A rotary dryer consists of a revolving cylinder horizontal or slightly inclined toward the outlet. Wet feed enters one end of the cylinder and dry material discharges from the other. The length of the cylinder may range from 4 to more than 10 times its diameter, which may vary from less than 0.3 to more than 3 m. Heating in rotary dryers usually is provided by direct contact of the hot gas with the wet material, but indirect heating (hot gas passing through an external jacket) may also be used.
If the “Use Hot Gas” option is checked:
● A wet feed stream will be heated and dried using a hot gas stream in the main drying section of the dryer.
● You must add a “Primary Gas In” stream (connected to the procedure icon’s mid-input port) and a “Primary Gas Out” stream (connected to the procedure icon’s mid-output port). (Hover the mouse pointer over the procedure icon’s I/O ports to see their names.)
● If the “Primary Gas In” Stream is not hot, you may check the “Preheat the Inlet Gas” option to simulate the preheating of the “Primary Gas In” stream before it enters the dryer. In this case, you must select a suitable heating agent and specify the temperature of the hot gas after preheating (“Hot Gas Temperature”).
● The “Main Drying” tab will appear in the operation’s data dialog. Through this tab, you can specify the required component evaporation, solids entrainment, dried product temperature, and “Primary Gas In” stream flow specification data.
If the “Preheat the Inlet Gas” option is checked, the program will solve the following enthalpy balance to calculate the heat duty of the (implicit) preheater:
GiHG,i + Qh = GhHG,h,
where Gi and HG,i are the respective mass flowrate and specific enthalpy of the cold “Primary Gas In” stream, Qh is the heat duty of the preheating step, and Gh and HG,h are the respective mass flowrate and specific enthalpy of the hot gas stream entering the dryer after preheating.
In the above enthalpy balance, Gh is taken to be equal to Gi. In addition, the physical state (PS) toolbox of the hot gas stream leaving the preheater is assumed equal to that of the “Primary Gas In” stream. Based on this toolbox, the physical states of components in the hot gas stream are determined, and the specific enthalpy of this stream (HG,h) is calculated.
If the “Use Cold Gas” option is checked, but the “Use Hot Gas” option is not checked:
● A hot feed stream will be cooled and dried using a cold gas stream in the main drying section of the dryer.
● Similarly to the case where the “Use Hot Gas” option is checked, you must add a “Primary Gas In” stream (connected to the procedure icon’s mid-input port) and a “Primary Gas Out” stream (connected to the procedure icon’s mid-output port).
Four options are available for specifying the flow of the “Primary Gas In” stream:
● Available In Stream
● Set Relative Amount (wt Primary Gas In/wt Evaporation)
● Set Volatile Content of Primary Gas Out Stream (wt Volatiles/wt Non-Volatiles)
● Calculated Based On Primary Gas Out Temperature
If the ‘Available In Stream’ option is selected, you must set the flow of the “Primary Gas In” stream directly (through its stream data dialog).
If this option is not selected, the flow of the “Primary Gas In” stream must be calculated first before solving the enthalpy balance in the preheater. More specifically:
If the ‘Set Relative Amount’ option is selected, you must specify the relative amount of the “Primary Gas In” stream, expressed as the ratio of the total mass flow rate of the “Primary Gas In” stream to the total mass flow rate of the evaporating volatile components in the main drying section.The program will calculate the total mass flow rate of the evaporating volatile components in the main drying section based on their evaporation percentages, and then it will multiply it by the specified relative amount to calculate the flow of the “Primary Gas In” stream.
If the ‘Set Volatile Content of Primary Gas Out Stream’ option is selected, you must specify the volatile content of the “Primary Gas Out” stream, defined as the ratio of the total mas flow rate of volatiles to the total mass flow rate of non-volatiles in that stream. Note that if the non-volatile content of the “Primary Gas Out” stream corresponds to dry air and the volatile content of the inlet gas corresponds to water, the value of the “Volatile Content of Primary Gas Out Stream” will correspond to the humidity of the air at the outlet. To calculate the flow of the “Primary Gas In” stream in this case, the program solves the material balances for evaporation, reactions, and solids entrainment in the main drying section (described below).
