Note: Descriptions are shown in the official language in which they were submitted.
WO96/21121 21 8 ~ 2 8 5 PCT~094/00215
A METHOD OF DRAINING A TANK AND A PLANT FOR USE IN SUCH
DRAINING.
The present invention relates to a method of draining a tank
containing a vapourized gas - after draining of its contents
of liquified gas - by supplying to the tank vapourized
nitrogen which is stored in liquid state in a supply.
Moreover, the invention relates to a plant for use in draining
of such a tank, comprising a supply of liquid nitrogen to be
supplied to the tank in vapourized state.
Nitrogen is supplied in order to remove residues of
combustible gases in tanks, and also in order to prevent the
tanks in containing a mixture of different gases when one type
of gas is to replace another type. Vapourized nitrogen, thus,
is supplied to the tanks after substantially draining their
contents of liquid gas, in order to expel the residues of the
vapourized gases present in the tanks. The residual gases may
be burnt or discharged into the atmosphere. Thereby a
contamination takes place, because of the combustion or
because the gases as such are contaminating. Moreover, a not
insignificant amount of residual gas is lost each time this is
carried out, which has been accepted as unavoidable.
Another aspect of flushing tanks with nitrogen is that the
nitrogen is stored in liquid state, resulting in consumption
of heat for vapourization of the nitrogen prior to supplying
it to the tanks. The surrounding air may indeed be used as a
heat emitting medium for the vapourization, but thereby the
large cooling capacity of the liquid nitrogen is not utilized
for anything else than cooling the air which causes the
vapourlzatlon.
According to the present invention a method and a plant have
been provided which at the same time permit recovery of the
contents of residual gases in vapourized state in tanks whose
contents of liquid gas have been drained and vapourization of
WO96/21121 ~ PcT~094/00215
2184285
liquid nitrogen to be supplied to the tanks by utilization of
the heat in the residual gases.
The method and plant according to the invention appear from
5 the succeeding claims.
According to the invention residual gases in vapourized state
are expelled from an incompletely drained tank, in order to
undergo heat exchange with liquid nitrogen to be supplied to
the tank. The heat of the vapourized residual gases is
utilized for vapourization of the nitrogen, and the residual
gases can be condensed into liquid state in order to be
recovered.
15 The invention may be utilized in landbased as well as in
floating plants, for tanks on vehicles (trucks, railway), gas
carrier ships, stationary onshore and offshore plants. In all
cases is achieved that discharge of residual gases to the
atmosphere or the development of combustion gases by
20 combustion of the residual gases is avoided.
For all of the most common gases transported in liquid state
the boiling point is substantially higher than that of
nitrogen, whose boiling point is - 197C (at atmospheric
25 pressure), and when the residual gases are at their boiling
point a large temperature difference will occur during the
exchange of heat. The residual contents in a tank after
incomplete drainage may for instance have the following
temperature, depending on the type of gas: ethylene -100C,
ethane -80C, propylene -45/-20C, propane -40/-20C, NH3 -
33C, butadiene and butylene -1C, butane +3C, i.e. slightly
higher than the boiling poin-t at atmospheric pressure. The
- residual contents of such gases in a tank after a usual
(incomplete) drainage may be estimated as follows, when the
volume of the tank is 6500 m3: ethylene 17000 kg, ethane 19500
kg, propylene 17000/42500 kg, propane 17000/35500 kg, NH3 5700
kg, butadiene l8500 kg, butylene 20800 kg and butane l9500 kg.
2184~
WO96/21121 PCT~094/00215
The plant may of course have several recovery tanks, at least
one for each type of residual gas to be collected.
The heat exchange may take place in two steps. Residual gas
supplied from the tank which is incompletely drained can be
conveyed to a first heat exchanger or a group of heat
exchangers, in which exchange of heat takes place between
vapourized, cold nitrogen gas from a storage tank and the
residual gas, and the nitrogen gas is conveyed to the
incompletely drained tank. The residual gas, which does not
necessarily have to condense, can be conveyed for condensation
in a separate circuit inside the nitrogen storage tank. This
condensation will cause that nitrogen is vapourized and
expelled from the storage tank and into the first heat
exchanger. The exchange of heat in the storage tank can be
carried out in such a manner that only a suitable degree of
supercooling of the condensate takes place.
