WO2013132047A1

WO2013132047A1 - Process for quenching a stream comprising essentially olefins and steam

Info

Publication number: WO2013132047A1
Application number: PCT/EP2013/054682
Authority: WO
Inventors: Jean-Pierre Thoret Bauchet; Eric Romers; Babua DAS; Laurent LEMINEUR; Wolfgang Garcia; Gaëtan MARY
Original assignee: Total Research & Technology Feluy
Priority date: 2012-03-09
Filing date: 2013-03-08
Publication date: 2013-09-12

Abstract

The present invention relates to a process for quenching a gas stream (A) comprising essentially (i) olefins and steam, (ii) a minor part of oxygenates and hydrocarbons and (iii) catalyst fines said process comprising : a) sending stream (A) to a quench column, fed with quenching water, to produce • an overhead gas stream consisting essentially of the olefins and the light hydrocarbons of stream (A) and • a water bottom stream (B) comprising the essential part of the oxygenates of stream (A), the catalyst fines of stream (A) and the essential part of the hydrocarbons of stream (A) which are liquid at the bottoms temperature and pressure, b) separating the catalyst fines from stream (B) to produce a stream having an enhanced catalyst fines content and a stream (C) having a reduced catalyst fines content, c) optionally recovering all or a part of stream (C), cooling it and recycling it to the quenching water inlet, Wherein, the quenching water is fed in the upper part of the quench column and dispersed in said column by spray nozzles, optionally there is no water level in the bottom of the quench column, optionally there is essentially no packing or trays between the spray nozzles and the bottom, optionally the quench tower bottom is sloped to drain quickly all liquid.

Description

PROCESS FOR QUENCHING A STREAM COMPRISING ESSENTIALLY

OLEFINS AND STEAM

[Field of the invention]

The present invention is a process for quenching a stream comprising essentially olefins and steam.

Olefins are traditionally produced from petroleum feedstocks by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s), such as ethylene and/or propylene, from a variety of hydrocarbon feedstocks. Ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds.

The limited supply and increasing cost of crude oil has prompted the search for alternative processes for producing hydrocarbon products. The MTO process produces light olefins such as ethylene and propylene as well as heavy hydrocarbons such as butenes. Said MTO process is the conversion of methanol or dimethylether by contact with a molecular sieve. The interest in the methanol to olefin (MTO) process is based on the fact that methanol can be obtained from coal or natural gas by the production of synthesis gas which is then processed to produce methanol.

The effluent produced by a MTO process is a complex mixture comprising the desired light olefins, unconverted oxygenates, by-product oxygenates, heavier hydrocarbons and large amounts of water. The separation and purification of this mixture to recover the light olefins and other valuable by-products is important to the overall efficiency and cost effectiveness of the process. Olefins can also be produced by dehydration of the corresponding alcohol. Ethanol can be obtained by fermentation of carbohydrates. Made up of organic matter from living organisms, biomass is the world's leading renewable energy source. The effluent produced by the ethanol dehydration comprises essentially unconverted ethanol, water, ethylene and acetaldehyde.

It means that both the MTO reactor and the alcohol dehydration reactor produce a stream comprising essentially olefins, steam and minor amounts of unconverted oxygenates, by-produced oxygenates and hydrocarbons. Said stream can comprise catalyst fines. Said stream has to be fractionated and purified to produce olefins. Said stream is usually at a temperature around 200°C to 500°C.

The output of the MTO or the alcohol dehydration reactor is sent to a quench column fed with cold water to recover an overhead olefins stream and a bottom water stream comprising the catalyst fines if any. A part of the water recovered at the bottoms is cooled and recycled on top of the quench column. Quench columns have already been described in the prior art.

[Background of the invention]

US 7,273,961 describes a process for quenching a reactor effluent stream. The reactor effluent stream comprises water, olefin product, and methanol and is further entrained with catalyst fines. The process removes water, catalyst fines, and methanol. In this prior art "... The catalyst fines are washed from the reactor effluent stream into the first liquid fraction. The mixture of gas, liquid and solid leaves the quench fitting through the outlet 18 into the first settling vessel 22. The first settling vessel 22 is configured to (1 ) separate the first liquid fraction and catalyst fines from the first effluent stream and (2) cause settling, or at least partial settling, of the catalyst fines that are entrained in the first liquid fraction. The first effluent stream passes through passage 44. The catalyst fines in the first liquid fraction settle into the lower part of the first settling vessel 22 into a boot 24 to improve the settling of the catalyst fines." (see col 13 lines 33+) Also in this prior art "...The second effluent stream travels from the bottom end to the top end. As it moves upward, the second effluent stream passes through a quench packing 68 to cause intimate contact between the second effluent stream and the second quench medium. .." (see col 15 lines 50+).

