REMOVAL OF VOLATILE ORGANIC COMPOUNDS BACKGROUND [0001] In three-dimensional printing a pre-wetting agent may be added to a printing powder to help a printing process, for example by adding a pre-wetting agent prior to printing a binder agent. Pre-wetting agents that are suitable for use in such printing processes may be volatile organic compounds (VOCs). BRIEF DESCRIPTION OF DRAWINGS [0002] The drawings provided are to illustrate various examples of the subject matter described in this disclosure related to methods and devices for removing VOCs from a powder bed and are not intended to limit the scope of the subject matter. The drawings are not necessarily to scale. [0003] FIG. 1 is a schematic showing a system for removing VOCs according to an example. [0004] FIG.2 is a flowchart illustrating an example method for removing VOCs from a powder bed in a 3D printer. [0005] FIG. 3A is schematic showing a module for removing VOCs as described herein according to an example. [0006] FIG. 3B is schematic showing an alternative module for removing VOCs as described herein according to an example. [0007] FIG. 4 is a schematic illustrating a 3D printing system as described herein according to an example. DETAILED DESCRIPTION [0008] A number of examples will be discussed in detail below. Each of the features disclosed in reference to the module may be applied to the method or the system. [0009] The pre-wetting of printing powder with a liquid of low surface energy may, in certain circumstances, dramatically enhance the quality of objects generated in 3D printing devices and additive manufacturing systems. Pre-wetting agents are added to printing powder to aid penetration of a binder agent, or binder, to the printing powder which then helps the binder to have more contact with the powder particles and aids bonding. A suitable VOC that may be used in 3D printing is ethanol. After printing, the pre-wetting agent is to be removed. [0010] As most VOCs are flammable, when these compounds are used in the printing process they should be used in low concentrations in an inert printing environment. Therefore, when using VOCs, the printing process may be carried out in an inert printing environment or the process may be carried out with at least a portion of a 3D printer in an inert printing environment for safety purposes. [0011] To create an inert environment, at least part of a printing chamber may be provided within a closed loop gas recycling system, which may include cooling to remove heat added as part of the printing process. An inert environment may be created or maintained using at least one inert gas. In such a gas recycling system, the gas may be re-circulated back into the 3D printer or printing chamber for reuse, to reduce the amount of inert gas to be used within the 3D printing system. In order to re-circulate the inert gas, any undesirable vapours, gases and powders, such as VOC vapour from a pre-wetting agent, should be removed from the gas stream. The method, system and module of the present disclosure may reduce the levels of VOC vapour present in a gas stream that is intended to be re-circulated back to the 3D printer or printing chamber. [0012] FIG.1 shows a schematic illustration of a system for removing VOCs. [0013] The system comprises apparatus for removing VOCs from a powder bed 150. The powder bed 150 comprises a printing powder, a VOC and a binder agent. The powder bed 150 is prepared in a 3D printer 120. The removal of the VOCs may be carried out when the powder bed 150 is in the 3D printer 120 or alternatively may be carried out when the power bed 150 is in a separate curing or heating station. When the powder bed 150 is heated, VOC vapour and water vapour are produced as a result of the heating of the printing powder, VOC and binder agent in the powder bed 150. The system also comprises an absorption element 110 in the form of a housing which contains a condensation coil 102 and a packing material 103. The condensation coil 102 is a device used to condense water vapour into a liquid state through a cooling process. The absorption element 110 may be a vertical packed bed or a packed column. The absorption element 110 may be configured such that the condensation coil 102 is located above the packing material 103 in the absorption element 110. The packing material 103 provides an increased surface area for condensed water to come into contact with VOC vapour. The packing material 103 may comprise structured packing. Structured packing may be formed from corrugated sheets of perforated metal, plastic or wire gauze. [0014] The system also comprises an exhaust line 104 and a disposal line 105. The exhaust line 104 routes VOC vapour and water vapour, that are produced in the powder bed 150 when the powder bed 150 is heated, to the absorption element 110. When water vapour comes into contact with the condensation coil 102 in the absorption element 110, condensed water is produced, which falls into the packing material 103. When VOC vapour comes into contact with the condensed water in the packing material 103, the VOC vapour dissolves into the condensed water. The condensed water and dissolved VOC may then be removed via the disposal line 105. The system shown in Fig.1 also comprises an outlet line 106. The