Amplitude attenuation of time-variant properties of fluid streams

#Fluid streams having time-variant properties are generated in many processes which operate in cyclic or discontinuous modes. The time variance of certain stream properties can have an undesirable effect on the use of the stream elsewhere in the process or in another process. For example, in the recovery of hydrogen from synthesis gas by pressure swing adsorption, blowdown and purge streams are generated in which the concentrations of combustible components vary periodically with time. These byproduct streams typically are used as fuel to fire a process furnace, and the performance of the furnace can be adversely affected by the resulting time-variant heating value of the fuel stream. Other examples can be found in cyclic or discontinuous processes such as the smelting of ore in the primary metals industries, operation of batch chemical reactors or the venting of process upsets to buffer tanks in the chemical process industries, and variations in wastewater composition in wastewater treatment processes.

#Typical time-variant stream properties are temperature, pH, and the concentrations of particular components in the stream. The amplitude of the time variance of the properties of such streams can be reduced or attenuated by two well-known methods.

#In one method, something is added to or removed from the stream in a controlled manner to compensate for the variations. For example, heat can be added to or removed from a stream to control temperature or another stream can be added under controlled conditions to the time-variant stream, such as the addition of acid or alkali to control the pH of a stream. This method often is used when the time variance of a desired property is largely random in nature.

#FIG. 1 is a plot of dimensionless residence time distribution vs dimensionless time for known mixing regimes and the preferred mixing regime of the present invention.

#FIG. 2 is a plot of amplitude ratio vs relative residence time illustrating the attenuation of the amplitude of a sinusoidally-variant inlet fluid property by known mixing regimes and the preferred mixing regime of the present invention.

#FIG. 3 is a schematic drawing of a mixing vessel according to an embodiment of the present invention.

#A fluid stream in a process can be a single-phase gas or liquid mixture, a multi-phase gas-liquid mixture (for example, an aerosol), a multi-phase liquid-liquid mixture (for example, an emulsion), or a multi-phase stream containing finely-divided solids suspended in a gas or a liquid. If the fluid stream properties vary with time, either randomly or periodically, the use of the stream in the process or in another process can be problematic. The present invention is a method to reduce or attenuate the amplitude of time-variant properties of a fluid stream by introducing the stream into a defined enclosed volume in a controlled manner such that the residence time distribution of the fluid exiting the enclosed volume approaches that of a preferred flat residence time distribution. This invention attenuates the amplitude of the time variance of the inlet stream more efficiently than known residence time distributions in defined volumes.

#The terms “time-variant”, “time-varying”, “time-variable”, “time variance”, and “time variability” as applied to a fluid stream refer to a physical property or properties of the stream which vary or change with time. The time variance can be random, symmetrically cyclic, non-symmetrically cyclic, or combinations thereof, and this variance occurs about a mean or average value of the property defined over a long period of time. The terms “cyclic” and “periodic” can be used interchangeably and have equivalent meaning. The cycle time of a periodic time-variant property is defined as the regular interval in which the time-varying property occurs, reoccurs, or repeats itself. The maximum deviation of the property value from the mean over a given time period is defined as the amplitude of the deviation. An attenuated time-variant property is defined as a time-variant property in which the amplitude or variation of the property from a mean value has been decreased or attenuated relative to that of an initial time-variant property. In the present invention, the amplitude of a time-variant inlet stream property is reduced by passing the inlet stream through a specifically-designed vessel, from which is withdrawn a time-variant outlet stream property having a reduced amplitude.

#Random variance of a process stream property can occur due to external and largely uncontrollable effects, for example, the effect of ambient atmospheric conditions on the operation of a given process. Symmetrically cyclic variance occurs in a regular repeatable pattern such as that described by a classic sine wave in which each half of a cycle is a mirror image of the other half. Non-symmetrically cyclic variance occurs in a repeatable pattern with time, but each half of a cycle is not a mirror image of the other half. In many processes, stream properties can exhibit combinations of random and cyclic behavior; for example, variance can be random over a short time period but cyclic over a longer time period. Cyclic stream property variations often occur in a process which consists of a series of discontinuous steps such as, for example, a pressure swing adsorption process used in gas separation.