WO1998033026A1

WO1998033026A1 - A method of reducing discharge of volatile organic compounds

Info

Publication number: WO1998033026A1
Application number: PCT/NO1998/000030
Authority: WO
Inventors: Ola Ruch; Terje ALMÅS; Otto Skovholt
Original assignee: Den Norske Stats Oljeselskap A.S
Priority date: 1997-01-29
Filing date: 1998-01-28
Publication date: 1998-07-30
Also published as: AU5886998A; NO970393D0; NO305853B1; NO970393L

Abstract

The present invention discloses a method of reducing the discharge of volatile organic compounds (VOC) into the atmosphere during loading, transport, unloading, processing and storage of hydrocarbons. By compressing and cooling during the condensation of VOC gas from oil, the condensed VOC being separated as a product stream from at least one separator, fed to at least one storage unit, and, when required, fed as a separate fuel stream to thermal machinery, an ecologically beneficial utilisation of the VOC gas is achieved as well as an economical beneficial use of this energy source.

Description

«A method of reducing discharge of volatile organic compounds))

The present invention relates to a process of reducing discharge of volatile organic compounds (VOC) into the atmosphere during loading, transport, unloading, processing and storage of hydrocarbons. This process can particularly be used for the reduction of discharge of volatile organic compounds (VOC) into the atmosphere during offshore loading of tankers and during their subsequent transport of oil to terminals ashore, in compressing and/or cooling the volatile organic compounds (VOC) during the offshore loading and the subsequent transport and total or partial operation of the thermal machinery of the vessel by these volatile organic compounds (VOC).

Further, the invention relates to the use of liquified volatile organic compounds (VOC) as a fuel for the thermal machinery of vessels, and the use of liquified volatile organic compounds (VOC) concurrently with the bunker fuel as engine fuel on vessels.

The discharge of volatile hydrocarbon gasses - in the following abbreviated to VOC (= volatile organic compounds) which in some cases will comprise N₂, C0₂ and some 0₂ implies a very severe contamination and thus environmental stress on the atmosphere. The modern society therefore considers this to be a problem, the solution of which is given a high priority.

The discharges are due to the combustion of engine fuel and heating fuels and the discharge of naturally occuring VOC in connection with the loading and transport of oil from oil reservoirs.

Under the influence of sunlight VOC will together with several nitrogen oxides NO_x be able to form ozon and pulmonary irritating gasses in the layer or air close to the ground.

A survey through a prolonged period of time of VOC discharges during offshore loading has shown that considerable and problematic amounts of VOC are discharged into the atmosphere by this type of operations. With respect to Norway, it is in 1996 calculated that about 55% of the total

VOC-discharge is due to discharge of VOC during offshore loading of oil to vessels offshore and during the subsequent transport of the oil to the terminals. Norway has as a nation within the year 2000 obliged itself to reduce its total discharge of the share of VOC, except for the methane part, which share for the sake of abbrevation is mentioned NMVOC (=non-methane volatile organic compounds), into the atmosphere by 30% of the total discharges in 1989. This task will most efficiently be solved by reducing the discharges of VOC during offshore loading and the subsequent transport, as the combustion gasses from machines and heating are difficult to scavenge.

Three principally different techniques may be used to prevent the discharge of VOC into the atmosphere: 1. The condensation of VOC to a liquid state and continued storage in this stage until the use thereof.

2. The transformation of the volatile hydrocarbons in VOC into gas hydrates and transport and storage until the use thereof in this form.

3. The absorbtion of evaporated VOC of the oil during transport and the required reabsorbtion in the oil again of recently evaporated VOC.

The hydrocarbons of VOC are mainly C^Cs hydrocarbons, but a smaller amount of C₆-hydrocarbons will also be present. The composition will vary considerably from one oil field to the other, but also during the period when the production and loading from the particular field takes place.

This variation of the composition constitutes a real challenge of being able to use the VOC components as engine fuel in thermal machinery and particularly in diesel engines.

The wellknown machinery can utilize high pressure injection of gaseous gasses directly into the combustion chamber, or may use the feeding of gaseous or liquified gas to the suction air of the engine. A provision for the gas operation of these well known engines is that the gas is refined or in another way has achieved a preset and stabilized composition having a predictable effect as a fuel in the diesel engine. If one of the wellknown engines, being constructed for gas feeding at a particular ignition ability, suddenly is fed by gas being considerably more able of ignition, a selfignition may occur during the compression stroke leading to severe operation disturbances of the engine. It has previously been suggested to scavenge the VOC damp during loading, liquifying the heavier components and pump the condensed part back to the crude oil at a depth of the crude oil tank at which the liquid pressure is sufficiently high to absorb the condensate. In this way «the gas is packed» into the crude oil. However, the problem of the release of gas from the crude oil arises again when the crude oil is to be handled in the next step of the transport chain. From the customer's point of view, this implies an undesirable supply form of the crude oil, and this will again have a negative impact on the market price of such a crude oil. In addition, several refineries have restrictions with respect to the amount of gas in a crude oil.

