Impinging streams (IS) are classified into gas-continuous and liquid-continuous ones (GIS and LIS). Large number of experimental data has shown GIS promotes transfer very efficiently; while it has the intrinsic faultiness of very short residence time, and its flow configuration is relatively complex, resulting in difficulty in arranging multistage process. Essentially, GIS is applicable only for rapid processes controlled by gas film diffusion. The effect of LIS enhancing transfer is negligible; while it has the features of efficient micromixing and strong pressure fluctuation both which are resulted from the intensive interaction between the opposing streams and can promote process kinetics. All the features of IS, including GIS and LIS, have great potential of application. Reviewing the results of number investigations, mostly worked by the authors, a somewhat detailed introduction to the features of IS and several cases of its successful applications, such as wet FGD, preparation of ultrafine or nano powders, successful industrial application of large scale LIS reactors and crystallizers, etc, are described.
Impinging streams (IS), as a novel technical method in chemical engineering, first presented by Elperin [
Original idea of impinging streams [
In addition, to make the concept clear, the definition of IS was proposed as follows [
Reviewing the results from a number of researches and developments, mostly worked by the authors, the present paper introduces the major features of IS of the two categories and some of their remarkable application cases.
In GIS heat and mass transfer are efficiently enhanced mainly by the following [
Another notable nature of GIS is strong collisions between particles in the impingement zone. In many cases, for example, when the particles are used as a catalyst, this property is harmful as it crushes particles and thus increases the loss of catalyst; while in the others strong collision can be used for crushing and grinding of solid materials. In fact grinding or milling of solids is a field where GIS earlier applied successfully in industries, and the well-known Trost Jet mill has been collected into the standard tool book [
On the other hand, GIS has the intrinsic faultiness of very short residence time in the active region, mainly the impingement zone; its flow configuration is relatively complex so that any arrangement of multistage GIS would be difficult and energy consuming. Therefore GIS alone is applicable essentially only for the processes involving rapid reaction(s) on surface of or inside the particles or droplets in dispersed phase and thus controlled by gas-film diffusion, such as powdery coal or sprayed liquid fuel burning and chemical absorption involving instant reaction in liquid, and so forth.
Liquid-continuous impinging streams (LIS) have been found to have the following major features of great significance for application.
With LIS, because of the properties of liquid, mainly high density and viscosity, the factors enhancing transfer in GIS mentioned in the last section are essentially disappeared so that the effect of LIS enhancing transfer is negligible. Tamir et al. [
On the other hand, high density of liquid leads to strong interaction between the opposing streams in LIS, resulting in a number of excellent features, as described below.
Micromixing is a critical condition for processes occurring on molecular scale in liquid phase. The interaction between opposing streams in LIS, including collisions, pressing, and shearing between fluid elements, reduces the segregation scale, and when the latter is down to the Kolmogoroff microscale,
A brief view of SCISR 1-drawing tubes; 2-propellers; 3 impingement zone; 4-overflow outlet IZ-impingement zone.
Mahajan and Kirwan [
The results of both the investigations mentioned above indicate clearly that LIS promotes micro-mixing very efficiently. Unfortunately, a new model for micromixing, correct and universal, cannot be established yet, and so further studies are needed.
Also, the interaction between opposing streams in LIS results in vibration of fluid elements and/or molecules, and the global behavior is just pressure fluctuation. Zhang et al. [ There exists considerably strong pressure fluctuation in SCISR. The standard deviation of instantaneous pressure can achieve 200 to 300 Pa, implying the maximum amplitude of fluctuation being about 1.6 kPa, under the condition of impinging velocity being about 0.2 The major frequency of fluctuation is concentrated in the range of <1000 Hz, while there are some lower peeks in acoustic wave range. The impinging velocity,
It is clear that both efficient micromixing and strong pressure fluctuation are resulted from the interaction between the two streams in impingement, including collision, pressing, and shearing between fluid elements, as mentioned above. Well micromixing must increase the possibility of collision between molecules. While pressure fluctuation, as the global behavior of molecules and/or fluid elements vibration, means a part of flowing energy converted into vibration energy of molecules and/or fluid elements. During the conversion, molecule energies and their profile may be changed so that more molecules achieve higher energy level. It is therefore inferred that the combined effect of strong micromixing and pressure fluctuation may increase the probability of effective collision and thus favors kinetics. This inference has been confirmed by experiments.
