Ammoxidation reactor control

A process is provided for control of an ammoxidation reactor. More specifically, the process includes controlling an amount of oxygen added to the reactor, steam temperature and linear velocity to minimize reactor temperature deviations.

A process is provided for control of an ammoxidation reactor. More specifically, the process includes controlling an amount of oxygen added to the reactor, steam temperature and linear velocity to minimize reactor temperature deviations.

BACKGROUND

Acrylonitrile is manufactured by an ammoxidation process where air, ammonia, and propylene are reacted in the presence of a catalyst in a fluidized bed reactor. This is an exothermic reaction, and the heat generated is removed by circulating water or steam through a set of cooling coils that remove the heat to generate steam or superheated steam. The reactor temperature and reactor linear velocity are key variables that need to be controlled to get the desired acrylonitrile yield. The reactor temperature is influenced by the amount of propylene added to the reactor, reactor pressure, the super heat steam temperature, and the number of cooling coils that are being used. The linear velocity is influenced by the amount of propylene, ammonia, and air added, and the reactor pressure. Cooling coil changes are very common in the acrylonitrile reactor. Coil changes are typically brought about by the process board operators during rate changes, or during the coil swapping process a process that takes out coils that have been in service for sometime, and replaces them with fresh coils that have better heat transfer capability.

It is desired to run the acrylonitrile reactors at the maximum possible linear velocity and at a fixed reactor temperature to get the best acrylonitrile yield. The main challenge in achieving this objective comes from the fact that the cooling coils for a given reactor have different cooling capacity, based on the number of passes they have. Thus, the temperature response varies depending on the type of coil that is added in or removed from service in the reactor. Traditional control schemes try to independently control the linear velocity and the reactor temperature, and usually take control action in a reactive manner, especially during coil changes inside the reactor. This invariably increases the response time of the controller, and takes a long time for the temperature to settle down after coil changes. A control scheme thus has to take into account the interaction between the key variables, and take preemptive control action during coil changes.

SUMMARY

An ammoxidation process provides for maximum reactor linear velocity and minimum deviations in reactor temperature. Increased linear velocity and minimal deviations in reactor temperature result in improved reactor efficiencies.

An ammoxidation process includes introducing a flow of a reactant stream into ari ammoxidation reactor. The reactant stream includes ammonia, an oxygen containing gas, a hydrocarbon selected from the group consisting of propane, propylene, isobutene, isobutylene and mixtures thereof. The process includes providing steam to coils disposed in the ammoxidation reactor to provide a reactor operating temperature of about 350° C. to about 480° C. The process further includes controlling an amount of oxygen added to the reactor and the steam temperature to maintain a superficial reactor linear velocity.

In another aspect, an ammoxidation process includes introducing a flow of a reactant stream into an ammoxidation reactor. The reactant stream includes ammonia, an oxygen containing gas, a hydrocarbon selected from the group consisting of propane, propylene, isobutene, isobutylene and mixtures thereof. The process includes providing steam to coils disposed in the ammoxidation reactor to provide a reactor operating temperature of about 350° C. to about 480° C. The process further includes controlling an amount of oxygen added to the reactor and the steam temperature to maintain a superficial reactor linear velocity within about 95% of a target superficial reactor linear velocity and within about 95% of a target reactor temperature.

An ammoxidation process introducing a flow of a reactant stream into an ammoxidation reactor. The reactant stream includes ammonia, propylene and oxygen containing gas. The process includes providing superheated steam to superheat coils disposed in the ammoxidation reactor. In one aspect, a set of manipulated variables includes reactor oxygen flow, superheated steam temperature, absorber pressure and amount of lean water to an absorber and the set of controlled variable includes a reactor linear velocity and a reactor temperature. Controlling at least one set of controlled variables includes controlling an amount of oxygen added to the reactor and the superheated steam temperature.

