Field of the Invention

[001] The present invention relates to a process for the preparation of acrylonitrile from glycerol.

Background of the Invention

[002] Acrylonitrile (ACN) is a man-made organic compound with the formula CH2=CH-CN. Acrylonitrile is used as a monomer in the production of the homopolymer polyacrylonitrile, as well as for producing copolymers such as styrene-acrylonitrile (SAN) acrylonitrile butadiene styrene (ABS), acrylonitrile styrene acrylate (ASA), and other synthetic rubbers such as acrylonitrile butadiene (NBR). It is further used as an intermediate in the production of nylon and acrylamide. Acrylonitrile also finds increasing use as a precursor in production of carbon fibre, which is a suitable material for high technology use especially in the aerospace and automotive industries. It is estimated that more than 6.5 million metric tons of ACN per year are produced worldwide.

[003] Worldwide over 95% of ACN is produced by the so-called SOHIO (Standard Oil of Ohio, discovered 1957) process, by means of gas phase ammoxidation of propylene:

CH ₃ CH=CH ₂ + NH ₃ + 3/2 O ₂ -> CH ₂ =CH-CN + 3 H ₂ O

By-products include the solvent acetonitrile (CH ₃ CN), hydrogen cyanide (HCN), and acrolein (CH?-CH- CHO). The reaction typically takes place in a fluidized reactor at temperature in the range of 400 to 500 °C, with an operation pressure usually below 3 bar. The feed typically contains a minimally stoichiometric excess of ammonia and oxygen over propylene. After quenching and acid neutralization to remove residual ammonia, ACN and by-products are recovered by water absorption in an absorption column. To increase the recovery rate of products, which is usually targeted to be more than 99.5% of the reaction products, the water is chilled to about 5 °C and a higher water/ACN ratio is applied. The SOHIO process further involves the use of multiple distillation columns to separate and purify the main product ACN and other by-products.

[004] Various alternative "green chemistry "routes to the SOHIO process have been developed, aimed at using renewable feedstocks for the synthesis of acrylonitrile, such as lignocellulosic biomass, glutamic acid and glycerol.

[005] Glycerol (1,2,3-propanetriol; glycerine) is a significant (about 10 wt%) by-product of the production of biodiesel, wherein e.g. vegetable oils, animals fats, or used cooking oils are subjected to a transesterification process to produce biodiesel, crude glycerol and water. Due to the large-scale production of biodiesel, efforts are being made to convert glycerol into more value-added chemicals.

[006] The preparation of acrylonitrile from glycerol generally involves a dehydration and an ammoxidation step, as described in for example US20100048850A1. The dehydration step involves the dehydration of glycerol in the presence of a tungsten-containing dehydration catalyst to produce acrolein and water according to

(CH ₂ OH) ₂ CHOH CH ₂ =CH-CHO + 2 H ₂ O (1)

The ammoxidation step involves the catalytic conversion of acrolein in the presence of ammonia and oxygen to acrylonitrile according to

CH ₂ =CH-CHO + 1/2 O ₂ + NH ₃ -> CH ₂ =CH-CN + 2 H ₂ O (2)

[007] Besides acrylonitrile, the gaseous ammoxidation product stream comprises smaller amounts of acetonitrile and other by-products. This effluent stream is typically subjected to quenching through a steam generator, acid neutralization column to remove residual ammonia and subsequently passed over an absorption column to which cooled water is supplied as an absorbent. At the bottom of the absorption column a concentrated stream of acrylonitrile, water and other by-products is obtained, from which the acrylonitrile product may be isolated by successive distillation steps.

[008] Due to the relatively high selectivity of the acrolein ammoxidation reaction, purification of the ammoxidation reaction effluent is simpler compared to the above-described (SOHIO) propylene ammoxidation process. However, the recovery of acrylonitrile and other potential by-products in the absorption column requires large amounts of cooled water, leaving room for improvement with regard to water use, water cooling and/or waste discharge of the process.

