Avoiding heat exchanger fouling after crude oil desalting

Abstract

Heat-exchanger fouling, for example in a crude-oil heat- exchanger train, is reduced by flashing the oil after desalting and then heating the oil further in a heat exchanger, instead of heating the oil in a heat exchanger immediately after desalting and before flashing. Preferably the flashing step is carried out by reducing the pressure on the oil to a pressure below the vapor pressure of free water at the flash-drum temperature. Preferably, the flash-drum temperature is between 250.degree. and 350.degree.F.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my application Ser. No. 184,738, filed Sept. 29, 1971 now abandoned.

Description

BACKGROUND OF THE INVENTION

The present invention relates to reducing fouling in heaters, particularly heat exchangers, used to heat oil.

The term "fouling" is used herein to mean deposition of material on heat transfer surfaces, usually with subsequent reduction in heat transfer coefficient, and usually also with an increase in pressure drop.

Essentially all oil refineries have crude oil distillation units. The crude oil distillation units basically consist of crude oil preheat exchangers, a desalter, a furnace, and a crude oil distillation column. The present invention is particularly concerned with reducing fouling in the crude oil preheat exchangers, although the process of the present invention can be applied to other heat exchanger surfaces where desalting is involved.

The basic function of a crude oil distillation unit is to split crude oil into different boiling-range fractions of the crude oil for further-downstream processing or for final use. To carry out splitting of the crude oil into various fractions by distillation, the crude oil must be heated to a high temperature to allow substantial vaporization as required for the crude oil distillation.

The heating of the crude oil ahead of the crude oil distillation column is generally carried out by heat exchange with hot products from the crude oil distillation column, and finally by heating in a furnace. The heat exchangers are usually shell and tube exchangers, with the crude oil usually flowing through the tubes and taking up heat from hot products flowing through the exchanger outside of the tubes; that is, through the shell side of the exchanger. A simplified crude-oil heating train is shown on page 38 of R. J. Hengstebeck's "Petroleum Process," McGraw-Hill, 1959.

Before the crude oil completes the flow through the heat-exchange train, it is usually subjected to desalting. Desalting is applied to remove brine from the crude oil. It is accomplished by mixing water with the crude oil whereby the water unites with the brine. The aqueous (brine-water) phase is separated from the oil by settling; in most modern plants removal of water droplets from the oil is aided by electrical means.

The brine which is separated from the oil contains salts such as sodium chloride, calcium salts, magnesium salts, etc.

Desalting of crude oil is described in more detail at page 265 of W. L. Nelson's "Petroleum Refinery Engineering," Fourth Edition, McGraw-Hill. As shown by data given on page 94 of Nelson's book, some crude oils are so low in salt content that desalting is not needed. But this is the exception, rather than the rule, and desalting nearly always is applied.

Also, before the crude oil completes the heating ahead of the crude oil distillation column, it is frequently flashed.

The word "flashed" is used to mean the vaporization of part of the oil mixture by reduction of the pressure on the oil mixture. Thus, in many crude oil units, after the oil is heated to about 350.degree. to 550.degree.F., it is flashed into a flash drum. The flash drum removes light components, such as water and low-boiling hydrocarbons, from the oil so that the flow through the final heating, the furnace, is reduced. The overhead from the flash drum is introduced usually as a vapor to the crude-oil distillation column. The bottoms are heated in the furnace and then fed to the crude-oil distillation column.

Many crude-oil units have a desalter but no flashing step. In either case, the desalting step is typically at a temperature of about 250.degree.F., that is, considerably below a typical flashing step temperature (assuming a flashing step is employed at all). Desalting is preferably carried out at the low temperature, because: (1) the separation of the brine from the oil is usually more efficient at the lower temperature, particularly if electrical means are used to aid in the separation; and (2) it is usually felt desirable from the standpoint of reduced heat-exchange fouling to reduce the salts before reaching a temperature above 300.degree.F. Flashing is typically done at a higher temperature, because light components are removed more effectively by flashing off from the majority of the crude oil at higher temperatures, compared to lower temperatures.

