A diphenyl carbonate reactive distillation production method based on overhead side line produced heat integration

Abstract

The application relates to the chemical technology field and discloses a diphenyl carbonate reaction rectification production method based on overhead side line produced heat integration. The method is configured with a reaction rectification tower and a flash separation tank, a high-temperature side line liquid phase is led out from the lower part of a rectification section or the upper part of a reaction section, mixed with fresh methyl phenyl carbonate raw materials, mixed fluid is adiabatically flashed in the flash tank to realize phase state reconstruction, separated gas phase is bypassed and transported to the bottom of the rectification section to quickly remove by-product dimethyl carbonate, and liquid phase is transported to the top of the reaction section to enrich methyl phenyl carbonate reactants. The application realizes dynamic and accurate control of the gas-liquid ratio and heat content state of the tower by constructing the pressure gradient between the devices and adjusting the side line circulation ratio. The method reduces the preheating energy consumption of raw materials by using side line sensible heat, breaks the reaction balance limitation by cooperating with gas-liquid double track staged feeding, and improves the diphenyl carbonate reaction conversion rate and the whole system operation stability.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, specifically to a reactive distillation method for producing diphenyl carbonate based on the integrated heat output from the top side stream of the column. Background Technology

[0002] Diphenyl carbonate is an important intermediate in the synthesis of engineering plastics such as polycarbonate. With the promotion of non-phosgene polycarbonate processes, the process route for synthesizing diphenyl carbonate via the disproportionation reaction of methyl phenyl carbonate has become the mainstream choice due to its green and environmentally friendly characteristics. Reactive distillation technology, by coupling the chemical reaction process with the distillation separation process in the same equipment, can overcome the thermodynamic equilibrium limitation by timely removal of reaction products, thereby improving the raw material conversion rate and selectivity, and is widely used in this disproportionation reaction system.

[0003] In the existing diphenyl carbonate reactive distillation process, fresh methyl phenyl carbonate feedstock is usually introduced from the outside in liquid form. In order to maintain the reaction temperature and gas-liquid balance in the column, a large amount of external heat source is required to preheat the feedstock before it enters the reaction section, or the heat load of the reboiler in the column bottom is increased to compensate for the sensible heat gap caused by the cold feedstock. This results in high energy consumption of utilities in the production process.

[0004] Furthermore, the disproportionation reaction is severely limited by chemical equilibrium. If the byproduct dimethyl carbonate cannot be quickly removed from the reaction system, it will inhibit the forward reaction. Traditional single-feed methods often result in an unsatisfactory local component concentration distribution at the feed location, making it difficult to achieve rapid separation of light and heavy components at the moment of feed. This causes dimethyl carbonate to accumulate in the liquid phase, which not only reduces the utilization rate of the reaction section but also increases the load on subsequent separation units, making it difficult to achieve stable production with high conversion rate under low energy consumption. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a reactive distillation method for producing diphenyl carbonate based on the integrated heat from the top side stream of the column, which solves the problems of high energy consumption for raw material preheating, limited reaction equilibrium, and low conversion rate caused by mismatch in component distribution at the feed location.

[0006] The first aspect of this invention provides a reactive distillation method for producing diphenyl carbonate based on the integrated heat from the top side stream of the column. This method is implemented through the following steps: A reactive distillation column and a flash separator are configured from top to bottom, consisting of a rectification section, a reaction section, and a stripping section. A high-temperature liquid side stream is drawn from the lower part of the rectification section or the upper part of the reaction section. The high-temperature liquid side stream is introduced into the mixing component with the fresh raw material based on an adjustable side stream circulation ratio for direct contact mixing to obtain a mixture. The pressure downstream of the mixing component is controlled by a throttling element, so that the mixture enters the flash separator for adiabatic flash evaporation, and is separated into a gas stream rich in dimethyl carbonate and phenol and a liquid stream rich in methyl phenyl carbonate and catalyst. A dual-track staged feeding method is adopted to construct a phase thermal coupling process in the feeding area, which transports the gas phase stream to the bottom of the rectification section and the liquid phase stream to the top of the reaction section; A liquid phase residence state that satisfies reaction kinetics is established in the reaction section of the reactive distillation column. The heat balance in the column is maintained by adjusting the side-stream circulation ratio, and the disproportionation reaction and gas-liquid mass transfer coupling process are executed. Dimethyl carbonate is removed from the top of the rectification section, and diphenyl carbonate product is collected from the bottom of the stripping section.

[0007] Preferably, in the step of drawing out the high-temperature liquid side stream, the selection of the side stream stage follows the following principles: based on the temperature distribution within the column, the tray where the temperature of the liquid stream is higher than the temperature of the fresh feed and the temperature difference between the two is greater than the set minimum heat transfer driving temperature difference is selected as the side stream stage; based on the component concentration distribution, the tray located in the transition region from the lower part of the rectification section to the upper part of the reaction section is selected as the side stream stage, where methyl phenyl carbonate and diphenyl carbonate are enriched and the molar fraction of dimethyl carbonate shows a decreasing gradient. Further, a liquid collection tray is provided at the location of the side stream stage, and the descending liquid phase is collected using the liquid collection tray and drawn out of the column through the side stream outlet.

[0008] Preferably, the direct contact mixing step specifically includes constructing an enhanced turbulent mixing flow field, wherein the mixing component is selected from a static mixer or a Venturi jet mixer. When using the static mixer, the side stream and the fresh feed stream are forcibly separated, rotated, and radially mixed as they flow through internal twisted blades or corrugated plate elements; when using the Venturi jet mixer, the high-temperature liquid phase side stream with a flow rate higher than that of the fresh feed is selected as the driving fluid, and the high-speed jet generated by the driving fluid is used to entrain the fresh feed as the entraining fluid.

[0009] Preferably, in order to ensure the mixing effect and prevent premature vaporization, the throttling element is used to establish back pressure in the mixing component, so that the operating pressure in the mixing component is higher than the bubble point pressure of the mixture at the current mixing temperature and mixing composition, ensuring that the material is mixed in a liquid single-phase state.

[0010] Preferably, during the adiabatic flash evaporation process, the vaporization fraction control follows the principle of heat balance: the total enthalpy of the mixed fluid before entering the flash separator is equal to the sum of the total enthalpy of the gas phase and the total enthalpy of the liquid phase under flash equilibrium conditions; by adjusting the opening of the gas phase flow control element connected between the gas phase outlet of the flash separator and the reactive distillation column, the operating pressure inside the flash separator is kept constant, thereby determining the molar distribution ratio of the gas phase and the liquid phase according to the enthalpy state of the mixed fluid.

