Shifting asynchronous

In the Varicol process, hnes are shifted asynchronously. In this case, the column distribution between zones does not stay the same during the period, because lines are shifted at different times, so that the allocation changes accordingly. Since the number of columns in one zone is not constant over a period of time, the configuration of columns contains non-integral numbers. For example, a configuration of O.5/1.5/1.5/O.5 in a 4 column system is possible. A Varicol process is more adaptable than an SMB one as more flexible options are available for the repartition of columns. It is commonly observed that Varicol is 15-25% more productive than SMB (typically 5 or 6 columns are used in Varicol , whereas 6 to 8 columns are used in SMB) [11, 18]. 

Asynchronous Pitch Shifting. Asynchronous pitch shifting, the simplest pitch shifting method, simply changes the clock rate of each output digital to analog converter (DAC) to vary the pitch. Each channel requires a separate DAC. Each DAC has its own clock whose rate is determined by the requested frequency for that channel. When a DAC clock occurs, the DAC issues a request to a memory controller that supplies a waveform sample to the DAC. The earliest samplers had a separate memory for each DAC. 

A more serious limit to this implementation is due to the volume of the recycling pump and associated equipment such as flowmeters and pressure sensors. As the pump moves with respect to the zones, its volume leads to a dead volume dissymmetry, which can lead to a decrease extract and raffinate purities. This decrease can be significant for SMB with short columns and/or compounds with low retention. However, it can be easily overcome by using a shorter column or asynchronous shift of the inlets/outlets [54, 55]. This last solution is extremely efficient and does not induce extra costs because it is a purely software solution. 

A similar study was done with methyl acrylate as the dienophile.28 The uncatalyzed and catalyzed TSs are shown in Figure 6.7. As with propenal, the catalyzed reaction is quite asynchronous with C(2)-C(3) bonding running ahead of C(l)-C(6) bonding. In this system, there is a shift from favoring the exo-s-cis TS in the thermal reaction to the endo-s-trans TS in the catalyzed reaction. A large component in this difference is the relative stability of the free and complexed dienophile. The free dienophile favors the s-cis conformation, whereas the BF3 complex favors the s-trans conformation. 

Numerous commercial instruments were built in the early 1980 s that used asynchronous pitch shifting, including the Fairlight Computer Music Instrument [Roads, 1996], Kurzweil 250 [Byrd and Yavelow, 1986], the E-mu Emulator and Emulator 2 [Massie, 1985], and the New England Digital Synclavier. 

These factors motivated a complete shift in sampler design from asynchronous to synchronous pitch shifting techniques in the middle 1980 s. 

In the serial mode, the digital word (number) is sent to the computer one bit at a time. Now a binary counter provides a parallel output since each of the output bits has its own data output channel and the value of each output bit is simultaneously available. To use a serial transmission scheme, this parallel output must be put into serial form. One way to accomplish this is to use a Universal Asynchronous Receiver Transmitter (UART). The detailed operation of the UART will not be given here as it is not germane to the subject of this book. It is sufficient to say that the heart of the UART is a shift register and the shift register is strobed by a signal from the computer that displaces the binary number, bit by bit, sequentially from the register to the computer. 

In the first case (Fig. 5.19a) the recycle pump is at a fixed position between two columns. Since all columns are moving upstream according to the SMB principle the recycle pump performs the same migration. This means that the flow rate of this pump has to be adjusted depending on the section it is located at present. This design results in a locally increased dead volume because of the recycle line and the pump itself. Such an unequal distribution of the dead volume can be compensated by asynchronous shifting of the external ports, as introduced by Flotier and Nicoud (1995). 

The quantity 0 indicates that there is a phase shift between the primary field and the induced current, that is, they oscillate asynchronously as shown in Fig. 1.47. In accord with eqs. 1.234 and 1.236, we have ... 

For maintenance small valves are easier and much cheaper to replace. However, the main advantage is that all ports can be shifted independently from each other. SMB plants are very often designed with a suction line of the recycling pump, which is much longer than the other lines between the columns. In the case of asynchronous shifting, it is necessary to compensate the unequal distribution of the dead volume. In the case of the Varicol process, an asynchronous multicolumn continuous system derived from SMB (Section 5.2.5.1) individual switching of the inlet and outlet lines is obligatory. 

The VariCol approach, as introduced by Ludemann-Hombourger, Bailly, and Nicoud (2000), increases the flexibility of the continuous separation system by an asynchronous movement of the injection and withdrawal ports. Within a complete process cycle, this leads to mean numbers of columns per section that are typically non-integer. As the minimum number of columns per section in an SMB system is 1, it is possible in VariCol systems to reduce the mean number to virtually any value less than 1. Owing to the asynchronous shift of the inlet and outlet lines in a VariCol process it may even happen that, during a certain interval, there are no columns in a section. In this case the inlet and outlet lines of this section coincide in one valve block. By placing the outlet lines upstream from the inlet lines a pollution of the product lines is avoided. 

The asynchronous way of a-oxide splitting is observed only when the arising cation centre is stabilized by a- or -participation The direction of the skeletal shift in such systems is determined only by the configuration of the breaking bond and does not depend on the structure of the syn-located bridge this factor, however, exerts an essential influence on the composition of the reaction products which also depends on the nature of the acid and on the reaction conditions. In all these rearrangements no products of homo-allylic conjugation with the double C —C bond are formed. 

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