Method for Handling Bituminous Crude Oil in Tank Cars

Abstract

Bituminous heavy crude oil is kept at elevated temperatures in order to flow and may be transported in one direction, and light hydrocarbons typically comprising of mixtures of components of which the majority will have molecular chain lengths of 2 to 12 carbon atoms, such as Liquefied Petroleum Gas (LPG), Natural Gas Liquids (NGL), light naphtha, natural gasoline and natural gas condensates may be transported in the opposite direction.

Description

BACKGROUND

This relates to a method for a rapid change in service for rail tank cars from carrying heated heavy oils or bitumen to transporting light hydrocarbon fractions.

Currently, when heavy Canadian crude oil is transported by rail to terminals or refineries in the U.S., the cars are sent back empty. At the same time, large quantities of light hydrocarbons such as condensate and naphtha travel north from the U.S. to the western Canada producers of the heavy oil to be used as diluent so that bitumen can be reduced in viscosity to a point where it can flow in a pipeline at ambient temperatures. At present, the empty railcars are, for the most part, not returned loaded with light hydrocarbons because it takes a long time for the insulated cars to cool down before they can be safely loaded with light hydrocarbons, which means that they either would have to be removed off-site or take up precious space inside terminals or refineries.

Currently, large quantities of bitumen are produced from tar sands either through surface mining or by steam assisted gravity drainage, notably in Alberta, Canada. At ambient temperatures, the viscosity of the bitumen thus produced is too high to allow transportation by pipeline or other conventional means of transportation such as unheated tank trucks, railcars or vessels. Therefore the bitumen is brought to distant markets either after conversion to lighter synthetic crude oil or after dilution with light hydrocarbon fractions, usually natural gas condensates, to a viscosity that allows conventional handling methods at ambient temperatures. The conversion to synthetic crude oil and the dilution with light hydrocarbon fractions are costly processes that result in end products that have properties that are less desirable and lead to yield losses and additional processing costs in refineries when compared to mixtures of conventional crude oil and undiluted bitumen.

It is in principle possible to transport undiluted bitumen in insulated railcars over great distances with only minimal heat loss, so that even after 15 to 21 days, the time required to bring a dedicated train of 100 or more railcars (referred to in the industry as “unit trains”) from the principal producing regions in Alberta, Canada, to major refining centers such as the US Gulf Coast, the bitumen is still hot enough to be discharged in liquid form. Even with longer transit times, only minimal heating, usually in the form of steam supplied to coils attached to the shell of the rail tank car, is necessary to be able to empty the rail car. The overall economics of transportation of undiluted bitumen by rail compare very favorably with the transport of diluted bitumen by pipeline, because of savings in transportation cost of the diluent, and the cost of the diluent itself. The advantage of transportation by rail and the overall energy efficiency of the process can be further enhanced by loading the railcars with a light hydrocarbon liquid as a backhaul cargo to be used as a diluent in the transportation of additional bitumen by pipeline. Overall, there is a shortage of suitable diluents such as natural gas condensates in the bitumen producing regions, while there is an increasing excess of light hydrocarbons in the US as a result of increased natural gas production.

Typically, a rail tank car must be cooled down first before the light hydrocarbons can be loaded in order to avoid the flashing off of flammable hydrocarbon vapors. Additionally, the rail tank cars may have to be purged with an inert gas such as nitrogen when a railcar is still containing air before the flammable light hydrocarbons can be introduced. The cooling down period and the purging with inert gas adds significantly to the turnaround time of the railcars, which translates into a considerable expense as well as space requirements for the parked rail tank cars. In addition, there are the costs of the inert gas used to purge the car and the losses of hydrocarbon vapor contained in the purged gas, which are usually not recoverable but are destroyed in special combustion systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments are described with respect to the following figures:

FIG. 1 is a schematic depiction of a rail tank car loaded with bitumen shortly after arrival at its destination according to a first embodiment;

FIG. 2 shows the same rail car after unloading at the same destination according to the first embodiment;