US 7,626,067 describes a quench system in an MTO process. At col 10 lines 4+ is mentioned ".. The quench tower typically includes internal elements to facilitate intimate contacting of the quench medium with the reactor effluent stream or portions thereof, including liquid distributors and contacting devices such as baffles, trays or structured packing. Intimate contacting with a liquid quench medium facilitates drawing catalyst fines out of the reactor effluent stream, into a free-flowing, dilute liquid phase and away from at least a portion of the olefins in a gaseous state WO 02 00579 describes an MTO process with a quench system; At page 17 lines 26+ is mentioned "...In quench tower 13 oxygenate conversion product stream 12 contacts directly with a quench medium consisting essentially of water at an initial temperature over a series of suitable contacting devices." US 7,396,513 relates to an apparatus for converting oxygenates to olefins which comprises:

• a fluidized bed reactor for contacting a feedstock comprising oxygenate with a catalyst comprising a molecular sieve under conditions effective to produce a deactivated catalyst having carbonaceous deposits and a product comprising said olefins;

• a separator for separating said deactivated catalyst from said product to provide a separated vaporous product which contains catalyst fines;

• a quench tower for quenching said separated vaporous product with a liquid medium containing water and catalyst fines, in an amount sufficient for forming a light product fraction comprising light olefins and catalyst fines and a heavy product fraction comprising water, heavier hydrocarbons and catalyst fines;

• a treater for treating said light product fraction by contacting with a liquid oxygenate substantially free of catalyst fines to provide a light product fraction having reduced catalyst fines content and a liquid fraction of increased fines content;

• a compressor for compressing said light product fraction having reduced catalyst fines content; and

• a recovery train for recovering said light olefins from said compressed light product fraction.

At Col 17 lines 6+ is mentioned "... Positioned above chimney tray 36 is a bed 39 providing surface area for vapor-liquid contacting, which bed can comprise trays or suitable irregular or regular packing as is known to those of skill in the art. Above the bed is an inlet for introducing liquid substantially free of catalyst fines via a liquid distributor 44 fed from line 46 which in turn can be fed by one or more suitable sources of liquid substantially free of catalyst fines...".

US 2009/0325783 discloses a method for recovering catalyst in an oxygenate to olefin process including removing a quench tower bottom stream containing catalyst from a quench tower and passing the catalyst containing stream to a drying chamber in order to dry it.

US 2007/0197845 describes a process for limiting the loss of catalyst particles through olefin product streams and regenerator flue gas stream exiting the reaction system with a removing of the catalyst particles from the reactor using a water stream and from the regenerator.

US 2004/0064006 describes a process for removing catalyst fines from an effluent stream in an oxygenate to olefin process, with the catalyst fines being separated from the effluent stream. US 2004/0152939 describes a process for quenching a reactor effluent stream comprising water, olefin and methanol. The process removes water, catalyst fines and methanol.

US 2006/01491 1 1 describes a process for converting oxygenate to olefins with removal of catalyst fines from a quenched vaporous effluent by contacting with a liquid low in catalyst fines content. US 2005/0065390 discloses a process for cleaning and using by product water from an oxygenate to olefin process to satisfy the water requirement of the oxygenate to olefin process.

The presence of a packing in the quench column may cause a serious fouling of the column. It has now been discovered to use a quench tower having no internals such as packing or trays to avoid any low turbulence area where catalyst particles could settle. The quench water at the top is dispersed by spray nozzles. Use of spray nozzles allows achieving a desired atomization of the liquid used to quench. The evaporation of the part of this quench liquid can remove the excess heat.

The Quench Tower bottom has to be a high turbulence area to avoid any catalyst settling.

Advantageously the quench column comprises one or more of the following features:

· No level of liquid in that bottom,

• The bottom is sloped to drain rapidly all liquid,

• A part of the bottom liquid is recycled directly in the bottom to wash all deposited solid, optionally in the upper part of the sloped bottom and optionally tangentially injected inside the column • The water from the bottom is separated from catalyst particles before partial use as Quench Water.