After determining the flow and temperature of the hot gas entering the dryer, the material balances for the evaporation can be performed. The physical states of the feed stream components are first determined based on the procedure’s PS toolbox, and the feed stream is split into a liquid/solid phase and a gas phase. The gas phase (if any) is directed to the “Primary Gas Out” port, where it is mixed with the contents of the hot gas stream. The remaining liquid/solid phase is used in the evaporation calculations.
Note that any gas phase present in the feed stream is ignored in evaporation calculations. For example, let’s assume that the feed stream contains 100 kg of water, 20% of which (20 kg) is in the vapor phase, and that the specified evaporation percentage of water is 40%. The program will split the feed stream into a liquid/solid phase containing 80 kg of liquid water and a gaseous phase containing 20 kg of water vapor. The water vapor will be sent to the “Primary Gas Out” stream, and the liquid/solid phase will be used in evaporation calculations. Since 40% of liquid/solid water is evaporated, the program will evaporate 32 kg of water. This amount will also be sent to the “Primary Gas Out” stream. Therefore, after evaporation the “Primary Gas Out Stream” will contain 52 kg of water (52% of total water) and the remaining liquid/solid stream will contain 48 kg (48% of total water).
The evaporation data for the main drying section is specified through the “Main Drying” tab. The material balances for the evaporation are as follows: For each pure component that is set as volatile, a percentage of its corresponding liquid/solid flow in the feed stream, equal to the specified evaporation percentage for this component, is evaporated. Therefore, it is removed from the feed stream and added to the “Primary Gas Out” stream as vapor.
If the “Perform Reaction Calculations in Liquid/Solid Phase” option is checked, the “Reactions” tab will appear in the operation’s data dialog. This tab allows you to specify one or more reactions that will be performed on the dried product.
The material balances for the specified reactions will be performed on the dried product of the main drying section. Note that only liquid/solid phase reactions are supported (i.e., only components contained in the dried feed are available to react, and since the dried feed does not contain any gaseous components, the specified reactions must not include any gaseous reactants). However, reaction products can be gaseous. After the reaction material balances are done, the gaseous reaction products will be directed to the “Primary Gas Out” stream, and the dried product will consist of the liquid/solid reaction products only.
The specified reactions are assumed to take place isothermally at the specified dried “Product Temperature” for the main drying section. To satisfy that temperature, we may need to supply additional heat to the dried feed or remove excess heat from it. To deal with this, this operation assumes that the “Primary Gas In” stream will be used to supply additional heat to the reacting mixure or “absorb” the excess heat that may be generated by one or more reactions. Hence, the external heat source term is assumed zero in the overall enthalpy balance that is solved for the entire heating section (see below).
For additional information regarding the material balance calculations in the case of reactions, see Stoichiometric Reaction Operations: Modeling Calculations.
To account for solids entrainment after the evaporation and reaction calculations are done in the main drying section, a percentage of the dried product, equal to the specified entrainment percentage through the “Main Drying” tab, is removed from the dried product and added to “Primary Gas Out” stream.
After performing evaporation, reaction and solids entrainment calculations, the “Primary Gas Out” stream will consist of the contents of the “Primary Gas In” stream, the gaseous phase of the feed stream (if any), the evaporated volatiles, the gaseous reaction products (if any), and the entrained solids (if any). The final product of the main drying section will consist of the liquid/solid phase of the feed stream after evaporation, reaction, and solids entrainment calculations.
