The process of CFD could be subdivided into 3 stages namely Pre-processing, Processing, Post- processing.In the pre processing stage simulation domain is defined and grid is generated and this study is carried out using CFD tools some simplication methods are applied for complex grids to make valid model. The mesh that is generated will surround the substance of interest say fluids and helps to get physical properties of the substance of interest. Fine mesh will be present in the areas where large gradients of fluid flow is expected and coarser in the areas of relatively less or no change. The second step is processing numerical cure is obtained by a sequentialmathematical process which uses high accuracy to achieve high precision with alarge amount of repetitions. The solution is tracked as the simulation continues, to decide whether a converged solution has been achieved. Final step is post processing in which desired flow properties are extracted andthe sensitivity is analysedusing the computed flow field¹⁸. A schematic representation of various steps involved in CFD is depicted in Figure 1.

Figure 1: Schematic representation of steps involved in computational fluid dynamics

In tableting CFD is applied in the various formulation process and evaluation tests like die cavity filling, granulation, granule drying, disintegration, and dissolution testing they help by predicting the velocity, temperature and certain other parameters related to particular process, predicting the efficiency of certain parameters and assessing the flow field in the system³. In aerosols, it is used to understand the behavior of the aerosols in respiratory disease treatment and trackingthe movement of aerosol particles^19,20. In the parenteral preparation and biphasic liquids preparation it is used in various unit operations²¹ which is discussed in this article. The design and construction of the equipment may have a direct effect on the process efficiency in any batch manufacturing process. This designing process is optimised initially by identifying the possible vulnerabilities, which forms the potential opportunity for optimisation. Next, criteria for this opportunity identified is defined so that possible alternatives are generated these alternatives are then evaluated to get the suggestion for redesigning the equipment²². Figure 2 depicts various applications of CFD in the pharmaceutical industry.

Figure 2: Pharmaceutical applications of computational fluid dynamics

CFD in parenteral products:

Manufacturing Premises:

Quality of the air, water, steam, and other gases used in the manufacturing process of sterile products should be of high grade, carefully controlled as they form a potential gateway for contamination⁷. In the overall processing, certain steps arecriticaland sometimes require special attention those include filling area, procedures for washing and sterilizing different components used in packaging, proper training to all thelabors who are responsible for subdivision of fluids to final containers. In the filling area the product is exposed to both personnel involved and the environment thus this forms a critical area in maintaining the quality of preparation²³. Sterile pharmaceutical product formulation and processing working area of is required "Grade A," and should have unidirectional airflow according to EU-GMP and WHO-GMP. The processing area of sterile pharmaceutical products can be divided into four grades as per EU GMP Annex 1 & Japanese pharmacopeia General information section 29. They can be described as follows Grade A which refers to the zone of high-risk operations E.g. filling, sealing, exposed ampules, vials, these working stations are equipped with a laminar airflow system which provide homogenous airspeed that ranges between 0.36 – 0.54 m/s. Grade B is employed for aseptic product formulation& filling. Grade C and Grade D are clean areas used to perform lesser critical activities. From a case study, using CFD it was observed that the airflow properties in Grade A has no significant changes when the velocity of supplied airflow is varied, and the particles generated from the operatorit was possible to exhaust the particles generated from the operator outside the Grade A area without contamination²⁴. The manufacturing of sterile products can be widely distributed into two types of those products that need only terminal sterilization, and the ones which need aseptic processing at some or all stages. The greatest source of particulate matter is the personnel working in that area and this also forms the difficult step to monitor CFD simulation is used to understand the risk of bioaerosol generation through human activity in the aseptic area²⁵. The room architecture greatly influences the HVAC airflow pattern,the air quality 3DHVAC program is employed to facilitate the development of optimum HVAC systems to maintain indoor air quality (IAQ)²⁶. Cleanability is very important in biopharmaceutical preparations as it forms the measurement of the quality and it is a basic step after completion of each batch manufacturing.In order to ensure sterile conditions in the filling area after completion of each batch production, the manufacturing premises, like clean rooms and isolators, are decontaminated; generally, hydrogen peroxide is used for this decontamination process. Disinfection and maintaining sterility with movement of working personnel is confirmed by irradiating entire premises with ultraviolet lamps²³. Research outlines the potential for the simulation of Computational Fluid Dynamics (CFD) to be applied to forecast effectiveness of upper-room ultraviolet germicidal irradiation(UVGI) in aiding device design and the creation of future guidance. However,this approach relies on several physical and biological factors, including the sensitivity of microorganisms to UVC under various environmental conditions, the strength of the UV field etc. CFD simulation now provides a way of deeper comprehension airflow trends in rooms and observations into optimal place for UV systems in the upper room (through a simulated dose) If the room would benefit from extra blending. One of the key benefits of CFD modeling are the room-wide dosage distribution can be expected, although it actually cannot be achieved in experimental set up²⁷.

