Stages of a process

The pH effect in chelation is utilized to Hberate metals from thein chelates that have participated in another stage of a process, so that the metal or chelant or both can be separately recovered. Hydrogen ion at low pH displaces copper, eg, which is recovered from the acid bath by electrolysis while the hydrogen form of the chelant is recycled (43). Precipitation of the displaced metal by anions such as oxalate as the pH is lowered (Fig. 4) is utilized in separations of rare earths. Metals can also be displaced as insoluble salts or hydroxides in high pH domains where the pM that can be maintained by the chelate is less than that allowed by the insoluble species (Fig. 3). 

A checklist analysis (CCPS, 1992) verifies the status of a system. It is versatile, easy and applicable at any life-cycle stage of a process. It is primarily used to show compliance with standards and practices by cost-effectively identifying hazards, chlorine Tar> <- liccklists provide commonality for management K.-, icw of hazard assessments. It may be used for controlling a proces.s from development to decommissioning. Approvals by appropriate authorities Cl i( V each stage of a project. 

A vvhat-if analysis identifies hazards, hazardous situations, and accident events with undesirable consequences (CCPS, 1992). What-if analysis considers deviations from the design, construction, modification, or operating intent of a process or facility. It is applicable at any litc stage of a process. 

Figure 1. Asphaltic substances in different stages of a process. The size and weight depend on the environment [modified from Ref. 27 and others].

Abstract There is a growing demand for hydrolyzable surfactants, i.e., sirnfactants that break down in a controlled way by changing the pH. Environmental concern is the main driving force behind current interest in these sirnfactants, but they are also of interest in applications where sirnfactants are needed in one stage but later undesirable at another stage of a process. This chapter summarizes the field of hydrolyzable sirnfactants with an emphasis on their more recent development. Surfactants that break down either on the acid or on the alkaline side are described. It is shown that the susceptibility to hydrolysis for many surfactants depends on whether or not the surfactant is in the form of micelles or as free unimers in solution. It is shown that whereas nonionic ester sirnfactants are more stable above the CMC (micellar retardation), cationic ester surfactants break down more readily when aggregated than when present as unimers (micellar catalysis). 

The occurrence of self-acceleration during curing of epoxy resins and epoxy-based compounds was proven by rheokinetic and calorimetric methods.53 This phenomenon can be treated formally in terms of an induction period (when the reaction is very slow in the initial stage of a process), followed by a constant rate. However, it seems preferable to use a single kinetic equation incorporating the self-acceleration effect to describe reaction as a whole. Such a kinetic equation contains only a limited number of constants (K and co in Eq. (2.33)) and allows easy and unambiguous interpretation of their dependencies on process factors. 

The increase in viscosity at the initial stages of a process is related to formation of a prepolymer, i.e., a transition from a monomer (or a monomer-polymer mixture) with viscosity in the range of 0.01 -1 Pa s to a prepolymer with viscosity 100 -1000 Pa s. The increase in viscosity occurs almost as a jump, i.e., very sharply in a relatively short induction period. A typical example is shown... 

Cycle stock the product taken from some later stage of a process and recharged (recycled) to the process at some earlier stage. 

Any process takes a certain amount of time and the length of the residence time often dictates the occasions when particular equipment or technology can be used. On the other hand, in almost all chemical unit processes the driving forces vary from time to time, and therefore time has the nature of non-equivalence, i.e., an equal time interval yields different, even greatly different, results for the early and later stages of a process. The result mentioned here means the processing amount accomplished, such as the increments of reaction conversion, absorption efficiency, moisture removal etc. Normally, these parameters vary as parabolic curves with time. Because of the nature of the non-equivalence of time, in addition to the mean residence time, the residence time distribution (RTD) affects the performance of equipment, and thus receives common attention. 

Typical life-cycle assessments are conducted during the product review stage of a process, after the plant, prototypes, and detailed designs of the product have been performed. However, Mueller et al.111 state that life-cycle evaluation should be conducted starting at the planning stage of product development. They illustrate this using an example of multifunctional chip cards that are used in a wide variety of electronics. They determine the amount of material used... 

C In one stage of a process for the manufacture of liquid air, air as a gas at 4 atm abs and 250K is passed through a long, insulated 3-in. ID pipe in which the pressure drops 3 psi because of frictional resistance to flow. Near the end of the line, the air is expanded through a valve to 2 atm abs. State all assumptions. 

The liquid outflow from the foam represents the last stage of a process which includes film thinning and rupture and liquid outflow through borders and films. That is why the term syneresis is often used in a wider sense involving the behaviour of foam dispersion medium (achievement of equilibrium liquid distribution, liquid sucking into the foam, etc.). In order to... 

A practical consequence of the pseudo second-order model is that, as market requirements dictate a lower sulfur content product, the reaction rate in the final stages of a process will be quite low. With a second-order model the reaction rate in the final incremental portion of a reactor system will be only one-tenth at the 0.3% sulfur level what it would be at the 1% sulfur level. 

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