The enthalpy balance for the main drying section can be expressed as:
(Fh,iHF,h,i + Gh,iHG,h,i)(1-floss) + Qrxn = Fh,oHF,h,o+Gh,oHG,h,o,
where
● Fh,i and HF,h,i are the mass flowrate and specific enthalpy, respectively, of the feed stream entering the main drying section,
● Gh,i and HG,h,i are the mass flowrate and specific enthalpy, respectively, of the hot gas stream entering the main drying section,
● floss is the percentage of radiation losses,
● Qrxn is the total enthalpy of all reactions,
● Fh,o and HF,h,o are the mass flow rate and specific enthalpy, respectively, of the product stream leaving the main drying section, and
● Gh,o and HG,h,o are the mass flowrate and specific enthalpy, respectively, of the “Primary Gas Out” stream.
If the preheating option is not enabled, the hot gas stream entering the main drying section is the “Primary Gas In” stream. If the preheating option is enabled, the hot gas stream entering the main drying section is the “Primary Gas In” stream after it is preheated to the specified temperature.
If the flow of the “Primary Gas In” stream is not calculated based on the specified “Primary Gas Out” stream temperature, the above enthalpy balance is solved for the specific enthalpy (and temperature) of the “Primary Gas Out” stream. If it is calculated based on the specified temperature of the “Primary Gas Out” stream, the enthalpy balance and the material balances of evaporation, reactions and solids entrainment, are solved iteratively to determine the flow of the “Primary Gas In” stream.
Note that the physical state toolbox of the “Primary Gas In” stream is also used for the “Primary Gas Out” stream to determine the physical states of its components and to calculate its specific enthalpy.
If the “Use Cold Gas” option is checked in addition to the “Use Hot Gas” option:
● The dryer is assumed to have two sections, a main drying section and a post-drying cooling section.
● The dried product from the main drying section will enter the dryer’s post-drying cooling section, where it will be cooled and dried (optional) using a secondary (cold) gas stream.
● You must add a “Secondary Gas In” stream (connected to the procedure icon’s bottom-input port) and a “Secondary Gas Out” stream (connected to the procedure icon’s top-output port).
● The “Post-Drying Cooling” tab will be displayed in the operation’s data dialog. Similarly to the “Main Drying” tab, through this tab you can specify the required specification data for this section, which include component evaporation, solids entrainment, dried product temperature, and “Secondary Gas In” stream flow specification data.
There are four options available for specifying the flow of the “Secondary Gas In” stream, similar to those for the “Primary Gas In” stream:
● Available In Stream
● Set Relative Amount (wt Secondary Gas In/wt Evaporation)
● Set Volatile Content of Secondary Gas Out Stream (wt Volatiles/wt Non-Volatiles)
● Calculated Based On Secondary Gas Out Temperature
If the ‘Available In Stream’ option is selected, you must set the flow of the “Secondary Gas In” stream directly (through its stream data dialog). If this option is not selected, the flow of the “Secondary Gas In” stream must be calculated first before solving the previous enthalpy balance in case of preheating. In more detail:
If the ‘Set Relative Amount’ option is selected, you must specify the relative amount of the “Secondary Gas In” stream, expressed as the ratio of the total mass flow rate of the “Secondary Gas In” stream to the total mass flow rate of the evaporating volatile components in the post-drying cooling section. The program calculates the total mass flow rate of the evaporating volatile components (based on the specified component evaporation percentages in the post-drying cooling section) and multiplies it by the specified relative amount to determine the flow of the “Secondary Gas In” stream.
If the ‘Set Volatile Content of Secondary Gas Out Stream’ option is selected, you must specify the volatile content of the “Secondary Gas Out” stream, defined as the ratio of the total mass flow rate of volatiles to the total mass flow rate of non-volatiles in that stream. Note that for dry air as the non-volatile content and water as the volatile content, this corresponds to the outlet air’s humidity. To calculate the flow of the “Secondary Gas In” stream, the program solves the material balances for evaporation and solids entrainment in the post-drying cooling section (described below).
If the ‘Calculated Based On Secondary Gas Out Temperature’ option is selected, you must specify the temperature of the “Secondary Gas Out” stream. To calculate the flow of the “Secondary Gas In” stream, the program solves the material balances for evaporation and solids entrainment in the post-drying cooling together with the overall energy balance for the post-drying cooling section (described below).
