Study on Separation of Water and Alumina Fine Particles by Cross-Flow Microfiltration

Zheng Bo; Tang Xiaojin; Hou Shuandi; Zong Baoning

(SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

Study on Separation of Water and Alumina Fine Particles by Cross-Flow Microfiltration

Zheng Bo; Tang Xiaojin; Hou Shuandi; Zong Baoning

(SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

A cross-fow microfltration process had been developed to separate alumina fne particles from the suspension using a stainless steel membrane tube with a pore size of 10 μm. The infuence of cross-fow velocity and trans-membrane pressure on the permeate fux and the solid holdup in permeate had been investigated. It was found that both the permeate flux and the solid holdup in permeate decreased with time. Moreover, the permeate flux increased with an increasing transmembrane pressure but the infuence of cross-fow velocity on the permeate fux was quite complex. Both the permeate fux and the solid holdup in permeate in long term fltration had been studied. The operation of cross-fow microfltration could be carried out stably for 10 hours with the permeate fux values ranging from 520.5 to 936 L/(m2·h) atrs=1%, while it could continue in 10 hours with the permeate fux values ranging from 226 to 432 L/(m2·h) atrs=5%. The solid holdup in permeate had been less than 10 mg/L during the whole operating cycle.

cross-fow microfltration; permeate fux; rejection rate; alumina fne particles

1 Introduction

Membrane separation technology has been widely used in environmental protection, biochemical engineering, and energy and chemical industries. Especially, the cross-fow microfiltration of inorganic membrane has advantages of high mechanical strength, good chemical stability and separation effciency in dealing with the suspension with high solid holdup. Compared with the dead-end microfiltration, the bulk fluid along with membrane surface could result in shear stress on the membrane, which would reduce the formation of a cake layer to extend the service life of filtration operation. Although the cross-flow microfiltration could limit the growth of the cake layer, it cannot prevent membrane from fouling. Some researchers have been studying on cross-flow microfiltration technology. Altmann, et al.[1]developed a mathematic model based on the force analysis of a single particle to describe the formation of a cake layer in cross-flow filtration, by which the permeate flux and the feature of cake layers could be predicted. Makardij, et al.[2]studied the effects of temperature, cross-flow velocity, and feed concentration on the permeate flux in a cross-flow microfiltration process and developed a simple model containing only two coeffcients. Makabe, et al.[3]investigated the influence of flow rate, particles diameter, and initial flux on the membrane fouling, and found that adopting larger solid particles and higher cross-flow velocity could prevent the membrane from fouling. Kwon, et al.[4]compared the critical permeate flux based on the mass balance (If it was below the critical permeate, particles would not deposit) and the increase in transmembrane pressure (If it was below the critical permeate, the membrane would not be fouled) respectively, and found that the critical permeate flux increased with the increase in particles size, and decreased with the increase in feed concentration. Kwon also found that the ionic strength of suspension only influenced the critical fux based on the increase in transmembrane pressure.

There are many applications of heterogeneous catalysis especially when the solid catalysts are applied in the energy and chemical industries, in which the catalysts with micron or sub-micron size grade used in the liquid-solid/gas-liquid-solid systems are playing an important role, so the separation of fine solid catalystsfrom the reaction systems is very essential. Cross-flow microfiltration is an effective method to solve these separation problems. In this study, the fine particles of alumina that were most widely used as catalyst carriers in the petroleum industry were selected as the solid phase, and the investigation on cross-flow microfiltration for suspension containing water and alumina fine particles was carried out.

2 Experimental

The schematic diagram of the cross-flow microfiltration setup is shown in Figure 1. The experimental setup mainly contains a feed tank, a cross-flow filter, a back-flushing tank and a permeate tank. The flter, 0.077 m in diameter and 0.948 m in height, is provided with a stainless steel membrane tube with a diameter of 0.037 m and a length of 0.274 m. The membrane tube has a nominal pore size of 10 μm, and the total fltration area is equal to 0.032 m2.

As shown in Figure 1, the suspension with fine solid particles was delivered from the feed tank into the flter by a feed pump. By making use of the pressure difference in the system, the permeate was able to pass through the membrane tube and then enter the permeate tank. The retentate in the flter shell pass was pumped into the feed tank by a retentate pump. In order to maintain a constant feed concentration, the permeate was also circulated into the feed tank. When the membrane was fouled, the backflushing method was used to solve this problem. After closing the permeate line valves, the back-flushing line valves were opened, so the liquid in the back-flushing tank would be instantly pushed into the membrane tube to clean the membrane by making use of the pressure difference between the flter and the back-fushing tank. After completion of the back-flushing operation, the separation could go on by closing the back-fushing line valves and opening the permeate line valves.

Figure 1 Schematic diagram of the experimental setup.

