Scholarly article on topic 'Investigating droplet separation efficiency in wire-mesh mist eliminators in bubble column'

Investigating droplet separation efficiency in wire-mesh mist eliminators in bubble column Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — Abdullah S. Al-Dughaither, Ahmed A. Ibrahim, Waheed A. Al-Masry

Abstract Effects of design parameters on performance of wire-mesh mist eliminators were experimentally investigated in 15cm bubble column. The demisters performances were evaluated by droplet collection efficiency as a function of wide ranges of operating and design parameters. These parameters include: droplet size exiting the demister (250–380μm), specific surface area (236–868m2/m3), void fraction (97–98.3%), wire diameter (0.14–0.28mm), packing density (130–240kg/m3), and superficial gas velocity (0.109–0.118m/s. All demisters were 15cm in diameter with 10cm pad thickness, made from 316L stainless steel layered type demister pad wires. Experiments were carried out using air–water system at ambient temperature and atmospheric pressure. The experimental data on the droplet removal efficiency were obtained using Malvern Laser Droplet Sizer. The removal efficiency was found to increase with the increasing the demister specific surface area, packing density, and superficial gas velocity. In contrast, the removal efficiency was found to increase with decreasing the demister void fraction and wire diameter. The separation efficiency is correlated empirically as a function of the design parameters. A good agreement was obtained between the measured values and the correlation predictions with ±5% accuracy.

Academic research paper on topic "Investigating droplet separation efficiency in wire-mesh mist eliminators in bubble column"

Journal of Saudi Chemical Society (2010) 14, 331-339

ORIGINAL ARTICLE

Investigating droplet separation efficiency in wire-mesh mist eliminators in bubble column

Abdullah S. Al-Dughaither a, Ahmed A. Ibrahim b, Waheed A. Al-Masry b *

a SABIC Research and Technology Complex, Riyadh, Saudi Arabia b Department of Chemical Engineering, King Saud University, Riyadh, Saudi Arabia

Received 10 November 2009; accepted 20 December 2009 Available online 14 April 2010

KEYWORDS

Mist eliminator; Wire mesh; Droplet separation; Bubble column

Abstract Effects of design parameters on performance of wire-mesh mist eliminators were experimentally investigated in 15 cm bubble column. The demisters performances were evaluated by droplet collection efficiency as a function of wide ranges of operating and design parameters. These parameters include: droplet size exiting the demister (250-380 im), specific surface area (236868 m 2/m3), void fraction (97-98.3%), wire diameter (0.14-0.28 mm), packing density (130240 kg/m3), and superficial gas velocity (0.109-0.118 m/s. All demisters were 15 cm in diameter with 10 cm pad thickness, made from 316L stainless steel layered type demister pad wires. Experiments were carried out using air-water system at ambient temperature and atmospheric pressure. The experimental data on the droplet removal efficiency were obtained using Malvern Laser Droplet Sizer. The removal efficiency was found to increase with the increasing the demister specific surface area, packing density, and superficial gas velocity. In contrast, the removal efficiency was found to increase with decreasing the demister void fraction and wire diameter. The separation efficiency is correlated empirically as a function of the design parameters. A good agreement was obtained between the measured values and the correlation predictions with ±5% accuracy.

© 2010 King Saud University. All rights reserved.

* Corresponding author.

E-mail addresses: Degether@sabic.com (A.S. Al-Dughaither), aidi@ ksu.edu.sa (A.A. Ibrahim), walmasry@ksu.edu.sa (W.A. Al-Masry).

1. Introduction

In many operations in chemical plants, it is frequently necessary to remove droplets from gas vapor streams. Droplets separation is required to recover valuable products, improve product purity, increase throughput capacity, protect down stream equipment from corrosive or scaling liquids, avoid undesired reactions, and to improve emissions control. Mist eliminators are devices that can remove entrained liquid from gas flow effectively. For example, in thermal desalinations plants, the droplets must be removed before vapor condensation over condenser tubes. If the mist eliminator doesn't separate efficiently the entrained water droplets, reduction of

1319-6103 © 2010 King Saud University. All rights reserved. Peerreview under responsibility of King Saud University. doi:10.1016/j.jscs.2010.04.001

Nomenclature

A (m2) bubble column cross sectional area z (m) distance between two successive layer