FIG. 3 shows the rail car after completion of a cooling down process according to the first embodiment;

FIG. 4 shows a rail tank car filled with hot bitumen shortly after arrival at its destination in accordance with a second embodiment;

FIG. 5 is a schematic depiction of the rail car shown in FIG. 4 after excess nitrogen has been vented off according to one embodiment;

FIG. 6 is a schematic depiction of the rail car shown in FIG. 5 after completion of bitumen discharge according to one embodiment;

FIG. 7 is a schematic depiction of the rail car shown in FIG. 6 as it is being loaded with liquid light hydrocarbons according to one embodiment; and

FIG. 8 is a front elevational view of a rail car according to one embodiment.

DETAILED DESCRIPTION

Bituminous heavy crude oil is kept at elevated temperatures on the order of 80 to 120° C. in order to flow and may be transported in one direction, and light hydrocarbons comprising of mixtures of components of which the majority will have molecular chain lengths of 2 to 12 carbon atoms, such as Liquefied Petroleum Gas (LPG), Natural Gas Liquids (NGL), light naphtha, natural gasoline and natural gas condensates may be transported in the opposite direction.

Alternatively, the railcar may be specially designed to withstand full vacuum. To prevent the collapse of a vessel under internal vacuum, a common practice in the industry is to provide the shell of the vessel with external or internal stiffening girders. In one embodiment, external heating coils function also as the stiffening rings that allow the railcar to withstand full vacuum without the risk of collapse.

In a first embodiment, heavy bitumen is transported in rail tank cars designed to withstand overpressure as well as full vacuum, such as widely used in the transportation of LPG and certain chemicals. The rail tank car is insulated with suitable, industry standard insulation materials, such as fiberglass, mineral wool or foam, usually in thicknesses in the order of 10 cm (4 inch) covered with suitable sheeting to prevent ingress of moisture into the insulating material.

The rail car is loaded at its origin with bitumen at a suitably high temperature, i.e., in a range of 90 to 120° C. (190 to 200° F.) in order to arrive at its destination after a journey that may take up to 10 days, at temperatures whereby the bitumen still flows freely, i.e., in a range of 70 to 90° C. (160 to 190° F.), at which temperatures the bitumen typically will have a dynamic viscosity in the order of 500 to 2,000 mPa·s.

The pressure in the vapor space of the rail tank car at arrival will correspond to the vapor pressure of the bitumen at the temperature at that time, which is close to full vacuum, after the bitumen has absorbed any remaining light hydrocarbon vapors of the previous cargo during the transit time. The presence of a vacuum in the vapor space allows a convenient check on the potential ingress of air, which might lead to unsafe conditions if highly flammable light hydrocarbon vapors are introduced into the rail tank car.

The rail tank car has connections for transfer of liquid and vapor as per industry standards. For LPG-type railcars these connections are usually at the top, whereby the liquid phase nozzle is provided with a dip-pipe that extends into the railcar to within a short distance from the bottom. At its destination, the rail tank car's liquid outlet is connected to a collection system for the bitumen, and the vapor nozzle is connected to a system that can supply light hydrocarbon vapors under pressure. The pressure of the light hydrocarbon vapors is used to fill the vacuum and subsequently to maintain sufficient pressure to displace the bitumen, which will typically be in a range of 3 to 4 bar g. A level switch in the liquid line is used to detect when the rail car is empty and closes a valve in the liquid line to prevent the breakthrough of light hydrocarbon vapors into the bitumen storage tank. Other means of preventing vapor breakthrough, as known to those skilled in the art, can also be used.

