Pressure flow disturbances

Schemes to control the outlet temperature of a process furnace by adjusting the fuel gas flow are shown in Figure 13. In the scheme without cascade control (Fig. 13a), if a disturbance has occurred in the fuel gas supply pressure, a disturbance occurs in the fuel gas flow rate, hence, in the energy transferred to the process fluid and eventually to the process fluid furnace outlet temperature. At that point, the outlet temperature controller senses the deviation from setpoint and adjusts the valve in the fuel gas line. In the meantime, other disturbances may have occurred in the fuel gas pressure, etc. In the cascade control strategy (Fig. 13b), when the fuel gas pressure is disturbed, it causes the fuel gas flow rate to be disturbed. The secondary controller, ie, the fuel gas flow controller, immediately senses the deviation and adjusts the valve in the fuel gas line to maintain the set fuel gas rate. If the fuel gas flow controller is well tuned, the furnace outlet temperature experiences only a small disturbance owing to a fuel gas supply pressure disturbance.

Any obstruction inserted into a duct or pipe that creates a measurable pressure difference can be used as a flow meter. The three basic standardized flow measurement devices presented above are perhaps more suitable for laboratory work than installation as permanent ductwork instruments in ventilation applications. They are sensitive to flow disturbances, relatively expensive, require considerable space, and have a narrow measurement range and a high permanent pressure loss. For these reasons, numerous attempts have been made to develop instruments without these drawbacks. Some of them, like the... 

The Mizushima Oil Refinery of Japan Energy Corporation first implemented an operation of vacuum residue hydrodesulfiirization in the conventional fixed bed reactor system in 1980. We have also conducted a high conversion operation to produce more middle distillates as well as lower the viscosity of the product fuel oil to save valuable gas oil which is used to adjust the viscosity. Vacuum residue hydrodesulfurization in fixed bed reactors mvolves the characteristic problems such as hot spot occurrence and pressure-drop build-up. There has been very little literature available discussing these problems based on commercial results. JafiFe analyzed hot spot phenomena in a gas phase fixed bed reactor mathematically, assuming an existence of the local flow disturbance region [1]. However, no cause of flow disturbance was discussed. To seek for appropriate solutions, we postulated causes ofhot spot occurrence and pressure-drop build-up by conducting process data analysis, chemical analysis of the used catalysts, and cold flow model tests. This paper describes our solutions to these problems, which have been demonstrated in the commercial operations. 

One of the simple ways to achieve active mixing is to induce a pressure field disturbance. Active micromixers relying on this strategy have been reported from different authors [43, 156-159], Deshmuck et al. [156, 157] proposed a T-junction microfluidic chip with an integrated micropump that alternatively drives and stops the flow within the microdevice to create a segmented flow. 

Example 8.6. Air at 200°F (93.3°C) is forced through a long, circular flue 36 in. (914 mm) in diameter. A pitot-tube reading is taken at the center of the flue at a sufficient distance from flow disturbances to ensure normal velocity distribution. The pitot reading is 0,54 in. (13.7 mm) HjO, and the static pressure at the point of measurement is 15.25 in. (387 mm) HjO, The coefficient of the pitot tube is 0.98. 

Spiral-wound modules of the type used for reverse osmosis (see Fig. 26.19) are widely used for UF. They are not as prone to plugging as hollow-fiber units, since the entrance is a narrow slit about 1 mm wide, but prefiltration of the feed solution is recommended. The velocity in the feed channels corresponds to laminar flow, but the flow disturbances caused by the spacers make the pressure drop and the mass transfer greater than for true laminar flow. 

Experimental approach In this case the physical equipment(s) of the chemical process is available to us. Consequently, we change deliberately the values of various inputs (disturbances, manipulated variables) and through appropriate measuring devices we observe how the outputs (temperatures, pressures, flow rates, concentrations) of the chemical process change with time. Such a procedure is time and effort consuming and it is usually quite costly because a large number of such experiments must be performed. 

Ideally, one would like to have a technique which could image the reactor in such a way as to be able to determine both the chemical identity and quantity of aU reactants, intermediates and products as a function of time and position imder identical conditions of temperature, pressure, flow rate etc. In addition, it is desirable that the probe method does not disturb the reaction, i.e., the method should be non-invasive. 