As (m2/m3) specific surface area

Dav (im) average droplet diameter exiting the demister Greek letter

Dd (im) droplet diameter e (-) void fraction

Dw (mm) demister wire diameter g (%) separation efficiency

Min (kg) mass of entrained droplet upstream the demister gsT (%) efficiency of single target

Mout (kg) mass of entrained droplet downstream the 1g (kg/ms) gas viscosity

demister p (-) constant (3.14)

n (-) number of layers pg (kg/m3) gas density

Qg (m3/s) volumetric flow rate pi (kg/m3 liquid density

St (-) Stokes number pp (kg/m3) packing density

Vg (m/s) superficial gas velocity

(Vol)liq (m3) bubble column liquid inventory volume

distilled water quality and formation of scale on the outer surface of the condenser tubes occurs. The last effect is very harmful because it reduces the heat transfer coefficient and enhances the corrosion of the tube material (Souders and Brown, 1934).

Another example is the two phase bubble column reactors. Bubble columns have been widely used in industry because of their simple construction and operation. Important applications include hydrogenation, oxidation, polymerization, Fischer-Tropsch synthesis, ozonolysis, carbonylation, carbox-ylation, alkylation reactions as well as for petroleum processes. Other important application area of bubble columns is their use as bioreactors in which microorganisms are utilized in order to produce industrially valuable products such as enzymes, proteins, and antibiotics. In the bubble column, the gas is introduced in the form of bubbles into a pool of liquid via a distributor. The mass transfer and hence the reaction takes place between the gas bubbles and the liquid. The gas stream leaving the liquid pool entrains droplets of liquid with it, which must be removed before it exits the reactor. Failure to do so will cause the reaction to continue in the exit streamlines. In polymerization reactions for example, the entrainment will cause plugging of the exit streams and overhead lines.

Mist eliminators belong to various groups that operate under different principles and are applied for the droplets removal with a specific size range. When selecting a mist eliminator, careful considerations should be given to performance parameters and one must weight several important factors so as to ensure a cost effective installation (Bell and Strauss, 1973; York, 1954). Collection efficiency is primarily a function of droplet size distribution, superficial gas velocity, mist loading and the mist's physical properties. Table 1 shows various groups of mist eliminators according to some performance parameters.

The knitted wire-mesh mist eliminator is one of these devices which have a widespread application in many industrial

plants. The separation process in the wire-mesh mist eliminator includes three steps; first 'inertia impaction' of the liquid droplet on the surface of wire. The second stage is the coalescence of the droplet impinging on the surface of the wires. In the third step, droplet detach from the pad. Wire-mesh mist eliminator has gained extensive industrial recognition as a low cost, easy installation, minimum tendency for flooding (re-entrainment), high capacity, small size, and efficient means for removal of entrained liquids droplets from vapor and gas streams. It is probably outnumber all other types of mist eliminators combined specially in petrochemical equipments such as scrubbers, evaporators and distillation columns. Although knitted wire mesh has been used by industry for broad ranges of entrainment elimination operations, the volume of fundamental work published regarding their performance characteristics is scant. The work of Satsangee (1948) was concerned primarily with wire mesh as column packing and contacting media and not specifically entrainment elimination. The detailed investigation of Carpenter and Othmer (1955) studied wire mesh as an entrainment separator in an evaporator handling salt solution and defined the efficiency, pressure drop, and capacity of knitted wire structure. As generally used, knitted wire-mesh mist eliminator consists of a bed, usually 10.1615.24 cm deep, of fine diameter wires interlocked by a knitting to form a wire-mesh pad with a high free volume, usually between 97% and 99%. The primary performance parameters affecting demister droplet removal are gas velocity, surface area, free volume, packing and hence, diameter of fibers used in mesh knitting and thickness of a demister.

2. Prediction of droplet separation efficiency

Semi-empirical equations based on the Souders-Brown relationship are commonly used for designing wire-mesh mist eliminators (York, 1954). However, their technique is rough

Table 1 Equipment selection versus mist particle size (Ziebold, 2000).

