Method for controlling a methanol plant with variable load

Abstract

A method for controlling a methanol plant, wherein: the plant includes a front end, a synthesis loop to produce crude methanol, an intermediate storage tank of crude methanol; a distillation section for purification of the crude methanol; a steam system configured to transfer heat from the synthesis loop to the distillation section; the method includes operating the methanol plant according to a first mode and a second mode wherein the crude methanol output of the synthesis loop differs from the crude methanol processed in the distillation section and the balance is sent to the storage tank in the first mode or withdrawn from the storage tank in the second mode; the method including the provision of supplementary heat to the distillation section when the heat transferred from the synthesis loop to the distillation section is insufficient for the distillation process.

Description

[0001] Method for controlling a methanol plant with variable load

[0002] DESCRIPTION

[0003] Field of application

[0004] The present invention relates to the field of methanol production. The invention relates to a method for controlling a methanol plant in fluctuating energy conditions.

[0005] Prior art

[0006] Methanol is produced industrially by reacting a make-up gas comprising hydrogen (H2) and carbon oxides (CO2 and CO) at elevated temperature and pressure through one or more beds of a suitable methanol synthesis catalyst. The catalytic beds are contained in one or more reactors (methanol converters). The product of the reaction is a methanol-containing gas which is cooled and condensed to obtain a liquid stream of crude methanol. Unreacted gas separated from said liquid stream is reintroduced in the converter via a circulation compressor, thus forming a so-called synthesis loop (synloop).

[0007] The crude methanol contains by-products of the synthesis including ethanol, ketones, higher alcohols, and some dissolved gases including H2, CO, CO2, N2 and CH4 and must be purified to meet the purity specification required by the market. The purification is typically carried out by distillation.

[0008] A methanol distillation section includes of one or more distillation columns, each comprising a number of separation stages which may be actual vapor-liquid contacting trays or sections of contactor packings equivalent to trays. Each stage contributes to the overall separation of the methanol by allowing a portion of the more volatile component to transfer to the vapor phase and the less volatile component to the liquid phase.

[0009] In a typical distillation set-up there are two types of distillation columns: the topping column, also known as a pre-run column, is used for initial processing, followed by one or more refining columns used to eliminate higher alcohols and water. The topping column receives the crude methanol and separates lighter components such as acetone and gases dissolved in the crude methanol from the synthesis loop, such as methane, hydrogen, carbon dioxide, nitrogen. The refining column then performs the main distillation, producing distilled methanol at the top and an aqueous solution at the bottom.

[0010] In each distillation column, a light-boiling stream is produced as an overhead vapor stream, also referred to as “top product”, effluent from the top stage of the distillation column. For the topping columns, said vapor stream is a stream of “light ends”, which are non-condensable gases or stream of gases lighter than the methanol byproducts; while for the refining columns the overhead vapor stream is almost pure methanol. Each distillation column might have a top reflux system, where a portion of the top product is recovered and fed back to the top of the column.

[0011] From the bottom stage of each distillation column, a heavy-boiling stream is produced as a liquid stream, known as "bottom solution". A portion of the bottom solution is heated using a heat exchanger, which employs hot steam or process gas. The heated bottom solution is reintroduced into the column to supply the necessary distillation heat. Heat is provided to the column with the aim of enriching the vapor in the light boiling stream and the bottom in the heavy boiling stream.

[0012] Typically, the bottom solution of a column becomes the feed stream of the following column, however other configurations are possible. Another stream known as "fusel oil" is separated from the refining column, typically at intermediate elevation, which is a mixture of methanol, water and by-products of the distillation. ln recent years, the production of hydrogen from renewable energy sources has emerged in order to reduce emissions and carbon intensity in the so-called e- methanol processes. E-methanol is, by definition, methanol produced from renewable energy with low carbon footprint. An example of relevant interest is the production of e-methanol by reacting a make-up gas comprising H2 and CO2, wherein H2 is produced from electrolysis of water using a low-carbon energy, preferably a renewable energy, and CO2 is preferably captured from fumes or from air. Typically, the make-up gas in the production of e-methanol contains little or no carbon monoxide and can be regarded as CO-free make-up gas. This differs substantially from production of grey methanol, wherein the make-up gas is typically obtained by reforming or gasification of a hydrocarbon, and contains mainly H2 and carbon monoxide CO.

[0013] The conversion of CO2 to methanol releases significantly less heat than the conversion of CO to methanol, according to the following reaction enthalpy carried out at 25°C and 1 bar: 49.8 kJ / mol;

[0014] A CO-free make-up gas inherently produces less reaction heat compared to a conventional CO-containing feedstock. The production of e-methanol therefore releases significantly less heat compared to the production of grey methanol. The heat of reaction is typically used in the distillation process, which means the production of e-methanol may suffer from a lack of heat for distillation.

[0015] Typically, in conventional grey plants the distillation section is oversized compared with the synthesis loop, to enable the distillation section to handle a situation in which some methanol has to be re-introduced in the distillation section for further purification, for off-spec or downtime of the distillation. For example, a typical set-up of a conventional grey plant comprises a synthesis loop designed to operate at 100% of its own capacity and a distillation section designed to process 120% of the synthesis loop capacity. Therefore, the capacity of the distillation section is generally about 10% to 20% higher than the capacity of the synthesis loop. The extra capacity of the distillation section can be balanced using steam heated reboilers. In e-methanol plants powered with renewable energy subject to fluctuation, the flow of the synthesis loop may be very low, therefore the missing heat for distillation may be much higher than 10-20%, making the use of steam heated reboilers insufficient to balance the difference in the flow rates.

[0016] The distillation section may also be provided with one or more storage tanks in which crude methanol or partially purified methanol (i.e. distilled methanol which does not meet an intended product specification of purity) can temporarily be stored in the event of maintenance or of small fluctuations in the synthesis loop. Typically, a crude methanol storage of a conventional grey plant stores some hours of production, for example 24 hours, and it is designed to be used to handle short stops of the distillation, or short periods where the distillation is not operated at its full capacity. The off-spec methanol may be collected in a separate tank which is typically used to store relatively small amount of methanol of insufficient purity which needs to be distilled again to reach the desired specification. Therefore, storages of methanol in the prior art are typically small tanks used as intermediate buffer. It will be understood that “small” tanks must not be interpreted in absolute terms, but in relation to a small-time duration required for filling it, for example one day.

