Gas hydrodynamics of a novel MTO high-speed loop reactor: The bypassing and backmixing along with average residence time

Journal Pre-proof Gas hydrodynamics of a novel MTO high-speed loop reactor: The bypassing and backmixing along with average residence time

Fenfen Wang, Shihan Ma, Jiajia Wen, E. Chenglin, Chunxi Lu PII:

S0032-5910(19)30758-2

DOI:

https://doi.org/10.1016/j.powtec.2019.09.032

Reference:

PTEC 14706

To appear in:

Powder Technology

Received date:

27 February 2019

Revised date:

2 July 2019

Accepted date:

11 September 2019

Please cite this article as: F. Wang, S. Ma, J. Wen, et al., Gas hydrodynamics of a novel MTO high-speed loop reactor: The bypassing and backmixing along with average residence time, Powder Technology(2018), https://doi.org/10.1016/j.powtec.2019.09.032

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© 2018 Published by Elsevier.

Journal Pre-proof Gas hydrodynamics of a novel MTO high-speed loop reactor: The bypassing and backmixing along with average residence time Fenfen Wang, Shihan Ma, Jiajia Wen, Chenglin E, Chunxi Lu* College of Chemical Engineering, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

Abstract: Methanol to olefins (MTO) is an attractive technique. In order to optimize the hydrodynamics of gas-solid phase and the olefins yield and selectivity, a novel MTO high-speed loop reactor (HSLR) is developed. The gas hydrodynamics of bypassing, backmixing and average residence time are explored in a large scale cold model setup. In addition, comparison between the

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gas backmixing as well as gas residence time in the HSLR and that in the traditional non-draft-tube fluidized bed or free fluidized bed (FFB) is made. Experimental results show that

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the gas bypassing fractions are closely related to the gas velocity. Based on the axial profiles of Dga and Peclet number, the gas backmixing in the HSLR is much lower than that in the FFB.

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Moreover, the average gas residence time in the HSLR is considerably short, indicating distinct transport capacity of gas. Furthermore, the proper operating conditions in the HSLR are given.

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Keywords: MTO; Hydrodynamics; Gas bypassing; Gas backmixing; Residence time

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Introduction

The production of olefins from methanol, the MTO process, is considered as an alternative

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for the thermal cracking. MTO is thus an effective way to solve the problems of decreasing oil resources and increasing olefins demands nowadays [1]. Based on the experimental results in the

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laboratory [2], the MTO reaction over SAPO-34 catalyst is characterized by rapid catalyst deactivation due to the coke deposition; and consequently short catalyst-gas contact time is required in order to avoid undesired byproduct. On the contrary, the catalyst deposited with some coke could favor the selectivity to ethene and propene at high methanol conversion [2, 3], which means the coke content should be kept within a proper value. It is introduced that the average 8 % coke in the reactor is the optimal operation window and the catalyst residence time should be controlled around 60 min [2]. Moreover, the MTO reaction over SAPO-34 catalyst is highly exothermal. As a result, the reaction heat must be removed from the reaction bed simultaneously to keep the reaction temperature within the designed range [2-4]. In conclusion, in order to maximize the ethylene and/or propylene yield and selectivity, four objectives are anticipated, i.e., 1) little gas backmixing or plug flow in the reaction region; 2) short gas residence time; 3) appropriate catalyst residence time; 4) quick heat transfer from the reactor [2,5,6]. Many researchers have developed different types of reactors for MTO reaction. Lattner et al. [7] disclosed a method for selectively converting oxygenates to light olefins. The method

Journal Pre-proof incorporated a fluidized bed reactor with a continuous regeneration. Avidan et al. [8] proposed an improvement in a process for the conversion of oxygenates to lower olefins by operation of a fluidized bed in a turbulent fluidization regime. It was further found that the bed of catalyst in the reactor could be least at 5 to 20 meters in height. Miller et al. [9] developed a fast fluidized bed reactor for carrying out an oxygenate conversion reaction with a significantly reduced catalyst inventory compared with traditional bubbling bed reactors. Liu et al. [10] adopted the dense phase turbulent fluidized bed reactor operating at a superficial gas velocity of 1~1.5 m/s due to the excellent gas-catalyst contact, high mass transfer efficiency and large solids holdup. In order to improve the yield of light olefins, Qi et al. [11] put forward a new method for MTO, i.e., the

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diameter of feed outlet was larger than that of feed inlet and the reaction area was divided into two

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zones. However, gas backmixing is serious and the residence time is quite long in those turbulent fluidized beds above [7, 8, 10, 11], which is believed to be harmful for the olefins yield and

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selectivity [4]. Besides, the fast fluidized bed [9] provides less gas backmixing nevertheless lower solids holdup than the turbulent fluidized bed [2], which may reduce methanol conversion. Further

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efforts are urgently needed to improve the performance of the traditional fluidized beds for the MTO.

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The airlift loop reactors (ALR) are featured by the addition of the draft tube to the traditional fluidized bed and the different gas velocities between the draft tube region and the annulus region.

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Researchers have conducted extensive experiments on the flow behaviors in the ALR [12-19]. Xie et al. [12] detected the gas circulation and its concentration as well as solids circulation rate in an

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internally circulation fluidized bed (ICFB). Luo et al. [13] measured the liquid velocity field, turbulent kinetic energy field, and distributions of sheer stresses in the draft tube airlift bioreactor.

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Liu et al. [14] studied the bed average voidage in the draft tube and the particle circulation velocity in a gas-solid ALR. Liu et al. [15] also investigated the mean voidage of the annulus region and the particle velocity in a novel gas-solids annulus-sparged airlift loop reactor. Furthermore, Liu et al. [16] discussed the influence of operating conditions and the geometric configuration on the distribution of bed density in a cold model annulus-lifted air loop reactor. Yan et al. [17-19] explored the time-averaged local solid fractions and particle velocities in a gas-solid ALR by experimental measurement and computational fluid dynamics (CFD). In summary, the gas-solid ALR provides a number of outstanding potential advantages [12-19] including 1) excellent mass and heat transfer rates; 2) high gas-solid contacting efficiency; 3) orderly solids circulation from one region to another resulting in regular and controllable solids trajectories; 4) regulating the residence time and reaction degree flexibly; 5) increasing the operating gas velocities; 6) simplicity in construction. This gas-solid ALR has been used in several different processes such as the petroleum coke combustion [17-19], the gas-solid

Journal Pre-proof dense-phase stripper [14, 20, 21], airlift loop heat exchanger [22], FCC naphtha olefin reformulation [23], etc. Combined the objectives of MTO process and the merits of gas-solid ALR as noted above, the gas-solid ALR can be applied to the MTO reaction. Therefore, a novel MTO high-speed loop reactor

(HSLR) is put forward. The HSLR consists of a fluidized bed and a draft tube. There are two main reaction regions, i.e., the draft tube region and the annulus region. The draft tube region is a turbulent or fast fluidized bed (Ugd= 0.8~2 m/s) the solids flow upward therein, whereas the annulus region is a bubbling or turbulent bed (Uga= 0.2~0.6 m/s) in which the solids move downward [24]. The solids circulation rate from the draft tube region to the annulus region in the

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HSLR can be easily controlled by regulating the ratio of the gas velocities in the draft tube to that

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in the annulus region. It is anticipated to enhance the hydrodynamics of gas-solid phase and the conversion level. As regard to this novel HSLR, the gas bypassing, the gas backmixing along with

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the average gas residence time are crucial for MTO reaction and have not been investigated yet. Furthermore, in this proposed paper, the gas backmixing and the average gas residence time in the

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traditional non-draft-tube fluidized bed, i.e., free fluidized bed (FFB) are also measured to provide a significant comparison with the HSLR. These gas hydrodynamics of bypassing, backmixing and

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the average residence time in the HSLR or in the FFB above are studied in a large scale cold model setup.