After determining the flow and temperature of the cold gas entering the post-drying cooling section, the material balances for evaporation in the post-drying cooling section can be done. The feed stream entering the post-drying cooling section is the dried product stream from the main drying section. Evaporation data is specified in the “Post-Drying Cooling” tab. Similarly to the evaporation calculations in the main drying section, the material balances for the evaporation are as follows: For each pure component that is set as volatile, a percentage of its corresponding liquid/solid flow in the section’s feed stream, equal to the specified evaporation percentage for this component, is evaporated. Therefore, it is removed from the section’s feed stream and added to the “Secondary Gas Out” stream as vapor.
A pure component of the liquid/solid phase of the section’s feed stream is considered volatile if the corresponding “Volatile?” option is checked through the “Post-Drying Cooling” tab. The evaporation percentages of the volatile components can be either specified by the user or calculated based on the specification of the ‘LOD After Evaporation’. The “LOD Before Evaporation” is defined as the fraction of volatiles in the section’s wet feed stream, whereas the “LOD After Evaporation” is defined as the fraction of volatiles in the section’s dried product stream. If the evaporation percentages of volatile components are set by the user, the total volatile mass flow rate of the section’s dried product stream is calculated by summing up the individual liquid/solid mass flow rates of all pure components that are set as volatile. Then, the “LOD After Evaporation” can be calculated by dividing the total volatiles mass flow rate of the section’s dried product stream by the total liquid/solid mass flow rate of that stream. If the “LOD After Evaporation” is specified, the program assumes that the evaporation percentage is the same for all volatile components. Then, the evaporation fraction can be calculated as 1 - (1 - “LOD Before Evaporation”) * “LOD After Evaporation”) / “LOD Before Evaporation” / (1 - “LOD After Evaporation”).
To account for solids entrainment after the evaporation calculations are done in the post-drying cooling section, a percentage of the section’s cooled (and dried) product, equal to the specified entrainment percentage through the “Post-Drying Cooling” tab, is removed from the dried product and added to “Secondary Gas Out” stream.
After evaporation and solids entrainment calculations in the post-drying cooling section, the “Secondary Gas Out” stream includes the contents of the “Secondary Gas In” stream, the evaporated volatiles (if any), and the entrained solids (if any). The final product of the post-drying cooling section is the remaining product from the main drying section, minus the evaporated volatiles and entrained solids.
The enthalpy balance for the post-drying cooling section is expressed as:
Fc,iHF,c,i + Gc,iHG,c,i + Qrxn = Fc,oHF,c,o+Gc,oHG,c,o,
where
● Fc,i and HF,c,i are the mass flowrate and specific enthalpy, respectively, of the feed entering the post-drying cooling section (dried product from the main drying section),
● Gc,i and HG,c,i are the mass flowrate and specific enthalpy, respectively, of the cooling gas (“Secondary Gas In”) stream entering the post-drying cooling section,
● Fc,o and HF,c,o are the mass flow rate and specific enthalpy, respectively, of the cold product stream leaving the post-drying cooling section, and
● Gc,o and HG,c,o are the mass flowrate and specific enthalpy, respectively of the cooling gas (“Secondary Gas Out”) stream leaving the post-drying cooling section.
If the flow of the “Secondary Gas In” stream is not calculated based on the temperature of the “Secondary Gas Out” stream, the above enthalpy balance is solved for the specific enthalpy (and temperature) of the “Secondary Gas Out” stream. If it is calculated based on the specified temperature of the “Secondary Gas Out” stream, the enthalpy balance and material balances for evaporation and solids entrainment, are solved iteratively to determine the flow of the “Secondary Gas In” stream.
Note that the physical state toolbox of the “Secondary Gas In” stream is also used to determine the component physical states and specific enthalpy of the “Secondary Gas Out” stream.