The handling of the volatile hydrocarbons as indicated above under item 2, is for instance disclosed a .o. in the Norwegian patent application 96 1666 (not publicly available).

By such a conversion to gas hydrates, it will be possible to scavenge all the lower volatile hydrocarbons in the form of gas hydrates and transport and store them as such in a cooled condition (within the equilibrium conditions) until the use thereof. When used, the gas hydrates are «melted» by heating to release the volatile hydrocarbons and feed them in a gaseous form to combustion means or reactors. The method requires cooling to relatively low temperatures during the formation of the gas hydrates as well as during the subsequent transport and storage demanding energy and which may also require the use of particular construction materials.

Another possibility is a direct absorption of preparated VOC into the oil again. Such a principle is f.i. disclosed in the Norwegian patent application 94 1704, in which VOC in a gaseous form is fed as a stream against the oil in an absorption column (i.e. the VOC gas is introduced at a low point of the column, whereas the absorption agent in the form of oil is introduced at a high point of the absorption column). The VOC-containing crude oil stream forming the product is then recycled to the main loading stream or to the loading tank, preferably at a sufficiently deep site where a sufficient liquid pressure prevails to prevent evaporation of the VOC parts. The principle for the introduction of an absorbate or condensate at the bottom of a tank of an oil tanker is also illustrated in the Norwegian patent 175 290.

However, a considerable problem occurs during the transport of oil supplied with gas in the bottom of the transport tank in this way. Due to the movements of the vessel, oil from the bottom including high concentrations of VOC will be mixed with the oil higher in the tank initially having low concentrations of VOC, thus increasing the concentration of VOC in the higher layers of the tank. In these layers the stable liquid pressure will not be sufficient to maintain said VOC absorbed. This results in a new evaporation of VOC from these layers. The alternative will then be to discharge this gas into the atmosphere or to absorb it again, which will require a continuous operation of a cooling or absorption plant. As mentioned above, this technique will in any case imply that the handling problem of the VOC of the oil is displaced to a later link of the transport chain. As mentioned above, the transformation of the volatile hydrocarbons of

VOC into gas hydrates results in an environmental beneficial form of storing the VOC gasses.

However, present calculations indicate that binding the volatile hydrocarbons of VOC as gas hydrates will be a more expensive process than a simple condensation of VOC into liquid form and storing it in this form until the use thereof. As already mentioned, this relates to the condensation of NMVOC, methane having such a low boiling point (-161.5°C at atmosphere pressure) that the condensation thereof will be inrentable for technical reasons.

The condensed hydrocarbons from VOC will therefore, as already mentioned, mainly constitute C₂-C₅ hydrocarbons, preferably C₃-C₅ hydrocarbons. A separate storage and transport of the condensed hydrocarbons on the tanker will imply logistic requirements when the product is to be supplied to the terminal. Today there is no reception system and no distribution net for this kind of a product. A beneficial solution will therefore be to find a utilization for these condensed NMVOC gasses as soon as possible after the condensation to avoid said logistic problems. According to the invention, the abovementioned problems are solved by performing compression and/or cooling discharging the condensed organic compound as a product stream from at least one separator separator, passing at least one storage unit, and when desired, feeding it as a separate engine fuel to thermal engines.

In this process it is preferred that the condensed VOC is fed to thermal machines aboard vessels, on offshore installations or in connection with plants ashore, particularly such ones where loading/unloading/processing of hydrocarbons are effected. It is parcularly preferred that the liquified VOC is fed to the machinery of the vessel pressurized, particularly that the liquified VOC is pressurized to at least 60 bar prior to injection into the vessel machinery as machine fuel. The vessel machinery may then have a construction as described in Danish patent application No. 0105/97 filed 29. January 1997 by MAN B & W Diesel A/S. The non-condensed part of the VOC gas is preferably discharged as a product stream from the separator(s) and directly used (without storage) as a separate fuel stream, preferably on board vessels, and particularly this gas stream is fed to the thermal machinery of the vessel during the loading of hydrocarbons offshore. So far VOC gas is available which cannot be liquified by means of equipment on board the vessel, this has to receive priority as a fuel as it is more difficult to store, and alternatively to be discharged into the atmosphere.

