WO2009123872A1

WO2009123872A1 - Procedure for the startup of a (meth)acrylic acid plant

Info

Publication number: WO2009123872A1
Application number: PCT/US2009/037922
Authority: WO
Inventors: Christopher T. Reeves; Fungau Ho; David O. Roark; Roger L. Roundy
Original assignee: Dow Global Technologies Inc.
Priority date: 2008-04-01
Filing date: 2009-03-23
Publication date: 2009-10-08

Abstract

The startup procedure for a process of producing (meth)acrylic acid is described. The procedure includes providing a gas reaction and purification apparatus in a shutdown state. The apparatus includes a reactor and dehydration and finishing columns. Liquids are drained from the apparatus, the apparatus is heated to a temperature above the dew point of the gases that will course through the apparatus, and a recycle flow of inert gas is started through the reactor and dehydration column. The finishing column is started in parallel but while decoupled from the reactor and dehydration column. After the recycle flow is established and the equipment is heated to the desired temperature, gradually the reaction feed gas is introduced to replace the inert recycle gas, and eventually the finishing column is coupled to the dehydration column.

Description

PROCEDURE FOR THE STARTUP OF A (METH)ACRYLIC ACID PLANT

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. patent application serial no. 61/041,385, filed on April 1, 2008, the entire content of which is incorporated by reference herein.

FIELD OF THE INVENTION

[0001] This invention relates to (meth)acrylic acid. In one aspect, the invention relates to a process for producing (meth)acrylic acid while in another aspect, the invention relates to a procedure for starting a (meth)acrylic acid process. In still another aspect, the invention relates to a startup procedure for an (meth)acrylic acid process that employs a recycle gas reaction system coupled to an intensified (meth)acrylic acid purification system.

BACKGROUND OF THE INVENTION

[0002] "(Meth)acrylic acid" means one or more of acrylic acid and methacrylic acid. "(Meth)acrolein" means one or more of acrolein and methacrolein. "(Iso)propylene". means one or more of propylene and isopropylene.

[0003] The commercial manufacture of (meth)acrolein and/or (meth)acrylic acid is well known, and U.S. Patent Application Publication No. US2005/0038291 is representative. (Meth)acrolein and (meth)acrylic acid are prepared predominantly by heterogeneously catalyzed, gas phase partial oxidation of suitable C₃/C₄ precursor compounds, in particular of propene and propane in the case of acrolein and acrylic acid, or of isobutene and isobutane in the case of methacrylic acid and methacrolein. The catalysts are typically solid-state, fixed-bed, multielement oxides. Depending upon the catalyst charge and reaction conditions, the reaction can produce predominantly (meth)acrolein, a mixture of (meth)acrolein and (meth)acrylic acid, or predominantly (meth)acrylic acid.

[0004] Typically, the gas phase partial oxidation is carried out in two successive steps. In the first step, the precursor compounds are passed over a first fixed catalyst bed that favors the production of (meth)acrolein. In the second step, the reaction effluent of the first step is passed over a second fixed bed that favors the production of (mcth)acrylic acid. The oxygen required for the oxidation reaction of the first step and the second step may be added to the feed of the first fixed bed or a portion may be added to the feed of the second fixed bed. The catalyst beds can be within the same reactor but in different zones, or in different reactors connected in series. The reaction effluent from the second catalyst bed is then passed to a purification system that usually comprises dehydration and finishing columns or towers. Typically, the reaction effluent from the second catalyst bed is passed to the dehydration column in which compounds whose normal boiling points are less than acrylic acid (e.g., nitrogen, by-product water, formaldehyde, acetic acid etc.) are removed overhead, and acrylic acid containing some water, acetic acid, and compounds whose normal boiling points are higher than acrylic acid (e.g. maleic acid/anhydride, furfural, bcnzaldchydc etc.) arc removed as a bottom stream. The bottom stream of the dehydration column is transferred to the finishing column in which technical grade (e.g., 99+ weight percent) (meth)acrylic acid product is recovered as a side draw,. The overhead from the finishing column is recycled to the dehydration column. The bottom stream from the finishing column can be further treated to recover acrylic acid, sent as feed to an acrylate esters production unit, purged from the system as waste, or any combination thereof. The overhead from the dehydration column is split with some returned to the reactor and the remainder vented to a waste treatment device such as a flare or thermal oxidizer.

