[TITLE OF Invention]

Method and Apparatus for Separating Liquid

[TECHNICAL FIELD]

[0001]

The present invention relates to a method for separating liquid, particularly for separating liquid from a solid-liquid mixture containing solid and liquid, especially solid particles. Specifically, the present invention relates to a method for recovering liquid from a solid-liquid mixture comprising solid catalyst particles and a liquid phase containing reaction raw materials and a reaction product which mixture is obtained by carrying out a liquid-phase reaction in the presence of the solid catalyst particles.

[BACKGROUND ART]

[0002]

As a method for recovering liquid from a solid-liquid mixture containing a reaction product obtained by carrying out a liquid-phase reaction in the presence of a solid catalyst as mentioned above, there is known a method for separating liquid from a solid-liquid mixture by supplying the solid-liquid mixture to a liquid cyclone so as to centrifugally sediment coarse particles contained in the solid-liquid mixture while discharging the coarse particles together with liquid, followed by subjecting a mixture containing fine particles obtained as a residue from the above centrifugally precipitating to a cross-flow filtration (see Patent Document 1 below).

[CITATION LIST]

[PATENT LITERATURE]

[0003]

[Patent Documents 1] JP08-332327 A

[SUMMARY OF INVENTION]

[TECHNICAL PROBLEM]

[0004]

In the methods of separating liquid as mentioned above, there is a problem that filter materials are required to be frequency cleaned, the apparatus for such method cannot be run for a long period of time, and the productivity decreases. Thus, the problem to be solved by the present invention is to provide a novel method for separating liquid for a long period of time.

[SOLTION TO PROBLEM]

[0005] The present invention provides a method for separating liquid from a solid-liquid mixture containing solid particles and liquid. The method is characterized in that it comprises a classification step of subjecting said solid- liquid mixture to wet classification so as to obtain a mixture containing solid particles each having a particle size larger than a predetermined particle size and the liquid, and a filtration step of subjecting the above obtained mixture to a cross-flow filtration so as to recover the liquid.

[0006]

In the present specification, the classification step means a step of subjecting a solid-liquid mixture containing liquid and solid particles to a field, wherein the gravity or a centrifugal force being applied, thereby dividing (or separating) the solid particles having various particle sizes into a coarser particle mixture, which contains a group of solid particles each having a particle size larger than a predetermined particle size (which is also referred to as "coarser particles") with the liquid and a finer particle mixture, which contains a group of remaining solid particles, usually a group of solid particles each having a particle size smaller than the predetermined particle size (which is also referred to as "finer particles") with the liquid by means of the difference of the gravities or the centrifugal forces applied to the solid particles. That is, it is a step of carrying out a so-called gravity classification or centrifugal- force classification. Usually, the predetermined particle size is determined by properties of the liquid and the solid particles that constitute the mixture as well as operation conditions and the device specifications for the classification step. Such a classification step can be carried out, for example using a liquid cyclone, a centrifugal separator, a super decanter, a gas floatation separation column and the like.

[0007]

Moreover, in the present specification, the filtration step is carried out by means of a cross-flow filtration. The present filtration step means a step of flowing the mixture to be filtered, which is obtained from the above classification step, substantially in parallel to a filter medium surface while using a pressure difference across a filter medium, thereby dividing (or separating) the mixture into a filtrate fraction obtained by passing through the filter member, which fraction does ^' not contain the solid particles each having a particle size larger than a predetermined particle size and a retentate fraction as a remainder, which fraction does not pass through the filter medium. Herein, the predetermined particle size is usually determined by the pore size of the filter member being used.

[0008] The solid-liquid mixture, which can be applied to the liquid separation method of the present invention described above, is not particularly limited as long as it contains solid particles and liquid. For example, when liquid is recovered from a reaction liquid containing a solvent, a reaction product and a catalyst which is obtained by carrying out a reaction in the presence of solid particles as the catalyst in a liquid phase as the solvent, the method of the present invention can be suitably used. Such a reaction includes a catalyst suspension reaction, a catalyst suspension fluidized bed reaction and the like.