Summary of the Invention

[009] In a first aspect of the present disclosure, there is provided a process for producing acrylonitrile from glycerol, comprising

(i) providing a feed stream comprising glycerol

(ii) subjecting the stream comprising glycerol to a dehydration step in the presence of a dehydration catalyst to produce an acrolein stream

(iii) optionally providing the acrolein stream obtained in step (ii) to a purification column to produce a concentrated acrolein stream and a residual stream comprising unconverted glycerol (iv) converting the acrolein streams obtained in step (ii) and optionally step (iii) in the presence of ammonia, oxygen and an ammoxidation catalyst to an acrylonitrile stream

(v) absorbing the acrylonitrile stream in an absorption column using a liquid absorbent comprising glycerol to provide a stream comprising glycerol and acrylonitrile

(vi) separating the stream comprising glycerol and acrylonitrile to produce an acrylonitrile product stream and an acrylonitrile-depleted glycerol stream

(vii) recycling at least a portion of the acrylonitrile-depleted glycerol stream as a glycerol absorbent feed stream to the absorption column.

[010] In this process, the glycerol stream mainly comprising glycerol and water recovered from the stripping column is at least partially recycled to the absorption column, where it is utilised to absorb acrylonitrile and optional by-products (such as acetonitrile) from the acrolein ammoxidation effluent. It was found by the present inventors that the use of glycerol or a glycerol/water mixture as an absorbent for the gaseous ammoxidation reaction products requires smaller volumes of absorbent as compared to pure water as used in the art for obtaining the same degree of acrylonitrile recovery. By using the acrylonitrile-depleted glycerol stream recovered from the acrylonitrile separation step as an absorbent in the ammoxidation effluent absorption step, significant improvements in water use and waste discharge can be obtained. Moreover, the use of a, preferably recycled, glycerol stream in the absorption process allows for operation at ambient temperatures without the need of cooling the absorbent, thus further reducing operating and/or capital expenses of the process.

[Oil] As will be described in more detail below, the process according to the present disclosure is particularly advantageous when crude glycerol is used as a feed for the acrylonitrile manufacturing process, and wherein a crude glycerol purification unit is used to purify the crude glycerol prior to providing it as a feed stream for the dehydration to acrolein.

[012] The process according to the present disclosure is particularly advantageous for processes for producing acrylonitrile from glycerol comprising one or more glycerol dehydration and acrolein ammoxidation steps. However, the use of according to the present disclosure can be generally integrated in acrylonitrile processes existing in the art, wherein an ammoxidation reaction effluent stream is subjected to absorption by a glycerol absorbent in an absorption column, and wherein at least a part of the glycerol absorbent is provided by a glycerol recycle stream, a purified crude glycerol feed or a combination thereof produced elsewhere in the process.

[013] Accordingly, in another aspect of the present disclosure there is provided a process for the recovery of acrylonitrile from an ammoxidation reaction effluent stream, comprising (i) providing the ammoxidation reaction effluent stream to an absorption column;

(ii) absorbing acrylonitrile from the ammoxidation reaction effluent stream using a liquid absorbent comprising glycerol, to produce a stream comprising glycerol and acrylonitrile; and

(iii) separating acrylonitrile from the stream comprising glycerol and acrylonitrile.

[014] With an integrated process for the manufacture of acrylonitrile from glycerol as described herein, crude glycerol from renewable sources can be used as feed stock, while glycerol recovered at various stages in the process can be partially purged and recycled to a crude glycerol purification unit to provide a full recycle and reuse of resources. Thus, more effective and efficient energy integration, a full raw material conversion and zero/near-zero waste discharge can be achieved.

Brief Description of the Drawings

Figure 1 is a schematic diagram showing a process according to an embodiment of the invention.

Figure 2 is a schematic diagram showing a process according to an embodiment of the invention.

