Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing

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MEK Accumulator

MEK Make Up

Inerts

To Vacuum Pump

Feed

Chiller Unit

Rotary Filter

Wax Rundown

Wax Stripper

Wax to Storage

De-waxed Oil to Storage

Oil Stripper

Steam

Blow Gas

Inert Gas

Figure 12.4. Conﬁguration of a MEK dewaxing process.

494 CHAPTER 12

Either neutral base oil or bright stock from the extraction unit is mixed with MEK

before entering a tubular chilling unit. During the chilling of the mixture the wax

content in the oil forms crystals in the presence of the MEK. The chiller tubes are

furnished with internal chain scrapers which collect the oily slurry. This stream is then

introduced to the bottom trough of a rotary ﬁlter. The outer drum of the ﬁlter is covered

by a ﬁlter cloth. The drum in turn is divided into two or sometimes three chambers.

The ﬁrst is under vacuum and draws the oily slurry in the trough through the ﬁlter

cloth. The oil phase free of wax crystals ﬂows though the cloth to the inner section of

the drum. The wax is retained ﬁrmly on the cloth. In some processes a stream of the

dewaxed oil is returned as a wash to the wax cake. As the drum rotates it enters the

second phase of its operation. Here a positive pressure in the form of an inert gas stream

is applied to the rear of the wax cake. This lifts the cake off the ﬁlter cloth sufﬁcient for

a disengaging scraper to remove the cake and deliver it to a heated trough where the

wax is melted. The molten wax is pumped from the trough to the top row of bafﬂes in

a wax stripper tower. Here the wax is stripped free of MEK using a stream of inert gas

passing upwards through several rows of bafﬂe trays counter current to the wax stream.

The stripped wax is pumped from the bottom of the stripper to storage and further

treating.

The dewaxed oil phase from the ﬁlter is pumped from the inner chamber of the ﬁlter

to a steam heater and then to the top row of disc and donut trays in the oil stripper

tower. Inert gas ﬂows upwards through several rows of trays counter current to the

oil ﬂowing downward. The oil is stripped free of MEK and leaves the bottom of the

tower to storage and use as blend stock.

The MEK from both strippers are collected, cooled, and routed to an accumulator

drum. Here the absorbed inert gas and the blow gas are ﬂashed off to be recycled or

vented. The MEK is returned to the system by a pump to join the waxy oil feed at the

chillers (Table 12.3).

12.2 Asphalt production

Almost invariably associated with the Lube Oil Reﬁnery are facilities for the produc-

tion of Asphalt. It is a popular belief, even among engineers that asphalt can be made

from the heavy end of any crude oil. Unfortunately this belief is far from being correct.

The asphalt product must have a series of properties which meet the speciﬁcations for

the different grades. Since many of these properties are interrelated, it is difﬁcult to

meet all the speciﬁcations at the same time. The penetration/softening point relation-

ship is probably the most important property to be considered from this stand point.

The ideal crude for asphalt production should be heavy, have a high, good quality

asphalt yield, and, of course, should be inexpensive. The oxidizing properties of the

THE NON-ENERGY REFINERIES 495

Table 12.3. Product yields and properties—MEK

dewaxing process

Feed A Feed B Feed C

Feed stock

Viscosity, SSU @210

◦

F 41 58 126

Pour point,

◦

F 95 110 125

Flash point,

◦

F 400 450 540

Products

De-waxed oil, vol% 70 77 80

Pour point,

◦

F151510

Cloud point,

◦

F201812

Slack wax

Yield, vol% 30 23 20

Oil content, wt% 25 20 15

asphalt must also be considered. Some crudes will produce ﬁne paving stock, but will

not be good feed for air blowing, when the speciﬁcation of the product calls for an

‘oxidized’asphalt.

Types of asphalt products

There are two major categories of asphalt products. These are:

Paving and liquid asphalt

Rooﬁng asphalt

The paving and liquid asphalt

The paving and liquid asphalt products are again sub divided into ﬁve paving grades of

paving and three liquid grades. The paving grades will have a penetration speciﬁcation

of 300 or less @ 77

◦

F and 100 g weight, while the softer liquid grades will have a

penetration of 300 and higher. The ﬁve paving grades fall into the following categories

based on their penetration:

40–50, 60–70, 85–100, 120–150, 200–300.

