Equipment for Recycling

Equipment for recycle operations differs from conventional HPLC equipment. For a recycle system to be useful, the extracolumn band spreading must be small relative to the band spreading of the column. This involves the solvent delivery system, transport tubing, and detector(s). Also, because a recycle system is a closed system with a finite volume, the operator must be aware that fast-moving materials could eventually overtake slower-moving materials and remix. To prevent peak overlap, a means must be provided to allow the operator to remove a portion of the sample components before overlap can occur. 

Catalytic reduction can be carried out in batches in steel or, preferably, in stainless-steel kettles equipped with agitators or in towers packed with a catalyst and equipped for recycling operation. Gas-liquid and liquid-liquid separators are required, as well as filters or centrifuges to remove the catalyst, which may be used as such or deposited on inert carriers. Where necessary, purification is carried out, as by fractionation in columns. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Equipment. Partial-oxidation gasification section equipment in many plants consists essentially of (/) the gasification reactor (2) the waste-heat exchanger for heat recovery from the hot reactor gas or direct quench system (J) the economizer heat exchanger for further heat recovery (4) the carbon removal system for separating carbon from the reactor product gas and (5) the carbon recovery system for recycle of carbon. 

Solvent Process. In the solvent process, or solvent cook, water formed from the reaction is removed from the reactor as an a2eotropic mixture with an added solvent, typically xylene. Usually between 3 to 10 wt % of the solvent, based on the total charge, is added at the beginning of the esterification step. The mixed vapor passes through a condenser. The condensed water and solvent have low solubiUty in each other and phase separation is allowed to occur in an automatic decanter. The water is removed, usually to a measuring vessel. The amount of water collected can be monitored as one of the indicators of the extent of the reaction. The solvent is continuously returned to the reactor to be recycled. Typical equipment for this process is shown in Figure 2. The reactor temperature is modulated by the amount and type of refluxing solvent. Typical conditions are ... 

The calcium cyanamide feed is weU mixed with the recycled slurry and filtrate ia a feed vessel. The calcium cyanamide is added at a rate to maintain a pH of 6.0—6.5 ia the cooling tank. The carbonation step can be conducted ia a turbiae absorber with a residence time of 1—2 min. After the carbonation step, the slurry is held at 30—40°C to complete the formation of calcium carbonate, after which the slurry is cooled and filtered. AH equipment for the process is preferably of stainless steel. The resulting solution is used directiy for conversion to dicyandiamide. 

Figure 3 shows a simple schematic diagram of an oxygen-based process. Ethylene, oxygen, and the recycle gas stream are combined before entering the tubular reactors. The basic equipment for the reaction system is identical to that described for the air-based process, with one exception the purge reactor system is absent and a carbon dioxide removal unit is incorporated. The CO2 removal scheme illustrated is based on a patent by Shell Oil Co. (127), and minimises the loss of valuable ethylene in the process. 

Peclet number independent of Reynolds number also means that turbulent diffusion or dispersion is directly proportional to the fluid velocity. In general, reactors that are simple in construction, (tubular reactors and adiabatic reactors) approach their ideal condition much better in commercial size then on laboratory scale. On small scale and corresponding low flows, they are handicapped by significant temperature and concentration gradients that are not even well defined. In contrast, recycle reactors and CSTRs come much closer to their ideal state in laboratory sizes than in large equipment. The energy requirement for recycle reaci ors grows with the square of the volume. This limits increases in size or applicable recycle ratios. 

Heavy-walled drums once used for lead antiknock chemicals have been used for water storage or as barbecue pits, with subsequent risk to the user from residual toxic material. Equipment from the industry cited has for many years been cleaned, cut up, and sent under supervision to steel mills for recycle to eliminate the possible misuse of scrapped containers. 

Typically, waste streams are either disposed of or forwarded to process sinks (equipment) for lecycle/reuse. In case of hnal discharge, the taiget composition of the undesirable species in each waste stream corresponds to the environmental regulations. On the other hand, if the intercepted waste stream is to be recycled to a process sink, its target composition should satisfy the constraints imposed by the process sink. 

When a mixture of compounds is to be treated, more limitations may be placed upon the selection of a suitable abatement method. There may be several compounds in the waste gas, some being unsuitable to one method, while others are unsuitable to another method. In such cases, thermal incineration may be the best solution. When recovering mixtures, additional separation equipment may be needed for recycling the reclaimed compounds. 

If the catalytic HBr oxidation reactor is required to serve as a central facility for recycling a variety of waste HBr streams and conditions that combust all of the organic contaminants cannot be discovered, then further bromine purification operations are probably required. The simplest operation is distillation of the bromine. Due to the high bromine vapor pressure, bromine distillation can be accomplished using relatively small equipment. This is expected to be a highly effective method of purification, particularly where the boiling points of any contaminants are greater than 10°C different from that of bromine. In other applications, absorption or extraction may be needed. 