[0011] Preferably, the method establishes a decreasing pressure gradient control strategy: controlling the operating pressure within the mixing component to be higher than the operating pressure within the flash separator, causing the mixed fluid to experience a pressure drop and flash evaporation when passing through the throttling element; controlling the operating pressure within the flash separator to be higher than the static pressure inside the reactive distillation column at the vapor inlet, allowing the vapor stream generated by flash evaporation to overcome pipeline resistance and flow by gravity into the reactive distillation column; and controlling the static pressure inside the reactive distillation column at the vapor inlet to be higher than the operating pressure at the top of the column, maintaining the upward flow of the vapor phase within the column.

[0012] Preferably, the spatial position of the dual-track staged feeding method satisfies the following conditions: the theoretical plate position number of the gas stream entering the reactive distillation column is less than or equal to the theoretical plate position number of the liquid stream entering the reactive distillation column, and the theoretical plate position number is defined as increasing downwards from the top of the column; the gas stream introduction position is located below the rectification section, and the liquid stream introduction position is located above the reaction section; a rising gas-liquid collection area is provided between the gas stream introduction position and the liquid stream introduction position, the rising gas-liquid collection area is used to collect the liquid descending from the rectification section and the liquid stream, and allows the gas stream to pass upwards.

[0013] Preferably, the phase-state thermal coupling process constructed in the feeding region by the dual-track staged feeding method includes: the reflux liquid stream of the rectification section directly contacts the incoming high-temperature gas stream when it flows through the free space of the gas stream introduction area; the high-temperature gas stream transfers heat to the reflux liquid stream, causing the reflux liquid stream to partially vaporize, while the reflux liquid stream washes the heavy phenol components entrained in the high-temperature gas stream and carries the heavy phenol components back to the reaction section.

[0014] Preferably, the step of maintaining the heat balance in the column by the side-stream circulation ratio includes: defining the side-stream circulation ratio as the ratio of the mass flow rate of the high-temperature liquid side-stream stream to the mass flow rate of the fresh raw material; when the temperature of the reaction section is detected to be higher than a set threshold, increasing the side-stream circulation ratio, increasing the temperature and vaporization fraction of the mixture when it enters the flash separator, increasing the latent heat load of the gas stream introduced from the top of the reactive distillation column, and reducing the sensible heat load of the liquid stream that directly enters the top of the reaction section.

[0015] Preferably, establishing a liquid phase residence state that satisfies reaction kinetics within the reaction section specifically includes: setting up high-weir plate trays or high-holding-capacity packing to form a deep liquid layer in the reaction section; controlling the total liquid phase residence time in the reaction section to be greater than or equal to the characteristic time required for the disproportionation reaction to reach a predetermined conversion rate; the total liquid phase residence time is determined by dividing the sum of the holding volumes of each stage in the reaction section by the average liquid volumetric flow rate flowing through the reaction section.

[0016] Preferably, the coupling process of the disproportionation reaction and gas-liquid mass transfer includes: utilizing the boiling point difference between the dimethyl carbonate and the reactants and diphenyl carbonate in the reaction system, the generated dimethyl carbonate is continuously vaporized from the liquid phase into the gas phase, keeping the reaction quotient in the liquid phase less than the equilibrium constant; at the gas-liquid interface, the mass transfer of the dimethyl carbonate component from the liquid phase to the gas phase is controlled, and the mass transfer of the methyl phenyl carbonate and the diphenyl carbonate components from the gas phase to the liquid phase is controlled, and the latent heat released by the condensation of the rising gas phase is used to supplement the latent heat of vaporization of the dimethyl carbonate and the endothermic heat of the reaction.

[0017] This invention provides a reactive distillation method for producing diphenyl carbonate based on the integrated heat from the top side stream of the distillation column. It has the following beneficial effects: 1. This invention directly mixes a high-temperature liquid stream from the side stream of a reactive distillation column with fresh methyl phenyl carbonate feedstock and performs adiabatic flash evaporation. The sensible heat of the side stream is used to heat the low-temperature feedstock and partially vaporize it, thus achieving internal recovery and utilization of the reaction heat. This not only reduces the external energy consumption required for feedstock preheating, but also provides rising steam for the distillation section directly through the flash evaporation, reducing the heat load on the reboiler. This solves the problems of low thermal efficiency and excessive utility consumption caused by direct feeding of cold feedstock in traditional reactive distillation processes.

[0018] 2. This invention constructs a gas-liquid dual-track staged feeding structure based on flash separation, introducing a flash vapor stream rich in dimethyl carbonate to the bottom of the rectification section and a flash liquid stream rich in reactants and catalysts to the top of the reaction section. This feeding method allows the byproduct dimethyl carbonate in the gas phase to quickly leave the reaction zone, breaking the chemical equilibrium limitation, while ensuring that the reactants at the top of the reaction section maintain a high concentration. This solves the problems of uneven component distribution in the reaction zone, severe inhibition of reverse reaction, and limited reaction conversion rate caused by traditional single-inlet feeding methods.