The gaseous VOC stream may be pressurized to at least 200 bar before the injection into the operation machinery of the vessel. When using the gaseous VOC stream for gas turbins or vessels, it will not be required to pressurize the gas to such high pressures.

Further, the total or parts of the product stream which is discharged in a gaseous form from the separator(s) may be passed to a hydrate forming plant. This may involve that the surplus of the gaseous part which is not fed to the operation machinery of the vessel as a fuel during the loading of oil, is passed to hydrate formation. When required, the stream of liquified VOC being fed to the operation machinery of the vessel will be supplied with a separate stream of bunker oil as a machine fuel.

During loading/unloading at a terminal the operation machinery of the vessel will preferably be fed liquified VOC alone, such an operation resulting in less polluting combustion gasses and thus a reduced environmental load close to rural sites.

Further, liquified VOC alone will preferably be used as machine fuel when the ship is in coastal waters. Correspondingly the part of liquified VOC being fed to the operation machinery of the vessel is, if required, reduced in the ratio to bunker oil when the ship is further away from coastal waters and in the open sea.

In general it is preferred that the liquified VOC only in the degree required is supplied by bunker oil as machine fuel for the machinery of the vessel, The mutual ratios of the streams of compressed gas, liquified VOC and optionally bunker oil being fed to the operation machinery of the vessel, will preferably be regulated by signals therefrom.

The liquifying plant of the VOC gas can preferably also be operated discontinuously, monitored by pressure sensors with appending equipment from the crude oil tanks of the vessel.

The storage tanks of this machine fuel is preferably situated on the deck of the vessel, thereby enabling the method to be performed by feeding the liquified volatile hydrocarbon gasses to the thermal machinery from such storage tanks.

The storage tanks may also be sited in the machinery room of the vessel. At the existing main platforms on the Norwegian sockel, liquified volatile hydrocarbon gasses (VOC) being scavenged by condensation during the loading of the tankers and during the transport by these, will be able to provide sufficient engine fuel for the machinery of the ship both to keep the vessel in the required position during the loading of oil at the platform offshore, and during transport of this oil to terminal plants and the return from the terminal plant to a new loading of oil at the platform offshore. In those case^'s where the oil being loaded does not include sufficient amounts of volatile hydrocarbon gasses after the liquifying to operate the machinery of the vessel alone as indicated above, it will be used with bunker oil for the operation of the machinery of the vesel during the loading, transport and return to offshore loading.

Thus the method of the invention in a new and unexpected way solves an environmental problem as well as a logistic object by using the volatile liquified hydrocarbon gasses (NMVOC) as machine fuel during loading and transport of the crude oil as well as the return of the vessel to a platform, A further advantage of this solution is that the bunker oil fuel tanks of the tankers can be substantially reduced - in some cases by up to 90-95%.

Further, the combustion of NMVOC will result in cleaner discharge gasses from the engines of the vessels, i.e. lower concentrations of sulphur-oxides, nitrogen oxides and PM. Depending on the conditions, a 40-80% reduction of NMVOC discharge will be achieved by the method of the invention by VOC condensation as disclosed herein.

This implies that a substantial part of the total Norwegian national contribution to the NMVOC reduction may be achieved by the use of the present method alone.

Particularly 50-90% reduction of sulphur oxides may be achieved, which is a directly correlated to the reduced combustion of bunker oil.

Further, a 50-90% reduction of PM (smoke), 20-30% reduction of nitrous gasses and some reduction of C0₂ can be achieved. In addition to these ecological gains, an economic gain by up to 90% reduction of combustion of fuel oil is achieved on these tankers. The fuel oil constitutes a main cost of the operation of the vessel.

Further, lower taxes will be available when loading ashore due to reduced VOC, NO_x, SO_x and PM discharges in connection with the loading of the oil ashore.

Further, VOC discharges from oil plants ashore can be scavenged and used as a fuel by the transport and loading in cases where the oil which is produced is so lean on VOC that the condensate will not cover the fuel requirements of loading, transport and return to platform.

Further the excess of hydrocarbon gas which normally escapes process plants could be used as a fuel. The fuel surplus which is possibly condensed unloading from oil source resources, particularly rich on gas in excess of that amount which is required to operate the tankers in re may be used as the fuel for vessels in coastal traffic close to the loading sites in re provided that the appropriate modifications are performed on their machinery. The following figures 1-3 elucidate the invention and the conditions on a shuttle tanker during loading, as well as the designs of machinery which are useful in connection with the present invention.

