Spray drying is a fast and cost-effective process which produces dry powders from a fluid material by rapidly drying with a hot gas medium (usually air). Spray drying techniques are used for a wide variety of processes associating mainly with the food and pharmaceutical industries. Although spray drying has shown its importance, the model of the processes depicting the performance of the spray dryers is poorly developed. As a result, an emphasis has been put on the flow stability of the dryer. Flow stability is a key issue because the spray dryer needs to avoid highly unstable flows, otherwise damages to the machine may occur. In other words, these highly unstable flows can result in partially dried product sticking to the walls of the machine. This results in the buildup of crust and the material can even catch on fire due to overheating. If this happens, then there are more frequent maintenance shutdowns which raises the overall cost and lowers the production rate 5.
The figure above shows a systematic plan of the conventional spray drying process. The spray drying process consists of four components: the atomization of the liquid feed, the drying of spray with the drying gas, the formation of dry particles, and lastly, the separation and collection of the dry product from the drying gas. The atomization of the liquid feed occurs when the fluid is fed into the drying chamber using a peristaltic pump through a nozzle or atomizer. Atomization is the process of breaking up bulk liquids into droplets and can occur through centrifugal, pressure or kinetic energy. Drying gas, usually air, is also added at the same time when the fluid is being fed into the chamber. The formation of the dry particles occur when the droplets, in the micrometer scale, are subjected to rapid solvent evaporation. The formation of dry particles are then separated from the drying gas by using the cyclone. The cyclone deposes the dry particles in a glass collector located at the bottom of the device. This is the final component which is the separation and collection of the dry product from the drying gas. Lastly, any exhaust gas is released into the surroundings. The liquid feed can be in the form of solutions, suspensions, emulsions, slurries, pastes or even melts. This gives spray drying more options for different dry particles due to the different viscosity and mixture of the liquid feed. Furthermore, the solid particles produced in the process have a higher chemical and physical stability when compared to its liquid counterparts. These solid particles can even be used as precursors for the production of capsules and tablets in drug delivery systems.
There are two operation configurations in the spray dryer, open-loop and closed-loop. The open-loop uses air as the drying gas that is not recirculated. The open-loop configuration is usually preferred because it is more cost effective and stable. The closed-loop, on the other hand, uses an inert gas that is recycled in the drying chamber. In addition, the closed-loop is used to prevent the mixing of explosive gases and to manipulate the oxygen sensitive substances. There are two directions in which the drying gas can flow with respect in the direction of the liquid atomization. It can flow in the same direction (co-current flow) or it can flow in the opposite direction (counter-current flow). The figure below shows the two different directions. In case A, the final product is in contact with the coldest air. As a result, case A is preferred for drying heat sensitive materials. In case B, however, the dry product is in contact with the hottest air. Therefore, case B cannot be used with temperature sensitive materials. But, in terms of higher thermal efficiency, case B is preferred. The direction does not only have to be with or against the liquid atomization. There are intermediate configurations containing mixed flow between co-current and counter-current 6.
There are 3 variables that can be manipulated which affect the characteristics of the result produced from spray drying. These variables are process parameters, properties of the liquid feed, and equipment design. The high flow rate of the liquid feed, large nozzle diameter and high formulation concentration are all parameters in which favor the formation of large particles. On the other hand, low surface tension, high atomization pressure and small nozzle diameter are parameters that promote the formation of small particles. In addition, faster solvent evaporation rate which indicates a lower boiling point usually produces more porous particles. This is due to the shorter time it takes for the droplets to shrink. The table below shows the different options within the 3 manipulated variables. By changing the options under each variable can significantly change the products of the process. Only by fully understanding what each of these parameters do, can one manipulate the products for the desired characteristics.
The equation to calculate the mass of the dried material is given below:
where mdm is the amount of dried material, mwm is the amount of wet material, Xwm is moisture content in %, and DMc is the dry matter content in %.
The total amount of heat produced in the spray drying process is given by QU=Q1+Q2. Q1 is the heat for water evaporation and Q1 = mha*h1a where mha is the required amount of hot (inlet) air and h1a is the enthalpy of inlet air. Q2 is the outlet air heat content and Q2 = mha *(h2a – h3a) where h2a is enthalpy of outlet air and h3a is the enthalpy of ambient air. It can be concluded that there is a transfer of heat during the spray drying process. This relates back to the Fourier’s Law and heat transfer. Spray drying, however, is a convective process. This means that heat is transferred through the bulk movement of molecules within fluids such as gases and liquids. This makes sense since the spray drying process uses liquid feed and drying gas to produce dry particles. When the dry particles are formed in the drying chamber, there is heat transfer via convection 7.
Due to its rapid, continuous, and reproducible abilities, the spray drying technique is very appealing for the laboratory and in the industry. This technique is scalable without making any major modifications. In addition, the final drying step is required in other techniques such as emulsion/solvent evaporation is not needed for spray drying. The technique greatly depends on the fulfillment of two conditions: scalability and cost-effectiveness. Another advantage spray drying has is the possibility to dry a large spectrum of compounds without any major detrimental effects. Heat sensitive substances are included in the spectrum. This is due to the atomization of the liquid which forms into small droplets with high surface area to volume ratio and therefore, leads to a very fast solvent evaporation. An example of this is during a co-current flow setup, the product temperature is 10 to 20 °C below the air outlet temperature. Although the droplets could be exposed to high temperature during the drying process, the exposure time is extremely short. It is usually in the range of seconds or even milliseconds. Under such conditions, drug degradation is not anticipated 8.