Process and plant

Abstract

A process for the catalytic cracking of a liquid ammonia feedstock to produce a cracked gas stream, comprising the steps of: heating the liquid ammonia feedstock to an intermediate temperature by heat

Description

Field of the Invention The present invention relates to processes and plants for producing hydrogen. More specifically, the present invention relates to a process and plant for the catalytic cracking of ammonia to form hydrogen and nitrogen. Background of the Invention There is renewed interest in using hydrogen as a green, carbon free, fuel in a variety of industrial settings. Hydrogen may be combusted to produce heat energy or electrical energy. Alternatively, hydrogen may be used as a fuel to produce electrochemical energy in, for example, a fuel cell. Ammonia has received interest as a possible compound to enable the storage and transport of hydrogen. Liquid ammonia has a higher hydrogen density than liquid hydrogen, and may be transported using existing infrastructure which is already in use for this purpose, such as that used for the transportation of ammonia in the agrochemical fertiliser industry. Once the liquid ammonia has been transported it may be converted to hydrogen by the process of cracking in an ammonia cracking plant. The catalytic cracking of ammonia into hydrogen and nitrogen has been known for many years. The reaction may be depicted as follows: 2 NH3 # N2 + 3 H2 The ammonia cracking reaction is endothermic and may usefully be achieved by passing ammonia over a suitable catalyst in externally heated catalyst-containing reaction tubes disposed in a furnace. Such furnaces are known, for example, for the steam reforming of natural gas or naphtha feedstocks. Processes for cracking ammonia are known in the art. Typically, liquid ammonia is vaporised and fed to an ammonia cracking reactor where a cracked gas, comprising hydrogen (H2) and nitrogen (N2) is produced. The cracked gas may then be cooled and heat recovered from it in the form of steam. Residual ammonia present in the cracked gas may be removed by scrubbing with water before a pure hydrogen product and a tail gas is obtained using, for example, a pressure swing absorption process. The tail gas from the pressure swing absorption process comprises hydrogen, nitrogen, and residual ammonia and may be recovered for further processing or to be combusted to produce heat. A block flow diagram of a typical ammonia cracking process is shown in Figure 1. WO2024112620A1 describes a process which is concerned with providing heat to a liquid ammonia feedstock to vaporise it. WO2024112620A1 is not concerned with how to provide or optimise cooling to downstream processes in an ammonia cracking process. It is envisaged that the majority of ammonia cracking plants may be situated at transport terminals, such as ports, or in industrial areas, where construction space is at a premium. There is therefore a need to minimise the footprint of ammonia cracking plants. Moreover, ammonia cracking plants may be situated in a variety of climates around the world. There is therefore a need to provide ammonia cracking plants and processes which may operate efficiently regardless of the indigenous climate. There is a need for improved ammonia cracking processes with improved heat integration and smaller plant footprint. Summary of the Invention The present invention provides a process for the cracking of ammonia which can reduce the footprint of an ammonia cracking plant and which may operate efficiently regardless of the indigenous climate. The present invention further provides an ammonia cracking plant with a reduced footprint and which may operate efficiently regardless of the indigenous climate. In a first aspect of the invention there is provided a process for the catalytic cracking of a liquid ammonia feedstock to produce a cracked gas stream, the process comprising the steps of: i) heating the liquid ammonia feedstock to an intermediate temperature by heat exchange with a liquid heat exchange medium to produce a cooled liquid heat exchange medium; and ii) using the intermediate temperature liquid ammonia feedstock to provide cooling to one or more downstream processes. In a second aspect of the invention there is provided an ammonia cracking plant configured to run the process of the first aspect of the invention. Heat management and heat integration in processes and plants for the catalytic cracking of ammonia are of paramount importance. Different processes within an ammonia cracking plant have very different temperature requirements and function optimally at different temperatures. The optimisation of heat energy is therefore critical in obtaining an economic and environmentally sustainable ammonia cracking process and plant. This is particularly true given the endothermic nature of the ammonia cracking reaction. The liquid ammonia feedstock used in the ammonia cracking reaction may be stored under pressure and at low temperature. Ammonia has a boiling point of about -33 °C at 1 atmosphere pressure. In order to vaporise liquid ammonia at the rate required to feed a commercial ammonia cracking reactor heat input is required. This is typically achieved by heat exchange with a medium such as steam (i.e. water vapour), cracked gas, or flue gas from the ammonia cracking reactor. However, direct vaporisation of ammonia means that at least one source of cooling (i.e. the liquid ammonia itself) is lost. The process of the present invention uses a liquid heat exchange medium which heats the liquid ammonia feedstock to an intermediate temperature. In doing so the temperature of the liquid heat exchange medium decreases. By heating the liquid ammonia feedstock to an intermediate temperature, a source of ultralow temperature cooling has been realised which is able to provide cooling to one or more downstream processes at consistent and low temperatures. The intermediate temperature liquid ammonia feedstock provides greater cooling to the one or more downstream processes than a water based cooling system. Moreover, the process of the invention allows the intermediate temperature of the liquid ammonia feedstock to be controlled by varying the heating provided by the liquid heat exchange medium. Accordingly, the intermediate temperature liquid ammonia feedstock provides a tuneable cooling source which may be present at a range of different temperatures. It is a further advantage of the present invention that, in addition to the intermediate temperature liquid ammonia feedstock, the cooled liquid heat exchange medium may also be used to cool one or more downstream processes. Surprisingly, the temperature of the cooled liquid heat exchange medium may be lower than that of plant process water, in particular plant process water in warmer climates and / or plant process water during hot summer months. It is a further surprising advantage of the process and plant of the present invention