Therefore, in the past, heat exchangers have been used after desalting and before flashing in order to increase the temperature from the desalting temperature to a more desirable flashing temperature (or to a higher temperature to prepare the oil for further heating or introduction to the distillation column).

SUMMARY OF THE INVENTION

In accordance with the present invention, in a process which comprises heating and desalting and then further heating a hydrocarbonaceous oil using heat exchangers and a desalter, the improvement is made which comprises flashing the oil to remove water after desalting the oil and before further heating of the oil in any heat exchanger.

The present invention is based in part on data which I have obtained and analyzed showing that flashed oil, particularly flashed crude oil, exhibits only mild fouling characteristics, such that relatively low fluid velocities, in the 2- to 10-feet-per-second range can keep the heat transfer surfaces clean. The surprising result that exchanger fouling can be dramatically reduced in many instances by flashing after desalting is believed by me to be at least in part attributable to the form the trace amount of salts achieves after desalting, if the oil is flashed.

In desalting, as in other processing, 100% efficiency is, of course, not achieved. Thus, there are at least small amounts of brine left in the oil after the desalting step. It is theorized that upon flashing the oil from the desalting step, remaining water -- that is, a substantial portion, such as 75% or more, of the remaining free water -- is flashed from the crude oil, and salts precipitate out as rather sizable aggregates, similar to dirt particles. If the water is left in the oil, the salt also stays dissolved in the oil by means of the water which is carried with the oil. It is theorized that the salt left in the oil in this form tends to act as a "cementing" means to result in very tenacious deposits to heat-exchanger surfaces, such as the inside of heat-exchanger tube walls. On the other hand, it is theorized that in the process of the present invention, when the water is substantially flashed out of the oil the salts precipitate out as aggregates, which aggregates in turn do not deposit extremely tenaciously upon the heat-exchange surfaces or cause extremely tenacious deposits on heat-exchange surfaces.

Thus, it is seen that the present invention is considerably different from prior art methods of attack, which in many instances were directed to efficient desalting per se, so as to remove the salt constituents from oil and thereby improve downstream operations. In fact, the data which I have obtained and analyzed show that in some instances extremely efficient desalting per se can increaase heat-exchanger fouling rather than decrease heat-exchanger fouling. It is theorized that the increased fouling resulting from more efficient desalting per se is caused by the fact that the desalting substantially dehydrates the oil in its efficient separation of the aqueous phase from the oil phase, with the result that little aqueous phase is left to maintain the remaining trace amounts of salts in solution. Thus, those tract amounts of salts left in the oil deposit tenaciously or cause tenacious deposits on the walls of the exchanger tubes as the temperature of the crude oil is increased when flowing through the downstream exchangers.

Connected with this, it should be pointed out that a number of salts exhibit inverse solubility as, for example, calcium sulfate, which is progressively less soluble at higher temperatures, so that as temperatures are increased it tends toward depositing out from solution in trace amounts of water in the oil. In addition, as the temperature is raised, water tends to dissolve in oil and/or vaporize so that less liquid-phase water is left to hold salts in solution.

Also, the process of the present invention can be contrasted to those prior art processes which suggest simple addition of an additive for reducing heat-exchanger fouling. For example, U.S. Pat. No. 3,390,073, entitled "Hydrocarbon Additive for Heat Exchanger Anti-Fouling," is directed to the addition of an anti-foulant additive at points such as downstream of a crude-oil desalter but ahead of crude oil preheat exchangers.

In the process of the present invention the crude oil desalting step is followed by a flashing step before passing the crude oil through any heat exchanger. The flash temperature should be selected at about the lowest temperature where crude oil can be flashed without foam problems. Foaming tends to be reduced if the crude oil is at a low viscosity. Thus, this is one of the factors which has caused the flashing step in the past to be carried out at relatively high temperatures compared to the desalting step, as viscosity is lower at higher temperatures. However, in the process of the present invention it is desired to operate at about the lowest temperature for the flash without going into the substantial-foaming-temperature regime. In the present invention, relatively low temperatures are desired for the flashing step because the flashing step immediately follows the desalting step, and it is generally desirable to carry out desalting at a low temperature, compared to the final temperature of the crude oil before the crude oil is fed to the crude-oil distillation column.