[0009] ⁰

Furthermore, the present invention provides a liquid separation apparatus, which is suitable for carrying out the liquid separation method which is described above and below. Such a liquid separation apparatus comprises a wet classification device and a cross-flow filtration device, which apparatus is constituted so that the mixture containing the liquid and the solid particles each having a particle size larger than the predetermined particle size, which mixture is resulted from the wet classification device, in other word, the coarser particle mixture is filtrated by the cross-flow filtration, device. Specifically, the liquid separation apparatus of the present invention comprises a conduit system which supplies the coarser particle mixture to the cross-flow filtration device.

[ADVABTAGEOUS EFFECTS OF INVENTION]

[0010]

With the liquid separation method according to the present invention, the coarser particle mixture containing the coarser particles, which mixture is resulted from the wet classification, is subjected to the cross-flow filtration and the particle sizes of the solid particles contained in the mixture are relatively larger compared with those in the conventionally performed liquid separation method, so that the filter medium becomes to be prevented from being easily clogged and the cleaning frequency of the filter medium to be used in the filtration step reduces, thereby the filtration step becomes be able to run for a longer period of time and/or the filtration treatment itself becomes to be easily performed, which makes the period of time to be required for the filtration step shorten (thereby the filtration device becomes compact in its size), and the productivity of the filtering step increases.

[BRIEF DESCRIPTION OF THE DRAWINGS]

[0011]

[Fig. 1] Fig. 1 shows a schematic view of a production process of propylene oxide wherein one preferable embodiment of the liquid separation method according to the present invention is adopted.

[Fig. 2] Fig. 2 shows a schematic view of a production process of propylene oxide wherein another preferable embodiment of the liquid separation method according to the present invention is adopted.

[EMBODIMENTS FOR CARRY OUT THE INVENTION]

[0012]

The liquid separation method of the present invention is useful when liquid is separated from a solid-liquid mixture containing the solid particles and the liquid. Hereinafter, the liquid separation method of the present invention will be illustrated in detail with reference to examples of a liquid phase reaction for producing propylene oxide by reacting reaction raw materials in a liquid phase in the presence of solid particles as a catalyst while referring to the accompanied drawings.

[0013]

It is noted that the liquid phase reaction for producing propylene oxide by the reacting reaction raw materials in the liquid phase in the presence of the solid particles as the catalyst itself is already known. For details of the reaction, for example, JP2009-256301 A1 may be referred to. Briefly explaining, a mixture gas containing oxygen, hydrogen and optionally nitrogen for dilution as well as propylene are supplied to a mixture of a mixed solvent of acetonitrile/water as well as palladium catalyst and titanium catalyst as the solid particles to cause them to react, thereby propylene oxide is produced.

[0014]

The above mentioned production process comprises ί reaction process and a post-treatment process fo separating acetonitrile/water as liquid from a reaction liquic as the solid-liquid mixture containing a reaction product unreacted raw materials and the catalysts.

[0015]

In the reaction process, solid particles as the catalyst are preliminarily suspended in acetonitrile/water in a reactor, into which the mixed gas and propylene are introduced and then they are reacted under the reaction conditions. The reaction liquid as the solid-liquid mixture resulted from the reaction process which contains the reaction product, the unreacted raw materials and the catalysts is discharged from the reactor and fed to the post-treatment process, wherein the liquid separation method of the present invention is carried out.

[0016]

The post-treatment process comprises the classification step and the filtration step. In the classification step, the reaction liquid is subjected to the wet classification, thereby obtaining a mixture comprising acetonitrile/water as the liquid and solid particles as the catalyst each having a larger particle size than a predetermined particle size and. As a result, from the above mixture, a mixture which contains acetonitrile/water as the liquid and the solid particles as the catalyst each having a smaller particle size than the predetermined particle size is separated.