[0017] The synthesis of e-methanol produces a higher concentration of water compared to a conventional grey process. In e-methanol synthesis, the obtained crude methanol may contain up to 50% mol water, while the crude grey methanol contains typically 5-20% mol water. The difference in the water content is related to the fact that the feed to the synthesis loop in grey methanol contains less CO2 with respect to the feed of e-methanol. As per the reaction above, the reaction from CO2 produces water as co-product of methanol. Since water must be separated from the methanol in the distillation step to achieve the commercial purity grade, the increased water content in crude methanol may lead to energy deficit in the distillation process.

[0018] Another problem which afflicts an e-methanol plant, where part or all of the hydrogen feed comes from a renewable energy source, is the fluctuation of the source itself as most renewable energies, like solar and wind, are naturally subjected to fluctuations. Fluctuation of the power generated from the renewables entails a fluctuation of power fed to the electrolysers used for the hydrogen production of the make-up gas, and consequently, fluctuation of the flow rate of hydrogen produced. Such fluctuation may come directly, if the renewable power supply is physically connected to the electrolysers, or virtually, if the power supply to the electrolysers is provided in relation to a power generation from renewables (e.g. via a power purchasing agreement or some other forms of agreed power supply).

[0019] The fluctuation of the power may be related also to other technical- or business- related reasons such as time variable amount of power provided to the electrolyser, and different form and shapes of the time profile of such power provision. In some cases, the electric power grid operator may impose conditions which result in a reduced power input to the electrolysis or to the e-methanol plant, for example a need to prioritize power supply to other users than the plant in a period or a specific season (winter or summer). Moreover, the price of kWh changes over time, creating a need to reduce power consumption at high price rates and vice-versa increasing it at low price rates. All these factors may cause a similar problem of intermittency or variability of the hydrogen flow rate, essential in the methanol make-up gas.

[0020] The intermittency of the energy source and / or of the hydrogen flow rate results in the synthesis loop operating under fluctuating conditions, ranging from 10% to 110% load. Method to control the synthesis loop at low load are known, e.g. from EP 3 819 261 , EP 4 054 979. However, the distillation section is inherently less flexible than the synthesis loop, for example a distillation section may have a load flexibility ranging from about 50% to about 110% of its nominal capacity, depending on the technology used, e.g. the fluid-dynamics load characteristics and ability of the column internal trays, stages and other components. Generally, the minimum load of the distillation section is limited by the flow distribution in the distillation columns and in most cases the minimum load for stable operation of the distillation section is around 50% of the nominal load. The 50% minimum load for the distillation is referred to the flow rate of crude methanol fed to it compared to the installed capacity flow rate of crude methanol. This value should be considered as an approximation, it is not intended that the internal flow rates of liquid and vapor in the distillation columns are exactly 50% of the nominal values. Consequently, the distillation section is not able to match the minimum load of the synthesis loop.

[0021] Another limitation to load flexibility of the distillation is the relatively slow load ramping rate up and down of the distillation columns. The inertia of the distillation limits its ability to quickly follow the load and flow rate changings, conversely to the electrolysis unit or to the synthesis loop which adapt to changes very quickly.

[0022] Finally, another reason for the limited load flexibility of the distillation is its cost. In fact, designing the distillation for load flexibility higher than 50%-110% would result in a larger installed size of the distillation, in a lower utilization factor and in a higher cost.

[0023] The heat generated from the synthesis loop is introduced in the distillation section with the purpose of driving the distillation process by heating the column bottom solution. However, when the synthesis loop operates at a very low load, such as 10%, the heat generated from the loop is normally insufficient to meet the distillation requirements. In e-methanol production, the energy source is provided from renewable energy which is subjected to fluctuations, depending on the availability of the source, causing great fluctuations in the load of the synthesis loop. Situations of low energy availability are common with renewable energy sources therefore the load of the synthesis loop may be lower than the minimum load of the distillation section. In such cases, the distillation section suffers of heat deficiency and the distillation process is not carried out properly. The difference between the minimum load of the synthesis loop and the minimum load of the distillation section creates a challenge in heat integration between the two sections across the full range of load operating conditions.

[0024] Additional prior art is W02020052979A1 , WO2018019875A1 ,

[0025] US20210188747A1 , WO2022136374A1 , WO2022152749A1 , EP3885335A1 , US20220064541A1 , WO2021148262A1 , CN218146429U, EP3935034B1 , WO20231 10479A1 .

[0026] Summary of the invention

[0027] The invention aims to overcome the above drawbacks of the prior art, with a new method of controlling a methanol plant according to the claims.

[0028] The invention is based on a combination of decoupling the synthesis loop and the distillation section by means of an intermediate storage tank of crude methanol and controlling the heat provided to the distillation section by activating one or more heat source(s) to provide supplementary heat, in addition to the heat recovered from the synthesis loop. Accordingly, the distillation section can receive sufficient heat to sustain the distillation process even when the synthesis loop is running at low load and the heat recoverable from the loop would be insufficient for this purpose. Thanks to the combination of the intermediate crude methanol storage tank and said control of the distillation heat, the distillation section can operate above its minimum load even when the synthesis loop falls to a very low load, such as 20% or less.

[0029] The invention focuses on minimizing the heat energy consumption of the distillation process and to maximize utilization of heat recovered from the synthesis of methanol in order to reduce the overall energy requirements of the production that the process must import from an external source. The invention provides an energy-efficient, environmentally friendly method for the production of methanol, particularly in the context of varying energy inputs and the need for a sustainable energy balance.

[0030] Description of the invention

[0031] The invention discloses a method for controlling a methanol plant wherein the plant includes at least a front end for generation of a methanol make-up gas comprising hydrogen and carbon dioxide; a synthesis loop configured to convert in a methanol converter said make-up gas into crude methanol; an intermediate storage tank of said crude methanol; a distillation section for purification of said crude methanol to obtain a distilled methanol product; a heat transfer system configured to transfer heat from the synthesis loop to the distillation section, to provide heat for distillation.