Style Brownian fiber beds Impaction fiber beds Mesh pads Vane separator

Collecting fiber diameter (im) Bed velocity (m/s) Pressure drop (mm H2O) Particle size collected (im) 8-10 0.05-0.25 100-450 <0.1-3 10-40 1.25-2.5 100-250 1-3 100-300 2-4 10-75 2-20 >300 2.5-5.0 3-25 >20

because it does not take into account neither the liquid load that can influence flooding of the pad, as indicated by York and Popple (1963), nor the droplet size, on which the collection efficiency is highly dependent. Recently, foretelling the separation performances based on a mechanistic description of the separation phenomena become possible. In theory, the collection efficiency of impingement-type demisters involves inertial, diffusion and interception captures (Holmes and Deckwer, 1984; Langmuir and Blodgett, 1946; Chotalal, 2004). Holmes and Deckwer (1984) showed that only inertial capture plays an important role in the separation efficiency for wire-mesh separators is largely contributed by inertial capture that entail neglecting the effect of interception and diffusion capture. Considering the dominancy of inertial capture, some models have been developed to evaluate the separation efficiency for a single wire target, gST (Langmuir and Blodgett, 1946). It was agreed for all models that the efficiency is a function of Stokes number, St, defined as

PiVgD,

18 igDw

where Vg is superficial gas velocity, ig the gas viscosity and Dd and Dw designate the droplet and wire diameters, respectively (Brunazzi and Paglianti, 1998).

Based on this sequence of study, the wire-mesh demister separation efficiency can be predicted by taking into account the separation efficiency for a single wire target as well as the demister geometry. Most of the published equations refer to the analysis proposed by Carpenter and Othmer (1955) who determined the wire-mesh demister efficiency in an evaporator. According to Carpenter and Othmer (1955), the theoretical effect of projecting a small water droplet at a single cylinder oriented with its axis perpendicular to the motion of the droplet approaching from a great distance has been determined by Langmuir and Blodgett (1946). Carpenter and Othmer (1955) assumed (a) uniform droplet size, (b) no re-entrainment, (c) no modifying buildup of liquid, and (d) constant entrainment removal efficiency in each layer of the demister. Based on that, Carpenter and Othmer suggested the following expression:

g = 1 - ( 1 - 2 AsgST p

where As is the specific surface area of the demister, Z is the distance between two successive layers, n is the number of layers that form the separator and gST is the single target efficiency.

Brunazzi and Paglianti (1998) performed an experimental study on wire-mesh demisters in horizontal and vertical arrangements using water spray generation circuit and carrier air circuit operating at ambient conditions. Brunazzi and Paglianti (1998) proposed the following relation:

g = 1 - (1 - gST)M ' + T' (1 - gST)

where M is the number of "reference" cells present in the pad, n is the number of layers necessary to fill each cell, and n' represents the number of layers that are not sufficient to form a complete cell. Expressions for M, n, and n' can be found in reference (Brunazzi and Paglianti, 1998). Brunazzi and Paglianti (1998, 2000) model is based on the following assumptions:

(a) no re-entrainment, (b) no buildup of liquid, and (c) no mixing after passage through each layer. The first two suppositions are common to the model suggested by Carpenter and Othmer (1955), whereas the last represents the difference between the two models. Brunazzi and Paglianti model and the model published by Carpenter and Othmer (1955) agree for packing with thicknesses greater than 65 mm. For thinner pads the model suggested by Carpenter and Othmer (1955) systematically underestimates the experimental efficiencies, whereas Brunazzi and Paglianti model enables good prediction of experimental removal efficiencies even for packing with thicknesses less than 65 mm (Brunazzi and Paglianti, 1998).

Eqs. (2) and (3) make it possible to calculate the separation efficiency if the efficiency of a single target, gST, is known. In the literature, many different equations are available to compute the efficiency of a single target and one of the most commonly used has been suggested by Langmuir and Blodgett (1946). Langmuir and Blodgett prepared a table of gST as function of the other parameters including wire radios, droplet diameter, and conditions of operation (Carpenter and Othmer, 1955). Since Langmuir and Blodgett (1946) were concerned with the formation of ice on the cylindrical leading edge of the wing of an airplane, their theoretical approach can induce underestimation of the separation efficiency when it is applied to an array of targets that are close to each other. This is the case of wire-mesh mist eliminators, and for this reason Bru-nazzi and Paglianti (1998) showed that separation efficiency of a common wire-mesh mist eliminator can be properly evaluated based on Stokes number (St) if the following empirical relation is considered:

gST = St for St < 1 (4)

while if

gST = 1 for St P 1 (5)

where St has been defined according to Eq. (1).