[0005] In one embodiment the catalyst beds constitute reaction zones within a single reactor shell while in another embodiment, each catalyst bed is within a separate reactor, the reactors connected in series. In still another embodiment, the multi-element oxide is capable of catalyzing both reactions simultaneously and as such, it is contained in a single bed within a single reactor shell.

[0006] For any one of a number of reasons, e.g., catalyst replacement or re-activation, equipment maintenance, clean-up after a reaction upset, etc., occasionally a reactor and its associated equipment, including the purification system, must be shutdown. After whatever action that necessitated the shutdown is completed, the reactor and its associated equipment must then be restarted. Restarting a (meth)acrolein or (meth)acrylic acid reactor and its associated equipment, or simply starting such equipment for the first time, requires attention to many details. Of particular importance is: (1) ensuring that the catalyst is protected from reducing atmospheres and liquid that may damage the performance of the catalyst, (2) avoiding flammable vapor compositions, (3) preventing the formation/deposition of polymer onto equipment surfaces, and (4) minimizing the time necessary to bring the equipment up to operating conditions.

BRIEF SUMMARY OF THE INVENTION

[0007] In one embodiment, the invention is a startup procedure for a (meth)acrylic acid process which includes recycling gas from an intensified (meth)acrylic acid purification system coupled to a process reactor, the process comprising the steps of:

A. Providing a gas reaction apparatus in an inactive state, the apparatus comprising (1) a gas-phase reactor containing a catalyst bed, the reactor in fluid communication with (2) a dehydration column which is in fluid communication with both (3) an cntrainment separator, and (4) a finishing column containing a base material, the entrainment separator in fluid communication with (5) a recycle gas compressor for recycling gas from the dehydration column to the reactor, the finishing column also in recycle communication with the dehydration column;

B. Draining the entrainment separator of liquid before startup and drawing off accumulated liquid during the startup procedure;

C. Heating the reactor, dehydration column, the finishing column, entrainment separator and recycle gas compressor to a temperature above the highest dew point of the gas recycled from the dehydration column to the recycle gas compressor;

D. Establishing a recycle flow of an inert gas, optionally containing oxygen, through the reactor;

E. Adding to the recycle flow through the reactor, an oxygen containing gas and propylene feed to the reactor, optionally including an inert gas feed, to maintain an oxygen:propylene mole ratio of between 1.6 and 2 in the reactor feed mixture to form a reaction product gas; and

F. Adding polymerization inhibitor(s) to the dehydration and finishing columns before the reaction product gas of (E) is fed to the dehydration column; with the proviso that the reactor and dehydration column are started together in parallel with and while decoupled from the finishing column.

[0008] In another embodiment, the procedure comprises the further proviso that heat is not added to the quench section of the dehydration column until the compressor is heated to a temperature above the highest dew point of the gas recycled from the dehydration column to the reactor. This procedure allows for a rapid startup of the process equipment while ensuring that the reactor conditions are maintained such that the catalyst will not be damaged, flammable mixtures are not produced, and the equipment is properly heated and protected against formation of polymer.