[0032] The method includes at least operating the methanol plant according to a first mode and a second mode; wherein in the first mode, the flow rate of crude methanol effluent from the synthesis loop is greater than the flow rate of crude methanol processed by the distillation section and the balance is sent to said intermediate storage tank of crude methanol; in the second mode, the flow rate of crude methanol effluent from the synthesis loop is less than the flow rate of crude methanol processed by the distillation section, the balance of crude methanol fed to the distillation section is withdrawn from said intermediate storage tank of crude methanol.

[0033] The method further includes that when during the first mode or second mode the heat transferred from the synthesis loop to the distillation section is insufficient for said distillation process, the missing heat for said distillation process is provided by at least one other heat source to supplement the heat from the synthesis loop.

[0034] Preferably, the nominal capacity of the distillation section is in a range between 40% to 100% of the nominal capacity of the synthesis loop. An e-methanol plant typically is connected to renewable energy sources but it could also be connected to the grid. According to an embodiment, the production of make-up gas and / or the operation of the synthesis loop may depend at least in part from a fluctuating or intermittent energy source, wherein in an embodiment said energy source is a renewable energy source. The term renewable energy denotes energy which is naturally replenished, such as solar and wind.

[0035] The make-up gas of e-methanol plants may receive hydrogen and carbon dioxide directly from a pipeline, however in a preferred embodiment the production of make-up gas in the front-end includes the production of hydrogen by means of splitting of water, preferably water electrolysis, which is powered at least in part by renewable energy. In an embodiment, at least part of the hydrogen produced by splitting of water or water electrolysis is collected into a hydrogen storage. Accordingly, at least part of the hydrogen in the make-up gas is produced by said process of splitting of water, preferably water electrolysis, more preferably powered at least in part by renewable energy. The specific cases of the e- methanol plant being directly connected to a renewable plant, with or without a backup grid power connection, in islanded or semi-islanded configurations, are non-exclusive embodiments of the invention.

[0036] The amount of crude methanol produced by the synthesis loop depends on the loop operating capacity which is preferably determined at least by one of the following: the flow rate of a hydrogen stream feeding the make-up gas; the current flow rate of make-up gas delivered by the front-end; the amount of hydrogen stored in a hydrogen storage; the amount of crude methanol collected in the intermediate crude methanol storage tank. The amount of crude methanol distilled in the distillation section depends on the distillation operating capacity which is preferably determined at least by one of the following: the flow rate of a hydrogen stream feeding the make-up gas; the load of the synthesis loop; the amount of crude methanol collected in the intermediate crude methanol storage tank.

[0037] Said hydrogen stream feeding the make-up gas may be withdrawn from a hydrogen storage and / or it may be produced from an electrolysis unit and / or from water splitting.

[0038] According to a preferred embodiment the method includes the steps of:

[0039] - selecting the first mode if the loop operating capacity is higher than the distillation operating capacity;

[0040] - selecting the second mode if the loop operating capacity is lower than the distillation operating capacity.

[0041] Typically, a distillation section comprises a topping column receiving the crude methanol effluent from the synthesis loop and one or more refining column(s) receiving a bottom solution effluent from the bottom of the previous distillation column as a feed stream. The refining columns are used to improve the quality of the crude methanol and typically they are in cascade configuration with decreasing pressure. The refining column and the topping column are two types of distillation columns.

[0042] In an embodiment, the distillation section comprises at least one refining column equipped with a condenser and a bottom solution heating boiler; said condenser producing a reflux flow reintroduced in the refining column and / or an overhead vapor stream and / or a stream of distilled methanol effluent from the condenser. The method includes the step of adjusting the temperature at the bottom of the column to a selected target temperature by means of said heating boiler, wherein the duty of the boiler depends on the reflux flow of the column. The amount of the reflux flow and / or the amount of distilled methanol is controlled at least by one of the following parameters:

[0043] - the load of the distillation section;

[0044] - the overhead vapour stream;

[0045] - hydrogen collected in a hydrogen storage;

[0046] - hydrogen produced in an electrolysis unit.

[0047] Therefore, the reboiler duty is regulated to maintain a setpoint temperature at the bottom of the column. As the reflux flow increases, the amount of liquid requiring heating also increases, resulting in a higher reboiler duty, and conversely, when the reflux decreases, the reboiler duty is reduced accordingly.

[0048] According to an embodiment, the heat transfer system, configured to transfer heat from the synthesis loop to the distillation section, includes a steam system and / or direct exchange with the methanol converter and / or direct exchange with a reaction effluent of the methanol converter and / or indirect heat exchange by means of a heating medium. Usually, steam is produced and possibly heated in the equipment of the synthesis loop and cooled / condensed in the distillation section, typically in bottom solution boilers of distillation columns.

[0049] In embodiments with indirect heat exchange, a circuit for a heat transfer medium can be provided between the synthesis loop and the distillation section. Said heat transfer medium receives heat from the synthesis loop and releases heat to the distillation section. According to a preferred embodiment, heat available from the reaction effluent is used to heat up said heat transfer medium, and the heat transfer medium releases heat to the topping column, preferably at the bottom solution of the topping column. Said heat transfer medium is preferably hot water.

[0050] During fluctuating conditions, the method is preferably operated such that the load of the synthesis loop ranges between 10% and 110% of its nominal load, while in fluctuating conditions the load of the distillation section does not fall below a minimum load of said section. Typically said minimum load is in a range of 30% to 80% of the nominal load of the distillation section, depending on the design of the distillation section. In preferred cases said minimum load is in the range 50% to 70%.

[0051] According to an embodiment, when the distillation section operates at low load, the reflux flow is increased to maintain a target liquid and vapor flow rate necessary to sustain the hydraulic profile in the one or more distillation column(s). According to this embodiment, it is possible to run a stable distillation even if the load of the section is less than 50%, for example if the withdrawal from the intermediate crude methanol storage tank is between 10% and 50%. The advantage of this embodiment is the possibility to use a small intermediate crude methanol storage tank, or even not to use it, although it requires higher heat consumption in the reboilers of the columns.