[0019] 3. This invention establishes a flow control loop based on the side-stream circulation ratio and a decreasing pressure gradient distribution between the tower and pipeline. By utilizing the coordinated control of side-stream flow and pressure parameters, the total enthalpy and vaporization fraction entering the flash tank are dynamically adjusted. This allows the system to flexibly adjust the gas-liquid ratio and temperature state returning to the tower according to production load fluctuations, ensuring the stability of gas-liquid mass transfer and reaction processes within the tower. This solves the problem of low operational flexibility and easy occurrence of flooding or leakage in complex reactive distillation coupled systems when facing feed fluctuations. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the overall process of an embodiment of the present invention; Figure 2 This is a flowchart of the side-line extraction site selection construction and raw material mixing process according to an embodiment of the present invention; Figure 3 This is a flowchart illustrating the pressure matching and control strategy according to an embodiment of the present invention. Figure 4 This is a flowchart illustrating the segmented structure and internal components of the reactive distillation column according to an embodiment of the present invention. Figure 5 This is a flowchart illustrating the dual-track graded feeding process according to an embodiment of the present invention. Figure 6 This is a flowchart illustrating the adjustment of the side-stream circulation ratio and thermal balance according to an embodiment of the present invention. Figure 7 This is a flowchart illustrating the implementation of the mass transfer and reaction mechanism within the tower in an embodiment of the present invention. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] See attached document Figure 1 This invention provides a reactive distillation method for producing diphenyl carbonate based on the integrated heat from the top side stream of the distillation column, comprising the following steps: S1 is configured with a reactive distillation column and a flash separator, which are divided into a rectification section, a reaction section and a stripping section from top to bottom. A high-temperature liquid side stream is drawn from the lower part of the rectification section or the upper part of the reaction section to the flash separator. The sensible heat of the high-temperature liquid side stream is used to directly contact and mix with the fresh raw material introduced into the flash separator. S2 controls the operating pressure of the flash separator, causing the mixture to undergo adiabatic flash evaporation within the tank. During this process, the material undergoes phase reconstruction, separating into two streams: a gaseous stream rich in light components and a liquid stream rich in reactants. S3, the gas stream is bypassed and transported to the upper part of the rectification section to remove by-products, and the liquid stream is directed to the reaction section to enrich the reactants; S4 maintains the disproportionation reaction and gas-liquid mass transfer balance in the reactive distillation column, and finally removes dimethyl carbonate from the top of the rectification section and collects diphenyl carbonate product from the bottom of the stripping section.

[0023] See attached document Figure 2 The construction and location of the side-stream feed stream directly determine the thermal integration efficiency and material balance of the distillation process system. The side-stream feed outlet of a reactive distillation column is located in the lower region of the rectification section or the upper region of the reaction section. The specific selection of tray locations and the construction of the feed structure are implemented through the following steps: S11. The heat source extraction stage is determined based on the temperature distribution characteristics within the column. Under steady-state operating conditions of the reactive distillation column, the tray with the liquid phase temperature meeting the minimum heat transfer temperature difference requirement is selected as the side-stream extraction stage. Specifically, the liquid phase temperature at the side-stream extraction stage must be higher than the temperature of the fresh feed at the inlet of the flash separator, and the temperature difference between the two must be greater than the minimum heat transfer driving temperature difference set by the distributed control system (e.g., 10 to 30 degrees Celsius). This temperature difference setting aims to ensure that the sensible heat carried by the side-stream stream is sufficient to drive the predetermined flash fraction after mixing, preventing incomplete gas-liquid separation due to excessively low mixing temperature.

[0024] S12, based on the component concentration distribution characteristics, determines the low-inhibition zone. The side stream sampling location must avoid the azeotropic region with the highest dimethyl carbonate concentration to reduce the inhibitory effect of the circulating material on the reaction equilibrium. In the transition region from the lower part of the rectification section to the upper part of the reaction section, the tray position is selected where the concentrations of methyl phenyl carbonate and diphenyl carbonate are relatively high, and the molar fraction of dimethyl carbonate shows a decreasing gradient. By sampling in this region, it can be ensured that the reactant concentration in the liquid stream recycled back to the reaction section is at a high level, maintaining the forward reaction kinetic advantage.

[0025] S13, Construct a side-stream liquid collection and extraction structure. Install a liquid collection tray at the designated side-stream extraction stage location. This tray includes a liquid collection surface and several gas riser channels. The gas riser channels penetrate the collection surface to allow lower-level vapor to rise, while the collection surface collects the liquid phase descending from the upper tray. The liquid collection tray connects to the side-stream extraction outlet on the tower wall via a flange interface, guiding the collected liquid phase to the external pipeline. Depending on process requirements, this liquid collection tray can be configured as a fully extracted or partially extracted type. When configured as a partially extracted type, an overflow weir and downcomer must also be installed on the liquid collection tray to allow the portion of the liquid phase not extracted by the side stream to overflow to the next stage tray. When configured as a fully recovered type, the liquid collection tray does not have a downcomer directly leading to the next tray. The collection tray, except for the gas riser channel, is a fully enclosed liquid-blocking structure, forcing all the descending liquid phase from the upper tray to collect on the tray surface and be completely led out of the tower through the side-stream outlet. This maximizes the utilization of the liquid phase heat source or precisely controls the flow rate. S14. Construct a side-stream flow control loop. Connect a flow regulating valve and monitoring instruments in series on the side-stream pipeline. Control the total enthalpy input to the flash tank by adjusting the side-stream extraction ratio (i.e., the ratio of side-stream flow to the total liquid flow descending in the column). The control logic is as follows: when the stripping section load is too high or there is a risk of flooding in the extraction stage, decrease the extraction ratio to increase the liquid flow descending in the column; when it is necessary to increase the preheating degree of the feedstock or increase the flash vapor phase, increase the extraction ratio. Furthermore, the pipeline design must match the pressure requirements. If a pressure differential exists, gravity flow combined with throttling and pressure regulation is used; if the pressure differential is insufficient, a booster pump is added to overcome the resistance.

[0026] Before the side-stream outflow and fresh feed enter the flash tank, they undergo an adiabatic mixing process to achieve uniform mixing and thermal equilibrium. This process occurs within the mixing component, and the specific implementation steps are as follows: S15, Constructing an Enhanced Turbulent Mixing Flow Field. To overcome the low heat transfer efficiency under laminar flow conditions, the side-stream extraction pipeline and the fresh feed pipeline are connected by a high-shear mixing component. This mixing component can be either a static mixer or a Venturi jet mixer. When a static mixer is used, the mixer shell is in the form of a pipe, with several sets of twisted blades or corrugated plate elements connected in series inside. The side-stream stream and the fresh feed stream enter the mixer inlet simultaneously. As the fluids flow through the blade elements, they are forcibly divided, rotated, and radially mixed, forming a uniform concentration and temperature distribution across the pipe cross-section. When a Venturi jet mixer is used, a high-flow-rate, high-temperature side-stream liquid phase stream is selected as the driving fluid entering the nozzle. The negative pressure created by the high-speed jet at the nozzle outlet acts as the entrainment fluid for the fresh feed fluid, and the two fluids complete momentum exchange within the mixing section.