outlet line 106 may be used to re-circulate water vapour back to the 3D printer 120 or to a separate curing or heating station. The system also comprises a control system 140. The control system 140 may control the exhaust line 104 and allow the opening and closing of the exhaust line 104. The control system 140 may also control the disposal line 105 and in particular allow the opening and closing of the disposal line 105. The control system 140 may also control the outlet line 106 and allow the opening and closing of the outlet line 106. In use, this system will operate to remove VOCs from a powder bed. A method using the system of FIG.1 is described with reference to FIG.2. [0015] FIG. 2 shows a flowchart illustrating an example method for removing VOCs from a powder bed in a 3D printer. [0016] The method comprises heating 20 a powder bed used in a 3D printer wherein the powder bed comprises a printing powder, a VOC and a binder, to evaporate the VOC and water from the powder bed to produce VOC vapour and water vapour; removing 30 the VOC vapour and water vapour from the powder bed as an exhaust stream; passing 40 the exhaust stream comprising the VOC vapour and water vapour through an absorption element comprising a condensation coil and packing material; contacting 50 the water vapour from the exhaust stream with the condensation coil to produce condensed water; dissolving 60 the VOC vapour in the condensed water in the packing material; and removing 70 the condensed water and dissolved VOC through a disposal line. [0017] In an example the VOC may be ethanol. Ethanol vapour is explosive above concentrations of 2.35% by mass. As most VOCs are flammable even at low concentrations, when a VOC is used as a pre-wetting agent, the VOC vapour (such as ethanol vapour) generated during printing should be removed in order to lower the concentrations of these compounds in the printing system. [0018] The method may comprise preparing a powder bed in a 3D printer wherein the powder bed comprises a printing powder, a VOC and a binder. The method may comprise printing the VOC, such as ethanol, into the printing powder before a binder is printed in the same powder. The VOC, binder and printing powder may be added at the same time or sequentially, in any order. The VOC may be added in sufficient quantities to occupy the space between the particles in a given layer or layers of the powder bed. In one example, the VOC may be applied to the regions where a binder has been previously applied. [0019] In one example, the prepared powder bed may remain in the 3D printer and the remainder of the method may be carried out in the 3D printer. In another example, the prepared powder bed may be removed from the 3D printer and placed into a separate curing station and the remainder of the method may be carried out in the curing station. [0020] The heating of the powder bed may be carried out in a curing station after all layers have been printed onto the powder bed. Alternatively, the heating of the powder bed may be carried out in the 3D printer after a single layer has been printed onto the powder bed, after multiple layers have been printed onto the powder bed or after all layers have been printed onto the powder bed. If the heating process is carried out after a single layer or multiple layers have been printed then the heating process may be repeated multiple times during the method. If the powder bed is heated in a curing or heating station, the curing or heating station may also thermally cure the binder in the powder bed. If the powder bed is heated in the 3D printer, the powder bed may be heated or the whole printing chamber may be heated. The heating process may be carried out at a temperature between about 60 ºC and about 200 ºC using any suitable technique. The method may also comprise hot air being passed through the powder bed. [0021] The removal of the VOC vapour and water vapour from the powder bed may occur in the 3D printer or in a separate curing or heating station depending on where the powder bed is located. Any VOC vapour and water vapour that has been evaporated from the powder bed is removed via the exhaust line and taken away from the powder bed and routed to the absorption element. An airflow passing through the exhaust line may help pass the VOC vapour and water vapour to the absorption element. The airflow may be passive or forced by mechanical means. The airflow passing through the exhaust line may come from the 3D printer or from a curing or heating station. The absorption element may be within the 3D printer or printing apparatus, or it may be external and/or separate to the 3D printer or printing apparatus. [0022] The water vapour that comes into contact with the condensation coil may come from the closed loop gas recycling or cooling system, and may be produced by evaporating water from the binding agent or the printing powder. [0023] When water vapour comes into contact with the condensation coil, it condenses to produce liquid water. The water may then drop into the packing material due to gravity. In the packing material, the water comes into contact with the VOC vapour. The water contacts the VOC vapour, and the contact between the water and VOC vapour is enhanced by the use of the packing material, which