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Firstly, in the traditional FFB, there is no draft tube and no gas bypassing exists. However, in the HSLR, consider gas streams are introduced into the draft tube region as well as the annulus

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region respectively and the gas velocities in the two regions are different, gas bypassing between the draft tube region and the annulus region must occur. The true gas flowrates in the two regions

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are closely linked with the gas residence time in the HSLR. If the gas bypassing in the HSLR is neglected, the true gas flowrates in the two regions cannot be quantified. Song et al. [25] and Zhang et al. [26] studied the gas bypassing in an internally circulating fluidized bed (ICFB) with a draft tube and dealt with coarse Geldart B particle. The obvious structure characteristic of ICFB is that the cross-sectional area of the draft tube region is much smaller than that of the annulus region [27]. In view of the different particle property and structure parameter, the gas bypassing in the ICFB is not helpful for our HSLR. Shen [28] investigated the gas bypassing in a gas-solid ALR under low gas velocities (Ugd= 0.08~0.54 m/s, Uga= 0.03~0.08 m/s). However, the gas bypassing under high gas velocities (Ugd= 0.8~2 m/s, Uga= 0.2~0.6 m/s) has never been introduced. Secondly, in the traditional FFB, the axial gas backmixing takes place inevitably [28]. It is closely related to the drag by the downward moving solids and must influence directly the gas residence time. The gas mixing in bubbling/turbulent /fast flow regimes with/without internals has

Journal Pre-proof ever been scrutinized [29-35]. Van Deemeter et al. [30] found that the axial gas dispersion coefficient increased approximately linearly with superficial gas velocity in the bubble flow regime. Cankurt et al. [29] presented that gas backmixing diminished with gas velocities beyond the bubbling regime and was negligible in the fast fluidized bed. Deshmukh et al. [31] and Zhang [35] demonstrated that the gas backmixing could be greatly suppressed by introducing internals, for example, the louver baffles. As for the gas-solid ALR, the gas backmixing must exist due to the influence of the draft tube [28]. However, the research on the gas backmixing in gas-solid ALR was quite limited. The gas in the draft tube region is pulled downward by the solids and sinks in the gas-solid interspace, while the gas in the annulus region is carried by both the

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circulation of downflow solids from the separation region to the annulus region and the macro

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downward moving solids in the annulus region. That means the gas backmixing in the gas-solid ALR exists concurrently in the draft tube region and in the annulus region. Shen [28] explored the

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axial gas dispersion coefficients only in the draft tube region of the gas-solid ALR and made comparison with that in the FFB under low gas velocities as mentioned before, while the gas

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backmixing in the annulus region of the gas-solid ALR was neglected therein. Under high operating gas velocities in the HSLR, the gas backmixing in the annulus region is bound to affect

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the average gas residence time and cannot be ignored. Therefore, the gas backmixing both in the draft tube region and in the annulus region of the HSLR requires being scrutinized thoroughly.

calculated and analyzed clearly.

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Furthermore, the specific gas backmixing flowrate in the annulus region of HSLR will be

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Finally, the gas residence time directly influences the production yield. In previous work, the gas residence time distribution was usually conducted in the risers of a gas-solid circulating

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fluidized bed [36, 37]. Nevertheless, the gas residence time in a gas-solid ALR system was seldom reported. It is imperative to gauge the gas residence time in the HSLR together with that in the traditional FFB and make an intuitive comparison of the performance between the two types of MTO reactors.

2. Experiment 2.1 Experimental apparatus The experimental setup of the novel MTO high-speed loop reactor is given in Fig.1.

Fig. 1. Schematic diagram of the experimental setup

The overall system basically comprises a high-speed airlift loop fluidized bed section, a riser section, a two-stage cyclone section and a recirculation section. The high-speed airlift loop fluidized bed is 0.38 m in inner diameter (I.D.) and 3 m in height. A draft tube (0.3 m I.D. with a height of 0.8 m) is installed in the center of the fluidized bed. The inner diameter and the height of

Journal Pre-proof the riser are 0.12 m and 5 m, respectively. A base cone distributor with 35×10 mm holes (opening area ratio of 3.9 %) and a tubular distributor with 18×6 mm holes (opening area ratio of 1.9 %) are located at the bottom of the HSLR. Except the gas distributors, the draft tube and the two-stage cyclone are made of steel, others are made of plexiglass for observing convenience purpose. The investigation of gas hydrodynamics in the FFB are conducted in the same setup with the HSLR, just replacing the base cone distributor, the tubular distributor and the draft tube by another base cone distributor of 0.38 m I.D.

2.2 Experimental materials In this paper, the methanol feed was instead of atmospheric air, which was supplied by a

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Roots blower. All the experiments were carried out at the atmospheric pressure and the room

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temperature of approximate 25 °C. The flowrates of two gas streams were mainly controlled by a couple of flowmeters. The gas in the draft tube was mainly introduced through the base cone

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distributor, while the gas in the annulus was inlet via the tubular distributor. Two-stage cyclone separator was utilized to separate the solids from gas. The SAPO-34 molecular sieves were

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usually employed as the MTO catalysts due to its high efficiency for the conversion of methanol to olefins [2]. The physical property of FCC catalyst is similar to that of SAPO-34 molecular

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sieves and these two types of particles are both classified as Geldart A [38], thereby the typical FCC catalysts were used as substitute for SAPO-34 molecular sieves. The mean diameter dp, bulk

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density ρb, particle density ρp and repose angel θ of the FCC catalysts were listed in Table 1.

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Table 1 Properties of FCC equilibrium catalyst

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2.3 Operating conditions

For the sake of convenient description, according to Liu et al. [14] and Yan et al. [19], the high-speed airlift loop section can be divided into four regions, as shown in Fig. 2: (I) the draft tube region, (II) the annulus region, (III) the bottom region, and (IV) the gas-solid separation region.

Fig. 2. Flow regions in the HSLR

In order to make the experimental conditions close to the actual commercial case, the cross-sectional average gas velocities in the HSLR must cover the operating velocities of the methanol feed in the industrial MTO reactors (Ug=1.0~1.2 m/s). Hence, the superficial gas velocity ranged from 0.8 m/s to 2.0 m/s in the draft tube region (U gd), and from 0.2 m/s to 0.6 m/s in the annulus region (Uga). The corresponding cross-sectional average gas velocity in the reactor

Journal Pre-proof (Ug) ranged from 0.59 m/s to 1.48 m/s and the gas velocity in the riser (Ugr) were from 5.89 m/s to 14.81 m/s. The overall solids circulation flux in the riser (Gs) varied between 46.2 kg/(m2·s) and 297.8 kg/(m2·s). The height of the static bed (H0) was 1.1 m and 1.6 m, which referred to the distance between the bed surface and that of base cone distributor.

2.4 Measurement of gas bypassing The gas bypassing between the draft tube region and the annulus region of the HSLR was measured by a gas steady-tracer technique. Qgd and Qga were the flowrates of the two gas streams in the draft tube and in the annulus respectively. Helium was used as the tracer gas and continuously injected into the pipeline of the base cone distributor with a constant flow rate Qgt.

