Process for Producing Liquefied Hydrogen

Field of the Invention

The present invention relates to a method for liquefying hydrogen gas, in particular to a method of arranging the necessary heat exchangers.

Background

Processes for the liquefaction of hydrogen typically comprise a first step of a pre-cooling part of the process, in which the hydrogen is cooled to a temperature of between -150 to -200 degC approximately by means of heat exchange with an external refrigerant such as nitrogen, and a second step of further cooling and liquefying the hydrogen by means of heat exchange with hydrogen or helium, both optionally admixed with neon.

So as to minimise ingress of heat, in current practice the necessary heat exchangers and associated equipment for the said second step of the process - further cooling and liquefaction - are usually enclosed in a vacuum-insulated vessel, as illustrated schematically in Drawing 1/2 with the equipment tags and stream numbers shown thereon. A stream of hydrogen [1] at ambient temperature flows to a hot passage of a first heat exchanger [A], having an outlet stream [2] with a temperature typically of -190 degC. The necessary cooling in heat exchanger [A] is provided by a stream (or streams) of a first refrigerant such as nitrogen [3], which flow through a cold passage of heat exchanger [A] and emerge as heated stream or streams [4], Cooled stream [2] flows to a hot passage of a second heat exchanger [B], having outlet steam [5] comprising liquefied hydrogen and having a temperature of below -240 degC. The necessary cooling in second heat exchanger [B] is provided by a stream or streams of a second refrigerant such as hydrogen or helium [6], which flow through a cold passage in second heat exchanger [B] and emerge as heated stream or streams [7],

Heat exchanger [A] is enclosed in a container or cold box [C], which typically contains a particulate insulating material such as perlite. A stream of dry inert gas [8] is admitted to the container [C] for the purpose of excluding air and moisture, and is vented to the atmosphere as stream [9], Heat exchanger [B] is enclosed a vacuum-insulated container [D],

Due to practical difficulties in provision of large vacuum-insulated vessels, the type of construction illustrated on Drawing 1/2 can only be used with a hydrogen liquefaction capacity of around 50 tonnes per day. Multiple vacuum- insulated heat exchangers would be needed for the much larger hydrogen liquefaction plants now under consideration with train capacities of up to 500 tonnes per day. Summary of the Invention

The invention relates to the final stage of a process for liquefaction of hydrogen, in particular to the part of the process with fluid temperatures below -150 degC.

The Applicant notes that in a typical hydrogen liquefaction installation the required heat exchanger capacity, commonly called UA, for the said first step of pre-cooling to a temperature of between -150 degC and -200 degC may be around 5 times the corresponding heat exchange capacity for the second step of further cooling and liquefaction of the hydrogen. Accordingly, it is expected that the total volume of the heat exchangers necessary for the first step will be larger than the total volume of the heat exchangers necessary for second step.

The invention consists of installing the heat exchangers required for the second step in a thermally insulated enclosure or cold box which is located inside a larger thermally insulated enclosure or cold box which also encloses the heat exchangers required for the first step.

With this arrangement, the temperature difference between the periphery of the enclosure of the second step, which may be at a temperature of around -150 degC, and the temperature of the liquid hydrogen product at around -250 degC is around 100 degC. This temperature difference is much smaller than the difference of around 280 degC between ambient temperature and liquid hydrogen when enclosure of the second step is installed independently of the enclosure of the first step. As a result the potential heat leakage into the enclosure of the second step is reduced, relative to the heat leakage into an independent enclosure of the second step. The need for a vacuum-insulated construction of the enclosure of the second step becomes less important, and use of a cold box construction of conventional type becomes feasible.

Thereby the capacity limitation of the said vacuum-insulated construction is avoided, with the associated need for multiple vacuum-insulated heat exchangers in the coldest part of the liquefaction process in plants with high output capacities.

The inner smaller cold box is filled with hydrogen or helium at a slightly elevated pressure so as to prevent nitrogen leaking in from the larger surrounding cold box and solidifying on the surface of the heat exchangers required for the second step.

Accordingly there is provided as follows a description of a process and apparatus for liquefying hydrogen, illustrating the main aspects of the invention (reference is made to Drawing 2/2 and the equipment tags and stream numbers shown thereon): providing a first stream of hydrogen gas [11] at ambient temperature; cooling stream [11] in a hot passage of a first heat exchanger [A], having an outlet steam [12] with a temperature of between -150 degC and -200 degC; providing a stream or streams [13] of a first refrigerant, such as nitrogen; passing [13] into a cold passage of a first heat exchanger [A], having outlet [14] with a temperature lower than the temperature of stream [11]; passing stream [12] into a hot passage of a second heat exchanger [B], having an outlet stream [15] comprising liquid hydrogen with a temperature lower than -240 degC; providing a stream or streams [16] of a second refrigerant, such as hydrogen or helium; passing [16] into a cold passage of second heat exchanger [B], having an outlet [17] with a temperature lower than the temperature of stream [12]; providing a first heat-insulated container [C], enclosing first heat exchanger [A]; providing a second heat-insulated container [D], enclosing first heat exchanger [B]; enclosing said second heat-insulated container [D] within the volume of said first heat-insulated container [C], providing a stream of nitrogen gas [18]; passing stream [18] through the volume of first heat-insulated container [C] so as to sweep out air and moisture in outlet stream [19]; providing a stream of hydrogen or helium gas [20]; - passing stream [20] through the volume of second heat-insulated container [D] so as to sweep out nitrogen in outlet stream [21]; providing a valve [E] with inlet stream [21] and outlet stream [22]; adjusting valve [E] so as to maintain the pressure in stream [21] at a higher level than the pressure of stream [19].

The heat-insulated containers [C} and [D] typically contain a particulate insulating material such as perlite.

If desired the condition of stream [15] may be that of a gas with a temperature lower than -200 degC.