disperses the water and the VOC vapour, providing more contact area to transfer the VOC vapour into the liquid. The condensing of the water vapour and the dissolving of the VOC vapour into the condensed water may occur continuously throughout the printing process to ensure the VOC vapour is continuously removed. The removal of dissolved VOC may be partial removal or complete removal. The percentage of VOC removed will depend on the amount of VOC present. [0024] The 3D printer, printing chamber, printing apparatus or curing station may be provided in an inert environment, e.g. in a closed loop gas recycling and/or cooling system. The inert environment may include at least one inert gas such as nitrogen or argon. The method may further comprise re-circulating water vapour and/or the inert gas back to the 3D printer via the outlet line 106. The method may also comprise re-circulating VOC vapour to the 3D printer, printing chamber or printing apparatus. When the VOC is ethanol, if the ethanol vapour is re-circulated, the re-circulated ethanol may be in a concentration of less than 1.5% by mass. [0025] In an example, additional water vapour may be present in the closed loop gas recycling system or the system in which the 3D printer is located, due to the presence of a water atomizer or water atomizers. Water atomizers may be included in a 3D printer or printing apparatus in order to cool and humidify the air being blown onto the print heads. The use of a water atomizer or water atomizers results in the air circulating within a print chamber being more humid and thus increases the amount of water vapour present. The water atomizers may be beneficial to the disclosed method as increasing the amount of water vapour allows additional water to be produced via the condensation coil and therefore results in more water in which to dissolve the VOC vapour, which may lead to improved removal of the VOC vapour. Where a water atomizer is already included in a 3D printer or printing apparatus, this feature may be utilized for the additional purpose of improving absorption of the VOC vapour in accordance with the described method. This may avoid the provision of additional water tanks or a water supply, which features may otherwise increase the cost and complexity of the 3D printer or printing apparatus, increase running costs, produce excess waste and/or increase maintenance costs. [0026] The described method offers a simple and effective way to utilize VOCs in an enhanced printing process, whilst ensuring the process is safe and minimizing the waste products that should be disposed of. [0027] In an example, the method may comprise preparing a powder bed by depositing printing powder onto the powder bed in a 3D printer. A VOC is then printed onto the printing powder in the powder bed. A binder is then printed onto the regions where the VOC has been printed. The amount of VOC and binder that is printed will depend on the final product being produced and the amount of printing powder that is used in the method. In this example, the powder bed remains in the 3D printer. The method comprises heating up the powder bed to evaporate the VOC and water from the powder bed to produce VOC vapour and water vapour. The VOC vapour and water vapour are removed from the powder bed as an exhaust stream. The exhaust stream comprising the VOC vapour and water vapour is passed through an absorption element 110 comprising a condensation coil 102 and packing material 103. The water vapour from the exhaust stream is contacted with the condensation coil 102 to produce condensed water. The VOC vapour is dissolved in the condensed water in the packing material 103 and then the condensed water and dissolved VOC are removed through a disposal line 105. [0028] In a variation of this example, the powder bed is removed from the 3D printer and placed into a curing station, and the heating of the powder bed takes place in the curing station instead of in the 3D printer.. [0029] FIG.3A and FIG.3B are schematics showing two alternative arrangements of a module 200, 300 for removing volatile organic compounds as described herein. [0030] Fig.3A shows a module 200 for removing a VOC from a 3D printing apparatus comprising: an absorption element 210, the absorption element comprising a condensation coil 202 and a packing material 203; an inlet 204 which routes VOC vapour and water vapour from a powder bed in a 3D printer to the absorption element 210; and a disposal line 205 to remove condensed water and dissolved VOC. [0031] The VOC may be ethanol. The module 200 for removing a VOC may be located in the 3D printer or printing apparatus or it may be external to the 3D printer or printing apparatus, in which case the module 200 may be located within a curing or heating station or it may be external to the curing station. [0032] The absorption element 210 corresponds to the absorption element 110 described above in connection with Fig.1 but, in the example of Fig.3A, does not include the outlet line 106. Therefore, the operation of the absorption element 110 described in connection with Fig.1 and the method of Fig.2 also apply generally to the module 200 of Fig.3A. [0033] The module 200 comprises an inlet 204 which routes VOC vapour and water vapour from a powder