This fraction is usually the first side stream of a conventional atmospheric distillation unit. It may be cut to meet a burning oil specification or become a component in Jet Fuel finished product. Its cut range is usually between 360?F and 480?F. Again this cut range may vary with the required heavy end of the naphtha and the front end of the Gas Oils. In most cases this Kero fraction must meet a flash point specification after it has been steam stripped, and its end cut point is usually set to meet a smoke point specification. Usually the sulfur content restriction is met by hydro-desulfurization, in some cases too the smoke point is reduced by other processes. These subsequent processes are aimed at removing or converting the aromatic content of the fraction. It can also be routed to the Gas oil pool as a precursor
for a diesel finished product.
There are usually two naphtha cuts produced from most crude. These are:
- Light naphtha (sometimes called light gasoline)
- Heavy naphtha.
Both these streams are the bottom product of the debutanizer unit. They are separated in a naphtha splitter fractionation tower. The light naphtha contains most of the crude’s C5’s and much of the paraffin portion of the crude’s C6’s. The purpose of making such a division is to produce a satisfactory heavy naphtha which will contain the heavier naphthenes and will be a suitable feed for a catalytic reformer.
The light naphtha has a TBP distillation range of C5 to around 190?F. The heavy naphtha as the feed to the catalytic reformer and is a cut on crude of about 190?–360?F. This cut point of 360?F can vary depending on the severity operation of the catalytic reformer, the volatility specification of the finished gasoline which the reformate will be a major precursor, and the refineries production requirements. In this latter case for example the refinery’s operating plan may call for maximizing kerosene in which case the atmospheric distillation unit would be operated to decrease the amount of overhead distillate in order to increase the kerosene (top side stream) fraction. Of course if the refinery plan is to maximize gasoline the atmospheric tower would be operated to increase the overhead distillate at the expense of the Kero fraction.
In many refineries most of theC4’s and lighter are removed from the atmospheric column overhead distillate in the first column of the light end unit. This is the unit’s debutanizer column. Some refineries however chose to separate the light naphtha and lighter from the heavy naphtha first. There is no specific reason one can assume its really a question of the specific refinery’s economic criteria. Having separated the C4’s and lighter as a distillate from the naphtha the distillate enters a depropanizer where C3 and lighter are separated as a distillate from the C4’s. This distillate is further fractionated in a deethanizer column where the C3 is removed as the bottom product. There is no distillate product from this unit but all the gas lighter than C3 leaves the tower as a vapor usually routed to the refinery fuel gas system.
The C4 portion of the overhead distillate is fractionated in the debutanizer so that it meets finished product specification with respect to itsC5 content. The fractionation in the depropanizer will be such that the C3 content of the bottom product—C4 LPG will meet the butane LPG specification with respect to RVP (Reid Vapor Pressure). The fractionation in this tower will also be such that theC4 content in the overhead distillate will meet the propane LPG specification which leaves as the bottom product from the deethanizer with respect to its C4 content. The fractionation in the de-ethanizer will be such as to ensure that the C2 and lighter content of the propane LPG will be such as to meet that LPG’s Reid Vapor Pressure specification.
This is not strictly a cut but consists of all the light material in the crude absorbed into the total overhead distillate from the crude tower. This distillate, and in most cases, together with similar distillates from other processes form the feed to the refinery’s light end unit. It is in the light end unit that the straight run LPG, light naphtha, and the Heavy Naphtha are separated. However, the end point of the heavy naphtha is determined by the cut point of the overhead distillate and the fractionation between it and the first side stream product from the atmospheric unit.
This is the bottom product from the vacuum distillation unit. Just as in the case of the atmospheric residue it has several options for its use in meeting the refinery’s product slate. In the case of the energy refineries it can be upgraded to prime distillate products by a recycling thermal cracking process, coking, deep oil fluid catalytic cracking or hydro-cracking or indeed a combination of these processes.
In the case of the production of lube oils it can be processed to remove the heavy asphaltene portion of the stream. This deasphalted product is excellent lube oil blending stock, commonly called ‘Bright Stock’, which, when de waxed and subject to other lube oil processes becomes the precursor for a variety of lube oil products . The asphalt portion of the de asphalting process has a wide spectrum as precursor to the many grades of bitumen used in building, and road making . The vacuum residue itself also figures in the production of bitumen. Where non-energy products are significant in the refinery’s product slate, the vacuum residue as produced is the key component in bitumen production.