[Brief summary of the invention]

The present invention is a process for quenching a gas stream (A) comprising essentially (i) olefins and steam, (ii) a minor part of oxygenates and hydrocarbons and (iii) catalyst fines said process comprising :

a) sending stream (A) to a quench column, fed with aqueous quenching liquid, preferably water, to produce

• an overhead gas stream consisting essentially of the olefins and the light hydrocarbons of stream (A) and optionally small amount of aqueous quenching liquid and

• a aqueous liquid stream (B) comprising the essential part of the oxygenates of stream (A), the catalyst fines of stream (A) and the essential part of the hydrocarbons of stream (A) which are liquid at the bottoms temperature and pressure,

b) separating the catalyst fines from stream (B) to produce a stream having an enhanced catalyst fines content and a stream (C) having a reduced catalyst fines content,

c) optionally recovering all or a part of stream (C), cooling it and recycling it to the aqueous quenching liquid inlet,

Wherein,

the aqueous quenching liquid is fed in the upper part of the quench column and dispersed in said column by spray nozzles,

optionally there is substantially no aqueous quenching liquid level in the bottom of the quench column,

optionally there is essentially no packing or trays between the spray nozzles and the bottom,

optionally the quench tower bottom is sloped to drain quickly all liquid. In an embodiment a part of the bottom liquid is recycled directly in the bottom, advantageously in the upper part of the sloped bottom, to wash all deposited solids. In a preferred embodiment, the liquid recycled is tangentially injected to the sloped bottom in order to wash the internal part of the quench column.

In an embodiment one or more (advantageously two) washing trays are located on top of the quench column above the spray nozzles. Said trays can be fed with a part of the quenching water sent to the spray nozzles.

In an embodiment a part of the quench water is mixed with stream (A) prior to the stream (A) entering the quench column. [Detailed description of the invention]

As regards the stream (A), It can be produced by an MTO process or an alcohol dehydration process or any other process, "comprising essentially (i) olefins and steam" means advantageously more than 70 w% of stream (A), and preferably up to 80% or more, "a minor part of oxygenates and hydrocarbons" means advantageously less than 30 w% of stream (A) and preferably between 2 and 20%. The amount of catalyst fines is advantageously less than 0.1 w% of stream (A). The temperature of stream (A) can be around 200°C to 500°C. The pressure of stream (A) can be around 0.1 barg (bar gauge) to around 15 barg.

By way of example in one embodiment, the MTO reactor effluent stream comprises ethylene and propylene, C4+ olefin(s), methane, C2+ paraffins, water, unreacted oxygenate(s), and oxygenate hydrocarbons. In another embodiment, the reactor effluent stream comprises from about 30 wt. % to about 70 wt. % water, preferably, from about 35 wt. % to about 70 wt. % water; more preferably from about 40 wt. % to about 65 wt. % water expressed as a percentage of the total weight of the reactor effluent stream.

As regards to the aqueous quench liquid, it is preferably water. It is in particular at least constituted of 50 wt % of water, preferably 75 wt %, most preferably 95 wt %. It may also be constituted of other components such as oxygenates. For instance, the aqueous quench liquid may contain methanol, dimethylether, ethanol, diethylether, propanol and/or mixture of thereof. Oxygenates can originate from the upstream process or be optionally recycled from the quench tower. The pH of the aqueous quench liquid may be adapted by addition of acidic or basic components, for instance oxygenated components, in order to avoid the catalyst fines coalescence or agglomeration.

As regards the overhead gas stream recovered at step a) consisting essentially of the olefins and light hydrocarbons of stream (A), it means advantageously that about 70 w% or more of this overhead comprises olefins. These olefins are advantageously the light olefins such as ethylene and propylene. Should the olefins of stream (A) are a mixture of light olefins and heavier olefins such as C4+ olefins, the light olefins go in the overhead and according to the temperature of the aqueous quenching liquid, preferably being water, and temperature of the bottom stream a proportion of the heavier olefins go in the aqueous quenching liquid bottom stream. Said overhead stream comprises the light hydrocarbons, if any, contained in stream (A). Such light hydrocarbon can be methane, ethane and propane. The stream (A) may contain hydrogen, of course said hydrogen goes in the overhead stream. The quench column overhead stream may contain water according to the temperature on top of said quench column and pressure. The content of the water in the overhead stream may advantageously be tuned depending on the operating conditions downstream. As regards the aqueous quenching liquid in the bottom stream, " it is preferably constituted of water and the essential part of the oxygenates of stream (A)" means advantageously more than 70w% of the oxygenates of stream (A) and preferably more than 80%. " the essential part of the hydrocarbons of stream (A) which are liquid at the bottoms temperature and pressure" means advantageously more than 70w% of the hydrocarbons of stream (A) which are liquid at the bottoms temperature and pressure and preferably more than 80%. Of course there is a splitting of the hydrocarbons between the overhead stream and the water bottom stream according to the temperature of the overhead and the temperature of the water bottom stream. pH of the water bottom stream can be set to an appropriate value to prevent catalyst agglomerates. Said adjustment can be made by injection in the quench water of an acidic or a basic component according to the pH required. Catalyst fines mean catalyst particles that are not retained in the upstream process and optionally particles of coke and/or corrosion residues.