It should be noted that the above relations is applicable if the range of the superficial gas velocity is in the range of

0.9.5.5 m/s and 1-2 m/s for Eqs. (2) and (3), respectively. However, in some occasions the application of bubble columns requires lower gas velocity (<0.2 m/s) which could make the above relations (Eqs. (2) and (3)) not reliable for bubble column applications. To the best of our knowledge, there is no study available in the open literature that deals with the application of the wire-mesh mist eliminator in bubble column.

In view of the previous discussion the following conclusions can be drawn:

1. We believe that the open research on performance evaluation of the wire-mesh mist eliminators is very limited despite the broad range of entrainment removal applications.

2. The available theoretical or empirical models that describe the performance of the wire-mesh mist eliminators are not adequate for implementing to the industrial units. The case of bubble column, however, is very extreme where there is no model to predict the performance of this type of demister.

The present investigation reports the results of experimental work using a knitted wire-mesh separator as an entrainment eliminator in bubble column. Various types of wire-mesh separators that are different in geometrical specifications are employed. The main objectives of this study include:

1. To investigate the design characteristics those affect the droplet separation efficiency in the wire-mesh demister employing a bubble column.

2. To develop a correlation for predicting droplets removal efficiency in the demister inside bubble columns. This correlation is established for the separation efficiency as function of the average droplet size exiting the demister, specific surface area, void fraction, wire diameter, packing density, and superficial gas velocity.

3. Experimental apparatus and procedure

The experimental setup is designed and built at the Department of Chemical Engineering of King Saud University. The experimental work is performed in 15 cm diameter bubble column which is fabricated from galvanized carbon steel. The upper flange is made of Plexiglas material. The experimental

apparatus is schematically sketched in Fig. 1 that includes the system components. Air flow rate is controlled using Omega Engineering Volumetric Flow Controller (Model No. FMA-2611). The flow set point is set by the digital readout device and the required flow is maintained accordingly. Air flows via perforated plate (sparger) through water pool in bubble column and detaches from the liquid surface towards the demister. This area is called disengagement zone which is fixed at 14 cm. The water droplets were carried over by air stream flowing towards the demister. Part of the large size droplets return back to the water pool as a result of gravity and most of the droplets continue flowing up towards the demister. The test demisters are supplied by RHODIUS GmbH. They are varied in geometric characteristics as shown in Table 2. All demisters are 15 cm diameter, 0.1 m pad thickness and made from Stainless Steal 316L without supporting grid. All experiments were carried out at ambient temperature and atmospheric pressure (T = 25 0C and P =1 atm).

Temperature controller

Transmitter unit

Flow controller -—,

To outside

Heater

Heater

Temperature controller

— Air

О О « _

0 0 о (P

О oo о

0 0 о о ОЧ о а 0.0° о о о о °00 „ОО ОСОООоО °о SJ? On» <Р>Р

W,14P, 0

= °оо°о°о?„

Control valve

Compressed air

Receiver unit

Wire mesh demister

Air bubbles Water

Sparger

Figure 1 Experimental test apparatus.

Table 2 Geometric characteristics of the test demisters.

Type Wire diameter (mm) Packing density (kg/m3) Specific surface area (m2/m3) Void fraction (%)

RHO-80-SS-0.28 0.28 80 145 99

RHO-110-SS-0.28 0.28 110 200 98.6

RHO-130-SS-0.28 0.28 130 236 98.3

RH0-145-SS-0.28 0.28 145 265 98.1

RH0-175-SS-0.28 0.28 175 320 97.8

RHO-240-SS-0.28 0.28 240 435 97

RH0-130-SS-0.14 0.14 130 472 98.3

RHO-240-SS-0.14 0.14 240 868 97

The droplet size is measured by a Malvern/INSITIC INline EPCS (Ensemble Particle Concentration and Size) system. This system is designed to continuously measure and feedback the droplet size distribution information of inlet and outlet stream of the demister. The EPCS system contains five primary components: the optical head, an electronic interface, the computer, computer interface cards and the software. The EPCS uses the He-Ne laser diffraction technique to measure the droplet size which allowed accurate measurements of the volumetric droplet distribution. Particular care was given to the acquisition of the experimental data in order to minimize the

M — Pl ■ V

Assume that the droplet sizes are symmetric, the volume the of water droplets is calculated as follows:

V — (No. Droplets x V(one droplet))

where V(one droplet) can be computed as 4

V(one droplet) — - ■ p ■ Did

By substituting Eqs. (8)-(10) in Eq. (7), droplet separation efficiency can be evaluated as

(p x No. Droplets x 4 ■ p ■ — (p x No. Droplets x | ■ p ■ D

(p x No. Droplets x | ■ p ■ Dd

i) out .