that the total volume of cooling medium (e.g. liquid heat exchange medium) required in processes and plants for the cracking of ammonia may be reduced. The reduction in the total volume of cooling medium means the duty of cooling systems (e.g. cooling towers) of the ammonia cracking plant can be decreased in size, thereby reducing the overall footprint of the plant. Brief Description of the Drawings Figure 1 shows a block flow diagram of an ammonia cracking process. Figure 2 shows a block flow diagram of an ammonia cracking process according to the invention. Detailed Description Preferred and / or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and / or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise. Lower and / or upper limits of any ranges disclosed herein are envisaged to be combinable with one another to provide new ranges, whether explicitly stated or not. The process of the invention is a process for the catalytic cracking of a liquid ammonia feedstock to produce a cracked gas stream. The process of the invention may be applied to an ammonia cracking plant. Accordingly, any feature described in relation to the process of the invention may equally be applicable to the plant of the invention. As used herein, by “downstream process” it is meant a process which is downstream of the process of heating the liquid ammonia feedstock to the intermediate temperature. For the avoidance of doubt, any processes and / or steps of the invention and the use of the intermediate temperature liquid ammonia feedstock in the one or more downstream processes may be independent of one another unless the context demands otherwise. In other words, it may not be essential for the intermediate temperature liquid ammonia feedstock to be used in every process and / or step of the invention unless the context requires otherwise. Furthermore, the intermediate temperature liquid ammonia feedstock of the invention may be used in combination and / or conjunction with other sources of cooling (e.g. plant process water and / or the cooled heat exchange medium). It may be preferable to use the intermediate temperature liquid ammonia feedstock as a final cooling step, thereby exploiting the low temperature of the intermediate temperature liquid ammonia feedstock. The process for the catalytic cracking of the liquid ammonia feedstock to produce the cracked gas stream typically comprises use of a fired ammonia cracking reactor. Suitable fired ammonia cracking reactors are known and may comprise a fuel combustion zone having a radiant section comprising one or more burners to which one or more fuel streams and an oxygen feed gas, such as air, oxygen enriched air, or oxygen, are fed. The radiant section may comprise one or more catalyst containing reaction tubes though which the ammonia feedstock is passed. Combustion of one or more fuel streams in the one or more burners of the fuel combustion zone, creates heat energy (e.g. radiant heat) for heating the one or more catalyst containing reaction tubes. There may be tens, hundreds, or thousands of catalyst containing reaction tubes in the radiant section. If desired, downstream of the radiant section, a flue gas from the combustion of the one or more fuel streams may be used to pre-heat one or more feed streams in a convection section. Reactors comprising a radiant section containing catalyst containing reaction tubes and a convection section for preheating feeds are known in steam methane reforming and may be applied to the present invention. Alternatively, fired ammonia cracking reactors may be used where the combustion of the one or more fuel streams in a fuel combustion zone is separate to the reactor comprising the catalyst containing reaction tubes. The catalyst in the catalyst containing reaction tubes may be any ammonia cracking catalyst. For instance, nickel catalysts and / or ruthenium catalysts may be used. Preferred catalysts are nickel catalysts. The catalyst may comprise 3 to 30% by weight nickel, preferably 8 to 20% by weight nickel, expressed as NiO, on a suitable refractory support, such as alumina or a metal aluminate. The catalyst may be in the form of pelleted shaped units, which may comprise one or more through holes, or may be provided as a wash coat on a structured metal or ceramic catalyst. Particularly preferred catalysts may be KATALCOR™ 27-2 which comprises 12 wt% nickel, expressed as NiO, on a cylindrical pellet formed from a high surface area calcium aluminate support, or KATALCORTM 27-200 which comprises 16 wt% nickel, expressed as NiO, on a quadralobe high surface area calcium aluminate support, both of which are available from Johnson Matthey PLC. The one or more catalyst containing reaction tubes may suitably be formed of an iron based alloy, a nickel based alloy, or a cobalt based alloy. The iron based alloy may be an ironchromium based alloy such as a stainless steel, preferably 316 stainless steel, or a high nickel steel such as those described by WO03 / 051771A1. Preferably, the one or more catalyst containing reaction tubes are formed of a nickel based alloy or a cobalt based alloy More preferably, the one or more catalyst containing reaction tubes are formed of a cobalt based alloy. The liquid ammonia feedstock may be derived from any source. The liquid ammonia feedstock may be produced by the catalytic combination of hydrogen and nitrogen, for example the ammonia feedstock may be produced from a Haber-Bosch ammonia synthesis process. The liquid ammonia feedstock may be produced in an ammonia production facility located upstream of the ammonia cracking reactor. Alternatively, the liquid ammonia feedstock may be provided from an ammonia gas storage facility, an ammonia storage unit, an ammonia storage tank, or an ammonia gas pipeline. The liquid ammonia feedstock may comprise 90 mol% ammonia or more, 95 mol% ammonia or more, 97 mol% ammonia or more, or 99 mol% ammonia or more. The liquid ammonia feedstock may be substantially 100 mol% ammonia. By “substantially 100 mol% ammonia” it is meant that any other component that may be present is an incidental impurity and may be present at an amount of less than 1 mol%, less than 0.5 mol%, or less than 0.1 mol% of the liquid ammonia feedstock. The cracked gas stream may comprise hydrogen (H2), nitrogen (N2), and residual ammonia (e.g. small amounts of unreacted ammonia). Preferably, the cracked gas stream may comprise from 72 mol% to 75 mol% hydrogen (H2), from 23 mol% to 25 mol% nitrogen (N2), and less than 4 mol% NH3 (e.g. less than 1 mol% NH3 or less than 0.1 mol% NH3). The process of the invention comprises the step of heating the liquid ammonia feedstock to an intermediate temperature by heat exchange with a liquid heat exchange medium to produce a cooled liquid heat exchange medium. In ammonia cracking processes of the prior art, the liquid ammonia feedstock may be heated and vaporised using (for example) steam directly to a temperature suitable