The permeate flux was calculated by the measured permeate volumetric rate and the filtration area. The size distribution of solid particles was measured by a laser particle size analyzer (Mastersizer). An electron microscope (SOIF) was used to observe the shape and dimension of the fine particles in permeate. The permeate turbidity was measured by a turbidimeter-2100P (HACH) and the solid holdupCin permeate was calculated by permeate turbidity according to Eq. (1)[5].in whichCis the solid holdup in permeate (mg/L),dpis the Sauter mean diameter of particles in the permeate (μm),dsis the Sauter mean diameter of particles in the suspension,Tis permeate turbidity (NTU), andT0is the pure water turbidity (NTU), which has a value of 0.13 NTU.

Pure water and alumina particles represent the liquid phase and the solid phase, respectively. Material properties are listed in Table 1.

Table 1 Material properties (T=20 °C,p=0.1 MPa)

The size distribution of alumina particles is shown in Figure 2.

Figure 2 Particle size distribution of alumina particles

The experimental conditions in this study are summarized in Table 2.

Table 2 Experimental conditions in this study

3 Results and Discussion

3.1 Rejection rate of membrane

Rejection rate can be used to evaluate the separation capability of membrane, and it can be calculated by Eq. (2).

in whichCfeedis solid holdup in the feed (mg/L),Cis solid holdup in the permeate (mg/L), andξis the rejection rate of membrane.

As an example, when the experimental conditions covered ΔP=0.065 MPa,UL=0.649 m/s, andrs=5% (Cfeed=5×104mg/L). The solid holdup in permeate calculated by Eq. (1) was 0.52 mg/L. Therefore the rejection rate of membrane could approach 99.99%. A comparison between the suspension and the permeate is shown in Figure 3. It can be found that the permeate is transparent just as the clean water. The fine particles in the permeate are shown in Figure 4.

Figure 3 Comparison between the suspension and the permeate

Figure 4 Fine particles in the permeate

According to all experimental data of the solid holdup in permeate, the highest value of solid holdup in the permeate is 10 mg/L (which will be described below), and the calculated rejection rate of membrane is equal toat least 99.9% as shown in Figure 5. Therefore the crossfow microfltration process we developed could separate solid fne particles from the suspension effectively.

Figure 5 Rejection rate of membrane for all permeate samples

3.2 Effects of cross-flow velocity and transmembrane pressure on cross-flow microfiltration

The permeate fluxJis an important parameter for evaluating the separation capacity of membrane. It is better to provide an optimized permeate fux to meet the demand of rejection rate and production. The permeate fux could be described with Dacy’s Law in Eq. (3).

in which ΔPis the transmembrane pressure (Pa),Ris the total resistance during filtration (m-1), andμis the permeate viscosity (Pa·s).

Eq. (3) shows that with the increase ofR,Jwould decline with time under a constant transmembrane pressure. In contrast with the traditional dead-end fltration, the crossflow filtration operation could remarkably reduce and control the resistance to extend the operating cycle at a satisfactory permeate fux. From this point, it is important to study the effects of operating parameters on the permeate fux.

3.2.1 Effects of cross-flow velocity on cross-flow microfiltration

Figure 6 and Figure 7 show the effects of cross-flow velocity on the permeate fux. It can be found that the permeate flux declines quickly in the initial filtration period (t<30 min) and then declines slowly. Because the membrane pores are blocked or covered by fine particles at the early stage of operation, the filtration resistance increases remarkably and rapidly. During the middle and later period of fltration, particles build up on the surface of membrane to form the cake layer, so the permeate fux declines slowly. The shear force caused by the cross-fow of fuid along with the surface of membrane could restrain the growth of cake layer and sometimes may destroy the cake layer. During this period, the tendency of reducing permeate flux slows down. These two Figures also show that the permeate fux decreases with the increase of cross-fow velocity especially in the middle and later period of operation. It might occur that with the increase of cross-flow velocity, the shear force along with the membrane surface increases and could destroy the cake layer. Owing to the lack of measures to keep the cake layer intact, some fine particles have the chance to block pores further, which would make the permeate flux reduce obviously[6].Figure 8 and Figure 9 show the effects of cross-flow velocity on the solid holdup in permeate. It could be found that the solid holdup in permeate decreased with time, because the fouled membrane could effectively catch the fne particles in suspension. The solid holdup in permeate was all less than 10 mg/L, so the rejection rate of membrane reached more than 99.9%.

Figure 6 Effects ofULonJatrs=1%

Figure 7 Effects ofULonJatrs=5%

Figure 8 Effects ofULonCatrs=1%

Figure 9 Effects ofULonCatrs=5%

3.2.2 Effects of transmembrane pressure on crossflow micro filtration

Figure 10 and Figure 11 show the effects of transmembrane pressure on permeate fux. It can be found that the permeate fux decreases with time and increases with the increase of transmenbrane pressure. According to Eq. (3), the transmembrane pressure plays a dominant role as a driving force of microfiltration and determines the value of permeate flux. The larger transmenbrane pressure leads to a higher permeate flux.