In modern refinery practice the distillation of atmospheric residue is accomplished under high vacuum conditions in a specially designed tower whose internal equipment ensures a very lower pressure drop. Normally the vacuum conditions in the flash zone of the tower allows about the same percentage of distillate based on the tower feed to be cut in this tower as the distillate on whole crude in the atmospheric unit. Again the flash zone temperature in the vacuum unit is kept below 700?F. Usually there are two or three vacuum distillates from this tower. In a pure energy related refinery there will be two. The heavier of the two say to a cut range of 750–930?F will be the feed to a distillate hydro-cracker or to a fluid catalytic cracker . In both these cases however a small heavier cut is taken off and returned to the tower bottom in order to correct the bottom distillate condradson carbon content to meet the specification required for either of the two downstream processes. This distillate product is usually titled HVGO (Heavy Vacuum Gas Oil).
For those refineries which produce lube oils as non energy products this bottom distillate may be split into two side streams in order to provide the flexibility required in the production of the lube oil blending stock specifications .
The light vacuum distillate is taken off as a top side stream and is usually routed to a hydro-desulfurizer to be sent either to the gas oil pool as heating oil stock or routed to the fuel oil pool as blending stock. This side stream is a cut range of 680–750?F and is usually labeled LVGO (Light Vacuum Gas Oil).
This is the bottom product from the atmospheric distillation of the crude oil. Most crude oils are distilled in the atmospheric crude oil tower to cut the atmospheric residue at a +650?F up to a +680?F cut point. Cutting the residue heavier than +680?F risks the possibility of cracking with heavier coke lay down and discoloring of the distillate products. Those atmospheric crude towers that do operate at higher cut points minimize the cracking by the recycle of cold quench into the bottom of the tower (below the bottom stripping tray) and minimizing the residue hold up time in the tower. The atmospheric residue may be routed to the fuel oil pool as the precursor to several grades of finished fuel oil products. The other options for this stream in a modern refinery are as follows:
- feed to a vacuum distillation unit. (This is the most common option.)
- feed to a thermal cracker (Visbreaker, or coking unit).
- feed to a deep oil fluid catalytic cracker.
- feed to a hydro-cracker or hydro-treater.
The factor of proportionality in the previous equations is called the Moody friction factor and is determined from the Moody resistance diagram shown in Figure 8-1. The friction factor is sometimes expressed in terms of the Fanning friction factor, which is one-fourth of the Moody friction factor. In some references the Moody friction factor is used, in others, the Fanning friction factor is used. Care must be exercised to avoid inadvertent use of the wrong friction factor.
In general, the friction factor is a function of the Reynolds number, Re, and the relative roughness of the pipe, £/D. For Laminar flow, f is a function of only the Re:
For turbulent flow, f is a function of both pipe roughness and the Reynolds number. At high values of Re, f is a function only of eD.
Table 8-1 shows the relative roughness for various types of new, clean pipe. These values should be increased by a factor of 2-4 to allow for age and use.
This equation, which is also sometimes called the Weisbach equation or the Darcy-Weisbach equation, states that the friction head loss between two points in a completely filled, circular cross section pipe is proportional to the velocity head and the length of pipe and inversely proportional to the pipe diameter. This can be written:
Equations 8-5 and 8-6 can be used to calculate the pressure at any point in a piping system if the pressure, flow velocity, pipe diameter, and elevation are known at any other point. Conversely, if the pressure, pipe diameter, and elevations are known at two points, the flow velocity can be calculated.
In most production facility piping systems the head differences due to elevation and velocity changes between two points can be neglected. In this case Equation 8-5 can be reduced to:
It is customary to express the energy contained in a fluid in terms of the potential energy contained in an equivalent height or “head” of a column of the fluid. Using this convention, Bernoulli’s theorem breaks down the total energy at a point in terms of
1. The head due to its elevation above an arbitrary datum of zero potential energy.
2. A pressure head due to the potential energy contained in the pressure in the fluid at that point.
3. A velocity head due to the kinetic energy contained within the fluid.
Assuming that no energy is added to the fluid by a pump or compressor, and that the fluid is not performing work as in a steam turbine, the law of conservation of energy requires that the energy at point “2” in the piping system downstream of point “1” must equal the energy at point “1” minus the energy loss to friction and change in elevation. Thus, Bernoulli’s theorem may be written: