Process for isomerizing an olefinically unsaturated alcohol in a reactor cascade

Abstract

A method is disclosed for isomerizing a feed containing an olefinically unsaturated alcohol by shifting a double bond to obtain an isomerized olefinically unsaturated alcohol. The method includes providing a cascade of n reactors, where n is an integer of at least 2. The method further includes merging an i-th supplementary stream with an i-th recycle stream to obtain an i-th reactor inlet stream; feeding the i-th reactor inlet stream into the i-th reactor and isomerizing the i-th reactor inlet stream on a heterogeneous isomerization catalyst to produce an i-th effluent stream; and splitting the i-th effluent stream into the i-th recycle stream and an i-th forward stream. In this method, i ranges from 1 to n; where for i=1, the feed constitutes the first supplementary stream, and for i=2 to n, the (i-1)-th forward stream constitutes the (i)-th supplementary stream; and the n-th forward stream is the product stream. This method allows for the isomerization of olefinically unsaturated alcohols with high conversion and improved selectivity.

Description

[0001] This invention relates to a method for isomerizing olefinic unsaturated alcohols, particularly isoprenol.

[0002] Alkenyl unsaturated alcohols, such as isopentenol (3-methyl-3-buten-1-ol), are important chemical intermediates, for example, used in the preparation of terpene-based fragrances such as citral. Therefore, such alkenyl unsaturated alcohols have significant industrial and economic implications.

[0003] WO 2008 / 037693 describes a method for producing 3,7-dimethyl-oct-2,6-dienal (citral), which includes the step of isomerization from 3-methyl-3-buten-1-ol (isopentenol) in the presence of hydrogen and a noble metal catalyst comprising palladium and selenium and / or tellurium to produce 3-methyl-2-buten-1-ol (prenol).

[0004] WO 2009 / 106622 describes a method for isomerizing olefinic unsaturated alcohols on supported noble metal catalysts containing carbon-based supports in an oxygen-containing atmosphere. Palladium and gold are preferred noble metals for isomerizing isopentenols to pentenols.

[0005] Known isomerization methods typically involve recycling the partially isomerized product mixture. Recycling increases the volumetric flow rate of the liquid through the reactor and reduces the concentration of the initial olefinic unsaturated alcohol. Therefore, the conversion per single pass and the adiabatic temperature rise can be limited. However, excessive recycling rates lead to the formation of byproducts. A trade-off exists between achieving an acceptablely high conversion rate and having appropriately high selectivity for the desired isomerized product.

[0006] Some byproducts, such as isoamyl alcohol, cannot be easily separated from unconverted isopentenol because their boiling points are close. When unconverted isopentenol is recycled to isomerization, a purge stream must be provided to prevent the accumulation of isoamyl alcohol. However, this is associated with the loss of valuable substances.

[0007] In view of the above, there is still a need for a method for isomerizing olefinic unsaturated alcohols with high conversion and improved selectivity.

[0008] This invention provides a method for isomerizing a feed containing an olefinically unsaturated alcohol by shifting a double bond to obtain an isomerized olefinically unsaturated alcohol, the method comprising:

[0009] Provide a cascade of n reactors, where n is an integer of at least 2;

[0010] The i-th supplementary flow and the i-th recirculation flow are combined to obtain the i-th reactor inlet flow;

[0011] The inlet stream of reactor i is fed into reactor i, and isomerized on heterogeneous isomerization catalyst to produce effluent stream i.

[0012] Divide the i-th outflow into the i-th recirculation flow and the i-th forward flow; and

[0013] Where i ranges from 1 to n; where for i = 1, the feed constitutes the first supplementary flow, and for i = 2 to n, the (i-1)th forward flow constitutes the (i)th supplementary flow; and

[0014] The nth forward flow is the product flow.

[0015] In a preferred embodiment, n is 2, 3, or 4.