for feeding to an ammonia cracking reactor. For instance, the liquid ammonia feedstock may be directly preheated or heated to a temperature of 140 °C or more, 200 °C or more, 300 °C or more, or 350 °C or more using steam, a hot process gas, or an electrical heater. The intermediate temperature may be a temperature of 0 °C or more, 2 °C or more, 3 °C or more, or 5 °C or more. The intermediate temperature may be a temperature of 30 °C or less, 28 °C or less, 25 °C or less, or 20 °C or less. For example, the intermediate temperature may be a temperature of from 0 °C to 30 °C, from 2 °C to 28 °C, from 3 °C to 25 °C, or from 5 °C to 20 °C, such as 10 °C or 15 °C. Advantageously in the process of the present invention, because the liquid ammonia feedstock is heated to an intermediate temperature (e.g. of 0 ° or more) before being superheated (e.g. to 450 °C or more) for cracking, multiple sources of cooling are provided. Advantageously, where the intermediate temperature may be above the freezing point of water, water which may not comprise an anti-freeze (e.g. plant cooling water) may be chilled for use as a cooling medium. As such, the process of the invention allows process cooling water to be cooled without the risk of freezing of pipes, values, and / or equipment. Where the liquid ammonia feedstock has been heated to the intermediate temperature, the liquid ammonia feedstock may be present at a pressure of 5 barg or more, 6 barg or more, or 7 barg or more. As such, the liquid ammonia feedstock at the intermediate temperature may be preferably present as a liquid. The liquid heat exchange medium may have a freezing point below -30 °C, for example a freezing point below -35 °C, -50 °C, -60 °C, or -70 °C. The liquid heat exchange medium preferably has a freezing point below the boiling point of ammonia (at 1 atmosphere pressure); e.g. below -33°C. This prevents freezing of the liquid heat exchange medium when in thermal communication with liquid ammonia. The liquid heat exchange medium may be an aqueous solution comprising an anti-freeze. The anti-freeze may be any suitable anti-freeze which forms a low freezing point aqueous solution. The anti-freeze may be a glycol (e.g. ethylene glycol or propylene glycol), or ammonia. In preferred processes of the invention, the anti-freeze may be ammonia. Where the liquid heat exchange medium is an aqueous solution comprising an anti-freeze, the anti-freeze may be present in the aqueous solution in a concentration sufficient to prevent the liquid heat exchange medium from freezing when heating the ammonia feedstock. For instance, the anti-freeze may be present in the aqueous solution in a concentration sufficient to decrease the freezing point of the liquid heat exchange medium to a temperature in the range as described hereinabove (e.g. below -33 °C). In preferred processes of the invention where the liquid heat exchange medium is an aqueous solution of ammonia, ammonia may be present in the aqueous solution in a concentration of 15 wt% or more, 20 wt% or more, or 25 wt% or more. There is no particularly upper limit to the concentration of ammonia, however, ammonia may be present in a concentration of 50 wt% or less, 40 wt% or less, or 30 wt% or less, but may be present in an amount of 60 wt% less, or 70 wt% or less. For example, ammonia may be present in a concentration of from 15 wt% to 50 wt%, from 20 wt% to 40 wt%, or from 25 wt% to 30 wt%. It is advantageous if the liquid heat exchange medium is an aqueous solution of ammonia because ammonia is readily available in an ammonia cracking process and plant, and aqueous solutions of ammonia have a freezing temperature of less than the boiling point of liquid ammonia. Furthermore, it is advantageous if the liquid heat exchange medium is an aqueous solution of ammonia because this minimises the risk of introducing contaminants into the ammonia cracking process streams (e.g. into the ammonia feedstock or the cracked gas stream). It may be advantageous if the anti-freeze consists of ammonia. It may be advantageous if the liquid heat exchange medium does not contain a non-ammonia-based anti-freeze (e.g. ethylene glycol, or propylene glycol). The cooled liquid heat exchange medium may have a temperature of 0 °C or more, 2 °C or more, 5 °C or more, or 10 °C or more. The cooled liquid heat exchange medium may have a temperature of 35 °C or less, 33 °C or less, 32 °C or less, or 30 °C or less. For example, the cooled liquid heat exchange medium may have a temperature of from 0 °C to 35 °C, from 2 °C to 33 °C, from 5 °C to 32 °C, or from 10 °C to 30 °C. The liquid heat exchange medium may be present in the process of the invention as part of a chiller circuit. Accordingly, the process of the invention may comprise a chiller circuit comprising the liquid heat exchange medium. The chiller circuit may supply the cooled liquid heat exchange medium to the one or more downstream processes. The chiller circuit may supply the liquid heat exchange medium from the one or more downstream processes to a heat exchanger for heating the liquid ammonia feedstock to the intermediate temperature. As will be understood, the heat exchanger for heating the liquid ammonia feedstock to the intermediate temperature cools the liquid heat exchange medium to produce the cooled liquid heat exchange medium. The chiller circuit may recirculate the liquid heat exchange medium from the heat exchanger for heating the liquid ammonia feedstock to the intermediate temperature to the one or more downstream processes and back to heat exchanger for heating the liquid ammonia feedstock. The process of the invention comprises the step of using the intermediate temperature liquid ammonia feedstock to provide cooling to one or more downstream processes. As will be understood the provision of cooling to the one or more downstream processes may involve the cooling of a piece of process equipment and / or a process fluid in the one or more downstream processes. The one or more downstream processes may be selected from one or more of: a) a downstream process of cooling the cracked gas stream from the ammonia cracking reactor; b) a downstream process of recovering ammonia from the cracked gas stream; and / or c) a downstream process of cooling and purifying the cracked gas stream to produce a purified hydrogen stream and a tail gas. The cooling of the one or more downstream processes using the intermediate temperature liquid ammonia feedstock may be affected directly (e.g. by direct heat exchange) or indirectly (e.g. by heat exchange with an intermediary fluid). The one or more downstream processes may comprise a downstream process of cooling the cracked gas stream from the ammonia cracking reactor. The one or more downstream processes may comprise a downstream process of cooling the cracked gas stream from the ammonia cracking reactor using the intermediate temperature liquid heat exchange medium. The cracked gas stream from the