Note that the two flows exist in rather different conditions. The first flow passes through a catalyst bed with definite hydraulic resistance, whereas the second flow is discharged to the atmosphere. Without equalization of the pressure drop values on both feed lines, the output Ar-normalized response of the He-He/Ar step change can be strongly distorted (Figure 51.2). Tlierefore, needle valve (3) should be installed between the four-port valve (1) and the four-port valve (2) on the line, bypassing the reactor (Figure 51.1) to avoid flow disturbance after the switch. 

Figure 5.9 Pressure-swing system 20% feed flow disturbances.

The emerging role of micro- and nanoscale hot-wire anemometry would likely accelerate the translation of in vitro devices to in vivo applications, thereby bridging the lab-to-patient gap. Real-time measurements of intravascular physical parameters, specifically shear stress, temperature, pressure, and flow rate, provide a basis to link hemodynamics with biochemical events in blood vessels. The complex curvature of the vascular system requires small, minimally invasive sensors to discretely measure in real time intravascular physical parameters with minimal blood flow disturbance. To achieve this, flexible micro-and nanoscale sensors would allow for steering in the complicated anatomy in biological systems (Fig. 11). In summary, the utilization of micro- or nanoscale sensors provides a quantitative assessment of vascular hemodynamics. This approach lends itself to applications in broad areas of medicine and physiology and is particularly relevant to quantitative studies of cancer biology as well as... 

These values relate to steady flow conditions in pipelines remote from the point at which the material is fed into the pipeline, bends in the pipeline and other possible flow disturbances. At the point at which the material is fed into the pipeline, the material will essentially have zero velocity. The material will then be accelerated by the conveying air to its slip velocity value. This process will require a pipeline length of many metres and this distance is referred to as the acceleration length . The actual distance will depend once again on particle size, shape and density. The process was illustrated earlier in Figure 4.29 in relation to the pressure drop across a bend. 

With the plethora of competing events, process engineers often find it difficult to operate the polymerization reactor in a way that maintains the production of a polymer with specified PSD characteristics. This is due, in some cases, to the fact that the control of the PSD is practiced by controlling its characteristic variables (e.g., the mean and variance) or olha- easily measured process variables (e.g., temperature, convasion and concentrations). These formulations are not enough for fine PSD control and thus fail in most circumstances when iQiplied to the real process. Moreover, the properties of the polymer formed are influenced by the process/kinetic history. Any unprecedented disturbances in the operating conditions (e.g., tanperature, pressure, flow rates, etc.) may cause drastic irreversible changes in product quality. One should add to this the fact that any reactants (e.g., monomer, initiator, and surfactant) introduced to the process cannot be removed. 

As an example of where cascade control may be advantageous, consider the natural draft furnace temperature control problem shown in Fig. 16.1. The conventional feedback control system in Fig. 16.1 may keep the hot oil temperature close to the set point despite disturbances in oil flow rate or cold oil temperature. However, if a disturbance occurs in the fuel gas supply pressure, the fuel gas flow will change, which upsets the furnace operation and changes the hot oil temperature. Only then will the temperature controller (TC) begin to take corrective action by adjusting the fuel gas flow. Thus, we anticipate that conventional feedback control may result in very sluggish responses to changes in fuel gas supply pressure. This disturbance is clearly associated with the manipulated variable. 

Note that some designers would choose to operate V7 fully open and let Pp float in order to save pumping costs associated with the pump in the reactor effluent line. We assume here that pressure control is necessary to maintain flash unit pressure constant. Disturbance sensitivity is assumed not to be an issue for this plant, so any need to control a flow rate or composition variable within the recycle loop will be satisfied by controlling x j). 

The exit pressure method assumes that flow is fully developed at the exit plane or that the extent of flow disturbances at the exit plane is negligibly small for all intents and purposes. The subject of flow disturbances near the exit plane of a die has been... 

It can therefore be concluded that, for all intents and purposes, the extent of flow disturbances near the die exit may be considered to be negligible for strongly viscoelastic polymer melts at high a (say, a > 25 kPa), justifying the extrapolation procedure used to obtain and thus making the exit pressure method valid (see Table 5.1). However, this conclusion cannot be extended to dilute polymer solutions and very weakly elastic polymer melts. 

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