bed in a 3D printer to the absorption element 210. Any VOC vapour and water vapour from the powder bed in a 3D printer or printing apparatus may be removed via the inlet 204 and routed to the absorption element 210. The VOC vapour may be present in the 3D printer or printing apparatus as VOCs may be used as a pre- wetting agent in the printing process. An airflow passing through the inlet 204 may be provided to help route the VOC vapour and water vapour to the absorption element 210. The airflow may be passive or forced by mechanical means. The airflow passing through the inlet 204 may come from a curing or heating station or from the 3D printer or printing apparatus. [0034] The water vapour that comes into contact with the condensation coil 202 may come from a closed loop gas recycling and/or cooling system in which the powder bed is located. The water vapour may be produced by evaporating water from the binder agent or the printing powder. The water vapour that comes into contact with the condensation coil 202 is condensed to create liquid water. [0035] The absorption element 210 may be a vertical packed bed or a packed column. In the example of Fig.3A, the module 200 is configured such that the condensation coil 202 is located above the packing material 203 in the absorption element 210. When water vapour comes into contact with the condensation coil 202, it condenses to produce liquid water. The water may then drop into the packing material 203 due to gravity. In the absorption element 210, and in particular in the packing material, the condensed water comes into contact with the VOC vapour. The packing material 203 provides a large surface area for the water to come into contact with the VOC vapour. The packing material 203 may comprise structured packing. Structured packing may be formed from corrugated sheets of perforated metal, plastic or wire gauze. The condensed water contacts the VOC vapour, and the contact area is enhanced by the use of the packing material 203, which disperses the water and the VOC vapour, providing more contact area to transfer the VOC vapour into the liquid. The condensed water and dissolved VOC, such as ethanol, may then be disposed of via the disposal line 205. This enables the safe disposal of the condensed water and dissolved VOC. [0036] Fig. 3B shows an alternative arrangement of a module for removing volatile organic compounds. The module 300 of Fig.3B comprises an absorption element 310 which comprises a condensation coil 302, a packing material 303, an inlet 304 and a disposal line 305, which corresponds to the equivalent components of the module 200 of Fig. 3A, but further comprises an outlet line 306, to re-circulate water vapour back to the powder bed in a 3D printing apparatus. The outlet line 306 may also re-circulate VOC vapour to a 3D printing apparatus. Further, the outlet line 306, may also re- circulate inert gases, such as nitrogen and/or argon, and airborne powder from the powder bed in the 3D printer or printing apparatus back to the 3D printer or printing apparatus. The absorption element 310 of module 300 therefore corresponds substantially to the absorption element 110 of Fig.1 and may be used in the system of Fig.1. [0037] The modules 200, 300 shown in Figs.3A and 3B may be located within a 3D printer, or may be connected to a 3D printing apparatus. The module 200, 300 may be located within a curing or heating station, or may be connected to a curing or heating station. Once a product has been printed, or single or multiple layers of printing powder have been printed, the powder bed may be removed from a 3D printing apparatus and placed into a curing or heating station. The curing or heating station may be used to heat the printing powder and remove any VOC vapour and/or water vapour. The curing or heating station may be used to evaporate solvents and/or thermally cure the binder. [0038] The present disclosure also provides a 3D printing system comprising a 3D printer, a module for removing VOCs and a control system. [0039] FIG.4 is a schematic of a 3D printing system. [0040] The 3D printing system comprises: a 3D printer 420; a module 400 for removing a VOC from the 3D printer 420, wherein the module 400 comprises an absorption element 410, the absorption element 410 comprising a condensation coil 402 and a packing material 403; an inlet 404 which routes VOC vapour and water vapour from the 3D printer 420 to the absorption element 410; and a disposal line 405 to remove condensed water and dissolved VOC; and a control system 440 for opening and closing the inlet 404 in the module 400. [0041] The VOC may be ethanol. The module 400 for removing a VOC may be located in the 3D printer 420 or printing apparatus or it may be external to the 3D printer 420 or printing apparatus. The module 400 may be located within a curing or heating station or it may be external to the curing station. The features of the condensation coil 402 and the absorption element 410 that are disclosed with reference to the examples and the method shown in the other figures also apply to the module 400. In particular, the module 400 corresponds substantially to the module 200 described in connection with Fig.3A [0042] The water vapour that comes into contact with the condensation coil 