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As shown in Fig. 3, the injecting point ‘A’ was far away from the base cone distributor. As a

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consequence, helium and gas were fully mixed before entering into the reactor. The investigated points of helium concentration at their corresponding axial cross sections and the dimensional

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radial positions were also given in Fig. 3, including ‘C’, ‘D’, and, ‘E’. The corresponding Helium concentrations are named as Cd1, Ca2, and Cd2. Helium and gas were uniform mixture in the HLSR,

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thus the helium concentrations of different dimensionless radial positons at each axial cross section in the draft tube region or in the annulus region were identical and could represent the

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cross-sectional average concentration of helium. The cross section of ‘D’ in the annulus region was chosen at 210 mm above the base cone distributor, which would not affected by the gas

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backmixing from the separation region to the annulus region. The detailed analysis will be given in the next text. These gas samples were the percentage concentrations of helium and measured by

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an Agilent GC7890B Gas Analyzer, which was calibrated by standard gas to be accurate enough for the measurements of this study. In addition, the helium concentration of the sampling point ‘B’,

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i.e., Ca1 equaled zero as no helium was injected into the annulus inlet.

Fig. 3. The investigated points of helium concentration for gas bypassing in the HSLR

A schematic of gas transport between the draft tube region and the annulus region of the HSLR is drawn in Fig. 4.

Fig. 4. Schematic of gas mass balance between the draft tube region and the annulus region

The net gas flowrate through the draft tube outlet (Qdn) and that of the annulus outlet (Qan) can be calculated from total mass balance equations of gas and helium between the two regions [25, 26]. The flowrate of gas bypassing from the draft tube region to the annulus region (Qda) and

Journal Pre-proof that from the annulus region to the draft tube region (Qad) in the HSLR can be calculated from mass balance equations of total gas and helium across the draft tube region or the annulus region. The overall mass balance for these two regions is

 g 1  Cd1   HeCd1   Qgd  Qgt    g 1  Ca1   HeCa1  Qga   g 1  C d 2   HCe d 2Q  d[n  1g C   a  C H] eQ a 2 2

an

(1)

The helium mass balance for the two regions is

Q

H eC

d 1

g d Q  g t

CH eQ a1  g a C HQe d 2

C QHe

d n

a 2

(2)

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The total mass balance for the draft tube region is

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From Eqs. (1) and (2), Qdn and Qan can be obtained.

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  g 1  Cd1   HeCd1  (Qgd  Qgt )    g 1  Cad   HeCad  Qad   g 1  Cd2   HeCd2  Qdn   g 1  Cda   HeCda  Qda

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(3)

The helium mass balance for the draft tube region is

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H eC d(1Q  g dQgt )   CH Q e a d a d C QH e  d 2 Cd nQ

He

da

da

(4)

Assuming that helium concentration of the bypassing gas is equal to that of the

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and (4) respectively.

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corresponding inlet gas (Cda=Cd1, Cad=Ca1=0), thus the Qda and Qad can be calculated from Eqs. (3)

The bypassing fraction of inlet gas (f) is defined by the volume percentage of the inlet gas

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bypassed into the other region. Then the bypassing fraction of the inlet gas of the draft tube region into the annulus region fda and that of the inlet gas of the annulus region into the draft tube region fad are

fda  100* Qda / (Qgd  Qgt ) fad  100* Qad / Qga

(5) (6)

Where the subscript da denotes gas bypassing from the draft tube region to the annulus region and ad represents gas bypassing from the annulus region to the draft tube region. Thus, the gas distribution law in the HSLR is obtained.

2.5 Measurement of gas backmixing The gas backmixing is dependent on the gas velocity. Under the operating gas velocities in this paper (Ugd=0.8 m/s ~2 m/s, Uga=0.2 m/s~0.6 m/s), solids flow upward with small bubbles or no bubbles in the draft tube region and solids move downward with bubbles in the annulus region.

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Journal Pre-proof The gas backmixing in the HSLR exists concurrently in the draft tube region and in the annulus region, as discussed before. A gas steady-tracer technique was employed to investigate the gas backmixing in the draft tube region and in the annulus region of the HSLR. The gas backmixing in the FFB was also measured at the same axial and radial locations as the HSLR. As shown in Fig. 5, tracer gas helium was continually injected downward through a point injector ‘d0’ in the centerline of the draft tube while the distance from the helium injector to the base cone distributor Hi was 0.94 m (near the outlet of the draft tube region) , which was generally different from the measurement of

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gas bypassing above.

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Fig. 5. Locations of gas sampling points for gas backmixing in HSLR and in FFB

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Fig. 5 (a) shows the specific locations of gas sampling points in the draft tube region and in the annulus region of HSLR. In the draft tube region, the gas at three different cross sections was

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sampled, i.e., Hi = 390 mm, 590 mm and 790 mm. Besides, there were four measuring dimensionless radial positions at each axial cross section, i.e., r/R = 0, 0.25, 0.5 and 0.75. As to the

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annulus region, there were five cross sections, i.e., Hi = 210 mm, 390 mm, 590 mm, 790 mm and 940 mm, and the corresponding dimensional radial position r/R = 0.95. There was another

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sampling tap mounted below the gas outlet tube of the bed (Hi = 2925 mm, r/R = 0) to measure the tracer gas concentration in the freeboard, named as Cf. These sampling gases were obtained at the

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same time and analyzed by the Agilent GC7890B Gas Analyzer. For the FFB, the gas sampling points were also located at three cross sections, i.e., Hi = 390

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mm, 590 mm and 790 mm, as shown in Fig. 5 (b). Accordingly, there were five sampling dimensionless radial positions at each axial cross sections, i.e., r/R = 0, 0.25, 0.5, 0.75 and 0.95. Similarly, another sampling tap (Hi = 2925 mm, r/R = 0) was also set at the same position as in the HSLR to detect the helium concentration in the freeboard. Axial gas mixing in fluidized beds can be characterized by three coefficients: the axial dispersion coefficient Dga, the backmixing coefficient Dgb and the radial gas mixing coefficient Dgr. According to van Deemter [30], these coefficients are related by

Dg a Ug D



Dg b U gD b Ug D Dg r

(7)

Where, Ug is the gas velocities and D is the inner diameter of bed. In fluidized beds with Geldart A particles, b ranges from 5×10-3~5×10-4 and is neglected [28]. Therefore, the radial dispersion term can be eliminated and hence Dgb=Dga.

2.6 Measurement of the average gas residence time

Journal Pre-proof Differing from the gas steady-tracer technique for the measurement of gas bypassing and gas backmixing, a gas pulse-tracer response technique was adopted to obtain the average gas residence time both in the HSLR and in the FFB. For the HSLR, the gas velocities in the draft tube region and that in the annulus region are different, thus the average residence time of gas flowing from the draft tube through bed (Tgd) and that of gas passing from the annulus through bed (Tga) ought to be explored respectively. As shown in Fig. 6 (a), in order to investigate the average residence time of gas flowing from the draft tube through bed (Tgd), the pulse helium (0.2 s) was injected at the point ‘R1’, which was far away from the base cone distributor rather than the inlet of the bed. Consider the time lag of helium flowing

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from the injecting point to the point below the outlet of bas cone distributor; a sampling tap was

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set at the point ‘S1’. At the same time, the gas was sampled at the point S3 close to the outlet of HSLR. Once the helium was injected from the point ‘R1’, ‘S1’ and ‘S3’ were connected with an

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SR-2050 online thermal conductance helium analyzer to detect the helium concentration curve varying as time. The axial cross sections and the corresponding dimensionless radial positions r/R

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of ‘S1’ and ‘S3’ were marked in Fig. 6 (a). Similarly, for the average residence time of gas passing from the annulus through bed (Tga), the pulse helium (0.2 s) was injected at the point ‘R2’ far away

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from the tubular distributor. Simultaneously, the gas was sampled at the point ‘S2’ below the outlet of tubular distributor and the point ‘S3’, which were also jointed to the SR-2050 online thermal

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conductance helium analyzer.