ammonia cracking reactor may be cooled directly following exit from the ammonia cracking reactor (e.g. before any further processing of the cracked gas stream) using the intermediate temperature liquid ammonia feedstock. Preferably, the cracked gas stream from the ammonia cracking reactor may be cooled using plant process water (e.g. to produce steam), further cooled using the cooled liquid heat exchange medium, and finally cooled using the intermediate temperature liquid ammonia feedstock. The downstream process of cooling the cracked gas stream from the ammonia cracking reactor may be carried out in any suitable process equipment, for example using a heat exchanger. The cracked gas stream from the ammonia cracking reactor may be cooled to a temperature of 10 °C to 50 °C, such as 20 °C, 30 °C, or 40 °C using the cooled liquid heat exchange medium. The cracked gas stream from the ammonia cracking reactor may be cooled from a temperature of from 600 °C to 800 °C, or from 700 °C to 750 °C to a temperature of 10 °C to 50 °C, such as 20 °C, 30 °C, or 40 °C using the intermediate temperature liquid ammonia feedstock. The one or more downstream processes may comprise the step of recovering ammonia from the cracked gas stream. The downstream processes of recovering ammonia from the cracked gas stream may comprise the step of cooling a wash water using the intermediate temperature liquid ammonia feedstock to produce a cooled wash water. The step of cooling the wash water using the intermediate temperature liquid ammonia feedstock to produce the cooled wash water may be carried out in any suitable process equipment, for example using a heat exchanger. The downstream processes of recovering ammonia from the cracked gas stream may comprise the step of scrubbing residual ammonia from the cracked gas stream using the cooled wash water to produce a scrubbed cracked gas stream and an ammonia laden wash water. The solubility of ammonia in the wash water is inversely proportional to the temperature of the wash water. In other words, the solubility of ammonia in the wash water increases as the temperature of the wash water decreases. It is therefore advantageous to provide the wash water at (i.e. cool the wash water to) as low of a temperature as possible. Preferably, the wash water is cooled to a temperature of less than 55 °C, less than 50 °C, or less than 45 °C, such as less than 40 °C. The lower limit of the temperature of the wash water is not particularly limited. However, the wash water may be cooled to a temperature of greater than 10 °C, such as greater than 15 °C, greater than 20 °C, or greater than 25 °C. Advantageously, providing the wash water at a lower temperature reduces the amount of wash water which may be required in the process and / or plant of the invention and may therefore reduce the size of the ammonia cracking plant. The ammonia laden wash water may comprise ammonia in an amount of from 2 mol% to 20 mol% ammonia, for example from 5 mol % to 15 mol% ammonia. The step of scrubbing ammonia from the cracked gas stream using the cooled wash water may be carried out in any suitable piece of process equipment. For example, the step of scrubbing ammonia from the cracked gas stream using the cooled wash water may be carried out in a scrubber, such as a wet scrubber. Advantageously, cooling of the scrubbed cracked gas stream with the intermediate temperature liquid ammonia feedstock allows additional water to be removed from the cracked gas stream thereby improving downstream purification and recovery of hydrogen product. The downstream process of recovering ammonia from the cracked gas stream may comprise the step of distilling the ammonia laden wash water in a distillation system. The downstream process of recovering ammonia from the cracked gas stream may comprise the step of distilling the ammonia laden wash water in a distillation system comprising an ammonia stripper condenser, recovering a light fraction comprising ammonia and water, and a heavy fraction comprising water and trace ammonia. The heavy fraction may be recovered from the bottom of a distillation column of the distillation system. The light fraction may be recovered from at or near the top of a distillation column of the distillation system. The ammonia stripper condenser may be a condenser in the distillation system used to recover ammonia from the light fraction. The ammonia stripper condenser may produce an ammonia rich stream and a water rich stream from the light fraction. The ammonia rich stream may be a gaseous stream comprising predominantly ammonia (e.g. greater than 90 mol% ammonia or greater than 95 mol% ammonia) and water vapour (e.g. less than 10 mol% water, or less than 5 mol% water). The water rich stream may be a liquid stream comprising water (e.g. greater than 60 mol% water, or greater than 70 mol% water) and ammonia (e.g. less than 40 mol% ammonia, or less than 30 mol% ammonia). Accordingly, the downstream process of recovering ammonia from the cracked gas stream may comprise the step of obtaining an ammonia rich stream and a water rich stream from the light fraction using the ammonia stripper condenser. The intermediate temperature liquid ammonia feedstock may be used to provide cooling to the ammonia stripper condenser of the distillation system. It is a surprising advantage of the present invention that the intermediate temperature liquid ammonia feedstock may maximise the recovery of ammonia from an ammonia laden wash water in the distillation system by separating more water from the light fraction. Separating more water from the light fraction consequentially improves the recovery of ammonia in the ammonia rich stream and thereby provides a more efficient ammonia cracking process by increasing ammonia utilisation. Moreover, the lower temperature of the intermediate temperature liquid ammonia feedstock of the present invention allows less cooling liquid to be present in the process and / or plant of the invention than in a conventional ammonia cracking process and plant. As will be appreciated, smaller volumes of cooling liquid may result in a reduction in ammonia cracking plant size and / or footprint. The downstream process of recovering ammonia from the cracked gas stream may comprise the step of cooling the heavy fraction comprising water and trace ammonia obtained from the distillation system to produce a cooled heavy fraction. The step of cooling the heavy fraction may be a step of cooling the heavy fraction using the intermediate temperature liquid ammonia feedstock, using the cooled liquid heat exchange medium, and / or using plant process water. The step of cooling the heavy fraction may be carried out in any suitable process equipment, such as a heat exchanger. The downstream process of recovering ammonia from the cracked gas stream may comprise the step of combining the heavy fraction (e.g. the cooled heavy fraction) comprising water and trace ammonia with the