease of 

typical gas speeds of conventional co/counter-flow technologies u~(20-40) m/s down to the typical LSST-speeds 

u~(10-20) m/s does not cause deterioration of the process on the whole; on the contrary, it enhances scrubbing 

efficiency while reduces control requirements. The main LSST merits and demerits can be generalized as 

follows.  

As indicated, both the process as a whole and some of its stages in particular (e.g., mixing, selffragmentation 

and phase separation) are very sensitive to the LSA geometric proportions. Hence, more sophisticated and careful 

study of the engineering design (at the stage of the project development) and manufacturing (at the stage of LSA 

implementation) are necessary. Design errors or inaccurate manufacturing results in dramatic deterioration of 

scrubbing efficiency.  

It may be concluded that LSST as a whole presents a successful and effective combination of well-known co-

current and counter current technologies. It provides for a stable scrubbing operation and allows for reasonably 

simple control and monitoring of the latter. The process is conducted by using scrubbers of special design LSA.    

Thus, the most specific features of LSA can be summarized as follows: 

1. LSA is free of any (e.g., mechanical and/or pressure) load; hence it is safe in operation.  

2. LSA has no unreliable structural units (e.g., injectors and/or any moving elements) that need careful 

maintenance; hence a highly skilled operator is not needed. It is a foolproof and serviceable structure.  



Vasili I. Dimitrov                                                                                                                                                     25 

  

  

25 | A c a d e m i c  J o u r n a l  o f  S c i e n c e ,  E n g i n e e r i n g  a n d  T e c h n o l o g y  

|  https://topjournals.org/index.php/AJSET 

3. LSA does not need any auxiliary facilities (e.g., re-circulation pumps).  

4. LSA has low hydraulic resistance.  

Nomenclature  

Design indices  

L[m], D [m] and V [m3] -  length, diameter and volume, respectively, Physico-chemical indices  

 [kg/m3] -  density,  

 [kg/s2] -  surface tension,  

 [kg/m.s] -  dynamic viscosity. Hydro/gas-dynamic indices  

d [m2/s] -  diffusion constant (Eq. 1, 2), H [Pa] -  hydraulic resistance, u [m/s] -  speed,  

Vg [m
3/s] -  total gas load, Gl [kg/m3] -  liquid-water content,  

gl [kg], vl[m
3] -  mass and volumetric holdup of liquid, respectively, gg [kg], vg[m

3] -  mass and volumetric holdup 

of gas, respectively,  

 [m3/m2.s] -  volumetric flux of liquid (current water content)   

Heat/mass exchange indices  

v,g [m
3/m3.s] -  coefficient of the gas mass exchange (volumetric flux from the gas phase to the liquid phase in 

relation to the gas volume),  

v,l [m
3/m3.s] -  coefficient of the liquid mass exchange (volumetric flux from the gas phase to the liquid phase 

in relation to the whole gas-liquid volume),  

v,S [m
3/m2.s] -  coefficient of the surface mass exchange (volumetric flux from the  gas phase to the liquid phase 

in relation to the specific interfacial surface),  

S [m2/m3] -  specific interfacial surface (interfacial area per unit volume), Subscripts and abridgements cont -  

contact, f -  fall, g -  gas, ini -  initial, l -  liquid, loc -  local, s -  surface, sep -  separation, sf -  self-fragmentation.   

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Vasili I. Dimitrov                                                                                                                                                     26 

  

  

26 | A c a d e m i c  J o u r n a l  o f  S c i e n c e ,  E n g i n e e r i n g  a n d  T e c h n o l o g y  

|  https://topjournals.org/index.php/AJSET 

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