[0017]

In the above case, the catalysts are in a condition of contacting with the reaction raw materials such as propylene during the filtration step in the liquid separation method of the present invention, however the period of time required for the filtration step is shortened as mentioned above so that side reactions caused by such contacting condition may be prevented. In addition, wet classification is particularly preferably carried out by the centrifugal- force classification using the liquid cyclone or the like in view of being capable of shortening the residence time. In such case, the side reactions may be further suppressed.

[0018]

For simplicity, the "solid particles as the catalyst each having a larger particle size than the predetermined particle size" are also referred to as "coarser particles" and the mixture containing the coarser particles and acetonitrile/water is also referred to as the "coarser particle mixture." In addition, the "solid particles as the catalyst each having a smaller particle size than the predetermined particle size" are also referred to as the "finer particles" and the mixture containing the finer particles and acetonitrile/water is also referred to as a "finer particle mixture."

[0019]

Next, the coarser particle mixture resulted from the classification step is sent to the filtration step where it is subjected to the cross-flow filtration treatment, thereby acetonitrile/water as the liquid is recovered from the coarser particle mixture.

[0020]

Fig. 1 schematically shows a process of the production of propylene oxide wherein a preferable embodiment of the present liquid separation method is employed. In the reaction process, the catalyst solid particles are preliminarily suspended in acetonitrile/water in an autoclave 10, into which a mixed gas 12 containing oxygen and hydrogen and nitrogen for dilution as well as propylene 14 is supplied and they are caused to react with each other under the reaction conditions. A reaction liquid 16 resulted from the above reaction is discharged from the autoclave 10, and sent to the post-treatment process wherein the liquid separation method of the present invention is carried out. The post-treatment process comprises a liquid cyclone 18 for carrying out the classification step and a cross-flow filtration device 20 for carrying out the filtration step.

[0021]

In the classification step, the reaction liquid 16 taken out from the autoclave 10 is supplied through a pump 22 to a liquid cyclone 18 wherein the reaction liquid is subjected to the wet classification. As a result, on the one hand, the coarser particle mixture containing the coarser particles and acetonitrile/water as the liquid is obtained as the coarser particle slurry 26, and on the other hand, the remainder, which is usually the finer particle mixture containing finer particles and acetonitrile/water as the liquid is obtained as the finer particle slurry 24.

[0022]

Upon the production of propylene oxide, the predetermined size of the finer particles and the coarser particles may be determined by the skilled person in the art based on the device specifications and the operation conditions of the liquid cyclone as well as the properties of the reaction liquid (for example, a density, a content and particle size distribution of the solid particles, a ratio of acetonitrile/water, a concentration of the reaction product, and concentrations of the unreacted raw materials).

[0023]

Next, the obtained coarser particle slurry 26 is supplied to the cross-flow filtration device 20, where the coarser particle slurry 26 is subjected to the cross-flow filtration. As a result, the coarser particle slurry is separated into a retentate fraction 28 and a filtrate fraction 30 wherein the retenate fraction 28 contains acetonitrile/water and the solid particles each having a particle size larger than the predetermined particle size, which is determined according to the device specifications and the operation conditions of the filtration device 20 as well as the properties of the coarser particle slurry; and the filtrate fraction 30 comprises acetonitrile/water preferably with substantially no solid particle. Upon the production of propylene oxide, the flow rate of the filtrate fraction in the cross-flow filtration may be optionally determined by the skilled person in the art based on the device specifications and the operation conditions for the cross-flow filtration device as well as the properties of the mixture resulted from the above classification step (for example, a density, particle size distribution and a shape of the solid particles, a viscosity of the liquid).