Figure 3: Computational fluid dynamics in parenteral products

Mixing:

Mixing is a process that maximizes the homogeneity in a given system. Instruments and equipment used in the sterile product manufacturing should be clean sterile and pyrogens fee. Factors influencing mixing include (a) The structure and configuration of the mixer, (b) Conditions of mixing, (c) operating conditions, (d) properties of components involved²⁸.

from the above parameters, (a)the type of impeller plays a significant role in both the hydrodynamics and the intensity of the liquid-solid mixing system²⁹. Preparation of products is done by using the best chemical grade. Simplified CFD models are utilized to investigate thesurge tank's cleanability. CFD can be acclimated tosimulate the flow of the cleaning solventto envision the normal wetted area and thus the cleanability of the tank. A study was conducted using the open-source CFD programming it was used to direct the simulation, a three-dimensional portrayal was drawn of the unit inside this the liquid stream was investigated. Thus, this process was able to provide optimum working and design of the washing process and valuable insight for tank redesign²². Mixing forms an important process of compounding sterile pharmaceuticals.CFD application in Mixing aid in selecting the mixer geometry, assessing mixing efficiency, for mixing simulations Reynolds-averaged Navier- Stokes equation (RANS) forms the most commonly used turbulent mixing simulation, 2D CFD can be reliably usedas it shows low computational cost at acceptable accuracy. Stirred tanks are the most commonly used equipment in mixing any pharmaceutical fluids, fluid flow in such tanks can be analyzed by using CFD. However, Experimental fluid dynamics and computational fluid dynamics are combined and usedto predict process performance³⁰. The rotating frame of reference technique also called multiple reference frames forms commonly applied tools.CFD and velocimetric techniques are used in combination to map experimentally and predict computationally the distribution of velocity within the apparatus as flow velocimetry process tend to generate data that are applicable only to certain locations and time, this may require frequent measurements from multiple locations, besides, CFD can produce data irrespective of time and location⁹. CFD is used to predict the mass transfer coefficient and this method is validated by performing experiments and it is proven its accuracy is comparable to that of empirical correlations. CFD can help assess the suspension of solid particles in slurry tanks.

Membrane filtration:

Membrane filtration is a commonly used method for sterilization of thermo-labile products that are unstable at a higher temperature. CFD fueled a study intending to pick the best suitable geometry of the membrane filter the study considered various dimensions of membrane filters possible i.e., quadrilateral, rectangular,andcircular narrow channels at distinctthree superficial velocities usually found in spiral wound membrane components were analyzed, i.e. 0.11, 0.16 and 0.2m/s. CFD analysis revealed that the circular channel provides a constant andbetter velocity stream compare to the others. As the circular shape provides a privilege by managing superficial feed flow inside the membrane channel, in comparison withquadrilateral and rectangularshaped channels³¹. CFD helps in modeling hydrodynamic aspects of the flow of substances across the membrane. A comparative study was done by comparing data obtained from direct observation by a high-speed camera and that obtained by CFD. Data obtained from direct observation revealed that the velocity of particles along the membrane increases with distance from the membrane surface, which correlates well with a ﬂuid velocity proﬁle obtained from CFD modeling. There are several membrane simulations packages available commercially.Koch Membrane Systems developed the ROPRO software, the ROSA software was developed by Dow Water and Process Solutions, the TorayDS software was developed by Toray Membrane, and hydranautics developed the InDesign softwareAre the most prominent examples of available membrane simulation packages,modelling of a wide range of membrane processes brings the tough problem of turbulence; 2D models can describe flow patterns to a considerable level while maintaining the cost reasonably low while considering the mass transfer process as well. However, 3D models can illustrate the complex flow patterns in membrane modulesandprovide the most easy implementation of the model possible due to larger expenses incurred by 3D models they find application in small representative samples³². The feature of periodic feed pressure techniques (PFPT) is highlighted using the results obtained from CFD simulations. Basically, PFPT is used to clean the membranefloor byreducing the dropletadheration to the membrane surface, giving the chance to the crossflow field to sweep off pinned droplets. CFDsimulations that have been stated for a droplet-pore combination in lab-scalemay notbe effective up scalable to large scale evaluation with the whole membrane system but this limitation can be overcome by using multicontinuamframework it is a unified framework of various microscopic findings obtained by CFD simulations^33,34.

