I would like to understand the formula or ways to calculate the drying time and other variable parameters required to dry a food product using hot air. Known data are - initial product moisture, weight and expected final moisture, drying air temperature, air velocity. Drying time needs to be calculated. Can some one help ?
I am not sure of your question, but I will use an example to try to answer you. lets say you have the following conditions below.
A material having a constant drying rate of 0.15 kg H2O/min (kg dry matter), and its critical moisture content is 1.10 kg H2O/kg dry matter. How long will it take to dry this material from an initial moisture content of 75% (wet basis) to a final moisture content of 8% (wet basis)?
Solution:
Convert the final and initial MC from wet to dry bases:
X0 = 3:0 kg water/kg dry matter
X = 0:0869 kg water/kg dry matter
Xc = 1:10 kg water/kg dry matter
Rc = 0:15 kg water/min (kg dry matter)
use the following equation
t = ((X0+Xc)/ Rc) + (Xc/Rc)*ln(Xc/X) the answer will be 31.3 min
NB: the constant drying rate (which is affected by air velocity) can be determined from the lab and complex problems could be solved using psychrometric charts.
I hope this will take you somewhere...
Hi!
There are many aspects involving drying. Drying time will depend on the material, equipment being used and also the process' parameters. Spray drying will bring a quite different time from fluid bed, for instance. According to the drying process you'll have a different drying profile, then different drying times. Other point, some materials are better dried in specific drying methods, something that also impacts on the drying time.
Depending on the drying method you can estimate the drying time, using specific equations for that. Normally you can see a mix of theory and practice. You estimate the time and check/follow in the field. If it is a commercial dryer, talk to the supplier, it will have almost all needed information. With all paramaters presented by you, the drying time will be a function of inlet flow rate, that will vary from one drying process to other.
If you can, it is possible to use NIR for following the drying in line and determine when the drying achieved the desired point. If not, you have to take samples and identify the right time. If you are not achieving the right moisture, look at the process parameters. A DCCR could be used in order to optimize your process.
You have some references bellow and attached.
http://www.dryingsystems.net/Calculations.html, http://www.fao.org/docrep/X5036E/x5036E0x.htm,
Hoping it helped.
Regards.
Thank you Osmar Almeida
The articles you have added are very good and better than other text books.
The Osmar Almeida's answer is excellent and complete. But I'd like to add that the experimental data are imperative. There are too many variables to the final result be precisely predicted. The time depends not only on microscopic characteristcs of the adhesion of water molecules to the object to be dried but also on macroscopic aspects like the shape of the object and efficiency of wind stream. One will always need to adjust the systems in a real run.
This will depend on drying temperature, air flow velocity humidity etc.
There are many empirical equations of drying kinetics and if one those match with your situation; it is very easy to calculate the drying time.
Otherwise, you can find it from theoretical models which consider many physical parameters. However, you need to chose the right one from single phase to multiphase; single scale to multiscale.
The contributions of Osmar Almeida and some others are very informative. if i must understand quite well the intention behind this question, i think you want to develop mathematical expression to Predict the drying time without conducting the experiment using the combination of parameters listed in your question.
it is possible , however, there must first be experiment to generate set of data you will need to work on to model such situation. such that subsequently, without measuring the drying time manually, you should be able to predict the drying time if the values of parameters used in the developed models are known.
http://www.sciencedirect.com/science/article/pii/S1877705813000520
Those interested at this discussion may possibly want to check this somewhat related thread: https://www.researchgate.net/post/How_do_I_calculate_cumulative_release_of_volatile_compound
Following publications include detailed discussion on food drying:
- Drying Technologies in Food Processing
- Handbook of Milk Powder Manufacture
- Badges
- Science topic