Method and system for storage and transport of liquefied petroleum gases

Abstract

A method for storage and transport of LPG on LPG carriers, in particular two cargoes of different LPG types on same shipment, having reliquefaction units in which vaporized gases are condensed and then returned into at least one cargo tank for the respective LPG cargo type. The method is further comprising: using the reliquefaction units, at a minimum one running, as to condense vapour from the first cargo type; passing the condensed vapour through a heat exchanger; simultaneously flowing vapour from the second cargo type through the heat exchanger as to condense vapour by means of heat exchanging with the condensed vapour; and returning the condensed vapours leaving the heat exchanger back into the respective cargo types. The present invention is also disclosing a system for storage and transport of LPG on LPG carriers.

Claims

1. A method for storage and transport of liquefied petroleum gas (LPG) on LPG carriers, in particular two cargoes of different LPG types on same shipment, having reliquefaction units in which vaporized gases are condensed and then returned into at least one cargo tank for the respective LPG cargo type, the method comprising: using the reliquefaction units, at a minimum one running, so as to condense vapor from a first one of the cargo types to yield a first condensed vapor; passing the first condensed vapor through a heat exchanger; simultaneously flowing vapor from a second one of the two cargo types through the heat exchanger so as to condense the vapor to yield a second condensed vapor by means of heat exchanging with the first condensed vapor; returning the first and second condensed vapors leaving the heat exchanger back into the respective cargo types; and if free flow back to at least one cargo tank for the second cargo type is impeded, pumping the second condensed vapor to be returned into the second cargo type.

2. A system for storage and transport of liquefied petroleum gas (LPG) on LPG carriers, in particular two cargoes of different LPG types on same shipment, having reliquefaction units in which vaporized gases are condensed and then returned into at least one cargo tank for the respective LPG cargo type, comprising: reliquefaction units configured, at a minimum one running, to condense first vapor from a first one of the two cargo types to yield a first condensed vapor; a heat exchanger configured to let the first condensed vapor pass therethrough; the heat exchanger being further configured to allow vapor from a second one of the cargo types to simultaneously flow therethrough to condense the vapor to yield a second condensed vapor through heat exchange with the first condensed vapor; and the heat exchanger having an outlet enabling the first and second condensed vapors leaving the heat exchanger to return back into the respective cargo types; wherein if free flow back to at the least one cargo tank loaded with the second cargo type is impeded, the second condensed vapor is pumped by means of a pump situated in a piping.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is discussed below with reference to preferred embodiments presented in the accompanying drawings, in which:

(2) FIGS. 1 and 2 schematically show a typical prior art reliquefaction unit and typical arrangement for a VLGC carrying two cargoes, respectively;

(3) FIG. 3 schematically shows an embodiment having two reliquefaction units, of which one is running and the other is in standby;

(4) FIG. 4 schematically shows another embodiment corresponding to FIG. 3, except that a throttle is arranged downstream of a heat exchanger;

(5) FIG. 5 schematically shows an out cut of the embodiments in FIGS. 3 and 4, respectively, and including a pump downstream of the heat exchanger;

(6) FIG. 6 schematically shows an out cut of the embodiments in FIGS. 3 and 4, respectively, and including a compressor upstream of the heat exchanger;

(7) FIG. 7 schematically shows an embodiment similar to FIG. 3 but including a separator downstream of the heat exchanger; and

(8) FIG. 8 schematically shows an embodiment in which running time by intermittently operating the reliquefaction unit based on pressure increase in the cargo tanks.

DETAILED DESCRIPTION OF THE INVENTION

(9) As mentioned above and illustrated in FIG. 3, for instance, the invention relates to a method and system for transporting and storing liquefied petroleum gases, in particularly two grades of products, on the same shipment. This allows for a reduced number of installed reliquefaction units compared to “Prior Art” all down to a minimum of two units including one running unit and is still providing the required redundancy set forth by international classification societies and the IGC Code. Ship owners additional requirements on refrigeration duty is also covered. During normal operations one out of the two units is in standby operations.

(10) Although the reduced number has a minimum of two reliquefaction units, other options is possible For instance, one reliquefaction unit with redundant rotating machinery could be used. Other configurations are also applicable, e.g. having three units.

(11) Note that the type of reliquefaction unit is not crucial when utilizing the invention. However, for convenience it is assumed same type of reliquefaction unit corresponding to the prior art but with typically twice the capacity.

(12) Vapor that evaporates from the first cargo type contained in one or more cargo tanks 100 flows via a line 1 to the reliquefaction unit 300 to be condensed and, thereafter, returned via a line 5. Condensate flows from the reliquefaction unit 300 via a throttle valve 600, in which the pressure is reduced to meet the pressure in the cargo tank(s) 100. After throttling, the condensate or, depending on the process conditions of the reliquefaction plant, the mixed phase fluid enters a heat exchanger 500, in which the condensate is used as the heat sink. At exit of the heat exchanger 500, the condensate leaves in the form of a mixed phase fluid and flows back to the cargo tank(s) 100. The heat exchanger 500 is preferably a free flow condenser.