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Fig. 6. Measurement of the average gas residence time in HSLR and in FFB

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As shown in Fig. 6 (b), in order to get the average residence time of gas flowing from the outlet of the base cone distributor to the outlet of the bed (Tgf) in the FFB, pulse Helium (0.2 s) was injected at the point ‘R3’ far away from the base cone distributor. In the meanwhile, gas was sampled at the point ‘S4’ below the outlet of base cone distributor and the point ‘S5’ exactly the same position as S3 in the HSLR. ‘S4’ and ‘S5’ were connected to the SR-2050 online thermal conductance helium analyzer likewise. The axial cross sections and their dimensionless radial positions of S4 and S5 were signed in Fig. 6 (b). Fig. 7 (a) ~ (d) shows several different examples of helium concentration C profile curve varying as time t. For the curve in Fig. 7 (a), a single well defined peak is obtained. While the curve in Fig. 7 (b) is less outspoken with a main primary peak together with several smaller peaks and followed by a long tailing signal. What’s more, Fig. 7 (c) and (d) demonstrate a clear peak as well as a limited tailing signal. Where, Ci is the measured helium concentration at the time point ti. In fact, the high or low helium concentration Ci at a certain time point ti indicates large or

Journal Pre-proof small turbulent instantaneous gas flowrate or velocity. In other words, the different instantaneous helium concentrations at the corresponding time points represent various transport capacity for gas flowrate or velocity. Despite various profiles of the measured helium concentration as plotted in Fig. 7 (a)~(d), Ci can always be taken as the weight factor to reckon the characterized time point when the helium is fully transported though a certain sampling cross section.

Fig. 7. Helium concentration distribution curve varying as time

The characterized time points TSk (k=1, 2, 3, 4, 5) at the five measured cross sections

N

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i

i

1

N

C

(8)

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TSk 

t C

f

( ‘S1’~’S5’) can be computed by Eq. (8).

i

1

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Once the characterized time points of the two sampling cross sections are achieved from Eq. (8), the deduction made of the two characterized time points is hence the time interval or duration

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within the space between the two cross sections, i.e., the average gas residence time. Therefore, for the HSLR, the average residence time of gas flowing from the draft tube

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through bed (Tgd) and that passing from the annulus through bed (Tga) are reckoned by Eqs. (9)

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rn

and (10).

Tg d  T S 3 T

S1

(9)

Tg a  T S 3 T

S2

(10)

Similar to the calculation of characterized time point by Eq. (8), the gas flowrate to the draft tube region Qgd and that to the annulus region Qga can also be used as the weight factor to obtain the total average gas residence time Tgh in the HSLR by Eq. (11).

Tgh 

QgdTgd  QgaTga Qgd  Qga

(11)

As to the FFB, the average residence time of gas flowing from the outlet of the base cone distributor to the outlet of the bed (Tgf) is given by Eq. (12).

Tg f  T S 5 T

S4

(12)

3. Results and discussion 3.1 Gas bypassing As discussed before, the gas bypassing must occur in the HSLR when methanol is fed to the

Journal Pre-proof draft tube region and the annulus region at the same time. It is ascribed to the fact that the gas pressure at bottom of the draft tube region is lower than that at the bottom of the annulus; and thus the inlet gas flowing to the annulus region will bypass to the draft tube region. Reverse gas bypassing from the draft tube region to the annulus region is also possible. Besides, the circulation rate of solids flow from the annulus region to the draft tube region will be affected by the countercurrent flow of gas and solids. However, if a large amount of gas bypasses into the annulus region from the draft tube region, gas may stay for a long time in the reactor. Consequently, undesired products may generate and the olefins yield reduces. Therefore, the gas distribution law between the draft tube region and the annulus region in the HSLR should be figured out.

f

3.1.1 The flow ratios at the inlet and outlet of the bed

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To have an initial knowledge of the dominant gas bypassing direction, the flow ratio was defined as the gas volumetric flowrate in the draft tube region to that in the annulus region. The

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flow ratios at the inlet ( FRin = Qgd / Qga ) and the outlet ( FRout = Qdn / Qan ) of the bed are plotted

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in Fig. 8.

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Fig. 8. The outlet to inlet flow ratio under all the operating conditions

As shown in Fig. 8, under all the operating conditions in the HSLR, FRout is greater than FRin,

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indicating the dominant gas bypassing direction is from the annulus region to the draft tube region.

3.1.2 Effect of the gas velocity in annulus region Uga on gas bypassing fractions

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The effects of the gas velocity in the annulus region (Uga) on both the gas bypassing fractions fda (the inlet gas bypasses from the draft tube region to the annulus region) and fad (the inlet gas

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bypasses from the annulus region to the draft tube region) are shown in Fig. 9. The gas bypassing fraction from the annulus region to the draft tube region fad (63.57 %~89.17 %) is considerably higher than that from the draft tube region to the annulus region fda (3.42 %~8 %). As shown in Fig. 9, at the same gas velocity in the draft tube region Ugd, fda increases slightly with the increase of Uga, whereas fad reduces as increasing Uga. Visual observations demonstrate that the annulus region is a bubbling bed and the voidage is relatively low. As a result, the energy consumption is quite large for gas to pass through the annulus region. Thus, the gas introduced into the annulus region flows to the draft tube, which is similar to the flowrate distribution in a parallel pipeline. In other words, gas always seeks to flows through the space where the resistance is the lowest, i.e., the principle of minimum stream power. With the increase of U ga, the voidage in the annulus region increases and the resistance therein reduces. Gradually the gas supplied can easily percolate through the annulus region from the inlet gas both in the base cone distributor and in the tubular distributor. The analysis above may be the intrinsic causes that fda tends larger and fad gets smaller

Journal Pre-proof with the increase of Uga.

Fig. 9. Effect of Uga on gas bypassing fractions: (a) fda; (b) fad

3.1.3 Effect of the gas velocity in draft tube region (Ugd) on gas bypassing fractions The effects of the gas velocity in the draft tube region (Ugd) on both the gas bypassing fractions fda and fad are shown in Fig. 10. Under the same gas velocity in the annulus region U ga, fda decreases whereas fad increases with the increase of Ugd. This can be also explained by the principle of minimum stream power as stated above. As the gas velocity in the draft tube region

f

Ugd increases, the voidage in the draft tube region grows large and the resistance of solids exerting

oo

on gas gets small. Gradually, more gas enters naturally into the draft tube region rather than the annulus region. On the other hand, the solids circulation from the annulus region to the draft tube

pr

region tends high as Ugd increases at the same Uga [19], which denotes more entrained gas from the annulus region to the draft tube region. The lower resistance and higher solids circulation

e-

signify the rise of fad and the decrease of fda under larger Ugd.

Pr

Fig. 10. Effect of Ugd on gas bypassing fractions: (a) fda; (b) fad

al

3.1.4 Dimensionless calculation models of gas bypassing fractions As described above, the gas bypassing fractions relate closely with the operating gas velocity.

rn

In this paper, the dimensionless calculation models of fad and fda are obtained by non-linear

of the HSLR:

Jo u

regression of Ugd/Umd, Uga/Umd, where Umd refers to the lowest gas velocity in the draft tube region

f da  9.06(

Ug d

U )0 . 5 6( 6 g )a U md U md

f ad  63.263(

Ug d

U )0 . 2 3( 3 g )a U md U md

0.278

0.101

(13)

(14)

As expressed by Eqs. (13) and (14), the gas bypassing fraction from the draft tube region to the annulus region (fda) grows large and that from the annulus region to the draft tube region (fad) turns small as the increase of Uga. On the contrary, fda becomes low and fad tends high with increasing Ugd. Based on the calculation models above, the comparison between the experimental data and the calculated values was given in Fig.11. In addition, the maximum positive and negative relative errors line were plotted in the meanwhile. As shown in Fig. 11, the maximum relative error of fda is less than ±17.2 % and that of fad is less than ±7 %. Therefore, a fair good agreement is achieved.

Journal Pre-proof

Fig. 11. Comparison between the data measured by experiment and calculated by models

3.2 Gas backmixing In this paper, the gas backmixing is also investigated to reveal the effects of draft tube on the gas flow patterns in fluidized bed. In general, gas flow mostly upward in the reactor, whereas the gas will be influenced by the solids movement all the time. For the HSLR, the gas in the draft tube region will be carried downward by the portions of downflow solids in the gas-solid interspace. On the other hand, the gas in the annulus region will be entrained by both the circulation of downward moving solids from the separation region to the annulus region and the macro

f

downflow solids in the annulus region. In summary, the gas backmixing in the HSLR exists

oo

concurrently in the draft tube region and in the annulus region. As to the FFB, the gas backmixing is caused by the combined action of gulf-streaming with the drag force of the downward moving

pr

solids. The gas backmixing in the FFB is a base case and provides a comparison with that in the HSLR. The schematic of gas backmixing in the HSLR and in the FFB are drawn in Fig. 12.

e-

Fig. 12 illustrates the flow process as the helium is injected downward and immediately enters into the upstream of the draft tube region in the HSLR or in the FFB. As shown in Fig. 12

Pr

(a), for the HSLR, the backmixing of helium or the gas backmixing concurrently occurs in draft tube region and in the annulus region, as discussed above. At first, some helium is carried by the

al

downward moving solids in the draft tube region. In the meanwhile, helium is continuously stripped by the gas. Thereby, helium concentration reduces as the distance from the cross section

rn

to the base cone distributor shortens. Besides, the rest of helium will flow upwards into the separation region together with solids. Under the influence of the draft tube, the solids will

Jo u

circulate from the separation region to the annulus region. Gradually, the helium will be initially entrained by the circulating solids from the separation region to the annulus region and then further dragged downward in the annulus region. Similar to the process in the draft tube region, the helium concentration in the annulus region gets low as the distance from the cross section to the base cone distributor reduces due to the continuous stripping of the upflow gas. As presented in Fig. 12 (b), as to the FFB, some helium will be dragged downward affected by the downward moving solids likewise. Similar to the process in the draft tube region of the HSLR, helium will be continuously stripped by the gas and the helium concentration tends less as the distance from the cross section to the base cone distributor decreases. Detailed analysis and demonstration will be given in the next text.

Fig. 12. The schematic of gas backmixing in the HSLR and in the FFB

Journal Pre-proof 3.2.1 Gas backmixing in the HSLR and in the FFB Fig. 13 shows the profiles of the helium concentration ratio (C/Cf) under different gas velocities both in the HSLR and in the FFB. Here Cf is the average helium concentration in the freeboard and Hi is the axial height between the sampling tap and the base cone distributor. Ugd and Uga refer to gas velocities in the HSLR, while the Ug refers to the gas velocity in the FFB. The cross sectional average gas velocity in the HSLR is equivalent to that in the FFB. The dimensionless radial position of draft tube is r/R=0.789, which is signed by a dash line in Fig. 13. As shown in Fig. 13 (a) – (f), under the same cross sectional average gas velocity, the helium concentration ratios in each dimensionless radial position in the HSLR are lower than the

f

corresponding ones in the FFB at the cross sections of Hi=390 mm, 590 mm and 790 mm,

oo

indicating less gas backmixing exists in the HSLR than in the FFB. The effect of draft tube is similar to that “diameter shrinkage”, thus increasing the gas velocity in the draft tube region in the

pr

HSLR. The high gas velocity inhibits the inner descending of solids distinctly, thus the gas

e-

backmixing is weakened.

Fig. 13. The radial profiles of helium concentration ratio at different cross sections in the

Pr

HSLR and in the FFB

al

In addition, it is seen that the helium concentration ratios reduce as Hi declines, which is coincident with the schematic of gas backmixing in Fig. 12. The direction of the main gas flow is

rn

upward and the helium will be stripped continuously. As a result, the helium concentration ratio gets smaller and smaller as helium move downward.

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Besides, the values of helium concentration ratio are low in the bed center and high near the bed wall, which means the gas backmixing in the bed center is obviously weaker than that near the bed wall. The helium concentration ratio in the bed center is related to the dragging by the inner solids circulation, which is believed to be quite small as a result of annular-core structure flow. On the contrary, the helium concentration ratio in the bed wall is the consequence of solids aggregation and weak exchange within the gas-solid phase. From the analysis above, the draft tube in the HSLR is extraordinarily effective on suppressing gas backmixing and the gas-solid flow is more close to plug flow in the HSLR than in the FFB, which is believed to be favorable to the methanol to olefins reaction.

3.2.2 Experimental data interpretation by one-dimensional dispersion model The gas backmixing in this paper is approximately treated by a one-dimensional quasi homogeneous phase model and represented by axial dispersion coefficient Dga. The steady one-dimensional dispersion equation is

Journal Pre-proof U g C 2 C  Dga 2  z z

(15)

The boundary conditions, z=Hj, C  C0 ; z=- ∞ , C  0 , where C0 is the volume concentration of the injecting helium and is substituted by Cf. The analytical solution of Eq. (15) is

In

C Ug 1  (z  H j ) Cf  Dga

(16)

Where C is taken as the cross-sectional average value of the five radial helium

oo

f

concentrations at every row of sampling points, and  is the cross-sectional average bed voidage

(z-Hj).

Ug H0

; here H0 is the static height of bed (1.1 m).

e-

Peclet number is expressed by Pe 

pr

obtained by experiments. The axial gas dispersion coefficient is evaluated by plotting In C / Cf vs.

 Dga

Pr

Peclet number reveals a relative ratio of gas convection vs. gas dispersion. A larger Peclet number denotes less gas axial dispersion. The corresponding axial gas dispersion coefficient Dga and

al

Peclet number of different cross sections are plotted in Fig. 14.

rn

As shown in Fig.14 (a), for the HSLR, the gas dispersion coefficient Dga gradually increases as Ug varies from 0.62 m/s to 1.41 m/s. In addition, Dga at the cross section Hi=390 mm ~ 590 mm

Jo u

is slightly larger than that at the cross section Hi= 590 mm ~ 790 mm, it may be attributed to the influence of the gas jets from the gas distributor. In the process of upward solids acceleration, the backmixing of solids and gas may be weakened. As to the FFB, at the cross sections both Hi= 390 mm ~ 590 mm and Hi=590 mm~790 mm, the gas dispersion coefficient Dga decreases as Ug increases from 0.62 m /s to 1.03 m/s; and then slightly rises with the increasing of Ug from 1.03 m/s to 1.41 m/s. That is to say, the gas dispersion coefficient Dga reaches the minimum value at Ug=1.03 m/s, which displays a different trend from that in the HSLR. Furthermore, the gas dispersion coefficient Dga at the cross section Hi=390 mm~590 mm is higher than that at the cross section Hi= 590 mm ~ 790 mm, coincident to that in the HSLR. On the whole, the values of D ga in the HSLR are considerably lower than that in the FFB, demonstrating the superiority of this novel HSLR in controlling gas backmixing. From Fig. 14 (b), the Peclet number both in the HSLR and in the FFB at different cross sections continues to increase as Ug changes from 0.62 m/s to1.41 m/s. This trend agrees with the

Journal Pre-proof results of Cankurt N T [29], Lee and Kim [32], Du [33], and Foka [34]. Moreover, under the same gas velocity Ug, the Peclet number in the HSLR is considerably higher than that in FFB, further showing the advantage of less gas backmixing in this novel HSLR. In conclusion, the gas-solid flow in HSLR is closer to the plug flow than in FFB and hence HSLR is more appropriate for the MTO reaction process, supporting the above analysis. Then the correlations for predicting the axial gas dispersion coefficient Dga and the Peclet number at different cross sections can be obtained by regressing the experimental data. As shown in Fig. 14, the profiles of Dga behave differently in the two subzones of the HSLR and the FFB. Therefore, the results in two subzones are given separately.

oo

In the upper subzone (590 mm＜ Hi ＜ 790 mm ):

f

The correlations in two subzones of the HSLR are obtained.

Dga  0.537U g 2  0.977U g  1.049

pr

In the lower subzone (390 mm＜ Hi ＜ 590 mm ):

(18)

e-

Dga  0.335U g 2  0.471U g  0.826

(17)

Accordingly, the equations in two subzones of the FFB are achieved.

Pr

In the upper subzone (590 mm＜ Hi ＜ 790 mm):

Dga  2.462U g 3  6.34U g 2  4.945U g  0.238

(19)

al

In the lower subzone (390 mm＜ Hi ＜ 590 mm):

rn

Dga  1.266U g 3  4.393U g 2  4.981U g  3.453

(20)

Jo u

3.2.3 Gas backmixing fraction from the separation region to the annulus region in HSLR The gas backmixing from the separation region to the annulus region and that in the annulus region will prolong the gas residence time in the novel HSLR, which is undesirable for the MTO reaction. Therefore, it is essential to quantify the gas backmixng flowrate above. In the annulus region, the upward gas from the distributor, the gas carried by the downward circulating solids and the gas constantly stripped by the upflow gas coexist. Therefore, a simplified model is required to specify the gas backmixing flowrate.

Fig. 15 Simplified calculation model of gas backmixing flowrate in annulus region of HSLR

As presented in Fig. 15, for the control volume at a certain cross section in the annulus region of HSLR, the net annulus gas flowrate Qan mainly passes through it. Besides, the gas flowrate entrained by the descending solids ΔQen and that stripped by the upflow gasΔQes , are simultaneously passing through the micro body. Thus the total variable of absolute gas flowrateΔ

Journal Pre-proof Qet in the control volume equals to the sum ofΔQen andΔQes. The total variable of absolute gas flowrate Qeti at the five cross sections in the annulus region of the HSLR can be calculated by Eq. (21).

Cei 

Qe t *i C f Qeti  Qan

(21)

Where, Cei and Cf refer to the helium concentration of the five sampling taps in the annulus region and the one sampling tap in the freeboard; Qeti reprensents the gas flowrate dragged by downward flow solids, m3·s-1; Qan refers to the net gas flowrate passing through the annulus outlet, m3·s-1.

f

To quantify the flowrate of gas backmixing in the annulus ΔQen, a simplified calculation

oo

model is proposed, i.e., ΔQen=ΔQes=1/2ΔQet. This is based on the fact that the gas backmixing from the separation region to the bottom of the annulus region will be completely stripped in the

pr

end. In other words, the net flowrate of gas backmixing at each cross section in the annulus region

e-

is equal to zero.

The gas backmixng fraction in the annulus region fba is defined as the volumetric percentage

Pr

of the gas backmixing flowrateΔQen divided by the total gas flowrate in the reactor, as given in Eq. (22).

al

f ba 

Qen 100 Qga  Qgd

(22)

rn

The effect of the gas velocity in the annulus region Uga and that in the draft tube region Ugd on the gas backmixing fraction of different cross sections in the annulus region of the HSLR are

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plotted in Fig. 16. Under the same Ugd, i.e., 1.4 m/s, at the cross sections of Hi=940 mm,790 mm and 590 mm in the annulus region, the gas backmixing fractions fba slightly reduce as the Uga increases from 0.2 m/s to 0.3 m/s, while fba rise as the Uga changes from 0.3 m/s to 0.6 m/s. That means Uga=0.3 m/s bring about the least gas backmixing and could be considered by the optimal gas velocity in the annulus region. The gas backmixing fraction fba at the cross section of Hi=390 mm tends slightly low when the Uga varies from 0.2 m/s to 0.6 m/s. Moreover, the gas backmixing fraction fba at the cross section of Hi=210 mm in the annulus region equals to zero under all the operating gas velocities, suggesting that the backmixing gas has been completely stripped upward at the bottom of the annulus region as illustrated by the above proposed model. As shown in Fig. 16 (b), when the gas velocity in the annulus region Uga is 0.3 m/s, the gas backmixing fractions fba of all the cross sections drop distinctly as the Ugd rises from 0.8 m/s to 2 m/s, which demonstrates less gas backmixing under high Ugd. On the other hand, it is evident to see that the gas backmixing fraction fba turns small as the distance from the the cross section of the annulus region to the base cone distributor shortens, further proving that the gas dragged by the

Journal Pre-proof descending solids from the upper cross sections will be continuously stripped upward by the gas from the distributors.

Fig. 16 The effect of gas velocity on the gas backmixing fractions: (a) Uga; (b) Ugd

3.2.4 Gas backmixing fraction fba calculation models of different cross sections in annulus region of HSLR In fact, the gas backmixing from the separation region to the annulus region and that in the annulus region is mainly linked to the operating gas velocities (Ugd and Uga), the solids circulation

gDd

) ( a

U ga 2 gDd

)b  (

Gs c ) pU gd

(23)

pr

f ba  k  (

U gd 2

oo

sections in the annulus region of fba can be expressed by

f

flux (Gs) and the diameter of draft tube (Dd). Thus the calculation models of different cross

e-

Table 2 gives the model parameters of the gas backmixing fraction fba at different cross sections in the annulus region by regressing the experimental data in Fig. 16. Accordingly, the

Pr

average relative errors between the calculated and experimental data are also given.

al

Table 2 Specific model parameters of fba at different cross sections in the annulus region

rn

From table 2, the average relative errors of fba reckoned by the models and the experimental data are quite small. Therefore, Eq. (23) can be used to estimate the gas backmixing fraction fba at

Jo u

different cross sections in the annulus region of HSLR.

3.4 Average gas residence time As stated above, the short average gas residence time denotes high turbulent instantaneous gas flowrate or velocity. That is to say, the average gas residence time represents the transport capacity for gas flowrate or velocity. In the following, the effect of gas velocity, static bed height on the average gas residence time in the HSLR is introduced first. Then the comparison between the average gas residence time in the HSLR and that in the FFB is discussed.

3.4.1 The effect of Uga and Ugd on the average gas residence time Tgh in the HSLR Fig. 17 illustrates the effect of gas velocity Uga and Ugd on the average gas residence time Tgh in the HSLR, respectively.

