Differential flow control

Gas Chromatograph. Aerograph Hy-FI, Wilkens model 610-C equipped with electron capture detector, isothermal proportional controller, and differential flow controller. 

Gas Chromatographic Oven. Aerograph Hy-FI, Wilkens model 550, equipped with electron capture detector, isothermal proportional controller, differential flow controller, and detector transfer switch. By using the detector transfer switch, this additional oven permits the use of two different columns without cooling which would otherwise be required to change columns. 

The operating conditions for both columns are the same. The glass-lined injection ports are held at 210°-220°C., and the glass columns are maintained at 175°C. The nitrogen carrier gas, which is dried by a molecular sieve (Linde type 13X), is regulated at 30 ml. per minute by the differential flow controllers. 

A block diagram of the experimental set-up is shown in Figure 3. The lower part of the reaction cell which held the liquid matrix extended into a Dewar. The temperature was controlled to =t2°C. by a cooled air stream which was regulated by a differential flow controller and flowmeter. The temperature could be decreased by increasing the flow rate. The temperature was monitored by a platinum resistance probe attached to a digital readout unit (Digitec Model 531). 

In temperature programming, even when the inlet pressure is constant, the flow rate will decrease as the column temperature increases. As an example, at an inlet pressure of 24 psi and a flow rate of 22 mL/min (helium) at 50°C, the flow rate decreases to 10 mL/min at 200°C. This decrease is due to the increased viscosity of the carrier gas at higher temperatures. In all temperature-programmed instruments, and even in some better isothermal ones, a differential flow controller is used to assure a constant mass flow rate. 

CSTRs and other devices that require flow control are more expensive and difficult to operate. Particularly in steady operation, however, the great merit of CSTRs is their isothermicity and the fact that their mathematical representation is algebraic, involving no differential equations, thus maldng data analysis simpler. 

Differential temperature as well as differential pressure can be used as a primary control variable. In one instance, it was hard to meet purity on a product in a column having close boiling components. The differential temperature across several bottom section trays was found to be the key to maintaining purity control. So a column side draw flow higher in the column was put on control by the critical temperature differential. This controlled the liquid reflux running down to the critical zone by varying the liquid drawn off at the side draw. This novel scheme solved the control problem. 

The flow transmitter (transducer block) senses the flow element differential pressure, converts this signal to a signal proportional to the process flow, and sends it to the flow controller. 

Catalysts were tested for oxidations of carbon monoxide and toluene. The tests were carried out in a differential reactor shown in Fig. 12.7-1 and analyzed by an online gas chromatograph (HP 6890) equipped with thermal conductivity and flame ionization detectors. Gases including dry air and carbon monoxide were feed to the reactor by mass flow controllers, while the liquid reactant, toluene was delivered by a syringe pump. Thermocouple was used to monitor the catalyst temperature. Catalyst screening and optimization identified the best catalyst formulation with a conversion rate for carbon monoxide and toluene at room temperature of 1 and 0.25 mmolc g min1. Carbon monoxide and water were the only products of the reactions. 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

In all these cases the reflux rate is simply set at a safe value, enough to nullify the effects of any possible perturbations in operation. There rarely is any harm in obtaining greater purity than actually is necessary. The cases that are not on direct control of reflux flow rate are (g) is on cascade temperature (or composition) and flow control, (h) is on differential temperature control, and (i) is on temperature control of the HTM flow rate. 

FIGURE 3.15 Differential EOF pumping rates were achieved from different gate voltages applied to the field-effect flow control electrodes. This produced (a) positive pressure for flow down and (b) negative pressure for flow up the field-free microchannel (ft) [371]. Reprinted with permission from the American Chemical Society. 

Monitoring differential pressure between the primary and secondary cooling systems would be one of the candidates for detecting the heat transfer tube rupture. Meanwhile, the pressure varies even at normal operation and therefore there is a concern of malfunction of CV isolation valves. In case of primary helium gas recovery flow rate, primary helium pressure control system only covers the rated operation since the system activates when the primary coolant pressure reaches about 3.95 MPa. On the other hand, the primary-secondary differential pressure control system covers start-up, shutdown and rated operations. The control system keeps supplying helium gas to the secondary cooling system during the scenario. Thus, monitoring the entire secondary helium gas supply would be an effective way to detect the tube rupture. 

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