[0027] S16 involves direct contact heat transfer without partitions. The high-temperature side-stream liquid stream and the low-temperature fresh raw material come into direct contact within the mixing component, with heat transferred from the high-temperature side-stream stream to the low-temperature fresh raw material. An insulation layer is laid on the outside of the mixing component and connecting pipes, and heat loss to the environment is ignored; this process is considered an isenthalpic mixing process.

[0028] The specific enthalpy of the mixed fluid is determined by the thermodynamic states of the two inlet streams and satisfies the following energy conservation relationship: ; In the formula, The molar enthalpy of the mixed fluid; Molar flow rate of fresh raw materials; The molar enthalpy of fresh raw materials at the import temperature; Molar flow rate of the lateral sampling material; The molar enthalpy of the side-stream extracted stream at the extraction temperature.

[0029] S17, Controlling the mixing pressure to maintain the liquid phase state. A throttling and pressure-reducing element (e.g., a regulating valve or orifice plate) is installed downstream of the mixing component and before entering the flash separator. This throttling and pressure-reducing element establishes back pressure within the mixing component, thus controlling the operating pressure during the mixing process. The following conditions must be met: ; In the formula, The operating pressure within the hybrid component. For the mixture at the mixing temperature and mixed composition The bubble point pressure below; among which, The temperature of the mixed stream, This represents the set of mole fractions of each component in the mixed fluid. By maintaining the operating pressure above the bubble point pressure, it is ensured that the side stream and fresh feed are mixed in a single-phase liquid state, preventing premature vaporization within the mixing pipeline that could lead to fluid pulsation or a decrease in mixing efficiency. At this point, the temperature of the mixed fluid is between the side stream temperature and the fresh feed temperature, and the specific value of this mixing temperature is determined by the aforementioned energy conservation equation and the specific heat capacity characteristics of the fluid.

[0030] At this point, the fluid remains in a supercooled or saturated liquid phase state, with its temperature between the two feed temperatures, thus establishing an energy basis for subsequent flash evaporation.

[0031] When the mixed fluid enters the flash separator, the original equilibrium is broken by a sudden pressure drop, and the material distribution is achieved through the difference in the volatility of the components, as follows: S21, performs adiabatic throttling expansion. When the high-temperature liquid phase flows through the inlet throttling valve, the pressure instantly drops to the flash pressure. This pressure value needs to be higher than the feed pressure of the reactive distillation column to ensure vapor transport. During this process, the fluid releases sensible heat, which is converted into latent heat, and some of the liquid vaporizes instantaneously, causing the overall temperature to drop to the flash equilibrium temperature.

[0032] S22, establishing phase reconstruction equilibrium. The gas-liquid mixture reaches a new thermodynamic equilibrium within the vessel, and the components are redistributed. Dimethyl carbonate and some phenol are mainly enriched in the gas phase, while methyl phenyl carbonate, diphenyl carbonate, and the catalyst remain entirely in the liquid phase. The molar vaporization fraction of the flash evaporation process. Controlled by the heat balance equation: ; In the formula, The molar enthalpy of the mixed fluid before it enters the flash separator; The flash vaporization fraction represents the ratio of the molar flow rate of the vapor phase generated by flash vaporization to the total molar flow rate of the feed. The molar enthalpy of the saturated gas phase at flash equilibrium temperature and pressure; This represents the molar enthalpy of the saturated liquid phase at the flash equilibrium temperature and pressure. The flash pressure is adjusted accordingly. Or adjust the upstream sideline extraction ratio to achieve the desired result. It can regulate the gasification fraction. This enables control over the distribution ratio of gas and liquid phase flow rates.

[0033] S23, implement gas-liquid physical separation. A tangential feeding method is used to separate droplets using centrifugal force. A wire mesh demister is installed at the top of the tank to capture mist, and a liquid seal is maintained at the bottom of the tank through level control. Finally, the gas phase is drawn out from the top of the tank, and the liquid phase is discharged from the bottom, completing the preparation for the material basis classification.

[0034] See attached document Figure 3 To ensure that the material flows in a predetermined direction between the side-stream extraction pipeline, flash separator, and reactive distillation column, a decreasing pressure distribution is established within each connecting device and in the connecting pipelines. This pressure distribution is maintained by fluid delivery equipment and regulating valve assemblies installed on the pipelines. The pressure matching and control strategy is implemented through the following steps: S24, Establish a pressure gradient. The operating pressure setpoints of the reactive distillation column, flash separator, and mixing components must meet the power requirements of fluid transport; that is, the operating pressures of the reactive distillation column and feed pumps must overcome pipeline resistance and the back pressure of the flash separator. The pressure distribution of the side-stream circulation and staged feed loops should satisfy the following relationship: ; In the formula, The operating pressure within the hybrid component; The operating pressure inside the flash separator; This refers to the static pressure inside the reactive distillation column at the vapor inlet. This is the operating pressure at the top of the reactive distillation column. This pressure gradient ensures that the mixed fluid can flash smoothly after throttling; the vapor generated by flashing can overcome pipeline resistance and back pressure inside the column and flow into the reactive distillation column by gravity; the vapor inside the reactive distillation column can rise smoothly to the top condenser.

[0035] S25, Flash pressure setpoint control. Operating pressure of the flash separator. It is an independent, controlled variable, and its stability directly affects the gas-liquid separation effect and the stability of subsequent feed. A gas phase feed regulating valve is installed on the pipeline connecting the gas phase outlet of the flash separator to the gas phase inlet of the reactive distillation column. A pressure controller (PIC) monitors the pressure inside the flash separator and adjusts the opening of the gas phase feed regulating valve according to the set value. When the monitored pressure is higher than the set value, the valve opening is increased, increasing the gas phase flow into the column, thereby reducing the pressure inside the tank; when the monitored pressure is lower than the set value, the valve opening is decreased, restricting the gas phase outflow, thereby increasing the pressure inside the tank. This set value... The power requirements for gas phase transport must be met, namely: ; In the formula, The control setpoint for flash pressure; This refers to the static pressure inside the reactive distillation column at the vapor inlet. The total pressure drop of the gas phase delivery pipeline and distributor; To ensure operational safety, a margin of 0.05 to 0.1 MPa is typically used.

[0036] S26, Coordinated control of sideline circulation and mixing pressure. The pressure of the sideline produced stream is provided by a circulation pump installed on the sideline produced line. The head design of this circulation pump must cover the total hydraulic losses of the sideline loop and the back pressure required by the mixing components. Mixing pressure It is controlled by a flash feed valve (i.e., the aforementioned throttling and pressure-reducing element) located downstream of the mixing component and upstream of the flash separator. The opening of this valve is not only regulated by pressure, but also forms a cascade control loop with the liquid level signal or side-stream flow signal of the flash separator.

