Process Reactors

A reactor is principally meant to bring the reacting species together under conditions in which chemical reactions are favorable, to supply the requisite energy, and ultimately to allow a separation of the reacted product phases. More often than not, the chemical treatment step that occurs in a reactor is the heart of the process. This step can be crucially important to make or break the process, on the basis of considerations of economic viability. 

At this stage of presentation a familiarization is provided for the reader with some representative types of reactors which are used for carrying out chemical metallurgical operations. It is also very much in order to indicate at this stage that later chapters contain accounts of different reactors in different fundamental positions. 

By way of an example, one may consider the case of hydrometallurgical reactors. Leaching is the most important of the different unit operations, and is prominently placed and assigned due emphasis in a typical hydrometallurgical process flowsheet. A representative list of the various types of reactors used for agitation leaching is given in Table 1.21. 

Pyrometallurgy, the dominant process in chemical metallurgy, uses reactor of different types and designs. In terms of the physical states of the reactants, one generally finds that the different reactions carried out in pyrometallurgy include principally, gas/liquid, liquid/ 

Pachuca tank The simplest and most inexpensive device for agitating the pulp. Agitation is achieved by the injection of air under pressure into the bottom of a tall tank. The rising gas draws the surrounding liquid with it to the surface 

---

This was a Hquid-phase process which used what was described as siUceous zeoUtic catalysts. Hydrogen was not required in the process. Reactor pressure was 4.5 MPa and WHSV of 0.68 kg oil/h kg catalyst. The initial reactor temperature was 127°C and was raised as the catalyst deactivated to maintain toluene conversion. The catalyst was regenerated after the temperature reached about 315°C. Regeneration consisted of conventional controlled burning of the coke deposit. The catalyst life was reported to be at least 1.5 yr. 

The polymerization of monomers to form hydrocarbon resins is typically carried out by either the direct addition of catalyst to a hydrocarbon fraction or by the addition of feed to a solvent—catalyst slurry or solution. Most commercial manufacturers use a continuous polymerization process as opposed to a batch process. Reactor temperatures are typically in the range of 0—120°C. 

Liquid nitrogen is used in cold traps to remove and recover solvents or volatile organic compounds from gas streams to reduce atmospheric emissions. Liquid nitrogen can be used to accelerate the cooldown time for process reactors (29). 

Silicones, an important item of commerce, are widely available commercially (9,494). The principal manufacturers of silicone operate direct-process reactors to produce dimethyl dichi orosilane and, ultimately, polydimethyl siloxane. Typical plants produce more than 450 t per year. The siUcone industry is a global enterprise in the 1990s, with principal producers in the United States (Dow Coming, GE, and OSi), Europe (Wacker Chemie, Hbls, Rhc ne-Poulenc, and Bayer), and Southeast Asia (Shin-Etsu, Toshiba SiUcones, and Dow Coming, Japan). Table 15 Hsts the approximate sales of the principal producers for 1991. 

The catalysts used in this CCR commercial service must meet several stringent physical property requirements. A spherical particle is required so that the catalyst flows in a moving bed down through the process reactors and regenerator vessel. These spheres must be able to withstand the physical abuse of being educated and transferred by gas flow at high velocity. The catalyst particles must also have the proper physical properties, such as particle size, porosity, and poresize distribution, to achieve adequate coke combustion kinetics. 

The catalyst is then transferred back to the first process reactor and is reheated to the reforming process temperature at the reactor inlet using a flow of hydrogen-rich process recycle gas, thereby achieving reduction of the platinum to a catalyticaUy active state. 

Chen (Process Reactor Design, Allyn Bacon, 1983) does the following examples mostly with simple calculus ... 

The system was operated cyclically. Steam was supplied to a process reactor to start an exothermic chemical reaction. Once the reaction began, cooling water was supplied to maintain process temperature. Cooling water at 200°F (93°C) entered the retiurn header after exiting the process reactor. 

Wastage is pronounced in equipment contacting high-pH fluids. Chemical process equipment, heat exchangers, water-cooled process reactors, valving, transfer pipes, and heating and cooling systems are often affected. 

Determine if placing the process reactor in a containment cell will significantly reduce risk. 

The licensor s basis for sizing has already been discussed and agreed to or changed. For an olefin plant, the number of steam crackers of the licensor s standard size is firm. For a new process, reactor scaleup methods have been agreed to. For a coal gasification plant, gasifier size. 

If tv o-phase flow situations are not recognized, pressure drop problems may develop which can prevent systems from operating. It requires very little percentage of vapor, generally above 7% to 8%, to establish volumes and flow velocities that must be solved by two-phase flow analysis. The discharge flow through a pressure relief valve on a process reactor is often an important example where two-phase flow exists, and must be recognized for its back pressure impact. 

Examination of the temperature control ranges of a process reactor reveals that the normal controls are to maintain a pressure of the reacting mixture of 80 psig, while the upper extreme could be 105 psig, which would be defined as the normal maximum operating pressure. 

It is clear that many of the chemical metallurgy processes must be carried out at high temperatures. In this respect, it is necessary to be acquainted not only with the process reactors but also with the methods of heat generation and with the refractories that are needed in the reactors to cope up with the high temperatures attained. 

The design optimization of an electrolytic cell aims at a high throughput with a low energy consumption at the lowest feasible cost. The throughput of an electrochemical reactor is measured in terms of the space time yield, Yt, defined as the volumetric quantity of the metal produced per unit time per unit volume of the process reactor. This quantity is expressed as ... 

Cobalt catalysts discharged from oxo -process reactors are frequently pyrophoric, owing to the presence of the carbonylcobalt. 

Before the details of a particular reactor are specified, the biochemical engineer must develop a process strategy that suits the biokinetic requirements of the particular organisms in use and that integrates the bioreactor into the entire process. Reactor costs, raw material costs, downstream processing requirements, and the need for auxiliary equipment will all influence the final process design. A complete discussion of this topic is beyond the scope of this chapter, but a few comments on reactor choice for particular bioprocesses is appropriate. 

Since long retention times are often applied in the anaerobic phase of the SBR, it can be concluded that reduction of many azo dyes is a relatively a slow process. Reactor studies indicate that, however, by using redox mediators, which are compounds that accelerate electron transfer from a primary electron donor (co-substrate) to a terminal electron acceptor (azo dye), azo dye reduction can be increased [39,40]. By this way, higher decolorization rates can be achieved in SBRs operated with a low hydraulic retention time [41,42]. Flavin enzyme cofactors, such as flavin adenide dinucleotide, flavin adenide mononucleotide, and riboflavin, as well as several quinone compounds, such as anthraquinone-2,6-disulfonate, anthraquinone-2,6-disulfonate, and lawsone, have been found as redox mediators [43—46]. 

Even though all three reactors share the same precursor delivery system, each tool offers specific advantages. For example, a cold-wall reactor (reactor B) helps prevent decomposition of the precursor before it reaches the substrate. A pulsed aerosol injection system at low pressure (reactor C) allows the film to grow under better-defined conditions than in a continuous process (reactor A) because of the minimization of undesirable transient effects caused by the high volatility of the solvents used.46 A more detailed description of each of the conditions for film growth, including reactor type, precursor type, delivery method, deposition temperature, growth time, and other parameters are summarized in Table 6.2. Depositions were done on bare and Mo-coated... 

Chen, Ning Hsing (1983), Process Reactor Design, Allyn and Bacon, Boston. 

CHEMICAL REACTIVITY CONSIDERATIONS IN PROCESS/REACTOR DESIGN AND OPERATION... 

---