Figure 1 is a principle sketch of the present method used on a tanker having the engine designed for operation by both liquified VOC (NMVOC), the separated still gaseous VOC gas and a common bunker oil as a fuel.

Figure 2 illustrates a tanker during loading where VOC is mainly liquified and stored in a storage tank.

Figure 3 shows an injection system for gas, liquified gas and fuel oil respectively to a combustion engine in a tanker. In Figure 1 a tanker 2 is indicated having handling and storage means for

VOC in section.

The storage tanks of the vessel are loaded by crude oil 3 through one or more loading hoses 1. During the filling the oil 3 releases hydrocarbon damp to the chambers 4, which for safety reasons are filled with neutral gas (N₂, C0₂ and some 0₂) in the tanks above the oil 3.

This VOC gas is passed from the tanks through circuits 5 to a process plant 14 on the ship deck. In this VOC process plant 14 the main part of the C₃-C₅ hydrocarbons as well as the minor part such as C₆₊ hydrocarbons are condensed, as well as possibly also a part of the C₂ hydrocarbons. In this process the gas is first compressed in a compressor 10 at 20 bar and passed as a stream 6 through a cooler 8 to a separator 9. In this separator 9 the compressed and cooled VOC gas is separated, the gaseous phase thereof being discharged as a stream 20 from the upper part of the separator 9, whereas the liqufied condensed VOC is discharged from the lower part of the separator 9. This liquified VOC is passed as a stream 18 to one or more thermally isolated storage tanks 13, where it is stored as a liquid at low temperatures, f.i. at below -42°C, and by atmospheric pressure, optionally in a pressurized tank. The tank pressure may f.i. correspond to the pressure of the separator.

In the VOC process plant 14 the components are monitored by a monitor device 7 which may receive monitoring impulses from the outside, f.i. as signals from the thermal machinery of the vessel as indicated by 16, or may include a monitoring circuit which receives information of the prevailing tank pressure.

This logistic results in the benefits of a homogenous condensate, and the condensate is kept cool in the storage tank(s) 13 by the supply of fresh, cold condensate.

When the liquified VOC in the tank(s) 13 is to be used as fuel for the machinery of the vessel of the diesel engine type, it is passed as a stream 11 through a compresssor 29 in which it is compressed to at least 60 bar, and further therefrom to the injection system of the diesel engine.

The non-liquified components methane and ethane are passed from the head of the separator 9 through a tube 20 to a multistep compressor compressing the non-liquified gas phase to an injection pressure which may typically be set to about 250 bar, and from this compressor a «common rail system* distributes the gas to the individual cylinders of the engine.

The thermal machines 24 of the vessel may in principle comprise piston engines, turbines and boilers. If the stream 23 is to be used in piston engines, a compressor device should compress it to 200 bar or higher as indicated on figure 1. When it is to be used in turbins and/or vessels, a pressure below 10 bar will be sufficient.

The surplus of gaseous VOC which is not fed to the thermal machinery 24 of the ship through the compressor 21 , is discharged as a sidestream upstreams to the compressor 21. This gas is preferably converted to a gas hydrate in a gas hydrate production plant as indicated by 25, whereas that part of the bi-stream which is not fed thereto is discharged into the atmosphere as a stream 26. The stream 26 is always kept as small as possible for ecological reasons.

Finally a tank 27 is provided for bunker oil which, when required but as little as possible, is passed as supplemental engine fuel in a feed circuit 28 to the machinery of the vessel.

During the loading of the vessel, the condensation plant is operated continuously, but in the following voyage a discontinuous operation of the system will be sufficiently monitored on the basis of measuring the pressure of the tank 13 to f.i. start the condensing system when the gasses of the tank 1 have a 0,14 bar gauge, and is stopped when the gasses have 0,05 bar vacuum. On figure 2 offshore loading of a vessel is shown. Different from what is shown on figure 1 , VOC is not passed from the storage tank as engine fuel to the ships machine, but is transported to a manifold ashore or is optionally returned to the oil being loaded. The texted figure is self- explanatory, but does apart from this, not show a process or use according to the present invention, but rather examples of wellknown technique.