[0016] Therefore, in one embodiment, the present invention relates to a method for isomerizing a feed containing an olefinically unsaturated alcohol by shifting a double bond to obtain an isomerized olefinically unsaturated alcohol, the method comprising:

[0017] A reactor cascade is provided, which includes a first reactor and a second reactor;

[0018] The feed is combined with the first recirculation flow to obtain the first reactor inlet flow;

[0019] The first reactor inlet stream is fed into the first reactor, and the first reactor inlet stream is isomerized on the heterogeneous isomerization catalyst to produce the first effluent stream;

[0020] The first outflow material is divided into a first recirculation flow and a first forward flow;

[0021] The first forward flow is combined with the second recirculation flow to obtain the second reactor inlet flow;

[0022] The inlet stream of the second reactor is fed into the second reactor, and the inlet stream of the second reactor is isomerized on a heterogeneous isomerization catalyst to produce a second effluent stream; and

[0023] The second outflow material is divided into a second recirculation flow and a product flow.

[0024] In another embodiment, the present invention relates to a method for isomerizing a feed containing an olefinically unsaturated alcohol by shifting a double bond to obtain an isomerized olefinically unsaturated alcohol, the method comprising:

[0025] A reactor cascade is provided, comprising a first reactor, a second reactor, and a third reactor;

[0026] The feed is combined with the first recirculation flow to obtain the first reactor inlet flow;

[0027] The first reactor inlet stream is fed into the first reactor, and the first reactor inlet stream is isomerized on the heterogeneous isomerization catalyst to produce the first effluent stream;

[0028] The first outflow material is divided into a first recirculation flow and a first forward flow;

[0029] The first forward flow is combined with the second recirculation flow to obtain the second reactor inlet flow;

[0030] The inlet stream of the second reactor is fed into the second reactor, and the inlet stream of the second reactor is isomerized on the heterogeneous isomerization catalyst to produce the second effluent stream;

[0031] The second outflow material is divided into a second recirculation flow and a second forward flow;

[0032] The second forward flow is combined with the third recirculation flow to obtain the third reactor inlet flow;

[0033] The inlet stream of the third reactor is fed into the third reactor, and the inlet stream of the third reactor is isomerized on the heterogeneous isomerization catalyst to produce the third effluent stream; and

[0034] The third outflow logistics is divided into the third recycling flow and the product flow.

[0035] In yet another embodiment, the present invention relates to a method for isomerizing a feed containing an olefinically unsaturated alcohol by shifting a double bond to obtain an isomerized olefinically unsaturated alcohol, the method comprising:

[0036] A reactor cascade is provided, comprising a first reactor, a second reactor, a third reactor, and a fourth reactor;

[0037] The feed is combined with the first recirculation flow to obtain the first reactor inlet flow;

[0038] The first reactor inlet stream is fed into the first reactor, and the first reactor inlet stream is isomerized on a heterogeneous isomerization catalyst to produce a first effluent stream;

[0039] The first outflow material is divided into a first recirculation flow and a first forward flow;

[0040] The first forward flow is combined with the second recirculation flow to obtain the second reactor inlet flow;

[0041] The inlet stream of the second reactor is fed into the second reactor, and the inlet stream of the second reactor is isomerized on the heterogeneous isomerization catalyst to produce the second effluent stream;

[0042] The second outflow material is divided into a second recirculation flow and a second forward flow;

[0043] The second forward flow is combined with the third recirculation flow to obtain the third reactor inlet flow;

[0044] The inlet stream of the third reactor is fed into the third reactor, and the inlet stream of the third reactor is isomerized on the heterogeneous isomerization catalyst to produce the third effluent stream; and

[0045] The third outflow material is divided into a third recirculation flow and a third forward flow;

[0046] The third forward flow and the fourth recirculation flow are combined to obtain the fourth reactor inlet flow;

[0047] The inlet stream of the fourth reactor is fed into the fourth reactor, and the inlet stream of the fourth reactor is isomerized on a heterogeneous isomerization catalyst to produce a fourth effluent stream; and

[0048] The fourth outflow material is divided into the fourth recycling flow and the product flow.

[0049] The double bond isomerization of olefinic unsaturated alcohols is an equilibrium reaction, and therefore complete conversion cannot be achieved even with prolonged residence time of the reaction mixture in contact with the isomerization catalyst. Furthermore, the formation of undesirable byproducts is observed, particularly with increasing residence time of the reaction mixture. For example, the selectivity for the hydrogenation isomerization of isopentenol to pentenol is typically in the range of about 91% to 94%, with byproducts including isopentenol, isopentenal, and methylbutene.

[0050] While conversion can be increased by extending the residence time of the reaction mixture in the reactor, this typically increases the amount of undesirable byproducts and leads to reduced selectivity of the method.