BRIEF DESCRIPTION OF THE DRAWING [0009] The figure is a schematic depiction of an acrylic acid production plant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0010] The numerical ranges in this disclosure are approximate, and thus may include values outside of the range unless otherwise indicated. Numerical ranges include all values from and including the lower and the upper values, in increments of one unit, provided that there is a separation of at least two units between any lower value and any higher value. As an example, if a compositional, physical or other property or process parameter, such as, for example, molecular weight, viscosity, melt index, temperature, etc., is from 100 to 1,000, it is intended that all individual values, such as 100, 101, 102, etc., and sub ranges, such as 100 to 144, 155 to 170, 197 to 200, etc., are expressly enumerated. For ranges containing values which are less than one or containing fractional numbers greater than one (e.g., 1.1, 1.5, etc.), one unit is considered to be 0.0001, 0.001, 0.01 or 0.1, as appropriate. For ranges containing single digit numbers less than ten (e.g., 1 to 5), one unit is typically considered to be 0.1. These are only examples of what is specifically intended, and all possible combinations of numerical values between the lowest value and the highest value enumerated, are to be considered to be expressly stated in this disclosure. Numerical ranges are provided within this disclosure for, among other things, the relative amount of reagents and catalyst in the reaction mixture or mass, and various temperatures and other process parameters. [0011] "Oxygen" means molecular oxygen, i.e., O₂.

[0012] The term "comprising" and its derivatives are not intended to exclude the presence of any additional component, step or procedure, whether or not the same is specifically disclosed. In order to avoid any doubt, any process or composition claimed through use of the term "comprising" may include any additional steps, equipment, additive, adjuvant, or compound whether polymeric or otherwise, unless stated to the contrary. In contrast, the term, "consisting essentially of excludes from the scope of any succeeding recitation any other component, step or procedure, excepting those that are not essential to operability. The term "consisting of excludes any component, step or procedure not specifically delineated or listed. The term "or", unless stated otherwise, refers to the listed members individually as well as in any combination. [0013] "Reaction mixture", "reaction mass" and like terms means the combination of materials necessary or ancillary to a reaction, typically under reactive conditions. Over the course of a reaction, a reaction mixture converts into a product mixture. Depending upon the moment in time in which the reaction mixture is characterized and other factors such as whether the process is batch or continuous, the physical state of the starting and product materials, etc., it will or can contain the reactants, catalyst, solvent, processing aids, products, byproducts, impurities and the like.

[0014] "Product mixture" and like terms means the combination of materials resulting from subjecting a reaction mixture to reaction conditions. A product mixture will always contain some product and/or byproduct and depending upon a multiplicity of factors (e.g., batch versus continuous, physical state of the starting materials, etc.), it may or may not contain unreacted starting materials, catalyst, solvent, processing aids, impurities, and the like. The typical product mixture that forms a part of this invention after the reaction has begun, typically at the end or near the end of the reaction, will include unreacted C₃ or C₄ hydrocarbon, oxygen, (meth)acrolein, (meth)acrylic acid and byproducts including water.

[0015] "Reaction conditions" and like terms generally refer to temperature, pressure, reactant concentrations, catalyst concentration, cocatalyst concentration, reactant conversion, product and by-product (or solids) content of the reaction mixture (or mass) and/or other conditions that influence the properties of the resulting product.

[0016] "Dehydration conditions" and like terms means the temperature, pressure, reagents and the like necessary to separate water of reaction, i.e., by-product water, from the reaction effluent or product mixture of the second stage reactor.

[0017] "Finishing conditions" and like terms means the temperature, pressure, reagents and the like necessary to separate and remove (meth)acrylic acid from water, acetic acid and other by-products present in a bottoms stream from a dehydration column. [0018] "Continuous process" and like terms means that the process is operated at a steady state, i.e., the reactants are fed to the reactor or reaction zone at a rate substantially in balance with the rate that product is removed from the reactor or reaction zone such that the reaction mass in the reactor or reaction zone is relatively constant in volume and composition. Continuous process does not include a batch or semi-batch process, the former characterized by a depletion of reactants and a growth of product over time, and the latter typically characterized by the unbalanced addition of reactant and removal of product over time.

