Investigation on the Separation Performance of a Multicyclone Separator for Natural Gas Purification

This study evaluated the separation efficiency and developed a pressure drop model for a multicyclone separator used for natural gas purification. The collection and grade efficiencies of a multicyclone separator with 15 single cyclone separators were evaluated at inlet velocities of 6–24 m/s and particle concentrations of 30–2000 mg/m at atmospheric pressure and room temperature. The experimental results show that although the collection efficiency of a multicyclone separator was 2%–10% less than that of a single cyclone separator at the same operating conditions, most of the particles larger than 10 μm in diameter can be removed by the multicyclone separator. Based on the theoretical pressure drop models of single and multicyclone separators at atmospheric pressure and the pressure drop data measured on site, a pressure drop model was developed that can be used to predict and calculate the pressure drops of multicyclone separators at different pressures and temperatures during natural gas transportation. The modeling results show that 80–90% of the overall pressure drop of a multicyclone separator can be attributed to the single cyclone separators in it. The pressure drop model proposed in this work can accurately predict the pressure drop for different high-pressure multicyclone separators.


INTRODUCTION
Many processing stations such as compressor station are necessary along the long-distance pipeline of natural gas transportation according to the requirement of pressurization and distribution.These stations are commonly equipped with multicyclone separators and filter separators to remove sand, rust water, light hydrocarbons, droplets, and other impurities from the natural gas.The multicyclone separator for natural gas purification is composed of several parallel single cyclone separators with cylinder diameters of 50-150 mm.The transported natural gas usually has relatively low dust concentrations less than 200 mg/m 3 , allowing low inlet particle concentration for the multicyclone separator.
Many scholars used experimental and numerical simulation method to study on the separation performance of single cyclone separators (Molerus and Glückler, 1996;Fassani and Leonardo, 2000;Yoshida et al., 2001;Su and Mao, 2006;Wan et al., 2008;Ji et al., 2009;Hsiao et al., 2011;Su et al., 2011;Karagoz et al., 2013;and so on).Moreover, some other scholars researched the parallel operation of cyclone separators.The overall collection efficiency of 14 small cyclone separators in parallel was determined (Koffman, 1953), the overall collection efficiency parallel operation was decreased to 92.2% compared to 96% for a single running cyclone separator.Based on the cold-state experiments, the parallel arrangement of cyclone separators can lead to multiple equilibrium states of the system flow and the degree of heterogeneity among channels increases with the inlet velocity (Broodryk and Shingles, 1995).However, few literatures have concluded the relationship and distinction of separation efficiency between single and multiple cyclone separators.
Many scholars researched on the pressure drop of single cyclone separators at atmosphere pressure.The resistance coefficient of cyclone separator can be described as a function of the inlet area and the vent pipe diameter and mainly depends on both the inlet size and the outlet size (Casal and Martinez-Benet, 1983).Many scholars (Iozia and Leith, 1989;Karagoz and Avci, 2005;Yang et al., 2009;Zhao, 2009;Zhao and Su, 2010; and so on) model several pressure drop of single cyclone separator based on the rotational energy dissipation.They calculated the velocity distribution in cyclone separator using angular momentum balance and estimated the overall pressure drop by combining the static pressure loss at the inlet and outlet and in the rotational flow.Furthermore, they pointed out that the reduced static pressure from the outer vortex to inner vortex rarely recovers in the standpipe in practice and can be as attributed to the dissipation lost.In these models, most of them claimed that the pressure drop of a cyclone separator is composed of three parts: the inlet loss, which is often ignored; the inner rotational flow loss, i.e., the kinetic energy loss caused by airflow viscosity during the formation of rotational flow; the rotational energy dissipation in the vent pipe.Another literature (Chen and Shi, 2007) proposed the theory of pressure drop distribution along the pipeline and developed the ESD pressure drop model.According to the model, the pressure drop is mainly composed of the expansion loss at the inlet, the inner rotational flow loss, and energy dissipation of the air flow in the vent pipe.Under the conditions of high pressure and room temperature, the pressure drop of the small cyclone separator with a diameter of 120 mm was measured (Zhu et al., 2008), and a pressure drop model of single cyclone separator at high pressure was established by using the semi-empirical approach.However, no model is used to calculate the pressure drop for multicyclone separator under high pressure conditions.
Few research reports have been published in areas such as the difference and correlation between multicyclone separator and single cyclone separator and how to calculate the multicyclone separator pressure drop under high pressure.In this study, the separation performance of multicyclone separator was measured by using the online measurement method as described in reference (Ji et al., 2009).The single separator and the multicyclone cyclone separator were compared in the separation performance and pressure drop under same inlet conditions.Furthermore, a pressure drop model of multicyclone cyclone separator at high-pressure conditions was built through theoretical analysis and verified by experiments.