DETAILED DESCRIPTION

FIG. 1illustrates a typical ammoxidation (acrylonitrile) reactor used. As shown, reactor10includes reactor shell12, air grid14, feed sparger16, a cooling system generally indicated at18including saturated cooling coils17and superheat cooling coils19, and cyclones20. WhileFIG. 1shows saturated cooling coils17and superheat cooling coils19being located on one side of reactor10and cyclones20being located on the other side, it will be understood that in actual practice these structures are positioned uniformly throughout the reactor. During normal operation, the process includes introducing a flow of reactant stream that includes ammonia, an oxygen containing gas, and a hydrocarbon selected from the group consisting of propane, propylene, isobutene, isobutylene and mixtures thereof. In one aspect, process air is charged into reactor10through air inlet22, while a mixture of propylene obtained from propylene supply line13and ammonia obtained from ammonia supply line15is charged into reactor10through feed sparger16. The flow rates of both are high enough to fluidize a bed44of ammoxidation catalyst in the reactor interior, where the catalytic ammoxidation of the propylene and ammonia to acrylonitrile occurs. A flowrate of propylene to the ammoxidation reactor is effective for providing a ratio of oxygen to propylene of about 2 to about 2.1 and a ratio of ammonia to propylene of about 1 to about 1.5. Ammonia is controlled by an NH3/C3controller.

Product gases produced by the reaction exit reactor10through reactor effluent outlet26. Before doing so, they pass through cyclones20, which remove any ammoxidation catalyst these gases may have entrained for return to catalyst bed44through diplegs25. Ammoxidation is highly exothermic, and so cooling system18is used to withdraw excess heat and thereby keep the reaction temperature at an appropriate level.

As further illustrated inFIG. 1, in addition to saturated cooling coils17and superheat cooling coils19, cooling system18also includes steam drum24, recirculating pump26, shut-off valve28and steam control valve30. The lower portion of steam drum24is filled with saturated liquid cooling water maintained at an elevated pressure and elevated temperature such as about 255° C. at about 4.2 mPaG. The upper portion of steam drum24is filled with saturated steam in equilibrium with this liquid cooling water. As is well understood in the art, water exists as a liquid at these elevated temperatures because it is also under greater than one atmosphere of pressure.

The primary way cooling system18removes heat from the interior of reactor10is by the recirculation of liquid cooling water from the lower portion of steam drum24through cooling coils17. For this purpose, recirculation pump26is arranged to pump liquid cooling water from the bottom of steam drum24through shut-off valve28and then through cooling coil17. In cooling coil17, some liquid vaporizes to steam and cooling water and steam produced is returned to steam drum24. Since the saturated cooling water fed to cooling coil17is composed of 100% liquid water, cooling coil17is typically referred to as a “saturated” cooling coil.

In actual practice, the flowrate of cooling water through saturated cooling coil17is selected so that a predetermined proportion of this cooling water, typically about 15% for example, is converted to steam. Accordingly, as shown inFIG. 1, the heated cooling water produced in saturated cooling coil17is returned to an upper portion of steam drum24, so that the vaporous fraction of this cooling water stream can remain in the upper portion of the steam drum while the liquid portion of this cooling water stream can fall to the lower portion of the steam drum for mixing with the liquid cooling water already there. The steam drum24may include make-up water conduit54.

In many designs, shut-off valve28is a simple on-off valve as opposed to a control valve capable of fine control of fluid flowrate. This is because other means are typically used for fine control of the reaction temperature inside the acrylonitrile reactor, and so a more complicated and expensive control valve is unnecessary. Also it is not desirable to convert to much of the liquid water into vapor inside the cooling coil as this can result in negative consequences such as erosion of the inside of the cooling coil pipe or scaling.

Each individual shut-off valve28on each individual coil is the only valve controlling whether or not cooling water flows through a particular saturated cooling coil17. That is to say, saturated cooling coil17is constructed without any additional valve or other flow control device for controlling the flow of cooling water through saturated cooling coil17. This is because such an additional valve is unnecessary to achieve the desired operation and control of the cooling coils in the manner described here. In addition, eliminating a valve on the outlet also eliminates the need for a safety valve, which would otherwise be necessary if such an outlet valve were used. Thus, the total flow through all of the cooling coils in service (that is for the saturated cooling coil17which have their valve open) is set by a discharge flow rate from pump26.

