Pilot units

Properties. Pilot-unit data indicate the EDS process may accommodate a wide variety of coal types. Overall process yields from bituminous, subbituminous, and lignite coals, which include Hquids from both Hquefaction and Flexicoking, are shown in Figure 14. The Hquids produced have higher nitrogen contents than are found in similar petroleum fractions. Sulfur contents reflect the sulfur levels of the starting coals ca 4.0 wt % sulfur in the dry bituminous coal 0.5 wt % in the subbituminous and 1.2 wt % sulfur in the dry lignite. 

The performance of SCWO for waste treatment has been demonstrated (15,16). In these studies, a broad number of refractory materials such as chlorinated solvents, polychlorinated biphenyls (PCBs), and pesticides were studied as a function of process parameters (17). The success of these early studies led to pilot studies which showed that chlorinated hydrocarbons, including 1,1,1-trichloroethane /7/-T5-6y,(9-chlorotoluene [95-49-8] and hexachlorocyclohexane, could be destroyed to greater than 99.99997, 99.998, and 99.9993%, respectively. In addition, no traces of organic material could be detected in the gaseous phase, which consisted of carbon dioxide and unreacted oxygen. The pilot unit had a capacity of 3 L/min of Hquid effluent and was operated for a maximum of 24 h. 

In cases where a large reactor operates similarly to a CSTR, fluid dynamics sometimes can be estabflshed in a smaller reactor by external recycle of product. For example, the extent of soflds back-mixing and Hquid recirculation increases with reactor diameter in a gas—Hquid—soflds reactor. Consequently, if gas and Hquid velocities are maintained constant when scaling and the same space velocities are used, then the smaller pilot unit should be of the same overall height. The net result is that the large-diameter reactor is well mixed and no temperature gradients occur even with a highly exothermic reaction. 

Analysis of a method of maximizing the usefiilness of smaH pilot units in achieving similitude is described in Reference 67. The pilot unit should be designed to produce fully developed large bubbles or slugs as rapidly as possible above the inlet. UsuaHy, the basic reaction conditions of feed composition, temperature, pressure, and catalyst activity are kept constant. Constant catalyst activity usuaHy requires use of the same particle size distribution and therefore constant minimum fluidization velocity which is usuaHy much less than the superficial gas velocity. Mass transport from the bubble by diffusion may be less than by convective exchange between the bubble and the surrounding emulsion phase. 

Economy of time and resources dictate using the smallest sized faciHty possible to assure that projected larger scale performance is within tolerable levels of risk and uncertainty. Minimum sizes of such laboratory and pilot units often are set by operabiHty factors not directly involving internal reactor features. These include feed and product transfer line diameters, inventory control in feed and product separation systems, and preheat and temperature maintenance requirements. Most of these extraneous factors favor large units. Large industrial plants can be operated with high service factors for years, whereas it is not unusual for pilot units to operate at sustained conditions for only days or even hours. 

Reactor Internals and Unit Hardware. Requirements for mixing feed components or separating products may determine minimum pilot unit size. If reactants caimot be premixed before they are passed into the reactor, the effectiveness of the inlet distributor in mixing the reactants can markedly affect reactor performance. This is especially tme for gases, multiple phases, or Hquid streams of greatly different kinematic viscosities. 

Additional definition of the operative mechanisms can obviate the need for the larger unit. It maybe possible to assess limitations in a smaller unit that has only a few injection points on the distributor. The unit could be used to evaluate distributor designs that permit a wide range of acceptable operating conditions. However, if the acceptable range proves smaller than desired, the larger pilot unit would then be needed to estabUsh acceptable performance. 

Sasol uses both fixed-bed reactors and transported fluidized-bed reactors to convert synthesis gas to hydrocarbons. The multitubular, water-cooled fixed-bed reactors were designed by Lurgi and Ruhrchemie, whereas the newer fluidized-bed reactors scaled up from a pilot unit by Kellogg are now known as Sasol Synthol reactors. The two reactor types use different iron-based catalysts and give different product distributions. 

Pilot plants utilizing a single-full-sized reactor tube from a commercial plant are generally used to assess the quaUty and performance of individual catalyst lots and to perform plant or customer ordered process tests. A weU-designed pilot unit is capable of simulating the performance of a commercial plant with great accuracy. 

Example 12 Segregated Flow The pilot unit of Fig. 23-11 with n = 9.3 has... 

Maximum impeller zone shear rate will be higher in the larger tank, but the average impeller zone shear rate will be lower therefore, there will be a mu(m greater variation in shear rates in a full-scale tank than in a pilot unit. 

In order to make the pilot unit more like a commercial unit in macro-scale characteristics, the pilot unit impeller must be designed... 

In general, the larger the pilot unit, the more reliable the pre-dic tion oflarge-scale performance. The pilot unit should be a prototype with all dimensions properly scaled down. 