wash water used in the step of scrubbing ammonia from the cracked gas stream. The heavy fraction may be combined with the wash water before or after the step of cooling the heavy fraction. Alternatively, the heavy fraction may be combined with the wash water without cooling the heavy fraction. Where the heavy fraction may be combined with the wash water without cooling the heavy fraction, it may preferably be combined with the wash water before the step of cooling the wash water. The process of the invention may comprise the step of supplying the ammonia rich stream to the ammonia cracking reactor as a fuel stream for providing heat to support the ammonia cracking reaction and / or as an ammonia feedstock which may be cracked in the ammonia cracking reactor. Advantageously, where the stripper condenser of the distillation system uses the intermediate temperature liquid ammonia feedstock to produce the ammonia rich stream, the water content of the rich stream may be particularly low for at least the reasons described hereinabove. As such, corrosion cause by the presence of water in the ammonia cracking reactor can be limited. The one or more downstream processes may comprise a downstream process of cooling and purifying the cracked gas stream to produce a purified hydrogen stream and a tail gas. In the downstream process of cooling and purifying the cracked gas stream, the cracked gas stream being purified may be the cracked gas stream from the ammonia cracking reactor or the scrubbed cracked gas stream. The cracked gas stream may be cooled using the intermediate temperature liquid ammonia feedstock. In the downstream process of cooling and purifying the cracked gas stream, the cracked gas stream may be cooled in any suitable equipment, such as in a heat exchanger. The downstream process of cooling and purifying the cracked gas stream may preferably comprise the step of purifying the cracked gas stream in a pressure swing adsorption (PSA) process. The downstream process of cooling and purifying the cracked gas stream may comprise a multi-stage cooling and compression of the cracked gas stream. For example, the cracked gas stream may be cooled (e.g. with plant process water, the cooled liquid heat exchange medium, and / or the intermediate temperature liquid ammonia feedstock), and compressed multiple times. In the downstream process of cooling and purifying the cracked gas stream, the cracked gas stream may be cooled to a temperature of 35 °C or less, 30 °C or less, 28 °C or less, or 25 °C or less. In the downstream process of cooling and purifying the cracked gas stream, the cracked gas stream may be cooled to a temperature of 15 °C or more, 17 °C or more, 18 °C or more, or 20 °C or more. For example, the cracked gas stream may be cooled to a temperature of from 15 °C to 35 °C, from 17 °C to 30 °C, from 18 °C to 28 °C, or from 20 °C to 25 °C. It has surprisingly been found that the intermediate temperature liquid ammonia feedstock can be used to provide the cracked gas stream (e.g. the scrubbed cracked gas stream) to a purification step, such as a pressure swing absorption system, at a temperature which maximises the efficiency of the purification step. It is a particular advantage of the invention that the cracked gas stream may be supplied to a pressure swing absorption system at a temperature of from 15 °C to 35 °C, from 17 °C to 30 °C, from 18 °C to 28 °C, or from 20 °C to 25 °C regardless of the climate in which the process or plant operates. The pressure swing adsorption process may produce a purified hydrogen product and a tail gas. The purified hydrogen product may comprise from 70 mol% to 100 mol% H2, from 75 mol% to 100 mol% H2, from 80 mol% to 100 mol% H2, from 85 mol% to 100 mol% H2, or from 90 mol% to 100 mol% H2. Preferably, the purified hydrogen product may comprise 90 mol% H2, greater than 95 mol% H2, greater than 98 mol% H2, or greater than 99 mol% H2. The tail gas typically comprises from 20 mol% to 95 mol% nitrogen (N2), from 10 mol% to 70 mol% hydrogen (H2), and residual ammonia (e.g. from 0 mol% to 5 mol%). The one or more downstream processes may comprise cooling and compressing a hydrogen gas stream. The hydrogen gas stream may be any gas stream comprising hydrogen gas (H2). Preferably, the hydrogen gas stream may be the purified hydrogen product. More preferably, the hydrogen gas stream may be the purified hydrogen product produced from the pressure swing absorption step. The intermediate temperature liquid ammonia feedstock may be used to cool the hydrogen gas stream or a compressor used to compress the hydrogen gas stream. The step of cooling and compressing a hydrogen gas stream (e.g. the purified hydrogen product) may be carried out in any suitable process equipment such as a heat exchanger and / or a gas compressor. The process of the invention may comprise the step of heating the liquid ammonia feedstock to a second temperature. The process of the invention may comprise the step of heating the liquid ammonia feedstock to a second temperature after the step of heating the liquid ammonia feedstock to the intermediate temperature. The step of heating the ammonia feedstock to a second temperature may be a step of heating and vaporising the liquid ammonia feedstock. The liquid ammonia feedstock may be heated to the second temperature by heat exchange with any suitable heat source. The liquid ammonia feedstock may be heated to the second temperature by heat exchange with steam, flue gas from the ammonia cracking reactor, and / or the cracked gas stream from the ammonia cracking reactor. The liquid ammonia feedstock may be heated to the second temperature directly, or may be heated to the second temperature in a stepwise fashion. For example, the liquid ammonia feedstock may be heated in a stepwise fashion to the second temperature by heat exchange with steam (e.g. to vaporise the liquid ammonia feedstock stream), followed by heat exchange with the cracked gas stream (e.g. to 450 °C) followed by heat exchange with the flue gas stream (e.g. to >500 °C). As will be understood, the liquid ammonia feedstock is not heated to the second temperature by heat exchange with the liquid heat exchange medium. Following heating to the second temperature, the ammonia feedstock may be fed to the ammonia cracking reactor and cracked to produce the cracked gas stream. The second temperature may be a temperature of 500 °C or more, 550 °C or more, 575 °C or more, or 600 °C or more. The second temperature may be a temperature of 800 °C or less, 775 °C or less, 750 °C or less, or 725 °C or less. For example, the second temperature may be a temperature of from 500 °C to 800 °C, from 550 °C to 775 °C, from 575 °C to 750 °C, or from 600 °C to 725 °C. The process of the invention may comprise the step of feeding the ammonia feedstock to the ammonia cracking reactor and cracking the ammonia feedstock to produce the cracked gas stream. The process of the invention