[0051] It is worth noting that the double bond isomerization of olefinic unsaturated alcohols is typically a moderately exothermic reaction; however, it is accompanied by highly exothermic side reactions. Reactor temperature is typically controlled by cooling the recirculated feed into the reactor. The ratio between the recirculated feed and the feed into the reactor (called the recirculation ratio) is adjusted so that the adiabatic temperature rise is not too large. For effective reactor temperature control, the proportion of recirculated feed in the reactor inlet stream must be sufficiently high.

[0052] In equilibrium reactions, the further the system is from equilibrium, the faster the reaction proceeds. Therefore, "backmixing," resulting from the recycling of a mixture of partially isomerized products near equilibrium, inevitably slows the reaction. It has been found that by performing isomerization in multiple reactors, improved reaction rates, associated with improved selectivity, can be obtained with desired high conversion rates. When multiple reactors are used, the adverse effects of backmixing from individual recycle streams are less pronounced, which in turn allows for increased selectivity.

[0053] In one embodiment, the weight ratio of the i-th recirculated stream to the i-th replenishment stream defines the i-th recirculation ratio, and the first recirculation ratio is greater than each of the subsequent recirculation ratios. Preferably, each i-th recirculated stream is cooled—preferably by indirect heat exchange—after which the i-th recirculated stream is combined with the i-th replenishment stream. Since the concentration of available olefinic unsaturated alcohols is highest in the first reactor inlet stream, the first reactor typically requires a greater degree of cooling and therefore a higher recirculation ratio than subsequent reactors.

[0054] Alkenyl unsaturated alcohols

[0055] Alkenyl unsaturated alcohols and isomerized alkenyl unsaturated alcohols each contain at least one carbon-carbon double bond.

[0056] In the method of the present invention, the double bond of an olefinic unsaturated alcohol is shifted to obtain an isomerized olefinic unsaturated alcohol. In a preferred embodiment, the terminal double bond of the olefinic unsaturated alcohol is shifted to obtain an isomerized olefinic unsaturated alcohol having an internal double bond.

[0057] For example, the olefinically unsaturated alcohol stream can be β,γ-unsaturated alcohol. Preferably, the olefinically unsaturated alcohol is selected from 3-buten-1-ol compounds having formula (I).

[0058] HR 1 C=CR 2 -CHR 3 -CR 4 R 5 -OH(I)

[0059] in

[0060] R 1 R 2 and R 3 Independently selected from hydrogen and C1-C 12 -alkyl, the C1-C 12 -Alkyl groups can be converted by OH, OR 6 (where R) 6 It is C1-C 12 -alkyl), COOH or halogen substitution;

[0061] R 4 and R 5 Independently selected from hydrogen and C1-C 12 -alkyl;

[0062] Where R 2 and R 5 Together with the carbon atoms located between them, they can form alicyclic rings.

[0063] The isomerized olefinic unsaturated alcohol is preferably a 2-buten-1-ol compound having formula (II).

[0064] H2R 1 CR 2 C=CR 3 -CR 4 R 5 -OH(II)

[0065] Where R 1 To R 5 As defined in equation (I).

[0066] Each C1-C 12 -The alkyl group is preferably selected independently from C1-C6-alkyl, especially C1-C3-alkyl.

[0067] R 1 To R 5 Preferably selected independently from hydrogen and C1-C 12 -alkyl, particularly selected from hydrogen and C1-C6-alkyl, such as selected from hydrogen and C1-C3 alkyl, most preferably selected from hydrogen and methyl.

[0068] In a preferred embodiment, the olefinic unsaturated alcohol is 3-methylbut-3-en-1-ol (isopentenol), and the isomerized olefinic unsaturated alcohol is 3-methylbut-2-en-1-ol (pentenol).

[0069] Heterogeneity

[0070] In each reactor, the isomerization of the reactor inlet stream on at least one heterogeneous isomerizing catalyst can be carried out by any suitable method known to those skilled in the art.

[0071] In one embodiment, isomerizing the i-th reactor inlet stream over a heterogeneous isomerization catalyst includes passing the stream through a fixed bed of catalyst.

[0072] For example, according to WO 2009 / 106622 or WO 2017 / 157897, isomerization can be carried out in an oxygen-containing atmosphere on a supported noble metal catalyst containing a carbon-based support. Palladium is preferred for isomerizing isopentenol to pentenol.

