Tubular Concepts

A constant value of the cell voltage is observed over 6000 hours. Life tests with self-sustaining type cells have shown that open-circuit voltage and power density remain practically constant over a period of more than three years. This is illustrated in Fig. 51. Several cyclings between 1000 °C and room temperature did not affect the performance [107], On the basis of these results, one may conclude that the tubular concept fulfills the life time requirement quite well. 

The concept of a tubular solid oxide hot water vapor electrolysis (Hot Elly) has been investigated in Germany within the frame of a technology program between 1975 and 1987. It was terminated with the successful demonstration of a 2 kW lab-scale pilot plant [23]. The tubular concept was replaced by the planar concept, after respective ceramics production technologies were sufficiently developed. 

There are mainly two different eoneepts under development - the tubular and the planar design. As far as proof of long term stability and demonstration of plant teehnology are eoneemed the tubular concept is far more advaneed. In eomparison, the planar development offers higher power density. In this ehapter the various design variants are presented, followed by a deseription of the main eompanies involved and the status of cell and staek teehnology. 

Depending on operation temperature the material chosen for the bipolar plate is graphite (or graphite composites) or iron-based alloys (stainless steel) for low- and medium-temperature fuel cells (PEFC, PAFC, MCFC) and chromium or ceramic-based materials for the high-temperature systems (SOFC). In SOFC, for sealing reasons, tubular concepts have also been developed (see Section 8.1.4.6). 

Two main concepts for solid oxide fuel cells (SOFCs) are currently under development the tubular and the planar designs. In terms of long-term stability, the tubular concept has demonstrated the best results, while the planar design promises higher power densities. 

For many years, also a tubular concept had been developed by Siemens Westinghouse, addressing the challenges in matching the thermal expansion coefficients of the different materials, namely for the electrolyte and the electrodes. 

Stack Manufacturing The differences in the stationary planar and integrated tubular concepts are also visible in the impact assessment, as shown in Fig. 21.26. The high amount of chromium in the stationary planar concept is decisive for the impacts in all emission-related categories. Also for energy-related emissions, the chromium production shows the highest figures. It has to be kept in... 

Solid Oxide Fuel Cells Past, Present and Future presents an initial historical background to Solid Oxide Fuel cells, then describes the materials and component parts required to make a working stack. It then compares different cell designs, particularly comparing planar with tubular concepts. Examples of SOFC units for different applications are then presented. 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

Experience in using the Z concept has demonstrated that the calculated Z factor should be modified by an efficiency factor to account for some of the aforementioned effects which are absent in the theoiy and, as such, this factor depends on the type of centrifuge. It is nearly 100 percent for simple spin-tube bottle centrifuge, 80 percent for tubular centrifuge, and less than 55 percent for di centrifuges. The... 

The idea of the effective plate number was introduced and employed by Purnell [4], Desty [5] and others in the late 1950s. Its conception was evoked as a direct result of the introduction of the capillary column or open tubular column. Even in 1960, the open tubular column could be constructed to produce efficiencies of up to a million theoretical plates [6]. However, it became immediately apparent that these high efficiencies were only obtained for solutes eluted at very low (k ) values and, consequently, very close to the column dead volume. More importantly, on the basis of the performance realized from packed columns, the high efficiencies did not... 

Common types of membrane materials used are listed in Table 3. This gets us into the concept of geometry. There are three types of modules generally used, namely Tubular, Spiral wound, and Hollow fiber. A comparison of the various geometries is given in Table 4. 

The concept of cross-flow microfiltration is shown in Figure 16.11, which represents a cross-section through a rectangular or tubular membrane module. The particle-containing fluid to be filtered is pumped at a velocity in the range 1-8 m/s parallel to the face of the membrane and with a pressure difference of 0.1-0.5 MN/m2 (MPa) across the membrane. The liquid penneates through the membrane and the feed emerges in a more concentrated form at the exit of the module.1617 All of the membrane processes are listed in Table 16.2. Membrane processes are operated with such a cross-flow of the process feed. 

Chapter 3 introduced the basic concepts of scaleup for tubular reactors. The theory developed in this chapter allows scaleup of laminar flow reactors on a more substantive basis. Model-based scaleup supposes that the reactor is reasonably well understood at the pilot scale and that a model of the proposed plant-scale reactor predicts performance that is acceptable, although possibly worse than that achieved in the pilot reactor. So be it. If you trust the model, go for it. The alternative is blind scaleup, where the pilot reactor produces good product and where the scaleup is based on general principles and high hopes. There are situations where blind scaleup is the best choice based on business considerations but given your druthers, go for model-based scaleup. 

The numbering-up concept demands achieving absolutely uniform flow equi-partition by placing special headers in front of the parallel micro channels [5]. Although this has been solved for tubes in conventional multi-tubular reactors, the expenditure for equipartition in micro channel stacks is assumed to be higher, since the small channels may have more relative differences in structural preci-... 

Testing, A., Heeguijuela, J. R., Development of a continuous segmented tubular flow reactor and the scale-out concept -in search of perfect powders, Chem. Eng. Technol. 26, 3 (2003) 303-305. 

The SA concept will prove to be particularly useful in the design of tubular reactors for gas phase reactions. 

In this chapter, we describe several ideal types of reactors based on two modes of operation (batch and continuous), and ideal flow patterns (backmix and tubular) for the continuous mode. From a kinetics point of view, these reactor types illustrate different ways in which rate of reaction can be measured experimentally and interpreted operationally. From a reactor point of view, the treatment also serves to introduce important concepts and terminology of CRE (developed further in Chapters 12 to 18). Such ideal reactor models serve as points of departure or first approximations for actual reactors. For illustration at this stage, we use only simple systems. 

---