1. Introduction
Continuous atomization of high viscous concentrates is an important task in different industrial processes, like combustion, spray drying or coating [
1,
2]. In particular in spray drying, the capability to atomize high viscous feeds into small and uniform droplets is of great interest, as it allows an increase the initial dry matter content prior to actual drying [
3,
4].
For atomization of high viscous feeds, external-mixing pneumatic (ExMP) atomizers are commonly used [
2]. In pneumatic atomizers the kinetic energy of a compressed gas stream, mostly air, is used as source of atomization energy [
5,
6,
7]. In ExMP atomizers, gas and liquid are mixed under atmospheric conditions, outside the atomizer. As both media are supplied in separated streams, ExMP atomizers provide easy and independent control of gas and liquid flow rates [
8]. Consequently, ExMP atomizers are widely used in lab and pilot scale spray drying. However, especially in spray-drying processes they do not meet the requirements of industrial applications on atomization efficiency, due to very high gas consumption rates [
3,
4]. In this case atomization efficiency is defined as a correlation between energy input and resulting spray droplet size. In pneumatic atomization, the energy input is typically correlated with the specific gas consumption under applied processing conditions, which is expressed as the air-to-liquid ratio (ALR) by mass (Equation (1)). Here,
MX are the mass flows,
Qx are volume flows and
ρx are densities of gas or liquid (
L).
For ExMP atomizers, ALR in a range from 1 to 10 is usually applied [
1,
3]. The specific atomization efficiency of an atomizer can be estimated by use of a suitable correlation function [
1]. Over the years, a variety of different correlation models has been developed for all kinds of pneumatic atomizers [
9]. Generic models rely on the weighting of basic parameters, like nozzle exit diameter, processing conditions and liquid properties [
10,
11,
12]. More recent correlation functions also contain so-called nozzle constants, which can only be determined experimentally [
13,
14]. Consequently, related correlations can be considered as “not complete”, as additional work is required for every change of the atomizer geometry. However, nozzle constants ensure the validity of correlations for a wide range of properties and nozzle geometries. A very popular correlation function for ExMP atomizers was proposed by Harari and Sher [
13] (Equation (2)).
Here B is a characteristic nozzle constant and We is the Weber number of the gas. The Weber number represents the ratio between the air inertia forces and surface tension forces and is defined by Equation (3). In this calculation, ρgas is the gas density, Wr2 is the relative velocity, d0 is the exit orifice diameter of the nozzle and σL is the surface tension of the liquid.
In contrast to ExMP atomizers, internal mixing pneumatic (IMP) atomizers are known for high atomization efficiency, as they allow for atomization of high viscous feeds at reduced ALR [
15,
16,
17]. Consequently, they are considered to have high potential for use in industrial spray drying [
18,
19]. In IMP atomizers, atomization gas and liquid stream are brought into contact in a mixing zone inside the atomizer and leave the atomizer through the exit orifice in a two-phase flow pattern. For this purpose, several gas injection geometries have been proposed (e.g., [
15,
17,
20,
21,
22,
23]). Although no targeted mixing of gas and liquid is performed in most cases [
24], it is well known for IMP atomizers that internal flow conditions determine the means of liquid disintegration and the spray development in time and space [
22]. Especially an annular flow pattern inside the exit orifice is considered to be beneficial for stable atomization results [
25,
26].
