Intensifying Mass and Heat Transfer using a High-g Stator-Rotor Vortex Chamber

Highlights

• Proof-of-concept experiments of an intensified centrifugal contactor STARVOC

• STARVOC uses energy flow of primary phase to drive rotor containing secondary phase

• Enhances heat and mass transfer by increasing density, uniformity, and slip velocity

• STARVOC reaches centrifugal accelerations in the range of 102 -104 g

Abstract

Vortex reactors take advantage of the synergy between enhanced heat and mass transfer rates and multifunctional phenomena at different temporal and spatial scales. Proof-of-concept experiments with our novel and innovative STAtor-Rotor VOrtex Chamber (STARVOC) confirm its advantageous features for the sustainable production of chemicals and fuels. STARVOC is a high-g contactor that uses carrier flow (gas or liquid) tangential injection to drive a rotor attached to low-friction bearings. The vortex chamber inside the rotor contains a secondary phase or phases, such as a solids bed, a liquid layer, or a suspension. Carrier fluid passes through the perforated rotor wall and contacts a densely and uniformly distributed secondary phase with enhanced slip velocities. Experiments focused on pressure profiles, rotor angular velocity, and solids azimuthal velocity. With air as the carrier fluid and different solid particle beds as the secondary phase, STARVOC reached bed azimuthal velocities up to four-fold compared to those reached in Gas-Solid Vortex Units with fully static geometry. These results show its potential to improve interfacial heat and mass transfer rates and take advantage of flow energy and angular momentum. Due to its process intensification capabilities, STARVOC is a promising alternative for the state-of-the-art chemical industry.

Introduction

Several industrially relevant gas-solid, gas-liquid, and gas-solid-liquid processes require intensive interfacial mass and heat exchange, and order-of-milliseconds contact time [1], [2], [3], [4], [5]. However, the Earth's gravitational acceleration limits the interfacial slip velocity and, consequently, the mass and heat exchange rate [6]. In addition, mesoscale non-uniformities [7,8] and low solids volume fraction are also detrimental and difficult to avoid [9]. Operating under centrifugal accelerations much higher than Earth gravity is a potential solution to these issues [6,[10], [11], [12]]. Classical means for generating the centrifugal field rotate a chamber around its central axis [13], [14], [15] or inject carrier fluid tangentially into a static chamber [16], [17], [18]. Here we present proof-of-concept experiments of an intensified centrifugal contactor that takes advantage of these two operation modes [19]. The STAtor-Rotor VOrtex Chamber or STARVOC uses the tangential injection of carrier fluid (gas or liquid) into a static cylindrical chamber to drive a rotor attached to a static shaft by low-friction bearings. The carrier flows radially inward through the perforated rotor wall, thus contacting a densely and uniformly distributed secondary phase (solids bed, liquid layer, or a suspension) contained in the rotor at high slip velocities. STARVOC minimizes the friction with walls and improves the operation with fine particles, thus opening promising process intensification perspectives.

Current solutions feature limited mass and energy interfacial exchange rates, high energy consumption, narrow slip-velocity operation range, and detrimental mesoscale non-uniformities. For example, Fluidized Beds (FBs) are ubiquitous in gas-solid processes [20], [21], [22]. Particulate solids in FBs transform into a fluid-like state in which the frictional force between particles and gas counterbalances the effective weight. Fluidization typically starts with bed expansion without bubbles (smooth fluidization) or a bubbling bed. However, increasing gas-solid slip velocity finally leads to larger bubbles and uncontrollable particle entrainment. Some of the limitations of gas-solid FBs are as follows [7,23]. First, the terminal velocity in the Earth's gravitational field represents an upper limit for gas-solid slip velocity. Second, the solids bed width-to-height ratio needs to be optimized to minimize non-uniformities. Lastly, light cohesive powders of less than ten microns do not form a fluid bed.

Two centrifugal-based technologies are attractive for gas-solid processes. The first one, known as Rotating Fluidized Bed or RFB, uses a rotating cylindrical chamber driven by electrical work, as shown in Fig. 1a [14,24,25]. The chamber features two flat end walls and a perforated cylindrical wall. Carrier gas flows radially inward through the perforations, contacts the solid bed that rotates at nearly the same rotational velocity of the chamber, and finally flows out through one of the end walls. However, significant particle entrainment, especially with fine powders, hinders its application [26], [27], [28]. Further limitations are the external energy supply for rotating the chamber and control difficulties due to the absence of balance between inertial and drag forces acting upon the solids bed.

Fig. 1b illustrates the second centrifugal-based technology, which uses carrier gas to sustain a rotating solids bed into a static chamber [16,[29], [30], [31]]. The chamber features tangential inlet slots evenly distributed on the cylindrical outer wall. The carrier gas enters at high velocity, typically exceeding 30 m s⁻¹, decelerates after transferring angular momentum to the solids, and flows out through a central opening in one of the end walls. Literature refers to this technology as Rotating Fluidized Bed in a Static Geometry (RFB-SG) [32], [33], [34], Gas-Solid Vortex Reactor (GSVR) [35], [36], [37], [38], or Gas-Solid Vortex Unit (GSVU) [39,40]. Limitations for their application are particle entrainment caused by near-end-wall gas jets, low solids azimuthal velocity caused by excessive particle-wall friction, and excessive energy consumption due to the relatively high carrier gas-to-solids mass ratio.

