Recycling continued problem materials

A problem may arise in processes that involve recycling. Suppose a material that enters with the fresh feed or is produced in a reaction remains entirely in a recycle stream, rather than being carried out in a process product. If nothing were done about this situation, the substance would continuously enter the process and would have no way of leaving it would therefore steadily accumulate, making the attainment of steady state impossible. To prevent this buildup, a portion of the recycle stream must be withdrawn as a purge stream to rid the process of the substance in question. 

Re-use, remanufacture and recycle. Focusing on re-use, remanufacture and recycling comes from a number of trends and drivers in society. Because of problems with waste disposal, many companies are trying to work towards a zero to landfill policy. As waste disposal costs rise, this trend will accelerate. At the same time, where raw materials are difficult or expensive to obtain, there is a real commercial incentive to recycle and reuse. Regulators also continue to push for the minimisation or elimination of the release of hazardous materials... 

With reversible reactions, sufficient improvement in conversion sometimes can be realized from removing the product to warrant a recycle operation. This can be done by sending the product to a separator and returning only unconverted material. Some systems, moreover, lend themselves to continuous removal of product in equipment integrated with the reactor. Extraction is thus employed in problem P4.06.13 and azeotropic distillation in problems P4.06.14 and P4.06.15. The gasoline additive, methyl-tert-butyl ether, is made in a distillation column where reaction and simultaneous separation take place. 

The process flow sheet was first tested for direct leaching of steel mill flue dust and production of zinc metal by electrowinning. The tests were performed in a continuously operating pilot plant, producing 10-20 kg/day zinc metal. The same pilot plant was then used for treating copper/zinc-rich brass mill flue dust in a closed loop operation, recycling all the zinc solvent extraction raffinate to the copper circuit leach section. In the zinc circuit leach section, only the amount of zinc rich dust necessary for neutralization of the copper solvent extraction raffinate was used. The results obtained from the pilot plant tests indicated contamination problems within the solvent extraction loops. The estimation of economic data showed a weak return on the assets compared with the alkali route, and sensitivity toward the raw material price. 

The older modular simulation mode, on the other hand, is more common in commerical applications. Here process equations are organized within their particular unit operation. Solution methods that apply to a particular unit operation solve the unit model and pass the resulting stream information to the next unit. Thus, the unit operation represents a procedure or module in the overall flowsheet calculation. These calculations continue from unit to unit, with recycle streams in the process updated and converged with new unit information. Consequently, the flow of information in the simulation systems is often analogous to the flow of material in the actual process. Unlike equation-oriented simulators, modular simulators solve smaller sets of equations, and the solution procedure can be tailored for the particular unit operation. However, because the equations are embedded within procedures, it becomes difficult to provide problem specifications where the information flow does not parallel that of the flowsheet. The earliest modular simulators (the sequential modular type) accommodated these specifications, as well as complex recycle loops, through inefficient iterative procedures. The more recent simultaneous modular simulators now have efficient convergence capabilities for handling multiple recycles and nonconventional problem specifications in a coordinated manner. 

The continually increasing demand for environmentally friendly industrial processes has also led to the development of techniques for recycling of the remaining 5-30% sulfate contained in the acidic wash water [2.55]. In modern processes, up to 99 % of sulfuric acid can be recovered and reused in production. In the chloride process, wastewater problems arise if the raw material contains < 90% Ti02. The metal chloride by products are sometimes disposed of in solution by the deep well method (e.g., at Du Pont). The metal chloride solutions are pumped via deep boreholes into porous geological strata. Special geological formations are necessary to avoid contamination of the groundwater by impurities. 

---