(No. Droplets x Dd)in — (No. Droplets x^D

i) out

(No. Droplets x Dii

noises due to the measuring system and to maximize the accuracy of the acquired data. Laser is radiated from the transmitter unit through a column to the receiver unit via two special glass windows that allow laser to penetrate without scattering. Due to continuous condensations occurred on the glass window inside surface, continuous hot air from electric air heaters flows to the windows to prevent such potential.

The volumetric mean droplet diameter reading D (Carpenter and Othmer, 1955; Fabian and Hennessy, 1993) has been taken three times for each test then the average value is calculated.

Superficial gas velocities are calculated using:

v = —

where Qg (in m3/s) is the volumetric flow rate at atmospheric pressure and ambient temperature. A is the cross sectional area of the column which is fixed at 0.018 m2.

It is attempted to find out the most suitable correlation from the open literature that can describe the droplet separation efficiency by the wire-mesh mist eliminator in bubble column. As described before, there are some semi-empirical correlation models which fall short to describe adequately the existing system. Therefore, it is essential to obtain a new correlation based on aspects of the available system. The applicability of Eqs. (2) and (3) was checked for minimum Stoke's number case (Dw = 0.28 mm, Dd = 270 im, Vg = 0.109 m/ s). It was found that Stoke's number is always higher than 1 based on Eq. (1) meaning that single target efficiency approaches 100% regardless the demister geometry which is not true. The reason for this appearance is the high generated droplet size for the present system (250-380 im) compared to the system used in the studies by Carpenter and Othmer, 68 im (Carpenter and Othmer, 1955) and Brunazzi and Pagli-anti, 2-20 im (Brunazzi and Paglianti, 1998, 2000).

Generally, the separation efficiency is a measurement of the droplets fraction in the gas swept out by the wire-mesh mist eliminator and given by:

Mm — Mp, " Mi„

■x 100

where Min and Mout are the mass of entrained water droplets upstream and down stream the demister, respectively. The mass of water droplets is defiined as

where Ddin and Ddout are the droplet diameters upstream and downstream of the demister, respectively. The droplet size upstream and downstream the demister is determined from the experimental data. The upstream droplet size is constant which is measured by running the column without demister.

Bell and Strauss (1973) described the number of mists entering and leaving a Louver mist eliminator at different superficial gas velocities. The air is flow through pool of water in a packed, cross flow scrubber at atmospheric pressure and normal temperature (20-22 0C) which is close to bubble column operation. Bell and Strauss experimental variables ranges are Dd (50-500 im), Vol (water loading, 0.045-0.09 m3), and Vg (3-5 m/s). The present experimental variables in this investigation fall in the following ranges: Dd (250-500 im), Vol (water loading, 0.04 m3), and Vg (0.109-0.118 m/s). Despite the gas velocity, these are close to the variable ranges of Bell and Strauss. For the gas velocity, there was no detected droplet observed below 0.109 m/s velocity. At gas velocity above 0.118 m/s, water reaches the demister and the entrained droplet cannot be detected, too. Bell and Strauss reported their data in graphical form. In this work, data was extracted from their plot and the following correlation was developed for determination of number of droplets upstream demister by employing regression analysis:

(No. Droplets)in — (—3 x 10—5Di + 0.0147Dd + 31.259)m xyj V/ x (Vol)0

The same procedure is used to determine the number of droplets leaving the demister as follows:

(No. Droplets)oUt — (—7 x 10—6Di — 0.0053Di + 14.343)

^V, 5 x (Vol)

Eqs. (11)—(13) have been used for the calculation of the separation efficiency.

Demisters are usually specified by means of their geometrical specifications like specific surface area (As), void fraction (e), wire diameter (Dw), and packing density (pp). These parameters are defined as

Surface area of wires

s Volume of demisters

Mass of wires qp Volume of demisters and

Volume occupied by wires Volume of demisters

4. Results and discussion

In this investigation, a series of hydrodynamic experiments are performed to study the effect of the design parameters on the droplet removal efficiency by wire-mesh mist eliminator in a bubble column. The design parameters affecting separation efficiency and pressure drop include specific surface area (As), void fraction (e), wire diameter (Dw), packing density (pp), and superficial gas velocity (Vg).