[0037] In one specific control logic, the side-stream flow controller (FIC) acts as the main controller, adjusting the opening of the flash feed valve to maintain a constant side-stream circulation flow rate. Simultaneously, back pressure naturally builds up before the flash feed valve; as long as this back pressure is higher than the bubble point pressure, the liquid-phase mixing requirements are met. If the distributed control system detects mixing pressure... If the pressure approaches the bubble point of the fluid, a safety interlock is triggered, forcibly closing the flash feed valve or increasing the speed of the circulating pump to rebuild the subcooled liquid phase state.

[0038] S27, maintain pressure balance within the tower. A feedforward control mechanism is introduced to handle tower pressure fluctuations; when pressure fluctuations within the tower are detected... When the pressure increases, the distributed control system automatically corrects the flash pressure setpoint. This causes them to rise synchronously, in order to maintain a constant pressure difference. This ensures the stability of the gas phase feed flow rate.

[0039] See attached document Figure 4 The reactive distillation column is divided into three sections in terms of physical structure and process function: the rectification section at the top, the reaction section in the middle, and the stripping section at the bottom. The internal components of each section are configured according to their specific mass transfer and reaction requirements. The functional division of the column sections and the specific configuration of the internal components are implemented through the following technical details: S31, Rectification Section (Separation Zone). Located at the top of the reactive distillation column, the rectification section separates the reaction byproduct dimethyl carbonate and refluxes it back into the column. The rectification section is packed with high specific surface area gas-liquid contact components. These components are selected from either wire mesh corrugated packing or plate trays. When using wire mesh corrugated packing, high-efficiency packing with a specific surface area of ​​500 to 750 square meters per cubic meter is selected to provide sufficient gas-liquid mass transfer interface, ensuring that the concentration of dimethyl carbonate in the gas phase approaches the azeotropic composition at that pressure. When using plate trays, sieve trays or valve trays are selected, with an opening ratio designed to be 10% to 15% to maintain gas velocity and prevent flooding. Furthermore, a gas inlet is provided at the bottom of the rectification section (i.e., the junction between the rectification section and the reaction section) to receive the gas stream from the flash separator. After entering from this position, the gas stream flows upward and comes into countercurrent contact with the liquid stream descending in the rectification section, thus achieving the separation of light and heavy components.

[0040] S32, Reaction Section (Reaction Zone). Located in the middle of the reactive distillation column, the reaction section is the area where the disproportionation reaction occurs. To meet the residence time requirements of reaction kinetics, the internal components of the reaction section are configured in two ways: homogeneous catalysis and heterogeneous catalysis. Implementation Method 1 (Homogeneous Catalysis): A high-weir tray column is used. The outlet weir height on the tray is set to 50mm to 100mm, and a liquid seal structure is installed at the bottom of the downcomer to force the liquid phase to form a deep liquid layer on the tray. The deep liquid layer increases the reaction volume of the liquid phase and prolongs the contact time between the dissolved catalyst and the reactants.

[0041] Implementation Method Two (Heterogeneous Catalysis): A catalytic distillation packing is used. This packing consists of alternating layers of wire mesh or fiberglass bags filled with solid catalyst particles and corrugated plates. The catalyst packing provides the solid acid / base sites required for the reaction, while the corrugated plates provide the channels needed for gas-liquid separation, thus achieving coupling between the reaction and distillation.

[0042] The total liquid phase residence time in the reaction section must meet the reaction conversion rate requirements, and the calculation relationship is as follows: ; In the formula, This refers to the total liquid phase residence time in the reaction section; This refers to the theoretical plate number or stage number of the reaction section; For the first The liquid holding volume on the trays or packing layers of a primary reaction tower; This represents the average liquid volumetric flow rate through the reaction section. The liquid stream from the flash separator is introduced into the top of the reaction section through the liquid inlet. Under the influence of gravity, the liquid stream flows downward through the internal components of the reaction section, while simultaneously coming into countercurrent contact with the upward-flowing gas stream.

[0043] S33, Stripping Section (Purification Zone). Located at the bottom of the reactive distillation column, the stripping section separates residual phenol and methyl phenyl carbonate from the descending liquid phase and returns them to the reaction section in vapor form. The stripping section is filled with structured packing or equipped with trays. Considering the higher temperature at the bottom of the stripping section, grid packing with low fluid resistance or large-aperture floating valve trays are preferred. A reboiler is connected to the bottom of the stripping section to provide the rising steam for the entire column. The liquid stream collected from the bottom of the column is the crude diphenyl carbonate product. The purity of the crude diphenyl carbonate product is controlled by adjusting the evaporation rate of the reboiler. The distributed control system typically selects the temperature of the stripping section's sensitive plate as the controlled variable. When a temperature fluctuation is detected, the flow rate of the reboiler's heat source medium is automatically adjusted to change the rising steam rate, thereby dynamically maintaining the stability of the purity of the bottom product.

[0044] See attached document Figure 5 After being separated in the flash separator, the gas stream and liquid stream enter the reactive distillation column through independently configured gas feed lines and liquid feed lines, respectively. Based on their respective physical states and compositional characteristics, these two material streams are introduced into different heights within the column, forming a dual-track staged feed configuration. The specific implementation structure and process steps of the dual-track staged feed are as follows: S34, Constructing the vapor phase feed trajectory and distribution structure. The flash vapor phase stream, rich in dimethyl carbonate, is introduced into the space at the bottom of the rectification section via a vapor phase feed manifold extending through the column wall and into the column body. Physically, this vapor phase feed manifold is located below the packing layer or tray layer of the rectification section and above the reaction section. To ensure that the incoming vapor phase uniformly covers the cross-section of the rectification section, a gas distributor is connected to the end of the vapor phase feed manifold. The specific structure of this gas distributor is selected from either a pipe-type distributor or a vane-type inlet box. When a pipe-type distributor is used, the distributor includes a main pipe perpendicularly connected to the feed manifold and branch pipes on both sides. A series of exhaust holes are opened on the lower side of the branch pipes. After the gas flow is ejected downwards, it impacts the lower components and is deflected upwards, thereby reducing the gas flow velocity and diffusing it to the entire column cross-section. When a vane-type inlet box is used, several guide vanes are installed inside the inlet box. These vanes guide the high-speed gas flow from the feed manifold tangentially or radially, converting it into a rotating or plug flow, preventing the gas flow from directly eroding the packing or trays above. After entering, the gas stream merges with the rising gas in the reaction section, together forming the rising vapor flow in the rectification section.