Detailed Description of the Invention

[015] The present disclosure provides a process and a system for producing acrylonitrile from glycerol.

[016] In this process, a liquid feed stream comprising glycerol is provided to a dehydration reactor comprising a catalyst bed comprising a suitable dehydration catalyst. The concentration of the feed stream is typically in the range of 5 - 80 wt% by proper dilution with water. Typically, the inlet temperature of the feed is set to be in the range of 250 - 300 °C using a preheater. The feed flow rate is generally set in the range of 0.5 to 15 per hr WHSV (weight hour space velocity) and the inlet pressure is normally set in the range of 1.1 - 2 bar (absolute).

[017] In the dehydration reactor, glycerol is dehydrated in the presence of the dehydration catalyst to produce acrolein and water according to

(CH ₂ OH) ₂ CHOH -* CH ₂ =CH-CHO + 2 H ₂ O

[018] Inside the dehydration reactor, the catalyst bed is typically separated into at least two stages with a heat exchanger installed in between, as this design allows to supply heat to sustain the temperature inside catalyst bed at least 250 °C.

[019] The dehydration catalyst may be any catalyst that is capable of converting glycerol to acrolein under the conditions as described herein. Suitable examples of such catalysts are disclosed in, for example, W02006087083A2 and WO2013017942 typically comprise a porous carrier containing at least one metal oxide selected from tantalum oxide (Ta ₂ O ₅ ), niobium oxide (Nb ₂ O5), alumina (AI ₂ Oa), titanium oxide (TiCh) , zirconia (ZrCh), tin oxide (SnCh), silica (SiCh) or silico-aluminate (SiCh - AI2O3), impregnated with acidic functions such as borate (BO3), sulphate (SO4), tungstate (WO3), phosphate (PO4), silicate (SiCh) or molybdate (MoOs).

[020] In an embodiment, the dehydration catalyst comprises about 20% WO3 on a commercially available ZrCh or SiCh porous support in pellet form, having a diameter of about 1.0 to 3.0mm, and with majority of pores in the meso pore size range (2-50nm).

[021] In an embodiment, at least two dehydration reactors comprising a dehydration catalyst are set in parallel in order to achieve continuous operation by keeping one on-line and the other off-line for catalyst regeneration. Thus, at least one dehydration reactor is online during the dehydration reaction and at least one dehydration reactor is offline during the dehydration step, and the dehydration catalyst in at least one of the offline dehydration reactors is regenerated. Catalyst regeneration typically involves nitrogen flushing, followed by dosing air into the nitrogen stream at the reaction temperature until pure air level is reached, whilst maintaining the temperature inside the reactor below 350 °C.

[022] The acrolein product stream from the dehydration step may optionally be purified by first passing the reaction product stream from the dehydration reactor through a quenching chamber with an outside cooling loop and injection inhibitor such as 4-hydroxy-2,2,6,6-tetramethylpiperidin-l-oxyl ("TEMPOL"), followed by feeding the cooled product stream into a distillation column where a purified is obtained from the top and a residual stream comprising unconverted glycerol may be withdrawn from the bottom.

[023] The acrolein stream obtained in the dehydration step, and optionally step purified as for example described above, is subsequently converted in the presence of ammonia, oxygen and an ammoxidation catalyst to acrylonitrile according to

CH ₂ =CH-CHO + 1/2 O ₂ + NH ₃ -> CH ₂ =CH-CN + 2 H ₂ O

[024] Typically, the optionally purified acrolein is mixed with ammonia, air, and or other inert diluents as needed and heated to 250 - 350 °C before feeding into an ammoxidation reactor. Molar ratios to acrolein in the feed are set in the range of 1.1 - 10 for oxygen, 1.1 to 1.5 for ammonia, and the acrolein concentration is typically set in the range of 1 to 5% v/v, with inert diluent gas making up the remaining balance..

