Mass-transfer dispensing

Of the mass-transfer dispensing methods, screen printing and stencil printing are the oldest and most widely used. Screen printing has been used for over 40 years in the electronics industry to apply thick-film conductors, resistors, and dielectrics in fabricating circuits on ceramic and plastic-laminate substrates. Screen printing is also used as a batch process for depositing electrically conductive and insulative adhesives to interconnect devices on thin-film and thick-film hybrid microcircuits. 

The parameter p (= 7(5 ) in gas-liquid sy.stems plays the same role as V/Aex in catalytic reactions. This parameter amounts to 10-40 for a gas and liquid in film contact, and increases to lO -lO" for gas bubbles dispersed in a liquid. If the Hatta number (see section 5.4.3) is low (below I) this indicates a slow reaction, and high values of p (e.g. bubble columns) should be chosen. For instantaneous reactions Ha > 100, enhancement factor E = 10-50) a low p should be selected with a high degree of gas-phase turbulence. The sulphonation of aromatics with gaseous SO3 is an instantaneous reaction and is controlled by gas-phase mass transfer. In commercial thin-film sulphonators, the liquid reactant flows down as a thin film (low p) in contact with a highly turbulent gas stream (high ka). A thin-film reactor was chosen instead of a liquid droplet system due to the desire to remove heat generated in the liquid phase as a result of the exothermic reaction. Similar considerations are valid for liquid-liquid systems. Sometimes, practical considerations prevail over the decisions dictated from a transport-reaction analysis. Corrosive liquids should always be in the dispersed phase to reduce contact with the reactor walls. Hazardous liquids are usually dispensed to reduce their hold-up, i.e. their inventory inside the reactor. 

Targeting in time interval (5.5-6 h) is shown in Fig. 12.24a. The associated block diagram is depicted in Fig. 12.24b. Since no contaminant load is removed from the B wash, dispensing with used water as effluent would certainly amount to inefficient use of available mass transfer driving forces in the system, as this water could still be reused in the next batch cycles if not reusable in the subsequent time intervals within... 

Pin- and daub-transfer processes are also mass-transfer methods, but comprise less than 10% of the adhesive-dispensing processes used in production. In pin transfer, hard tooling consisting of an array of pins corresponding to the positions on the circuit board where components are to be attached is fabricated. The pin arrays are dipped into atray containing the... 

As indicated before, the interfacial concentrations are dispensed with by defining overall mass-transfer coefficients ... 

One way around this problem is to dispense with the trapping and separation stage by continuous sampling of the atmosphere around the Aermal probe. This can be achieved by using a small-bore silica-glass capillary transfer line which also serves to reduce the pressure fix)m atmospheric to that which could be accommodated by the ion source of the mass spectrometer. The capillary tube... 

Internal Standard Solution—Weigh 20 g of 0.500 mass % cadmium, cobalt, or yttrium (or any other suitable metal) organometallic concentrate into a 1 L volumetric flask and dilute to 1 L with the dilution solvent. Prepare fresh, at least weekly, and transfer this solution into a dispensing vessel. The concentration of the internal standard element is not required to be 100 pig/mL however, the concentration of the internal standard element in the test specimen solution should be at least 100 times its detection limit. 

---