[0052] The distillation load may be controlled as function of one or more of the following: a) the flow rate of crude methanol effluent from the synthesis loop; b) the level of crude methanol in the intermediate crude methanol storage tank.

[0053] In option a), the distillation load may be proportional to the load of the synthesis loop, reducing the need for external inputs.

[0054] In one embodiment, when the distillation load is controlled as function of the level of crude methanol in accordance with option b), the load of the distillation depends on one or more operational threshold(s) and one or more alarm threshold(s) of crude methanol in the intermediate storage tank; optionally the one or more operational threshold(s) include(s) one or more tolerance zone(s). Typically, the threshold levels may include levels marked internally as “low”, “very low”, “high”, “very high” or similar, with reference to one or more threshold(s). The threshold levels of crude methanol in the storage tank may be distinguished in alarm thresholds and operational thresholds.

[0055] Alarm thresholds are set at critical levels in the process to trigger emergency interventions when the system approaches unsafe conditions. These thresholds are typically located at the extreme ends of the process limits and are used to ensure the system stays within a safe operating range. In particular, when a high alarm level is reached, actions are triggered to reduce the synloop operational load to avoid overload. On the other hand, when lower alarm level is reached, different actions can be taken to keep the process running, including:

[0056] - recirculating the refined methanol to the intermediate crude methanol storage tank to keep the minimum liquid level;

[0057] - reducing the feed flowrate to the distillation section below minimum turndown and increasing the column reflux flows to ensure minimum required fluxes in the columns;

[0058] - increasing the synloop operating load, optionally by using additional energy or grid power.

[0059] Alarm thresholds are binary, meaning that once they are crossed, a predefined emergency action is immediately triggered. They are used to respond to immediate, critical deviations in the system’s operational parameters and are typically located at the extreme high and low ends of the process's safe operating range.

[0060] Operational thresholds are used within the normal operating range of the process to gradually adjust the process load of the distillation section as required by the operational conditions. These thresholds are used to optimize the system performance and help the transitioning between different operational states without abrupt changes. The transition between thresholds is gradual, following a ramping procedure to avoid abrupt changes in system behaviour. The aim of these thresholds is to maintain efficiency and stability in the process controlling in a stepwise manner.

[0061] When an operational threshold is reached, the system transitions to a new load level, either increasing or decreasing the operational capacity of the distillation process. These actions are less urgent than those triggered by alarm thresholds but are important for maintaining steady, efficient operation. Transitions between operational thresholds follow predefined ramps with controlled rates (e.g., >10% / h or >30% / h), ensuring smooth changes in the process load. The number and position of these operational thresholds can be adapted depending on the specific process requirements, allowing for flexible load management.

[0062] According to a preferred embodiment, when the distillation load depends on the level of crude methanol in the intermediate crude methanol storage tank, the distillation load is controlled according to any of the following: b1 ) the distillation load is reduced when the quantity of crude methanol in the intermediate storage tank transitions from a value above said one or more operational threshold(s) to a value below said one of more operational threshold(s); b2) the distillation load is increased when the quantity of crude methanol in the intermediate storage tank transitions from a value below said one or more operational threshold(s) to a value above said one or more operational threshold(s).

[0063] The method further includes that when the quantity of crude methanol in the intermediate storage tank is in a range delimited by two consecutive of said operational thresholds, the distillation load changes according to the filling of the intermediate storage tank, preferably the distillation load changes proportionally to the filling of the crude methanol storage tank.

[0064] The distillation load must fulfil the maximum and the minimum ramping rates of the distillation section. The control of the load is performed by means of a control valve which opens or closes modulating the load of the distillation section. To prevent the control valve from opening and closing too frequently in response to small fluctuations in the level of crude methanol of the intermediate storage tank, a tolerance zone may be added to stabilize the load change process. Tolerance zones can be applied around the operational thresholds to prevent the control system from reacting to minor fluctuations that do not significantly impact the process.

[0065] For example, it is considered a situation in which the level of crude methanol in the intermediate storage tank is above 50%. Without a tolerance zone, if the level of crude methanol decreases to 49.9% the control valve would be activated changing the load of the loop. The same would happen if, for example, the level would increase from 49.9% to 50.1 %, activating the control valve and changing again the load of the loop. This could lead to unstable control and unnecessary changes in the load for small fluctuations. The introduction of a tolerance zone, of for example 5%, allows the control valve to activate only if the level of crude methanol in the storage falls below 45% or above 55%. The value of the operational threshold(s) and of the tolerance zone(s) may be selected depending on the specific needs of the case.

[0066] The choice between the two modes of controlling the distillation section described above depends on the availability of the renewable energy used to power the methanol plant.

[0067] The intermediate crude methanol storage tank is sized based on the selected mode of control of the distillation load. The amount of crude methanol, that should be stored to ensure continuous and stable operation of both the synthesis loop and the distillation section, is computed through mathematical modelling, starting from a specific power profile and following the operating profile during an hour (or less), at a specific time, throughout an entire year (or more years). Maintaining the constraint of stable operation, the tank is designed to have the minimum possible size according to an economically optimal criterion.

[0068] The advantage of the first mode of distillation load control is that the additional heat required is minimized because the distillation follows the load of the synthesis loop, however the intermediate crude methanol storage tank is larger in size. Conversely, the advantage of the second mode of control of the distillation load is that the size of the intermediate methanol tank is minimized, while the need for an external supply of heat increases.

[0069] The method further includes modes of operation wherein: the crude methanol from the synthesis loop is entirely sent to the distillation section, and / or the crude methanol from the synthesis loop is entirely sent to the intermediate crude methanol storage tank, and / or the crude methanol processed by the distillation section is entirely taken from the intermediate crude methanol storage tank. Preferably, all the crude methanol effluent from the synthesis loop is sent to the intermediate storage without by-passing it, in order to equalize the composition of crude methanol before sending it to the distillation section.

[0070] Modes where all the crude methanol is sent to the intermediate crude methanol storage tank without any withdrawn to the distillation section from the intermediate crude methanol storage tank, or all the feed to the distillation section is taken from the intermediate crude methanol storage tank without any feed to the intermediate methanol tank from the synloop, may be maintained for short periods, depending on the capacity of the intermediate crude methanol storage tank, because it could lead to the complete filling or emptying of the crude methanol intermediate storage.