Aseptic filling of parenteral:

Parenteral are products that are very delicate especially the vaccines and highly concentrated preparations, preparations with a monoclonal antibody.Precise and in-depthknowledge of filling operation inside the different dosing systems can be procured with Computational fluid dynamics. In most of the formulations of liquid parental products fill volume is generally calculated for each vial or container and is added to ensure the required quantity of drug formulation is available for dosing. A study aiming at finding analliance between different shear rates and observed proteindamage was performed rotating piston pump, linear peristaltic pump, circular peristaltic pumpwhere compared. In this study, CFD modeling was used to dig up and understand the flow patterns during low volume aseptic filling and the corresponding shear rate calculation was done.This study showed that the piston pump creates a recirculation zone, which causes product degradation by exposing it repeatedly to surfaces that produce shear stress. The CFD model could consistently simulate the effect of mechanical shear during the filling process³⁵. Blow seal apparatus often equipped with filling overlay in the filling area, over the ampoules. The pressure inside this overly directs airflow which in turn creates a clean environment around the ampoules and their immediate environment. CFD can be used to simulate this air velocity magnitude and flow rate of this mass. It also provides a better understanding of the different parameter settings that affect airborne particle concentrations in filling areas, also a clear understanding of the route of particle dispersion. CFD also suggests the means to alter the particle concentration for example a study showed that blow fills seal machine the particle concentrations could be reduced by reducing Mandrel velocity. However, these results are limited to the particular type of BFS machine that was used²¹.CFD modeling can be utilized to learn and comprehend the fluid displacement characteristics of diverse rheological qualities.

Freeze drying:

Freeze drying is one of the complex and energy consuming and critical unit operation in injectable manufacturing³⁶ it requires at least 2 times higher energy in drying process when compared to conventional dryers such as hot air oven, fluidised bed dryers³⁷. This technique isapplied in few parenteral preparations that are unstable in liquid form the process is also known as Lyophilization.The aim of using freeze-drying process extraction of the solvent in a manner such that the delicate drug's active ingredient is least disturbedand it is rehydrated rapidly and fully on water addition³⁸. Simulation of different parts employed in a freeze dryer can be done with CFD.CFD and statistical modeling wereused to acquire a better knowledge of flow dynamics and determine condenser performance, gain deeper insights intoa deposition in the condenser, ice condensation, and design improvement and methods used in this process.CFD inquiries greater understanding of how certain geometric features of the equipment affect the distribution of vapor and thus, the efficiency of the condenser may be achieving useful data for a potential enhancement of its design^39,37. Zhu .T et al applied CFD to simulate lyophilizers of both industrial and laboratory scale to understand the effect of change in design on the performance of the equipment. For a given formulation, independent variables such as storage temperature , chamber pressure, and response variables such as product resistance, product temperature, and primary drying time could be predicted by coupling of CFD with steady-state mass and heat transfer of vialfor various lyophilizers, the models were then verified experimentally⁴⁰. CFD forecasts and deposition dynamics of the water vapour flow field provide very useful knowledge to understand if the water vapor in the investigated equipment is well distributed and if the cooling surfaces are used effectively or, on the contrary, whether there is stagnant fluid zones and areas of underused refrigerating components. An articleIntroduces a two-dimensional simulation of a laboratory scale dryer with an emphasis on the significance of hardware architecture, drying conditions, and for process control and a three-dimensional simulation of industrial dryer were introduced along with a comparison of the findings produced with the analytical viscous.The findings revealed that the presence of sterilize in place (SIP)/ clean in place (CIP) pipes in the duct has a substantial impact on the flow area properties. The results of simulation obtained from the vapor flow rate in the industrial freeze dryer was compared with the gravimetric measurements andTunable diode laser absorption spectroscopy (TDLAS)³⁸. A schematic diagram of industrial freeze dryer is shown in Figure 4.