As regards step b), it can be made by any separation apparatus such a decanter, a filter or a hydrocyclone. One or more hydrocyclones in parallel or in series can be used. Stream (C) can be purified by any separation apparatus such a decanter, a filter or a hydrocyclone. One or more hydrocyclones in parallel or in series can be used.

As regards step c), advantageously a part of the aqueous quenching liquid bottom stream, corresponding to the aqueous quenching liquid brought into the quench column by stream (A) minus the aqueous quenching liquid going with the overhead stream has to be discarded.

Advantageously the aqueous quenching liquid inlet is at a top portion of the quench column. The aqueous quenching liquid is advantageously fed into the quench column through spray nozzles that can optionally be in series and/or feeder pipes. The spray nozzles may be designed to provide full coverage across the quench tower cross section and to finely atomize the aqueous quenching liquid. Use of spray nozzles allows providing an intimate contact between the gas and liquid. Spray nozzles such as hydraulic spillback lances or gas assisted lances or any type suitable for the operation may be used. The aqueous quenching liquid is evenly distributed across the top of the column. At the bottom of the quench column is a gas inlet where the stream (A) enters the quench column. The vapor components move up the column countercurrent to the quenching water moving down the column. The outlet for the liquid is at the bottom of the column, typically below the gas inlet. The outlet for the gas phase (overhead) is at the top of the quench column, typically above the liquid inlet.

In an embodiment the washing trays on top of the column, located above the spray nozzles are fed with clean water. Clean water means water capable to wash the trays and prevent any plugging or fouling of these trays. Said clean water can be the water from the stream (C) further purified by any means e.g. filtration, centrifugation, stripping, distillation.

Fig 1 describes an embodiment of the invention. In fig 1 the water bottom stream is sent via a pump to a cyclone producing a stream at bottom of the cyclone having an enhanced catalyst fines content and a stream (C) on top of the cyclone having a reduced catalyst fines content.

As regards the quench column, the wording "no level in the bottom" or "no water level in the bottom" or even "no liquid level in the bottom" means that the bottom part has a reduced cross section area as compared with the cross section of the column and the water level is maintained in this part having the reduced cross section area. Advantageously this part having the reduced cross section area is linked to the upper part of the quench column by a part having essentially a conic shape. This conic shape provides a slope to drain quickly all liquids. This avoids settling and keep the water flowing to prevent as much as possible a decanting of solids, e.g. catalyst particles. The wording "substantially no aqueous quench liquid level in the bottom quench column"

Optionally in the lower section of said column there are no or reduced internal surfaces, this part is empty or there are only baffle trays. The lower section means the part from the bottom up to 30 or 40% of the total height.

The quench column can be divided in two or more sections, optionally physically separated. The stream to be quenched goes from one section to the other. As separation, Chimney trays with liquid level can be used.

By using two or more sections we can create different quench loops, with their own water quality (catalyst fines content, temperature). Each section can be fed with a specific aqueous quenching liquid and a bottom stream is recovered at each bottom of each section. In an embodiment aqueous quenching liquid is not recovered separately at each section bottom but aqueous quenching liquid from a section bottom moves to a lower section. In another embodiment there are sections wherein aqueous quenching liquid is recovered at each bottom of said sections and sections wherein aqueous quenching liquid moves to a lower section. In an embodiment all sections are not fed with aqueous quenching liquid but at least the upper section is fed with aqueous quenching liquid, which means the upper section and optionally one or more of the remaining sections are fed with aqueous quenching liquid.

In another embodiment, the process comprises several quenching steps. The reactor effluent stream is quenched in a first quench stage with a first aqueous quenching liquid. The first aqueous quenching liquid can be a pure aqueous solution. The quenching in the first quench stage forms a first liquid fraction and a first effluent stream. The first liquid fraction, typically, has no more than 20 wt. % water based upon the weight of water in the reactor effluent stream and also has a majority of catalyst fines from the reactor effluent stream. Then, the first effluent stream is quenched in a second quench stage with the first quench medium. The quenching in the second quench stage produces a second liquid fraction and a second effluent stream. The quenching in the second quench stage also optionally removes a majority of water from the reactor effluent stream or optionally clean the gas stream and remove heat. The second effluent stream is then quenched in a third quench stage with a second quench medium comprising substantially oxygenate free quench medium. The quenching in the third quench stage optionally removes a majority of the oxygenates from the second effluent stream or optionally clean the gas stream and remove heat.