At completion of the discharge of the bitumen, the connection to the bitumen system is broken and the liquid phase nozzle of the rail tank car is then connected to a system that can deliver liquid light hydrocarbons. Depending on the composition of the liquid light hydrocarbons and the temperature of the rail tank car after completion of the discharge of bitumen, a certain portion of the light hydrocarbon liquid may vaporize when brought into contact with the still hot tank wall of the rail tank car and the liquid heel of bitumen still remaining at the bottom. During this phase, the flow of the light hydrocarbon liquid may be carefully controlled to avoid excessive build up of pressure in the rail tank car, preferably by a pressure controlled automatic flow control valve. The vaporized light hydrocarbon vapors are vented through the vapor phase connection of the rail tank car to the same system through which the light hydrocarbon vapors were supplied to displace the bitumen. The flow in this system is now reversed and the vapors are led from the vapor space of the rail tank car to a condenser where the vapors are recovered in liquid form. After the rail tank car and its contents are cooled down, for example to a temperature in the range of about 30 to 40° C. at which no further flashing off of liquid occurs, the filling of the rail tank car can proceed at loading rates of for example greater than 100 cubic meters per hour.

Using the process described above, and handling up to 60 rail tank cars simultaneously, it is possible to complete the unloading of a 120-car unit train with bitumen and reloading it with light hydrocarbon diluent in less than 12 hours.

In a second embodiment, the rail tank cars may be specially built Dual Purpose cars such as described in the prior art, or general insulated and coiled bulk liquid rail tank cars. Typically, such rail tank cars are designed for pressures up to 5 bar g (75 psi) but are not designed to withstand full vacuum and are protected by a vacuum relief valve that will let in ambient air if the pressure in the vapor space of the rail tank car falls below atmospheric pressure. At their origin, after having been discharged from light hydrocarbon liquids and subsequent loading of bitumen at a suitably high temperature, the rail tank cars are now pressurized with an inert gas such as nitrogen to a pressure that is above ambient and below the maximum allowable working pressure of the rail tank car. Pressurizing the rail tank cars with nitrogen after loading with hot bitumen will prevent a drop in pressure in the vapor space to below atmospheric pressure as the car cools down during transit, which could lead to ingress of air as the rail tank car's vacuum relief valve would open.

On arrival at its delivery destination for the bitumen, the nitrogen pressure allows a convenient check against the possible ingress of air prior to the introduction of highly flammable light hydrocarbon vapors. The excess pressure of nitrogen is vented off to the atmosphere before offloading the rail tank car. Depending on the amount of hydrocarbon present in the nitrogen and on local environmental regulations, such venting may require the use of a Vapor Destruction Unit (VDU) or a Vapor Recovery Unit (VRU). After venting off the nitrogen, as with the first embodiment, pressurized light hydrocarbon vapors are used to displace the bitumen and empty the rail tank car. Once empty, the rail tank car is reloaded with light hydrocarbon liquid. This time, because of the nitrogen now present in the vapors resulting from the initial flashing off and the vapors later displaced from the rail tank car as it is filled, the condensation process will not be complete and uncondensed inert gases will have to be vented. Since these gases will contain certain amounts of hydrocarbons, such venting will have to take place by means of a VDU or VRU, as may be required by local environmental regulations.

The first embodiment may be advantageous over the second in terms of operational consideration, in some cases, because there is one fewer process step because it does not require venting of nitrogen, the cost of nitrogen is avoided, and there are no venting losses of light hydrocarbon vapors. The first embodiment also will require less capital, because there is no need for a knock-out separator and VDU or VRU. However, whether the first embodiment in its totality offers a financial advantage over the second or not will depend on market factors such as the differential in leasing cost for LPG type rail tank cars versus more common bulk liquid tank cars. The latter also have a slight advantage in that they can carry a larger backhaul cargo of light hydrocarbon liquids (for high-density bitumen, the carrying capacity in both cases is determined by the maximum allowable weight per rail car).