may comprise the step of heating the liquid ammonia feedstock to a second temperature and feeding the ammonia feedstock at the second temperature to the ammonia cracking reactor and cracking the ammonia feedstock in the ammonia cracking reactor to produce the cracked gas stream. In preferred processes of the invention the process may comprise the step of using the cooled liquid heat exchange medium to provide cooling to one or more downstream processes. In preferred processes of the invention the process may comprise the step of using the cooled liquid heat exchange medium to cool a process fluid and / or process equipment. In preferred processes of the invention the process may comprise the step of using the cooled liquid heat exchange medium as a source of cooling for one or more downstream processes. The cooled liquid heat exchange medium may be used as a source of cooling for one or more downstream processes either directly or indirectly. In preferred processes of the invention the cooled liquid heat exchange medium and the intermediate temperature liquid ammonia feedstock may be used to cool the same downstream process(es). In preferred process of the invention the process may comprise the step of using the intermediate temperature liquid heat exchange medium to cool a plant process water. In preferred process of the invention the process may comprise the step of using the intermediate temperature liquid heat exchange medium to cool a plant process water and using the cooled plant process water to cool one or more downstream processes. In preferred processes of the invention, the process may comprise the steps of: i) heating a liquid ammonia feedstock to an intermediate temperature by heat exchange with a liquid heat exchange medium to produce a cooled liquid heat exchange medium; ii) using the intermediate temperature liquid ammonia feedstock to provide cooling to one or more downstream processes, wherein, the one or more downstream processes may be selected from one or more of: a) a downstream process of cooling the cracked gas stream; b) a downstream process of recovering ammonia from the cracked gas stream, and bi) optionally, cooling a wash water in an ammonia wash water cooler to produce a cooled wash water; bii) optionally, scrubbing ammonia from the cracked gas stream using the cooled wash water and producing a scrubbed cracked gas stream and an ammonia laden wash water; biii) optionally, distilling the ammonia laden wash water in a distillation system comprising an ammonia stripper condenser and recovering a light fraction comprising ammonia and water and a heavy fraction comprising water and trace ammonia; biv) optionally, obtaining an ammonia rich stream and a water rich stream from the light fraction using the ammonia stripper condenser; bv) optionally, supplying the ammonia rich stream to the ammonia cracking reactor as a fuel stream for providing heat to support the ammonia cracking reaction and / or as an ammonia feedstock which may be cracked in the ammonia cracking reactor; bvi) optionally, cooling the heavy fraction comprising water and ammonia obtained from the distillation system using a heavy fraction cooler to produce a cooled heavy fraction; bvii) optionally, combining the cooled heavy fraction with the wash water; and / or c) a downstream process of cooling and purifying the cracked gas stream and separating a purified hydrogen stream from a tail gas, and ci) optionally, feeding the tail gas to an ammonia cracking reactor and combusting the tail gas with a source of oxygen to provide heat energy to support the ammonia cracking reaction. In some process of the invention, the one or more downstream processes may be selected from one or more of: a) a downstream process of cooling the cracked gas stream; and / or b) a downstream process of recovering ammonia from the cracked gas stream. In some process of the invention, the one or more downstream processes may be selected from one or more of: b) a downstream process of recovering ammonia from the cracked gas stream; and / or c) a downstream process of cooling and purifying the cracked gas stream to produce a purified hydrogen stream and a tail gas. In some process of the invention, the one or more downstream processes may be selected from one or more of: a) a downstream process of cooling the cracked gas stream; and / or c) a downstream process of cooling and purifying the cracked gas stream to produce a purified hydrogen stream and a tail gas. In a second aspect of the invention there is provided an ammonia cracking plant. The ammonia cracking plant of the invention is configured to run the process of the first aspect of the invention. In the ammonia cracking plant of the invention the liquid heat exchange medium may form part of a chiller circuit. Accordingly, the chiller circuit may comprise the liquid heat exchange medium. The chiller circuit may by the chiller circuit described in relation to the first aspect of the invention. The chiller circuit may supply the cooled liquid heat exchange medium to the one or more downstream processes. Examples Figure 1 shows a block flow diagram of a typical ammonia cracking process not according to the invention and depicts process steps which may be present. For the avoidance of doubt, the block flow diagram of figure 1 may not comprise all steps in an ammonia cracking process, nor is it suggested or implied that all of the process steps depicted need necessarily be present. In Figure 1, liquified ammonia (1) is fed to a pre-heater / vaporiser (2) where the liquid ammonia is directly vaporised and heated from a temperature of below -33 °C to a temperature in excess of 450 °C (e.g. 600 °C or more). The liquefied ammonia is heated in the pre-heater / vaporiser by heat exchange with the cracked gas from the ammonia cracking reactor (3), the flue gas (not shown) from the ammonia cracking reactor(3), and the steam circuit (101). The vaporised and heated ammonia is fed to a fired ammonia cracking reactor (3), where the ammonia is cracked in the plurality of catalyst containing reaction tubes, to produce a cracked gas stream comprising hydrogen gas and nitrogen gas. The cracked gas stream is cooled (4) by heat exchange to produce stream in a steam circuit (101). The cracked gas stream may be further cooled using a plant cooling water circuit (102a / b) fed from a plant cooling water reservoir (6). The cooling circuit (102a / b) is typically filled with water and circulates to different process steps (102a) and returns (102b) to the plant cooling water reservoir (6). The plant cooling water reservoir (6) may have cooling means (e.g. electrically powered refrigerators, or liquid / air heat exchangers) to cool the water in the cooling circuit (102). Steam raised on recovery of heat from the cooling of the cracked gas stream in (4) is used to vaporise and heat the liquid ammonia (1) in the pre-heater / vaporised (2). The cooled cracked gas stream is fed to