A drum dryer consists of one or more heated metal rolls on the outside of which a thin layer is evaporated to dryness. Dried solid is scraped off the rolls as they slowly revolve. The rolls of a drum dryer are 0.6 to 3 m in diameter. Drum dryers are operated in continuous mode. In some cases, the rolls are covered and the dryer operates under vacuum.
The evaporation calculations are based on the liquid/solid portion of the wet feed stream only. Any gaseous phase present in the wet feed stream is ignored in evaporation calculations and it is simply sent to the vapor stream. For example, let’s assume that 20% of water contained in the wet feed stream is already in the vapor phase and that the evaporation percentage of water in the dryer is set as 10%. In that case, the program will transfer 20% of water to the vapor stream automatically. From the remaining 80% of water that appears in the liquid/solid phase of the wet feed stream, the program will evaporate another 10%, or equivalently, another 8% of water contained in the wet feed stream. Consequently, a total 28% of water contained in the wet feed stream will leave the dryer through the vapor stream.
Also, the program assumes that the dried product (final solids) stream appears entirely in the liquid phase and the outlet vapor stream appears entirely in the gaseous phase.
The component material balances are done based on the evaporation percentages of pure components that are set as volatile. For each non-volatile pure component, its mass flow rate in the dried product stream is set equal to its liquid/solid mass flow rate in the wet feed stream, and its mass flow rate in the vapor stream is set equal to zero. For each volatile pure component, its mass flow rate in the vapor stream is calculated by multiplying its liquid/solid mass flow rate in the wet feed stream by its evaporation percentage. Also, its mass flow rate in the dried product stream is set equal to the remaining mass flow rate after subtracting its evaporating mass flow rate from its liquid/solid mass flow rate in the wet feed stream.
The evaporation percentages are either specified by the user or calculated based on the specified ‘Final LOD’ value. That is the loss on drying (LOD) of the dried product stream. A stream’s LOD is calculated as the ratio of total liquid mass flow rate to total liquid/solid mass flow rate for that stream. If a pure component appears in the liquid/solid phase of the wet feed stream, then that liquid/solid phase is considered either liquid or solid depending on whether that component is set as volatile or not. The ‘Initial LOD’ (i.e., the LOD of the wet feed stream) is first determined by dividing the total liquid mass flow rate of the wet feed stream by the total liquid/solid mass flow rate of that stream. Then, the total solids mass flow rate of the wet feed stream (which is the same as that of dried product stream) is calculated as (1-Initial LOD). If the evaporation percentages of volatile components are set by the user, the total liquid 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 ‘Final LOD’ can be calculated by dividing the total liquid mass flow rate of the dried product stream by the total liquid/solid mass flow rate of that stream. If the ‘Final LOD’ is specified, then the program assumes that all volatile components have the same evaporation percentage and that percentage is calculated as 100 (Initial LOD-Final LOD) / Initial LOD / (1-Final LOD).
In addition to evaporation specifications, a suitable heating agent must be specified. The program expects that the wet feed stream is heated by the heating agent. In order for this to happen, the inlet and outlet temperatures of heating agent must be higher than the specified dried product temperature.If the above constraint is not true, then the transfer of heat from the heating agent to the wet feed is not possible.
The enthalpy balance can be stated as follows: the sum of wet feed stream enthalpy and heating duty must be equal to the sum of vapor stream enthalpy and dried product stream enthalpy. In that case, the user specifies the final solids temperature and the program solves the enthalpy balance based on the assumption that the vapor temperature is the same as the dried product temperature in order to calculate the heating duty. Then, the heating agent requirement is calculated by dividing the heating duty by the heating agent’s mass-to-energy factor and by the heating agent’s efficiency.
By default, the operating pressure in the dryer is assumed equal to the minimum pressure of the two inlet streams (wet feed and inlet drying gas). Optionally, the user may set his/her own value for the operating pressure. The pressure of the two outlet streams (dried product and outlet drying gas) 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 drum 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 specifies the operation’s specific evaporation rate (expressed as evaporation rate per unit area of drum), and the program calculates the drum area by dividing the total mass flow rate of evaporated volatile components by the specified specific evaporation rate. If the calculated drum area exceeds the specified maximum drum area, multiple identical units of smaller diameter are assumed that operate in parallel. The calculated number of units and the calculated diameter of each unit are such that the total volume of all the units is the same as the total volume requirement 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 and drum area. By default, the program calculates the operation’s specific evaporation 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) or the specific evaporation rate (and let the program calculate the drying time). The Drying Capacity is also specified and it is used to calculate the operation’s equipment capacity utilization. If the operation’s equipment drying capacity requirement exceeds the drying capacity of the equipment, the program displays a warning.
See Vacuum Pump Auxiliary Equipment Calculations.
1. McCabe W. L., J. C. Smith, and P. Harriott. (1993). Unit Operations of Chemical Engineering, McGraw-Hill, 5th ed., pp. 803-805.
2. Coulson J. M. and J. F. Richardson, (1978). Chemical Engineering, Vol. 2, Pergamon Press, 3rd ed., pp. 733-735.
The interface of this operation has the following tabs:
● Oper. Cond’s, see Drum Drying: Oper. Conds Tab
● Utility Data, see Rotary Drying: Utility Data 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