Experimental Object
Fig. 1(a) shows the multicyclone separator used in the experiments.The multicyclone separatore is composed of 15 single cyclone separators with the same structure and size of the cylinder diameter of 150 mm as shown in Fig. 1(b).All the 15 single cyclone separators share the intake chamber, dust discharge chamber, and exhaust chamber.To find out the similarities and differences between multicyclone separator and single cyclone separator in the separation performance, same methods were used to determine the collection efficiency and pressure drop of the multicyclone separator and single cyclone separator as shown in Fig. 1.

Experimental Setup
The experiment setup for multicyclone separator at atmosphere pressure is shown in Fig. 2  system, a flow measurement system, a dust particles inspection system, and an experiment object.The experiment was carried out under negative pressure with the gas directly exhausted by the blower.The gas velocity was determined by the Pitot tube in the intake pipe of cyclone separator.The weighted dust particles were fed in by a BEG-1000 (Palas Company) feeder before being carried by air flow through the intake pipe into the cyclone separator for separation.In the experiment, a Welas aerosol particle spectrometer was employed to measure the particle size distribution and concentration at the inlet and outlet to obtain the collection efficiency and grade efficiency of the cyclone separator.Furthermore, both the sampling position and the position of sampling nozzle in the inlet and outlet tubes for the multicyclone separator are in strict accordance with the requirements for isokinetic sampling achieved by changing the size of the sampling nozzle as shown in Fig. 2. The experiment was conducted using air as the medium at room temperature and atmospheric pressure.To simulate the dust in actual natural gas transportation, an 800-mesh talc powder was used.The particle size is in the range of 0-40 µm with a median particle diameter of 10-12 µm and the density of 2,700 kg/m 3 , close to the values of the dust particles in actual natural gas pipeline.The multicyclone separator has a flow rate of 794-3,174 m 3 /h, which is 6-24 m/s, converted to the average inlet velocity (V in ) of the internal single cyclone separators, and the particle concentration is 30-2,000 mg/m 3 .

Collection Efficiency of Multicyclone Separator
The relationship between the collection efficiency of multicyclone separator and the inlet particle concentration were obtained at four different inlet velocities (for the convenience of comparing with the single cyclone separator, the flow rate of the multicyclone separator was converted to the inlet velocity of single cyclone separator under the same conditions).The collection efficiency for each particle concentration was measured for at least three times and averaged values were used.Fig. 3 shows that at very low inlet particle concentrations, the collection efficiency varies considerably for different inlet velocities.While at high concentrations, the collection efficiency is less sensitive to the inlet velocities.It can be concluded that the collection efficiency of the multicyclone separator varies at different concentrations and increases with the inlet particle concentration in the range of 30-2,000 mg/m 3 , but the collection efficiency of multicyclone separator is not high at low particle concentrations.For example, when the inlet velocity is 6 m/s and the inlet particle concentration is 30 mg/m 3 , the collection efficiency is only about 60%.In addition, Fig. 3 also indicates that the collection efficiency can be enhanced by increasing inlet velocity.
Although the collection efficiency of multicyclone separator is enhanced as the inlet velocity and particle concentration increases, the collection efficiency of the multicyclone separator is not as high as that of a single cyclone separator.The multicyclone separator is compared with the single cyclone separator in terms of the collection efficiency as shown in Table 1, in which the data for the   collection efficiency of multicyclone separator are the same as those in Fig. 3.Under the same inlet velocities and inlet particle concentrations, the collection efficiency of a multicyclone separator is lower than that of a single cyclone separator, and the D-value between the collection efficiencies increases with the inlet velocity with the largest D-value slightly higher than 10%.In most situations, the collection efficiency of the multicyclone separator, for example, at an inlet velocity of 16 m/s, is a few percent lower than that of the single cyclone separator, and sometimes the D-value is very small.This fact can be explained by two reasons that the uneven distribution of inlet airflow among single cyclone separators and no isolation among the dust outlets at the bottom of the cyclone separator.The phenomena of the cross "blow-by" or flow interference will occur when two or more cyclone separators operate in parallel and share one collection chamber or one ash hopper.The uneven distribution of the airflow into the cyclone separators leads to different pressures of cyclone separators, causing the "air leak" at one or several cyclone separators and the "blow-by" of the rest cyclone separators in counteraction.The separation performance of the "blow-by" cyclone separators is damaged in some extent, reducing the collection efficiency of the multicyclone separator.As the inlet velocity rises, the volume difference of the air entering the single cyclone separators increases and the "air leak" and "blow-by" becomes more serious, generating larger D-values in collection efficiencies between the multicyclone separator and the single cyclone separator.This phenomenon can be validated by comparing the collection efficiencies between the single and multicyclone separators as shown in Table 1.
The flow distribution of cyclone separators can be improved by staggering the internal and external single cyclone separators in the multicyclone separator to make the external cyclone separators higher than the internal ones.Similarly, it can also be improved by lifting the outlet for the gas that enters the collection chamber to the bottom of the collection chamber for changing the height-to-distance ratio for air entering the internal and external cyclone separators.However, the inlet flow is not necessarily the same for every single cyclone separator.

