Parallel fouling

A trickle-bed reactor was used to study catalyst deactivation during hydrotreatment of a mixture of 30 wt% SRC and process solvent. The catalyst was Shell 324, NiMo/Al having monodispersed, medium pore diameters. The catalyst zones of the reactors were separated into five sections, and analyzed for pore sizes and coke content. A parallel fouling model is developed to represent the experimental observations. Both model predictions and experimental results consistently show that 1) the coking reactions are parallel to the main reactions, 2) hydrogenation and hydrodenitrogenation activities can be related to catalyst coke content with both time and space, and 3) the coke severely reduces the pore size and restricts the catalyst efficiency. The model is significant because it incorporates a variable diffusi-vity as a function of coke deposition, both time and space profiles for coke are predicted within pellet and reactor, activity is related to coke content, and the model is supported by experimental data. 

If coke forms mainly by route 1 only, then this is parallel fouling if mainly by routes 2 and 3, it is series and if equally by all three routes, then it is independent fouling. 

A model based on parallel fouling was developed to represent these experimental observations. The following assumptions were made ... 

A parallel fouling model has been developed to represent experimental observations for hydrotreating a coal oil in a trickle-bed reactor over a commercial NiMo/Al catalyst. This model accurately predicts coke profiles with time and reactor position, and hydrogenation and hydrodenitrogenation as functions of coke content. The following conclusions can be drawn from this study. 

Eor the reaction under consideration, several combinations of series and parallel fouling schemes may be chosen, involving either one or more of Aj, A2, A3 or one or more of Aj + A2, A2 + A3, A3 + Aj, or Aj + A2 + A3. The different forms of the function/3 in Equation (CS3.2) resulting from these fouling reactions are summarized in Table CS3.2. Eurthermore,/2 and/4 are assumed to be given by... 

Figure I Measurement of carbon profiles, (a) Experimental data. (6) Calculated profile. Parallel fouling mechanism (from Richardson [36]).

Distributors in industrial units typically have large numbers of injection points of quite diverse design characteristics, some of which are depicted in Eigure 16 for fluidized-bed appHcations. Flow variations through these parallel paths can lead to poor flow distributions within a reactor, thus reducing product yields and selectivity. In some circumstances, undesirable side products can foul portions of the distributor and further upset flow patterns. Where this is important, or where the possibiHties and consequences are insufficiently understood and independent means caimot be employed to assure adequate distribution, the pilot plant must be sized to accommodate such a distributor. Spacing should be comparable to those distributors that are anticipated to be... 

Spiral-wound modules consist of several flat membranes separated by turbulence-promoting mesh separators and formed into a Swiss roll (Figure 16.18). The edges of the membranes are sealed to each other and to a central perforated tube. This produces a cylindrical module which can be installed within a pressure tube. The process feed enters at one end of the pressure tube and encounters a number of narrow, parallel feed channels formed between adjacent sheets of membrane. Permeate spirals roward the perforated central tube for collection. A standard size spiral-wound module has a diameter of about 0.1m, a length of about 0.9 m and contains about 5 m2 of membrane area. Up to six such modules may be installed in series in a single pressure tube. These modules make better use of space than tubular or flat sheet types, but they are rather prone to fouling and difficult to clean. 

Hollow fiber modules, or permeators, are precisely machined units containing bundles of fine hollow fibers, positioned parallel to and around a perforated center FW tube, with only one or two bundles per pressure vessel. They are widely used for brackish and seawater supply applications. Hollow fiber modules exhibit a low flux rate (permeate flow per unit membrane per unit time) and foul easily, but... 

Sheppard, J.D., Thomas, D.G., and Channabasappa, K.C., "Membrane Fouling Part IV Parallel Operation of Four Tubular Hyperfiltration Modules at Different Velocities with Feeds of High Fouling Potential," Desalination 11, 385-398 (1972). 

However, problems associated with the reproducibility between electrodes derived from the screen-printing process and the partial electrode fouling have compromised the sensitivity of the biosensors. Work is in progress to improve both the reproducibility and the limits of detection by the use of new types of electrodes. The toxin overestimation observed with the amperometric biosensor, in the case of the microcystin analysis, suggests the use in parallel to other analytical techniques in order to minimise the risk of false-positive results. Nevertheless, the electrochemical strategy is appropriate to discriminate between toxic/non-toxic samples. 

Process "simulation. This technique is applied to equipment that is easily fouled and for which spare parallel units are provided. The fouled equipment s isolated, drained of process fluid, and filled with the cleaning solution the process operation is then simulated, thereby cleaning the equipment. An example of this route is the removal of iron oxide and copper deposits from high-pressure steam generators, using ammoniated ethylenediaminetetraace-tic acid (EDTA) solution. 

The parallel-passage reactor (PPR) and the lateral-flow reactor (LF ) are fixed-bed reactors suitable for the treatment of large volumes of gas at relatively low pressure, as are typical for end-of-pipe cleaning of combustion gases and other stack gases. In such applications, a low pressure drop, e.g., below 10 mbar, is generally required, and this demand can be met by these reactors. In addition, resistance to fouling by dust particles in the gas is important in a number of cases, and the PPR is particularly suitable for such cases. 

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