Wu et al. [
The kinetics data of Na2HPO4 crystal growth were measured in the same SCISR operated at the impinging velocity ranged from 0.1 to 0.2 m
The kinetics data of alcohol ester saponification were measured, respectively, in the SCISR with an effective volume of 3.6 L and a traditional stirred tank reactor (STR) of 0.6 L operated under the same conditions. The results indicate that the values for the rate constant of the second-order irreversible reaction measured in SCISR,
Later, a comparative investigation was also made for hydrolysis kinetics of sugar by Chen et al. and similar results were obtained, as reported in [
What can be concluded or, at least, inferred, from the results above are the following LIS promotes process kinetics significantly; or, more generally, process kinetics depends not only on the natures of the system involved and operating conditions such as temperature, and so forth, but also on flow configuration in the reactor employed. The fact that no obvious differences between the values for the active energies measured in different reactors or crystallizers are observed suggests that LIS increases the frequency factors,
Also, the problem rises from the results: How to get, or what reactor should be used for, accurate kinetic data. SCISR cannot yet be considered as the ideal one for, at least, its micromixing time,
Obviously, the features of IS, including GIS and LIS, described above have great potential of application, provided that the target systems are chosen properly. In fact, in a number of application cases they exhibited very well performances, as briefly described in what follows.
As mentioned in [
In addition to the above mentioned applications, a remarkable application of GIS should be for flue gas desulfurization (FGD), a very important problem of environment protection as well known. Wu et al. [
As described above, GIS alone is suitable only for rapid or instant processes governed by gas-film diffusion.
With dilute Ca(OH)2-in-water suspension as the absorbent for wet FGD, the major reactions in liquid are instant and irreversible with new solid products forming [
The equipment used for the pilot plant test of wet FGD is the patented impinging stream gas-liquid reactor (ISGLR) [
A brief view of GIS gas-liquid reactor.
The eddy pressure nozzle.
The essential goal of the study is to develop FGD equipment of industrial interest, and so understanding its general performance is of significance. With fully mixed SO2 and air as the pseudoflue gas (concentration of CO2 equals zero), several sets of typical operation data measured in stable operations are listed in Table
Typical operation data of the impinging stream FGD system.
Gas flow rate m3 | Initial SO2 in gas mg m−3 | Flow rate ratio | Mole ratio of Ca/S Mol | Impinging velocity m | Contacting time, s | Final SO2 in gas mg | Efficiency of sulfur- removal, % | Pressure drop over reactor, Pa |
---|---|---|---|---|---|---|---|---|
0.08 | 3200 | 0.84 | 1.4 | 7.0 | 3.02 | 240 | 92.5 | 405 |
0.08 | 2400 | 0.84 | 1.4 | 7.0 | 3.02 | 132 | 94.5 | 405 |
0.08 | 2000 | 1.20 | 1.4 | 7.0 | 3.02 | 80 | 96.0 | 405 |
0.08 | 2400 | 0.85 | 1.0 | 3.02 | 3.02 | 276 | 88.5 | 405 |
0.10 | 2000 | 0.65 | 1.4 | 10.2 | 2.42 | 91.8 | 91.8 | 380 |
As the most important indexes, both gas-film and volumetric mass transfer coefficients and the influences of some factors on them were investigated. The major results obtained are: The impinging velocity, With the impinging velocity Essentially, SO2 concentration in flue gas has no influence on
Table
Comparison between volumetric mass transfer coefficients measured in ISGLR and RPB, respectively.