However, in the process of the present invention the flash temperature should be sufficiently high so that the vapor pressure of steam is sufficiently high to result in at least 75% of the free water flashing off or vaporizing from the crude oil in the flash drum. Alternatively, this may be phrased in terms of pressure in the flashing step or flash drum; the pressure in the flash drum preferably is below the vapor pressure of free water at the temperature in the flash drum. "Free water" refers to that water over and above the water which is soluble in the oil as a true solution.

Preferred flashing temperatures are usually between about 250.degree.-350.degree.F., contrasted to more typical flash-drum temperatures between about 350.degree.-550.degree.F. The flash-drum temperature or flashing temperature can be taken, for sake of specificness, as the temperature of the crude oil liquid remaining after the flash, i.e., the crude oil liquid in the bottom part of the flash drum as opposed to the vapor in the upper part.

Together with the usual preferred temperature of about 250.degree.-350.degree.F. for the flashing step, in accordance with the present invention preferred pressures are usually between about 10-120 psig.

Preferred oil feeds for the process of the present invention are whole crude petroleum oils. Arabian light petroleum is a particularly preferred feedstock for the process of the present invention, because the Arabian light petroleum has a relatively low viscosity compared to other whole crudes, and therefore can be flashed at a relatively low temperature without substantial foaming problems. The viscosity of Arabian light petroleum is about 1.0 cp at 280.degree.F. The gravity of Arabian light petroleum is about 34.5.degree.API.

Other whole crudes having a relatively low specific gravity, for example a gravity between about 33.degree.API and 36.degree.API, are also preferred whole crude feedstocks for the process of the present invention.

In the process of the present invention, as applied to the Arabian light petroleum, it is preferred to flash the Arabian light petroleum at a temperature between about 260.degree.-320.degree.F., for example about 280.degree.F. Carrying out the flashing step of the process of the present invention, as applied to relatively light whole crude petroleum oils having a gravity between about 33.degree.API and 36.degree.API, at the relatively low flashing temperature of 260.degree.-320.degree.F. is a particularly preferred embodiment of the present invention.

According to a specific embodiment of the present invention, a process is provided for reducing the fouling of crude-oil distillation unit preheat exchangers downstream of a desalting step, which process comprises: (a) heating feed crude oil to a temperature between 150.degree.-450.degree.F. by passing the oil through heat exchanger tubes at a pressure between 20 and 500 psig and transferring heat from a hot fluid across the tube walls to said oil; (b) removing salts from the oil in a desalter by contacting the oil with water and then separating the oil from the water at a pressure between 20 and 500 psig and a temperature between 150.degree.-450.degree.F.; (c) flashing off remaining water from the oil, before further heating in any heat exchangers, by reducing the pressure below the vapor pressure of free water at the temperature of the flashing step; and (d) then further heating the oil to a temperature above the flashing temperature and between about 350.degree.-900.degree.F. by passing the oil through heat-exchange tubes and transferring heat from a hot fluid across the heat-exchange-tube walls to the oil.

When using the process of the present invention with some crude oil feedstocks, particularly heavy, viscous feedstocks having a gravity above about 36.degree.API, it is desirable to obtain some heating between the desalting and flashing steps, but without the use of heat exchangers, as omission of the use of shell and tube heat exchangers between the desalting and flashing steps is critical to the process of the present invention. One manner of obtaining further heating between the desalting and flashing steps is by injecting a hot oil, for example a recycle oil, from downstream of the post-flash-drum heat exchangers and/or furnace. This manner of directly heating the crude oil is described further in my co-inventorship application with John H. Arndt, entitled "Crude Oil Processing," filed on or about Jan. 26, 1973, Ser. No. 326,616, now U.S. Pat. No. 3,798,152.

For crude oil feedstocks with a gravity below 36.degree.API, it is usually preferable in the process of the present invention to omit any heating, direct or by heat exchange, between the desalting and flashing steps. But for relatively viscous crude oil feedstocks with a gravity above 36.degree.API, it is usually preferred to directly heat the crude oil by hot oil injection or by steam injection or the like between the desalting and flashing steps.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic process flow diagram illustrating a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWING

Referring now more particularly to the drawing, crude oil is introduced to the process via line 1 and pumped in liquid form to a pressure of about 100 to 500 psig using pump 25. The oil is then heated by heat exchanger 2 from a temperature of about 70.degree.F. to about 250.degree.F. Usually exchanger 2 will not be simply one exchanger, but will be two or more heat exchangers.