Differences in materials of construction between the pilot unit and the production unit should be considered. These may have a bearing on caking, abrasion, and electrostatic effec ts. 

Limiting flow rates are hsted in Table 23-16. The residence times of the combined fluids are figured for 50 atm (735 psi), 400°C (752°F), and a fraction free volume between particles of 0.4. In a 20-m (66-ft) depth, accordingly, the contact times range from 6.9 to 960 s in commercial units. In pilot units the packing depth is reduced to make the contact times about the same. 

One can to outline a general approach for medium selection along with a test sequence applicable to a large group of filter media of the same type. There are three methods of filter media tests laboratory- or bench-scale pilot-unit, and plant tests. The laboratory-scale test is especially rapid and economical, but the results obtained are often not entirely reliable and should only be considered preliminary. Pilot-unit tests provide results that approach plant data. The most reliable results are often obtained from plant trials. 

For example, if the larger reactor is twice the size of the pilot unit (i.e., SUFy = 2), and the two units share the same aspect ratio, then the heat transfer area only increases by 1.59 from Equation 13-28 ... 

The control of the reaction was based on the assumption that stopping the flow of chlorine would stop all reaction this was true on the pilot unit but not on the full-scale plant. On the pilot unit, there was no stirrer, as the incoming chlorine gave sufficient mixing. When chlorine addition stopped, mixing also stopped and so did the reaction. On the full-scale plant, a stiirer was necessary, and this continued in operation after chlorine addition stopped. In addition, on the pilot unit the cooling was sufficient to hide any continuing reaction that did occur. 

Figure 9-6 is a simplified flow diagram of a TEA dehydration pilot unit. 

Obrecht, M. F., Sastor, W. E. and Keyes, J. M., Integrated Design of Field Test Panel Pilot Unit for Investigating Pitting Corrosion of Copper Water Tube by Potable Water Supplies , Proc. 4th Int. Congr. Met. Corr., 1969, 576 (1972)... 

Ross (R2) measured liquid-phase holdup and residence-time distribution by a tracer-pulse technique. Experiments were carried out for cocurrent flow in model columns of 2- and 4-in. diameter with air and water as fluid media, as well as in pilot-scale and industrial-scale reactors of 2-in. and 6.5-ft diameters used for the catalytic hydrogenation of petroleum fractions. The columns were packed with commercial cylindrical catalyst pellets of -in. diameter and length. The liquid holdup was from 40 to 50% of total bed volume for nominal liquid velocities from 8 to 200 ft/hr in the model reactors, from 26 to 32% of volume for nominal liquid velocities from 6 to 10.5 ft/hr in the pilot unit, and from 20 to 27 % for nominal liquid velocities from 27.9 to 68.6 ft/hr in the industrial unit. In that work, a few sets of results of residence-time distribution experiments are reported in graphical form, as tracer-response curves. 

For the pilot unit, taking the impeller diameter as ( >7 /3), then ... 

A pilot unit was built to confirm effective heat control and to establish long-term catalyst stability (Fig. 1) (ref. 17). The pilot unit was designed around a single tube... 

Fig. 1. Schematic of the pilot unit used for catalyst life and heat management testing.

Table 1. Composition of the aqueous HBr feed used for pilot unit testing...

Mass flow through full-scale unit ipQ)fuii-scaie Mass flow through pilot unit pQ)pUot-scale... 

Mass inventory in the full-scale unit ipV)fuU-scaie Mass inventory in the pilot unit pV)pUot-scaie... 

The primary goal of scaleup is to maintain acceptable product quahty. Ideally, this will mean making exactly the same product in the large unit as was made in the pilot unit. To this end, it may be necessary to alter the operating conditions in the pilot plant so that product made there can be duplicated upon scaleup. If the pilot plant closely approaches isothermal piston flow, the challenge of maintaining these ideal conditions upon scaleup may be too difficult. The alternative is to make the pilot plant less ideal but more scaleable. 

This chapter assumes isothermal operation. The scaleup methods presented here treat relatively simple issues such as pressure drop and in-process inventory. The methods of this chapter are usually adequate if the heat of reaction is negligible or if the pilot unit operates adiabatically. Although included in the examples that follow, laminar flow, even isothermal laminar flow, presents special scaleup problems that are treated in more detail in Chapter 8. The problem of controlling a reaction exotherm upon scaleup is discussed in Chapter 5... 

If kiAi is known with good accuracy, it may be possible to back out the intrinsic kinetics using the methods of Section 7.1. Knowing the intrinsic kinetics may enable a scaleup where kiAj(af — ai) is dilferent in the large and small units. However, it is better to adjust conditions in the pilot reactor so that they are identical to those expected in the larger reactor. Good pilot plants have this versatility. The new conditions may give suboptimal performance in the pilot unit but achievable performance in the full-scale reactor. 

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