Pump Around Packie Curves

Figure 2.5. Vapor and liquid traffic for Types U, R and A atmospheric towers.

10 Petroleum Refinery Distillation

A typical light hydrocarbon separation is that of fractionating between propane and n-butane. This problem requires that the designer provide a sufficiency of trays and reflux to satisfy the composition specifications. Satisfying the trays-reflux requirements depends upon the specified separation requirements of the process and the inherent physical chemistry of the system involved. Talking in terms of a binary system, two terms which will be used later to clarify the language used in heavy oil work must be defined.

Degree of separation can be defined in terms of product purities or in terms of component recoveries. The greater the degree of separation, the greater will be the recovery of the light component in the distillate and the heavy component in the bottoms. This will result in higher product purities.

Degree of difficulty of separation is defined as the relative difficulty encountered in separating the two compounds in question, regardless of the purity requirements set by the process specifications. In light ends terminology, it may be considered as inversely proportional to the relative volatility between the two components.

From one's past experience, the following become obvious.

1. For a given system, the tray requirements increase markedly as purity requirements become greater, but reflux requirements increase only a small amount once a relatively high purity level is reached.

2. For a fixed separation, tray and reflux requirements increase as the relative volatility decreases, i.e., the separation becomes more difficult. For example, the propane and n-butane separation is easier than propane-propylene but more difficult than propane and n-pentane.

Speaking qualitatively, at reflux conditions exceeding minimum requirements, tray requirements are directly proportional to the required degree of separation and to the degree of difficulty of separation inherent in the physical-chemical system under consideration. Conversely, for a fixed number of trays, reflux requirements are directly proportional to the degree of difficulty inherent in the system and, to a somewhat lesser extent, to the required degree of separation.

The above terminology is the author's own. However, it is straightforward and, when applied to any distillation system involving discrete components, will result in a rapid qualitative assessment of tray-reflux requirements.

In the refinery, two terms are used to discuss product composition and the degree of separation between adjacent fractions. ASTM boiling range defines the general composition of the fraction and is usually one of the key specifications for most distillates from both the atmospheric tower and the vacuum tower.

The second term, (5-95) Gap, defines the relative degree of separation between adjacent fractions. It is determined by subtracting the 95 volume percent ASTM temperature of a fraction from the 5 volume percent ASTM temperature of the adjacent heavy fraction.

Packie's (2) classic paper was the first to disclose criteria for defining fractionation between atmospheric tower distillate streams. Figure 2.6 is Packie's curve for fractionation between the overhead fraction and the adjacent side-stream.

The nomenclature for this correlation is as follows:

1. Lj^. = gallons per hour reflux from the top tray measured as 60 degree F liquid.

2. Dj^ = gallons per hour total distillates (vapor and liquid) to top tray, measured as 60 degree F liquid.

3. Nj = number of actual trays in the section, i.e., trays M through N inclusive = N - (M - 1) = N - M + 1. Note that each tray in pumparound heat removal service counts as one-third of an actual tray.

Figure 2.7 is Packie's curve for fractionation between sidestream products. The nomenclature for this correlation is as follows:

1. Lj^ = gallons per hour reflux from the upper draw tray measured as 60 degree F liquid.

2. Pj^ = gallons per hour total product vapors, measured as 60 degree F liquids, to the upper draw tray, i.e., stream Dj^ plus all lighter products.

3. Nj = number of actual trays in the section, i.e., M through (N — 1) inclusive = N — M. Note that each tray in pumparound heat-removal service counts as one-third of an actual tray.

4. At(50%) = (ASTM 50 percent temperature of the lower sidestream product, D^j) - (ASTM 50 percent temperature of the total products lighter than D^).

Figure 2.7 does not apply to fractionation between the lowest sidestream and the bottoms stream, nor does it apply to vacuum fractionation although it is often used for the latter purpose due to lack of anything better. Note that these curves apply only for the case where steam stripping of sidestreams is practiced at rates of at least 0.2 pounds steam per gallon of product (8.4 pounds steam per barrel product). Reboiling of sidestreams will also satisfy this stipulation as long as the portion of the sidestream vaporized back is at least equal to that which would be produced by the above-mentioned steam rate. In cases where this

Atmospheric Tower 11 «

Atmospheric Tower 11 «

Packie Refinery

Figure 2.6. Fractionation between total overhead and highest sidestream product, atmospheric crude towers (used with permission of the American Institute of Chemical Engineers).

Figure 2.6. Fractionation between total overhead and highest sidestream product, atmospheric crude towers (used with permission of the American Institute of Chemical Engineers).

12 Petroleum Refinery Distillation -50 -40 -30 -20 -10

12 Petroleum Refinery Distillation -50 -40 -30 -20 -10

Figure 2.7. Fractionation between adjacent sidestream products, atmospheric crude towers (used with permission of the American Institute of Chemical Engineers).

PRODUCTS (D5+V5) 100

Was this article helpful?

0 0

Post a comment