[0019] "Fluid communication" and like terms mean that two or more pieces of equipment are connected or coupled to one another in a manner that allows for the passage of a fluid, e.g., a gas or liquid, from one piece of equipment to another. The fluid can pass through intervening equipment, e.g., pipes, valves, regulators, tanks etc., in its passage from the source equipment to the destination equipment. The passage can be regulated such that at any moment in time, no fluid can pass (e.g., the passage is blocked by a closed valve), some fluid can pass (e.g., the fluid is partially blocked by a partially closed valve), or all the fluid can pass.

[0020] "Coupled", "connected" and like terms means that two pieces of equipment are linked or attached to one another, either directly or indirectly. Two pieces of equipment are indirectly coupled to one another through one or more other pieces of equipment, e.g., heat exchangers, compressors, regulators, etc.

[0021] Certain embodiments of the invention are illustrated by reference to the Figure. Various items of equipment such as valves, fittings, sensors, surge and holding tanks, transmitters, steam and cooling water sources, and the like are omitted so as to simplify the description. However, those skilled in the art will recognize that such conventional equipment can be, and is, employed as desired.

[0022] The Figure illustrates a gas reaction and purification apparatus for a (meth)acrylic acid process. In this apparatus an intensified (meth)acrylic acid purification system is coupled to a two stage reactor. While the equipment and procedure are described in the context of acrylic acid manufacture, the equipment and procedure is also applicable, with some adjustment of the process reagents and parameters well known to those of skill in the art, to the manufacture of methacrylic acid.

[0023] In the Figure, first stage reactor 10 is in fluid communication and connected in series to second stage reactor 1 1. In operation, air and propylene and/or propane (typically principally propylene) are fed to first stage reactor 10 in which they are exposed to reaction conditions such that at least a part of these reactants are converted to acrolein over a multi-element catalyst bed (not shown) designed to convert propylene and/or propane and oxygen to acrolein. The reaction effluent, i.e., the product mixture, from this reaction is then passed directly to second stage reactor 11 in which it is exposed to reaction conditions such that the acrolein is further oxidized, i.e., converted, to acrylic acid. Second stage reactor 11 also contains a multi-element oxide catalyst bed (not shown), this catalyst designed to promote the conversion of acrolein to acrylic acid. In another embodiment, not shown, the first and second stage reactors can be combined into a single reactor in which the respective catalysts of the first and second stage reactors are positioned within first and second reaction zones with the single reactor, or the two catalysts are combined (or replaced with a single catalyst) and positioned into a single reaction zone. [0024] Second stage reactor 1 1 is in fluid communication and coupled to dehydration column 12 which in turn is in fluid communication and coupled to finishing column 13. In practice, the reactor effluent of second stage reactor 11 is transferred to dehydration column 12 in which it is subjected to dehydration conditions. The effluent of the second stage reactor 11 is transferred to dehydration column 12 via a quench device (not shown in Figure), which can be external or internal to the dehydration column 12. The noncondensibles, e.g., nitrogen, unreacted propylene and oxygen, the lighter by-products such as most of the formaldehyde and water, etc., are removed overhead while the condensibles, e.g., most of the acrylic acid and some of the byproduct water, acrolein and acetic acid, are removed underneath, i.e., as a bottoms stream. [0025] The overhead stream of the Dehydration column 12 is passed to a partial condenser where some of the condensable compounds such as water, acetic acid, and acrylic acid are condensed and returned to the Dehydration column 12 as reflux. Dehydration column 12 is in recycle fluid communication with first stage reactor 10, which means that some of the vent stream of the partial condenser is returned to first stage reactor 10. For example and as shown in the Figure, the vent stream from dehydration column 12 partial condenser is divided in any desired manner and amount such that part of the stream is directed to entrainment separator 14 (otherwise known as a knock-out pot) in which any entrained liquid in the vapor stream can be removed. The vent of the entrainment separator 14, containing nitrogen, unreacted propylene and oxygen, and water vapor passes through recycle gas compressor 15, and is then returned to first stage reactor 10. The part of the partial condenser vent from dehydration column 12 not directed to entrainment separator 14 is typically incinerated.