SafeChem, a subsidiary of Dow, has developed a handling system for chlorinated solvents that allows them to be used in closed-loop degreasing systems. The Safe-Tainer system uses two dedicated double wall containers one to hold fresh solvent and the other used solvent. The containers are connected to the cleaning equipment with zero dead volume, leak-free connections that prevent spills, leaks or vapour emissions during use. Used solvent is collected for recycling and professional disposal of any residues. The system minimises solvent use and release to the environment. A study carried out by Dow during a trial in... 

If the program continues and additional reductions are desired, more expensive and more complex projects begin to emerge (Phase II). These are often associated with equipment modifications, process modifications and process control and may include the addition or adaptation of auxiliary equipment for simple source treatment, possibly for recycle. This phase usually has little immediate ROI, and more inclusive approaches to assessing the economics of the operation (estimating costs for waste handling, long-term liability, risk) are needed to justify the continued pollution-prevention operation. 

Shredded circuit boards. Circuit boards are metal boards that hold computer chips, thermostats, batteries, and other electronic components. Circuit boards can be found in computers, televisions, radios, and other electronic equipment. When this equipment is thrown away, these boards can be removed and recycled. Whole circuit boards meet the definition of scrap metal, and are therefore exempt from hazardous waste regulation when recycled. On the other hand, some recycling processes involve shredding the board. Such shredded boards do not meet the exclusion for recycled scrap metal. In order to facilitate the recycling of such materials, U.S. EPA excluded recycled shredded circuit boards from the definition of solid waste, provided that they are stored in containers sufficient to prevent release to the environment, and are free of potentially dangerous components, such as mercury switches, mercury relays, nickel-cadmium batteries, and lithium batteries. 

When recycling material to the reactor for whatever reason, the pressure drop through the reactor, separator (if there is one), the heat transfer equipment upstream and downstream of the reactor, control valves, and so on must be overcome. This means the pressure of any material to be recycled must be increased. Again, for the case of a liquid recycle, the cost of this pressure increase is usually small. On the other hand, to increase the pressure of material in the gas phase for recycle requires a compressor and is expensive. 

Convention on Control of Transboundary Movements of Hazardous Wastes and their Disposal and 69 ratified the ban on all kinds of hazardous waste export from wealthy OECD-countries to non-OECD countries, large amounts of waste electrical and electronic equipment (WEEE) are shipped overseas for recycling, the majority to China as reported by Brigden et al. [2] and Puckett et al. [3], lesser quantities to India and Western Africa reported by Kuper and Hojsik [4]. WEEE contains a variety of harmful substances like endocrine disruptors and persistent organic pollutants (POPs). Additionally, hazardous substances may be formed during informal recycling. This often practised informal treatment without proper equipment for metal extraction and labour safety heavily affects the environment and human health of workers and the inhabitants of whole stretches of land. 

If A has significant economic value then it should be separated from the reactor effluent stream and recycled for subsequent use. Since the conversion level is higher in the plug flow reactor, the recycle rate will be much smaller and the demands on the separation equipment for reclaiming species A will also be somewhat smaller. Even when species A is of relatively little economic value, there may be circumstances when the costs associated with meeting the pollution control requirements for the process effluent will dictate separation and recycle of this reactant as the most economic alternative. 

An exact calculation of inventory is difficult in the conceptual design phase, since the size of equipment is not usually known. The mass flows in the process are however known from the design capasity of the process. Therefore it is practical to base the estimation of inventory on mass flows and an estimated residence time. Consequently the inventory has been included to the ISI as a mass flow in the ISBL equipment including recycles with one hour nominal residence time for each process vessel (e.g. reactor, distillation column etc). For large storage tanks the size should be estimated. The total inventory is the sum of inventories of all process vessels. 

The German Environmental Ministry is reported to have attacked European proposals to ban substances such as specific flame retardants in forthcoming regulations for recycling electrical and electronic equipment. Initial proposals from the EU Commission on the disposal of waste electrical and electronic equipment, include a phaseout of PBDEs, despite preliminary findings under EU risk assessment that there is no need for risk reduction from the two types, decaBDE and octaBDE mainly used in such equipment. The Ministry is said to be concerned at the excessively prescriptive and restrictive system being proposed, and that substance restrictions should not be addressed in waste legislation, but should be based on life cycle risk assessments. 

For recycling to improve the performance of an MCFC network, it must provide benefits that outweigh its inherent disadvantages. If carbon dioxide is not separated from the anode-anode recycle, the concentration of carbon dioxide in the anode is increased. This reduces the Nemst potential. The Nemst potential is similarly reduced by the anode-cathode recycle if steam is not condensed out, since recycled steam dilutes reactant concentrations in the oxidant. In addition, part of the power generated by the network is consumed by the equipment necessary to circulate the recycle streams. Such circulation equipment, along with the additional ducting required by recycling, also increases the capital cost of the MCFC network. 

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