What Is a Mixed Episode?

In many chemical operations, agitation and mixing of materials is involved. At this time, knowledge about the time required to achieve uniform mixing is often important. Mixing time is usually defined as the time required to reach homogeneity on a molecular scale. However, since measurement technology on this scale is difficult to achieve, researchers can only measure the final mixing time required to achieve uniformity by observing to the extent possible. ^[1]

Roughly speaking, the time actually required to make the feed uniformly distributed over the whole tank is the mixing time. But when does it count as the starting point of the mixing process? Someone is operating normally

In the mixing of powder and granules, the particles of each component have been fully mixed with each other, and the mixing and separation effects have reached a dynamic equilibrium, which has become a statistically complete mixing state. If the mixing operation is continued, the mixing state has not changed, so the required mixing time is called the complete mixing time.

In addition, when the mixed powder and granular components have different physical properties such as particle size, density, shape, and internal friction coefficient, during the mixing process, sometimes the two effects of mixing and separation are in a state of migration before the dynamic equilibrium state is reached. A point is expressed as an apparently complete mixing state, and the mixing time at this time is sometimes called a complete mixing time. In any kind of mixed operation. The complete mixing time refers to the most appropriate mixing time. ^[3]

The batch mixing time refers to the time from when all the raw materials enter the mixer to when the discharge door is opened to discharge. The general mixing time is 3 to 5 minutes. The mixing stage of the batch is divided into three stages: dry mixing, adding water, and wet mixing. Each stage is obtained by means of the mean square error test according to the process conditions and the gradation of the raw material particle size.

The following illustrates the calibration method of mixing time.

The water adding time of the raw material batching system of a production line is set to 30s according to the program. First, according to production experience, the ratios of dry and wet mixing time are preset to be 1: 0.8, 1: 1, 1: 1.2, 1: 1.5, and the dry mixing time is scheduled to be 60s. Mix the batches according to different ratios, measure the mean square error of different ratios, and choose a reasonable ratio of 1: 1.5. Then, the dry-mixing time was changed in the same way, the mean square error was measured, the lowest point of the mean-square error was found, and the best time for dry-wet mixing was determined.

The figure below is the mixed time calibration chart.

The mixing time t _M is an important performance index of the fermenter, and is defined as the time required to reach a certain uniformity after the tracer pulse is injected into the reactor. In fermentation production, especially in fed-batch or continuous operation, it is always desirable that t _{M in the} reactor is as short as possible, so that the fed-batch is quickly and uniformly distributed. To quantitatively describe the uniformity, the so-called "non-uniformity h" is used. It is defined as the relative deviation of the actual concentration c from the average concentration c when completely mixed:

In continuous operation, the ratio of t _M to the residence time of the same material in the reactor determines the degree of uniformity achieved by the overflow. MacDonald et al. Studied a continuous stirred reactor for chemical reactions and pointed out that the final mixing time should be 5% less than the average residence time of the feed liquid to ensure that the deviation between the overflow concentration and the average composition of the product at any time is 5 Within%. Mechanically stirred fermentation tanks can meet such conditions. In small reactors, the mixing of liquids is fast, but as the volume and viscosity of the fermenter increases, t _M increases significantly (see table below).

Methods for detecting t _M include density method, thermal method, and conductivity method. Conductivity method refers to adding a certain amount of electrolyte to a reaction system (mechanical stirred tank or airlift reactor system), such as a certain concentration of KCl, and the reading from the addition of the electrolyte to the conductivity meter is no longer due to the fluctuation of the electrolyte concentration The period of change, ie the mixing time. To achieve complete mixing, in theory, it takes a long time, so the time required to reach 95% of the initial electrolyte concentration is often the mixing time of the reaction system. ^[7]