Processing recovery, systems approach

P is the partial pressure of the gases at the reformer exit. Tr is usually less than 10-15°C throughout the catalyst lifetime. The thermal efficiency of a tubular steam reformer with waste heat recovery system approaches 95% of which 50-60% is transferred to the process. " The remaining heat is recovered by production of steam, preheating of air, feedstock, etc. 

In this approach accident cases and design recommendations can be analysed level by level. In the database the knowledge of known processes is divided into categories of process, subprocess, system, subsystem, equipment and detail (Fig. 6). Process is an independent processing unit (e.g. hydrogenation unit). Subprocess is an independent part of a process such as reactor or separation section. System is an independent part of a subprocess such as a distillation column with its all auxiliary systems. Subsystem is a functional part of a system such as a reactor heat recovery system or a column overhead system including their control systems. Equipment is an unit operation or an unit process such as a heat exchanger, a reactor or a distillation column. Detail is an item in a pipe or a piece of equipment (e.g. a tray in a column, a control valve in a pipe). 

Practical conversion processes can only approach the theoretical efficiencies shown in Table 3. The coal conversion reactions do not proceed to completion at ambient temperatures within practical time limitations. As a result, a portion of the coal feedstock must be burned to supply heat so that the reactions can be carried out at elevated temperatures and pressure where the rates of conversion are rapid. In practical systems, this additional heat can only be partially recovered. Consequently, practical conversion processes have actual heat recovery efficiencies of about 60-70% for production of high H/C ratio products. Production of secondary fuels having somewhat lower H/C ratio, i.e. about 2.0, permits attainment of heat recovery efficiencies of 70 to 80j. 

The entire system is based on a tiered approach where three layers of technology are integrated into the overall treatment system, as illustrated in Chart 2. First, a distributed process control system is network linked to the various component subunits of the waste management system such as pH control, ion-exchange control, tank level control, etc. Next, are the recovery/treatment processes themselves. The final tier is a monitoring system which controls both the performance of the treatment systems and the discharge assurance of the plant effluent... 

Process Synthesis, an important research area within chemical process design, has triggered during the last three decades a significant amount of academic research work and industrial interest. Extensive reviews exist for the process synthesis area as well as for special classes of problems (e.g., separation systems, heat recovery systems) and for particular approaches (e.g., insights-based approach, optimization approach) applied to process synthesis problems. These are summarized in the following ... 

Influence of the downstream plants. Up to now, we have regarded the coal gasification reactor with the waste heat recovery system as an isolated unit. In the event that the gas generated is intended to be used as fuel gas, for example in a combined power station, this approach is justified. If, however, the gas is to be used as synthesis gas, the effect of the downstream units must be taken into consideration. In such cases it is necessary to feed the gas to a CO shift conversion unit in order to obtain the C0/H2 ratio required for the synthesis process. Apart from gasification at atmospheric pressure, which requires an intermediate compression step, it has proved advisable to locate the CO shift conversion directly downstream of the gasification section. A stage in which dust particles are removed from the gas is situated between these two units. It is assumed that exergy losses do not occur in this unit. 

A portion of the sodium carbonate melt is withdrawn from the molten salt reactor, quenched, and processed in an aqueous recovery system. The recovery system removes the ash and inorganic combustion products (mainly sodium salts such as NaCl and Na2S) retained in the melt. Unreacted sodium carbonate is returned to the molten salt furnace. The ash must be removed when the ash concentration in the melt approaches 20 to 25 wt % in order to preserve the melt fluidity. The inorganic combustion products must be removed before all of the sodium carbonate is completely converted to noncarbonate salts. For the case of a waste containing a valuable mineral resource, the valuable mineral resource is retained in the melt during the gasification process and may be recovered as a by-product of the regeneration process. 

In the past, the retrofit methods based on the proeess pineh concept was widely used (Tjoe and Linnhoff, 1986 Policy etal., 1990 Shokoya and Kotjabasakis, 1991). However, the process pinch is developed for grassroots design and is fundamentally irrelevant to the retrofit scenario. The method (Zhu and Asante, 1999) to be discussed here is a two-stage approach identify the network pinch, the true bottleneck of an existing heat recovery system, and determine modihcations with minimum capital costs to overcome the network pinch— that is the reason why this method is called the network pinch method. 

None of the above patents address the crucial solvent recovery issues. While recovery of acetone is possible, re-creating concentrated phosphoric acid from either dilute solutions or phosphates appears as difficult as the recovery of sulphuric acid from sulphates in the viscose process. The viscose approach is of course to extract and sell the neutralization product and use fresh concentrated sulphuric acid to make up the recovery shortfall— an option that is apparently imattractive in phosphate systems. 

The removal of aromatics and relatively heavy hydrocarbons from gas streams with fixed beds of activated carbon is essentially the same process as. solvent recovery, and similar adsorbents and equipment are used. The principal differences are that in hydrocarbon recovery the feed is typically a natural gas or other combustible gas stream rather than air, and adsorption is usually (but not always) conducted at elevated pressure. The basic design approach for hydrocarbon recovery systems follows the same general logic as that described for solvent recovery systems. A brief outline of the key design steps is given in the Calgon Carbon Corporation bulletin, Heavy Hydrocarbon Removal or Recovery from Gas Streams (1987). 

Output-pulled versus input-pushed Products, processes, and systems should be output-pulled rather than input-pushed through the use of energy and materials. This approach is based on Le Chatelier s principle, which states that when a stress is applied to a system at equilibrium, the system readjusts to relieve or offset the applied stress. Stress is defined as temperature, pressure, or concentration gradient. It is possible to increase the productivity of a process by overwhelming the feed with a specific reactant. That leads to incredible excess of the reactant in the product stream, which requires separation, recovery, and recycle. On the other hand, if you can remove a product from the process stream as it is produced, then the reaction will drive to greater quantities of product, without any need for use of excess reactants. 

Electrolysis. Electrowinning of zirconium has long been considered as an alternative to the KroU process, and at one time zirconium was produced electrolyticaHy in a prototype production cell (70). Electrolysis of an aH-chloride molten-salt system is inefficient because of the stabiUty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stabiUty of in solution, decreasing the concentration of lower valence zirconium ions, and results in much higher current efficiencies. The chloride—electrolyte systems and electrolysis approaches are reviewed in References 71 and 72. The recovery of zirconium metal by electrolysis of aqueous solutions in not thermodynamically feasible, although efforts in this direction persist. 

In comparison with classical processes involving thermal separation, biphasic techniques offer simplified process schemes and no thermal stress for the organometal-lic catalyst. The concept requires that the catalyst and the product phases separate rapidly, to achieve a practical approach to the recovery and recycling of the catalyst. Thanks to their tunable solubility characteristics, ionic liquids have proven to be good candidates for multiphasic techniques. They extend the applications of aqueous biphasic systems to a broader range of organic hydrophobic substrates and water-sensitive catalysts [48-50]. 

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