[025] The temperature inside the ammoxidation reactor is typically controlled below 450 °C by either external cooling or inter-stage cooling. External cooling can be achieved by applying, for example a multi-tubular reactor, while inter-stage cooling typically applies a multi-stage fixed bed reactor with heat exchangers built in between two stages or a fluidized bed reactor with built-in cooling loops.

[026] Suitable ammoxidation catalysts are known in the art and typically comprise one or more mixed oxides chosen for example from molybdenum, bismuth, iron, antimony, tin, vanadium, tungsten, antimony, zirconium, titanium, chromium, nickel, aluminium, phosphorus or gallium.

[027] The ammoxidation step produces a gaseous effluent stream comprising acrylonitrile and possible by-products such as acetonitrile. The ammoxidation effluent stream is subjected to purification steps known in the art, typically comprising quenching through a steam generator and absorption in an acid neutralization column to remove residual ammonia.

[028] Subsequently, the ammoxidation product stream is provided as a feed to the bottom stage of an absorption column, to which a liquid absorbent stream comprising glycerol is continuously supplied at the top stage. In this absorption step, the ammoxidation product stream is intimately contacted with liquid absorbent to produce a liquid stream comprising mainly glycerol and acrylonitrile, as well as possible by-products. The liquid absorbent stream may consist essentially of glycerol, or may be a mixture comprising glycerol and water. Typically, the liquid absorbent comprises at least 5 wt% glycerol, preferably at least 10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt%, 70 wt%, 80 wt% or 90 wt% glycerol. Typically, the liquid absorbent comprises at most 100 wt% glycerol, preferably at most 95 wt%, 90 wt%, 85 wt%, 80 wt%, 70 wt%, 60 wt%, 50 wt%, 40 wt%, or 30 wt% glycerol. The glycerol content in the liquid absorbent may be varied in order to control the desired concentrations of acetonitrile in addition to acrylonitrile. For example, a concentration of glycerol in the range of 30 - 60 % wt may be preferred range in to recover both acrylonitrile and acetonitrile in desired amounts.

[029] The use of a glycerol-containing absorbent allows for using the absorbent for the ammoxidation reaction products at ambient temperatures rather at than at temperatures of about 5 °C which is customary for process using water as the sole absorbent in the art. Typically, the liquid absorbent comprising glycerol is provided at a temperature in the range of 10-40 °C, preferably in the range of 20- 30 °C, more preferably in the range of 22-28 °C, most preferably about 25 °C.

[030] A liquid stream comprising mainly glycerol, acrylonitrile and possible by-products such as acetonitrile is withdrawn from the absorption column. The acrylonitrile and possible by-products can be recovered from the glycerol stream by a suitable separation process to produce an acrylonitrile product stream and an acrylonitrile-depleted glycerol stream. In an embodiment, the liquid product stream from the glycerol absorption step is fed to a distillation column in order to separate the products from the glycerol absorption solvent. In another embodiment, typically when a high glycerol concentration is used in the liquid absorbent, a stripping column is used to separate the glycerol absorbent from the acrylonitrile and possible by-products.

[031] The separated acrylonitrile, possibly further comprising acetonitrile, stream is sent to product purification unit for further separation and purification by means known in the art.

[032] At least a portion of the acrylonitrile-depleted glycerol stream is recycled as a glycerol absorbent feed stream to the absorption column. Typically, at least 50 %, preferably at least 80 %, more preferably at least 90 %, most preferably at least 95 % by volume of the acrylonitrile-depleted glycerol stream is recycled as a glycerol absorbent feed stream to the absorption column. Thus, the liquid absorbent used for absorbing nitrile products from the ammoxidation reaction effluent is at least partially provided by glycerol recovered from the subsequent nitrile-glycerol separation (e.g., distillation or steam-stripping) step. As compared to conventional designs, wherein cooled pure water is used for recovering products from the gaseous ammoxidation effluent, the use of glycerol or a glycerol/water mixture as an absorbent for the gaseous ammoxidation reaction products requires smaller volumes of absorbent while obtaining the same degree of acrylonitrile recovery. Moreover, by using the acrylonitrile-depleted glycerol stream recovered from the acrylonitrile separation step at least partially as the glycerol absorbent in the ammoxidation effluent absorption step, significant improvements in overall solvent use and waste discharge can be obtained.