The liquid asphalt grades are typiﬁed as, RC (Rapid Curing), MC (Medium Curing),

and SC (Slow Curing) each of these grades will also have 4 viscosity grades as given

below:

70 grade—RC/MC/SC 70 to 140 cS @ 140

◦

250 grade—RC/MC/SC 250 to 500 cS.

800 grade—RC/MC/SC 800 to 1,600 cS.

3,000 grade—RC/MC/SC 3,000 to 6,000 cS.

496 CHAPTER 12

Rapid curing cutbacks are penetration asphalt and naphtha blends having a viscosity

range from 75 SSF @ 77

◦

F to 600 SSF @ 180

◦

F. The naphtha content may be as

high as 75% vol. Cutbacks are hot sprayed onto existing roads as a binding medium

for new wearing surfaces.

Medium curing cutbacks are penetration grade asphalt and kerosene blends with four

grades having the same viscosities as the RC cutbacks. These are used in road building

in the same way as the RC cutbacks.

Slow curing cutbacks are penetration asphalt and gas oil blends normally produced

directly from the crude oil atmospheric or vacuum distillation. The volume produced

is small and they are used mainly as a gravel dust layer or mixed with aggregates for

cold patching of asphalt surfaces.

Rooﬁng asphalt

The second largest asphalt use is that for rooﬁng. Most of these asphalts are pro-

duced by air blowing. There are three major rooﬁng grades and may be classiﬁed by

penetration and softening points. These are:

Pen@77

◦

F Softening point

◦

Type 1 25–50 140–150

Type 2 20–30 166–175

Type 3 15–25 190–205

Type 1 grade is used on ‘Dead Flat’roof while the other two are used on intermediate

and steep slope roofs respectively.

Asphalt oxidizing mechanism

Asphalt is basically a colloidal dispersion of asphaltenes in oil with resins as the

stabilizing agent. The quantities of these can vary widely with the type of crude.

Asphaltenes can be hydrogenated to resins, resins to oils. Resins can be oxidized to

asphaltenes, oil to resins. Thus:

Asphaltenes + H2 H2 → Oil

Resins

Asphaltenes + O

2 O2 + Oil

Oxidation is really a misnomer, as air blown asphalt has essentially the same oxygen

content as the charge. Air blowing asphalt increases the asphaltene content, hardens it,

THE NON-ENERGY REFINERIES 497

decreases penetration, increases softening, and reduces ductility. Basically air blowing

is a polymerization process following the route below:

1. Addition of O

to form unstable compounds from which H

O is eliminated leaving

unsaturated compounds which polymerize.

2. Addition of O

to form carboxyl derivatives from which CO

is eliminated followed

again by polymerization.

3. Formation and elimination of volatile oxidation products other than H

O and CO

followed again by polymerization.

It is worth noting, that the best crudes for asphalt air blowing are those with high

percentage of asphaltene fraction and low in parafﬁn hydrocarbons, and waxes. Resins

can be oxidized to asphaltenes relatively easily, so crudes rich in resin are good raw

materials for asphalt manufacture. Oils can also be oxidized to asphaltenes, but they

must be oxidized to resins ﬁrst, which requires a more severe operation. Cracked

residua simply do not make good asphalt.

Asphalt yields

In general heavier crudes contain a higher content of asphalt or asphaltic material and

this is demonstrated in Figure 12.5.

The actual yield of these materials depends mainly on the composition of the crude

itself in terms of parafﬁnic, oleﬁnic, asphaltenic, and naphthenic fractions it contains.

Good reliable crude oil assays provide such data for speciﬁc crude oils. However

if such data is absent then a reasonably close estimate of the asphalt yield can be

obtained using a method by S. Patel (Oil & Gas Journal Feb 1964).

This predicting method correlates Asphalt yield vs. ‘Asphalt Factor’a parameter

obtained by Patel’s formula, which is:

F =

(A)(CC)

(B/C)(K − 10.4)

where:

F = asphalt factor

A = the slope between 400

◦

F and the vapor pressure at which 100 penetration

starts to boil (see Figure 12.6).

CC =Conradson carbon of 750

◦

F plus residue.