Figure 4: Industrial freeze dryer

Evaluation of parenteral: Drug release:

In the evolutionandassessment of drug-delivery systems (DDS) in vitro drug release testing forms the important process of biopharmaceutical characterization. There are different aspects of consideration while applying a tool that claims to be predictive of in vivosituationsfor pharmaceutical products administered by the parenteral path, there is no specific in vitro dissolution procedure in contrast to oral drug delivery systems where basket and paddle type are generally used according to shen J et al⁴¹. However, methods of in vitro release testing that mimics in vivo performance with strong discrimination power are important for quality control and the application of the data to obtain invitroinvivo correlation (IVIVC). Several applications of CFD in dissolution testing are available but their application in dissolution testing of parenteral preparations is less explored. A study of parenteral dissolution testing was carried out by Frenning G et al they used a modified reservoir method with rotating disc as per Ahnfelt E et al with the primary aim ofsimulating fluid flow in this method. The secondary aim was to understand the effect on convective drug exchangeamong the donor compartment andmedium of release and during hydrodynamic conditions in the donor compartment andthe impact ofthe nylon mesh filter. This study makes use of miniaturized dissolution apparatus this forms an advantageous method for in vitro dissolution testing during the initial phase of drug development as the amount of drug available during the initial stage of drug development is minimal^41,42. A schematic representation of miniaturised rotating disc apparatus for drug release testing is depicted in Figure 5.

Figure 5: Miniaturised rotating disc apparatus

CFD in emulsions:

The available computational fluid dynamics (CFD) models are referred to as CFD simulationsare used to research droplet formation in coflowing fluid streams, particularly with an end goal to offer a hypothetical expectation for emulsion droplet arrangement and explain thestream systems of narrowing jetting and broadening jetting. Besides, an experiment on drop formation in a coflowing liquid stream was conducted to give a comparison and validation for the simulation results⁴³. An analytical model to represent the subsequent droplet size can be provided based on the results from the comprehensive CFD simulations. The impact of a few significant boundaries follows directly from this model. The volume of fluid method incorporated (VOF) in CD-Adapco's Star-CD, version 4.0, was used as a simulation tool utilized to provide a view of the creation of spontaneous droplets in terrace-based microchannelsas microscopic observation provides an only two-dimensional view of the droplet and shapes, on the other hand, CFD can be used to get a detailed three-dimensional view thus, forms effective tool in obtaining a clear picture of curvature of the oil phase, in accordance with the pressure in the system. Besides, the impact of process variables such as contact angle, surface tension, channeldimensions, viscosity, and applied pressure on droplet formation could be predicted hence,an analytical model that expresses droplet size as a result of applied pressure was developed⁴⁴. CFD was used to study the impacts of the type and physical properties of the dispersed oil phase and the surfactant concentration on droplet development from a straight-through microchanneland the reliability of simulation can be confirmedby experimentation. CFD could successfully demonstrate the transformation of the oil phase to micrometer-sized droplets after passing through microchannel thus helping in designing the droplet formation process⁴⁵.