Figure 12 and Figure 13 show the effects of transmembrane pressure on the solid holdup in permeate. It can be seen that the solid holdup in permeate decreases with time and is less than 7 mg/L along with a rejection rate of more than 99.9%.

Figure 10 Effects of ΔPonJatrs=1%and UL=0.743 m/s

Figure 11 Effects of ΔPonJatrs=5%and UL=0.649 m/s

Figure 12 Effects of ΔPonCatrs=1%and UL=0.743 m/s

Figure 13 Effects of ΔPonCatrs=5% andUL=0.649 m/s

3.3 Long term operation of filtration

The cross-flow microfiltration should keep running continuously and stably to meet the needs of production. So it is important to study the long term operation of membrane. Figure 14 and Figure 16 show the permeate flux curve with time in long term operation under different conditions, while Figure 15 shows the relationship between ΔJ/Δtand the operating time. It can be seen from Figure 14 that the curve of permeate flux, which declined with time, could be divided into three stages. In the first stage, the permeate flux declined rapidly. As shown in Figure 15, during the period of 5 to 15 min, the range of |ΔJ/Δt| covered from 56 to 3000 (L/(m2·h))/ min. In the second stage, permeate flux declined slowly. During the period of 15 to 100 min, the range of |ΔJ/Δt| covered from 0.4 to 6 (L/(m2·h))/min. In the third stage, the permeate flux changed extremely slowly, and almost reached a steady state[7]. During the period of 100 to 600 min, the values of (ΔJ/ Δt) were close to zero with a variance of ±0.25 (L/ (m2·h))/min. It could be supposed that permeate flux had reached a quasi-stationary value.

Figure 14 Permeate flux in long term operation atrs=1%

Figure 15 Relationship between (ΔJ/Δt) and time in different stage atrs=1%, ΔP=0.27 MPa, andUL=0.743 m/s

Figure 16 Permeate flux in long term operation atrs=5%

During the first stage, a large amount of solid fine particles would block the interior and cover the surface of membrane pores, causing membrane fouling and a remarkable increase of filtration resistance, so the permeate flux reduced rapidly as shown in Eq. (3). In the second stage, the rest of solid particles could not enter the interior of pores, so they would deposit on the membrane surface to form a cake layer. Because the particles deposition rate was slow, the fltration resistance caused by cake forming increased slowly. In this way, the permeate flux would decrease slowly with time. Being different from the traditional dead-end filtration, the shear stress along with the situation of membrane surface caused by fluid cross-flow could reduce or restrain the formation of cake layer, which could make the fltration process reach a quasi-stationary state for the long term just as the third stage did[4,8-11].

Figure 14 and Figure 16 show that the permeate flux could keep a quasi-stationary value for 10 hours without back-fushing. The quasi-stationary values of the permeate fux in Figure 14 and Figure 16 are listed in Table 3.

Table 3 Quasi-stationary values of permeate flux

Figure 17 and Figure 18 show the solid holdup in permeate curve with time during the long term operation. It can be seen that atrs=1%, the solid holdup in permeate was less than 2.5 mg/L with a rejection rate of more than 99.9% after 10 hours. It can be also found that atrs=5%, the solid holdup in permeate was less than 7 mg/L with a rejection rate of more than 99.9% after 10 hours. The investigations of cross-flow microfiltration in this study have shown the effcient separation capacity to deal with suspension containing fne particles.

Figure 17 Solid holdup in permeate in long term operation atrs=1%

Figure 18 Solid holdup in permeate in long term operation atrs=5%

4 Conclusions

The cross-flow microfiltration process provided with a stainless steel membrane tube was used to separate the fine alumina particles from suspension. Rejection rates of membrane for all experiments have been calculated, the influence of cross-flow velocity and transmembrane pressure on the permeate flux and the solid holdup in permeate has been investigated. Separation performance of the membrane tube in long term operation is analyzed. The following conclusions can be drawn:

1) The cross-fow microfltration process developed in thisstudy could be used to separate the fne alumina particles from suspension with a rejection rate of more than 99.9%.

2) The permeate flux decreased with the time until a quasi-stationary state was reached, and it increased with the increase of transmembrane pressure. The values of the solid holdup in permeate were all less than 10 mg/L.

3) The cross-flow microfiltration operation could run continuously and stably without back-fushing for 10 hours. The steady permeate flux values covered the range from 226 to 936 L/(m2·h) and the values of solid holdup in the permeate were mostly less than 1 mg/L in this study. The total curve of permeate fux with time could be divided into three stages according to the rate of declining permeate fux and the membrane fouling mechanism.

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Novel NWS-1 Type Vanadium Catalyst for Treating Sour Tail Gas

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date: 2017-01-18; Accepted date: 2017-03-14.

Prof. Zong Baoning, Telephone: +86-10-82368011; E-mail: zongbn.ripp@sinopec.com.