[0073] However, isomerization is preferably carried out in the presence of hydrogen. This method can be called hydroisomerization. In this case, isomerization is preferably carried out on a noble metal catalyst, particularly a supported noble metal catalyst. In one embodiment, the noble metal catalyst is a fixed-bed catalyst.

[0074] In a preferred embodiment, the precious metal includes palladium. The catalyst may contain 0.1% to 2.0% by weight, preferably 0.2% to 0.8% by weight, and particularly 0.4% to 0.6% by weight, of palladium based on the total weight of the catalyst.

[0075] It is worth noting that when pure palladium is used in the presence of hydrogen, significant hydrogenation of the double bonds in the compound may occur, potentially forming saturated products. Furthermore, low-boiling-point compounds such as hydrocarbons and aldehydes may form as byproducts, for example, through hydrogenation and isomerization. Hydrogenation of the double bonds is undesirable, especially since separating the hydrogenation byproducts and isomerization products by distillation is difficult.

[0076] In a preferred embodiment, the noble metal catalyst comprises palladium, and further comprises selenium, tellurium, or a mixture thereof.

[0077] In one embodiment, the precious metal catalyst comprises 0.01% to 0.2%, preferably 0.02% to 0.08%, particularly 0.04% to 0.06% by weight of selenium, tellurium, or mixtures thereof, based on the total weight of the catalyst.

[0078] The BET surface area of ​​noble metal catalysts can range from 80 to 380 m². 2 / g, preferably 100 to 150 m 2 / g, especially 110 to 130 m 2 The BET surface area is within the range of / g. It can be determined by nitrogen adsorption according to DIN 66131.

[0079] In one embodiment, the pore volume of the noble metal catalyst can range from 0.6 to 0.95 cm² in the pore size range of 3 nm to 300 µm. 3 / g, preferably 0.8 to 0.9 cm 3 / g, especially 0.8 to 0.85 cm 3 The pore volume is 1 / g, wherein 80% to 95%, preferably 85% to 93%, of this pore volume is in the pore size range of 10 to 100 nm. The pore volume can be determined by the Hg porosity determination method.

[0080] In addition to the active components mentioned, other metals may be present on the catalyst in small amounts. Preferably, only palladium, selenium, and / or tellurium, especially only palladium and selenium, are present on the support.

[0081] In one embodiment, the noble metal catalyst is a supported noble metal catalyst, and the support is preferably selected from refractory materials, such as silica, alumina, and mixtures thereof. In a particularly preferred embodiment, the support is a silica support containing at least 90 wt.% silica, preferably at least 98 wt.% silica.

[0082] Further details regarding suitable catalysts and their production are described, for example, in EP 0 841 090 A2.

[0083] Ultimately, isomerization is achieved, in which the double bond migrates within the molecule of the olefinic unsaturated alcohol.

[0084] The reactor used in the method of the present invention is not particularly limited. In one embodiment, each isomerization is carried out in an upflow mode in a tubular reactor containing a fixed-bed catalyst as described above. The tubular reactor preferably includes a gas distributor in its lower section, for example in the form of a filter plate, static mixer, or nozzle. The gas distributor is used to feed hydrogen, preferably distributing the hydrogen uniformly across the reactor cross-section. The reactor inlet stream is introduced into the reactor from below. Hydrogen can be injected into the reactor inlet stream or introduced into the reactor at the bottom or lower region of the reactor.

[0085] The conversion level gradually increases between reactors. The term "conversion level" is intended to refer to the (cumulative) conversion rate achieved after reactor i relative to the feed (which is considered to have a conversion level of 0%). The final conversion level achieved relative to the feed in the effluent stream from reactor n (and therefore in the product stream) is preferably in the range of 30% to 70%, more preferably 34% to 68%, and particularly 36% to 65%.

[0086] Typically, the contributions of each reactor to the final conversion level are not equal, and preferably the first reactor makes the greatest contribution to the conversion level.

[0087] In a cascade of two reactors, the conversion level reached after the first reactor is suitably in the range of 20% to 38%, and the conversion level reached after the second reactor is suitably in the range of 34% to 60%.