As a rough check of validity of the specified evaporation data, the program calculates the dew point of the outlet gas stream from each section (“Primary Gas Out” stream in the main drying section; “Secondary Gas Out” stream in the post-drying cooling section), by flashing the outlet gas stream based on the assumption that Raoult's law is applicable to all volatile components contained in it. Since the outlet gas stream is supposed to be entirely in the gaseous phase, its dew point is expected to be lower than its temperature. If the calculated dew point for this stream based on Raoult’s law is higher than or equal to its temperature, a warning is displayed to indicate that the specified evaporation may not be feasible. Possible reasons are that the specified evaporation percentages of one or more components may be too high, or the specified inlet gas stream flow may be too low (if the option to specify the flow of the inlet gas stream is selected), or the specified outlet gas stream temperature may be too low (if the option to specify the outlet gas stream temperature is selected).
By default, the operating pressure in the dryer is assumed equal to the pressure of the feed stream. Optionally, the user may set his/her own value for the operating pressure. The pressure of all outlet streams is set equal to the operating pressure.
You may choose among three options for specifying the power requirement for this operation: you can specify the specific power (defined as the ratio of total power to total dryer area), or the total power (per cycle, if the procedure operating mode is set to batch), or the power per equipment unit (and per cycle, if the procedure operating mode is set to batch).
In Design Mode, the user may specify either the specific evaporation rate or the specific feed rate through the operation’s data dialog. Additionally, the user specifies the maximum drum diameter and the length-to-diameter ratio of the drum through the equipment data dialog.
The specific evaporation rate is defined as the evaporation rate per unit drum volume. If this option is chosen, the program calculates the required drum volume by dividing the total mass flow rate of evaporated components by the specified specific evaporation rate.
The specific feed rate is defined as the feed mass flow rate per unit drum volume. If this option is selected, the program calculates the required drum volume by dividing the feed stream’s mass flow rate by the specified specific feed rate.
Once the drum volume is calculated, the program calculates the drum’s length, diameter and area based on the specified length-to-diameter ratio. If the calculated drum diameter exceeds the maximum, the program assumes the use of multiple identical units of smaller diameter operating in parallel. It calculates the number of units and their diameter such that the total volume of all units matches the required drum volume for the operation. Additionally, the drying capacity of the equipment is calculated by dividing the total mass flow rate of evaporated volatiles by the number of units.
In Rating Mode, the user provides the number of units, drum length, drum diameter, and drum capacity. The program then calculates the drum area and volume. By default, it also calculates the specific evaporation rate and specific feed rate. However, if the process and procedure operating modes are set to batch, the user has the option to either specify the drying time (allowing the program to calculate the specific evaporation and feed rates) or specify one of the specific rates (allowing the program to calculate the drying time). If the total available drying capacity exceeds the required drying capacity for the operation, the program displays a warning.
See Vacuum Pump Auxiliary Equipment Calculations.
1. Perry R.H. and D.W. Green (1984). Perry’s Chemical Engineers’ Handbook, 6th ed. McGraw-Hill, section 20 pp. 29-33.
2. McCabe W. L., J. C. Smith, and P. Harriott. (1993). Unit Operations of Chemical Engineering, McGraw-Hill, 5th ed., pp. 795-798.
3. Coulson J. M. and J. F. Richardson, (1978). Chemical Engineering, Vol. 2, Pergamon Press, 3rd ed., pp. 727-733.
The interface of this operation has the following tabs:
● Oper. Cond’s, see Rotary Drying: Oper. Conds Tab
● Heating, see Rotary Drying: Main Drying Tab
● Cooling, see Rotary Drying: Main Drying Tab
● Reactions, see Stoichiometric Reaction/Fermentation Operation: Reactions Tab
● Labor, etc, see Operations Dialog: Labor etc. Tab
● Description, see Operations Dialog: Description Tab
● Batch Sheet, see Operations Dialog: Batch Sheet Tab
● Scheduling, see Operations Dialog: Scheduling Tab