[033] In an embodiment, the glycerol feed stream provided to the dehydration reactor is at least partially provided as a purified glycerol stream from a crude glycerol purification unit. Thus, in this embodiment a purified glycerol stream is withdrawn from a crude glycerol purification unit upstream of a first dehydration reactor and provided to the dehydration reactor for conversion to acrolein.

[034] The crude glycerol purification unit generally comprises in an inlet for receiving a crude glycerol feed and one or more outlets for discharging a purified glycerol stream and a water stream, and may comprise any combination of elements necessary to process a crude glycerol feed to the desired level of purity and suitability for any subsequent steps, including dehydration to acrolein. Generally, this comprises means for removing any potential high boiling species, such as residual fatty acids and their esters, that may coke and deactivate the dehydration catalyst. Furthermore, the crude glycerol purification unit is typically equipped with means for recovering any water that may be present in the crude feed. It typically comprises a wiped film evaporator located upstream of a distillation column, wherein the wiped film evaporator may be used as a preheater, as well as for removal from any heavies and salt from the crude glycerol feed. The column preferably is of a multiple side-draw design allowing to separate various components present in the crude glycerol feed. The column reboiler preferably comprises a down flow tubular evaporator that is suitable for handling fluids of high viscosity. The entire crude glycerol purification unit is typically operated under vacuum (e.g., about 15 mbar).

[035] The crude glycerol feed provided to the crude glycerol purification unit may at least partially be formed by a crude glycerol stream resulting from industries such as biodiesel plants.

[036] In an embodiment, a minor portion, for example between 5-20 % by volume, of recycled glycerol from the glycerol absorption and separation cycle (the major portion typically being recycled to the absorption column as described above) is provided as a feed to the crude glycerol purification unit reboiler. As such, this minor recycle stream acts as a bleeding stream to remove any heavy components that are potentially generated during the glycerol absorption and subsequent separation cycle.

[037] A purified glycerol stream is withdrawn from the crude glycerol purification unit, which is typically combined with recovered water withdrawn from the same unit and used as a feed for the glycerol dehydration reactor.

[038] In an embodiment, at least a portion of the purified glycerol stream withdrawn from the crude glycerol purification unit is provided as the glycerol-containing absorbent in the absorption step. Thus, in the absorption step the glycerol liquid absorbent may comprise a combination of glycerol recovered and recycled from the acrylonitrile separation step and purified glycerol from the crude glycerol purification unit.

[039] It is further possible to recycle any unreacted glycerol and possible reaction intermediates from the dehydration step to the crude glycerol purification unit for purification and provide the purified glycerol to the dehydration reactor for conversion to acrolein. Thus, in one embodiment, optionally at least a portion of the residual stream obtained in the acrolein purification step is recycled as a feed stream to the crude glycerol purification unit. The amount of unreacted glycerol to be recycled to the crude glycerol purification unit depends on the extent of glycerol conversion in the dehydration reaction; it may be up to 100% considering that the crude glycerol purification unit is designed to separate and remove any un-wanted components, and only provide water and glycerol for supplying to the dehydration reaction. Thus, by this design, a full raw material conversion and zero/near-zero waste discharge can be achieved.

[040] The present invention, wherein glycerol or a glycerol-water mixture instead of pure water is used for the recovery of ammoxidation products, including acrylonitrile and possible by-products such as acetonitrile, from a gaseous reaction product stream, advantageously utilizes considerably smaller volumes of absorbent and allows the absorption to be carried out at ambient temperatures. Moreover, the use of glycerol as the main absorbent permits the use of multiple (recycled) glycerol streams obtained elsewhere in the process.

