Reactor counter current flow

Fluid Iron Ore Reduction (FIOR) is a process for reducing ore to iron with a reducing gas in a fluid bed. For thermodynamic efficiency, iron ore reduction requires counter current flow of ore and reducing gas. This is achieved in FIOR in a multiple bed reactor. Precautions are necessary to prevent significant back mixing of solids between beds, since this would destroy counter current staging. 

Describe the advantages and disadvantages of the following reactor types with reference to heat and mass transfer. For each reactor discuss one reaction for which it may be appropriate to use that reactor, (a) fluidized bed reactor, (b) A continuous counter-current flow reactor, (c) A monolith reactor. 

The final step is a hydrolyzing step with sulfatase enzymes (E.C. number 3.1.6.1), such as limpet sulfatase, Aerobacter aerogenes sulfatase, Abalone entrail sulfatase, or Helixpomatia sulfatase. This step was suggested to be carried out in a CSTR or fluidized bed reactors, with counter-current flow between the aqueous and the oil phase. A more efficient removal of the sulfate into the aqueous stream is expected to occur in this cross-flow manner. A final separation of the reacting mixture was suggested to obtain sulfur-free product and aqueous enzyme solution for recycle. 

Gasification processes can be separated into three major types (1) moving-bed (counter-current flow) reactors (2) fluidized-bed (back-mixed) reactors and (3) entrained-flow (not back-mixed) reactors. Figure 19.11 shows the types of gasification reactors together with temperature profiles and locations of feed and product streams. Table 19.12 summarizes the important characteristics of each type of gasifier, and Table 19.13 presents the performance characteristics of selected gasifiers. 

The Laporte hydrogenator contains a series of tubes which dip just below the surface of the liquid. Hydrogen is then fed into the bottom of each tube, and small gas bubbles are formed. A counter current flow is set up due to the density difference between the solutions in the tube and the reactor. The palladium catalyst suspension is drawn into the tubes by a continuous movement of the working solution. 

The modeling and design of a three-phase reactor requires the knowledge of several hydrodynamic (e.g., flow regime, pressure drop, holdups of various phases, etc.) and transport (e.g., degree of backmixing in each phase, gas-liquid, liquid-solid mass transfer, fluid-reactor wall heat transfer, etc.) parameters. During the past decade, extensive research efforts have been made in order to improve our know-how in these areas. Chapters 6 to 8 present a unified review of the reported studies on these aspects for a variety of fixed bed columns (i.e., co-current downflow, co-current upflow, and counter-current flow). Chapter 9 presents a similar survey for three-phase fluidized columns. 

Applying an isothermal and plug>flow membrane reactor (on both sides of the membrane) to the above reactions, Itoh and Xu [1991] concluded that (1) the packed-bed inert membrane reactor gives conversions higher than the equilibrium limits and also performs better than a conventional plug-flow reactor without the use of a permselective membrane and (2) the co-current and counter-current flow configurations give essentially the same conversion. 

Itoh and Govind [1989b] further analyzed an isothermal packed-bed inert membrane reactor but under a counter-current flow configuration. Under the conditions studied, the authors found that the counter-current flow configuration provides a much greater conversion than the co-current flow mode. 

Co-current versus counter-current flows. It is noted that in the operation of a separation system, a counter-current flow has always given a larger average concentration gradient than a co-current flow. Thus, it is expected that counter-current flow configuration is preferred between the two in a membrane unit. In a membrane reactor, however, an additional factor needs to be considered. To obtain a high conversion of a reversible reaction, it is necessary to maintain a high forward reaction rate. 

As will become evident later, the counter-current flow conflguration appears to provide a clear-cut advantage over the co-current flow configuration with respect to the reaction conversion in a dense membrane reactor. 

Most of the simulators allow heat input or removal from a plug-flow reactor. Heat transfer can be with a constant wall temperature (as encountered in a fired tube, steam-jacketed pipe, or immersed coil) or with counter-current flow of a utility stream (as in a heat exchanger tube or jacketed pipe with cooling water). 

Flow direction. This must be examined from two points of view. First we consider the flow direction of the reactants in relation to one another. In an entrained-bed process the reactants flow by definition in co-current flow, but in a moving-bed reactor one has a choice. The most well-known moving-bed reactor, the Lurgi-Sasol dry bottom gasifier, operates in counter-current flow, but there are a number of smaller gasifier designs that use cocurrent flow to reduce the tar make. 