The air-core-liquid-ring (ACLR) atomizer was designed especially to induce an annular flow pattern in the exit orifice by injection of a continuous core of compressed gas in the middle of the liquid stream [
27]. In contrast to ExMP atomizers, atomization gas gets into contact with the liquid in a compressed state. By leaving the atomizer via the exit orifice, it expands and reaches atmospheric pressure. Inside the exit orifice, gas properties depend on the specific pressure loss of the atomizer, which in turn depends on the atomizer geometry, the ALR and the viscosity of the liquid. This is mainly based on the fact that the gas and the liquid stream have to share the cross-sectional area of the exit orifice in annular flow [
28]. Consequently, ALR can be increased by a decrease in liquid flow rate (
QL) at constant gas pressure (
pgas), or by an increase in
pgas at constant liquid flow rate (
QL) [
29]. In the latter case, the mass flow of gas (
Mgas) increases at constant volume flow (
Qgas) by an increased gas density (
ρgas). In practical use, this case of ALR adjustment is very important, as subsequent processes often require the adjustment of certain droplet size distributions at constant liquid flow rate and constant atomizer geometry. An increase in
pgas also increases the internal energy of the atomization gas stream by means of a higher expansion potential. This additional energy might contribute to disintegration of the liquid flow into droplets. This consideration is specific to IMP atomizers. In ExMP atomizers, the influence of
pgas can be neglected, as full gas expansion to atmospheric conditions can be assumed before the first contact with the gas stream. Consequently, velocities of gas and liquid are determined by atomizer geometry and ALR in ExMP atomizers. This is not the case for IMP atomizers, as the share of cross-sectional occupation of the exit orifice for liquid and gas is dependent on the pressure-dependent gas density. Accordingly, the aforementioned correlation function by Harari and Sher [
13] is not applicable for IMP atomizers without knowledge of the gas pressure distribution inside the exit orifice, as the velocities of gas and liquid are unknown and change over the length of the exit orifice. As the expansion energy of the gas stream is not directly considered in the calculation of ALR (see Equation (1)) it is possible to apply equal ALR at different gas pressures with different gas expansion potentials in IMP atomizers. Based on the assumption that the energy of gas expansion contributes to the atomization energy, we hypothesize that spray droplet sizes decrease with increasing
pgas at constant ALR. In this case, ALR is not sufficiently defined to represent the energy input of IMP atomizers. However, it is important to keep in mind that an increase in gas pressure is always connected to a change in gas density. Consequently, an increase in gas pressure always leads to an increase in ALR, when liquid flow rate and atomizer geometry are kept constant in IMP atomization. Therefore, it is not possible to investigate the sole influence of gas pressure on the atomization efficiency of an IMP atomizer with given geometry at constant ALR. Therefore,
pgas and
QL have to be increased simultaneously in a specific ratio to keep the ALR constant. Hence, the influence of
pgas can only be estimated in a comparative way, using an alternative reference parameter for quantification of the energy input.
As an alternative parameter of energy input, the volume-specific energy density (
EV) can be used. This parameter was originally proposed to evaluate the energy input of emulsification processes in different dispersing systems. According to Equation (4) it is defined as power input (
P) per volume flow of liquid (
QL) [
30].
As an alternative correlation function to estimate the atomization efficiency, a characteristic droplet size
xchar can be described as a function of
EV by a power-law equation with two fit parameters
b and
c (Equation (5)) [
30].
The characteristic droplet size is mostly expressed as Sauter mean diameter (SMD). However, other characteristic values of the generated droplet size distribution can also be used, depending on the objective of the investigation. To characterize the breakup of a bulk phase at a specific energy input, the characteristic droplet size
x90,3 can be used, representing the largest droplets of a distribution [
31].
Stähle et al. introduced the volume specific energy density (
EV) in the field of atomization, in order to compare the atomization efficiency of atomizers that use different energy sources [
32]. According to the authors, this value is very useful for atomizer selection and scale up of atomization processes [
32]. In contrast to the ALR, which is only applicable in pneumatic atomization,
EV is defined as a discrete value (power per volume flow of processed liquid). This allows direct evaluation of energy consumptions in different atomization processes, independent of the used atomizer.
For internal mixing atomizers
EV is calculated according to Equation (6), where
R is the gas constant for dry air,
T is the temperature, and
p0 is the ambient pressure.
In contrast to the ALR, the expansion potential of the gas stream is considered in
EV in form of the energy that is needed for isothermal gas compression [
33]. In order to review the correlation of ALR and
EV, Equations (1) and (4) can be combined. Therefore, the liquid volume flow rate (
QL) is expressed as a quotient of liquid mass flow rate (
ML) and liquid density (
ρL). This leads to Equation (7).
Consequently, the mass flow ratio of gas and liquid can be replaced by the ALR (Equation (8)):
Based on this correlation, it can be seen that EV increases with increasing gas pressure (pgas) at constant ALR, temperature (T) and liquid density (ρL). Therefore, as described above the increase of internal energy of the gas stream at increased pgas is considered in the calculation of EV.
Nevertheless, it is not yet clear to which extent this energy is transformed into atomization energy in ACLR atomizers. Investigations of Stähle et al. were performed at constant pgas of 0.4 MPa. To the best of our knowledge, no further work on the applicability of EV as a reference parameter of ACLR atomization efficiency has been published yet. However, it can be assumed that differences in spray droplet sizes, caused by variation of pgas at constant ALR, are reflected by the value of EV. This assumption can apply, if the portion of energy that is converted into atomization energy is larger than the energy of thermal losses. Thermal losses are not reflected by the described calculation of EV, as the energy of the gas stream is calculated on the basis of isothermal gas compression.
In order to investigate the hypothesis of decreasing droplet sizes with increasing gas pressure, we performed atomization experiments with a pilot scale ACLR atomizer at constant pgas and different QL, as well as at constant QL and different pgas. In these experiments, model solutions of different dry matter concentrations and hence different viscosities were used. From all processing conditions, ALR and EV were calculated. Thereafter, resulting droplet sizes were correlated to ALR, as well as to EV, in order to identify the applicability of both parameters as the reference of energy input in ACLR atomization. Additionally, the proposed advantages of using EV in the evaluation of scale up-related differences in atomization efficiency were investigated. As described above, energy efficiency of an ACLR atomizer depends on the pressure loss over the exit orifice, which in turn is dependent on geometry and processing conditions. Therefore, an up-scaled ACLR atomizer with larger exit orifice diameter was designed and used in atomization experiments with increased QL. Finally, the atomization efficiencies of both atomizers were compared.
2. Materials and Methods
Aqueous solutions of maltodextrin (C*Dry MD01958, Overlack GmbH, Mönchengladbach, Germany) were used as model system in the performed investigations. Dry matter contents c
MD of 40, 45% and 47% were adjusted under consideration of the moisture content (X = 4.5%, dry basis) of the used maltodextrin powder. The maltodextrin solutions were characterized by viscosity (
µ), surface tension (
σ), density (
ρ) and refractive index (RI). All measurements were executed in triplicate at a temperature of 25 °C. Mean values and relative uncertainty values
ur(γ) of these parameters are summarized in dependency of the dry matter content c
MD in
Table 1. Viscosity measurements were performed using a rotary rheometer (MCR 101/301, Anton Paar GmbH, Graz, Austria) with coaxial cylinder geometry (CC27). Within the measurement range of shear-rates between 1 and 1000 s
−1 all solutions showed Newtonian behavior. Surface tensions were measured with a Wilhelmy plate system (DCAT 21, DataPhysics Instruments GmbH, Filderstadt, Germany). For measurement of refractive indices (RI) a refractometer (Carl-Zeiss, Oberkochen, Germany) was used. Densities were measured by use of a pycnometer.
2.1. Air-Core-Liquid-Ring Atomizers
In this study, two different self-manufactured ACLR atomizers prototypes were used. The geometries of both atomizers are shown in
Figure 1. Atomizer 1 is a prototype, designed for atomization in pilot scale spray-drying processes. This geometry was already used in several former studies [
27,
29,
32,
34,
35,
36]. Atomizer 2 is a scale up version of atomizer 1. In comparison to atomizer 1, mainly diameter and length of the exit orifice were increased. The most important dimensions of both atomizers are given in
Table 2.
2.2. Spray Test Rig
For spray experiments a modular test rig was used. The atomization gas was compressed air supplied by a compressor (Renner RSF-Top 7.5, Renner GmbH, Güglingen, Germany) with a pressure vessel volume of 90 L. Relative pressures of 0.4, 0.6, and 0.8 MPa were adjusted with a pressure regulator. Corresponding gas volume flows (Qgas) were measured by a gas flow meter (ifm SD6000, ifm electronic, Essen, Germany). Measurement values of Qgas are reported at normal conditions (273.15 K, 0.1013 MPaabs) in normal cubic meters per hour (Nm3∙h−1). In experiments with atomizer 1, an eccentric screw pump (NM011BY, Erich Netzsch GmbH & Co. Holding KG, Waldkraiburg, Germany) was used for the liquid supply. The flow rate was adjusted between 21 L·h−1 and 40 L·h−1 by an electronic frequency converter. In experiments with atomizer 2, a pressure vessel was used to supply flow rates of 100 L·h−1, 120 L·h−1, and 140 L·h−1, adjusted by a manually operated needle valve. In both cases, the liquid flow rate was measured by a flow meter (VSI 044/16, VSE GmbH, Neuenrade, Germany). The liquid pressure was not monitored. All trials were performed at room temperature (25 °C).