Referring to FIG. 1, a rail tank car 9 suitable for the transportation of liquefied petroleum gas (LPG) and designed to withstand overpressure as well as full vacuum. Such cars usually have all vapor and liquid connections at the top, whereby the liquid connection will have a dip pipe that extends into the liquid phase until close to the bottom. Liquid light hydrocarbons are drawn from a tank 1 by a pump 2. The liquid light hydrocarbons 3, now under pressure, are vaporized in a heater 4, using steam or another suitable form of heat. The light hydrocarbon vapors 6 pass into the vapor space of the rail tank car 9 through a valve in the vapor delivery and collection system 7 and the vapor connection valve 8 of the rail tank car, which may be connected by a loading arm with swiveling joints or a flexible hose.

In the rail tank car 9 the pressure of the light hydrocarbon vapors displaces the bitumen, which is pushed out through the liquid phase connection(s) of the rail tank car 10 through a loading arm with swiveling joints or a flexible hose connecting the rail tank car through a valve 11 into a heated and insulated line that transfers the hot bitumen, labeled 12, into a heated and insulated storage tank 13. As another example, the hot bitumen can be mixed with lighter crude oils immediately after discharge, through in-line blending or other suitable means as known to those skilled in the art, after which it can be stored in normal storage tanks without the need for insulation and heating (not shown).

FIG. 2 shows a rail tank car 9 upon completion of discharging the bitumen. The connection between the rail tank cars liquid nozzle 10 and the bitumen collection system, valve 11 is broken after closure of valve 11. The rail tank car may have a small remaining amount of liquid bitumen in it, referred to in the industry as the “heel” of the tank. As before, liquid light hydrocarbons are drawn from a storage tank 1 by a pump 2, but this time, the flow, labeled 3 and 14 passes through valve 15 and a loading arm or flexible hose (not shown) into the rail tank car's liquid connection 10. Flow through the vaporizer 4 is prevented by closure of valve 5. Liquid light hydrocarbons are thus introduced in the still hot rail tank car where they come into contact with the bitumen heel and the hot shell of the rail tank car. As a result of the heat contained in the heel and the tank car, a certain amount of the liquid light hydrocarbons flash off and the vapors are forced out of the tank car through the vapor connection 8 into the vapor collection system 6. Throughout the cool down phase of the operation, the flow of the liquid light hydrocarbons may be carefully controlled. This can be achieved through a flow control valve 14 controlled by a pressure sensor in the vapor system 15 or other conventional methods known to those skilled in the art (not shown). The light hydrocarbon vapors are led to a condenser 19 and are returned as a stream 20 to the light hydrocarbon storage tank 1. The condenser is depicted as an air-cooled fin-fan condenser, but other means of condensing such as shell and tube exchanger using cooling water can be more advantageous depending on available infrastructure at the site.

FIG. 3 shows the rail tank car 9 after completion of the cooling down process as it is being filled with liquid light hydrocarbons. The valve positions and connection to the railcar are unchanged from the line up in FIG. 2 and the only difference is that upon completion of the cooling down phase, the flow rates of the liquid light hydrocarbons can be increased to normal filling rates for rail tank cars. During the filling process, the heel of bitumen that was still present in the car when the liquid light hydrocarbons were introduced, dissolves in the light hydrocarbons and retains its commercial value when the light hydrocarbons are used as diluent after the rail tank cars are returned to the bitumen production location.

According to the second embodiment, shown in FIG. 4, a rail tank car 90 is filled with hot bitumen shortly after arrival at its destination. However, the rail tank car 90 is a standard insulated and coiled rail tank car that is designed for moderate overpressure but not for vacuum, against which such rail tank cars are usually protected by a vacuum relief valve that will admit air in case the pressure in the car were to fall below atmospheric pressure. At its origin the vapor space of the rail tank car has been pressurized with nitrogen to prevent ingress of air that might otherwise occur when the pressure in the car drops as it cools down during transit. FIG. 4 shows how on arrival, this nitrogen is vented to the atmosphere by opening valve 21 and valves 7 and 8 connecting the rail tank cat to the facility's vapor collection system. In this example the nitrogen 23 is shown to be passed through a Vapor Destruction Unit 24 to be released as a stream 25 in full compliance with locally applicable environmental regulations.

