VACUUM TRAY DRYER (VTD)

MEANING OF VTD OR WHAT DOES VTD STAND FOR:

VTD Or Vacuum Tray Dryer, is one of the most popular vacuum drying equipment, which is economic, as compared with other vacuumized drying equipments. Just like the RCVD, VTD too is majorly used in Pharmaceutical (especially API) Drying operations. VTD is also used in other industries for Drying purpose such as Chemicals, Food, Cosmetics etc.

POPULAR CAPACITIES OF VTD:

Usually VTDs are known with the Number of Trays they can accommodate. Mostly, 48 Trays and 96Trys VTD are common in the industry. However, the smallest VTD can be of 3 trays and the biggest could be of 120 trays also. In the rarest cases, a VTD of 192 trays can also be manufactured, however, a lot of operational challenges comes along, such as, the number of trays increases, the loading/ unlading of trays becomes challenging and laborious making it difficult to manage ergonomically. Also, the overall chamber size increases, which makes it difficult for the manufacturer to be able to produce a rectangular box like structure that can withstand high vacuum without bulging.

ADVANTAGES OF VTD OVER TRADITIONAL DRYERS OR POPULAR VACUUM DRYERS:

• Minimum Loss/ Wastage of Material: VTD works on the principle of static bed. Hence, the LOD (loss on dying) majorly is nothing but the mass of solvent Or water evaporated. There are possibilities of minute and light powder particles getting lost due to vacuum, however, the loss is negligible.
• Safe to Operate: Since, it is a Static Dryer, chances of operational hazards are less (unlike other dryers like RCVD, ANFD, AVD Or FBD which are rotary in nature and can cause hazard if rotary mechanism goes out of control).
• Space Saver: Many a times, the body of the VTD is taken off the operational area (viz. VTD Body is taken off the Clean Room) and only the Door is inside the operation area. This saves a lot of space and offers a great amount of operational flexibility to the operator and/or user.
• Simplicity of Operation: Operating is very simple and straight forward. With minimal training, operators can use it easily.
• Ease of Cleaning: Washing/ Cleaning is easy as compared to other types of dryers. Internal Hot Plates Or Heating Shelves are completely removable, hence, can be cleaned once in a while. Otherwise, the entire interiors of the chamber are washable in place.
• Provision of Solvent Recovery: Solvent recovery rate is very high, as compared with other Or traditional dryers.
• Energy Saver: Power Or Electricity consumption is minimum ( as comparted to any rotary kind of a dryer).
• Easy to Automate: Level of automation is mostly Non-Complex Or Straightforward, as, it is a static dryer.
• Economical: VTD is indeed economical, when compared with the other types of Vacuum Dryer, because it is simple cabinet dryer with static bed vacuum drying.
• Choice of MOC (Material of Construction): Just like RCVD or any other Vacuum Dryer, the MOC can be SS316 Or Hastelloy Or Halar/ PU/ PFE coated SS304. This can take care of the corrosive material and its vapors.
• Possibility of integrating it with a Flexible or Rigid Isolator: Small size VTDs can be used for Containment Application, thus avoiding the product and vapor contacts with the operator.

CHALLENGES THAT COMES ALONG WITH VTD:

• Loading and Unloading of the Wet/ Dry mass respectively: Bigger the VTD, more difficult is the loading and unloading. It is not only Time Consuming, but also stressful (as mostly single Or maximum two operators usually has to manage the entire Unit Operation.
• Shuffling/ Racking of Wet Mass between the Batches: Many a times, a batch of material has to undergoes a series of Drying Cycles (ranging from a few minutes to a few hours). In this process, usually, after end of each Drying Cycle, the VTD door is opened and the trays filled with wet mass are taken off for Shuffling Or Racking and are loaded back to their respective positions. Handling the heavy trays loaded with the process mass is challenging. Also, the trays can be very very hot at times, hence, unloading-shuffling-putting back can be painful.
• Handling Wastages: Since the operation involves handling of metal trays filled with powder/wet mass, and that too sometimes very hot, there are chances of material spilling over due to improper racking/shuffling, loading and/or unloading. 
• Size and Capacity wise Limitation: As a usual practice, a typical tray in Pharma API is usually of size 800mm (32") Long x 400mm (16") Wide x 25mm Or 30mm (1") Deep. The approximate tray volume in this case is nearly 10 litres. With a bulk density of  500 kg/m3 and say 50% loading, the mass of powder Or material is approximately 2 to 2.5 Kgs (or maximum 3 kgs). Hence, a typical 48 Tray VTD can dry approx 145 Kgs, and a 96 Trays VTD can dry approx 290 kgs. The other types of Vacuum Dryers can have more material handling capacity.
• Human Exposure to Hazardous Vapors and Wet mass: Since a lot of manual handling of material is involved in this process, VTD is necessarily recommended to handle Only Non-Hazardous and Non-Toxic chemicals. 