2.3. Droplet Size Measurement
For spray droplet size measurements a laser diffraction spectroscope (Spraytec, Malvern Instruments, Malvern, UK) was mounted to the test rig. The spectroscope was equipped with a 750 mm focal lens, offering a droplet size measurement range of 2–2000 µm. The laser beam crossed the full cone of the spay angle at the atomizers centerline in a vertical distance of 250 mm. The spray was collected in a vessel below the measurement zone, which was connected to an exhaust fan. Measurements were conducted at a frequency of 250 Hz over a time of 25 s, leading to the recording of 6250 droplet size distributions per measurement. Based on this data, an average droplet size distribution was calculated under consideration of the refractive indices of the used model solutions. For beam steering correction, the method described in one of our former studies was used [
36]. In this study, the commonly used Sauter mean diameter (SMD) was chosen as standard reference parameter. Additionally, the characteristic value
x90,3 was regarded to review the correlation between energy input and resulting droplet sizes. This parameter represents the largest droplets of a distribution and is known to be very sensitive to changes in processing conditions of ACLR atomization. Results of
x90,3 are only partly shown, in order to ensure readability of the paper.
2.4. Calculations and Statistical Analysis
OriginPro software, version 2017G (OriginLab Corporation, Northampton, MA, USA) was used for calculation of fitting curves and statistical analysis. For fitting curves, the Levenberg Marquard iteration algorithm was used. As goodness of fit parameters, the corrected determination coefficient R2 was used. The effects of ALR and EV on resulting spray droplet sizes were evaluated in dependency of the liquid viscosity by a 1-way analysis of variance (ANOVA). Scheffé’s test was used for comparison of means. In the performed tests, probability of p < 0.05 was used for the identification of significant differences.
4. Conclusions
In the presented study, the influences of gas pressure (pgas) and atomizer scale up on atomization efficiency of air-core-liquid-ring (ACLR) atomization were investigated. It was assumed that pressure-dependent expansion energy of atomization gas contributes to liquid disintegration in ACLR atomization. For this reason it was hypothesized that droplet size decreases with increasing pgas at constant ALR. In this case ALR would not be sufficient for distinct determination of processing conditions. Therefore, the volumetric energy density (EV) was considered as an alternative reference parameter.
For correlations between SMD/x90,3 and the two investigated reference parameters of energy input, ALR and EV, only slight differences in determination coefficients R² of applied fit functions were found. In total, an influence of gas pressure on the atomization efficiency could not be determined to full extend in the range investigated. Instead, the mechanism of liquid disintegration seems to be similar to that of ExMP atomizers. Hence, resulting droplet sizes mainly dependent on the liquid sheet thickness at the outlet of the atomizer, the relative velocity of gas and liquid phase, and the viscosity of the liquid. Consequently, the use of established correlation functions for ExMP atomizers should be considered for description of the atomization performance of ACLR atomizers in future studies.
Experiments with an up-scaled ACLR atomizer revealed that similar droplet sizes can be generated with increasing liquid flow rates, when the diameter of the exit orifice is increased. However, in this case also specific energy consumption increases, leading to a decreased atomization efficiency as the kinetic energy of the gas, travelling in the middle of the gas core, cannot be used for liquid disintegration. As a conclusion, the direct correlation of energy input and resulting droplet sizes might not be sufficient for determination of atomization efficiency of differently scaled ACLR atomizers. Instead, the liquid sheet thickness has to be taken into account for assessment of atomization efficiency and scale up, although it is difficult to evaluate in practical use.
Further research will be addressed to the flow conditions inside the exit orifice of ACLR atomizers, in order to gain a better understanding of the mechanism of droplet formation. In particular, the dependency of the liquid sheet thickness and the relative velocity of gas and liquid phase has to be investigated for further development of ACLR atomization.