an ammonia scrubbing system (5) for recovery of ammonia from the cracked gas, and where ammonia is recovered into a wash water. The ammonia in the ammonia laden wash water may then be recovered by distillation in a distillation system (7). In the distillation system (7) the ammonia stripper condenser is cooled using plant cooling water from the cooling circuit (102a / b). The cooled cracked gas stream exiting the scrubbing system (5) is cooled in a cooler (8) which uses cooling water from the cooling circuit (102a / b). The cracked gas is then fed to a purification step comprising a pressure swing adsorption device (9) where hydrogen and nitrogen are separated from one another to produce a purified hydrogen stream (104), and a tail gas (105) comprising nitrogen, hydrogen, and small amounts of residual ammonia. It is to be recognised that gas exiting the cooler (8) may not be at an optimised temperature for the pressure swing adsorption process, especially where the ammonia cracking process or plant is operated in hotter climates. Here, the plant cooling water (6) may not be capable of being chilled to a sufficiently low temperature to achieve the optimum temperature required for certain processing steps, for instance the cracked gas stream may not be at a temperature sufficiently low to allow optimum separation of hydrogen from nitrogen in a pressure swing adsorption process. Figure 2 shows a block flow diagram of an ammonia cracking process according to the invention. As for the depiction of Figure 1, Figure 2 shows process steps which may be present. For the avoidance of doubt, the block flow diagram of Figure 2 may not comprise all steps of an ammonia cracking process of the invention, nor is it suggested or implied that all of the process steps depicted need necessarily be present in the process or plant of the invention. In Figure 2, a chiller circuit (202a / b / c) is provided which comprises a liquid heat exchange medium fed from a storage reservoir (60). Liquified ammonia (10) is fed to a pre-heater (20a) where the liquid ammonia is heated from a temperature of below -33 °C to an intermediate temperature of 0 °C or more (e.g. 0 °C to 30 °C) by the liquid heat exchange medium (202a). The liquid heat exchange medium exists the pre-heater (20a) as a cooled liquid heat exchange medium (202b). The cooled liquid heat exchange medium (202b) may be fed to downstream processes and be returned (202c) to the reservoir (60) after performing its cooling duty. The intermediate temperature liquid ammonia feedstock is used to provide cooling duty to one or more downstream processes. The cooling duty to the one or more downstream processes using the intermediate temperature liquid ammonia feedstock is provided by direct heat exchange in the downstream process by sending the ammonia feedstock (210a) to the process and returning it (210b) to the pre-heater (20a). Further cooling to the one or more downstream processes is provided by indirect heat exchange of the intermediate temperature liquid ammonia feedstock with an intermediary fluid. The intermediary fluid is cooled in a cooling loop (201a / 201b) by heat exchange in a heat exchanger (31). The intermediate temperature liquid ammonia feedstock is fed to a heating means (20b). In the heating means (20b), the liquid ammonia feedstock is heated and vaporised to a temperature of 450 °C or more (e.g. 600 °C or more). Heating is provided to the heating means (20b) by recovering heat from the hot flue gas and the cracked gas stream from the ammonia cracking reactor (30) which is not shown here for clarity. The cracked gas stream is cooled (40) and the heat recovered as steam in the steam circuit (not shown). Further cooling of the cracked gas stream using the cooled liquid heat exchange medium (202b) may be applied, and / or further cooling using plant cooling water may be applied (not shown). In the present example, the chiller circuit (202a / b / c) comprises an aqueous solution of ammonia (in this instance 20 wt% ammonia). The cooled cracked gas stream exiting the cooler (40) is fed to an ammonia scrubbing system (50), where ammonia is recovered into a wash water as an ammonia laden wash water. The ammonia in the ammonia laden wash water may then be recovered by distillation in a distillation system (70) comprising an ammonia stripper condenser. The ammonia stripper condenser is cooled by indirect heat exchange with the intermediate temperature liquid ammonia feedstock using an intermediary fluid which flows through a cooling loop (201 a / b). The ammonia distillation system comprises a distillation column which produces a heavy fraction comprising liquid water and trace ammonia (207) and a light fraction comprising ammonia and water (not shown). The heavy fraction (207) is returned to the ammonia scrubbing system (50) where it is combined with the wash water. Optionally, and not shown, the heavy fraction (207) may be cooled using the cooled liquid heat exchange medium (202b) before being combined with the wash water in the ammonia scrubbing system (50). The light fraction (not shown) is fed to the ammonia stripper condenser in the distillation system (70). The ammonia stripper condenser is used to separate the light fraction into an ammonia rich stream comprising gaseous ammonia and water vapour (206) and a water rich stream (208) comprising an aqueous solution of ammonia. The water rich stream (208) is recirculated to the distillation column of the distillation system (70) for further separation. The ammonia rich stream (206) is sent to the ammonia cracking reactor (40) where it is used to provide heat to support the ammonia cracking reaction and / or as an ammonia feedstock which may be cracked in the ammonia cracking reactor (40). The cracked gas stream exiting the scrubbing system (50) is cooled in a cooler (80) which uses the intermediate temperature ammonia feedstock (210a / 210b) to cool the cracked gas stream. The increased cooling power provided by the intermediate temperature liquid ammonia feedstock over process cooling water allows more efficient ammonia recovery. The cracked gas is fed to a pressure swing adsorption device (90) where hydrogen and nitrogen are separated from one another to produce a pure hydrogen stream (204), and a tail gas (205) comprising nitrogen, hydrogen, and small amounts of residual ammonia. The tail gas from the pressure swing adsorption device contains hydrogen and is cooled using the cooled liquid heat exchange medium (not shown) and then compressed (95) before being sent to the ammonia cracking reactor (40) for use as a fuel. In the process and plant of the present invention the gas exiting the cooler (80) can be at an optimised temperature for the pressure swing adsorption process, regardless of where the ammonia cracking presses or plant is operated. Accordingly, the process and plant of the invention allows for improved separation of hydrogen over processes of the prior art.

