Rerunning operations

Rerunning operations are characterized by large volumes of distillate products and relatively small residue yields. Frequently, the product is withdrawn as a sidestream with undesirable light fractions passing overhead and polymers being withdrawn from the bottom of the tower. Lube rerun stills usually have several sidestreams which permit close control of flash point and viscosity while producing a wide range of stocks. 

Next came the use of pipestills and bubble towers where the vapor in the tower bubbles through the liquid on the plates in the tower. Bubble towers allow fractionation to take place while a mixture of rising vapor is scrubbed by a stream of falling oil. Bubble towers are used in topping plants, rerun operations, cracking plants and natural gas stabilization. 

A large amount of spent alkylation add is becoming available to the industry and it has a strength of 90 to 93 per cent because of inactive hydrocarbons and certain oxidation products contained in it. It has been found satisfactory for the treating of naphthas, furnace oils, and lubricating oils (if a subsequent rerunning operation is conducted). Hesults with wax and other special uses have not been consistent, but ways of utilizing the acid have been devised by most refiners. 

Cracking imposes an additional penalty in a vacuum unit in that it forms gas which cannot be condensed at the low pressures employed. This gas must be vented by compressing it to atmospheric pressure. This is accomplished by means of steam jet ejectors. Ideally, it would be possible to operate a vacuum pipe still without ejectors, with the overhead vapors composed only of steam. In practice, however, leakage of air into the system and the minor cracking which occurs make it necessary to provide a means of removing non-condensibles from the system. In addition to the distillation of atmospheric residuum, the lube vacuum pipe still is also used for rerunning of off specification lube distillates. 

Rerunning is a second distillation step applied to distillate stocks in order to remove undesirable higher boiling materials from the product. These materials may be present because of poor fractionation in the primary distillation step more frequently they are heavy polymers formed in treating operations. Rerunning may be combined with solvent removal, as in the case of heavy lube distillates which are diluted with naphtha to lower their viscosity before being chemically treated. 

The C4 Olex process is designed with the full allotment of Sorbex beds in addition to the four basic Sorbex zones. The C4 Olex process employs sufficient operating temperature to overcome diffusion limitations with a corresponding operating pressure to maintain liquid-phase operation. The C4 Olex process utilizes a mixed paraffin/olefin heavy desorbent. In this case it is an olefin/paraffin mix consisting of n-hexene isomers and -hexane. A rerun column is needed to remove heavy feed components such as Cs/C because they would contaminate or dilute the hexene/hexane desorbent. Table 8.5 contains the typical feed and product distributions. 

The process is reported capable of dewaxing any wax-bearing stock, either raw or after selective solvent refining, and waxes with a relatively low oil content may be obtained, by a two-stage operation in which the wax from the primary stage is rerun after further dilution with cold solvent on a second battery of centrifuges. 

Typical Motor Alkylate from, HF Alkylation Unit Operating without Refrigeration and without Rerun Column... 

Coking in reboilers and rerun columns is caused in many cases by operating at excessive temperatures. In these cases, the difficulty can be reduced and sometimes be eliminated by reducing the temperature of the reboiler. Temperatures of 400°F. and above should be avoided. The breakdown of esters and other sulfur compounds in the rerun reboiler usually results in the production of corrosive alkylate and corrosion in the rerun column. A reduction in the reboiler temperature is sometimes sufficient to clear up this situation, but in some cases, an injection of mono- or triethanolamine into the feed is required. 

The essential equipment requirements for a tetramer unit are a combined feed-charge drum, charge pump, catalyst chamber, depropanizer, recycle column, rerun column, and attendant pumps and exchangers. Figure 1 shows such a unit as designed to produce tetramer and cumene alternatively. Table VII shows yields from the tetramer operation when a 350°-485°F. boiling-range tetramer is produced. 

The equipment required for the production of cumene is almost identical to that shown in Fig. 1. The recycle column takes the recycle benzene overhead while the cumene is removed as an overhead product of the rerun column. Table VIII shows typical yields from cumene operation. 

For the sake of comparison the case 2 problem of Table 6.4 was rerun with the same product specifications on the main-cuts but without any off-cut production (the STN for this operation is shown in Figure 6.7). 

If the chromatographic peak separation between 13Ci2-2,3,7,8-TCDD and 13Ci2-l,2,3,4-TCDD is not resolved with a valley of <. 25% on the DB-5 (or equivalent) column, or 2,3,7,8-TCDD is not resolved from the closest eluting isomer with a valley of 25% on the SP-2331 (or equivalent) column, then the Contractor shall adjust the GC/MS operating conditions and rerun the affected sample. This criterion applies to sample analyses. If this criterion is not met for a calibration standard, all associated samples must be rerun. 

A pump designated to operate at 50% of its best efficiency point, when provided with 3 ft of NPSH, was tested at a constant value of NPSH and found to operate perfectly. The pump test was then rerun in the presence of a witness. This time, however, the pump failed completely. 

In-line analysis and control systems that prevent the rerunning of a nonspecification product are safer and certainly better way to run a complex operation. While samples are still taken and analyzed in the laboratory, the research is mostly used for calibration purposes and to check online analysis. 

The type of metal you purchase for your shearing operation can prohibit part quality as much as a poorly adjusted machine. If your sheared parts are inaccurate after you have checked the above nine tips for accurate shearing, the material used is probably the cause of the inaccuracies. Inferior metal that is full of stresses, buckled sheets, second stock, or rerun steel cannot be accurately sheared as first-quality material. 

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