[0071] When the synthesis loop and the distillation section are running with the same flowrate of methanol, the crude methanol effluent from the synthesis loop may be fed directly to the distillation section, bypassing the intermediate crude methanol storage tank. In another mode of operation, the flow rate from the synthesis loop to the intermediate crude methanol storage tank may equal the flow rate from intermediate crude methanol storage tank to distillation, so that the stored amount does not vary.

[0072] The intermediate crude methanol storage tank disclosed by this invention is sized to handle some days of production. According to an embodiment, the crude methanol storage tank is sized at least to store the amount of crude methanol produced in 48 hours of operation at maximum capacity of the synthesis loop of the methanol plant. The dimensioning of the intermediate storage enables to operate the distillation section at different loads and at different maximum capacities. When the synthesis loop operates at or near full capacity, the amount of crude methanol that cannot be processed by the distillation section is stored; the stored crude methanol is then used when the synthesis loop operates at reduced load, to keep a stable operation of the distillation section.

[0073] In an advantageous embodiment, the capacity of the distillation section is undersized compared to the output of the synthesis loop. According to this embodiment, the distillation section has a nominal load which corresponds to processing and purifying an amount of crude methanol smaller than the output of crude methanol from the methanol synthesis loop at nominal load.

[0074] For example, it is considered a methanol plant with the maximum capacity of the distillation ranging from 70% to 90% of the maximum capacity of the synthesis loop. Considering a wind profile, a distillation section according to the invention may be dimensioned to produce a maximum quantity of AA grade methanol in the range of 35 MTD to 45 MTD, with a synthesis loop maximum production of crude methanol of 50 MTD.

[0075] Undersizing the distillation section is possible by increasing the capacity of the intermediate crude methanol storage tank and is very useful in conditions of high fluctuation of the production. A related advantage is a reduction in the capital cost due to the smaller distillation section. Additionally, undersizing the distillation section has the advantage to reduce the extra heat required for the distillation process when the synthesis loop works at a minimum load. According to this embodiment, the intermediate crude methanol storage tank is sized to hold many days of operation of the synthesis loop, therefore it is oversized compared to other embodiments. This has the great advantage to reduce the capital expenditure, since a bigger crude methanol storage costs less than bigger distillation section, and also the energy input is reduced.

[0076] The first mode of operating the methanol plant may be selected during period of high availability of said energy source, and the second mode may be selected during period of low availability of said energy source. For example, in a solar- powered application the first mode can be selected in daytime when solar energy is available, while the second mode can be selected overnight. The distillation of methanol is maximized when the energy production from the synthesis is at its maximum. In condition of high energy availability, when both the synthesis loop and the distillation section are operated at their maximum loads, there may be no need of extra heat input because the heat integration between the two sections is sufficient to perform the distillation. In condition of low energy availability, the heat recovered from the synthesis loop is generally insufficient to run a stable distillation process. In such cases, the invention provides that at least one other heat source is used to provide the missing distillation heat.

[0077] Additional heat is provided to the distillation section when the heat produced in the synthesis loop is below a minimum threshold. Preferably, when the plant operates according to the first mode, the heat provided by said at least one other heat source is in the range between 0% to 25% of the total heat required by the distillation process; when the plant operates according to the second mode, the heat provided by said at least one other heat source is in the range between 10% to 90%, preferably between 25% to 80%, of the total heat required by the distillation process. This percentage depends also on the type of selected mode of control of the distillation load.

[0078] Said at least one other heat source, used to provide distillation heat when heat retrieved from the synloop is insufficient, may include one or more of the following heat sources: burning of a synthesis loop purge gas, preferably in a heating boiler; burning of a hydrogen stream withdrawn from a hydrogen storage and / or from an electrolysis unit and / or from water splitting, preferably in a heating boiler; burning of byproducts effluent from the distillation section, such as fusel oil, preferably in an engine to produce combined heat and power; one or more electrical heater(s); steam produced by an electrical heated steam boiler.

[0079] According to an embodiment, at least part of the distillation heat is provided by one or more electrical heater(s) arranged to heat at least a bottom solution effluent from the bottom of at least one distillation column. The heated bottom solution is then reintroduced into the column after heating providing distillation heat. If the distillation section includes more than one distillation column, each distillation column may be equipped with an electrical bottom heater. Electrical heaters may be powered by the grid.

[0080] According to a preferred embodiment, one electrical heater is an electrically heated steam boiler which provides steam to one or more steam reboiler(s) heating the bottom solution of one or more distillation column(s). This embodiment has the advantage to significantly reduce the capital expenditure, keeping the same velocity of ramping up and ramping down. Preferably, at least part of the distillation heat is provided by said at least one other heat source during a ramp-up or ramp-down of the duty of distillation. In accordance with an embodiment, during a ramp-up or ramp-down of the duty of distillation, or during operation at low load of distillation, the methanol stream of the distillation section and / or at least part of the crude methanol collected in the intermediate crude methanol storage tank may be heated electrically, preferably by means of resistive heating.

[0081] Supplementary heat sources for the distillation may include fired heaters or electrical heaters that directly provide heat to the distillation section. In preferred embodiments, fired heaters may use one or more fuel streams recovered from the synthesis loop, such as purge gas. In preferred embodiments, hydrogen-fired heaters are used, to avoid carbon emissions.

[0082] The heat provided by said at least one other heat sources is preferably controlled to keep the load of the distillation section not lower than 30% of the nominal load of the synthesis loop and / or not lower than 50% of the nominal load of the distillation section.

[0083] The load of the distillation section is given by the flow rate of crude methanol sent to the section; the load of the synthesis loop is given by the flow rate of crude methanol produced by the loop.

[0084] When reference is made to a load in percentage, this shall be understood with reference to a nominal load. For the distillation section, the nominal load is the flow rate of crude methanol distilled by the section under design conditions; for the synthesis loop the nominal load is the flow rate of crude methanol produced by the synthesis loop under design conditions.