FIG. 5 shows the rail tank car 90 of FIG. 4 after the excess nitrogen has been vented off. As explained before for FIG. 1, light hydrocarbon vapors are drawn from storage tank 1 by pump 2 and vaporized in a heater 4, and used to pressurize the rail tank car and force out the hot bitumen, flow 12 into storage tank 13. Depicted here is a rail car with a bottom liquid outlet, which is customary for standard bulk liquid rail tank cars. All other aspects of the operation are identical to the process described for FIG. 1.

FIG. 6 shows the rail tank car of FIG. 4 upon completion of the discharge of the bitumen, when liquid light hydrocarbons are introduced in the rail tank car for cooling it down through a flashing off process. The process is identical to the one described for FIG. 2, except that because of the presence of the nitrogen that was present in the vapors before discharge, the light hydrocarbon vapors will contain some incondensable inert gases. After passing through condenser 19, the vapor/liquid mixture is passed through a knock-out separator 26, after which the liquids 20 are returned to the storage tank 1 and the vapors 23 are fed to a Vapor Destruction unit or Vapor Recovery Unit 24 from which the gases 25 are vented to the atmosphere. The flow of liquids from the knock-out drum 26 may be under control of a level gauge in the drum 28 and the venting of the inert gas may be under pressure control 27, processes well understood by those skilled in the art. Other configurations are also possible to separate the inert gases, for instance, when a shell-and-tube heat exchanger is used, vapors can be vented by pressure control directly from the shell (not shown).

FIG. 7 shows the rail tank car of FIGS. 4 through 6 as it is being loaded with liquid light hydrocarbons. The procedure and line up is identical to that described for FIG. 3, except that the vapor collection and the discharge of inert gases follow the process described for FIG. 5, whereby some incondensable inert gases present in the displaced light hydrocarbon vapors are separated by a knock-out drum (26) and vented to the atmosphere after removal of the hydrocarbons present in the inert gas stream (23) by a VDU or VRU.

FIG. 8 shows a railcar (1) that is fitted with external heating coils (2) that are welded to the shell of the tank car. The coils (2) are evenly spaced over the length of the shell and are interconnected at or near the top the rail tank car (1) by means of an inlet header (3) that will allow a heating medium such as steam to enter the header (3) via inlet valve (4) to be evenly distributed across the coils (2). The inlet valve (4) is placed at or near the manhole assembly (5), where customarily all other valves such as inlet, outlet and vent valves and pressure relief valves (not shown) are also located. At or near the bottom of the tank car's shell, a second header (6) is provided to collect the spent heating medium such as condensate. Detail A shows an example of how the coil (8) and header (9) can be welded to the shell (10). Shown is a coil with a semi-circular cross section, but other profiles such as angular, rectangular or full circle tubular profiles will also work. The required cross sectional area will depend on such factors as the spacing of the coils acting as stiffeners, material properties and dimensions of the coils and the shell, calculations that are well defined and known to those skilled in the art.

References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application.

While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.

Claims

1. A method for changing a rail tank car from transporting heated bituminous crude oil to transporting light hydrocarbons comprising:

transporting the bituminous crude oil at a temperature of 70° C. or higher under its own vapor pressure; and

discharging bituminous crude oil from a rail car at a destination without reheating using light hydrocarbon vapor.

2. The method of claim 1 including using the presence of near vacuum in the rail tank car's vapor space as a safety check against the potential ingress of air during transit before the introduction of light hydrocarbons, using light hydrocarbon vapors under pressure to discharge the bituminous crude oil from the rail tank car, and refilling the rail tank car with light hydrocarbons to be transported back to the location where the bituminous crude oil was loaded.

3. The method of claim 2 including controlling the introduction of light hydrocarbon liquids such that the vapor generated by the boil off when these liquids come into contact with the rail tank car and a small quantity of remaining bitumen liquids does not cause the pressure in the rail tank car to exceed its design pressure, and the vaporization of the light hydrocarbons is used to cool down the rail tank car and its contents.