In accordance with a preferred embodiment of the present invention, as referred to under "Summary of the Invention," water is introduced via line 26 to aid in the reduction of fouling in exchanger 2.

The heating fluid from which heat is transferred to the crude oil flowing through heat exchanger 2 usually passes through the shell side of exchanger 2 as indicated by line 3, and then line 4. This heating fluid can be one or more of streams 20-24 withdrawn from crude oil distillation column 19.

The crude oil, at a temperature of about 300.degree.F., is passed via line 5 from exchanger 2 to desalter 6. Additional water can be added to the crude oil immediately ahead of the desalter as indicated by line 27. Typically, the amount of water in the crude oil introduced to the desalter is a total of about 5 pounds per 100 pounds of oil.

The desalter is used to remove the brine from the crude oil in accordance with well-known desalting techniques. The separation of the aqueous brine phase from the oil phase is frequently aided by electrical means, so that a substantially water-free oil is obtained after the desalting step.

In prior art processes the substantially water-free oil, particularly as obtained after highly efficient electrical desalting, frequently causes severe exchanger heat transfer surface fouling downstream of the desalter. However, in the process of the present invention, the oil withdrawn from the desalter is not passed immediately through a heat exchanger; instead, a flashing step follows the desalting step. Then further heating of the oil in heat exchangers and/or a tubular furnace is carried out, with usually a reduced fouling rate and/or fouling of a less severe nature that can be kept at a tolerable level for long periods (a quarter-year and more), by velocities such as 2- to 10-feet-per-second for crude oil through heat exchanger tubes.

Referring again more particularly to the drawing, separated water is withdrawn from the desalter as indicated by line 30. Oil is withdrawn in line 7 and is flashed across pressure-drop valve 8 into flash drum 10 via line 9.

Vapor is withdrawn from flash drum 10 via line 11 and is then introduced to the crude oil distillation column.

The majority of the crude oil remains in liquid phase and is withdrawn via line 12 for further heating in heat exchanger 13. The heating of the oil in exchanger 13 is typically accomplished by exchange with one or more of streams 21-24 from the crude oil distillation column. A hot product stream such as stream 24 is passed through the heat exchanger, as indicated by lines 14 and 15.

The crude oil is thus heated in exchanger 13 (or in several exchangers which may be grouped under the designation 13) from the flash-drum temperature of about 300.degree.F. to a temperature of about 450.degree.F. The crude oil is then heated further to a temperature of about 600.degree.-800.degree.F. in furnace 17 before introduction to the crude oil distillation column via line 18.

EXAMPLE

Data were obtained from one operation (designated as "PN"). The crude oil heating train in the PN operation consisted of a first heat exchange, desalting, further heat exchange (second heat exchange), flashing, still further heat exchange (third heat exchange), furnace heating, and then introduction to a crude oil distillation column. The water used for desalting in this operation was zeolite-softened water. The connate water in the crude oil was basically sea water, having large amounts of sodium chloride and only relatively small amounts of calcium salts. In this operation the data which were obtained and analyzed showed severe fouling for the second heat exchange, that is, fouling in the heat exchanger between the desalting and the flashing steps. The deposits were very tenacious and could be successfully cleaned from the heat-exchange surface (i.e., in this instance, inside of the exchanger tubes) only by vigorous means such as reaming out the deposits from the exchanger tubes.

The main salt component carried over by the entrained water from the desalter in the subject operation was, of course, expected to be sodium chloride. Metals analysis of the desalted crude oil confirmed that the sodium content (1 ppm) was greater than the calcium content (0.4 ppm). Analysis of the unwashed fouling deposit, however, showed that the calcium content was two orders of magnitude higher than the sodium content, and that the main constituent was CaSO.sub.4 -- an inverse solubility salt. The principal salt components in the fouling deposit were: Na 0.12 weight percent Ca 16.8 do. Cl 0.08 do. SO.sub.4 45.2 do.