In addition to saturated cooling coils17, cooling system18also uses superheat cooling coils19for removing heat from the interior of acrylonitrile reactor10. Superheat cooling coils19differ from saturated cooling coils17in that superheat cooling coils19are connected by means of steam inlet header32to an upper portion of steam drum24so that the feed to these cooling coils is superheated steam rather than saturated steam. The steam entering superheat cooling coil19is at a saturation temperature corresponding to the steam drum pressure. The steam drum pressure increases as it flows through superheat cooling coil19and thus becomes superheated. Accordingly, cooling coils19are typically referred to as “superheat cooling coils.” In this aspect, the process includes providing the superheat steam at a temperature of about 350° C. to about 480° C., in another aspect, about 355° C. to about 400° C., in another aspect, about 360° C. to about 390° C., and in another aspect, about 370° C. to about 380° C.

An important function of superheat cooling coils19is to raise the temperature of steam produced in coils19so as to provide superheated steam for driving the steam turbines used in other parts of the acrylonitrile plant as liquid droplets in wet steam may damage turbine internals. For this purpose, the superheated steam passing out of superheat cooling coils19is typically discharged through steam outlet header34to steam supply conduit35for transfer directly to these steam turbines.

Common practice in many acrylonitrile plants includes connecting steam inlet header32and steam outlet header34with bypass line33so that the temperature of the steam passing into steam supply conduit35can be controlled by adjusting the amount of steam supplied to this conduit directly from steam drum24. Because the temperature of the steam in steam drum24is necessarily lower than the temperature of the superheated steam passing out of superheat cooling coil19, increasing the flowrate of steam passing through bypass line33necessarily lowers the temperature of the steam reaching steam supply conduit35. So, it is also customary in most commercial acrylonitrile plants also to include steam control valve30in bypass passageway33, whose operation is controlled by controller39in response to the measured temperature T1of the steam in steam supply conduit35. Control valve30is then operated to maintain the measured temperature T1of the steam in steam supply conduit35at a constant temperature somewhere between about 340 to 385° C.

In order to keep an acrylonitrile reactor operating in peak condition, it is desirable to maintain its operating temperature within a target temperature range of about 200 to about 480° C., in another aspect, about 215 to about 440° C., and in another aspect, about 215 to about 230° C., when modern molybdenum based ammoxidation catalysts are used. In this aspect, it is more desirable to maintain the reactor temperature as close as possible to a single control point temperature rather than to allow the operating temperature to drift up and down within a range of temperatures. Although control of reaction temperature can be carried out by adding to or subtracting from the number of cooling coils in active service, this approach does not provide precise temperature control. Rather, the addition and subtraction of cooling coils alone does not necessarily achieve the precise reactor operating temperature.

Accordingly, precise temperature control of acrylonitrile reactor10is commonly done by increasing and decreasing the flowrate of propylene supplied to the acrylonitrile reactor in response to the measured temperature TRof the ammoxidation reaction occurring inside the reactor. For this purpose, as illustrated inFIG. 1, propylene control valve37in propylene supply line13and controller41are provided to control the flow of propylene into acrylonitrile reactor10in response to the measured ammoxidation reaction temperature, TR. Thus, a certain number of cooling coils are put into service to provide reactor temperature control within a desired temperature range, and a propylene feed rate is adjusted up or down to achieve a more precise temperature adjustment.

In one aspect, the process provides improved temperature control and reduced reactor temperature deviations during changes in heat transfer areas of coil. In this aspect, reactor temperature deviations are maintained at about 10° C. or less during changes in heat transfer area of the coils, in another aspect, about 6° C. or less, in another aspect, about 5° C. or less, and in another aspect, about 3° C. or less.

Every reactor may have a different target temperature within the ranges described herein. In one aspect, an amount of the oxygen added to the reactor and the steam temperature are controlled to maintain a reactor temperature within about 95% of a target reactor temperature, and in another aspect, within about 98% of a target reactor temperature.

In another aspect, a total available superheat coil area per reactor cross sectional area (ft2/ft2) is about 1 to about 7, in another aspect, about 2 to about 6, and in another aspect, about 3 to about 5. The superheat coil area (ft2) per heat removed by the superheat coils (Kcal) per metric ton of acrylonitrile produced is about 275,000 to about 475,000, in another aspect, about 300,000 to about 400,000, and in another aspect, 325,000 to about 375,000.