(13) Although, only one heat exchanger is shown in the drawings, it should be understood that more heat exchanger 500 could be installed. In such an instance the condensed vapor from the reliquefaction unit 300 is divided in an appropriate manner and passed through the respective heat exchangers.

(14) Vapor that evaporates from the second cargo type contained in at least one cargo tank 200 flows via a line 6 to the heat exchanger 500 and the vapor is condensed and returned back to the cargo tank(s) 200 via a line 7. The vapor flow is by means of natural circulation. No compressors or other mechanical means are needed, such as e.g. an ejector, to propel the vapor from cargo tank 200 into heat exchanger 500 to be condensed and returned.

(15) The refrigerant duty required to condense all vapor associated with the second cargo type is taken from the available spare refrigerant capacity of the reliquefaction unit handling all vapor associated with the first cargo type. Condensate from the refrigeration unit 300 is thus used as a refrigerant in the heat exchanger 500 to condense the vapor from the second cargo type.

(16) The heat exchanger 500 is preferably installed on a high point location on the LPG carrier allowing the condensed vapor to freely flow back to the cargo tanks 100, 200. A high point location can be on top of the cargo compressor room, on the pipe rack running along the LPG carrier, on a high point on any existing deck module or on a dedicated high point structure.

(17) Handling of all associated vapor from the first cargo type is in principle identical to the “Prior Art” but differs with respect to the increased vapor flow rate caused by the fact that the condensate returned to the tank(s) 100 is first used to condense all associated vapor from the second cargo type before returned to the cargo tank(s) 100. The net condensate returned to the first cargo type in the cargo tank(s) 100 corresponds to the net evaporated cargo vapor being evaporation caused by heat added to the cargo tank(s) 100.

(18) The function according to the invention is based on the fact that each reliquefaction unit is designed for handling a ship being fully loaded with its coldest design cargo, typically propane and when some of this cargo capacity is taken up by a warmer cargo, e.g. butane, it is available an excessive refrigeration capacity that can be used to condense the warmer part cargo.

(19) The excessive refrigeration capacity is utilized by transferring heat added to the warmer cargo side into the colder cargo side and, thus, circulating a higher cold vapor flow than if two segregated arrangements are in operation.

(20) The present example illustrates the operations for a LPG carrier loaded with two grades on board a VLGC. Iso-butane is loaded in two cargo tanks, tank A & B, and propane is loaded in two other cargo tanks, tank C & D.

(21) Approximately 690 kg/hr of iso-butane flows naturally towards heat exchanger 500 and typically enters the heat exchanger at a temperature of −3° C. The total refrigerant duty required to cool and condense this flow of iso-butane is about 7/kW. The total refrigerant duty required to cool and condense the propane flow is about 219 kW. One reliquefaction unit has a total refrigeration capacity of 427 kW.

(22) Other sizes of reliquefaction units occur for other sizes of LPG carriers.

(23) As depicted in FIG. 4, the throttle valve 600 is alternatively located downstream of heat exchanger 500.

(24) When needed, the heat exchanger 500 can alternatively be located at a lower elevation than the piping running back to the cargo tanks 100, 200 but, then, a circulation pump 700 must be installed, see FIG. 5 not showing the correct location of the heat exchanger relatively to the piping.

(25) Alternatively, a small blower or compressor 800 can be installed upstream of heat exchanger 500 providing a slightly elevated condensation pressure and thus also allowing for a more flexible location of heat exchanger 500, see FIG. 6.

(26) As shown in FIG. 7 the mixed phase fluid leaving the heat exchanger 500 via the line 5 enters a separator 900, in which the vapor and liquid phases are separated. Liquid leaves via a line 8 and is introduced back into the first cargo type in the cargo tank(s) 100. Vapor leaves via a line 9 and is mixed with vapor flowing in line the 1. By this arrangement, the required vapor handling capacity of each liquid tight container and associated piping is reduced.

(27) To minimize running time on machinery the reliquefaction units are operated intermittently. This is done by allowing the pressure in the cargo tanks to increase to a high level, then start the reliquefaction units and reduce the pressure in the cargo tanks. Actual running time is governed by several factors as e.g. ambient temperatures, amount of volatile components in the cargo and sea conditions. Volatile components in the LPG are typically ethane and normally varied between 0 and 5 mol %. Higher concentrations of ethane may occasionally occur.

(28) The compressor 1.100 and 1.200 shown in FIG. 1 is commonly two compression stages of one large reciprocating compressor 2.000, see FIG. 8. More than two compression stages are also common, not shown. An electric motor 1.900 drives the compressor.

(29) A reciprocating compressor is a positive displacement compressor where for a given compressor its volumetric capacity is given by its design and thus operates at its maximum volumetric capacity at any given time. Since not only running time but also the compression work is governed by conditions as ambient temperatures and amount of volatile components in the gas to be compressed the electric motor 1.900 does not necessarily run on its maximum continuous rating.

(30) To utilize the power potential of the electric motor it is proposed to speed up the motor rpm above normal rpm when conditions described above allows for it, this will be done by increasing the frequency of the power supply 1.950 to frequencies above normal. The volumetric capacity of a displacement compressor increases proportionally with speed and hence the refrigerant capacity also increases and thus running time reduces.