The main objective of OCR process is demetallization of residues. To decrease reactor volume and to make an effective use of the catalyst Chevron developed a technology in which oil and catalyst are fed to the reactor in counter-current flow (figure 5). The beaded catalyst is added once or several times a week to the top of the reactor and moves in plug flow to the bottom of the reactor, where the spent catalyst is removed. Chevron carried out extensive experiments to develop a safe and reliable system to feed fresh and to withdraw spent catalyst. 

A counter-current flow of solid and fluid offers the advantage of continuous operation. The easiest concept of such a reactor is the true moving bed reactor (TMBR). It is a direct adoption of the true moving bed (TMB) process explained in Chapter 5.3.4. 

The simulated moving bed reactor (SMBR) based on the simulated moving bed (SMB) process is a practical alternative for implementing counter-current continuous reactors. Counter-current movement of the phases is simulated by sequentially switching the inlet and outlet ports located between the columns in direction of the liquid flow (Fig. 8.4). As with the SMB process, two different concepts are known to realize the counter-current flow. One is based on switching the ports and the other on the movement of columns. However, both require elaborate process control concepts to realize the movement. Owing to the periodical changes of the set-up the pro-... 

Two different modeling approaches are used for simulated moving bed reactors. The first approach combines the model of several batch columns with the mass balances for the external inlet and outlet streams. By periodically changing the boundary conditions the transient behavior of the process is taken into account. The model is based on the SMB model introduced in Chapter 6 and is, therefore, referred to as the SMBR model. The second approach assumes a true counter-current flow of the solid and the liquid phase like the TMBR. Therefore, this approach is called the TMBR model. 

The iron blast furnace, which is the first step in the production of steel, provides an example of a counter-current flow moving-bed reactor in which numerous gas-solid reactions occur. Iron ore, coke, and limestone are charged to the top, while hot air is fed to the bottom of a refractory-lined reactor vessel. The solids have a residence time of 6-8 hr, whereas the air residence time is only 6-8 sec. The iron ore may contain 50-70% iron, while the molten iron product will typically contain about 95% iron, 4% carbon, and 1% of a number of compounds including silicon, manganese, titanium, phosphorus, and sulfur. 

Tubular reactors are commonly used in laboratory, pilot plant, and commercial-scale operations. Because of their versatility, they are used for heterogeneous reactions as well as homogeneous reactions. They can be run with cocurrent or counter-current flow patterns. They can be run in isothermal or adiabatic modes and can be used alone, in series, or in parallel. Tubular reactors can be empty, packed with inert materials for mixing, or packed with catalyst for improved reactions. It is often the process that will dictate the design of the reactor, as discussed in this entry. 

Fig. 5 Fixed-bed reactors with gas-liquid flow. (A) Trickle-bed reactor with cocurrent downflow (B) trickle-bed reactor with counter-current flow and (C) packed bubble-flow reactor with cocurrent upflow.

For section I, one unique counter-current flow reactor is set up. At the inlet. Oxygen must be separated from SO2 and hydriodic acid must be concentrated. 

Fig. 9. Yield vs. product concentration in shrinking-bed and non-shrinking-bed counter-current flow reactor. Substrate yellow poplar reaction conditions acid concentration = 0.08 wt% sulfuric acid, T = 230°C, reactor length = 6 inches...

Here the reacting mixture and coolant flow in opposite directions for counter current flow of coolant and reactants. At the reactor entrance, V = 0, the reactants enter at temperature To, and the coolant exits at temperature At the end of the reactor, the reactants and products exit at temperature T while the coolant enters at T. ... 

Figure 5.19 Temperature profiles for a tubular reactor with jacket cooling (a) Parallel flow, high coolant rate (b) parallel flow, moderate coolant rate (c) counter-current flow, moderate coolant rate. 

With regard to other significant factors, oxygen transfer can be singled out as of paramount importance. To enhance this transport step, we operated the spiral wound reactor counter-currently. In other words, a special provision was incorporated into the reactor design to allow flow of pure oxygen countercurrent... 

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