ChE 473 Process Drying

Dryer Control In order to control any process, we need a good understanding of the process itself What is the drying process? Dryer classifications and types Process analysis Macro vs. Nano, Micro

Dryers – A common yet costly unit operation Dryers used in chemical processing, food processing and pharma Batch or continuous Energy intensive Frequently over dried at added costs, dusting, product loss Drying accounts for ~12% manuf. costs

A common household example … Clothes dryer appliance

What is the Drying Process … Removal of small amount of liquid, usually water – Large amounts of water normally removed by press or centrifuges. Thermal methods employed. Heat and Mass transfer

Solid drying process is very complex with micro and nano mechanisms Liquid movement due to capillary forces Diffusion due to concentration gradients Liquid vapor flow due to pressure differences Vapor diffusion due to vapor pressure differences, concentration differences Osmotic pressure created by colloidal bodies has soluble and insoluble fractions Vapor Effusion – A relationship of vapor flow to pore diameter Thermodiffusion Vaporization-condensation mechanism

Macro Drying Process This program will not study these nano and micro relationships; we will develop our controls based on the macro mechanisms

What is the Drying Process … Drying - water liquid vaporization; not as efficient as centrifuge, 1050 BTU/lb of water removed. Final moisture varies “dried” table salt contains 0.5 % water, dried coal 4%. Solids can have many different forms, flakes, granules, crystals, powders, etc. The liquid can be on the surface, within the surface in cellular structures, such as wood. Consider the method of handling, dusting, rough or gentle treatment.

Equilibrium Moisture The solid’s moisture content is a function of the humidity of the drying air. The moisture cannot be lower than the equilibrium moisture content corresponding the humidity of the incoming air. 50% RH air equilibrium moisture Wool 12.5 %Newspaper 5.5%

How is the moisture reported? Moisture content can be expressed as: wet / (wet + dry) wet / dry

The Drying Process can be described in several ways… Batch or Continuous; how the material is processed. A single charge – Batch Continuous input and output.

The Drying equipment can be described as “dryer types” Dryer Types; the classification as to the method solids travel through the heated zone, the heat source and transfer method.

The Drying Process can be classified as: Classifications Adiabatic Dryers are the type where the solids are dried by direct contact with gases, usually forced air. With these dryers, moisture is on the surface of the solid. Non-Adiabatic Dryers When a dryer does not use heated air or other gasses to provide the energy required the drying process is considered a non- adiabatic.

In the case of Adiabatic Dryers The process can be considered to be two related processes: Solids Drying Air Humidification We will view dryer control from the air humidification process

Adiabatic dryers, solids are exposed to the heated gasses in various methods : Blown across the surface cross circulation Blown through a bed of solids, through- circulation; solids stationary; wood, corn etc Dropped slowly through a slow moving gas stream, rotary dryer Blown through a bed of solids that fluidize the particles; solids moving; frequently called fluidized bed dryer Solids enter a high velocity hot gas stream and conveyed pneumatically to a collector Flash Dryer

What can the Psychometric Properties tell us about the drying process? In many ( or most ) cases, the nano and macro drying mechanisms are not know. However, we do know air properties Lets make use of the air properties to control our dryer

Psychometric chart - displays phase conditions of water vapour in air

The Psychometric chart computer program Akton Associates Post Office Box 2076 Edmond, Oklahoma

Properties shown on psychometric chart … The air temperature - dry bulb temperature of the stable air water vapour mixture; on the x axis The dew point temperature - temperature where condensation begins to form as the water is condensed from the wet air; not shown on the chart The wet bulb temperature is the temperature at which adiabatic heat is transferred during the drying of solid or humidification of air. For a dryer, moisture in the solid is transferred to the air. The air will gain moisture while the solid looses moisture, therefore or humidification of the air occurs. This process will occur at a constant wet bulb temperature. The dry bulb air temperature will decrease during this process and be lower exiting the dryer or chamber. This temperature is shown as a series of curved lines sloping downward.

Properties shown on psychometric chart … Relative humidity is the ratio of the water vapour pressure at the dew point to the water vapour pressure at the dry bulb temperature. This ratio is usually expressed as a percent. This ratio is multiplied by 100 to obtain the percentage reading. These lines are the curved lines sloping upward. Vertical line on the right shows the absolute moisture; pounds of moisture per pound of dry air.

Relative Humidity The relative humidity is calculated as a ratio of partial pressures: is the water vapor pressure at the dew point temperature is the water vapor pressure at the dry bulb temperature.

Relative Humidity The water vapor pressure can be calculated by an exponential equation: p in psia and T in DegF

Drying is in one of two zones or periods… Constant rate and Falling rate zones

Constant Rate Zone a.k.a. first period of drying Layer of saturated air on solid surface This rate is determined by the capacity and properties of the inlet gas or vapor Solid temperature is equal to the wet bulb temperature during this period Free water drying

Falling Rate Zone a.k.a. second period of drying inflection point at the “critical moisture” begins when the surface or free water is removed solid temperature increases form wet bulb temp to that approaching the inlet air, gas, temperature

Batch Drying If air is passed over a moist solid, air temperature will be reduced as the water is evaporated. Calculated through an enthalpy balance: T i = Inlet Dry Bulb Temperature T o = Outlet Dry Bulb Temperature G = Air Mass Flow C = Air Heat Capacity F w = Mass rate of water evaporation H v = Heat of vaporization

Batch Drying The outlet temperature value will be between the inlet and the wet bulb temperature. The rate of evaporation dF w is equal to: T i Inlet Dry Bulb Temperature T w Wet Bulb Temperature a Mass transfer coefficient R Rate coefficient dA Surface Area

Control of the drying process Drying is considered a self regulating process A change is heat input will, after time, result in a change in product moisture, assuming all other conditions are constant

Drying Rate Control To control the drying rate, you control the temperature differences. Ti = Inlet Dry Bulb Temperature To = Outlet Dry Bulb Temperature G = Air Mass Flow C = Air Heat Capacity Fw = Mass rate of water evaporation Hv = Heat of vaporization

Why should we control the drying rate? Some products sensitive to excessive heat - examples: lumber, drying too fast causes the wood to crack Pharmaceuticals

Drying Rate Control But the outlet temperature lags the inlet by some amount This lag is due to the thermal time constant of the solid We need to compensate for this time difference for proper control In our experiment we will measure this lag time as well as calculate it, knowing the properties of the material being dried.

Drying Rate Control We want the temperature difference to be the difference between the inlet and the outlet temperatures, but the inlet temperature must be lagged before the difference is taken. We must reference the inlet temperature at a previous time that caused the current outlet temperature.

Drying Rate Control First order lag must be applied to the inlet temperature before the difference is calculated. The reset setting in the temperature difference controller is set to the same time as the first order inlet temperature lag