[0088] In a cascade of three reactors, the conversion level reached after the first reactor is suitably in the range of 16% to 31%, the conversion level reached after the second reactor is suitably in the range of 29% to 52%, and the conversion level reached after the third reactor is suitably in the range of 38% to 65%.

[0089] In a cascade of four reactors, the conversion level reached after the first reactor is suitably in the range of 14% to 27%, the conversion level reached after the second reactor is suitably in the range of 25% to 47%, the conversion level reached after the third reactor is suitably in the range of 35% to 61%, and the conversion level reached after the fourth reactor is suitably in the range of 42% to 70%.

[0090] The introduction of hydrogen is set to vary with temperature and total pressure, thereby maintaining a hydrogen partial pressure of 0.5 to 5 bar, preferably 0.5 to 2 bar, and particularly 0.6 to 1 bar. Hydrogen that has passed through the reactor can be discharged as waste gas after condensation of low-boiling-point substances, or it can be recycled back into the method.

[0091] Isomerization is carried out at temperatures ranging from 50°C to 150°C, preferably from 50°C to 120°C, and more preferably from 80°C to 100°C. Depending on the starting compound used, a catalyst space rate of 0.5 to 5 L / L (catalyst) × h, preferably 0.5 to 1.5 L / L (catalyst) × h, is employed.

[0092] Isomerization can be carried out with or without an inert organic solvent. Suitable inert organic solvents include, for example, ethers such as diethyl ether, dioxane, or tetrahydrofuran; alcohols such as ethanol or isobutanol; aromatic or aliphatic hydrocarbons such as heptane or benzene; or mixtures thereof. Preferably, the method is carried out in the absence of an inert organic solvent.

[0093] The product stream obtained from the nth reactor contains isomerized olefinic unsaturated alcohols and unconverted olefinic unsaturated alcohols. In one embodiment, the method includes separating the stream of isomerized olefinic unsaturated alcohols and the stream of unconverted olefinic unsaturated alcohols from the product stream.

[0094] For this purpose, the reaction product mixture containing the isomerized olefinic unsaturated alcohol obtained from the equilibrium reaction is preferably post-treated directly by distillation. Separating the second olefinic unsaturated alcohol from the first olefinic unsaturated alcohol by distillation and returning the first olefinic unsaturated alcohol increases the economic feasibility of the isomerization method. The separation by distillation is preferably carried out continuously in a suitable apparatus, particularly in a partition wall column.

[0095] In one embodiment, the method includes recycling at least partially a stream of unconverted olefinic unsaturated alcohols back into the feed.

[0096] In another embodiment, the method includes subjecting a stream of unconverted olefinic unsaturated alcohols at least partially to oxidative dehydrogenation to produce olefinic unsaturated aldehydes. Specifically, unconverted isopentenol can be at least partially oxidized to produce isopentenal. This is particularly important because isopentenol (an undesirable byproduct of isopentenol isomerization) can only be separated from isopentenol with great difficulty due to their similar boiling points, thus subjecting unconverted isopentenol to oxidative dehydrogenation.

[0097] The oxidative dehydrogenation of unconverted olefinic unsaturated alcohols, particularly isopentenols, typically involves contacting a reaction stream containing the unconverted olefinic unsaturated alcohol, particularly a gaseous reaction stream, with at least one heterogeneous oxidative dehydrogenation catalyst, particularly at least one silver-containing heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen. This at least one heterogeneous catalyst may consist of an inert support having a smooth surface containing a silver active layer. Alternatively, a solid (all-metallic) silver body may be used.

[0098] The present invention will be further described with reference to the following figures and examples.

[0099] Figure 1 A cascade of two reactors suitable for carrying out the present invention is shown.

[0100] according to Figure 1 The reactor cascade includes a first reactor 101 and a second reactor 102.

[0101] The feed 103, containing an olefinic unsaturated alcohol, is combined with a first recirculation stream 104 to obtain a first reactor inlet stream 105. The first reactor inlet stream 105 and a hydrogen stream 106 are fed into a first reactor 101, wherein the first reactor inlet stream 105 isomerizes on a fixed bed of a heterogeneous isomerization catalyst to produce a first effluent stream 107, which is taken out as a side draw. A gaseous effluent stream 108 is taken out via the top of the first reactor 101. The first effluent stream 107 is split into a first recirculation stream 104 and a first forward stream 109.