402 may come from the closed loop gas recycling and/or cooling system. The water vapour may be produced after evaporating water from the binder agent or the printing powder. The water vapour that comes into contact with the condensation coil 402 is condensed to create liquid water. [0043] The absorption element 410 is in the form of a housing that contains the packing material 403 and the condensation coil 402. The absorption element 410 may be a vertical packed bed or a packed column. The module 400 may be configured such that the condensation coil 402 is located above the packing material 403 in the absorption element 410. When water vapour comes into contact with the condensation coil 402, liquid water is produced. The water may then drop into the packing material 403 due to gravity. In the absorption element 410, the condensed water comes into contact with the VOC vapour. The packing material 403 provides a large surface area for the water to come into contact with the VOC vapour. The packing material 403 may comprise structured packing. Structured packing may be formed from corrugated sheets of perforated metal, plastic or wire gauze. The condensed water contacts the VOC vapour, and the contact area is enhanced by the use of the packing material 403, which disperses the water and the VOC vapour, providing more contact area to transfer the VOC vapour into the liquid. The condensed water and dissolved VOC, such as ethanol, may then be disposed of via the disposal line 405. This enables the safe disposal of the condensed water and dissolved VOC. [0044] The inlet 404 routes VOC vapour and water vapour from a powder bed in the 3D printer 420 to the absorption element 410. Any VOC vapour and water vapour from the powder bed in the 3D printer 420 may be removed via the inlet 404 and routed to the absorption element 410. The VOC vapour may be present in the 3D printer 420 as VOCs may be used as a pre-wetting agent in the printing process. An airflow passing through the inlet 404 may help route the VOC vapour and water vapour to the absorption element 410. The airflow may be passive or forced by mechanical means. The airflow passing through the inlet 404 may come from a curing or heating station or from the 3D printer. [0045] The module 400 may further comprise an outlet line (not shown) to re-circulate water vapour back to the powder bed in a 3D printing apparatus, as described in connection with Fig.1. [0046] The control system 440 may be used for opening and closing the inlet 404 in the module 400. The control system may also be used for opening and closing an outlet line in the module 400, where provided. The control system may also be used for opening and closing the disposal line 405. The inlet 404 may comprise one or more valves to allow vapour into the absorption element 410. The control system may be used for the opening and closing of such a valve in the inlet 404. An outlet line, where provided, may also comprise one or more valves which may be controlled by the control system. The disposal line 405 may also comprise one or more valves, which may be controlled by the control system. [0047] The control system 440 may comprise a processor and a memory storing commands. When executed by the processor the commands may cause the control system to open or close the inlet 404 and/or the disposal line 405, as well as an outlet line, where provided, for example by opening or closing any of the valves described above. The commands when executed by the processor may cause printing of the printing powder, a VOC and a binder in the powder bed in the 3D printer. The commands when executed by the processor may cause heating of a powder bed in a 3D printer. The commands when executed by the processor may cause heating of a powder bed in a separate curing or heating station. [0048] The present disclosure also provides a device that includes a processor and a memory storing commands which when executed by the processor cause the device to perform a method comprising: heating a powder bed used in a 3D printer wherein the powder bed comprises a printing powder, a volatile organic compound (VOC) and a binder to evaporate the VOC and water from the powder bed to produce VOC vapour and water vapour; and controlling an inlet to allow removal of the VOC vapour and water vapour from the powder bed by passing the VOC vapour and water vapour through the inlet to an absorption element comprising a condensation coil and a packing material. [0049] The method may also comprise opening and closing the inlet via a control system. The inlet may comprise at least one valve. The method may comprise controlling an outlet line which allows water-vapour to be re-circulated to a 3D printer. The method may comprise opening and closing the outlet line via a control system. The outlet line may comprise at least one valve. The method may comprise opening and closing, via a control system, a disposal line for removing condensed water and dissolved VOC from the absorption element. The disposal line may comprise at least one valve. [0050] While the method, module, system and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above- mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims. [0051] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.