Because the nitrogen temperature in the heat exchangers for
nitrogen and residual gas is lower than the freezing point of
the residual gas, freezing-up problems can arise in the heat
exchangers. This problem can be avoided by interposing a
cooling agent system between condensing residual gas and
vapourizing nitrogen, so that no direct heat exchange takes
place between nitrogen and residual gas. The cooling agent
system keeps the temperature of the vapourized nitrogen below
the freezing point of the residual gas, while the cooling
agent is kept above the freezing point of the residual gas.
Such a system may for instance work with propane. Among other
mediums which can be used are mentioned propene, ethane,
mixtures of hydrocarbons and halocarbon Rl3. The cooling
agent, in liquid state, is circulated by pumps, for exchange
of heat with the nitrogen and the residual gas, respectively.
The exchange of heat with the nitrogen may take place in two
auxiliary heat exchangers. The cooling agent is pumped through
a first auxiliary heat exchanger which causes vapourization of
nitrogen. In a nitrogen tank sorrounding this first auxiliary
Wo96121121 PCT~094/00215
8428~ , ` ,
heat exchanger the nitrogen is kept at such a high pressure,
and a correspondingly high temperature, that the cooling agent
does not freeze, but the temperature may be lower than the
freezing point of the residual gas. Another pump pumps the
cooling agent through the residual gas condenser, whereupon
the agent is conveyed for heat exchange with the nitrogen
vapour in a second auxiliary heat exchanger, in order to ~
adjust the temperature of the nitrogen vapour prior to
conveying it to the heat exchanger for cooling of residual gas
supplied from the tank to be drained. Thus, the agent in the
cooling agent system, being kept at a higher temperature than
the freezing point of the residual gas, is utilized for
condensation of the residual gas. This is achieved by a
control valve. The heating of the nitrogen can be adjusted by
adjustment of the flow of the liquid cooling agent through the
second auxiliary heat exchanger. The system may comprise
indicators and controllers which permit adaption of the
temperatures to different types of residual gases having
different freezing points.
The invention will be explained more detailed in the
following, with reference to the accompanying drawings, which
diagrammatically show two examples of plants according to the
lnventlon .
Fig. 1 shows a first embodiment, which is solely based on
exchange of heat between nitrogen vapour and
residual gas.
Fig. 2 shows a second embodiment, having a separate cooling
agent system which prevents freezing of the residual
-- gas.
The plant shown in Fig. 1 comprises three main units; a
recovery unit 1, in which residual gas and nitrogen are
subjected to exchange of heat and separation, an insulated
tank 2 having a heat exchanger (condenser) in a supply of
liquid nitrogen and an insulated tank 3 for receiving
WO96/21121 2 1 8 4 2 8 5 PCT~094/00215
recondensed residual gas.
From the tank (not shown) having been incompletely drained of
its contents of liquid gas and containing a residual gas in
vapour state, the residual gas is conveyed into the recovery
unit l through a conduit lO. In the conduit lO is shown a fan
6-, for pumping of the residual gas. The residual gas is
conveyed to a heat exchanger 4, to which also vapourized, cold
nitrogen from the tank 2 is supplied via a conduit 14.
Nitrogen in vapour state from the heat exchanger 4 passes a
heater 7, whereupon it is conveyed into the tank containing
residual gas, via a conduit ll. In the first phase nitrogen
and residual gas in the tank will be stratified. The nitrogen
will act to force residual gas out of the tank and into the
recovery unit l. The fan 6 may in the principle be omitted,
but it will accelerate the transport of residual gas to the
plant. The heat exchanger 4 may for instance be a tube heat
exchanger of a known type.
Below the heat exchanger 4 is mounted a collector 9, which
receives condensed residual gas and uncondensed residual gas
having an increasing amount of nitrogen gas. The condensed
residual gas is conveyed further to a separator 5, from which
the condensed residual gas is transported through a conduit 16
to the collector tank 3.