4.1. Effect of specific surface area

Fig. 2 elucidate the obtained droplets separation efficiency as function of superficial gas velocity at three specific surface areas for two different wire diameter demisters. As it can be seen, all curves show similar trends where the removal efficiency increases with the increase of the specific surface area. As defined by Eq. (14), the specific surface area represents the ratio of the total surface area of the wires to the total volume of the demister. As the wires surface area increased, the free space for gas flow will be decreased. This will increase the number of captured droplets carried by gas flow on the

wires surface. This description may give reason behind this event where the removal efficiency enhances with the increase of the specific surface area. The maximum efficiency obtained for 0.28mm wire diameter demisters was 86% at 435m2/m3 surface area, 264.3 im average droplet size, and 0.115 m/s superficial gas velocity. Alternatively, the minimum efficiency obtained was 62.4% at the conditions of 236 m2/m3 surface area, 344 im average droplet size, and 0.109 m/s superficial gas velocity. For 0.14 mm wire diameter demisters, the maximum efficiency acquired was 88% at 868 m2/m3 surface area, 252 im average droplet size, 0.118 m/s superficial gas velocity. However, the lowest efficiency recorded was 65.4% at the conditions of 472 m2/m3 surface area, 334 im average droplet size, 0.109 m/s superficial gas velocity.

4.2. Effect of void fraction

Void fraction (e) represents the ratio of the volume of the demister interstices to its total volume. The interstices can be quantified as the subtraction of the total demister volume to the volume occupied by the wires. The void fractions of demisters with 0.28 mm wire diameter are 97%, 97.8% and 98.3% while 0.14mm wire diameter demisters have 97% and 98.3% void fractions. The efficiency is plotted for each void fraction against gas velocity as shown in Fig. 3 at different void fractions. The results indicate that the removal efficiency is being enhanced as the void fraction is reduced. The void fraction is associated with the reduction of the surface area. As much the volume of interstices increased, the surface area for the wires is decreased. This may give reason for the removal efficiency enhancement with the reduction of the void fraction. The maximum efficiency obtained for 0.28 mm wire diameter

As, m2/m3

o 435 0 o X o X o o X o

o X X o

o o (a)

0.112 0.116

Vg (m/s)

100 90 80 70 60

Vg (m/s)

Figure 2 Effect of surface area on the separation efficiency at different gas velocities: (a) Dw = 0.28 mm; (b) Dw = 0.14 mm.

Sä S)

O 98.3

X 97.8

o 97 0 X 0 X

o X 0 o

° X ©

o (a)

0.112 0.116

Vg (m/s)

O 98.3

X 97 X X X o X o X o

o (b)

Vg (m/s)

Figure 3 Effect of void fraction on the separation efficiency at different gas velocities: (a) Dw = 0.28 mm; (b) Dw = 0.14 mm.

demisters was 86% at the conditions of 97% void fraction, 264 im average droplet size, and 0.115 m/s superficial gas velocity. In contrast, for 0.14 mm wire diameter, the maximum efficiency was 88% at 97% void fraction, 252 im average droplet size, and 0.118 m/s superficial gas velocity. The minimum efficiency found for 0.28 mm wire diameter demisters was 62.4% at the conditions of 98.3% void fraction, 353.7 im average droplet size and 0.109 m/s superficial gas velocity. Alternatively, for 0.14 mm wire diameters demisters, the lowest efficiency obtained was 65.4% at 98.3% void fraction, 333.7 im average droplet size and 0.109 m/s superficial gas velocity.

