Distillation: processes – separatory – Particular type of heating
Reexamination Certificate
2000-05-04
2002-11-26
Manoharan, Virginia (Department: 1764)
Distillation: processes, separatory
Particular type of heating
C202S158000, C203SDIG001, C203SDIG001, C210S295000, C210S634000, C261S128000, C261S114200, C261S114400, C261S114500, C261S005000, C261S114100, C585S800000
Reexamination Certificate
active
06485613
ABSTRACT:
The present invention relates to a mass transfer process in which, in two or more contact stages, two or more liquid or gaseous phases are brought into contact with one another, one or more components being transferred between the phases and the phases moving countercurrently with respect to the overall configuration.
Mass transfer processes have already been in use for many years and in recent decades have undergone systematic consideration. In the present application, mass transfer processes are understood to mean processes in which at least one component is transferred from one phase to another phase. It relates to processes such as distillation, extraction and washing.
Depending on the manner of contact between the phases, mass transfer processes can be divided into the following classes:
Steady-state and non-steady-state processes. Steady-state processes are those processes in which the conditions, such as concentration at any location in the system, the volume of the different flows and the compositions of the different flows, are constant over time.
With regard to direction of flow of the phases through the process with respect to one another. In this way, processes are divided into concurrent, counter-current and crosscurrent.
The way in which the phases flow through the stages in which the contact between the phases takes place. In this context, the following distinction is made: continuous/continuous: both phases flow through the contact stages continuously; batch/batch: both phases flow through the contact stages in a batchwise manner; and continuous/batch: the flow through the contact stages is continuous for one phase and batchwise for the other phase.
In order to achieve efficient, large-scale mass transfer operations, the professional process engineering world nowadays primarily selects processes in which
the conditions are steady-state;
the phases move countercurrently in order to obtain the maximum number of equilibrium stages and thus the maximum level of mass transfer
both phases flow continuously through the contact stages.
These processes can therefore be referred to as steady-state, continuous/continuous countercurrent processes.
The general reasons cited by the professional process engineering world for selecting the steady-state design rather than the non-steady-state batch/batch design for liquid or gaseous phases when efficient mass transfer is desired are as follows:
Steady-state countercurrent processes make it possible, in equipment of identical size, to obtain a better separation of components than that which is achieved in non-steady-state process types.
To achieve the same separation level, choosing a steady-state counter-current process allows the equipment to be smaller than if a non-steady-state process type were to be selected.
In addition, processes with continuous/continuous flow are preferably selected owing to the fact that temporary storage is required for batch/batch processes.
These arguments in favour of selecting steady-state countercurrent processes are based on the fact that there is no knowledge, in the above-mentioned phase sector, of using non-steady-state countercurrent mass transfer processes with a plurality of stages. For this reason, comparisons are generally made between a single-stage non-steady-state process and a multistage steady-state countercurrent process. The result is what one might call an unfair comparison. This fact will be referred back to later.
An example of a steady-state continuous/continuous countercurrent distillation process is described in Dutch Patents 105668 and 105668. Although the descriptions of the processes given in these documents refer to a stepwise method, the phases are fed to and discharged from the contact stages continuously; both the liquid and the gaseous phases flow continuously through the contact stages.
An example of a steady-state continuous/continuous extraction process is described in U.S. Pat. No. 2,009,347. The device which is described in this document is designed in such a manner that each stage of the device comprises both a mixing zone and a separation zone. As soon as this process is fully operational, the feed and discharge of the heavy and light phases to and from each stage take place continuously.
An intermediate form between batch/batch processes and continuous/continuous processes is formed by the batch/continuous processes. In these processes, one of the phases flows through the contact stages in a batchwise manner and another phase flows through the contact stages continuously. These processes too can be of countercurrent design with more than one stage.
Since it is considered that steady-state continuous/continuous processes are the most efficient processes, non-steady-state, batch/continuous countercurrent processes with more than one stage have hitherto been designed only for solid/liquid extractions. The reason for this is that in processes of this nature there are practical drawbacks in carrying out a continuous/continuous steady-state process.
Thus far there is no knowledge of carrying out non-steady-state mass transfer processes between gaseous and liquid phases countercurrently, with the result that it has not been recognised that with a design of this nature the performance of the process is improved enormously if the phase with most back mixing flows in a batchwise manner.
It has now been found that it is also possible to carry out mass transfer processes in which the phases are liquid or gaseous under non-steady-state, countercurrent conditions. Therefore, the present invention provides a mass transfer process in which, in two or more contact stages, two or more liquid or gaseous phases are brought into contact with one another, one or more components being transferred between the phases and the phases moving countercurrently with respect to the overall configuration of the process, characterized in that the flow through one or more of the contact stages is batchwise for one phase and continuous for another phase.
According to the present invention, it has been found that it is possible to carry out countercurrent mass transfer processes with more than one stage under non-steady-state conditions. For example, if a distillation column is considered, the present invention offers the following advantages:
Owing to the lower strip factor (lower hydraulic load) in a batch/continuous design according to the invention, the diameter of a column can be reduced by up to 50% compared to the design which uses a continuous/continuous countercurrent process.
The power consumption of a batch/continuous process is lower by a factor of from 1.5 to 2 than that of a continuous/continuous design. In a batch/continuous process, the column height can be smaller by a factor of 2 than that of a continuous/continuous design.
Further advantages of the invention will be explained below with reference to specific embodiments.
The following definitions will be used in the present text:
A “steady-state process” is understood to mean a process in which the concentration at any location in the system, the volume of the different flows and the compositions of the different flows are independent of time.
A “non-steady-state process” is understood to mean a process in which the parameters mentioned above are dependent on time.
“Contact stage” is understood to mean that part of a process in which the phases are brought into contact with one another and—after mass transfer has taken place—are separated from one another again.
“Batch(wise)” flow through the contact stage is understood to mean that there is no significant feed or discharge of the relevant phase from and to the contact stage over a certain period of time.
“Continuous” flow through the contact stage is understood to mean that the relevant phase is fed and discharged continuously to and from the contact stage.
In this connection, it should be noted that if, with batch flow to a contact stage, so-called weeping or entrapment, for example, occurs, this flow is still considered to be batchwise.
In the case of continuous flow, the
Continental Engineering B.V.
Manoharan Virginia
Young & Thompson
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