In another aspect, a total available saturated coil area per reactor cross sectional area (ft2/ft2) is about 8 to about 18, in another aspect, about 8 to about 15, and in another aspect, about 10 to about 13. The saturated coil area (ft2) per heat removed by the saturated coils (Kcal) per metric ton of acrylonitrile produced is about 2,375,000 to about 2,900,000, in another aspect, about 2,400,000 to about 2,800,000, and in another aspect, about 2,500,00 to about 2,700,000.

In an aspect, the process includes operating or reacting in a reactor a hydrocarbon, wherein the effluent flow has a linear velocity of about 0.5 to about 1.5 m/sec, in another aspect, about 0.7 to about 1.0 m/sec, in another aspect, about 0.75 to about 0.8 in/sec, and in another aspect, about 0.75 to about 0.77 in/sec (based on effluent volumetric flow and reactor cross-sectional area (“CSA”) excluding cooling coils and dip legs area, i.e., ˜90% of open CSA). It has been found that it is possible to design and operate the reactor system using this velocity whilst also achieving good fluidization/catalyst performance and reasonable catalyst entrainment/catalyst losses from cyclones, such that velocities may be maintained in about this range to the extent possible as reactor capacity is increased. In an embodiment, the reactor may be operated to maintain a top pressure of about 0.25 to about 0.5 kg/cm2, and in another aspect, about 0.2 to about 0.45 kg/cm2. In another aspect, an amount of the oxygen added to the reactor and the steam temperature are controlled to maintain a superficial reactor linear velocity within about 95% of a target superficial reactor linear velocity, and in another aspect, within about 98% of a target superficial reactor linear velocity.

In one aspect, a ratio of cyclone inlet velocity in meters/second to a reactor effluent velocity in meters/second is 20 or greater, in another aspect, about 20 to about 30, in another aspect, about 22 to about 25, in another aspect, about 23 to about 26, and in another aspect, about 27 to about 29.

In another aspect, the process includes controlling reactor top pressure at about 3.8 psig to about 5.0 psig, in another aspect, 4.0 psig to about 5.0 psig, and in another aspect, about 4.0 to about 4.5 psig.

FIG. 2is a schematic flow diagram of an embodiment in accordance with aspects of the disclosure as applied to the manufacture of acrylonitrile. Referring to the figure, an apparatus100comprises reactor10, quench vessel20, effluent compressor30, and absorber40. Ammonia in stream1and hydrocarbon (HC) feed in stream2may be fed as combined stream3to reactor10. HC feed stream2may comprise a hydrocarbon selected from the group consisting of propane, propylene and isobutylene, and combinations thereof. A catalyst (not shown inFIG. 2) may be present in reactor10. Oxygen containing gas may be fed to reactor10. For example, air may be compressed by an air compressor (not shown inFIG. 2) and fed to reactor10.

Acrylonitrile is produced in reactor10from the reaction of the hydrocarbon, ammonia, and oxygen in the presence of a catalyst in reactor10. Reactor10may be run at reactor or first pressure P1, wherein the first pressure may be characterized as the pressure at inlet17, such as a first-stage inlet of cyclone22. In accordance with the disclosure, cyclone22may be the first cyclone of a multi-stage cyclone system that may be configured to convey a stream comprising acrylonitrile to a plenum (not shown inFIG. 1). The stream comprising acrylonitrile may exit the plenum and out of a top portion of reactor10as reactor effluent stream4. In an aspect, cyclone22may be configured to separate catalyst that may be entrained in the stream comprising acrylonitrile that enters inlet17, and return the separated catalyst back to the catalyst bed in reactor10through a catalyst return dip leg (not shown inFIG. 1). Reactor effluent stream4comprising acrylonitrile produced in reactor10may be conveyed through line11to quench vessel20. In this aspect, the first pressure is about 140 kPa or less, in another aspect about 135 kPa or less, in another aspect about 130 kPa or less, in another aspect about 125 kPa or less, in another aspect, about 101 kPa to about 140 kPa, in another aspect, about 110 kPa to about 1400 kPa, in another aspect, about 125 kPa to about 145 kPa, in another aspect, about 120 kPa to about 140 kPa, in another aspect, about 130 kPa to about 140 kPa, in another aspect, about 125 kPa to about 140 kPa, in another aspect, about 125 kPa to about 135 kPa, in another aspect, about 120 kPa to about 137 kPa, and in another aspect, about 115 kPa to about 125 kPa.