[0085] Electrical heating may be used for trim heating at least one methanol feed stream of a distillation column, preferably by resistive heating. The term trim heating denotes the heat regulation provided to the distillation column to run a stable distillation. The trim heating of the feed stream to the distillation column establishes more quickly vapor flows in the column, allowing for fast startup, while the electrical heating of the crude methanol in the intermediate crude methanol storage tank enables to maintain a lower load of the synthesis loop by complementing the heat recovered from the production of methanol. This integrated approach ensures efficient energy utilization and minimizes the need for external energy sources.

[0086] According to an embodiment, the distillation section includes one or more off- spec methanol storage tank(s) and the method of controlling the methanol plant includes one or more of the following steps:

[0087] - monitoring the purity of the distilled methanol, preferably in the off-spec methanol storage tank;

[0088] - storing in the off-spec storage tank the distilled methanol that does not meet the required purity;

[0089] - storing in the off-spec storage tank the distilled methanol during maintenance and / or small fluctuations in the synthesis loop;

[0090] - feeding the distillation section with off-spec distilled methanol retrieved from the off-spec methanol storage tank for further distillation to achieve the desired purity.

[0091] Preferably, the measuring of the purity level of distilled methanol is function of the liquid level of distilled methanol in the off-spec storage tank and / or the current purity of distilled methanol effluent from the distillation section and / or the flow rate of distilled methanol. Preferably the heat provision from the at least one other heat source is increased if distilled methanol does not meet the required purity. This allows to optimize the energy usage and saving on extra energy at low loads.

[0092] In an embodiment, the crude methanol is processed in the distillation section passing through a topping column and a single stabilization column, producing a methanol-to-olefine (MTO) grade methanol and / or a fuel grade green methanol. The topping column receives the crude methanol effluent from the synthesis loop and produces a bottom solution which is directed to the stabilization column. The stabilization column refines the methanol removing volatile components, such as light hydrocarbons, and ensures the methanol product meets the specific quality requirements. Heat is supplied to the stabilization column through electrical heating or a heating boiler which is heated using steam or another heat transfer medium.

[0093] This alternative embodiment provides an efficient and cost-effective solution for producing lower-grade green methanol, suitable for applications where high purity is not a prerequisite. The single stabilization column simplifies the distillation process, resulting in significant reduced energy consumption, which can be up to 50% lower than the expected energy consumption for a distillation process performed with one or more refining columns in cascade. The reduction in energy consumption in this embodiment is primarily attributed to the different product specifications required for MTO grade or fuel grade green methanol compared to the higher purity levels necessary for the methanol produced with more refining columns.

[0094] Preferably, the control of the methanol plant is based on a set of process parameters and a mathematical model configured to calculate and optimize said process parameters and the operation of the plant. A methanol plant should be configured adjusting the size of the water electrolysis unit, of the hydrogen storage device, of the synthesis loop, of the intermediate methanol tank and of the distillation section. The design optimization of these entities may be provided starting from the collection of historical data based on renewable energy production and the computation of an average value of energy production, from which the stability of the energy production may be retrieved. Depending on the magnitude and rate of change of the energy production the above-said entities may vary in size identifying the configuration with the minimum production cost.

[0095] The method proposed by the present invention is particularly advantageous in situations where high duty for distillation is required, for example when methanol is produced starting from CO2, with consequently high-water content in the product, and when the scope of the plant is producing methanol with high specification. The invention is not limited to cases where the e-methanol is connected to a renewable power plant or to a grid that receives power plant, as it can be applied to any case which results in a need of the e-methanol loop to operate in a load changing or load flexible mode.

[0096] Another advantage of the present invention is the minimization of the additional energy required for the distillation process thanks to the decoupling system. The minimization of additional heat to the distillation process is particularly relevant for methanol synthesis under fluctuating energy conditions. The load of the synthesis loop is decoupled from the load of distillation by means of the intermediate crude methanol storage tank. The need of extra heat for the distillation process is satisfied with the production of energy burning the synthesis loop purge and / or hydrogen in heating boilers; by means of engines fed with byproducts of the distillation; or with electrical heaters. This method maximizes the utilization of heat recovered from the synthesis loop and reduces the need of adding energy from external sources.

[0097] The present invention optimizes the energy consumption for the full range of operating loads of both the synthesis loop and the distillation section in methanol processes operated under fluctuating energy condition. The invention proposes a more energy-efficient and environmentally friendly method for the production of methanol.

[0098] Detailed description

[0099] Fig. 1 illustrates a methanol distillation section according to an embodiment of the invention.

[0100] Figs. 2a, 2b, 3a, 3b, 4a, 4d and 5 are plots of various parameters useful to show the advantages of the invention.

[0101] Fig. 1 illustrates a simplified diagram of a methanol distillation section of a crude methanol 1 , including a topping column 4 and a refining column 16. Other embodiments may include a greater number of columns, for example a typical embodiment has three refining columns arranged in a cascade and working at different pressure (high-, medium- and low-pressure).

[0102] The crude methanol 1 is produced in a synthesis section, not shown.

[0103] The topping column 4 is equipped with a reflux condenser 10 and a heat exchanger 7; the refining column 16 is connected to a condenser 24 and a heat exchanger 19. Part of the bottom solution 5 of the topping column 4 is fed to the refining column 16, for further distillation, via a pump 13 and a let-down valve 14. The refined methanol is obtained at line 23 from top of the refining column 16; this stream 23 is condensed in the condenser 24 obtaining a liquid refined methanol product 25.

[0104] The distillation section further comprises an intermediate crude methanol storage tank 2, to which the feed line of the crude methanol 1 is connected. The crude methanol is fed to the topping column 4 from the intermediate crude methanol storage tank 2, via a pump 3 and line 35.

[0105] The bottom solution 5 from the topping column 4 is partly sent to the heat exchanger 7 via line 36 and reintroduced in the column as heated solution 8, and partly sent to the refining column 16. In the heat exchanger 7, the hot fluid is for example steam 34 produced in the methanol synthesis section (e.g. from cooling of hot reaction effluent). According to a preferred embodiment, the line 36 is also connected to an electrical heater 6. A stream of light components 9 emerging from top of the column 4 is sent to the reflux condenser 10, which releases a mixture of light gases in the overhead vapour stream 12 and a condensed effluent which is reintroduced in the topping column 4 as reflux flow 11 .