Advantageously the lowest part of the quench column has a conical shape. Such shape allows a better flow of the aqueous quench liquid inside the quench column.

The quench column can comprise adapted internals in each section. An object is to improve catalyst removal. It means, as an exemple:

- Spray nozzles and no internals in the bottom section,

- Spray nozzles and baffles trays in the middle section,

- Packing or vane separator in top section, optionally spray nozzles.

With such design the less fouling sensitive internals are in the bottom section where catalyst fines are mostly trapped.

The parts of the column in which there are high velocities can be made of specific alloys to withstand attrition.

The quench water can be fed not only in the upper section but also in the medium and/or the lower section of the column. Said quench water is advantageously dispersed by spray nozzles.

In an embodiment there are baffle trays in the medium section.

In an embodiment in the upper section of the column, there are means to separate gas and droplets. By way of example it is a packing or a vane chevron separator.

Said means to separate gas and droplets can be located on top of the top section, the overhead stream goes through said separator before leaving the quench column or can be located under the spray nozzles.

One can cite the mesh-type mist eliminator. In a mesh-type mist eliminator each strand acts as an obstruction around which gas must flow. Within a very short distance upstream of a filament, the gas turns aside sharply, but some mist droplets are unable to follow. They strike the filament, adhere, and coalesce to form droplets that are large enough to trickle down and fall away. Such mesh-type mist eliminator are sold by the company amistco© in texas, USA. One can cite the vane separators also known as chevron or plate type. The mist eliminators of the vane type separator consist of closely spaced corrugated plates that force mist-laden gas to follow serpentine paths, vanes bend the path of mistladen gas into relatively tight curves. As the gas changes direction, inertia or momentum keeps mist droplets moving in straighter paths, and some strike adjacent vanes. There, they are held by surface forces and coalesce (merge) with other droplets, eventually trickling down. If the vane material is wettable, a surface film promotes coalescence and drainage. In the case of upward flow, coalesced liquid disengages from the bottom of the vanes as droplets large enough to fall through rising gas. In the case of horizontal flow the liquid trickles down vanes to a drain below. Such vane separators are sold by the company amistco© in texas, USA and The King Tool Company© in texas, USA.

Claims

1 Process for quenching a gas stream (A) comprising essentially (i) olefins and steam, (ii) a minor part of oxygenates and hydrocarbons and (iii) catalyst fines said process comprising :

a) sending stream (A) to a quench column, fed with aqueous quenching liquid, to produce

• a aqueous quenching liquid bottom stream (B) comprising the essential part of the oxygenates of stream (A), the catalyst fines of stream (A) and the essential part of the hydrocarbons of stream (A) which are liquid at the bottoms temperature and pressure,

c) optionally recovering all or a part of stream (C), cooling it and recycling it to the quenching water inlet,

Wherein,

the aqueous quenching liquid, preferably being water, is fed in the upper part of the quench column and dispersed in said column by spray nozzles,

2 Process according to claim 1 wherein there is no water level in the bottom of the quench column.

3 Process according to any one of the preceding claims wherein there is essentially no packing or trays between the spray nozzles and the bottom. 4 Process according to any one of the preceding claims wherein the quench tower bottom is sloped to drain quickly all liquid.

5 Process according to any one of the preceding claims wherein a part of the bottom liquid is recycled directly in the bottom, advantageously in the upper part of the sloped bottom and optionally with a tangential direction toward the section of the quenching column, to wash all deposited solids.

6 Process according to any one of the preceding claims wherein one or more, advantageously two, washing trays are located on top of the quench column above the spray nozzles.

7 Process according to claim 6 wherein said trays can be fed with a part of the quenching water sent to the spray nozzles.

8 Process according to any one of the preceding claims wherein a part of the quench water is mixed with stream (A) prior to the stream (A) entering the quench column.

9 Process according to claim 1 , 2, and 4 to 8 any one of the preceding claims wherein the quench column is divided in two or more sections.

10 Process according to claim 9 wherein each section is fed with a specific quench water and a bottom stream is recovered at each bottom of each section. 1 1 Process according to claim 9 wherein water is not recovered separately at each section bottom but water from a section bottom moves to a lower section.

12 Process according to claim 9 or 1 1 wherein there are sections in which water is recovered at each bottom of said sections and sections wherein water moves to a lower section.

13 Process according to claim 9 wherein the upper section and optionally one or more of the remaining sections are fed with quench water.

14 Process according to claims 9 to 13 wherein the quench column comprises adapted internals in each section.

15 Process according to any of the preceding wherein the quenching tower is constituted of at least three separated quenching section.