In quench vessel20, reactor effluent stream4may be cooled by contact with quench aqueous stream5entering quench vessel20via line12. Quench aqueous stream5may comprise an acid in addition to water. The cooled reactor effluent comprising acrylonitrile (including co-products such as acetonitrile, hydrogen cyanide and impurities) may then be conveyed as quenched stream6to effluent compressor30via line13.

Quenched stream6may be compressed by effluent compressor30, and exit effluent compressor30as compressor effluent stream7. Compressor effluent stream7may have a second or compressed pressure P2. Compressor effluent stream7may be conveyed to a lower portion of absorber40via line14. In absorber40, acrylonitrile may be absorbed in a second or absorber aqueous stream8that enters an upper portion of absorber40via line15. The aqueous stream or rich water stream18that include acrylonitrile and other co-products may then be transported from absorber40via line19a recovery column. (not shown inFIG. 2) for further product purification.

The non-absorbed effluent9exits from the top of absorber column40through pipe16. Non-absorbed effluent9may comprise off-gases, which can be burned in absorber off-gas incinerator (AOGI) or absorber off-gas oxidizer (AOGO).

In an aspect, effluent compressor30functions by pulling quenched stream6through line13. Effluent compressor30may compress quenched stream6so that it exits effluent compressor30as compressed effluent compressor stream7that has a higher pressure (second pressure) than the reactor pressure (first pressure). In an aspect, the pressure in line14of compressed effluent compressor stream7is about 2 to about 11.5 times greater than the operation pressure of reactor10, in another aspect, about 2 to about 12.5 times, in another aspect, about 2.5 to about 10, in another aspect, about 2.5 to about 8, in another aspect, about 2.5 to about 5, in another aspect, about 2.5 to about 4, in another aspect, about 2.5 to about 3.2, in another aspect, about 2 to about 3.5, in another aspect, about 2 to about 3, in another aspect, about 3 to about 11.25, in another aspect, about 5 to about 11.25, and in another aspect, about 7 to about 11.25 (all based on an absolute comparison). In an aspect, the second pressure (absolute) is about 300 to about 500 kPa, in another aspect, about 340 kPa to about 415 kPa, in another aspect, about 350 kPa to about 400 kPa, in another aspect, about 250 kPa to about 500 kPa, in another aspect, about 200 kPa to about 400 kPa, in another aspect, about 250 kPa to about 350 kPa, in another aspect, about 300 kPa to about 450 kPa, and in another aspect, about 360 kPa to about 380 kPa.

In an aspect, the second pressure is such that the absorber may be operated with a flow rate of aqueous stream8of about 15 to about 20 kg/kg of acrylonitrile final product produced when aqueous stream8is uncooled or unrefrigerated and/or is 4 to about 45° C. and wherein the absorber rich water stream contains about 5 weight percent or more organics, in another aspect, about 6 weight percent or more organics, and in another aspect, about 7 weight percent or more organics. In another aspect, the flow rate of aqueous stream8may be about 15 to about 19 kg/kg of acrylonitrile, in another aspect, about 15 to about 18 kg/kg of acrylonitrile, and in another aspect, about 16 to about 18 kg/kg of acrylonitrile. In another aspect, the uncooled or unrefrigerated aqueous stream is about 20 to about 45° C., in another aspect, about 25 to about 40° C., in another aspect, about 25 to about 35° C., and in another aspect, about 25 to about 30° C.

A cooling system (not shown inFIG. 2) may be located at or downstream of compressor30, wherein the cooling system is configured to cool compressed effluent compressor stream7to a predetermined temperature, e.g., about 105° F. (about 40.5° C.) prior to entering absorber40.

In an aspect, absorber40may include forty to sixty (40-60) trays. In an aspect, absorber40may include fifty (50) trays. Compressed effluent compressor stream7may enter absorber40below the bottom tray of the absorber. In an aspect, absorber40may be operated with variable flow rates of refrigerated water in second aqueous stream8, including zero amount of refrigerated water.

