Flow loops

A desirable attribute of a control loop is a response that is independent of the operating point, or a linear response. To this end, good practice requires offsetting nonlinearities in the loop to create an overall linear response. For example, when using a head flow device, a square-root extractor is used to linearize the flow signal. The square-root extractor is a device that simply takes the square root of the signal in order to linearize it. 

Another factor that affects the linearity of the response is the characteristic of the valve selected. Three of the most common valve characteristics include quick opening, linear, and equal percentage. If the majority of the system pressure drop is taken across the valve, then a linear valve should be used since its installed characteristic will also be linear, giving the linear response desired. However, if the pressure drop across the valve is a small part of the total line drop and is not constant, then an equal percentage valve can be used since its installed characteristic will be close to linear. Quick-opening valves are most commonly used with on-off controllers where a large flow is needed as soon as the valve begins to open. More information on these valve flow characteristics can be found in Chapter 2. 

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Feedforward control can also be applied by multiplying the liquid flow measurement—after dynamic compensation—by the output of the temperature controller, the result used to set steam flow in cascade. Feedforward is capable of a reduction in integrated error as much as a hundredfold but requires the use of a steam-flow loop and dynamic compensator to approach this. 

Minimum-flow loop left open at normal rates, or byj>ass around control valve is open. 

Close minimum-flow loop or control valve byj)ass valve. 

A similar catalytic dimerization system has been investigated [40] in a continuous flow loop reactor in order to study the stability of the ionic liquid solution. The catalyst used is the organometallic nickel(II) complex (Hcod)Ni(hfacac) (Hcod = cyclooct-4-ene-l-yl and hfacac = l,l,l,5,5,5-hexafluoro-2,4-pentanedionato-0,0 ), and the ionic liquid is an acidic chloroaluminate based on the acidic mixture of 1-butyl-4-methylpyridinium chloride and aluminium chloride. No alkylaluminium is added, but an organic Lewis base is added to buffer the acidity of the medium. The ionic catalyst solution is introduced into the reactor loop at the beginning of the reaction and the loop is filled with the reactants (total volume 160 mL). The feed enters continuously into the loop and the products are continuously separated in a settler. The overall activity is 18,000 (TON). The selectivity to dimers is in the 98 % range and the selectivity to linear octenes is 52 %. 

Select flow loop geometry, i.e., type reboiler arrangement. 

Repeat calculations, adjusting flow loop geometry if necessary, until assumed x gives the proper boilup rate. 

A = point Ain flow loop, Figure 10-110 B = point B in flow loop, Figure 10-110 C = point C in flow loop. Figure 10-110 E = reboiler exit system... 

A = point A in flow loop B = point B in flow loop... 

Hetsroni et al. (2003a) investigated two-phase air-water flow in 21 triangular parallel micro-channels of dh = 130 pm. The experimental test facility and flow loop, for liquid flowing through micro-channels is shown in Fig. 5.13. 

Fig. 1. Schematic diagram of the capillary flow loop apparatus. From reference [54], reproduced with permission. 1995, Wiley-Liss, Inc, a subsidiary of Wiley, New York...

Fig. 2. Variation of specific death rate with average shear stress, under laminar and turbulent conditions, in the capillary flow loop apparatus [54,121]...

A more sophisticated implementation is full metering control (Fig. 10.6). In this case, we send the signals from the fuel gas controller (FC in the fuel gas loop) and the air flow transmitter (FT) to the ratio controller (RC), which takes the desired flow ratio (R) as the set point. This controller calculates the proper air flow rate, which in turn becomes the set point to the air flow controller (FC in the air flow loop). If we take away the secondary flow control loops on both the fuel gas and air flow rates, what we have is called parallel positioning control. In this simpler case, of course, the performance of the furnace is subject to fluctuations in fuel and air supply lines. 

Heat can be added to or removed from stirred-tank reactors via external jackets (Figure 7.5a), internal coils (Figure 7.5b) or separate heat exchangers by means of a flow loop (Figure 7.5c). Figure 7.5d shows vaporization of the contents being condensed and refluxed to remove heat. A variation on Figure 7.5d would not reflux the evaporated... 

Harris, P.C. Harms, W.M. Norman, L.R. "Study of Continuously Mixed Crosslinked Fracturing Fluids With a Recirculating Flow Loop Viscometer", SPE paper 17044, 1987 SPE Eastern Regional Meeting, Pittsburgh, October 21-23. 

Zigrye, J.L., Osborne, M.W., and Westbrook, G.H. "Field Analysis and Flow Loop Tests Diagnose Problems with Delayed Action Crosslinked Fracturing Fluid Systems," SPE paper 15633, 1986 Annual Technical Conference and Exhibition of the SPE, New Orleans, October 5-8. 

Lescarboura, J.A., Sifferman, T.R. and Wahl, H.A. "Evaluation of Frac Fluid Stability Using a Heated, Pressurized Flow Loop," SPE paper 10962, 1982 SPE Annual Technical Conference and Exhibition, New Orleans, September 26 29. 

A- FLOW LOOPS. PI controllers are used in most flow loops. A wide proportional band setting (PB — ISO) or low gain is used to reduce the effect of the noisy flow signal due to flow turbulence. A low value of integral or reset time (t, - 0.1 minute per repeat) is used to get fast, snappy setpoint tracking. 

Various materials have been used as separators in zinc—bromine cells. Ideally a material is needed which allows the transport of zinc and bromide ions but does not allow the transport of aqueous bromine, polybromide ions, or complex phase structures. Ion selective membranes are more efficient at blocking transport then nonselective membranes.These membranes, however, are more expensive, less durable, and more difficult to handle then microporous membranes (e.g., Daramic membranes).The use of ion selective membranes can also produce problems with the balance of water between the positive and negative electrolyte flow loops. Thus, battery developers have only used nonselective microporous materials for the separator. 

Hydrate formation is a substantial problem in deepwater production and flowlines. Pipelines that transport condensed hydrocarbon phases such as gas condensate or crude oil have limited possibilities for removing hydrates once the plugs have formed. Earlier work by Scauzillo (1956), indicating that formation may be inhibited by the input of hydrocarbon liquids, cannot be confirmed by thermodynamic calculations, and Skovborg (1993) has found counter-examples. Thus, the construction of large-scale pilot flow loops have been completed by large corporations such as ExxonMobil (Reed et al., 1994), Tulsa University, and Institut Francais du Petrole. Such experiments are discussed briefly in Chapter 6. 

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