[0102] The first forward flow 109 is combined with the second recirculation flow 110 to obtain the second reactor inlet flow 111. The second reactor inlet flow 111 and the hydrogen flow 112 are fed into the second reactor 102, wherein the second reactor inlet flow 111 isomerizes on a fixed bed of heterogeneous isomerization catalyst to produce a second effluent flow 113, which is taken out as a side feed. A gaseous effluent flow 114 is taken out via the top of the second reactor 102. The second effluent flow 113 is split into a second recirculation flow 100 and a product flow 115.

[0103] Example 1

[0104] The effect of the number of reactors in a cascade on the selectivity of pentenol in isopentenol isomerization was examined using the software CHEMASIM. This set of parameters is based on operating data and equilibrium measurements from an apparatus using Pd / Se on SiO2 as the isomerization catalyst in the presence of hydrogen.

[0105] Examine setups utilizing one, two, and three reactors respectively. Each reactor is equipped with a recirculation line through which a sub-stream of the effluent is recirculated back to the reactor inlet. The recirculation ratio, along with the reactor temperature and adiabatic temperature rise, is indicated in the table below.

[0106] Each reactor includes a fixed bed of isomerization catalyst. The total isopentenyl alcohol conversion is set at 50%, with an isopentenyl alcohol feed rate of 6.8 t / h and a final distillation pentenyl alcohol flow rate of 3.1 t / h.

[0107]

[0108] Temperature difference between the inlet flow and the effluent flow of reactor i

[0109] The weight ratio of the i-th recirculated flow to the i-th replenished flow

[0110] Clearly, reactor cascades with multiple reactors exhibit increased selectivity at the same conversion rate.

Claims

1. A method for isomerizing a feed containing an olefinically unsaturated alcohol by shifting a double bond to obtain an isomerized olefinically unsaturated alcohol, the method comprising: Provide a cascade of n reactors, where n is an integer of at least 2; The i-th supplementary flow and the i-th recirculation flow are combined to obtain the i-th reactor inlet flow; The inlet stream of the i-th reactor is fed into the i-th reactor, and the inlet stream of the i-th reactor is isomerized on the heterogeneous isomerization catalyst to produce the i-th effluent stream; as well as The i-th outflow material is divided into the i-th recirculation flow and the i-th forward flow; Where i ranges from 1 to n; For i = 1, the feed constitutes the first supplementary flow, and for i = 2 to n, the (i-1)th forward flow constitutes the (i)th supplementary flow; and The nth forward flow is the product flow.

2. The method according to claim 1, wherein, The weight ratio of the i-th recycle stream to the i-th supplementary stream defines the i-th recycle ratio, and the first recycle ratio is greater than each of the subsequent recycle ratios.

3. The method according to claim 1 or 2, wherein the method comprises cooling each i-th recirculation stream—preferably by indirect heat exchange—and then merging the i-th recirculation stream with the i-th supplementary stream.

4. The method according to any one of the preceding claims, wherein, This isomerization occurs in the presence of hydrogen.

5. The method according to any one of the preceding claims, wherein, The inlet flow of these reactors is liquid.

6. The method according to any one of the preceding claims, wherein, This heterogeneous isomerization catalyst contains noble metals.

7. The method according to claim 6, wherein, This precious metal includes palladium.

8. The method according to claim 7, wherein, The heterogeneous isomerization catalyst contains selenium, tellurium, or a mixture thereof.

9. The method according to any one of claims 6 to 8, wherein, The precious metal is loaded on a carrier, which is preferably a refractory material, especially silicon dioxide.

10. The method according to any one of the preceding claims, the method comprising separating a stream of isomerized olefinic unsaturated alcohols and a stream of unconverted olefinic unsaturated alcohols from the product stream.

11. The method of claim 10, wherein the method comprises at least partially recycling the stream of unconverted olefinic unsaturated alcohol back into the feed.

12. The method of claim 10, further comprising subjecting the stream of the unconverted olefinic unsaturated alcohol to at least partially oxidative dehydrogenation to produce an olefinic unsaturated aldehyde.

13. The method according to any one of claims 10 to 12, wherein, Separating the stream of isomerized olefinic unsaturated alcohols and the stream of unconverted olefinic unsaturated alcohols from the product stream involves distilling the product stream, preferably in a separatory wall column.

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