[0106] The remainder of the bottom solution 5 forms the feed stream 15 of the refining column 16. The bottom solution 17 is at least in part heated through the heat exchanger 19 and optionally an electrical heater 18 via line 37. A stream of aqueous solution is removed from the refining column 16 via line 32. The heat exchanger 19 receives hot steam 30 from an external source 31 (e.g. process steam) or from a heating boiler 26. Said heating boiler 26 produces steam from feed water 29. The fuel for the heating boiler 26 may include one or more of: a purge gas 27 from the synthesis loop, a stream of hydrogen 28 from a hydrogen storage or an electrolysis unit (not shown in figure).

[0107] As above, the refined methanol product is obtained from the column 16 as stream 25, after condensation.

[0108] A stream of by-products known as fusel oil 21 , which is a mixture of methanol, water and higher alcohols, is also removed from the refining column, typically at intermediate elevation. Said fusel oil 21 can be used as fuel for an engine 22 to produce power and heat 33 to be used in the distillation section.

[0109] The intermediate crude methanol storage tank 2 allows to decouple the synthesis loop and the distillation section. Thanks to said intermediate crude methanol storage tank 2, the instant flow rate of crude methanol produced by the synthesis loop can differ from the flow rate of methanol processed by the distillation section. The load of the synthesis loop may therefore vary substantially independently from the load of the distillation section, which is particularly useful when the production of methanol in the synthesis loop relies on a fluctuating source of energy, e.g. production of hydrogen from a renewable energy source. In the above case, and in a condition of high availability of the fluctuating source, the flow rate in line 1 coming from the synthesis loop may be higher than the flow rate processed by the distillation section, and the difference is stored in the intermediate crude methanol storage tank 2. In a subsequent condition of low availability of the source, the synthesis loop may run at reduced load and the flow rate in feed line 35 may be greater than the input of line 1 , the difference being provided by the intermediate crude methanol storage tank 2.

[0110] When the synthesis loop runs at a low flow rate, the heat recovered in the loop may be not sufficient for a stable distillation process, therefore one or more additional heat inputs may be provided by the electrical heaters, 6, 18, and / or the heating boiler 26.

[0111] The following examples illustrate the operation of the invention.

[0112] Example 1

[0113] Fig. 2a and Fig. 2b relate to the production of e-methanol in a synthesis loop and distillation section, assuming 50 MTPD (metric tons per day) peak production with photovoltaic energy. The synthesis loop operates between 20 - 110 %, whereas the distillation section operates at a stable load of 70% with feed from intermediate crude methanol storage tank. The distillation steam requirement is 1 .13 t / h (tons per hours) of medium pressure steam, and the steam produced in the synthesis loop varies between 0.21 - 1 .18 t / h. The additional heat required to run a stable distillation process, is shown in Fig. 2b and is provided by the one or more additional sources, for example the electrical heaters 6 and 18. In case of electric heating, an additional specific consumption of 219 kWh / t is required on average daily production, and the peak power required for electric heating is 745 kW.

[0114] Figs. 3a and 3b relate to the example of Figs. 2a, 2b wherein the distillation section operates with a variable load between 60 - 80%, as illustrated in Fig. 3a. This situation leads to a steam required in the distillation section between 0.97 - 1.30 t / h as shown in Fig. 3b. In this case the additional source needed to run distillation is decreased to 214 kWh / t and the maximum required additional heating power is 614 kW.

[0115] Example 3

[0116] This example is a variation of example 2, wherein the distillation is run with a wider variable load between 50 - 85%, Fig. 4a, leading to steam required in the distillation section of 0.81 - 1 .38 t / h, Fig. 4b. In this situation the additional heating source needed for the distillation is still 214 kWh / t as in example 2, however the peak power consumption is decreased to 530 kW.

[0117] Example 4

[0118] This example has the same working conditions of example 3, therefore Fig. 4a describes the loop and distillation load condition for this example. The difference in this case with respect to example 3 is the use of purge combustion to increase the steam production necessary for the distillation section. Purge combustion generates 1 .2 t / h of steam at 50 MTPD loop production, as shown in Fig. 5. Considering the same ranges of load described in example 3, using purge combustion at variable load as per the synthesis loop, the additional heating source needed for the distillation is reduced to 42 kWh / t, with a peak electric power consumption of 288 kW.