[0026] The bottoms stream from dehydration column 12 is directed to the top of finishing column 13 in which it is subjected to finishing conditions. The light fraction, e.g., water, acetic acid and some acrylic acid, are removed overhead and recycled to the bottom of dehydration column 12. The heavy fraction, e.g., acrylic acid including oligomers and polymers of acrylic acid, is removed underneath and recovered as an ester grade form of acrylic acid, i.e., acrylic acid of a purity less than 99 weight percent. Technical grade (99+ weight percent) acrylic acid is recovered as a vapor side draw.

[0027] In the initial startup of the gas reaction and purification apparatus shown in the Figure or in the startup of this apparatus after it has been in operation and then shutdown for whatever reason, the catalyst or catalysts disposed within the reactors or reactor stages must be protected against damage, e.g., due to contact with liquid or reducing atmospheres. Moreover, the equipment must also be protected against polymer fouling, and the creation of flammable mixtures avoided to guard against explosions.

[0028] Accordingly, liquids are drained from entrainment separator 14 and preferably, any process lines, normally used to carry vapor streams, in which liquid may have accumulated during shutdown. Moreover, preferably the entrainment separator, recycle gas compressor, and vapor carrying process lines are kept free of liquids as best as possible until the startup procedure is complete and the equipment is running under normal operating conditions. To this end, the reactor, entrainment separator, recycle gas compressor and any vapor carrying process lines are heated to and maintained at a temperature above the dew point of the material coursing through the equipment during the startup procedure. Typically, this temperature is between 20 and 400⁰C. These temperatures also contribute to minimizing unwanted polymerization of acrylic acid and acrolein during the later stages of the startup procedure.

[0029] The dehydration column and finishing columns are heated to and maintained at a temperature equal to or above the dew point of the corresponding vapor composition within the column in order to minimize heat losses from the column surfaces to the surrounding environment which can lead to unwanted condensation of uninhibited acrylic acid and in turn result in unwanted polymer formation. This temperature is typically between 20 and 15O⁰C. Polymerization inhibitors should be added to the dehydration column, finishing column and associated equipment before reaction product gas is introduced into the dehydration column to prevent unwanted polymer formation due to condensation of uninhibited acrylic acid.

[0030] The heating can be accomplished by any desirable means, e.g., heating jackets, coursing heated gases through the equipment, etc. The heating of the recycle gas compressor, however, is typically accomplished by heat tracing, and/or running heated gas through the equipment, before the rest of the startup procedure is commenced. The recycle gas compressor is allowed to warm by running on by-pass before flowing gas from the dehydration column to the reactor.

[0031] In the course of heating these pieces of equipment, particularly if the heating is accomplished by coursing heated gases through the equipment, most, if not all, associated pieces of equipment, e.g., pipes, valves, surge and holding tanks, etc., are also heated.

[0032] The reactor is started by first establishing a recycle flow of an inert gas, e.g., nitrogen, optionally with oxygen, through it. The presence of oxygen, usually in the form of air, in the recycle flow acts to prevent the reduction of the catalyst that might otherwise occur due to carryover of organics from other parts of the apparatus, e.g., the dehydration column, connecting piping, condensers, etc. The concentration of oxygen in the recycle flow is typically limited to between 0.01 and 10, preferably between 0.1 and 5 and more preferably between 0.5 and 3 volume or mole percent of the recycle gas.

[0033] The reactor and dehydration column arc started together such that the gas flowing through the reactor is passed to the dehydration column and then recycled back to the reactor through the entrainment separator and recycle gas compressor. While the finishing column is typically started at the same time as the reactor and dehydration column, it is started while it is decoupled from the dehydration column. At this time, the finishing column is operated under total reflux conditions and run in this standby mode until a flow of dehydration column bottoms material to the top of the finishing column is established, typically near to or at the end of the startup procedure.