Claims

1. A process for the catalytic cracking of a liquid ammonia feedstock to produce a cracked gas stream, the process comprising the steps of:i) heating the liquid ammonia feedstock to an intermediate temperature by heat exchange with a liquid heat exchange medium to produce a cooled liquid heat exchange medium; andii) using the intermediate temperature liquid ammonia feedstock to provide cooling to one or more downstream processes.

2. A process according to claim 1, wherein the one or more downstream processes are selected from one or more of:a) a downstream process of cooling the cracked gas stream from the ammonia cracking reactor;b) a downstream process of recovering ammonia from the cracked gas stream; and / orc) a downstream process of cooling and purifying the cracked gas stream to produce a purified hydrogen stream and a tail gas.

3. A process according to claim 1 or claim 2, wherein the intermediate temperature is a temperature of from 0 °C to 30 °, from 2 °C to 28 °C, from 3 °C to 25 °C, or from 5 °C to 20 °C, such as 10 °C or 15 °C.

4. A process according to any one of the preceding claims, wherein the liquid heat exchange medium has a freezing point below -30 °C, -35 °C, -50 °C, -60 °C, or -70 °C.

5. A process according to any one of the preceding claims, wherein the liquid heat exchange medium is an aqueous solution comprising an anti-freeze.

6. A process according to any one of the preceding claims, wherein the liquid heat exchange medium is an aqueous solution comprising an anti-freeze and the antifreeze is selected from a glycol (e.g. ethylene glycol or propylene glycol), and ammonia, preferably ammonia.

7. A process according to any one of the preceding claims, wherein the liquid heat exchange medium is an aqueous solution of ammonia and ammonia is present in theaqueous solution in a concentration of from 15 wt% to 50 wt%, from 20 wt% to 40 wt%, or from 25 wt% to 30 wt%.

8. A process according to any one of the preceding claims, wherein the cooled liquid heat exchange medium has a temperature of from 0 °C to 35 °C, from 2 °C to 33 °C, from 5 °C to 32 °C, or from 10 °C to 30 °C.

9. A process according to any one of the preceding claims, wherein the one or more downstream processes comprise a downstream process of cooling the cracked gas stream to a temperature of 10 °C to 50 °C, such as 20 C, 30 °C, or 40 °C.

10. A process according to any one of the preceding claims, wherein the process comprises the downstream process of recovering ammonia from the cracked gas stream and further comprises the step of cooling a wash water using the intermediate temperature liquid heat exchange medium to produce a cooled wash water.

11. A process according to claim 10, wherein the cooled wash water has a temperature of less than 55 °C.

12. A process according to claim 10 or claim 11, wherein the downstream process of recovering ammonia from the cracked gas stream comprise the step of scrubbing ammonia from the cracked gas stream using the cooled wash water to produce a scrubbed cracked gas stream and an ammonia laden wash water.

13. A process according to claim 12, wherein the downstream process of recovering ammonia from the cracked gas stream comprises the step of distilling the ammonia laden wash water in a distillation system.

14. A process according to claim 12 or claim 13, wherein the downstream process of recovering ammonia from the cracked gas stream comprises the step of distilling the ammonia laden wash water in a distillation system comprising an ammonia stripper condenser, recovering a light fraction comprising ammonia and water, and a heavy fraction comprising water and trace ammonia.

15. A process according to claim 14, wherein the downstream process of recovering ammonia from the cracked gas stream comprises the step of cooling the heavy fraction comprising water and trace ammonia obtained from the distillation system to produce a cooled heavy fraction.

16. A process according to claim 14 or claim 15, wherein the downstream process of recovering ammonia from the cracked gas stream comprises the step of combining the heavy fraction comprising water and trace ammonia with the wash water.

17. A process according to any one of claims 14 to 16, wherein the downstream process of removing ammonia the cracked gas stream comprises the step of obtaining an ammonia rich stream and a water rich stream from the light fraction using the ammonia stripper condenser18. A process according to claim 17, wherein the downstream process of recovering ammonia from the cracked gas stream comprises the step of supplying the ammonia rich stream to the ammonia cracking reactor as a fuel stream for providing heat to support the ammonia cracking reaction and / or as an ammonia feedstock to be cracked in the ammonia cracking reactor.

19. A process according to any one of the preceding claims, wherein the one or more downstream processes comprises the downstream process of cooling and purifying the cracked gas stream to produce a purified hydrogen stream and a tail gas.

20. A process according to claim 19 when dependent on any one of claims 12 to 18, wherein the step of purifying the cracked gas stream is a step of purifying the scrubbed cracked gas stream.

21. A process according to any one of the preceding claims, wherein the step of purifying the cracked gas stream is carried out in a pressure swing adsorption (PSA) process.

22. A process according to any one of the preceding claims, wherein the cracked gas is cooled to a temperature of from 15 °C to 35 °C, from 17 °C to 30 °C, from 18 °C to 28 °C, or from 20 °C to 25 °C in the downstream process of cooling and purifying the gas stream to produce a purified hydrogen stream and a tail.

23. A process according to any one of the preceding claims, the process comprising the further step of heating the ammonia feedstock to a second temperature.

24. A process according to claim 23, wherein the second temperature is a temperature of from 500 °C to 800 °C, from 550 °C to 775 °C, from 575 °C to 750 °C, or from 600 °C to 725 °C.

25. A process according to claim 1, wherein the process comprises the steps of:i) heating a liquid ammonia feedstock to an intermediate temperature by heat exchange with a liquid heat exchange medium to produce a cooled liquid heat exchange medium;ii) using the intermediate temperature liquid ammonia feedstock to provide cooling to one or more downstream processes,wherein, the one or more downstream processes may be selected from one or more of:a) a downstream process of cooling the cracked gas stream;b) a downstream process of recovering ammonia from the cracked gas stream, and bi) optionally, cooling a wash water in an ammonia wash water cooler to produce a cooled wash water;bii) optionally, scrubbing ammonia from the cracked gas stream using the cooled wash water and producing a scrubbed cracked gas stream and an ammonia laden wash water;biii) optionally, distilling the ammonia laden wash water in a distillation system comprising an ammonia stripper condenser and recovering a light fraction comprising ammonia and water and a heavy fraction comprising water and trace ammonia;biv) optionally, obtaining an ammonia rich stream and a water rich stream from the light fraction using the ammonia stripper condenser;bv) optionally, supplying the ammonia rich stream to the ammonia cracking reactor as a fuel stream for providing heat to support the ammonia cracking reaction and / or as an ammonia feedstock which may be cracked in the ammonia cracking reactor;bvi) optionally, cooling the heavy fraction comprising water and ammonia obtained from the distillation system using a heavy fraction cooler to produce a cooled heavy fraction;bvii) optionally, combining the cooled heavy fraction with the wash water; and / orc) a downstream process of cooling and purifying the cracked gas stream and separating a purified hydrogen stream from a tail gas, andci) optionally, feeding the tail gas to an ammonia cracking reactor and combusting the tail gas with a source of oxygen to provide heat energy to support the ammonia cracking reaction.

26. A process according to any one of the preceding claims, wherein the processcomprises the step of using the cooled liquid heat exchange medium to provide cooling to one or more downstream processes.

27. An ammonia cracking plant configured to run the process of any one of claims 1 to 26.

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