4.3. Effect of wire diameter

The effect of the wire-mesh diameter on the separation efficiency with the increase of superficial gas velocity is shown in Fig. 4 for tow different packing densities (130 and 240 kg/ m3). For each packing density, two different wire diameter demisters are utilized (0.14 and 0.28 mm). The results are obtained for a maximum detectable droplet size of 400 im. As it can be observed, the separation efficiency is insensitive to the increase of the wire diameter especially for 130 kg/m3 packing density. The droplet separation efficiency improves a little with the decrease of the wire diameter. This is caused by the fact that the surface area of the wires at constant packing density and depth is directly related to the wire diameter. As a result, more droplets with smaller sizes can be trapped by mesh wire with smaller diameter. The number of liquid droplets touching the wire is primarily determined by the ratio of the wire diameter and droplet size. As the wire diameter is reduced, the surface area increases (e.g., for qp = 130 kg/m3, As = 236 m2/m3 for 0.28 mm Dw and 472 m2/m3 for 0.14 mm Dw). Therefore, the thinner wires provide dense packing that can trap the entrained droplets by capillary action between the wires. Capillarity action can be explained by considering the effects of two opposing forces: adhesion, the attractive (or repulsive) force between the molecules of the liquid droplets and those of the wire surface, and cohesion, the attractive force between the molecules of the liquid. Adhesion causes water to wet the demister wires and thus causes the water's surface to rise. If there were no forces acting in opposition, the water would creep higher and higher on the demister wires and eventually overload the demister. Although, the results show good performance of demisters with smaller wires; on the other hand, use of larger diameter wire is necessary to facilitate demister washing and cleaning. Also, the use of larger diameter wire gives adequate mechanical strength and opera-

tional stability. The calculated efficiencies are in the range of 62.4-81.7% for 130 kg/m3 packing density, and from 75.1% to 88% for 240 kg/m3 packing density. Carpenter and Othmer (1955) emphasized that the reduction in the wire diameter would provide the most effective improvement for capturing the smaller particles, but the physical properties for the material used in making the wire and fabricating it in a machine will control the extent to which a reduction in wire diameter becomes possible or practical.

4.4. Effect of packing density

The packing density can be expressed as the mass of the wire over the total volume of the demister. The increase of the pad packing density is associated with the reduction of the void fraction (El-Dessouky et al., 2000). As a result, the number of entrained droplets that approach the wires and the amount of captured droplets increase as the void fraction of the demister is diminished. This fact can be employed to explain the steady augmentation of the separation efficiency with the increase of the demister packing density as illustrated in Fig. 5 for two different wire diameters. Six different demisters were tested for 0.28 mm wire diameter at different packing densities. These are 80, 110, 130, 145, 175, 240 kg/m3. The droplet sizes leaving 80 and 110 kg/m3 demisters are large enough so they cannot be detected by the instrument. It is interesting to note that the effect of the pad density on the separation efficiency is more pronounced at low superficial gas velocity. This is mainly due to the increase of the droplet re-entrainment and liquid holdup at higher air velocities. The maximum efficiency obtained for 0.28 mm wire diameter demisters was 85% at 240 kg/m3 packing density, 270 im average droplet size and 0.118 m/s superficial gas velocity. For the same wire diameter, the minimum efficiency acquired was 62.4% at 130 kg/m3 packing density, 353 im average droplet size, and 0.109 m/s superficial gas velocity. For the case of 0.14 mm wire diameter, two different packing density demisters (130 and 240 kg/m3) were tested. The best efficiency obtained was 88% at the conditions of 240 kg/m3 packing density, 251 im average droplet size and 0.118 m/s superficial gas velocity. On the other hand, the minimum efficiency determined was 65.4% at 130 kg/m3 packing density, 334 im average droplet size, and 0.109 m/s superficial gas velocity.

4.5. Effect of superficial gas velocity

The previous figures (Figs. 2-5) illustrate the percentage of droplets removed as a function of superficial gas velocity in

60 0.108

Dw, mm

O 0.14

X 0.28

o X o X

O 0 x X (a)

0.112 0.116

Dw, mm O 0.14 X 0.28 « O O X 8 o X

X (b)

0.112 0.116 (m/s)

Figure 4 Effect of wire diameter on the separation efficiency at different gas velocities: (a) pp = 130 kg/m3; (b) pp = 240 kg/m3.