[041] Accordingly, in another aspect there is provided process for the recovery of acrylonitrile from an ammoxidation reaction effluent stream, comprising providing the ammoxidation reaction effluent stream to an absorption column; absorbing acrylonitrile from the ammoxidation reaction effluent stream using a liquid absorbent comprising glycerol to produce a stream comprising glycerol and acrylonitrile; and separating acrylonitrile from the stream comprising glycerol and acrylonitrile.

[042] Typically, as described above, the liquid absorbent comprises at least 5 wt% glycerol, preferably at least 10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt%, 70 wt%, 80 wt% or 90 wt% glycerol.

Typically, the liquid absorbent comprises at most 100 wt% glycerol, preferably at most 95 wt%, 90 wt%, 85 wt%, 80 wt%, 70 wt%, 60 wt%, 50 wt%, 40 wt%, or 30 wt% glycerol.

[043] Advantageously, at least a portion of the liquid absorbent comprising glycerol is obtained

(a) as a glycerol recycle stream withdrawn from an acrylonitrile separation section located downstream of the absorption column;

(b) as a purified stream, optionally a purified recycle stream, withdrawn from a crude glycerol purification unit located upstream of the absorption column; or a combination thereof.

[044] Thus, the process combines the advantages of using glycerol rather than water for absorbing acrylonitrile and optional other products and the utilization of various glycerol recycle streams produced elsewhere in the process, including recycle streams that have first been subjected to purification in a front end crude glycerol purification unit.

[045] By the integration of the crude glycerol purification unit as disclosed herein, the process design can potentially reduce raw material consumption and be more efficient in energy integration between acrylonitrile production and direct use of crude glycerol as feed stock.

[046] In another aspect there is provided a system for carrying out the process as described herein, the system comprising

A dehydration section comprising an inlet for receiving a glycerol feed stream and an outlet for discharging an acrolein stream, the dehydration section comprising a dehydration reactor, wherein the dehydration reactor is configured to convert the glycerol stream to an acrolein stream;

Optionally, a purification section, comprising an inlet fluidly connected to the outlet of the dehydration section, an outlet for discharging a concentrated acrolein stream, and an outlet for discharging a residual stream comprising unconverted glycerol, the purification section comprising a purification column configured to produce a concentrated acrolein stream and a residual stream comprising unconverted glycerol;

An ammoxidation section comprising an inlet fluidly connected to the outlet of the purification section, and an outlet for discharging an acrylonitrile stream, wherein the ammoxidation section comprises an ammoxidation reactor configured to receive an acrolein feed stream and to convert the acrolein feed stream to an acrylonitrile stream;

An absorption section comprising an inlet fluidly connected to the outlet of the ammoxidation section, an inlet for receiving a glycerol absorbent stream, and an outlet for discharging a glycerol absorbent stream comprising an acrylonitrile product, wherein the absorption section comprises an absorption column configured to produce a glycerol absorbent stream comprising an acrylonitrile product;

A separation section comprising an inlet fluidly connected to the outlet of the absorption section, an outlet for discharging a acrylonitrile-depleted glycerol stream, and an outlet for discharging an acrylonitrile product stream, wherein the separation section comprises one or more apparatuses configured to separate the acrylonitrile product and optional other products from the glycerol absorbent stream; and

Means for recycling at least portion of the acrylonitrile-depleted glycerol stream from the outlet of the separation section to the inlet or receiving a glycerol absorbent stream of the absorption section.

[047] In an embodiment, the system further comprises a crude glycerol purification unit as described herein above, located upstream of the dehydration section, wherein an outlet of the crude glycerol purification unit is fluidly connected to the inlet for receiving a glycerol feed stream of the dehydration section. In an embodiment, an outlet of the crude glycerol purification unit is fluidly connected to the inlet for receiving a glycerol absorbent stream of the absorption section.

