Type of Recycle

This chapter presents a comparison of the steady-state economics of four alternative tubular reactor systems. The entire process will be considered, not just the reactor in isolation, because the optimum economic steady-state design can be determined only for the entire plant. The type of recycle, the phase of the reaction, and the heat transfer configuration all affect the optimum design. 

The recovery of unreacted components in the reactor exit stream can produce a liquid recycle or a gas recycle. Liquid recycle systems typically involve distillation columns to separate products from the material to be recycled. The recycle stream is usually a liquid that can be inexpensively pumped back to the reactor. If the reaction is liquid-phase, the liquid recycle is added to the liquid fresh feedstreams. If the reaction is vapor-phase and the recycle is liquid, a vaporizer is used. The major cost with liquid recycle is energy, in both the separation step and the vaporizer, if it is required. 

Gas recycles usually involve a liquid-vapor phase separation in which the lighter reactants come off the top of the flash drum as vapor and are compressed so that they can be recycled back to the reactor inlet. A gas recycle almost always implies a gas-phase reaction. The major costs with gas recycle are compressor capital investment and compressor work. 

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Problems of removal of mercury from aqueous effluents are more comphcated in plants that manufacture a variety of inorganic and organic mercury compounds it is generally best to separate the effluent streams of inorganic and organic mercurials. When phenyhnercuric acetate is precipitated from its solution in acetic acid by addition of water, the filtrate is collected and reused for the next precipitation. This type of recycling is necessary not only for economic reasons but also to minimise recovery operations. 

Waste Minimization source reduction, certain types of recycling and reclamation. Waste minimization does not include recycling activities whose uses constitute disposal and burning for energy recovery. 

The hazardous waste identification process (as discussed in Chapter 1) describes how to determine whether a material is a solid and hazardous waste. How a material is regulated under RCRA (i.e., whether or not it is a solid and potentially a hazardous waste) when it is recycled depends on what type of material it is and what type of recycling is occurring. If the recycled material is not a solid waste, then it is not a hazardous waste and is not subject to RCRA Subtitle C requirements. However, if the material qualifies as a solid and hazardous waste, it is subject to RCRA Subtitle C jurisdiction. 

Materials that are recycled are a special subset of the solid waste universe. When recycled, some materials are not solid wastes, and therefore not hazardous wastes, but others are solid and hazardous waste, but are subject to less-stringent regulatory controls. The level of regulation that applies to recycled materials depends on the material and the type of recycling (Figure 13.2). Because some types of recycling pose threats to human health and the environment, RCRA does not exempt all recycled materials from the definition of solid waste. As a result, the manner in which a material is recycled will determine whether or not the material is a solid waste, and therefore whether it is... 

Additives often form a problem in recycling processes. Material recycling is often not possible or only with a considerable loss of quality. Plastics recycling is notoriously difficult due to the mixed composition of the plastics waste stream. The recycled material can only be used in certain applications that do not demand a pure material. Recycling of the additives themselves is theoretically possible only for metals, but in practice this type of recycling is not feasible. The metals occur only in low concentrations. Recovery from fly ash and bottom ash is possible, but expensive in view of growing scarcity problems it may become a viable options for at least some metals. 

The following hypothetical data give the participation rates in a particular type of recycling program and the number of tracks purchased for collection by ten towns in a small mid-Atlantic state ... 

Figure 8.20 shows another type of recycle design in which a recycle loop increases the concentration of the permeable component to the point at which it can be removed by a second process, most commonly condensation [38], The feed stream entering the recycle loop contains 1 % of the permeable component as in Figures 8.16-8.19. After compression to 20 atm, the feed gas passes through a condenser at 30 °C, but the VOC content is still below the condensation concentration at this temperature. The membrane unit separates the gas into a VOC-depleted residue stream and a vapor-enriched permeate stream, which is recirculated to the front of the compressor. Because the bulk of the vapor is recirculated, the concentration of vapor in the loop increases rapidly until the pressurized gas entering the condenser exceeds the vapor dew point of 6.1 %. At... 

However, this type of recycle policy has the following disadvantages ... 

Onsite recycling It is the reuse of waste materials at the site of generation. It may be used in the same or another process (like the reuse of cleaning washes and of solvents in the production process). This type of recycling reduces the cost of raw materials as well as the cost of waste disposal. 

There are two types of recycling processes. One is preconsumer recycling and the other is postconsumer recycling. The preconsumer recycling involves raw materials, products, and by-products that have not reached a consumer for an intended end-use but they are typically reused within an original process. The postconsumer-recycled materials are those that have served their intended end-use and have been separated from solid waste for the purpose of recycling. 

Direct sampling of free soil gas requires that a sampling probe be inserted into the ground to collect a soil gas sample. The deeper the penetration, the more difficult and expensive the procedure becomes, eventually requiring that analysis be conducted on drilling fluids or rock samples recovered from a hole. Deeper holes almost always encounter water, which also influences the collection of free gases, forcing one to analyse the gas content of some type of recycled water or mud system which is used to drill the hole. 

Ratios of wood chip from palette, crating or packing material, concrete-form plywood, and others were 18%, 14%, 13%, and 4%, respectively. Mills that can use this type of recycled wood were limited. They selected the waste wood resources according to their own locations. 

There are two types of recycled wood chip obtained from house construction and demolition. These are called P-chip and S-chip (8). P-chip is defined as high quality knife-cut chips obtained from sound timbers or lumbers with a relatively large cross section as used in house construction. P-chips are mainly used in the pulp and paper industry. S-chip is a general term for crushed chips obtained from demolition wood or recycled wood. S-l chip is a high grade chip in for paper or board making. S-2 is for the board industry, and S-3 a low quality chip is for fuel. 

Looking at the shares occupied by different types of recycled HMA in Fig. 5, we find that these proportions have not changed much since the first survey. Table 1 shows the changes in the penetration of old bitumen included in RAP. The penetration of old bitumen is an important factor in the recycled HMA mix design. The average penetration has dropped, if only slightly, between the second and third surveys, and the rate of this change is on the increase. 

Crust-mantle chemical mass-balance models offer important constraints on compositional variations in the mantle, but their constraints on the size of the various reservoirs involved depend critically on uncertainties in the estimates of the bulk composition of the continental crust, the degree of depletion of the complementary depleted mantle, and the existence of enriched reservoirs in Earth s interior, for example, possibly significant volumes of subducted oceanic crust. This last item was left out of the mass-balance models that suggested that the upper and lower mantle are chemically distinct. Chapter 2.03 makes it clear that much of the chemical and isotopic heterogeneity observed in oceanic volcanic rocks reflects various mixtures of depleted mantle with different types of recycled subducted crust. With this realization, and excepting the noble gas evidence for undegassed mantle, some of the characteristics of what was once labeled... 

For cost reasons, even if only a limited selection of process equipment is used, simulating the continuous operation of an entire production line is normally not possible during testing. In those cases where in the actual plant recycle will be produced and returned in one way or another, product is first made from the fresh feed during its evaluation, the expected type of recyclate is produced, and, for further testing, the anticipated amount is mixed with the fresh feed or other material streams, such as, for example, the crusher or screen feeds, to simulate the conditions in a continuously operating system. Installation of a pilot plant on-site and/or in-line should be considered if the risks that are always connected with new installations, are to be minimized. 

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