[0024]

The retentate fraction 28 is recycled to the autoclave 10 and re-used as the catalyst for the reaction since the fraction 28 contains the solid particles which do not pass through the filter medium of the filtration device 20. In the shown embodiment, the finer particle mixture 24 which is resulted from the liquid cyclone 18, is also recycled to the autoclave 10 and re-used as the catalyst for the reaction, since the mixture 24 also contains the solid particles. It is noted that the conduits in the apparatus are arranged so that the retentate fraction 28 and the finer particle mixture 24 are merged into the line 32 for supplying make-up acetonitrile/water. Since the filtrate fraction 30 contains the reaction product and the unreacted raw materials, the fraction 30 may be subjected to a purification treatment step wherein the reaction product is separated.

[0025]

The mixed gas 12, which is supplied to the autoclave 10, is taken into a liquid phase in which the solid particles as the catalyst are suspended by a stirrer 34, and reacts with propylene. As illustrated in Fig. 1, the gaseous phase resulted from the autoclave 10 is recycled through the circulating line 36 and supplied again to the autoclave 10 together with the newly fed mixed gas 12.

[0026]

Fig. 2 schematically shows a process of the production of propylene oxide wherein another preferable embodiment of the present liquid separation method is employed. The reaction process in this case is substantially the same as the reaction process which is explained in the above with reference to Fig. 1. That is, the catalyst solid particles are preliminarily suspended in acetonitrile/water in an autoclave 10, into which a mixed gas 12 containing oxygen and hydrogen and nitrogen for dilution as well as propylene 14 is supplied and they are caused to react with each other under the reaction conditions. A reaction liquid 16 resulted from the above reaction is discharged from the autoclave 10, and sent to the post-treatment process wherein the liquid separation method of the present invention is carried out.

[0027]

The post-treatment process comprises a gas flotation separation device 38, where the wet classification step is carried out, and the cross-flow filtration device 20 where the filtration step is carried out. The gas floatation separation device 38 comprises a separation column 40 and a sparger 42 located at around the bottom thereof. The upper part of the separation column 40 is in fluid communication with the gaseous phase in the autoclave 10 through an upper connecting tube 44, and the lower part of the separation column 40 is in fluid communication with the liquid phase in the autoclave 10 through an lower connecting tube 46. As a result, the reaction liquid in the autoclave 10 enters the separation column 40 through the lower connecting tube 46.

[0028]

The gaseous phase in the separation column 40 is supplied to the separation column 40 through the sparger 42 to form air bubbles and ascends through the separation column 40 together with the liquid phase within the separation column 40. At this time, the particles having particle size larger than the predetermined particle size, i.e., the coarser particles fail to get into the ascending stream and descend through the separation column 40, thereby sediment at the bottom. Such coarser particles are discharged as the coarser particle slurry 48 from the bottom portion of the separation column 40 and supplied to the cross-flow filtration device 20.

[0029]

The discharged coarser particle slurry 48 is then subjected to the filtration treatment, in which it is separated into the retentate fraction and the filtrate fraction 30 as with the same manner as explained above with reference to Fig. 1. That is, the filtrate fraction 30 is recovered, thereby the filtrate fraction as the liquid is separated from the reaction I iq u id .

[0030]

It is noted that in the embodiment shown in Fig. 2, a portion of the gaseous phase in the autoclave 10 is sent to the separation column 40 through the upper connecting tube 44, and a portion thereof is supplied to the sparger 42 from the upper part of the separation column 40 and is used for the floatation separation, and the remainder is recycled to the autoclave 10.

[EXPLANATION OF REFERENCE NUMERALS]

[0031]

10 - autoclave

12 - mixed gas

14 - propylene

16 - reaction liquid

18 - liquid cyclone

20 - cross-flow filtration device

22 - pump

24 - finer particle slurry

26 - coarser particle slurry

28 - retentate fraction

30 - filtrate fraction

32 - makeup

34 - stirrer

36 - circulating line

38 - gas flotation separation device

40 - separation column - sparger

- upper connecting tube - lower connecting tube - coarser particle slurry