In an experiment of size-based sorting of emulsion droplets, CFD was employed solely to obtain droplet surface area simulated droplet trajectories sensitivity of droplet sorting protocol was assessed further optimization of operating parameters was made possible for efficient separation of particles within the size range. Quantification of changes in droplet surface area along the guiding track as possible and a good agreement between the experimental value and CFD simulations were observed⁴⁶. It was found by a study that the CFD dependentdual-phasic model was able to predict more constricted and safer windows atsimilar conditions as compared to the experimental results⁴⁷. Gallo-Molina et al performed simulations with two propellers: a propeller and a straight paddle turbine homogenization was simulated in a steady state.Here, Computational Fluid Dynamics (CFD) has been applied to understand better the relationship between process variables and other related responses. Process variables here refers to impeller Relationships between elastic modulus, zeta potential, mean droplet diameter, stability andbuilt-in energy measurements that could be developed. The simulation permitted the observation of three-dimensional gradients in relative viscosity, distributed phase volume fractionanddroplet diameter, and flow information for two of the impeller geometries studied⁴⁸.

CFD in suspensions:

Suspended particle flow:

Usually, chemical engineering processes deal with dense suspensions of large solid particles in a liquid. Predicting such systems behavior becomes difficult suspended particles affect the effect of the impeller(29). Accurate simulation of particles of suspension is of interest both for academic and industrial purposes for particle suspensions and porous media have complicated boundary conditions⁴⁹. Talib Dbouk developed a new approach of the computational frameworkwith adaptive meshrefinement (AMR) under theOpenFOAMopen source CFD framework.The phenomenon of shear-induced migration in different 2D and 3D geometrical configuration can be predicted using this open source framework⁵⁰. Mengmeng Zhou et al used CFD approach coupled with the Discrete Element Method (DEM) to understand the hydraulic conveying of coarse particles (suspension 6). A study was conducted to determine the change in flow behaviorby change of feed duct from circular to square and rectangular using 3D CFD simulations⁵¹. Multi- fluid CFD study based on Euler-Euler modeling is applied to study wellness in mixing in a desupersaturation tank where continuous agitation is required to prevent fouling problems and deposition of sludge⁵².

Runrunlu et al proposeda contrast simulation experiment for cohesive force quantification in a suspension at the microscopic level. A combined computational fluid dynamics anddiscrete element method (CFD-DEM) approach is the basis of the method.They've been utilized to investigate and explain the role of cohesion in particle fluidization. The DEM's enables a dynamic simulation of the motion of the solid phase by monitoring individual particles, while a CFD algorithm is widely used to simulate the flow field of the continuous fluid phase⁵³. With the help of both High and Low Reynolds k-ε Turbulence closures, horizontal tubing a 3D Computational Fluid Dynamics (CFD) numerical studies are described, using the Mixture Model for concentrated solid-liquid suspension flow. Numeric Simulations were performed to evaluate the effect of the closure of the turbulence in the model's capacity to predict the behavior of fully formed solid-liquid concentrated settling flows by spherical particles.⁵⁴. Forexample, Xiaoxia Duan et al used CFD software Fluent for numerical simulation and verified the model by comparing it with experimental values.Here the software was used to simulate the turbulent flow field and micromixing in solid-liquid stirred tankstetrahedral and hexahedral mesh was generated usingGAMBIT mesh generation toolwhich was used to discretize the computational domain.In this function, a typical simulation run was carried out. In general, around 1-1.5 h of operating time in total was required to solve mixture fraction and mixture fraction distributions and the variation thereof. Then the simulation was conducted to check if it falls within the predicted range obtained by the experimental method⁵⁵. G. Frenning E et alexperimented-on simulation of suspensions. Resin particles ranging from 250 microns to 300 microns in the size distribution range were used to perform this experiment with particle suspension. The distribution of particle size and the density of the particles were experimentally determined using the known values of particle size and densitythe trajectories of the suspended particles in the vessel could be simulated with CFD. The good match between the experimental results and the simulation shows that CFD can be used to conduct particle suspension studies by this experiment⁵⁶. In the process of preparation of dense suspensions of nanoparticles methods like wet stirred media milling method is most commonly used method extensive review was carried out with a total of 71 studies on 41 materials and 30 drugs was reviewed finally they concluded that the potential future in this field is in grounds of improving the mechanistic and accurate CFD-DEM-PBM(Computational fluid dynamics -discrete element method-population based model) coupling or MHD-PBM(microhydrodynamic- population-based model) coupled with CFD⁵⁷.