[0045] S35, Constructing the liquid feed trajectory and distribution structure. The flash liquid stream, rich in methyl phenyl carbonate, catalyst, and some diphenyl carbonate, is introduced into the space at the top of the reaction section via a liquid feed pipe that passes through the column wall. To prevent the cold reflux liquid descending from the upper rectification section from directly mixing into the high-temperature reaction section, a riser tray is installed between the gas feed distributor and the liquid feed pipe. This riser tray includes an impermeable bottom plate and several riser pipes protruding from the bottom plate. The descending liquid from the rectification section and the flash liquid stream converge on this bottom plate, while the rising gas from the lower layer passes through the riser pipes. A liquid distributor is installed below the riser tray or directly connected to the end of the liquid feed pipe. This liquid distributor is located above the internal components of the reaction section and is either a gravity-type orifice flow distributor or a spray distributor. In an embodiment employing a gravity-type orifice flow distributor, a number of liquid distribution holes are provided on the distributor plate, with a distribution density of 50 to 100 spray points per square meter. Under the action of gravity, the liquid phase forms a uniform precipitating liquid flow through the liquid distribution holes, wetting the catalytic packing or reaction tray below.

[0046] The dual-track feeding method described above achieves a match between the feed location and the material composition. The feed location is determined based on the concentration matching principle between the feed stream composition and the fluid composition within the column; that is, the difference between the concentration of the key component in the feed stream and the concentration of the key component in the fluid within the column at the feed location is within a preset range. The spatial interval between the dual-track feed locations is determined by simulation calculations of the reactive distillation column, satisfying the following positional relationship: ; In the formula, Number the theoretical plate where the gas phase feed inlet is located (define the first plate at the top of the column as 1, and increment downwards); This refers to the theoretical plate location of the liquid phase feed inlet. The gas phase feed inlet is located immediately adjacent to the bottom of the rectification section, while the liquid phase feed inlet is located immediately adjacent to the top of the reaction section. and The difference corresponds to the theoretical plate equivalent number occupied by the above-mentioned gas-liquid collection tray and gas-liquid distribution space.

[0047] This dual-track feed structure also includes a phase-state thermal coupling mechanism. The reflux liquid phase from the rectification section, as it flows through the free space where the gas phase inlet is located, comes into direct contact with the newly entered high-temperature flash vapor phase stream. The high-temperature gas phase releases heat to the low-temperature reflux liquid, causing some of the reflux liquid to vaporize. Simultaneously, the reflux liquid washes away trace amounts of heavy phenol components entrained in the gas phase, carrying them back to the reaction section.

[0048] Subsequently, by adjusting the flow ratio of the side-stream outflow to the fresh feedflow (i.e., the side-stream circulation ratio), the enthalpy state of the mixture entering the flash separator can be changed, thereby controlling the gas-liquid phase ratio and temperature returning to the tower and achieving the regulation of the heat balance within the tower.

[0049] See attached document Figure 6 The specific steps for adjusting the side-stream circulation ratio and heat balance are as follows: S36, Set the sidestream recirculation ratio. The sidestream recirculation ratio is defined as the ratio of the mass flow rate of the sidestream outflow stream to the mass flow rate of the fresh feed stream. This parameter determines the degree to which the fresh feed is diluted by the materials in the column. Sidestream recirculation ratio The calculation formula is as follows: ; In the formula, Sideline circulation ratio; The mass flow rate of the lateral extraction stream; This refers to the mass flow rate of the fresh methyl phenyl carbonate feedstock stream. In actual operation, the mass flow rate of the fresh methyl phenyl carbonate feedstock stream... It is usually determined by the upstream production load, and is considered an uncontrollable variable or a feedforward variable. As an operating variable, the side-stream circulation pump speed is adjusted or the opening of the flow control valve on the side-stream pipeline is controlled. When the temperature in the reaction zone of the reactive distillation column is too high or local hot spots appear in the catalyst bed, the controller increases the side-stream circulation ratio. Since fresh phenol feedstock is usually in a supercooled state (temperature much lower than the bubble point inside the tower), increasing the circulation ratio means introducing more sensible heat sources, while utilizing a large amount of circulating liquid to absorb the heat of reaction, thus suppressing the temperature rise.

[0050] S37, Establish a mixed heat equilibrium model. The heat exchange process within the mixing component follows the law of conservation of energy. The equilibrium temperature after mixing the side-stream outflow (high temperature) and the fresh feed stream (low temperature) directly determines the vaporization rate of the subsequent flash evaporation process. The specific enthalpy of the mixed stream... The following heat balance equation is satisfied: ; In the formula, This represents the total mass flow rate of the mixture entering the flash separator. The specific enthalpy of the mixed stream under mixing pressure, The mass flow rate of the lateral extraction stream; The specific enthalpy of the side-stream produced stream at the production temperature. The mass flow rate of the fresh methyl phenyl carbonate feedstock; Let be the specific enthalpy of the fresh feed stream at the feed temperature. Since the specific heat capacity of the phenol and diphenyl carbonate mixture changes little with temperature, the above equation is transformed into a temperature control model in engineering control: ; In the formula, The temperature of the mixed stream; The temperature of the fluid inside the tower at the side sampling location; The supply temperature for fresh raw materials; The average isobaric specific heat capacity of the lateral stream; The average isobaric specific heat capacity of fresh raw materials, This refers to the sideline circulation ratio. This is achieved through monitoring... and Based on real-time data, the controller calculates the target mixing temperature. Required cycle ratio And adjust the output of the side-line circulation pump accordingly.