The presence of iron sulfate and iron sulfide was also noted.

The above salt analysis supports other data which I have analyzed indicating that substantial fouling occurs after the desalting step when the crude oil is heated prior to being subjected to a flashing step. It is theorized that the fouling is caused or connected to precipitation of inverse-solubility salts from continuing liquid water droplets; that is, entrained fine water droplets or the like remaining in trace amounts after the desalting step. As stated before, the inverse-solubility salts, such as calcium, have decreased solubility in water as temperature is increased.

Contrasted to the severe fouling in the second heat exexchange, the fouling in the third heat exchange, that is, the heat exchange after flashing, was very low, specifically, lower by a factor of at least 10. Of course, the salts which were present in the oil leaving the second heat exchange are still present in the oil after the flashing step, as the salts are not volatile. However, it is believed, in view of the data which have been analyzed, that the salts are in a much less harmful form after the flashing, perhaps because of the removal of remaining water and resultant precipitation of the salts in the flashing step as aggregates such as dirt particles. It should be mentioned that the term "removal of the remaining water" by flashing does not mean that all of the remaining water is removed as, of course, the water sets up an equilibrium with the oil and, also, even equilibrium is not reached 100% completely, and there are some inefficiencies in vapor liquid separation.

In another operation (designated as "PG"), crude oil was passed through a series of exchangers, desalted, and then passed through more exchangers and a furnace before introduction to a crude oil distillation column. No flashing step was used. When the desalter was operating relatively inefficiently, such that about 0.5 to 1.0 weight percent of water remained in the oil from the desalter, heat-exchanger fouling after the desalter was lower than when the desalter was operating more efficiently with better electrical means, so as to reduce the water to about 0.2 weight percent. Furthermore, the addition of more water when the desalter was operating efficiently resulted in the substantial reduction of exchanger-tube fouling rate. Thus, the addition of water to aid in the reduction of fouling was shown by the operation designated as PG.

Claims

What is claimed is:

1. In a process which comprises heating and desalting and then further heating hydrocarbonaceous oil using heat exchangers and a desalter, the improvement which comprises flashing the oil to remove water after desalting the oil and before further heating of the oil in any heat exchanger.

2. A process in accordance with claim 1 wherein the oil is substantially dehydrated in the flashing step by flashing the oil into a flash drum at a pressure below the vapor pressure of free water, at the temperature of the liquid in the bottom part of the flash drum.

3. A process in accordance with claim 1 wherein the oil is a whole crude petroleum oil.

4. A process in accordance with claim 1 wherein the oil is heated in a heat exchanger ahead of the desalting step to a temperature between 100.degree. and 350.degree.F., and between 0.1 and 5 pounds of water per 100 pounds of oil are injected into the oil before it passes through said heat exchanger.

5. A process in accordance with claim 1 wherein the flash is carried out at a temperature between 250.degree. and 350.degree.F.

6. A process in accordance with claim 1 wherein the flash is carried out at a temperature between 260.degree. and 320.degree.F., and the crude oil is an Arabian light crude oil.

7. A process for reducing the fouling of crude oil distillation unit preheat exchangers downstream of a desalting step, which process comprises:

a. heating feed crude oil to a temperature between 150.degree. and 450.degree.F. by passing the oil through heat exchangers at a pressure between 20 and 500 psig, and transferring heat from a hot fluid across the tube walls to said oil;

b. removing salts from the oil in a desalter by contacting the oil with water and then separating the oil from the water at a pressure between 20 and 500 psig, and a temperature between 150.degree. and 450.degree.F.;

c. flashing off remaining water from the oil, before further heating of the oil in any heat exchanger, by reducing the pressure below the vapor pressure of free water at the temperature of the flashing step; and

d. then further heating the oil to a temperature above the flashing temperature and between about 350.degree. and 900.degree.F. by passing the oil through heat exchangers and transferring heat from a hot fluid across the heat exchange tube walls to the oil.

8. A process in accordance with claim 1 wherein the gravity of the crude oil is below about 36.degree.API and wherein there is no heating of the crude oil by hot oil injection between the desalting and flashing steps.