Device | System | Rotary speed, rpm | Reference | |
---|---|---|---|---|
ISGLR | SO2—Ca(OH)2 | 0 | 0.58–1.04 | [ |
RPB | CO2—NaOH | ~1400 | 0.21–0.44 | [ |
RPB | CO2—NaOH | 1400 | 0.35 | [ |
RPB | CO2—NaOH | 1550 | 1.14 | [ |
RPB | CO2—NaOH | 1369 | 0.66 | [ |
The device shown in Figure
Brief view of HFISGLR 1-tower; 2-screen foam-remover; 3-gas outlet tube; 4-liquid outlet tube; 5-gas conduit; 6-eddy pressure nozzle; 7-liquid feeding tube; 8-damper I, II…: impinging stream components.
Atomization of absorbent is an important operation in the wet FGD technology discussed here. GIS itself calls finely dispersed absorbent, on one hand; while atomization provides great interface area to increase transfer rate, on the other. Therefore atomizer is an important auxiliary device to HFISGLR. Since flue gas from coal-burning is generated in huge amount, the amount of absorbent needing to be atomized is also very large, even if a small liquid-to-gas ratio is used. So atomizers of high production must be used. Otherwise, over thousand atomizers may be needed in one GIS absorber, making installation and maintenance extremely difficult. Based on plentiful experience obtained in successful industrial applications of the eddy pressure nozzle developed earlier [
The large flowrate eddy pressure nozzle [
The combination of HFISGLR with the large flowrate eddy pressure nozzles forms a novel technology for wet FGD. In comparison with the most popular technology for wet FGD, at least in China, the so-called limestone-gypsum (LSG) process, in which fine limestone powder-in-water suspension is used as the absorbent and absorption is carried out in a spray tower, the novel technology is predicted to have the following major outstanding advantages: (
SCISR shown in Figure
Reaction precipitation is an important category among various methods for preparation of ultrafine materials because of its economic reasonability and convenient and feasible operation. Essentially, the major process in this method is crystallization from a solution. For production of ultrafine product with narrow size distribution the key is to create high and uniform supersaturation environment for precipitation. In addition, the surface of nucleus or microparticles newly formed is very active and so usually needs to be aged for certain time to avoid or ease assembling of new microcrystals with each other to form larger particles.
High (average) supersaturation can always be achieved without difficulty; while uniform one is not easy. As mentioned above, SCISR, one of LIS devices, has the feature of very efficient micromixing and thus can provide the condition of very uniform supersaturation. On the other hand, SCISR has the special flow configuration of circulative perfect mixing-plug flow (essentially no mixing) in series [
With SCISR with an effective volume of 3.6 L shown in Figure
Chen [
The product obtained under the optimized conditions is average sized 5.1 nm, while the average size of the mixed products from over 20 repeated runs is less than 10 nm, suggesting well stability of operation. A comparison of the mean size of the products with those by other processes and/or reactors is given in Table Using the mixed product in lubricating grease as an additive leads to an increase in lubricating efficiency by 50%, higher than those with grinded Nano copper powder as the additive by about 20% [ By properly controlling the conditions both the products of sphere and needle forms can be obtained, as shown in Figure
Comparison of average sizes of nanocopper products obtained by various processes and with various reactors.
Copper salt | Reducing agent | Temperature, | Reactor | Mean size of product, nm | Reference |
---|---|---|---|---|---|
CuSO4 | HCHO | 70 | STR | 100 | [ |
CuSO4 | Ascorbic acid | 85 | STR | [ | |
CuSO4 | N2H4 | 60 | STR | 50–500 | [ |
CuSO4 | NaH2PO2 | 55–65 | STR | 50 | [ |
CuSO4 | KBH4 | 20 | STR | 40–100 | [ |
CuCl2 | KBH4 | 20 | SCISR | [ | |
CuCl2 | Na2S2O4 | 60–70 | STR | 20 | [ |
TEM photos of products with different forms.
Sphere form
Needle form
Chen also studied the preparation of Cu-Ag double metal powder from Nano copper particles of sphere form by substitution, with Ag-ammonium solution as the reagent and gelatin as the protecting agent, in order to improve anti oxidation nature of product. A comparison of the experimental product with those by other methods and/or equipments is given in Table
Comparison between Cu-Ag double metal powders obtained by various processes and/or with various reactors.
