Recycle operation fractionated

Conversion is the fraction of reactant carbon monoxide and/or hydrogen converted to other compounds. It is generally expressed in terms of the fresh feed components, but sometimes in terms of the total feed components for recycle operation. 

Utilities savings for a once-through vs. recycle operation arise from lower pumping and compression costs as a result of the possible lower design pressure and also of lower hydrogen consumption. Additional savings are realized as a result of the lower oil and gas circulation rates required, because recycle of oil from the fractionator s bottom is not necessary. 

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. 

Experiments have recently been completed by Kummer, DeWitt, and Emmett (75), using C as a tracer in the synthesis on an iron catalyst. The results are inconclusive. If the total catalyst surface is uniformly active in the synthesis, the results show that only a small fraction of the reaction proceeds by way of the carbide. However, if only occasional active patches of the surface are participating in the synthesis, then it is possible to interpret the results as indicating that all of the reaction proceeds by way of the carbide. The carbide intermediate hypothesis for the mechanism of the synthesis on iron catalysts, however, is probably incorrect. Thus, the results of recycle operations at low temperatures on iron catalysts show that alcohols are formed earlier in the synthesis than olefinic hydrocarbons. 

A possible answer to these issues could be the preventive separation of the different plastics prior to processing. Actually, since recycling has attracted the interest of industry, the separation processes are becoming more numerous and efficient achieving almost total segregation of the different plastic fractions. However, despite the improvement of the effectiveness of the pretreatment steps, there is still the possibility to find other polymers as contaminants in a main stream, and even at those low amounts, they can disturb the regularity of the recycling operation. 

Owing to the fast catholyte flow rate necessary to avoid side reactions, the conversion of acrylonitrile to adiponitrile per pass of the cathoylte through the cell was only 0.2%. This necessitated the use of an external reservoir and determined the mode of operation of the process, i.e. batch recycle. A fraction of the reservoir was bled continuously into the extraction plant and further... 

Clearly, the time chart shown in Fig. 4.14 indicates that individual items of equipment have a poor utilization i.e., they are in use for only a small fraction of the batch cycle time. To improve the equipment utilization, overlap batches as shown in the time-event chart in Fig. 4.15. Here, more than one batch, at difierent processing stages, resides in the process at any given time. Clearly, it is not possible to recycle directly from the separators to the reactor, since the reactor is fed at a time different from that at which the separation is carried out. A storage tank is needed to hold the recycle material. This material is then used to provide part of the feed for the next batch. The final flowsheet for batch operation is shown in Fig. 4.16. Equipment utilization might be improved further by various methods which are considered in Chap. 8 when economic tradeoffs are discussed. 

Ethylene Stripping. The acetylene absorber bottom product is routed to the ethylene stripper, which operates at low pressure. In the bottom part of this tower the loaded solvent is stripped by heat input according to the purity specifications of the acetylene product. A lean DMF fraction is routed to the top of the upper part for selective absorption of acetylene. This feature reduces the acetylene content in the recycle gas to its minimum (typically 1%). The overhead gas fraction is recycled to the cracked gas compression of the olefin plant for the recovery of the ethylene. 

Recycling of HDPE. Polyolefins, including HDPE, are the second most widely recycled thermoplastic materials after PET (110). A significant fraction of articles made from HDPE (mostly bottles, containers, and film) are collected from consumers, sorted, cleaned, and reprocessed (110—113). Processing of post-consumer HDPE includes the same operations as those used for virgin resins blow mol ding, injection molding, and extmsion. 

The solvent is 28 CC-olefins recycled from the fractionation section. Effluent from the reactors includes product a-olefins, unreacted ethylene, aluminum alkyls of the same carbon number distribution as the product olefins, and polymer. The effluent is flashed to remove ethylene, filtered to remove polyethylene, and treated to reduce the aluminum alkyls in the stream. In the original plant operation, these aluminum alkyls were not removed, resulting in the formation of paraffins (- 1.4%) when the reactor effluent was treated with caustic to kill the catalyst. In the new plant, however, it is likely that these aluminum alkyls are transalkylated with ethylene by adding a catalyst such as 60 ppm of a nickel compound, eg, nickel octanoate (6). The new plant contains a caustic wash section and the product olefins still contain some paraffins ( 0.5%). After treatment with caustic, cmde olefins are sent to a water wash to remove sodium and aluminum salts. 

Oxidation of cumene to cumene hydroperoxide is usually achieved in three to four oxidizers in series, where the fractional conversion is about the same for each reactor. Fresh cumene and recycled cumene are fed to the first reactor. Air is bubbled in at the bottom of the reactor and leaves at the top of each reactor. The oxidizers are operated at low to moderate pressure. Due to the exothermic nature of the oxidation reaction, heat is generated and must be removed by external cooling. A portion of cumene reacts to form dimethylbenzyl alcohol and acetophenone. Methanol is formed in the acetophenone reaction and is further oxidized to formaldehyde and formic acid. A small amount of water is also formed by the various reactions. The selectivity of the oxidation reaction is a function of oxidation conditions temperature, conversion level, residence time, and oxygen partial pressure. Typical commercial yield of cumene hydroperoxide is about 95 mol % in the oxidizers. The reaction effluent is stripped off unreacted cumene which is then recycled as feedstock. Spent air from the oxidizers is treated to recover 99.99% of the cumene and other volatile organic compounds. 

Selection of Fractionator 11 gives pure hexane, which can be recycled to Mixer 1. The distillate Dll, however, is a problem. It cannot be distilled because of its location next to a distillation boundary. It is outside of the two-phase region, so it cannot be decanted. In essence, no further separations are possible. However, using the Recycle heuristics, it can be mixed into the MSA recycle stream without changing the operation of Mixer 1 appreciably. However, as both outlet streams are mixed together. Fractionator 11 is not really needed. The mixture of hexane and isopropanol, 07, could have been used as the MSA composition in the first place. 

The final loose end in the process is the aqueous decanter product, A7. The hexane must be removed before the mixture can be sent to wastewater treatment, ie, accepted as a water by-product. Two opportunistic separations. Fractionators 12 and 13, are possible. Selection of Fractionator 13 gives pure water underflow, and a distillate similar to D5. Distillate D13 can be recycled back and mixed with D5 without affecting the operation of Mixer 1. AH streams are processed and the flow sheet produces both desired products (Fig. 5b). 

In one possible sequence the MSA composition is chosen as water-saturated methylene chloride expected to be regenerated by decantation. The boundary-crossing strategic operation is to mix the feed with the MSA. The resulting two-phase mixture is opportunistically fractionated to produce the 2-propanol product as bottoms, and a mixture of water—methylene chloride as distillate. This distillate is opportunistically decanted to recover water-saturated methylene chloride MSA for recycle. The aqueous decanter phase is the water product, which optionally may be further purified by... 

---