Simulation at the interphase:

In the case of suspension, the interaction forces between solid-liquid interphase forms and the important problem concerning the prediction of its rheological behavior Alexander zubou el al proposed a model to understand fluid viscosity in the presence of solid spherical particles. The existence of spherical particleshelps to simulate interactions of fluid and particles. A two-step process is used in this case where initially the rheology is observed to plain Newtonian fluids using CFD after which DEM is used along with CFD to simulate particles in suspensions. The results showed qualitative agreement with the literature data available⁸. Suspension balance direct-forcing immersed boundary model (SB-DF-IBM)solver based on mixed Eulerian-Langrangian approach is used in OpenFOAM® open-source Computational Fluid Dynamics (CFD) software. This forms a powerful tool in simulating non-Newtonian flows over obstacles.Here study was conducted with monodisperse suspension flows in both wide and miniature channels, to test theability of SB-DF-IBM solver^58,59. Various computational softwares and their applications are enlisted in Table 2.

Table 2: Computational softwares and applications

Software	Simulation application	Reference
OpenFOAM®	Incompressible laminar flows of fluid with immersed obstacles and Equipment redesign	8, 22, 58
ANSYS Fluent software	Turbulent flow field and micromixing	31, 55, 26, 42
ORCA CFD package	Velocity of fluids in the dissolution apparatus	30
ROPRO® ROSA®TorayDS	Membrane simulation in membrane filtration in membrane filtration	32
COMSOL 4.4	Particle Tracing in suspensions	10
CFX	Particle movement detector in suspensions	10
Comsol Multiphysics 5.4	Filtration cell simulation in membrane filtration	34
Gambit™ and Fluent™	Mesh generator
Flow- 3D v. 11.2 (FlowScience Inc., Santa Fe, USA)	Study of flow properties during low volume aseptic filling	35

CONCLUSION:

CFD forms a qualitative and quantitative method of assessing fluid flow with the help of (a) mathematical models, (b) numerical analysis, (c) suitable software.It minimizes the number of experimental works thus reducing the expenses and giving fast results.The Computational Model is a simple technique to employ in the design of potential fluid flow systems andaidsin capturing the systems mechanistic interpretation. CFD will show data at any moment and at any time and place but accuracy forms the limitation of the simulation. Greater the accuracy more will be the computational cost. However, CFD simulations are actually suffering from their failure to readily incorporate the physical properties distribution that are fixed or evolving from time to time. Advancement in computer technology can lead to accurate numerical solution of mathematical equations and also minimize simulation time.

ACKNOWLEDGEMENTS:

Authors are grateful to Manipal College of Pharmaceutical Sciences, Manipal Academy of Higher Education for the consistent support.

CONFLICT OF INTEREST:

The authors hereby declare that there is no conflict of any interest.

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Received on 20.10.2022 Modified on 08.03.2023

Research J. Pharm. and Tech 2023; 16(12):5935-5943.

DOI: 10.52711/0974-360X.2023.00963

Field	Application	Reference
Automotive industry	To understand the process variables and their effects, in turn, lead to meeting the progressive demands	11
Biomedical science	To understand the flow of fluids in the human body	12
Chemical and mineral processing	Combines complex processes into a single compact process thereby improving the efficiency of the process and assisting to extend the process to a large scale effectively	13
Civil and Environmental Engineering	Assist in ensuring compliance with certain regulations and also exploring all possible design before beginning the construction process	14
Food industry	In optimizing ventilation, sterilization, drying, refrigeration, cold display, food processing, etc.	15
Nuclear safety and metallurgy	Helps to ensure nuclear safety and redesigning certain systems to get access to operating conditions, qualitative effects of process parameters.	16
Pharmaceutical industry	Troubleshoot equipment's, determining flow properties of materials, Optimising process involved such as mixing,	17