Moreover, the collector 9 is connected to a condenser 8 for
the residual gas. The uncondensed residual gas and the
nitrogen present is conveyed from the collector 9 through the
condenser 8, where an almost complete condensation and
supercooling of the residual gas takes place. The supercooled
residual gas and the nitrogen present is conveyed through the
conduit 13 to the separator 5. The nitrogen is discharged from
the separator, and the condensed residual gas is conveyed
through the conduit 16 to the collector tank 3.
Moreover, the collector tank 3 for condensed residual gas is
WO96/21121 PCT~094/00215
2;~4~85 "
connected to the conduit l0 for incomi~g residual gas to the
plant, through a conduit 18. Overpre-ssure in the collector
tank 3 will cause recirculation of residual gas.
The plant may comprise shut-off valves, being shown in the
drawing as a valve l9 in the conduit 15, a valve 20 in the
conduit 14, a-valve 21 in the conduit 18 and a valve 22 in the
- conduit 16. Of course additional valves may be included, for
instance safety valves.
The plant may be designed for, but is not limited to, all
types of liquified gases having a gas pressure of more than
2,8 kp/cm2 (a~s.) at a temperature of 37,8C. The plant can
only be utilized with one residual gas at a time. If the plant
is to be utilized with plural types of residual gases, the
residual gas side of the plant has to be flushed with nitrogen
gas prior to admitting another type of gas. Moreover, another
collector tank must be connected.
Of course, the plant comprises, in addition to valves, (not
shown) sensors and controllers.
When residual gas in vapour state, for instance ethylene at a
temperature of approx. -100C, is conveyed to the heat
exchanger 4 and cold nitrogen gas at approx. -190C is
simultaneously supplied to the heat exchanger 4 from the
storage tank 2, the residual gas is cooled, but the nitrogen
is heated further prior to flowing to the tank to be drained.
A further heating is in some cases desired in order to achieve
a favourable stratification of nitrogen and residual gas in
the tank to be drained.
The nitrogen will force the residual gas from the tank to the
plant, possibly aided by the fan 6. After some time from the
starting of the plant a mixture of residual gas and nitrogen
will flow in the conduit l0. On the residual gas side of the
heat exchanger 4 the separator 5, therefore, is connected, in
W096/21121 21 8 4 2 8 ~ PCT~094/00215
order that the nitrogen accompanying the condensed residual
gas be separated and discharged through the conduit 17. The
condensate of residual gas is conveyed from the separator 5 to
the collector tank 3. The uncondensed residual gases in the
collector 9 and the accompanying nitrogen are conveyed to the
condenser 8 situated in the nitrogen storage tank 2.
- Provisions may be made in order that the condensate of
- residual gas be only supercooled to a limited degree, i.e.
that the cooling does not proceed until the condensate is in
the vicinity of the temperature of the liquid nitrogen. The
accompanying nitrogen will still be in gaseous state. From the
condenser 8 the condensate of residual gas and the nitrogen
present are conveyed to the separator 5, where the condensate
is mixed with condensate coming directly from the collector 9.
The nitrogen is separated and discharged through the conduit
17.
The result, thus, is that the residual gas can be recovered
almost completely. The residual gas in tanks which have been
incompletely drained may comprise in the order of l % of the
contents of the tank when filled. Thus, the quantities of
residual gas in question which can be recovered are large. In
addition to the contamination which is avoided, residual gas
representing a large value is collected.
Another example of a plant according to the invention,
including a system with heat exchange by use of a cooling
agent, is explained in the following, with reference to Fig.