[0051] S38 controls the flash vaporization rate and return enthalpy. After entering the flash separator, the mixed stream undergoes partial vaporization due to a sudden pressure drop. The vaporization rate determines the ratio of gas to liquid phases returned to the reactive distillation column. Vaporization rate The relationship between the circulation ratio and pressure is as follows: ; In the formula, The mass gasification rate; This refers to the gaseous mass flow rate generated by flash evaporation. Specific enthalpy of the mixed stream under mixing pressure; The operating pressure inside the flash separator. This represents the total mass flow rate entering the flash separator. The side-stream circulation ratio is adjusted in conjunction with this. With flash pressure This allows for precise control of the heat removed. Specifically, the flash evaporation process converts some sensible heat into latent heat. The resulting gas stream carries a large amount of latent heat and enters from the top of the column, while the liquid stream carries sensible heat and enters from the bottom. When it is necessary to lower the temperature in the lower part of the reaction section, the controller increases the side stream circulation ratio while maintaining a constant total feed rate. , to make the mixing temperature Increase, thereby increasing the gasification rate. This allows more material to be introduced from the top of the tower in gaseous form, reducing the sensible heat load of the liquid phase directly entering the top of the reaction section, while utilizing the latent heat of vaporization to remove more heat from inside the tower.

[0052] The reactive distillation column has an internal flow field structure with countercurrent gas-liquid contact. The externally introduced liquid stream and the reflux liquid stream from the top of the column wet the packing or tray surface under gravity, forming a descending liquid film. The externally introduced gas stream and the rising vapor stream from the reboiler in the bottom of the column pass through the packing voids or tray channels under the influence of pressure difference. At the gas-liquid interface, momentum, heat, and mass transfer processes occur simultaneously, coupled with a disproportionation chemical reaction.

[0053] See attached document Figure 7 The mass transfer and reaction mechanism within the tower is carried out through the following physicochemical processes, with the following steps: S41, implementing a reaction equilibrium shift mechanism. In the reaction section, methyl phenyl carbonate in the liquid phase undergoes a disproportionation reaction at the catalyst active sites. Because the boiling point of the product dimethyl carbonate is lower than that of other components in the reaction system, within the temperature range where the reaction occurs, the generated dimethyl carbonate molecules, utilizing their high volatility, rapidly cross the gas-liquid interface, vaporizing from the bulk liquid phase into the bulk gas phase. This phase transfer process reduces the concentration of the product dimethyl carbonate in the liquid phase, thus shifting the reaction quotient... Always less than the equilibrium constant This maintains the chemical driving force for the forward reaction. During this process, the latent heat and sensible heat released by the condensation of the rising vapor stream compensate for the latent heat required for the vaporization of dimethyl carbonate and the endothermic reaction, keeping the temperature of the reaction section within the optimal activity temperature window of the catalyst.

[0054] S42 implements non-equilibrium mass transfer and kinetic matching. At the gas-liquid interface within the column, each component migrates directionally based on its chemical potential difference. For the dimethyl carbonate component, the mass transfer direction is from the liquid phase to the gas phase, exhibiting a desorption process; for the methyl phenyl carbonate and diphenyl carbonate components, the mass transfer direction is from the gas phase to the liquid phase, exhibiting an absorption process. Components The mass transfer flux at the gas-liquid interface satisfies the following rate equation: ; In the formula, Components molar mass transfer flux; The overall mass transfer coefficient is based on the gas phase mole fraction. Components The actual mole fraction in the gas phase bulk; To achieve an equilibrium molar fraction of the gas phase with the bulk liquid phase, and to overcome the kinetic limitation of the slow disproportionation reaction rate, this process increases the residence time of the liquid phase in the reaction section through the aforementioned high liquid holdup internal components. To make it consistent with the characteristic reaction time Matching, that is, satisfying This ensures that the reaction progress is close to the equilibrium conversion rate at that temperature as the liquid phase flows through each theoretical plate, avoiding incomplete reaction due to excessive flow rate.

[0055] S43, establish a concentration gradient distribution along the column height. Based on gas-liquid countercurrent contact and reaction consumption, a stable component distribution curve is formed along the column height. In the rectification section, dimethyl carbonate in the rising gas stream is washed by the reflux liquid and returned to the reaction section, ensuring the top gas phase reaches the specified dimethyl carbonate purity. In the reaction section, the reactant concentration gradually decreases from top to bottom, the product concentration gradually increases from top to bottom, and the dimethyl carbonate concentration in the gas phase gradually increases from bottom to top. In the stripping section, unreacted methyl phenyl carbonate and residual dimethyl carbonate in the descending liquid phase are stripped by the rising steam and returned to the reaction section, ensuring the bottom liquid phase reaches the specified diphenyl carbonate purity. The gas-liquid flow rate ratio in the column is controlled by adjusting the reboiler heat load. To maintain the stability of the above concentration distribution curve when disturbed by feed fluctuations.

Claims

1. A method for producing diphenyl carbonate by reactive distillation based on heat integration from the top side stream of the column, characterized in that, Includes the following steps: A reactive distillation column and a flash separator are configured from top to bottom, consisting of a rectification section, a reaction section, and a stripping section. A high-temperature liquid side stream is drawn from the lower part of the rectification section or the upper part of the reaction section. The high-temperature liquid side stream is introduced into the mixing component with the fresh raw material based on an adjustable side stream circulation ratio for direct contact mixing to obtain a mixture. The pressure downstream of the mixing component is controlled by a throttling element, so that the mixture enters the flash separator for adiabatic flash evaporation, and is separated into a gas stream rich in dimethyl carbonate and phenol and a liquid stream rich in methyl phenyl carbonate and catalyst. A dual-track staged feeding method is adopted to construct a phase-state thermal coupling process in the feeding area, which transports the gas phase stream to the bottom of the rectification section and the liquid phase stream to the top of the reaction section; A liquid phase residence state that satisfies reaction kinetics is established in the reaction section of the reactive distillation column. The heat balance in the column is maintained by adjusting the side-stream circulation ratio, and the disproportionation reaction and gas-liquid mass transfer coupling process are executed. Dimethyl carbonate is removed from the top of the rectification section, and diphenyl carbonate product is collected from the bottom of the stripping section.

2. The method for producing diphenyl carbonate by reactive distillation based on the integrated heat from the top side stream of the column according to claim 1, characterized in that, The extraction of the high-temperature liquid phase side stream specifically includes the following steps: Based on the temperature distribution inside the tower, the tray with the liquid stream temperature higher than the fresh feed temperature and the temperature difference between the two greater than the set minimum heat transfer driving temperature difference is selected as the side-stream sampling stage. Based on the component concentration distribution, the tray located in the transition area from the lower part of the rectification section to the upper part of the reaction section is selected as the side-stream sampling stage. Methyl phenyl carbonate and diphenyl carbonate are in an enriched state at the tray and the molar fraction of dimethyl carbonate shows a decreasing gradient. A liquid collection tray is installed at the side-line extraction stage location. The liquid collection tray is used to collect the descending liquid phase and lead the descending liquid phase out of the tower through the side-line extraction outlet.

