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 “Heat the Feed Using Heating Gas” option is checked:
● The wet feed stream will be heated and dried using a hot gas stream in the heating section of the dryer.
● You must add a “Heating Gas In” stream and a “Heating Gas Out” stream (hover the mouse pointer over the procedure icon’s i/o ports to find the “Heating Gas In” and “Heating Gas Out” ports).
● If the “Heating Gas In” Stream is not hot, you may check the option “Use Heating Agent to Preheat the Gas” to simulate preheating of the “Heating Gas In” stream before that stream enters the heating section of the dryer. In that case, you must select a suitable heating agent and you must also specify the temperature of the hot gas stream entering the dryer after preheating (“Hot Gas Temp.”).
● The “Heating” tab will also appear on the operation’s data dialog. Through this tab, you may specify the “Heating Gas In” stream’s flow requirement as well as the component evaporation data, solids entrainment data and product temperature after heating.
If the “Use Heating Agent to Preheat the Gas” option is checked, the following enthalpy balance is solved to calculate the heat duty of the (implicit) preheater: GiHG,i + Qh = GhHG,h, where Gi and HG,i are the mass flowrate and specific enthalpy of the “Heating Gas In” stream, Qh is the heat duty of the preheating step, and Gh and HG,h are the mass flowrate and specific enthalpy of the hot gas stream.
Four options are available for specifying the heating gas requirement:
● Available In Stream
● Set Relative Amount (wt Heating Gas In/wt Evaporation)
● Set Volatile Content of Heating Gas Out (wt Volatiles/wt Non-Volatiles)
● Calculated Based On Heating Gas Out Temperature
If the ‘Available In Stream’ option is selected, you must set the flow of the inlet drying gas stream directly, through its stream data dialog.
If the heating gas requirement is not ‘Available In Stream’, then the flow of the “Heating Gas In” stream is unknown and it must be calculated first before solving the above enthalpy balance for preheating. In more detail:
If the ‘Set Relative Amount’ option is selected, you must specify the relative amount of the “Heating Gas In” stream, which is expressed as the ratio of the total mass flow rate of the “Heating Gas In” stream to the total mass flow rate of the evaporating volatile components in the heating section. In this case, the program first calculates the total mass flow rate of the evaporating volatile components in the heating section (from the specified component evaporation percentages in that section), and then it multiplies it by the specified relative amount to calculate the flow of the “Heating Gas In” stream.
If the ‘Set Volatile Content of Heating Gas Out’ option is selected, then you must specify the volatile content of the “Heating Gas Out” stream, which is 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 inlet gas is dry air and the volatile content is water, then the value of the “Volatile Content of Heating Gas Out Stream” corresponds to the humidity of the air at the outlet. To calculate the flow of the “Heating Gas In” stream in this case, the program must solve first the material balances in the heating section, i.e. the material balances for the evaporation that takes place, and after that the material balances for the reactions that may take place, and finally, the material balances for the solids entrainment that may take place. These will be described in detail later (see below).
After the flow and temperature of the hot gas entering the dryer are determined, the material balances for the evaporation can be done. But first, the physical state of feed stream components (i.e., the component vapor fractions) must be determined based on the procedure’s physical state (PS) toolbox. Based on this toolbox, the feed stream is split into a liquid/solid phase and a gas phase. The gas phase will be directed to the “Heating Gas Out” port together with the hot gas stream, and the liquid/solid phase will be used for the evaporation calculations.
As described above, the evaporation calculations are based on the liquid/solid portion of the feed stream only (as this is determined based on the procedure’s PS toolbox). Consequently, any gaseous phase present in the feed stream will be ignored in the evaporation calculations (it will simply be sent to the “Heating Gas Out” stream). For example, consider the case that the feed stream contains 100 kg water and 20% of water (20 kg) is in the vapor phase and the specified evaporation of water is 40%. In this case, the program will transfer the entire water vapor (20 kg) to the “Heating Gas Out” stream right away, and it will also evaporate another 40% of the remaining 80 kg of water liquid. So, after evaporation, 52 kg of water (52% of total water) will be in the vapor phase and 28 kg (28% of total water) will be in the liquid phase.
