Mechanically operated kill agent injection safety system and method to stop a runaway chemical reaction

Disclosed is a method to safely terminate a runaway reaction within a reaction vessel, comprising: sensing an increase in pressure in the reaction vessel, opening a barrier blocking a flow path into the reactor, wherein the barrier opening is achieved via a mechanical response to the sensed increase in pressure, and injecting a kill agent into the reaction vessel via the opened flow path, thereby terminating the reaction. Also disclosed is a system for performing the method. The system functions without an external electrical source and is therefore compliant with ASME standards.

FIELD OF THE INVENTION

This invention generally relates to injection kill systems and/or safety relief systems to protect tanks under positive pressure relative to the atmosphere. More specifically, the invention relates to a mechanically operated safety injection kill system to prevent extreme over pressurization of a reaction vessel due to a runaway chemical reaction.

BACKGROUND OF THE INVENTION

In the field of safety system design for vessels under positive pressure relative to the atmosphere, devices to relieve excess pressure are desirable for safety purposes. Excessive pressurization may be caused by an emergency such as a loss of power, a loss of reactor cooling water, and/or a fire surrounding a reaction vessel and heating the fluids therein. Likewise, heat may be generated for example by a runaway chemical reaction within a vessel. In the case of a catalyzed chemical reaction, catalyst activity levels have increased over the years through research and development, and the risk of such runaway reactions and safety concerns related thereto has increased.

Safety relief systems are typically provided for an emergency vent in a pressure vessel and are sized according to expected over pressurizations due to an emergency to quickly and safely relieve excessive system pressure without rupturing the pressurized vessel. The American Society of Mechanical Engineers (ASME) states that safety and protection relief systems cannot be electrically driven, so that they will operate in the event of a power failure. Examples of such ASME safety relief systems include mechanical relief valves, rupture disks, etc. However, activation of traditional relief systems may have an adverse environmental impact and/or may require significant maintenance work and loss of production time to bring the vessel back to service. Therefore, a common practice in the industry is to install an Engineered Control System (ECS) to prevent the pressure to reach the relief set point. Such practices as early relief, depressurization, dumping, etc are typically employed. For a reactor vessel, injection of a kill agent at the onset of a runaway to stop the reaction is normally used. However, the reliability of a reactor “kill” system depends upon the availability of external energy sources. A typical reactor kill system requires electricity to power a control system including an instrument signal, programmed logic interlock, electrical solenoids, etc. It also requires the availability of pneumatic power (i.e. instrument air) to drive the valves' actuators. In certain emergency scenario, i.e. power failure, the external energy sources may not be available and rendered the reactor kill system useless. Thus, a need exists for a mechanically driven safety injection kill system for protecting against vessel over pressurizations to replace or supplement existing traditional systems.

Note that throughout the following description,FIGS. 3aand3b, though not directly referred to, can optionally be used to illustrate any reference toFIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the description that follows, like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawing figures are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. The present invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. Specifically, the emergency safety injection kill process and system of the present invention may be used for any pressure vessel or reactor wherein a chemical reaction occurs with a potential for over pressurizing the reaction vessel in the event of an emergency situation, such as a runaway reaction. In an embodiment of the present invention, the emergency safety injection kill process and system of the present invention is integrated with a bulk loop polypropylene reactor, and the remainder of the detailed description will focus on this embodiment with the understanding that the present invention may have broader applications such as slurry reactors, slurry loop reactors, stirred tank reactors, gas phase reactors, liquid phase reactors, and combinations thereof including with or without bulk loop reactors. Furthermore, the remainder of the detailed description may focus on catalyzed reactions and kill agents suitable therefore, with the understanding that the process and system may also be used with uncatalyzed reactions with the selection of an appropriate kill agent.

