Flash Drying in the Coffee industry

Introduction:

Clair Engineer’s Private Limited (CEPL), got a call from client – a coffee supplier located in Andhra Pradesh and is one of the leading sellers oflisted products in coffee. The concern was regarding the re-commissioning of the de-watering and drying systems of the spent coffee production. The technical parameters are shared in Table 1.

Here the working principle includes a Pneumatic drying system /Flash dryer is used for drying of the spent coffee.

The dewatered spent coffee is fed to the flash dryer from the feed hopper by a feed screw to dry up the spent coffee moisture from 65% to 35% by using the hot gases.

Boiler exhaust gases shall be used for the drying purpose. The heat available from the Boiler is about 6, 00,000. Kcal/hr. (As reported by VCL). The total moisture evaporated is about 1 ton.

The hot gas entered at the bottom of the dryer below the feed point @ 180 °C. The dryer exit gas temperature will be 80 to 85°C. Then the dryer gases are vented through the bag filter for further reduction and finally after much more filtration , the gases are let out.

Data/ Table:

Table 1:The technical specifications

Observations:

• Short term: immediately modify the support system to compensate for expansion
• Long term: Install a single/twin cyclone with refractory lining or a replace with a new SS316 Multi cyclones
• Direct heating method: bleed 10-20% of the hot gas with the natural drying air and maintain the temperature about 50°C will result in moisture reduction to 30%
• Indirect heating method: An APH has to be installed and air to be pre heated about 60dgrs and use in the dryer
• Material transportation system from dryer to Yard has to be installed
• Minor issues will be solved at the site
• Bags to be installed with all accessories
• The pneumatic conveying is working as per the design and the material fed to the dryer section
• The RAL spare rotor is recommended for screw press discharge RAL
• Natural drying with dryer reducing the moisture about 8-10% from the SP discharges moisture
• There is no major problem observed in the boiler with the dried material except minor coating observed on area between fluidization streams to secondary air inlet. that is because of moisture in the feed material to the boiler
• Further increase in air temperature relatively reduction in spent coffee moisture
• The dryer material has to be fed directly to the yard and mix with husk and feed to the boiler
• The equipment is operating normally at about 650-700 °C. There could be some run away temperatures sometimes
• Inlet & outlet plenum area are damaged at welding joints and also some cracks on the plates are reported
• Weld joints on the tube sheets are also frequently failing
• Based only on temperature control, complete combustion cannot be ensured /confirmed. Possibility of low intensity explosions cannot be ruled out
• The casing in front and back of the multi cyclones is damaged/ bulged to a large extent. The sides of the casing are also bulged to a lesser extent
• The total multi cyclone unit has been supported on a structural frame and is welded to the structure
• The main reason appears to be the high temperature and the expansion problems associated with it
• Another reason for the failure is the unsuitability of SS 304 for temperatures higher than 750 °C