MATERIAL OF CONSTRUCTION:

Since Pharmaceutical, Food and even Cosmetics follows GMP guidelines (sometimes due to statutory mandates), usually the Contact Parts are SS316 Or Higher Grade, and Non Contact Parts are SS304. Non - Contact can be of Mild Steel, in case of Non-GMP set up. Incase of Highly Corrosive chemicals, it is highly recommended that the Contact Parts are Hastelloy (C22 Or C276), Or Halar Coated SS304 Or PU/ PFA Coated SS304.

SURFACE FINISH: 

As per the GMP (Good Manufacturing Practice) guidelines, the product contact surface needs to be extremely smooth. This also helps in preventing the surface corrosion due to electrolytic cell formation. Hence the Product Contact Surface usually is minimum 240 grit Mirror Finish and Non Contact Surface is 180 grit matt finish. Incase of a Non-GMP set up, if Mild Steel is used (for Product Non Contact in some cases), the surface is either epoxy painted Or coated with protective layer.

MAJOR COMPONENTS OF A VTD:

The components can be listed as under:

1. VTD Chamber: The Rectangular chamber is built in such a way that it can safely handle high vacuum and high temperature. It is most of the times limpeted (for hot utility circulation) on the two lateral sides (so as to emit and transfer heat by radiation). All four sides (including the door) are insulated with hot insulation material such as Mineral Wool Or Rock Wool. The Top and Bottom of the Chamber are usually Not Insulated. Inside clits are provided for supporting the removable heating shelves inside the chamber. Number of nozzles are provided on the chamber including vapor nozzle (connected to the condenser and receiver). The backside of the chamber has a series of openings, through which the inlet and outlet nozzles of each heating shelf protrude through. These openings needs to be properly sealed with high density gasketing material so as to avoid any sort of leakage.

2. VTD Door and Hinges: VTD door is designed to withstand high vacuum as well as high temperature. In rare cases, VTD door too is limpeted for circulation of the Hot Fluid. In any case, VTD door is insulated with thermal insulating material such as Mineral Wool Or Rock Wool. 

3. Heating Shelves: Heating Shelves are nothing but a series of Hot Plates which are heated by circulation of Hot Fluid (viz Hot Water Or Low Pressure Steam) inside. The rectangular heating shelves are made up of a Top and Bottom plates, which are joined to from a hollow box with inside stiffeners and channeling rods or channels.

4. Inlet and Outlet Headers: The Vertical Cylindrical closed ended headers are usually located at the back of the VTD chamber. Incase of bigger capacity VTDs, the headers are split in two or more parts (each dedicated to a certain number Hot Plates Or Heating Shelves). 

OTHER PARTS OF THE VTD SYSTEM:

The other parts Or components of  Vacuum Tray Drying systems are almost same as any other Vacuum Drying system viz. RCVD, AVD, ANFD etc., and can be listed as under:

1. Condenser: Usually a Shell and Tube Type Heat Exchanger is used to condense the vapors coming out of the VTD chamber. Usually the process side fluid (vapors) are passed through tubes, whereas, on the Shell side the utility fluid (mostly chilled water Or cooling water) is circulated. However, it can be the other way around too (depending on the heat load calculations). Condenser can be insulated and cladded for avoiding heat loss to the ambience.

2. Receiver: A solvent recovery receiver is provided at the bottom of the STHE condenser. Receiver can be either Vertical Cylindrical with legs supports Or Horizontal Cylindrical with saddle supports. A vacuum breaking arrangement is provided with a Control/ On-Off valve on the receiver.

3. Vacuum Pump: A suitable capacity Vacuum Pump is connected to the system (mostly to the receiver), which creates vacuum inside the VTD chamber, and, also facilitates the extraction of the vapors, hence complements the condensation process. 

4. Water Heating system Or Low Pressure Steam (for heating the VTD Hot Plates and Limpet): Incase of a Hot Water System, a dedicated skid mounted vertical cylindrical vessel is provided with external heating i.e. either with steam (LPS) Or with Electrical Heaters. A circulation pump pushes the hot water into the VTD Heating Shelves and Limpet, and the return water goes back into the Hot Water Generation tank. The level guage/switch on this tank ensure water doesn't spill over, and, make up water valves gets ON when level goes down. The Temperature sensor on the Hot Waters Tank, as well as, sensor on the Inlet Header of VTD together controls the flow of Steam (through PID control valve) Or the Electrical Heater, thus ensures the inlet water temperature stays in control. Incase of direct steam heating, the PID valve ensures steam is throttled and uniform temperature is maintained in the Hot Plates and VTD limpet.