B =Slope of high boiling material. (see Figure 12.6).

C =Slope of lower boiling material (see Figure 12.6).

K = Characterization factor at 750

◦

n = Empirical exponent, normally 1.0.

498 CHAPTER 12

° API Gravity of Crude.

% Vol of 100Pen Asphalt.

100

5152535

Figure 12.5. Asphalt in average crudes (approximate).

Figure 12.7 shows the relationship between the ‘F ’f actor and the approximate yield

of 100 pen asphalt.

The calculation is by trial and error, and this is given in the example for the design of

an asphalt air blowing process in Appendix 12.1 of this chapter.

Asphalt blowing process

Blowing air into a vessel containing asphalt from vacuum distillation or the de-

asphalting process will change its penetration and softening properties. Among the

variables that affect the manufacture of air blown asphalt are:

THE NON-ENERGY REFINERIES 499

Figure 12.6. How to obtain crude oil slopes for use in the Patel equation.

The rate of the air injected

The temperature of the asphalt

Retention time of the asphalt

The system pressure

These variables and their effect on the process are discussed below.

The rate of air injected

This is the most important variable in the process. If the temperature, pressure, and

residence time are kept constant in the oxidizer, the softening point of the asphalt

(the ring & ball) can be remarkably increased by increasing the air rate. This increase

is larger at low air rates than higher values A point is reached where the increase in

air rate has little effect on the softening point and this is shown in Figure 12.8. The

penetration properties of the asphalt are also affected by the air rate and this is shown

in Figure 12.9.

For continuous operation normal air rates will be between 0.3 SCFM/BPSD of oil to

1.0 SCFM/BPSD. For Batch processes this rate will be between 1.5 SCFM/Bbl to 9.5

SCFM/Bbl this will depend of course on the length of each batch and the number of

cycles to be processed each day. The air supply must not contain free water but need

500 CHAPTER 12

Yield of 100 Pen Asphalt % Vol of Whole Crude.

‘F’ Factor from Patel Equation

1.0

10 20 30 40 50 70 100 200 500

100

23456789 234567891

Figure 12.7. Correlation of ‘F ’f actor versus yield of bitumen.

not be dried. If the air is delivered by a reciprocating compressor then a well designed

knock out pot is sufﬁcient to remove the free water.

The reaction temperature

If the other variables are kept constant increase in the temperature of the oxidizer

increases the softening point of the asphalt. The effect of temperature increase on the

softening point is larger at the lower temperature levels than when the temperature is

already high. A point is reached when the increase in temperature has little effect on the

softening point. Figure 12.10 shows this effect. The ratio of softening point and pene-

tration increases by decreasing the temperature. To increase the softening point with-

out affecting the penetration it is necessary to reduce the reaction temperature, blow

air for a longer period, or increase the residence time of the asphalt. Figure 12.11

shows this behavior.

THE NON-ENERGY REFINERIES 501

Figure 12.8. Effect of air rates on softening point.

Figure 12.9. Effect of air rates on softening point/penetration ratio.

502 CHAPTER 12

Figure 12.10. Effect of temperature on softening point (or hardness).

There are certain limitations to the temperatures at which the oxidizer can be designed

or operated. The design of the oxidizer should not be higher than 50

◦

F below the ﬂash

point of the feedstock or any of its components. It must however be recognized that

effective oxidizing reaction does not occur at temperatures below 420

◦

F. Generally

speaking batch processes operate at a temperature between 430

◦

F and 475

◦

F while

continuous processes operate at between 500

◦

F and 550

◦

F. The asphalt oxidizing

reaction is exothermic and as such must be controlled by some form of cooling. The

most common means of cooling in continuous processes is to recycle a portion of

the reacting feed through a cooler and returning it to the reactor mixed with the feed.

Another method is to install cooling coils into the reactor itself. This method is used

mostly for batch processes. Steam is injected above the oil level in the reactor. The

main purpose of this steam is to act as snufﬁng steam to avoid explosion in the air/oil

atmosphere existing in this zone.

Retention and contact time

If all other variables are kept constant, the softening point can be remarkably increased

by increasing the retention time of the asphalt in contact with air in the oxidizer. In

the case of continuous processes this is achieved by varying the throughput or by