Fig. 17 The effect of gas velocity Uga and Ugd on the average gas residence time in the HSLR

As shown in Fig.17 (a), under the same Ugd, the average gas residence time Tgh shortens

Journal Pre-proof markedly when Uga rises from 0.2 m/s to 0.3 m/s, then slightly ascends as Uga increases from 0.3 m/s to 0.6 m/s. It indicates the priority of Uga = 0.3 m/s with the strongest transport capacity for gas flowrate or velocity and can be regarded as the design point for the novel HSLR. Combined with the analysis above in Fig. 16 (a), at the cross sections of Hi= 940 mm, 790 mm, and 590 mm, the gas backmixing fractions show the same trend as the average gas residence time when Uga changes from 0.2 m/s to 0.6 m/s and likewise present the lowest value at Uga = 0.3 m/s, which is one of the main causes why the average gas residence time Tgh is shortest for the case of Uga = 0.3 m/s. On the whole, the average gas residence time varies from 0.25 s to1.78 s under all the operating velocities, which meets the demands of the MTO reaction.

f

As shown in Fig.17 (b), the average gas residence time Tgh tends short with the rise of Ugd

oo

under the same Uga, denoting the large transport capacity for gas flowrate or velocity at high Ugd. Firstly, it can be traced back to the results of gas bypassing fraction as stated above. As described

pr

in chapter 3.1.3, the gas bypassing fractions both fda and fad reduces with the increase of Ugd, bringing out the high total gas flowrate or velocity in the draft tube region and less gas residence

e-

time in the HSLR. Secondly, as depicted in Fig. 14 (b), the Peclet number in the HSLR grows large with the increase of Ug or Ugd, which means low gas backmixing and short gas residence

Pr

time under high value of Ugd. At last, focusing on the gas backmixing fraction fba in Fig. 16 (b), fba drops sharply as Ug increases from 0.2 m/s to 0.6 m/s, which coincide with the trend in Fig. 17 (b).

al

That is to say, the average gas residence time in the HSLR is the comprehensive consequence of gas bypassing fraction fda and fad, Peclet number and gas backmixing fraction fba in the HSLR.

rn

3.4.2 The effect of static bed height H0

Fig. 18 depicts the effect of static bed height H0 (1.1 m and 1.6 m) on the average gas

Jo u

residence time Tgh in the HSLR. It is obviously seen that the average gas residence time Tgh in the HSLR at H0=1.1m is always smaller than that at H0=1.6 m, indicating the large capacity to transport gas when the static bed height is set at 1.1 m rather than 1.6 m. In addition, under the same Uga=0.3 m/s, for the case of H0 = 1.1 m, Tgh diminishes from 0.72 s to 0.25 s with the increase of Ugd from 0.8 m/s to 2 m/s; while for H0= 1.6 m/s, Tgh reduces from 1.22 s to 0.56 s as Ugd increases from 0.8 m/s to 2 m/s. The maximum time difference between H0=1.1 m and H0=1.6 m is 0.7 s while Ug equals to 1.0 m/s. It is probably ascribed to that the resistance from the solids is larger for gas to pass through at the case of H0 =1.6 m than that of H0=1.1 m. Based on the comparison above, the static bed height of H0 = 1.1 m is superior than that of H0 =1.6 m in HSLR for the shorter average gas residence time, i.e. the static bed height of H0=1.1 m is more proper for the MTO reaction under present research.

Fig. 18 The effect of static bed height on the average gas residence time in the HSLR

Journal Pre-proof

3.4.3 The comparison of average gas residence time in the HSLR and in the FFB To obtain an overall recognition on the performance of the HSLR and the FFB, the average gas residence time is used to analyze the transport capacity for gas flowrate or velocity in both of the two types of reactors. Fig. 19 gives the comparison between the average gas residence time in the HSLR Tgh and that in the FFB Tgf with the increase of the cross-sectional average gas velocity Ug. It is intuitively shown that the average gas residence time in the HSLR is far less than that in the FFB, implying the high ability of conveying gas in the HSLR. The absolute maximum time difference between

f

the average gas residence time in the HSLR Tgf and that in the FFB Tgh is 5.14 s when Ug is 0.62

oo

m/s. It can refer to the detailed analysis of the Peclet number both in the HSLR and in the FFB in previous text 3.2.2, the Peclet number in the HSLR is much larger than that in FFB all the time,

pr

indicating less gas backmixing in the HSLR. Besides, both the average gas residence time in the HSLR Tgh and that in the FFB Tgf behave the same tendency of monotonic drop with increasing Ug

e-

from 0.62 m/s to 1.41 m/s, presenting the favorable transport capacity of high gas velocity all

Pr

along.

4. Conclusions

al

Fig. 19 Comparison between average gas residence time in the HSLR and that in the FFB

rn

(1) The gas bypassing fractions of fda and fad obtained by the gas steady tracer technique are used to quantify the true gas flowrate in the draft tube region and in the annulus region of the

Jo u

HSLR. The dominant gas bypassing direction is from the annulus region to the draft tube region under all the operating gas velocities. Based on the experimental results, the dimensionless calculation models for gas bypassing fractions are established. The maximum relative error of fda between the experimental value and the calculation data is less than ±17.2 % and that of fad is less than ±7 %. (2)The gas backmixing both in the HSLR and in the FFB are scrutinized and comparison is made between them. Results show that the gas backmixing in the HSLR is considerably less than that in the FFB. The one-dimensional dispersion model is adopted to represent the gas

backmixing by the axial dispersion coefficient Dga and the Peclet number. Under the same gas velocity Ug, the Peclet number in the HSLR is much larger than that in FFB, demonstrating that the gas-solid flow in the HSLR is closer to the plug flow than that in the FFB. The non-linear correlations for predicting the axial gas dispersion coefficient Dga and the Peclet number at different cross sections are given by regressing the experimental data.

Journal Pre-proof The gas backmixing fractions fba from the separation region to the annulus region and that in the annulus region at the cross sections of Hi=940 mm,790 mm and 590 mm are achieved by another proposed model. Under the same gas velocity in the draft tube region, the case of U ga = 0.3 m/s provides the least gas backmixing and is beneficial for the MTO reaction. At last, the dimensionless calculation models for gas backmixing fraction fba at different cross sections are also listed. (3) The average gas residence time in the HLSR and in the FFB are both explored by a gas pulse tracer technique. For the case of Uga = 0.3 m/s under the same Ugd in the HSLR, the average gas residence time is the shortest, i.e., the strong transport capacity for gas flowrate or velocity.

f

Therefore, Uga = 0.3 m/s can be regarded as the design point for the novel HSLR. On the other

oo

hand, it is obviously seen that the average gas residence time Tgh in the HSLR at H0=1.1m is always smaller than that at H0=1.6 m, indicating the large capacity to convey the gas when the

pr

static bed height is set at 1.1 m rather than 1.6 m. In the end, the comparison between the average gas residence time in the HSLR and that in the FFB is discussed. It is intuitively found that the

e-

average gas residence time in the HSLR is far less than that in the FFB, further demonstrating the

Pr

superiority of the HSLR.