5. Nitrogen Purging system: A  nozzle can be provided with control valve, that can facilitate the intermediate and periodic gushing or N2 gas, which pushes the vapors out towards the condenser. Vacuum needs to be stopped temporarily while purging.

6. Product carrying Trays: The trays are usually rectangular open trays of size 800mm (32)" x 400mm (16)" x 25mm to 30mm (1 or 1½)" with Or without bidding.

FACTORS TO BE TAKEN INTO CONSIDERATION WHILE DESIGNING A VTD:

1. Orientation of VTD, Available Floor Space, Availability of the Utilities and Floor Layout/ Elevation: An important point to be noted here is that, VTD can be installed (a) either completely inside the Clean Room Or (b) Only Door and Skirting  Frame inside and rest of the body in the Technical Area by passing it through the cleanroom wall.

Many a times, project team overlooks the area and space available and later have to regret the decision as VTD along with its door opening radius doesn't fit in that size. Similarly, the utility equipments such as Hot Water Tank, Vacuum Pump, Chilled Or Cooling Water, Steam, Nitrogen and Electrical Panel etc if not planned favorably can cause a lot of trouble such as financial loss due to structural rearrangement work, major construction work and sometimes production time loss too. As a usual practice all these things are planned simultaneously and in sync with each other. Also, a GAD (General Arrangement Drawing) is superimposed on Autocad Layout Drawing to see the feasibility of installation during the design stage.

2. Material of Construction: As mentioned above, the right MOC will decide the adherence to the quality and process parameters, as well as, longevity of the VTD.

3. Level of Automation: Depending on the needs of the Production Department, as well as, budget allocation, the level of automation can be decided.

4. Statutory and Safety compliance: Sometimes, due to company policy and FDA mandate, special features are incorporated such as Touch Screen HMI (Human Machine Interface) with PLC (Programmable Logic Control). In some cases, there can be further sophistication by adopting special Data Security measures such as 21 CFR Chapter 11 compliance, which allows password protected individualistic logins and maintains record of each activity by each user. Overall, Safety, GMP guidelines along with respective FDA mandates has to be followed while designing an VTD.

5. Batch Size Calculations: A basic Process Calculations must be done including Heat Load Calculations. This helps in deciding the optimum batch size, as well as, the time cycle, hence monthly Or timely production throughput.

6. Product Specific Operational Challenges (based on the previous experience Or Studies): Since it is a capital purchase, the careful attention must be given to the specific needs of the Production Team, concerns and apprehensions of the Engineering and Maintenance, as well as, the Safety and Statutory department, which helps ensure the production goes smooth.

7. Mechanical and Process Design, as well as, Sizing of the Equipments: Stress analysis/ calculations must be done under varying parameters such as temperature, vacuum and pressure. Similarly, the thickness calculations must be done keeping in mind the static and dynamic loads.

HOW TO SELECT THE RIGHT VTD MODEL/ VTD VENDOR (Tips and Tricks):

Here are a few tips that will help you choose/ select the most appropriate VTD brand Or Vendor who will manufacture the VTD for you. These tip and suggestions are over and above your Vendor Qualification protocols and procedures, and purely focuses on the Product/ Machine under discussion.

1. Flatness of the Heating Shelves: Flatter and smooth the top surface of the heating shelf, better the contact between Tray and Shelf, hence, better is the Heat Transfer, and, faster the Drying.

2. Flatness of the Door Frame, Body Frame/ Uniformity of the engrooved Gasket Channel: Door Frames and Body Frames, as well as, the welded Or Engrooved channels for Gasket, should be accurately machined. If those are done through Planer machine tool, they give amazing uniformity. However, many a times, vendor choose to use grinder to achieve the accuracy, which is humanly impossible. 

3. Type and Quality of Door-Gasket: One needs to see to it that the Gasket fitted on the door is (a) of High Density and Good Quality (b) D type (c) Doesn't loose shape and size on washing/ cleaning (d) is Seamless Or Moulded and not made by joining the two ends by glue.

4. Welding Quality of the Heating Shelves: Although the operator is bound to adhere to the design pressure of the hot utility, however, there have been rare cases reported of bursting of the Heating Shelves Or Leakages in Heating Shelves causing the entire Product Batch contamination. Hence, it is very important to carefully examine the quality of the welding and buffing of the heating shelves.

5. Close to Zero or Minimum Leakage of Vacuum:  The Success of any VTD primarily depends on (a) Vacuum Holding Capacity (b) Heat Transfer rate. It is very important to ensure that air leakage doesn't take place when the VTD chamber is vacuumized.

6. Other Engineering and Design Aspects: Right from the Heat Load calculations to Nozzle Sizing to Material Selection to Workmanship, all aspects needs to be studied properly.

Please feel free to contact me regarding any questions, queries Or additional assistance needed on Design, Sales, Qualification or other aspects at   shg.techo@gmail.com