Injecting carrier gas through stationary inclined blades covering an annular space at the bottom results in higher gas-solid slip velocities than those in FBs [41], [42], [43]. The carrier gas jets suspend the solid particles forming a rotating solids bed, as shown in Fig. 1c. The device, known as TORoidal BED (TORBED®), is a Mortimer Technology Holdings Ltd registered trademark. The jets minimize the boundary layer surrounding solid particles, which, together with higher gas-solid slip velocities, enhances interfacial mass and energy transfer. TORBED certainly improves gas-solid processes, but its operation still relies on the gravitational force to balance the gas-solid drag force: the centrifugal acceleration diminishes with bed height to a point where the bed behaves as a conventional fluidized bed. Although TORBED partially suppresses mesoscale non-uniformities, the limited gas-solid slip velocity finally restricts mass and energy transfer rates.

Bubble and slurry bubble columns are well-known gas-liquid-solids contactors [44], [45], [46]. Typically, gas bubbles enter through a distributor into a liquid or a suspension. The gas flows either co-currently or counter-currently to the liquid flow direction. Different flow regimes exist for gas-liquid contact, starting from a homogeneous bubbly flow in which the superficial gas velocity lies in the range of 0.03-0.08 m s⁻¹ [46]. Higher superficial gas velocities lead to a slug-type or heterogeneous bubble flow, depending on the column's cross-sectional area. Several limitations are inherent to bubble and slurry bubble columns. For a column of a given size, the homogeneous bubbly flow features small and uniform bubbles. However, the gas throughput is limited to keep the superficial gas velocity below 0.08 m s⁻¹. The homogeneous bubbly flow regime shows the best results in mass transfer, but industrial bubble columns mainly operate under heterogeneous flow conditions leading to excessive column heights. Under these conditions, bubble terminal velocities and interfacial shear stress decrease while bubble size increases.

Existing centrifugal-based contactors can potentially enhance gas-liquid and gas-liquid-solid processes. For example, the Rotor Stator Spinning Disk Reactor or RSSDR simultaneously applies centrifugal forces and high-shear on an order-of-millimeters thin liquid film [47], [48], [49]. For its implementation, hurdles are limited gas throughput and excessive energy consumption to keep the shear rate and rotor movement. Additionally, applying a centrifugal field to a cocurrent multiphase flow tends to separate the phases. An alternative is the Centrifugal Bubbling Reactor (CBR) or Gas-Solid Liquid Reactor (GLVR) [50], [51], [52]. GLVR forms a highly dispersed gas-liquid mixture with stability over a wide range of centrifugal acceleration, relatively high volumetric interfacial surface area, high renewal rate of the interface, and high gas throughput. Some GLVR limitations are liquid entrainment caused by near-end-wall gas jets and relatively low rotational velocity caused by excessive friction with static walls. Another development is the Spinning Fluids Reactor or SFR [53], which uses tangential gas and liquid injection to generate a high shearing force that decreases bubble size. The relatively high interfacial area per bulk liquid is a salient SFR advantage, although the interfacial slip velocity still limits its performance.

Advanced energy conversion processes and the sustainable production of chemicals and fuels call for innovative reactor technologies. STARVOC can potentially provide orders of magnitude reduction in reactor size, improved control, and enhanced heat and mass transfer rates. STARVOC's main strength is that it could effectively use the primary phase energy and angular momentum to drive a rotor containing a secondary phase. Centrifugal accelerations in the range of 10² -10⁴ g should result in a densely and uniformly distributed secondary phase and higher slip velocities. A potential application is the valorization of natural gas in the form of ethylene and higher hydrocarbons via Oxidative Coupling of Methane (OCM) [5,54]. STARVOC provides effective catalyst retention, order-of-milliseconds contact time, narrow residence time distribution, and enhanced heat removal. The effective retention of fine particles (< 200 μm) and enhanced mass transfer indicate potential process intensification opportunities. These capabilities are also attractive for catalytic short-contact-time reactions such as partial oxidation of methane and oxidative dehydrogenation of ethane [2]. Another potential application is the adsorption of SO₂ and NOx from flue gases on a particulate sorbent [55]. These benefits also apply to Fluid Catalytic Cracking (FCC), which uses a 1g-Geldart A type catalyst [34]. Recent gas-liquid experimental results in a static vortex unit show improved micromixing time [52]. STARVOC could improve these results due to the enhanced centrifugal force, which opens perspectives for CO₂ capture from flue gases or air in liquid sorbents.

The remainder of this paper is organized as follows. First, the experimental set-up, operational procedures, and measurement devices are described. Next, the first experimental data obtained over a broad range of gas mass flow rates in our STARVOC demo unit are discussed, such as the rotor and solids bed azimuthal velocities next to pressure profiles. Finally, differences between STARVOC and other centrifugal-based contactors are quantified, such as carrier gas usage, solids bed's azimuthal velocity, and centrifugal acceleration. The process intensification capabilities could make STARVOC a promising alternative for the state-of-the-art, as indicated by the proof-of-concept experiments discussed here.