In an aspect, absorber40may be operated at pressure that is higher than the pressure in an absorber in a conventional process. By operating absorber40at this higher pressure, the absorber may be operated more efficiently than an absorber in a conventional process. Due to the higher absorber efficiency achieved in the process of the present disclosure, the same recovery of acrylonitrile in rich water stream18may be achieved as in a conventional process, but less water is required to absorb acrylonitrile in the absorber. In this aspect, rich water refers to water having about 5 weight percent or more organics, in another aspect, about 6 weight percent or more organics, and in another aspect, about 7 weight percent or more organics. In an aspect, the water used to absorb acrylonitrile in the absorber may be process or municipal water (e.g., having a temperature of about 4-45° C.). In this aspect, process or municipal water is more than about 95 weight percent water, in another aspect, about 97 weight percent or more water, in another aspect, about 99 weight percent or more water, and in another aspect, about 99.9 weight percent or more water. In an aspect, the temperature of second aqueous stream8may be in the range of about 4 to about 45° C., in another aspect, about 10 to about 43° C., and in another aspect, about 27 to about 32° C.

Advanced Process Control

A model predictive control (MPC), also known as advanced process control (APC), uses a process model to predict the behavior of a process into the future, and then implements an optimized control action to counter process deviation from a desired target. Along with controlling the process, MPC also tries to drive the process towards the most “economic” condition by moving the key process variables. One aspect of the present process includes an MPC to control the reactor temperature and the linear velocity. The process includes using MPC to achieve the maximum linear velocity possible for the reactor and a constant reactor temperature with minimum deviations during coil changes.

As used herein, the term “manipulated variable” refers to variables that are adjusted by the advanced process controller. The term “controlled variables” refers to variable that are kept by the advanced process controller at a predetermined value (set point) or within a predetermined range (set range). “Optimizing a variable” refers to maximizing or minimizing the variable and to maintaining the variable at a predetermined value.

One aspect of model predictive control is that future process behavior is predicted using a model and available measurements of the controlled variables. The controller outputs are calculated so as to optimize a performance index, which is a linear or quadratic function of the predicted errors and calculated future control moves. At each sampling instant, the control calculations are repeated and predictions updated based on current measurements. In this aspect, a suitable model is one that includes a set of empirical step-response models expressing the effects of a step-response of a manipulated variable on the controlled variables.

An optimum value for e parameter to be optimized can be obtained from a separate optimization step, or the variable to be optimized can be included in the performance function.

Before model predictive control can be applied, one determines first the effect of step changes of the manipulated variables on the variable to be optimized and on the controlled variables. This results in a set of step-response coefficients. This set of step-response coefficients forms the basis of the model predictive control of the process.

During normal operation, the predicted values of the controlled variables are regularly calculated for a number of future control moves. For these future control moves a performance index is calculated. The performance index includes two terms, a first term representing the sum over the future control moves of the predicted error for each control move and a second term representing the sum over the future control moves of the change in the manipulated variables for each control move. For each controlled variable, the predicted error is the difference between the predicted value of the controlled variable and a reference value of the controlled variable. The predicted errors are multiplied with a weighting factor, and the changes in the manipulated variables for a control move are multiplied with a move suppression factor. The performance index discussed here is linear.

Alternatively, the terms may be a sum of squared terms, in which case the performance index is quadratic. Moreover, constraints can be set on manipulated variables, changes in manipulated variables and on controlled variables. This results in a separate set of equations that are solved simultaneously with the minimization of the performance index.

Optimization can be done in two ways; one way is to optimize separately, outside the minimization of the performance index, and the second way is to optimize within the performance index.

When optimization is done separately, the variables to be optimized are included as controlled variables in the predicted error for each control move and the optimization gives a reference value for the controlled variables.

Alternatively, optimization is done within the calculation of the performance index, and this gives a third term in the performance index with an appropriate weighting factor. In this case, the reference values of the controlled variables are predetermined steady state values, which remain constant.

The performance index is minimized taking into account the constraints to give the values of the manipulated variables for the future control moves. However, only the next control move is executed. Then the calculation of the performance index for future control moves starts again.

The models with the step response coefficients and the equations required in model predictive control are part of a computer program that is executed in order to control the liquefaction process. A computer program loaded with such a program that can handle model predictive control is called an advanced process controller. Commercially available computer programs that may be utilized include for example, DMCplus® by Aspen Technology and PredictPro® by Emerson.