Figure 3 shows an embodiment of an injection system to a single engine cylinder, wherein a secondary injector 16 is provided for injecting gaseous gas and a liquid injector 17 for injecting liquified gas as well as a pilot injector 18 for injecting oil. The three injectors may be separately mounted in individual houses of the appending cylinder lid. It is also possible to fuse two of the injectors in a common house to a so-called «dual fuel» injector. Albeit the pilot injector obviously can be included in such a dual fuel injector, it is preferred in those cases where all three injector types are present on one single cylinder that the secondary injector 16 and the liquid injector 17 are unified in the dual fuel injector, thereby feeding the gasses to the same injector house which facilitates the encapsulation of the gass feed systems. Dual fuel injectors are f.i. further disclosed in the Danish patents 153240 and 155757, and in W095/24551 a gas blowing injector is closer described. Several injectors of each type may be mounted on the same cylinder to among other achieve a better distribution of the fuel in the cylinder. The description set forth in connection with the figures 1 and 3 are only to be construed as examples of an embodiment of the invention. It will be obvious that variations thereof are possible within the scope of the invention, and these are to be considered as equivalents for which protection is also claimed.

The equipment for collecting and preparations of VOC to be used as engine fuel in a vessel machinery may f.i. be supplied as a retrofit equipment to an existing turbin operated tanker.

The scope of the invention is defined by the following claims:

Claims

Patent claims

1. A method of reducing the discharge of volatile organic compounds (VOC) into the atmosphere from a tanker including mainly organic liquids during such handling as loading, transport, discharge, processing and storage of hydrocarbons, characterized in effecting compression and/or cooling the condensed organic compounds being separated as a product stream from at least one separator, and passed to at least one storage unit, and feeding them when required as a separate engine fuel stream to thermal machinery.

2. The method of claim 1 , characterized in feeding the liquified organic compounds to thermal machinery sited aboard vessels, offshore installations and/or in connection with plants ashore.

3. The method of claim 1 , characterized in feeding the liquified organic compounds under pressure to the machinery of the vessel.

4. The method of claim 3, characterized in pressurizing the liquified organic compounds to at least 60 bar prior to the injection thereof to diesel machinery of the vessel as a fuel.

5. The method of claim 1 , characterized by discharging the non-liquified part of the volataile organic compounds as a product stream from the separator(s) and use thereof (without storage) as a separate fuel stream, preferably in the thermal machinery of vessels.

6. The method of claim 5, characterized in feeding said product stream preferably to the thermal machinery of the vessel as engine fuel during the loading of hydrocarbons.

7. The method of claim 5 and 6, characterized in pressurizing the gaseous fuel in the form of said product stream to at least 200 bar prior to the injection in the piston engine(s) of the vessel.

8. The method of claims 5 and 6, characterized in pressurizing the gaseous fuel to a pressure below 10 bar prior to feeding it to the turbins and/or boiler(s) of the vessel.

9. The method of claim 5, characterized in passing the non-liquified organic compounds as a product stream from the head of the separator, as a feed to hydrate formation.

o 10. The method of claim 5-8, characterized in passing a possible surplus of the non-liquified organic compounds which are not fed to the thermal machinery of the vessel as a fuel during loading of hydrocarbons, to hydrate formation.

11. The method of claim 1-8, characterized in feeding only bunker oil to the s required extent as an ignition aid as required when the actual fuel requirement of the machinery exceeds the feeding of volatile organic compounds, to the machinery of the vessel.

12. The method of claim 1-4, characterized in feeding preferably exclusively o liquified organic compounds to the thermal machinery of the vessel during loading/unloading at a terminal.

13. A method of claim 1-4, characterized in feeding preferably liquified organic compounds to the machinery of the vessel when the vessel is in coastal waters. 5

14. The method of claim 1-4, characterized in reducing the percentage of condensed organic compounds being fed to the machinery of the vessel in relation to the bunker oil which is fed to the machinery of the vessel when the vessel is further away from coastal waters and in the open sea. 0

15. The method of claims 1-14, characterized in regulating the fuel stream of the compressed VOC gas and/or condensed organic compounds to the thermal machinery of the vessel by a monitor unit based on predetermined information of when the fuel may be combusted at an environmentally beneficial time.

16. The method of claims 1-15, characterized in operating the liquifying plant for the volatile organic compounds discontinuously monitored by signals from the crude oil tanks of the vessel.

17. The method of claim 1 , characterized in passing the liquified organic compounds stored in plants ashore when required, to the storage tanks for liqiufied organic compounds on board vessels.

18. The use of volatile organic compounds as engine fuel for thermal machinery of vessels.

19. The use of volatile organic compounds concurrently with bunker oil as an engine fuel for the machinery of vessels.