Grade Efficiency of Multicyclone Separator
Fig. 4 shows the changing rule of the grade efficiency of the multicyclone separator with different inlet particle concentrations at different inlet velocities for internal single cyclone separators.Fig. 5 gives the changing rule of the grade efficiency at 100 and 2000 mg/m 3 inlet particle concentrations and different inlet velocities.Similar to the change rule obtained by reference [7] of single cyclone separator, it can be concluded that higher inlet velocity and inlet particle concentration lead to higher grade efficiency of multicyclone separator.The multicyclone separator can only remove the particles larger than 15 µm at an inert velocity of 6 m/s, while the multicyclone separator can remove clearly the particles larger than 10 µm with significantly improved separation accuracy at inlet velocities greater than 10 m/s.
The grade efficiency curves of single cyclone separator at inlet velocity of 16 m/s are plotted in Fig. 6.The single cyclone separator as shown in Fig. 1(b) can remove clearly the particles larger than 7µm.Although the multicyclone separator has lower collection efficiency and grade efficiency compared to the single cyclone separator, the collection efficiency and grade efficiency of multicyclone separator is still high under the conditions of low inlet velocity and particle concentration, which can satisfy the requirement of natural gas gas-solid separation.

Pressure Drop of Multicyclone Separator
Compared to the single cyclone separator, many more complicated factors contribute to the pressure drop of a multicyclone separator, namely, the pressure loss at the curved pipe between the inlet of multicyclone separator