Nomenclature

Constant number, 5×10-3~5×10-4

C

The helium concentration, (v) %

C

The cross-sectional average value of five radial helium concentrations, (v) %

C0

The concentration of the injecting helium, %

Ca1

The helium concentration that was near tubular distributor in HSLR, (v) %

Cad Cd1

rn

Jo u

Ca2

al

b

The helium concentration in annulus region of HSLR, (v) % The helium concentration of bypassing gas from annulus to draft tube, (v) % The helium concentration that was near base cone distributor in HSLR, (v) %

Cd2

The helium concentration in draft tube region of HSLR, (v) %

Cda

The helium concentration of bypassing gas from draft tube to annulus, (v) %

Cei

The helium concentration of the five sampling taps in annulus region, (v) %

Cf

The helium concentration in the freeboard, (v) %

Ci

The instantaneous helium concentration, (v) %

dp

Particle mean diameter，μm

D

Inner diameter of bed, m

Dga

The axial dispersion coefficient

Dgb

The gas backmixing coefficient

Journal Pre-proof Dgr f

The radial gas mixing coefficient Gas bypassing fraction, vol. % of inlet gas bypassed to the other region

fad

Gas bypassing fraction from annulus to draft tube, %

fda

Gas bypassing fraction from draft tube to annulus, %

fba

The vol.% of the net gas backmixing divided by the total gas flowrate, %

FRin FRout

Flow ratio at the inlet of the bed,=Qgd/Qga Flow ratio at the outlet of the bed, =Qdn/Qan Solids circulation flux in the riser of HSLR，kg·m-2·s-1

H0

The height of the static bed，m

Hi

The axial height between sampling tap and base cone distributor，m

Hj

The axial height between injecting point of Helium and base cone distributor，m

Pe

Peclet number

Qad

The gas flowrate bypassing from the annulus to the draft tube, m3·s-1

Qan

The net gas flowrate passing through the annulus outlet, m3·s-1

Qda

The gas flowrate bypassing from the draft tube to the annulus, m3·s-1

Qdn

The net gas flowrate passing through the draft tube outlet, m3·s-1

Qen

The gas flowrate entrained by descending solids, m3·s-1

Qes

The gas flowrate stripped by the upflow gas, m3·s-1

Qet

The total absolute gas flowrate, m3·s-1

Qga

The gas flowrate in the annulus of HSLR, m3·s-1

Qgd

The gas flowrate in the draft tube of HSLR, m3·s-1

Qgt

Constant flowrate of tracer gas for gas bypassing in HSLR, m3·s-1

r/R

Dimensionless radial position

TSk Tga

oo

pr

e-

Pr

al

rn

Jo u

ti

f

Gs

The instantaneous time point, s The characterized time points, s The average gas residence time from the annulus through bed, s

Tgd

The average gas residence time from the draft tube through bed, s

Tgf

The average gas residence time in FFB, s

Tgh

The average gas residence time in the HSLR, s

Ug

Average cross-sectional superficial gas velocity in FFB and in HSLR，m·s-1

Uga

Superficial gas velocitiy in the annulus region of HSLR，m·s-1

Ugd

Superficial gas velocity in the draft tube region of HSLR，m·s-1

Ugr

Average cross-sectional superficial gas velocity in riser of HSLR，m·s-1

Umd

The lowest gas velocity in the draft tube region of HSLR，m·s-1

z

Axial height，m

Journal Pre-proof

Greek letters 

The cross-sectional average bed voidage

ρb

Particle bulk density，kg·m-3

ρg

Gas density，kg·m-3

ρHe

Helium density，kg·m-3

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[2] P. Tian, Y.X. Wei, M. Ye, Z.M. Liu, Methanol to Olefins (MTO): From fundamentals to

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[7] J.R. Lattner, H.N. Sun, S.N. Vaughn, K.H. Kuechler, D.C. Skouby, Process for converting oxygenates to olefins using molecular sieve catalysts comprising desirable carbonaceous deposits [P]:

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an airlift loop reactor. Particuology 9 (2) (2011) 130-138.

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petroleum coke combustor. Powder Technology 192 (2) (2009) 143-151. [20] Y.M. Zhang, C.X. Lu, M.X. Shi, Large-scale cold pilot experiment on a new annular catalyst

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[24] J.R. Grace, Cotacting modes and behaviour classification of gas-solid and other two-phase suspensions. The Canadian Journal of Chemical Engineering 64 (1986) 353-363. [25] B.H. Song, Y.T. Kim, S.D. Kim, Circulation of solids and gas bypassing in an internally circulating fluidized bed with a draft tube. Chemical Engineering Journal 68 (1997) 115-122. [26] H.Y. Zhang, R. Xiao, D.H. Wang, J.M. Cho, G.Y. He, S.S. Shao, Hydrodynamics of a novel biomass autothermal fast pyrolysis reactor: Solid circulation rate and gas bypassing. Chemical Engineering Journal 181-182 (2012) 685-693. [27] M.X. Liu, C.X. Lu, M.X. Shi, Advances in gas-solids airlift loop reactor. CIESC Journal 64 (1) (2013) 116-123. [28] Z.Y. Shen, Hydrodynamic characteristics of a gas-solid draft tube-lifted air loop reactor [D]. Beijing: China University of Petroleum (Beijing) (2012) . [29] N.T. Cankurt, J. Yerushalmi, Gas backmixing in high velocity fluidized beds. Fluidization Ⅱ.Cambridge: Cambridge University Press (1978). [30] J. J. van Deemter, Mixing patterns in large-scale fluidized beds. In Fluidization. Eds. Plenum Press.

Journal Pre-proof New York, USA (1980) 69-89. [31] S.A.R.K. Deshmukh, M. van Sint Annaland, J.A.M. Kuipers, Gas back-mixing studies in membrane assisted bubbling fluidized beds, Chemical Engineering Science 62 (2007) 4095-4111. [32] G.S. Lee, S.D. Kim, Gas mixing in slugging and turbulent fluidized beds. Chemical Engineering Communications 86 (1989) 91-111. [33] B. Du, L-S Fan, F. Wei, W. Warsito, Gas and solids mixing in a turbulent fluidized bed. AIChE Journal 48(9) (2002) 1896-1909. [34] M. Foka, J. Chaouki, C. Guy, D. Klvana, Gas phase hydrodynamics of a gas-solid turbulent fluidized bed reactor. Chemical Engineering Science 51(5) (1996) 713-723. [35] Y.M. Zhang, Hydrodynamic and mixing properties of a novel baffled fluidized bed [D]. Beijing:

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China University of Petroleum (Beijing) (2008).

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[36] G.S. Patience, J. Chaouki, Gas phase hydrodynamics in the riser of a circulating fluidized bed, Chemical Engineering Science 48 (1993) 3195–3205.

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[37] S. Mahmoudi, J.P.K. Seville, J. Baeyens, The residence time distribution and mixing of the gas phase in the riser of a circulating fluidized bed. Powder Technology 203 (2) (2010) 322-330.

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[38] D. Geldart, Types of gas fluidization, Powder Technology 7 (5) (1973) 285-292.

Journal Pre-proof Table 1 Properties of FCC equilibrium catalyst

Particle mean diameter dp, μm

Particle bulk density ρb,kg·m-3

Particle density ρp,kg·m-3

Repose angle θ

dP,μm 66.9

843

1500

31.3

。

Table 2 Specific model parameters of fba at different cross sections in the annulus region

Hi=940 mm

Hi=790 mm

Hi=590 mm

Hi=390 mm

Hi=210 mm

k

22.778

3.3525

1.831

0.7743

0

a

-1.1076

-1.2967

-1.0946

-1.0183

0

b

-0.077

0.078

0.2785

-0.3763

0

c

0.7975

0.334

-0.0592

0.5668

0

average relative error, %

2.35

5.73

5.27

3.79

0

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Parameter

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A novel high-speed loop reactor (HSLR) for Methanol to Olefins (MTO) is developed. The dimensionless calculation models for gas bypassing fractions are established. The gas backmixing in the HSLR is considerably less than that in the FFB. Uga = 0.3 m/s and H0=1.1 m are proper for MTO in the HSLR. The transport capacity of gas in the HSLR is more distinct than that in the FFB.

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Journal Pre-proof Credit author statement

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I have made substantial contributions to the conception or design, the acquision, analysis, and interpretation of data for this work. I have drafted the work or revised it critically for important intellectual content. I agree to be accountable for all aspects of the work in ensuring accuray or integrity of any part of the work. All persons who have made substantial contributions to the work are listed in the manuscript, including Shihan Ma and Jiajia Wen who participated the experiment work, Chenglin E who reviewed and edited this paper, and Professor Chunxi Lu who supervised this work.

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