[048] In an embodiment, the system further comprises means for recycling at least portion of the residual stream comprising unconverted glycerol from the outlet of the purification section to the inlet of the crude glycerol purification unit. [049] In an embodiment, the system further comprises means for recycling at least portion of a glycerol bleed stream from an outlet of the separation section to an inlet of the crude glycerol purification unit.

[050] The invention will now be described in detail with reference to the non-limiting embodiments shown in the Figures. In the following, for the sake of understanding, elements of embodiments are described in operation. However, it will be apparent that the respective elements are arranged to perform the functions being described as performed by them. Further, the subject matter that is presently disclosed is not limited to the embodiments only, but also includes every other combination of features described herein or recited in mutually different dependent claims. It will be clear to the skilled person that as a schematic diagram this figure does not show all inputs and optional other elements.

[051] Referring to Figure 1, a schematic diagram illustrating a system 100 for producing acrylonitrile from glycerol according to an embodiment of the present disclosure is shown. During operation, a liquid feed stream 107 comprising glycerol is provided to a dehydration reactor 101. A dehydration reaction product stream is withdrawn from the dehydration reactor 101 and, following quenching and distillation steps (not shown) provided as a purified acrolein feed stream 108 to ammoxidation reactor 102.

[052] In ammoxidation reactor 102, acrolein is converted in the presence of a suitable ammoxidation catalyst, ammonia, oxygen or air and inert diluent gases as needed, to acrylonitrile.

[053] A gaseous effluent stream comprising acrylonitrile and possible by-products such as acetonitrile is withdrawn from the ammoxidation reactor 102. Residual ammonia present in the effluent may be removed by acid neutralization (not shown). The ammoxidation product stream is then provided as a feed 109 to the bottom stage of absorption column 103, to which a liquid absorbent stream 110 comprising glycerol is continuously supplied at the top stage. A liquid stream 111 comprising glycerol, acrylonitrile and optional by-products is withdrawn from absorption column 103 and provided as a feed to stripping column 104, wherein the absorbent solution is separated from the acrylonitrile and optional by-products to provide an acrylonitrile stream 112 and an acrylonitrile-depleted glycerol stream 113. At least a portion of the acrylonitrile-depleted glycerol stream 113 is recycled to the absorption column

103 for use in liquid absorbent stream 110. The acrylonitrile stream 112 obtained from stripping column

104 may be sent to one or more purification units for further separation and purification by means known in the practice.

[054] Referring to Figure 2, a schematic diagram 200 illustrating a system for producing acrylonitrile from glycerol according to another embodiment of the present disclosure is shown. In this embodiment, a crude glycerol purification unit 205 is provided upstream of a dehydration reactor 201, to which a crude glycerol feed stream 214, preferably from a renewable source, may be provided.

[055] During operation, crude glycerol is purified in glycerol purification unit 205 to produce a purified glycerol stream 215. At least a portion of purified glycerol stream 215 is employed as a feed stream 207 for dehydration reactor 201. A dehydration reaction product stream 218 is withdrawn from the dehydration reactor 201, quenched and provided to a purification section 206, wherein the dehydration product stream is purified by distillation. A purified acrolein feed stream 208 is obtained from the top of distillation column and provided to ammoxidation reactor 202. The bottom of the column may be discharged as waste stream or optionally recycled back as a glycerol recycle stream 219 to the crude glycerol purification unit 205 unit if any unconverted glycerol or intermediate reaction products need to be recovered.

[056] In ammoxidation reactor 202, acrolein is subjected to ammoxidation as described above to produce an ammoxidation product stream which is provided as a feed 209 to the bottom stage of absorption column 203, to which a liquid absorbent stream 210 comprising glycerol is continuously supplied at the top stage. The glycerol absorbent stream 210 may at least partially be provided as a purified glycerol effluent stream 216 from crude glycerol purification unit 205. Alternatively or additionally, glycerol absorbent stream 210 may at least partially be provided as a recycled glycerol stream 213 from a stripping column 204 located downstream of the absorption column 203.