The material balances for the evaporation are as follows: for each pure component that is set as volatile, a percentage of the corresponding liquid/solid flow in the feed stream equal to the specified evaporation percentage is evaporated. That is, it is removed from the feed stream and added to the “Heating Gas Out” stream as gas.
The evaporation percentages are either specified by the user or calculated based on the specified ‘LOD After Evaporation’ value, which is the amount of volatile components in a dried material sample measured using a Loss On Drying (LOD) test (which is performed after evaporation). Similarly, the “LOD Before Evaporation” is the amount of volatiles in a wet sample measured using a LOD test (which is performed before evaporation). A stream’s LOD can be calculated by dividing the stream’s total volatiles mass flow rate by the stream’s total liquid/solid mass flow rate. If a pure component appears in the liquid/solid phase of the feed stream, then that component is considered either volatile or non-volatile depending on whether the corresponding “Volatile?” option is checked or not. The “LOD Before Evaporation” (i.e., the LOD of the wet feed stream) is first determined by dividing the total volatiles mass flow rate of the wet feed stream by the total liquid/solid mass flow rate of that stream. Then, the total non-volatiles mass flow rate of the wet feed stream (which is the same as that of dried product stream) is calculated as 1- “LOD Before Evaporation”. If the evaporation percentages of volatile components are set by the user, the total volatiles mass flow rate of the 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 dried product stream by the total liquid/solid mass flow rate of that stream. If the “LOD After Evaporation” is specified, then the program assumes that the evaporation percentage of all volatile components is the same. Then, it can be calculated as 100 (“LOD Before Evaporation” - ”LOD After Evaporation”) / “LOD Before Evaporation” / (1 - “LOD After Evaporation”).
After evaporation, if the “Perform Liquid Phase Reaction Calculations” is checked, the dried feed will be used to do the reaction mass balances for the specified reactions. Note that only liquid 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 “Heating Gas Out” stream and the dried feed will consist of the liquid/solid reaction products.
The specified reactions are assumed to take place isothermally at the specified “Hot Product Temperature”. 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 “Heating Gas In” stream will be used to supply additional heat to the reacting mixure or “absorb” the excess heat generated by the reactions. Then, 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, a percentage of the dried feed equal to the specified solids entrainment percentage is removed from the dried feed and added to “Heating Gas Out” stream.
Overall, after doing the evaporation, reaction and solids entrainment calculations, the “Heating Gas Out” stream will include the contents of the “Heating Gas In” stream as well as the gaseous phase of the feed stream (if any) (calculated based on the procedure’s PS toolbox), the evaporated volatiles, the gaseous reaction products (if any), and the entrained solids (if any). The final product of the heating section will be the liquid/solid phase of the feed stream after evaporation, reaction, and solids entrainment calculations.
The overall enthalpy balance around the heating section of the dryer can be written as follows: (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 inlet feed stream in the heating section, Gh,i and HG,h,i are the mass flowrate and specific enthalpy, respectively, of the heating gas stream entering the heating section, floss is the percent radiation losses, Qrxn is the total enthalpy of all reactions, Fh,o and HF,h,o are the mass flow rate and specific enthalpy of the product stream leaving the heating section, and Gh,o and HG,h,o are the mass flowrate and specific enthalpy of the “Heating Gas Out” stream. Note that depending on whether the preheating option is checked or not, the heating gas stream entering the heating section is either the heating gas stream after preheating or the “Heating Gas In” stream.
If the flow of the “Heating Gas In” stream is not calculated based on the specified temperature of the “Heating Gas Out” stream, the above enthalpy balance is solved for the specific enthalpy (and temperature) of the “Heating Gas Out” stream. If the flow of the “Heating Gas In” stream is calculated based on the specified temperature of the “Heating Gas Out” stream, the above enthalpy balance and material balances of evaporation, reactions and solids entrainment, are solved iteratively to calculate the flow of the “Heating Gas In” stream.
If the “Cool Down the Hot Feed Using Cooling Gas” option is checked:
● Either the feed stream (if the “Heat the Feed Using Heating Gas” option is not checked) or the hot dried feed leaving the heating section of the dryer (if the “Heat the Feed Using Heating Gas” option is checked) will be cooled down and dried (optional) using a cold gas stream in the dryer’s cooling section.