2. The same reference numerals as in Fig. l are used for
elements being similar to or equivalent with elements in Fig.
l. A system is described which uses propane as cooling agent. -
- A first auxiliary heat exchanger 30, in the form of a set of
tubes for propane, is mounted in a nitrogan tank 2 to which
nitrogen is supplied from a supply (not shown) through a valve
52, the propane being forced by a pump 31, through tubes 32,
33. A valve 40 keeps the nitrogen at such a high pressure in
W096/21121 PCT~094/00215
218~285
8 ;
the tank 2 that the propane does not freeze. The pressure may
for instance be approx. 2,8 kp/cm2 (abs.). A second pump 35
brings the propane through the residual gas condenser 8, via
tubes 36 and 37. A valve 38, controlled by a regulator 44,
controls the propane temperature in such a manner that it is
somewhat higher than the freezing point of the residual gas
supplied to the condenser 8. The nitrogen vapour flowing from
the tank 2 to the heat exchanger 4 is directed through a
second auxiliary heat exchanger 39, through which also propane
having passed through the condenser 8 flows, via tubes 41 and
42. Thereby the nitrogen vapour is heated prior to flowing to
the heat exchanger 4 via the tube 14. Residual gas is supplied
to the heat exchanger 4 through a conduit 10, which may be
equipped with a valve 53. The degree of heating of the
nitrogen vapour in the auxiliary heat exchanger 39, for
accommodation to various types of residual gases, is
controlled by a valve 43, which regulates the flow of propane
through the auxiliary heat exchanger 39 in such a manner that
the nitrogen vapour attains a somewhat higher temperature than
the freezing point of the residual gas. The regulation during
operation may take place automatically, in that temperature
controllers 44 and 45 control the valves 38 and 43, and these
controllers can be set somewhat higher than the freezing point
of the residual gas.
Fig. 2 also shows a nitrogen heater 7 having a fan, for flow
through of nitrogen vapour to be supplied to the tank to be
drained, via a conduit 11. The conduit 11 may be equipped with
a valve 54, and also a discharge valve 55 is shown. A conduit
12 conveys residual gas from the heat exchanger 4 to the
condenser 8. The residual gas may be caused to condense by the
pressure in the tank to be drained. This pressure is
determined by the amount of nitrogen supplied to the tank.
Also a pump may be used, if the tank to be drained cannot
withstand the necessary inner pressure. Because residual gas
and nitrogen are mixed in the tank to be drained, a nitrogen
separator 5 is used, to which the residual gas is supplied via
W096/21121 21 8 4 2 8 ~ PCT~094/00215
a conduit 13, and residues of nitrogen are discharged through
a conduit 17, which is equipped with a controller valve 51,
while condensed residual gas is conveyed to a collector tank
(not shown) through a conduit 16 equipped with a pump 47, a
filter 48 and valves 49 and 50. A condensate collector 9 is
mounted in association with the heat exchanger 4 and receives
partially condensed residual gas. Condensate of residual gas
from the collector 9 is conveyed to the separator 5 via a
conduit 15 equipped with a valve 56.
Moreover, the propane circuit is equipped with an expansion
tank 46 for propane, in order to allow thermal expansion.
Fig. 2 also shows various auxiliary valves, indicators and
controllers, having the following designations:
DPI: Pressure difference indicator.
LIC: Liquid level indicator and controller.
M : Servo motor.
P : Pressure gauge.
PSV: Safety valve.
PIC: Pressure indicator and controller.
SC : Speed controller.
TIC: Temperature indicator and controller.
TI : Temperature indicator.
UC : Level controller.
While residual gas flows from the tank to be drained through
the conduit lO, nitrogen vapour flows to the tank through the
conduit ll, after having passed the auxiliary heat exchanger
39, the heat exchanger 4 and the heater 7. The residual gas
flows throu~h the heat exchanger 4, the condenser 8 and the
nitrogen separator 5, to a not shown collector tank. In the
cooling agent system the liquid cooling agent flows in a
circuit through the first auxiliary heat exchanger 30 and in a
second circuit through the condenser 8 and thereupon to the
second auxiliary heat exchanger 39. The indicators and the
WO96/21121 2 1 8 4 2 8 5 PCT~094/00215
10 ~. ~
controllers make it possible to control the temperature of the
nitrogen vapour in such a manner that the residual gas being
cooled in the heat exchanger 4 and being condensed in the
condenser 8 does not freeze, and the temperature can be
adjusted in accordance with the type of residual gas.
