Value analysis, recycling

A value analysis should be made, to determine how much of the product, or which sections of it, can effectively be recovered. As noted already, the decision may not be wholly on economic groimds. Other non-market factors, such as legislation and image/good-will may also have to be considered. Overall, however, is the fact that, almost irrespective of the value of the materials recovered, the time and therefore the cost of dismantling will usually be disproportionate. Consideration should be given to incinerate rather than recycle when certain materials provide no benefit to the environment particularly cost wise. 

Economic Analysis. The economic success of recycling programs is subject to the following inequaUty where X = the cost to recover recyclable materials, Y = the cost of disposal, and Z = the value of the resource recovered. 

Before Southdown accepts any waste materials for recycling as fuel, a chemical analysis must be performed to identify their chemical composition. Wastes that cannot be blended to meet standards for content, heat value, and compatibility with cement production are not accepted. For instance, cement cannot be made with fuels that have a high chlorine content. 

Table V summarises the data of the sulphur analysis of the hydrocracked liquids and the various bpt fractions for CoMo and NiMo catalysed experiments. The sulphur contents of neither the total hydrocracked liquids nor the individual bpt fractions showed any dependence on repeat contact or catalyst type. The values did show that the sulphur concentrated in the recycle solvent fraction (275-450°C), suggesting that, even under the relatively strong conditions used, certain sulphur-containing compounds will survive to be recycled in the solvent However, the sulphur content of the coal liquid feed was reduced by about 50% and the sulphur content of the likely upgradable product was low.

Refer to Figure GISa (from Ref 14c) for a flow chart of a typical acid prepn plant involving on site tnanuf of nitric, purchase of 65% oleum, sulfuric, and recycling of spent acid the figures are for MA for MG prepn and would be dfferent for NC, TNT, etc. The pre- or semi-mix consists of a blend of 97% nitric and 40% oleum, sulfuric (contg 6% nitric as antifreeze). Analysis of the pre-mix gives the values for the variables in the two formulas. 

Since the total concentration a + r + s follows the time evolution d(a + r + s)/ dt = F - k(a + r + s), it approaches the steady state value F/k with a relaxation time 1 /k. This is a consequence of unbiased outflow (Eq. 47) of all reactants with the same rate k. Consequently, even though we are dealing with an open system under a flow, the analysis is similar to the closed system by replacing the total concentration c with the steady state value F/k. Instead of recycling, therefore, constant supply of the substrate allows the system to reach a certain fixed point with a definite value of the order parameter 0i, independent of the initial condition. 

Inspecting Equation (5.29), we notice that three of the state variables (namely, Mr, My, and Ml) are material holdups, which act as integrators and render the system open-loop unstable. Our initial focus will therefore be a pseudo-open loop analysis consisting of simulating the model in Equation (5.29) after the holdup of the reactor, and the vapor and liquid holdup in the condenser, have been stabilized. This task is accomplished by defining the reactor effluent, recycle, and liquid-product flow rates as functions of Mr, My, and Ml via appropriate control laws (specifically, via the proportional controllers (5.42) and (5.48), as discussed later in this section). With this primary control structure in place, we carried out a simulation using initial conditions that were slightly perturbed from the steady-state values in Table 5.1. 

The optimization variable is the flow rate of the recycled benzene. As a constraint, the outlet reactor temperature is limited at 250 °C. The first term decreases, while the second and third terms increase with higher B /P ratio. As a numerical example, we consider the following prices 72 /kmol (600 /ton) cumene, 0.150 /kWh hot utility (high-temperature thermal fluid), as well as 0.015 /kWh for the generated LP steam. The Aspen Plus optimization routine finds an optimum at a B/P ratio around 7. Note that the optimum is rather flat, but also very sensitive to prices. For lower values of the hot utility (probable) the optimum shifts to the high B/P bound, in this case 10. This analysis demonstrates that the reaction selectivity toward the main product is the key optimization variable. 

From the previous analysis, we conclude that a robust plantwide control structure will fix the combined isobutane + recycle (Fj) and fresh butene flow (F0), as illustrated in Figure 9.4. The desired production rate and selectivity could be achieved in a 3-m3 reactor, operated at 268 K. The operating point shows low sensitivity to errors in the manipulated variable Fj (Figure 9.5). This design seems to ensure feasible operation even if the temperature decreases to 260 K (Figure 9.6) or the catalyst activity becomes 40% of the initial value (Figure 9.7), irrespective of the purity of the butene feed stream (Figure 9.8). 

In the above analysis, it was assumed that product prices and the value of products as a function of their impurity content are well defined. Often, this is not so, for example, when one of the products is recycled to a reactor. In this case, the product value needs to be calculated as a function of its impurity content from data on the expected effect of impurity and recycle on the reactor performance. This analysis can become complicated. 

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