3. The method for producing diphenyl carbonate by reactive distillation based on the integrated heat from the top side stream of the column, as described in claim 1, is characterized in that... The direct contact mixing specifically includes the following steps: A reinforced turbulent mixing flow field is constructed, wherein the mixing component is selected from a static mixer or a Venturi jet mixer; When the static mixer is used, the side stream and the fresh raw material stream are forcibly separated, rotated and radially mixed as they flow through the internal twisted blades or corrugated plate elements; When the Venturi jet mixer is used, the high-temperature liquid phase side stream with a flow rate higher than that of the fresh raw material is selected as the driving fluid, and the high-speed jet generated by the driving fluid is used to entrain the fresh raw material as the entraining fluid. The throttling element is used to establish back pressure in the mixing component, so that the operating pressure in the mixing component is higher than the bubble point pressure of the mixture at the current mixing temperature and mixing composition, ensuring that the material is mixed in a liquid single-phase state.

4. The method for producing diphenyl carbonate by reactive distillation based on the integrated heat from the top side stream of the column according to claim 1, characterized in that, During the adiabatic flash evaporation process, the vaporization fraction control follows the principle of heat balance, including: The total enthalpy of the mixed fluid before entering the flash separator is equal to the sum of the total enthalpy of the gas phase and the total enthalpy of the liquid phase under flash equilibrium conditions; By adjusting the opening of the gas phase flow control element connected between the gas phase outlet of the flash separator and the reactive distillation column, the operating pressure inside the flash separator is kept constant, thereby determining the molar ratio of the gas phase to the liquid phase based on the enthalpy state of the mixed fluid.

5. The method for producing diphenyl carbonate by reactive distillation based on the integrated heat from the top side stream of the column according to claim 1, characterized in that, The pressure downstream of the hybrid component is controlled through a process including: The operating pressure inside the mixing component is controlled to be higher than the operating pressure inside the flash separation tank, so that the mixed fluid generates a pressure drop and flashes when passing through the throttling element; The operating pressure inside the flash separator is controlled to be higher than the static pressure inside the reactive distillation column at the vapor inlet, so that the vapor stream generated by flash evaporation can overcome pipeline resistance and flow into the reactive distillation column by gravity. The static pressure inside the reactive distillation column at the gas phase inlet is controlled to be higher than the operating pressure at the top of the column, thereby maintaining the upward flow of the gas phase inside the column.

6. The method for producing diphenyl carbonate by reactive distillation based on the integrated heat from the top side stream of the column according to claim 1, characterized in that, The spatial location conditions for the dual-track graded feeding method include: The theoretical plate position number of the gas stream entering the reactive distillation column is less than or equal to the theoretical plate position number of the liquid stream entering the reactive distillation column, and the theoretical plate position number is defined as increasing downwards from the top of the column; the gas stream inlet position is located below the rectification section, and the liquid stream inlet position is located above the reaction section; a rising gas-liquid collection area is provided between the gas stream inlet position and the liquid stream inlet position, the rising gas-liquid collection area is used to collect the liquid descending from the rectification section and the liquid stream, and allows the gas stream to pass upwards.

7. The method for producing diphenyl carbonate by reactive distillation based on the integrated heat from the top side stream of the column according to claim 1, characterized in that, The dual-track staged feeding method, in which the phase-state thermal coupling process is constructed in the feeding region, includes: When the reflux liquid stream of the rectification section flows through the free space of the gas stream introduction area, the gas stream comes into direct contact with the incoming high-temperature gas stream. The high-temperature gas stream transfers heat to the reflux liquid stream, causing partial vaporization of the reflux liquid stream. At the same time, the reflux liquid stream washes away the heavy phenol components entrained in the high-temperature gas stream and carries the heavy phenol components back to the reaction section.

8. The method for producing diphenyl carbonate by reactive distillation based on the integrated heat from the top side stream of the column according to claim 1, characterized in that, The adjustment steps for maintaining the heat balance within the tower by the side-stream circulation ratio include: The side-stream circulation ratio is defined as the ratio of the mass flow rate of the high-temperature liquid phase side-stream stream to the mass flow rate of the fresh raw material; When the temperature of the reaction section is detected to be higher than the set threshold, the side-stream circulation ratio is increased, the temperature and vaporization fraction of the mixture entering the flash separator are increased, the latent heat load of the gas stream introduced from the top of the reactive distillation column is increased, and the sensible heat load of the liquid stream directly entering the top of the reaction section is reduced.

9. The method for producing diphenyl carbonate by reactive distillation based on the integrated heat from the top side stream of the column according to claim 1, characterized in that, The establishment of a liquid-phase residence state that satisfies reaction kinetics within the reaction section includes: By setting up high-weir plate trays or high-liquid-holding packing, the liquid stream can form a deep liquid layer in the reaction section; The total liquid phase residence time of the reaction section is controlled to be greater than or equal to the characteristic time required for the disproportionation reaction to reach the predetermined conversion rate. The total liquid residence time is determined by dividing the sum of the liquid holding volumes at each stage of the reaction section by the average liquid volume flow rate through the reaction section.

10. The method for producing diphenyl carbonate by reactive distillation based on the integrated heat from the top side stream of the column according to claim 1, characterized in that, The coupling process of the disproportionation reaction and gas-liquid mass transfer includes: By utilizing the boiling point difference between the dimethyl carbonate and the reactants and diphenyl carbonate in the reaction system, the generated dimethyl carbonate is continuously vaporized from the liquid phase into the gas phase, keeping the reaction quotient in the liquid phase less than the equilibrium constant. At the gas-liquid interface, the mass transfer of the dimethyl carbonate component from the liquid phase to the gas phase is controlled, and the mass transfer of the methyl phenyl carbonate and the diphenyl carbonate component from the gas phase to the liquid phase is controlled. The latent heat released by the condensation of the rising gas phase is used to supplement the latent heat of vaporization of the dimethyl carbonate and the endothermic reaction.

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