60 0.108

pp, Kg/m3

o 130 X 145 o 175 A 240

A A 8 <cx o A O x o

A 8 » O

O O (a)

Vs (m/s)

60 0.108

0.112 0.116 Vg (m/s)

Figure 5 Effect of packing density on the separation efficiency at different gas velocities: (a) Dw = 0.28 mm; (b) Dw = 0.14 mm.

m/s. The overall experiments conducted for different demister design parameters show that the efficiency is low at lower superficial gas velocity and increases as the velocity is raised. This could be caused by the fact that only smaller droplets pass upward through the disengagement zone to reach the separator at low velocities and that many of these are carried along with the gas around the wire. However, as the velocity increases, even the smaller droplets will be less likely to be carried around the wires in the gas stream lines, owing to the inertial forces overcoming the tendency of these droplets to follow the path of the gas stream lines. Then, the size of the droplets passing disengagement zone and reaching the separator will increase with increasing superficial gas velocity. Since the large droplets will be less likely to be carried in the gas stream lines, a larger portion of them will be captured by collision with the wires. This increases the efficiency continuously and to reach 88% maximum for 0.14 mm wire diameter at the conditions of 240 kg/m3 packing density, 251 im average droplet size and 0.118 m/s superficial gas velocity. For similar conditions, 0.115 and 0.113 m/s air velocities give 87.1% and 85.8%, respectively, so they have close efficiencies as for 0.118 m/s air velocity. The same findings are observed for the most of the experiments whereas at 0.113 m/s velocity, the separation efficiency remains nearly at the same value. In some of the experiments, it is observed that the separation efficiency starts to decrease at 0.115 m/s with increasing the superficial gas velocity as shown in Fig. 5 for different demisters packing densities. This phenomenon could be described by the fact that above 0.115 m/s, the free drainage of the separator is impeded by the rising gas, and the separator begins to give evidence of liquid holdup or overload on its component wires. At sufficiently high velocities, the effect of gravity and surface tension is overcome by the pressure of the rising gas, and at least some of the liquid film enveloping the wires is swept upward to the top layer of the demister. Therefore, for higher velocities, most of the droplets loaded in the demister are stripped off from the wires to the top surface and are carried down stream from the demister as re-entrainment. Thus, re-entrainment may be defined as entrainment that is initially removed by the demister, and eventually escaped being torn from the elements or wires of the demister (El-Dessouky et al., 2000).

S 60 -

« 40 -

20 40 60 80 Measured effeciency ( n ),%

Figure 6 Parity plot for the calculated efficiency.

sign parameters of the wire-mesh mist eliminator in bubble column is produced. These include the average droplet size exiting the demister (Dav), surface area (As), void fraction (e), wire diameter (Dw), packing density (pp), and superficial gas velocity (Vg). These correlations are obtained based on 15 cm bubble column diameter and SS wire-mesh demister of 15 cm diameter and 0.1 m pad thickness at standard environmental conditions (T = 25 0C and P = 1 atm). Taking a regression fitting of the experimental data gives the following empirical correlation:

g = 5693.9(Dav x 10-6) 0:7534 x (Dw x 10-3)-0

>( [Pp

0.0042

The considered ranges of the experimental variables were Dav (250-380 im), Dw (0.14-0.28 mm), pp (130-240 kg/m3), e 97-98.3%), As (236-868 m2/m3), and Vg (0.109-0.118 m/s). Eq. (17) is in-line with the experimental observation data where the droplet separation efficiency is proportional to the increase of surface area, packing density, and superficial gas velocity. On the other hand, the droplet separation efficiency is reduced with the increase of void fraction and wire diameter. The comparison between the obtained separation efficiency data and the calculated values using the developed correlation is shown in Fig. 6. This figure demonstrates clearly that the correlation can be used to evaluate the separation efficiency with an accuracy of ±5%.

5. Correlation of the experimental data

6. Conclusions

0.0012/ -, -0.0074

As part of this study, an empirical correlation for predicting the droplet separation efficiency as a function of the main de-

The available theoretical models developed for the performance of the wire-mesh demister are not adequate to apply

to bubble columns. Hence, the current study gained more emphasis to understand the performance of the wire-mesh demister in bubble column. In this work, the experimental investigations showed that the droplet separation efficiency augments steadily with the increase of the demister specific surface area and packing density. Also, the separation efficiency is enhanced clearly as the void fraction is decreased. However, similar effect is small for the case of the demister wire diameter, and become more obvious at higher superficial gas velocities especially for 130 kg/m3 packing density demister. The separation efficiency rises steadily as the superficial gas velocity is increased. This continues up to certain velocity after which the separation efficiency remains constant with the increase of the gas velocity. The empirical correlation obtained gives sound model for predicting the separation efficiency with accepted accuracy (±5%).

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