4. The method of claim 3 including condensing the boiled-off light hydrocarbon vapors and the vapors displaced when filling the rail tank car with light hydrocarbons by reversing the flow in the system that delivered the light hydrocarbon vapors for the discharge of the bituminous crude oil and leading the vapors to a condenser to recover liquids.

5. The method of claim 1 including filling the rail tank car with liquid light hydrocarbons at a fill rate of greater than 100 cubic meters per hour.

6. The method of claim 1 using the presence of overpressure in the rail tank car's vapor space as a safety check against the potential ingress of air during transit before the introduction of light hydrocarbons, and venting off the excess nitrogen pressure upon arrival of the rail tank cars at the destination.

7. The method of claim 6 including venting incondensable inert gases containing the nitrogen introduced in the vapor space at the loading point in a controlled manner, using a vapor recovery unit or vapor destruction unit to remove any hydrocarbons contained in the incondensable inert gases.

8. The method of claim 1 including providing the rail car with external coils that allow the transfer of heat from a suitable heating medium to reheat the bitumen, said coils acting as external stiffeners to enable the rail tank car to withstand full internal vacuum.

9. A method comprising:

receiving a tank car with bituminous crude oil at a temperature of at least 70° C.;

discharging the bituminous crude oil at a temperature of at least 30° C. using light hydrocarbon vapors; and

refilling the rail tank car with light hydrocarbons.

10. The method of claim 9 including using the presence of near vacuum in the rail tank car's vapor space as a safety check against the potential ingress of air during transit before the introduction of light hydrocarbons, using light hydrocarbon vapors under pressure to discharge the bituminous crude oil from the rail tank car, and refilling the rail tank car with light hydrocarbons to be transported back to the location where the bituminous crude oil was loaded.

11. The method of claim 10 including controlling the introduction of light hydrocarbon liquids such that the vapor generated by the boil off when these liquids come into contact with the rail tank car and a small quantity of remaining bitumen liquids does not cause the pressure in the rail tank car to exceed its design pressure, and the vaporization of the light hydrocarbons is used to cool down the rail tank car and its contents.

12. The method of claim 11 including condensing the boiled-off light hydrocarbon vapors and the vapors displaced when filling the rail tank car with light hydrocarbons by reversing the flow in the system that delivered the light hydrocarbon vapors for the discharge of the bituminous crude oil and leading the vapors to a condenser to recover liquids.

13. The method of claim 9 including filling the rail tank car with liquid light hydrocarbons at a fill rate of greater than 100 cubic meters per hour once the rail tank car and its contents have been cooled down sufficiently to allow rapid filling.

14. The method of claim 9 using the presence of overpressure in the rail tank car's vapor space as a safety check against the potential ingress of air during transit before the introduction of light hydrocarbons, and venting off the excess nitrogen pressure upon arrival of the rail tank cars at the destination.

15. The method of claim 14 including venting incondensable inert gases containing the nitrogen introduced in the vapor space at the loading point in a controlled manner, using a vapor recovery unit or vapor destruction unit to remove any hydrocarbons contained in the incondensable gases.

16. A rail car terminal comprising:

a condensate vaporizer to vaporize liquid light hydrocarbons and to supply the hydrocarbons to a rail car;

a condenser to condense light hydrocarbon vapors from said car; and

a pressure control system to control the pressure of condensate vapors.

17. The terminal of claim 16 said vaporizer to fill the car at a rate greater than 100 cubic meters per hour.

18. The terminal of claim 16 including a vapor recovery unit or vapor destruction unit to remove hydrocarbons contained in condensable inert gases.

19. The terminal of claim 16 to discharge bituminous crude oil from said car using light hydrocarbon vapors.

20. The terminal of claim 19 to discharge bituminous crude oil from said car at a temperature of at least 30° C.

21. A rail car comprising:

a shell;

external heating coils extending around said shell to heat the contents of said shell; and

said coils acting as stiffeners to enable said shell to withstand full internal vacuum.