Lower boiling feeds

The usual feed is a virgin gas oil that is, the part of crude oil boiling between about 60 °F. and 1050°F. Sometimes material below 600°F will be included into the cat feed but more often, it is put into diesel fuel or home heating oil. The heavy material above 1050°F is not normally used as cat feed because it often contains metallic compounds that contaminate the catalyst. Even if metals are not present, there are sometimes tarry materials that end up on the catalyst. This deposit increases the load on the regenerator, and, hence, the 1050 °F+ material is less desirable than lower boiling feeds. 

Figure 33.10 Distillation column and control volumes for the rectification and stripping section (x and y refer to lower-boiling feed component A, i.e., to Xa and yA in the text).

Another method for separating very close boiling components takes advantage of the chemical dissimilarity of the feed compounds. In extractive distillation, the other component (solvent) that is added to the column is a high-boiling liquid that alters the relative volatilities between feed components. While normally the lower boiling feed components... 

The olefin product contains 1.1% of residual / -paraffins. Essentially similar results have been obtained in commercial operations on Cg—C q and C g feedstocks. The desorbents used are generally hydrocarbon mixtures of lower boiling range than the feed components. The concentrated olefin stream may then be used for production of detergent alcohols. 

Of the main reactions, aromatization takes place most readily and proceeds ca 7 times as fast as the dehydroisomerization reaction and ca 20 times as fast as the dehydrocyclization. Hence, feeds richest in cycloparaftins are most easily reformed. Hydrocracking to yield paraffins having a lower boiling point than feedstock proceeds at about the same rate as dehydrocyclization. 

An efficient feed injection system produces extremely small droplets that vaporize quickly. Rapid vaporization minimizes the amount of non-vaporized hydrocarbons that block the active sites. An effective feed nozzle system must instantaneously vaporize and crack asphaltenes and polynuclear aromatics to lower boiling entities. 

As long as this middle section operation leaf intersects with those for the top section (above the entrainer feed) and the bottom section (below the feed point for the feed mixture), the column design will be feasible. Note that there will always be a maximum reflux ratio, above which the separation will not be feasible because the profiles in the top and bottom sections will tend to follow residue curves, which cannot intersect. Also, the separation becomes poorer at high reflux ratios as a result of the entrainer being diluted by the reflux of lower boiling components. 

The catalyst components are generally dissolved in methyl acetate which acts as both reactant and solvent. Other solvents may be used and in fact, upon several batch recycles where lower boiling products are distilled off, the solvent is an ethylidene diacetate-acetic acid mixture. Any water introduced in the reaction mixture will be consumed via ester and anhydride hydrolysis, therefore anhydrous conditions are warranted. Typical batch reaction examples are presented in Table 1. There is generally sufficient reactivity when carbon monoxide and hydrogen are present at 200-500 psi. Similar results were obtained from the pilot plant using a continuous stirred tank reactor (CSTR). The reaction can also be run continuously over a supported catalyst with a feed of methyl acetate, methyl iodide, CO, and hydrogen. 

With 5% nCg in feed, conversion of the nonane was less than 5% with either the 40 S/A or 525 S/A additives. If less than 10% additive had been used, the conversion of nC9 due to the additive would have been almost undetectable. Since the rate of cracking of lower boiling paraffins is likely to be lower than that of nC9, those components should also be unaffected. This suggests that any octane enhancement observed with the additives has little to do with cracking of n-paraffins. 

This chapter reports results of applying a catalytic hydrorefining process to four coal liquids solvent-refined coal (SRC) process filter feed, SRC extract product, Synthoil, and H-Coal process hydroclone underflow. The achieved upgrading is evaluated in terms of reduction in benzene and heptane insolubles, reduction in sulfur, nitrogen, and oxygen, an increase in hydrogen content, and a yield of lower boiling products. 

Of the 29% SRC contained in the feed, a considerable amount was converted to lower boiling materials. However, an unambiguous product distribution is difficult to establish because of the preponderance of process solvent in the feed. Therefore, SRC was hydrotreated without addition of extraneous solvent. 

Several factors determine the best feeds for catalytic crackers. Heavy feeds are preferred thus, the lower boiling point is about 650°F. The feed should not be so heavy that it contains an undue amount of metal-bearing compounds or carbon-forming material. Deposition of metals and coke can quickly deactivate the catalyst. 

In the second method a part of the condensate is returned (known as reflux) so that it comes in contact with the rising vapor on its way to the condenser. This technique is known as rectification. The efficiency of separation depends on the ratio of condensate returned to the still to the liquid product removed (which is known as the reflux ratio). The vapor and the reflux are made to come in intimate contact in a tall column located on top of the still. In the column the low boiler (liquid with a lower boiling point) spontaneously diffuses from the liquid reflux to the vapor, while the high boiler (the liquid with the higher boiling point) diffuses from vapor to liquid. So as the vapor rises in the column, it becomes enriched in low boiler and, as the liquid descends the column, its content of high boiler increases. A reboiler is provided at the bottom to vaporize the liquid and feed it up the column (Fig. 3.5). 

The natural relative volatility of the system is enhanced when the activity coefficient of the lower-boiling pure component is increased by the solvent addition (Yl/Th increases and Pl /Pu > 1). In this case, the lower-boiling pure component will be recovered in the distillate as ej ected. In order for the higher-boiling pure component to be recovered in the distillate, the addition of the solvent must decrease the ratio Tl/Th such that the product of the and Pl Ph (ie., a n) in the presence of the solvent is less than unity. Generally, the latter is more difficult and requires higher solvent-to-feed ratios. It is normally better to select a solvent that forces the lower-boiling component overhead. 

Higher-boiling feed stocks are more readily cracked and, therefore, heavy gas oils require less-severe operating conditions than light gas oils. This requirement is met in Houdry fixed-bed units by the use of lower oil partial pressure or lower catalyst activity, and in moving-bed and fluid units by the use of higher space velocity, lower temperature, or lower catalyst/oil ratio. 

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