Fig. 4. (continued).
and the outlet of intake pipe, the expansion loss in the intake chamber, the friction loss of the gas on the inner wall of intake chamber of multicyclone separator and the outer wall of single cyclone separator, the inlet loss of all single cyclone separators, the pressure loss after gas entering the single cyclone separator, the pressure loss of the gas entering the ash hopper, the expansion loss of the gas entering the collection chamber from single cyclone separator and the outlet loss of collection chamber.Among these factors of the pressure drop of multicyclone separator, the single cyclone separators play the key role.Fig. 7 is used to compare the pressure drop of multicyclone separator with that of single cyclone separator.It can be seen that at the same inlet velocity, the pressure drop of the single cyclone separators in a multicyclone contributes to 88-90% of the overall pressure drop of multicyclone separator, and therefore the overall pressure drop can be expressed as 0.88 ~0.9 where Δp m is the pressure drop of multicyclone separator at atmosphere pressure; Δp s is the pressure drop of single cyclone separator at atmosphere pressure.The effect of the operating pressure on the flow field in a single cyclone separator is analyzed (Xiong et al., 2005;Shi et al., 2006).The density strongly affects the flow field.So the effect of high pressure on the multicyclone separator can be reduced to the effect of density on the pressure drop.In this way, the pressure drop of multicyclone separator at high pressure Δp m-h can be calculaetd as Fig. 6.Grade efficiency of the single cyclone separator as a function of particle concentration.where m is the dimensionless factor; ρ g is the gas density at atmosphere pressure; ρ g-h is the gas density at high pressure; Δp m-h is the pressure drop of multicyclone separator at high pressure.Therefore, as long as the dimensionless factor m is determined, the pressure drop of a multicyclone separator at high-pressure conditions can be calculated.
To obtain a better knowledge of the changing rule of the pressure drop of multicyclone separator at high-pressure conditions and determine the value of m, the onsite measurement of the pressure drop of multicyclone separators at multiple stations along the natural gas pipeline was carried out as shown in Fig. 8. Considering the short measuring distance, the measured values can be deemed as the pressure drop of the multicyclone separator at high pressure with a differential gauge accuracy of ± 1.5‰FS.Table 2 lists the operating pressure, flow rate, temperature and pressure drop of multicyclone separators at different stations at actual operating pressure (high pressure).In the table, the measured values of three different natural gas pipelines at compressing stations are listed in rows 1-2, 3-6 and 7, respectively.For the first pipeline, the single cyclone separator is of the axial inlet type and for the second and third pipelines, tangential inlet of single cyclone separators are used.The compositions of natural gas in the three gas transportation pipelines are as shown in Table 3.
The data in Tables 1 and 2 can be used to obtain the density and the compression ratio of natural gas at different pressures.By using the calculation formula (1) for multicyclone separator at atmosphere pressure, the pressure drop of single cyclone separator at atmosphere pressure and the pressure drop value of multicyclone separator measured on site at high pressure, the value of m can be obtained by curve fitting.It was found out that m ranges approximately 1.51-1.54for the first pipeline and 1.17-1.21for the second and third pipelines.By substituting the mean value of m into the Eq. ( 2), the pressure drop of multicyclone separator at high-pressure conditions can be calculated as shown in Table 4.It can be seen that the calculated and measured values for pressure drop match well with each other with small deviations.Consequently, as long as the pressure drop of thesingle cyclone separators at atmosphere pressure is determined, the pressure drop of a multicyclone separator used for natural gas transportation at high-pressure conditions can be preferably predicted using Eqs.( 1) and ( 2).

CONCLUSIONS
In this paper, by means of online testing, the collection efficiency of multicyclone separator at atmosphere pressure is discussed in detail.Also the collection and grade efficiencies between single cyclone separator and multicyclone separator are compared.A pressure drop model of multicyclone separator at high-pressure was built through experiments at atmosphere-pressure and high-pressure conditions.The results can be generalized as following: (1) The collection efficiency of multicyclone separator is 2-10% less than that of single cyclone separator at the same inlet conditions due to the structure of multicyclone separator.The flow in the intake chamber cannot be evenly distributed into all single cyclone separators, leading to different separation performance of the single cyclone separators, and therefore, the collection efficiency of multicyclone separator is reduced; (2) If the inlet velocity is more than 10 m/s, most of the particles larger than 10 µm can be removed by the multicyclone separator while the single cyclone separator can clearly remove the particles larger than 7 µm; (3) The pressure drop of the single cyclone separators in a multicyclone separator contributes 80-90% of the overall pressure drop of the multicyclone separator; (4) The obtained equations for multicyclone separator used in natural gas purification at high-pressure conditions can accurately predict the pressure drop of the multicyclone separator.

)Fig. 3 .
Fig. 3. Collection efficiency of the multicyclone separator at different particle concentrations and inlet velocities.

Fig. 4 .
Fig. 4. Grade efficiency of the multicyclone separator as a function of particle concentration.

mg/m 3 Fig. 5 .
Fig. 5. Grade efficiency of the multicyclone separator as a function of inlet velocity

Fig. 7 .
Fig. 7. Comparison of pressure drop between the single and multicyclone separator.

Fig. 8 .
Fig. 8. Schematic diagram of measuring pressure drop for multicyclone at high-pressure conditions.

Table 2 .
Pressure drop of multicyclone measured at high-pressure.

Table 3 .
Composition of natural gas in different gas transportation pipelines (Mol%).

Table 4 .
Comparison of pressure drop between experimental and calculated values for the multicyclone separator at highpressure conditions.