● You must add a “Cooling Gas In” stream and a “Cooling Gas Out” stream (hover the mouse pointer over the procedure icon’s i/o ports to find the “Cooling Gas In” and “Cooling Gas Out” ports).
● The “Cooling” tab will appear on the operation’s data dialog. Similarly to the “Heating” tab, through this tab you may specify the “Cooling Gas In” stream’s flow requirement as well as the component evaporation data, solids entrainment data and the product temperature after cooling.
For a description of the cooling gas requirement options, evaporation calculations and solids entrainment calculations in the cooling section, please see the respective descriptions of the heating gas requirement options, evaporation calculations and solids entrainment calculations for the heating section (just think of cooling and cooling gas whenever heating and heating gas is mentioned.
Overall, after doing the evaporation and solids entrainment calculations, the “Cooling Gas Out” stream will include the contents of the “Cooling Gas In” stream as well as the gaseous phase (if any) of the feed stream entering the cooling section, the evaporated volatiles (if any), and the entrained solids (if any). The final product of the cooling section will be the remaining entering stream after removing the evaporated volatiles (if any) and entrained solids (if any), which is cooled down to the specified temperature. Note that depending on whether the “Heat the Feed Using Heating Gas” option is checked or not, the feed stream entering the cooling section will be either the dried feed leaving the heating section or the procedure’s feed stream. In either case, the procedure’s PS toolbox will be used to determine the stream’s gaseous phase.
The overall enthalpy balance around the cooling section of the dryer can be written as follows: 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 inlet feed stream in the cooling section, Gc,i and HG,c,i are the mass flowrate and specific enthalpy, respectively, of the cooling gas stream entering the cooling section, Fc,o and HF,c,o are the mass flow rate and specific enthalpy of the product stream leaving the cooling section, and Gc,o and HG,c,o are the mass flowrate and specific enthalpy of the “Cooling Gas Out” stream.
If the flow of the “Cooling Gas In” stream is not calculated based on the specified temperature of the “Cooling Gas Out” stream, the above enthalpy balance is solved for the specific enthalpy (and temperature) of the “Cooling Gas Out” stream. If the flow of the “Cooling Gas In” stream is calculated based on the specified temperature of the “Cooling Gas Out” stream, the above enthalpy balance and material balances of evaporation and solids entrainment, are solved iteratively to calculate the flow of the “Cooling Gas In” stream.
Note that the physical state (PS) toolbox of the “Cooling Gas In” stream is also used for the “Cooling Gas Out” stream (to determine the physical states of its components and to calculate its specific enthalpy).
As a rough check of validity of the specified evaporation data, the program calculates the dew point of the outlet gas stream (“Outlet Gas Dew Point”) 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 the specified temperature for this stream, 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 amount of drying gas may be too low, or the outlet drying gas temperature may be too low.
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 equipment 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 (expressed as evaporation rate per unit volume of drum) or the specific feed rate (expressed as feed mass flow rate per unit volume of drum). If the specific evaporation rate is specified, the program will calculate the drum volume by dividing the total mass flow rate of evaporated components by the specified specific evaporation rate. If the specific feed rate is specified, the program will calculate the drum volume by dividing the mass flow rate of the feed stream by the specified specific feed rate. In addition, the user specifies the length/diameter ratio of the drum and the program calculates the drum length, diameter and area. If the calculated drum diameter exceeds the specified maximum drum diameter, the program will assume multiple identical units of smaller diameter operating in parallel and it will calculate the required number of units and diameter so that the total volume of all units is the same as the required volume for this operation. In addition to the above, the program will also calculate the drying capacity of the equipment by dividing the total mass flow rate of evaporated volatiles by the number of units.
In Rating Mode, the user specifies the number of units as well as the drum length and drum diameter, and the program calculates the drum area and drum 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 may choose to specify either the drying time (and let the program calculate the specific evaporation rate and specific feed rate) or one of the specific evaporation rate and specific feed rate (and let the program calculate the drying time). In addition, the user specifies the drying capacity of the equipment. If the total available drying capacity exceeds the required drying capacity for this 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: Heating/Cooling Tab
● Cooling, see Rotary Drying: Heating/Cooling 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