Claims

CLAIMS1 ) A method for controlling a methanol plant wherein: the plant includes at least a front end for generation of a methanol make-up gas; a synthesis loop configured to convert in a methanol converter said make-up gas into crude methanol; an intermediate storage tank of said crude methanol; a distillation section for purification of said crude methanol to obtain a distilled methanol product; a heat transfer system configured to transfer heat from the synthesis loop to the distillation section, to provide heat for distillation; wherein the method includes at least operating the methanol plant according to a first mode and a second mode; in the first mode, the flow rate of crude methanol effluent from the synthesis loop is greater than the flow rate of crude methanol processed by the distillation section, the balance being sent to said intermediate storage tank of crude methanol; in the second mode, the flow rate of crude methanol effluent from the synthesis loop is less than the flow rate of crude methanol processed by the distillation section, the balance of crude methanol fed to the distillation section being withdrawn from said intermediate crude methanol storage tank; the method includes that when during the first mode or second mode the heat transferred from the synthesis loop to the distillation section is insufficient for said distillation process, the missing heat for said distillation process is provided by at least one other heat source to supplement the heat from the synthesis loop.2) A method according to claim 1 , wherein the nominal capacity of the distillation section is in a range of 40% to 100% of the nominal capacity of the synthesis loop.3) A method according to any of the previous claims, wherein the production of make-up gas and / or the operation of the synthesis loop depends at least in part from a fluctuating or intermittent energy source, preferably said energy source is a renewable energy source; the method includes the steps of producing hydrogen for the make-up gas by means of splitting of water, preferably water electrolysis, which is powered at least in part by said renewable energy source; optionally at least part of the hydrogen produced by splitting of water or water electrolysis is collected into a hydrogen storage.4) A method according to any of the previous claims, wherein the loop operating capacity is determined at least by one of the following: the flow rate of a hydrogen stream feeding the make-up gas, the current flow rate of make-up gas delivered by the front-end, the amount of hydrogen stored in a hydrogen storage, the amount of crude methanol collected in the intermediate crude methanol storage tank; the distillation operating capacity is determined at least by one of the following: the flow rate of a hydrogen stream feeding the make-up gas, the load of the synthesis loop, the amount of crude methanol collected in the intermediate crude methanol storage tank; said hydrogen stream feeding the make-up gas being withdrawn from a hydrogen storage and / or being produced from an electrolysis unit and / or produced from water splitting; the method including the step of: selecting the first mode if the loop operating capacity is higher than the distillation operating capacity; selecting the second mode if the loop operating capacity is lower than the distillation operating capacity.5) A method according to any of the previous claims, wherein the distillation section comprises at least one distillation column equipped with a condenser and a bottom solution heating boiler; said condenser producing a reflux flow reintroduced in the distillation column and / or an overhead vapor stream and / or a stream of distilled methanol effluent from the condenser; the method including the step of regulating the temperature at the bottom of the column to a selected target temperature by means of said heating boiler; the duty of the heating boiler depends on the reflux flow of the column wherein the amount of the reflux flow is controlled at least by one of the following: the load of the distillation section, the flow rate of the overhead vapor stream, the hydrogen for the make-up gas collected in a hydrogen storage unit or produced in an electrolysis unit.6) A method according to any of the previous claims, wherein said heat transfer system includes a steam system and / or direct exchange with the methanol converter and / or direct exchange with a reaction effluent of the methanol converter and / or indirect heat exchange by means of a heating medium.7) A method according to any of the previous claims, wherein during fluctuating conditions, the method is operated such that the load of the synthesis loop ranges between 10% and 110% of its nominal load.8) A method according to any of the previous claims, wherein during fluctuating conditions, the method is operated such that the load of the distillation section does not fall below a minimum load of the distillation section, said minimum load being preferably in the range 30% to 80% of the nominal load of the distillation section, more preferably in the range 50% to 70%.9) A method according to any of the previous claims, including the step of increasing the reflux flow of at least one distillation column when the distillation section operates at low load, said increase of reflux flow being determined tomaintain a target liquid and vapor flow in said distillation column.10)A method according to any of the previous claims, wherein the distillation load is controlled as function of one or more of the following: a) the flow rate of crude methanol effluent from the synthesis loop; b) the level of crude methanol in the intermediate crude methanol storage tank.11 )A method according to claim 10, wherein when the distillation load is controlled as function of the level of crude methanol in accordance with option b), the load of the distillation depends on one or more operational threshold(s) and one or more alarm threshold(s) of crude methanol in the intermediate storage tank; optionally the one or more operational threshold(s) include(s) one or more tolerance zone(s).12)A method according to claim 11 , wherein when the distillation load depends on the level of crude methanol in the intermediate crude methanol storage tank, the distillation load is controlled according to any of the following: b1 ) the distillation load is reduced when the quantity of crude methanol in the intermediate storage tank transitions from a value above said one or more operational threshold(s) to a value below said one of more operational threshold(s); b2) the distillation load is increased when the quantity of crude methanol in the intermediate storage tank transitions from a value below said one or more operational threshold(s) to a value above said one or more operational threshold(s).13) A method according to claim 11 or 12 wherein, when the quantity of crude methanol in the intermediate storage tank is in a range delimited by two consecutive of said operational thresholds, the distillation load changesaccording to the filling of the intermediate storage tank, preferably the distillation load changes proportionally to the filling of the crude methanol storage tank. )A method according to any of the previous claims, further including modes of operation wherein: the crude methanol from the synthesis loop is entirely sent to the distillation section, and / or the crude methanol from the synthesis loop is entirely sent to the intermediate crude methanol storage tank, and / or the crude methanol processed by the distillation section is entirely taken from the intermediate crude methanol storage tank. )A method according to any of the previous claims, wherein the intermediate crude methanol storage tank is sized at least to store the amount of crude methanol produced in 48 hours of operation at maximum capacity of the synthesis loop of the methanol plant. )A method according to any of the previous claims, wherein when the plant operates according to the first mode, the heat provided by said at least one other heat source is in the range between 0% to 25% of the total heat required by the distillation process; when the plant operates according to the second mode, the heat provided by said at least one other heat source is in the range between 10% to 90%, preferably between 25% to 80%, of the total heat required by the distillation process. )A method according to any of the previous claims, wherein the at least one other heat source includes one or more of the following heat sources:burning of a synthesis loop purge gas, preferably in a heating boiler; burning of a hydrogen stream withdrawn from a hydrogen storage and / or from an electrolysis unit and / or from water splitting, preferably in a heating boiler; burning of byproducts effluent from the distillation section, such as fusel oil, preferably in an engine to produce combined heat and power; one or more electrical heaters; steam produced by an electrical heated steam boiler. )A method according to any of the previous claims, wherein distillation heat is provided by said at least one other heat source during a ramp-up or rampdown of the duty of distillation. )A method according to any of the previous claims, wherein the heat provided by said at least one other heat source is controlled to keep the load of the distillation section not lower than 30% of the nominal load of the synthesis loop and / or not lower than 50% of the nominal load of the distillation section. )A method according to any of the previous claims, wherein the distillation section includes one or more off-spec methanol storage tank(s), the method includes one or more of the following steps: monitoring the purity of the distilled methanol, preferably in the off-spec methanol storage tank; storing in the off-spec storage tank the distilled methanol that does not meet a required purity; storing in the off-spec storage tank the distilled methanol during maintenance and / or during small fluctuations in the synthesis loop; feeding the distillation section with off-spec distilled methanol retrieved fromthe off-spec methanol storage tank for further distillation. )A method according to any of the previous claims, including measuring the purity level of distilled methanol from the liquid level of distilled methanol in an off-spec storage tank and / or from the purity of distilled methanol effluent from the distillation section and / or from the flow rate of distilled methanol.

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