Channel volume

When the space-time yield is referred to the total reactor volume (and not only to the micro-channel volume), the large share of inactive construction material has to be taken into account. Consequently, the space-time yields per micro-channel volume have to differ by orders of magnitude, e.g. more than a factor of 1000, from... 

GP 2] [R 2] The definition of space-time yield in a micro reactor depends on the definition of the reactor volume . Owing to the large amoimt of construction material relative to the reaction channels and the neglect of some reactor parts ( abstraction to the real reaction zone ), several more or less useful definitions can be made. In the following, two definitions concerning the time yield divided by the pure reaction channel volume and the platelet volume were used. 

Material thickness 100 pm between reaction and heat transfer channels Volume of system without, < 1 ml with one and with two delay < 3 ml loops < 5 ml... 

Micro channel volume 1.3 pi Typical residence time 21 s... 

The above-mentioned space-time yields were referred solely to the reaction volume, i.e. the micro channel volume. When defining this quantity via an idealized reactor geometry, taking into accoimt the construction material as well, natarally the difference in space-time yield of the micro reactors from the laboratory bubble column becomes smaller. Still, the performance of the micro reactors is more than one order of magnitude better [38], The space-time yields for the micro reactors defined in this way ranged from about 200 to 1100 mol monofluorinated product... 

GL 18] ]R 6a]]P 17/Using the same experimental conditions and catalysts with the same geometric surface area, the performance of micro-channel processing was compared with that of a fixed-bed reactor composed of short wires [17]. The conversion was 89% in the case of the fixed bed the micro channels gave a 58% yield. One possible explanation for this is phase separation, i.e. that some micro channels were filled with liquids only, and some with gas. This is unlikely to occur in a fixed bed. Another explanation is the difference in residence time between the two types of reactors, as the fixed bed had voids three times larger than the micro channel volume. It could not definitively be decided which of these explanations is correct. 

FIGURE 1.33 Top left membrane unit with 1 mL channel volume (A = inert material B = membrane) Top right membrane unit with 10 flh channel volume. Bottom hollow fiber membrane unit with 1.3 /rL acceptor channel (lumen) volume.151 (Reproduced with permission from IUPAC.)... 

A twin electrode thin layer Kissinger cell was designed in which the channel volume could be varied through the use of PTFE spacers [174]. The working and counter electrodes were carbon paste (3.14 m ) and the reference electrode was Ag/AgCl. The performance of the cell was tested on 50 pL portions of chlorpromazine solutions in 0.01 M HCl, and the three cited methods were compared. Linear sweep voltammetry was found to be the simplest to apply and showed moderate sensitivity. 

SV geometric surface area (surface area of channels/volume of monolith) t time... 

Define active channel volumes where electrochemistry occurs—this defines the material capacitance of the system regarding the electrochemistry... 

Because surface binding causes bulk concentration depletion, protein solution was flowed continuously through the channels until the bulk concentration became stabile. This required an aliquot roughly equal to 4 or 5 times the channel volume in the microchannels with the lowest protein concentration, while the highest concentration channels required considerably less flow, as expected. A line profile generated from the epifluorescence image of these channels is shown in Fig. 6.10b. The majority of the signal emanated from the bulk. 

Because numerical errors due to discretization of a convective term introduce an additional, unphysical diffusion mechanism, termed numerical diffusion (ND), the diffusion coefficient D was set to zero [152], The resulting concentration fields nonetheless are indicative of the distribution of a solute within the micro channel volume. In this way, convective patterns can be derived for the redistribution of the liquid transverse to the flow direction. Accordingly, the stretching, tilting and thinning of liquid lamellae can be followed. 

The authors provided various drivers for the application of micro channels, namely the safe operation of the explosive mixture [44], higher surface to channel volume compared with conventional ceramic monoliths and finally smaller pressure drop compared with packed beds or porous solid foams. 

Reactor Description Channel width (pm)/length (mm) Per channel volume (mm3)/surface area (mm2) Enzyme "loading" g"E"/g PDMS Urea Feed Solution Flow rate Mean residence (mL/min) time (min) ... 

The channel volume divided by the net volumetric flow rate gives the mean residence time... 

We now extend the model to the positive net flow situation, and assume that the differential volume moves axially. Although the axial flow is not plug flow, this is not an unreasonable approximation because as we recall the RTD is rather narrow. In this case, the elapsed time t becomes the mean residence time in the extmder given by the ratio of screw channel volume and net flow rate... 

N-isopropylacrylamide 1 is added to the polymerization mixture to increase hydro-phobicity of the monolith required for the separations in reversed phase mode. Vinylsulfonic acid 12 provides the chargeable functionalities that afford electroosmo-tic flow. Since the gelation occurs rapidly already at the room temperature, the filling of the channel must proceed immediately after the complete polymerization mixture is prepared. The methacryloyl moieties attached to the wall copolymerize with the monomers in the liquid mixture. Therefore, the continuous bed fills the channel volume completely and does not shrink even after all solvents are removed. Fig. 6.8 also shows scanning electron micrograph of the dry monolithic structure that exhibits features typical of macroporous polymers [34],... 

The sedimentation forces, which apply uniformly throughout the channel volume, are equally well-defined in terms of particle size, particle density, and fluid density. Thus migration rate can be rigorously related to particle size, as will be shown in the theory section. 

Figure2.20 Examples of microreactors (150 mm x 150 mm), with two functional layers. Outside tempering. Inside preliminary heating, mixing, dwell channel (volumes approximately 3 ml), with friendly permission of the Little Things Factory GmbH, llmenau, Germany.

The void time t,j is by definition equal to the void or channel volume, V0=bLw, divided by the channel flow rate V, which leads to ... 

FIGURE 12.1 Flat-membrane modules for membrane extraction in flow systems, (a) Spiral-channel membrane module with 1 mL channel volume (A, blocks of inert material B, membrane). (From Jonsson, J.A. and Mathiasson, L., Trends Anal. Chem., 11, 106, 1992. With permission from Elsevier Science.) (b) Straight-channel membrane module with 10 p-L channel volume. (From Lindegard, B., Bjork, H., Jonsson, J.A., Mathiasson, L., and Olsson, A.-M., Anal Chem., 66, 4490, 1994. With permission.)... 

For online connection to HPLC, SLM is the preferred extraction technique, as the extract obtained is aqueous and therefore in principle compatible with HPLC. With large membrane units (channel volumes around 1 mL), direct transfer of the entire volume... 

Here, AT is the temperature drop across the channel, which is set by the user, and D is the ordinary (mass) diffusion coefficient. The parameter V° is the geometric volume of the channel, which is constant for a given instrument. Note that V, is the same parameter used to define retention in SEC and that the ratio on the left side of Eq. (1) is the number of channel volumes required to flush a sample component through the thermal FFF channel. Although Eq. (1) is an approximation, it becomes accurate to within 3% when V/V° > 10. More important for this discussion, Eq. (1) characterizes the influence of analyte parameters D and on retention in thermal FFF. 

---