[057] A liquid stream 211 comprising glycerol, acrylonitrile and possible by-products is withdrawn from absorption column 203 and provided as a feed to stripping column 204, wherein the absorbent solution is separated from the acrylonitrile and optional by-products to provide an acrylonitrile stream 212 and an acrylonitrile-depleted glycerol stream 213.

[058] At least a portion of the acrylonitrile-depleted glycerol stream is recycled as glycerol absorbent stream 213 to the absorption column 203. Another, typically smaller, portion may be recirculated as a purging stream 217 to crude glycerol purification unit 205 where any heavy components are removed to avoid their accumulation in the absorbent solution.

[059] The acrylonitrile stream 212 obtained from stripping column 204 may be sent to one or more purification units for further separation and purification by means known in the practice.

Examples

The invention will now be described by the following non-limiting examples. Example 1

[060] The absorption of an ammoxidation product stream using a liquid absorbent in an absorption tower was simulated using ASPEN® Rad-Frac model. Herein, an ammoxidation product stream comprising acrylonitrile (ACN), acetonitrile (AN), acrolein (ACR), nitrogen (N2), oxygen (O2), and H2O with composition (mole %) as shown in Table 1 is fed into an absorption column. The feed stream is preheated to 60 °C.

[061] Table 1

[062] The absorption column has 10 theoretical stages, and the absorption solvent is fed at the top (stage 1) of column. The ammoxidation product stream is fed at bottom (stage 10). The solvent enters the column at ambient temperature (25 °C) at a flow rate required to achieve 99.5% recovery of acrylonitrile from the product stream. Different absorption solvents, i.e. pure water, 50 wt% glycerol in water, 80 wt% glycerol in water, and 90 wt% glycerol in water, were simulated using the ASPEN® Rad- Frac model. The results are shown in in Table 2 below.

[063] The mass ratio is defined as the ratio between the mass flow rate of absorption solvent that is required to achieve the targeted 99.5% recovery of ACN and the mass flowrate of ACN in the feed stream. The numbers corresponding to each component represent the proportion of recovery (recovery rate) for the component by the corresponding absorption solvent. The simulations were run under identical conditions, except for the exit temperature of the liquid solvent which was allowed to vary due to differences in heat capacity of each solvent.

[064] Table 2

[065] As shown in Table 2, using pure water for the absorption solvent to recover the same amount of ACN, requires at least 3 times more weight of absorption solvent as compared to a glycerol-water mixture. The data in the table also show that an increased glycerol concentration in the absorption solvent results in a decrease in the recovery rates of AN and ACR while the ACN recovery rate remains the same. This finding is potentially applicable in improving efficiency of ACN and AN separation in the extractive distillation by adding glycerol as an assisting solvent.

Example 2

[066] The recovery of acrylonitrile (ACN), acetonitrile (AN), acrolein (ACR) from the liquid absorption product stream of example 1 was simulated using the ASPEN® Rad-Frac model. The liquid product stream from example 1 containing recovered reaction products and absorption solvent (mainly glycerol and water) is fed to a distillation column to further separate products from the absorption solvents. The feed is preheated to 60 °C. The column is set at 10 theoretical stages, the reflux ratio at 1.0, and the bottom withdraw ratio is set at 0.95. The simulations were run under identical conditions except for the reboiler temperature which was allowed to vary so that the same bottom withdraw ratio could be maintained.

[067] In either case, more than 99% of all three components are recovered and concentrated in the distillate from the top of the column. The resulting distillate stream forms a crude product stream and is subjected to further separation and purification by means well known in practice. There is no loss of glycerol and very minor loss of water to the distillate stream. The recovered solvent is readily recycled back to the absorption column.

[068] Table 3