[0034] After the equipment is heated, air and propylene/propane feeds are introduced to the reactor. The air and hydrocarbon feed are increased together to maintain an oxygen:hydrocarbon ratio between 1.6 and 2, preferably between 1.7 and 1.9. Optionally, an inert gas, e.g. nitrogen, can be included with the air/hydrocarbon feed to guard against the formation of a flammable mixture. The temperature of the gaseous feed at the time it is fed to the reactor is above its dew point, and it is maintained above its dew point while within the reactor.

[00351 Analyzers are used throughout the gas reactor and purification apparatus to monitor the oxygen, hydrocarbon and other components in the feed and recycle loops to protect against the formation of flammable mixtures and to control conversion. Polymerization inhibitors, such as 4-hydroxy TEMPO, hydroquinone, methyl ether of hydroquinone, phenothiazine, manganese salts, or copper salts are added to the dehydration and finishing columns to protect against unwanted polymerization of acrylic acid and acrolein. These inhibitors are usually added before vapor traffic has been established in the columns. Inhibitors can be added to the dehydration column through the reflux from the condenser (not show in the Figure) and/or the top of the column. Inhibitors can be added to the finishing column through the feed to the column and/or into the overhead condenser (not shown in the Figure). During the initial startup of the finishing column, i.e., before feed is introduced into it, the base material containing high levels of polymerization inhibitors can be used as the polymerization inhibitor solution. [0036] If the startup of the gas reactor and purification apparatus is a new startup, i.e., it is the first time that the apparatus is used to make (meth)acrylic acid, then the liquid inventories in the dehydration column bottom, the finishing column bottom and finishing column overhead condensate receiver can come from actual process material produced in another apparatus or it can be a synthetic mixture including acrylic acid, water, acetic acid and polymerization inhibitors.

[0037] The startup procedure is completed once operating conditions have been established, i.e., once a steady state of feed to the reactor and product recovery from the finishing column is established. A startup procedure will require between one hour and several days, typically less than a day.

[0038] Although the invention has been described in considerable detail by the preceding specification, this detail is for the purpose of illustration and is not to be construed as a limitation upon the following appended claims. For purposes of United States patent practice, the contents of any referenced patent, patent application, or publication are incorporated by reference in their entirety (or its equivalent US version is so incorporated by reference) especially with respect to the disclosure of synthetic techniques, definitions (to the extent not inconsistent with any definitions specifically provided in this disclosure) and general knowledge in the art.

Claims

CLAIMS What is claimed is:

1. A startup procedure for a (meth)acrylic acid process which includes a recycle gas reaction system coupled to an intensified (meth)acrylic acid purification system, the process comprising the steps of:

A. Providing a gas reaction apparatus in an inactive state, the apparatus comprising (1) a gas-phase reactor containing a catalyst bed, the reactor in fluid communication with (2) a dehydration column which is in fluid communication with both (3) an entrainment separator, and (4) a finishing column containing a base material, the entrainment separator in fluid communication with (5) a recycle gas compressor for recycling gas from the dehydration column to the reactor, the finishing column also in recycle communication with the dehydration column;

2. The procedure of Claim 1 in which the reactor, dehydration and finishing columns, entrainment separator and recycle gas compressor are heated and maintained at a temperature between 20 and 400⁰C.

3. The procedure of Claim 2 in which the reactor, dehydration column, finishing column, entrainment separator and recycle gas compressor are heated by heat tracing.

4. The procedure of Claim 1 in which the inert gas of the recycle flow is nitrogen and oxygen is present in an amount between 0.1 and 10 volume percent.

5. The procedure of Claim 1 in which the process is an acrylic acid process.

6. The procedure of Claim 1 with the further proviso that heat is not added to the quench section of the dehydration column until the recycle gas compressor is heated to a temperature above the highest dew point of the gas recycled from the dehydration column to the reactor.