Reducing agent and/or reactor for preparation of nano copper | Average size of nano copper | Amount of Ag coated, %mass | Mean size of Cu-Ag powder | Antioxidation nature at room temperature | Reference | |
NaH2PO2 | 22–85 | ≥100 nm | Yes | [ | ||
N2H4 | 50–500 nm | — | ≥300 nm | Yes | [ | |
Ascorbic acid | ≥500 nm | ≥500 nm | Yes | [ | ||
KBH4 | STR | Yes | [ | |||
SCISR | 5–30 nm | Yes | [ |
Li [
The sizes and crystalline types of products obtained under the optimal conditions while calcined at different temperatures are listed in Table
Particle sizes and crystalline types of nano-Titania calcined at different temperatures.
Temperature, | Mean size, nm | Crystalline type |
---|---|---|
No calcined | Immeasurable | Amorphous |
400 | 5.47 | Incomplete anatase |
600 | 8.84 | Anatase |
800 | 26.84 | Rutile |
TEM photos of Nano Titania calcined at 600 and
Calcined at 600
Calcined at 800
To verify the superiority of the reactor, a comparative study was also made in the SCISR with an effective volume of 3.6 L and a traditional stirred tank reactor (STR) of 0.6 L, respectively, under the same conditions. The results are shown in Table
Comparative results of SCISR with STR.
Reactor | Yield of Ti | Size of TiO2, nm | ||
Maximum | Minimum | Average | ||
SCISR | 0.962 | 16.0 | 2.0 | 5.68 |
STR | 0.927 | 20.85 | 3.34 | 11.28 |
Hydroxyapatite (HAP), Ca5(PO4)3(OH) or Ca10(PO4)6(OH)2, has important application in medicine because of its well bio compatibility. Yuan et al. [
The influences of mixture pH, dropping rate of (NH4)2HPO4 solution, and heating time after reaction are examined and the products are characterized by energy spectrum, thermal difference, infrared spectroscopy with Fourier transformation, X-ray diffraction, scanning, and transmission electron microscopy to determine composition, mainly Ca/P mole ratio, and appearance of the microcrystals. The optimal conditions determined in the primary study are Dropping rate of (NH4)2PO4 solution 54 mL
The TEM photo of the product prepared under primarily optimized conditions is shown in Figure
TEM photo of nano Hydroxyapatite prepared under primarily optimized conditions.
In addition, with the same SCISR Zhou et al. studied experimentally the preparation of nanomagnesium oxide [
All the results related to the subject indicate that LIS is a powerful tool for preparation of ultrafine materials by reaction precipitation, and that improvement of technical device is also an effective way to enhance preparation technologies of Nano materials.
As described above, SCISR has been used in a number of fundamental and applied investigations and has exhibited excellent performance in preparation of ultrafine powders. While it was also found to have some disadvantages caused by its horizontal structure, such as the following (
The non-rotating flow vertical circulative impinging stream reactor [
The reactor employs the flow configuration of vertical co axial two impinging streams plus circulation after impingement. The basic design is as follows. The upper and lower drawing tubes of identical smaller diameter are placed co axially in a reaction container of larger diameter. Two propellers with opposing screw directions are mounted at the inlets of the tubes, respectively, on the same shaft and driven by the same motor. The streams are drawn by the propellers and transported through the drawing tubes towards the center of the container and impinge against each other to form vertical coaxial two impinging streams. After impingement, the streams flow through the annular chambers between the drawing tubes and the wall of the container to circulate. Thus, like SCISR, NRFVCISR has the special flow configuration of circulative complete mixing flow—no mixing flow in series. The circulation is originally arranged for increasing residence time to meet requirement of most reaction systems, while it is of especial significance for certain systems, as will be described later.
Because the impingement plane is at a depth far from the level, NRFVCISR can be operated at much higher impinging velocity stably; while the placement of bearing being over the reaction mixture keeps from possible detrition and leakage.
On the other hand, since the two propellers have opposing screw directions while are driven by the same shaft, their rotation will cause both axial flows in opposite directions and rotating flows in the same tangential direction. As the result of the superposition of the two rotary momentums, very strong rotating flow occurs, yielding great centrifugal force and thus concave level with large height difference, destroying normal operation. In the design of NRFVCISR, as the measure of restraining rotating flow, several vertical buffers are mounted on the inside walls of the drawing tubes. Because of acting at the beginning, they restrain the rotating flow very efficiently without additional energy consumption.
The application cases described below show that the design of NRFVCISR has attained completion.
According to the results of batch runs reported in [
Instead of SCISR, the pilot plant test employs the NRFVCISR with an effective volume of 0.3 m3 for the reaction, of which the production is 200
Principle system scheme of pilot plant test for preparation of “ultrafine” white carbon black by impinging stream reaction
Near 30 runs were made for both experimental data and probationary product. The measured average sizes of products from a part of runs, which are for probationary product and reconfirming the feasibility and so employ essentially the same conditions, are listed in Table
Average sizes of products from a part of runs.
Run nos. | Operating conditions | Average size | ||||
Concentration of Na2SiO3 SiO2% m | Concentration of H2SO3% m | Reaction temperature | Time for adding acid S | Reaction time min | ||
14 | 35 | 55 | 8–11 | 10 | 2.4885 | |
15 | 35 | 55 | 8–11 | 10 | 2.2698 | |
16 | 35 | 55 | 7–10 | 15–20 | 20 | 2.3120 |
17 | 35 | 55 | 7–10 | 15–20 | 20 | 2.9426 |
18 | 35 | 55 | 7–10 | 15–20 | 20 | 2.0024 |
19 | 35 | 55 | 7.1–10.5 | 15–20 | 20 | 2.4689 |
20 | 35 | 55 | 7.1–10.5 | 15–20 | 20 | 1.7418 |
21 | 35 | 55 | 7.1–10.5 | 15–20 | 20 | 2.2162 |
Size distribution probability of typical “ultrafine” white carbon black.
From the data listed in Table
On a quasi commercial scale, the results of the pilot plant test above prove again that the superior micromixing property favors reaction-precipitation processes to produce ultrafine powders with smaller particle sizes and narrower size distribution.
The application of NRFVCISR in commercial production of pentaerythritol as the condensation reactor is a very successful example.
In 2008, a group company built a new production system for pentaerythritol with monopentaerythritol as the main product. Monopentaerythritol and bipentaerythritol are produced by condensation of acetaldehyde with formaldehyde and caustic soda. This is a complex system, and the main reactions are
The reactions are carried out with large excess of formaldehyde, and acetaldehyde is the key component. Other by-reactions are also possible to occur. If acetaldehyde at high concentration contacts caustic soda directly, undesired by-reactions will occur to produce colored by-products, resulting in reduced yield, purity and, whiteness of product with poor market competition ability. Therefore feeding to and mixing in the condensation reactor are of essential importance.
The company is the earliest one producing pentaerythritol in China, and its skills including achieved yield, and so forth, are in the head, at least, in China. The original production system employs special stirred tank reactors (STRs) with external circulation; multiagitator, each contains several paddles of different forms, is used in the tank in order to enhance mixing. The acetaldehyde is fed into recycled mixture, and so the feeding point is far from that for caustic soda.
In the new system, three nonrotating vertical circulative impinging stream reactors (NRFVCISRs), effective volume of 28 m3 each, are employed for condensation. The acetaldehyde and caustic soda are fed at the inlets of the upper and lower drawing tubes, respectively; a coil cooler is settled between the wall of the container and the drawing tubes to remove the great reaction heat.
Up to date the reactors have been operated constantly for over one year without any fault, and exhibited excellent performances greatly prior to the stirred tank reactor with external circulation in the following.
This is expressed by the following. (a) The chromaticity of product is obviously reduced, and the whiteness of the products primarily crystallized and recrystallized is higher by 35 and 25 (Pt-Co standard color), respectively, than those from the original production system, and so the appearance of product is significantly improved. (b) The consumption of acetaldehyde is reduced to an extent, and the statistic yield is increased by 1.1%.
The installed total capacity for the STR with external circulation in the old system is 70 kW; while that designed for NRVCISR is 10 kW only for the same production, and the practically installed as requested by the user is 15 kW.
The operation for long term shows that the feeding and mixing performances of NRFVCISR are very well. It is evaluated that, at an impinging velocity of 0.3 m
NRFVCISR provides the superior features of efficient micromixing and strong pressure fluctuation, the inherent features of LIS, and so large recycling ratio although; its power consumption is very small. This is because of the special mode of fluid transportation. As well known, the propellers transport the reaction mixture at very small head that is needed to cover the resistance only. Generally, NRFVCISR needs an installed total capacity under one third of that for STR of the same effective volume without external circulation, and so its energy-saving effect is significant.
Because of its high efficiency, great energy saving, and the special functions in producing finer nanomaterials and enhancing selectivity of reactions, and so forth, NRVCISR can be expected to be the ideal substitute of the traditional stirred tank reactor (STR).
Crystallization is an important unit operation for producing number solid products. The major aim of the operation is to obtain product with larger and uniform particle sizes; while the key is to create proper and uniform supersaturation, at which crystals grow quickly and, in the case of no crystal seeds feeding, nucleation can occur to supply proper number of nucleuses [
Non-rotating flow impinging stream crystallizer.
Since 2007, the nonrotating flow impinging stream vacuum and heat-exchanging crystallizers has been applied industrially; now, totally 23 sets of large scale, 18 sets of effective volume 32 m3, and others of 20 m3 are in operation in three companies in China for production of various products, and all they are working well.
In addition, recently the reactor shown in Figure
It is trusted that, in addition to those described above, impinging streams can find more and more applications because of their excellent features.
Impinging streams (IS) are classified into two categories: gas-continuous impinging streams (GIS) and liquid-continuous impinging streams (LIS). They have individual and quite different features and thus are applicable for various occasions. All the features of IS, including GIS and LIS, have great effects on enhancing processes. One very important thing in application is that the target system has to be chosen properly according to the properties of both the specific IS and the target system.
The major feature of GIS is enhancing efficiently transfer between phases; while it has the intrinsic disadvantage of very short residence time. So, GIS alone is applicable especially and only for rapid or instant processes controlled by diffusion through gas film. Except to combustion, the most important application of GIS may be for wet FGD with dehydrated lime as the absorbent; an equipment system employing GIS of quasiindustrial scale can be expected to be built in the near future.
The effect of LIS enhancing transfer is negligible; while, due to strong interaction between the opposing streams, it has the important features of efficient micromixing and considerably strong pressure fluctuation favoring process kinetics and so is suitable for processes occurring on molecular scale in liquid, especially those involving reaction(s), and its application field should be much wider than GIS for such processes are widely involved in many process industries. Results of number investigations have shown that preparation of ultrafine materials by reaction precipitation is a potential area of LIS application, and that improving reaction device is an effective way to enhancing technologies for preparation of nano materials, too. Since 2007, a number of reactors and crystallizers employing LIS, NRFVCISR, NRFISVC and NRFISHEC, of large scales, have been, as the first time, successfully applied in industry, and operated for long term without any faultiness. Because of its high efficiency, great energy saving, and the special functions in producing finer nanomaterials and enhancing selectivity of reactions, and so forth, NRVCISR can be expected to be the ideal substitute of the traditional stirred tank reactor (STR).
It is trusted that, in addition to those described here, impinging streams can find more and more applications in various processing industries because of their excellent features.
Specific area, m2
Mole ratio of calcium to phosphorus
Mole flow rate ratio of calcium to sulfur
Diameter of absorption chamber, m
Diameter of gas flow conduit, m
Impinging velocity, m
Height of absorption chamber, m
Height of gas conduit from the lower edge of the cylinder chamber, m
Overall crystal-growth rate coefficient in IS crystallizer, m
Overall crystal-growth rate coefficient in fluidized-bed crystallizer, m
Gas-film mass transfer coefficient, m
Liquid-film mass transfer coefficient, m
Reaction rate constant in IS device, m3 kmol−1 s−1
Reaction rate constant in stirred tank reactor, m3
Impinging distance, m
Micromixing time, milliseconds
Kolmogoroff microscale.
The projects supported by The National Natural Science Foundation of China (No. 29276260, 20176043, and 20424002).