Disclosed herein is an emergency safety injection kill system and method for use of same. The system has a pressure activated mechanical barrier including a non reclosing pressure relief device and a valve structure to selectively inject a kill agent into a reaction vessel in the event of a runaway chemical reaction. The emergency safety injection kill system of this disclosure can utilize ASME approved devices, such as a pilot operated safety relief valve and a rupture disk, that require no electrical instruments or automated controls to initiate a reaction kill. Such a system has improved reliability for relieving capabilities in the event of a total loss of power during a runaway reaction. To ensure that the emergency safety injection kill system does not undesirably kill the reaction in a non-emergency situation, the pressure activated mechanical barrier isolates the reactor from the kill agent. The mechanical barrier prevents the injection of the kill agent into the reactor as long as the reactor pressure stays below the set pressure of the devices used in the mechanical barrier. If the pressure of the reactor exceeds the set pressures of the mechanical barrier, the mechanical barrier will open and the kill agent will be injected into the reactor. The emergency safety kill system may be used as a stand alone system or may be used in conjunction with one or more additional safety systems as described herein. Additional safety relief valves would normally be used to relieve the vessel in the event of vessel exposure to fire and normally require a much smaller relief valve.

Referring toFIG. 1, a reaction vessel2is coupled to an emergency safety injection kill system125for safely killing a chemical reaction within the reaction vessel in the event of over pressurization due to a runaway reaction. The emergency safety injection kill system125further comprises a pressurized supply of kill agent, for example container25; a means for providing the kill agent to a chemical reaction in the reaction vessel2, for example lines15,22, and26; a means for mechanically isolating the reaction vessel from the kill agent, for example a rupture disk10and a modified pilot operated relief valve20; and a means for opening the means for mechanically isolating the reaction vessel from the kill agent, for example a pressure sensory line27and a pilot valve29to mechanically operate a main relief valve23. All lines and devices of the emergency safety kill injection system are desirably sized accordingly to accommodate the maximum system pressure, for example the pressure exerted by the pressurized container of kill agent25.

Reaction vessel2is coupled to the emergency safety injection kill system125by line15, which ultimately terminates at an injection port located in reaction vessel2. As used herein, the terms line and stream include any conventional physical means for conveying a fluid such as a piping, tubing, conduit, and the like. Any suitable type of injection port, either previously existing or newly added, may be used to couple the emergency safety injection kill system125to reaction vessel2such that reaction kill agent may be transferred from the injection kill system to the reaction vessel. A plurality of injection ports may be used, for example spaced about equally around the lower elevation of the reactor to ensure a quick and evenly dispersed injection of kill agent into reaction vessel2.

In an embodiment of the present invention, the reaction vessel2may also be configured such that one or more injection ports7located in lower portions of each leg of the reactor connect to a second kill system47via line4. The second kill system refers to a system used in the normal, non-emergency operation of the reaction vessel (sometimes referred to as an engineered control system), for example to kill a polymerization reaction upon reaching a desired level of polymerization or during a shut down for cleaning or maintenance of the vessel. The second kill system typically uses electrical components, such as controller, solenoid, and pumps, that may become inoperable should a power failure occur. An example of a second kill system for a polypropylene bulk loop reactor is supply and means for injection of alcohol, e.g., methanol, isopropanol; water; water and alcohol; carbon monoxide, pure or mixed with diluent gases such as nitrogen; a N,N-Bis(2-hydroxyethyl) alkylamine, such as ATMER 163 available from ICI Americas Inc.; a polymeric nitrogen and sulfur compound known as STADIS 425 available from Dupont; and the like. Mixtures of these materials can also be used. Any compound or material that is known to those of ordinary skill in the art of killing a reaction to be useful can be used with the present invention.

In the embodiment ofFIG. 1, the emergency safety injection kill system125is coupled to the reaction vessel2via injection port7, which may be part of an existing, second kill system47or alternatively may be a separate, dedicated injection port. The injection port7may be configured to receive a clean fluid, such as propylene, from flush line5for purging and flushing any solid material from the injection point to prevent plugging thereof. Flush line5contains a tee8, and the emergency safety injection kill system125is coupled to tee8via line15. Additionally, a check valve12is inserted in flush line5to prevent back flow. Other close/open valves97as shown inFIGS. 1,2,3aand3bare located at various points throughout the system disclosed and can be used to isolate streams as needed for maintaining the system.

Reaction vessel2may be any pressure vessel or reactor with any reaction occurring within that has a potential for over pressurizing in the event of a runaway reaction. In an embodiment of the present invention, reaction vessel2is a polymerization reactor; more specifically, a poly alpha-olefin reactor; more specifically, a polypropylene reactor; and more specifically, a bulk loop polypropylene reactor, for example a bulk loop reactor having a plurality of loops, with each loop having a plurality of segments or “legs” connected in series. A polypropylene reactor system typically comprises a catalyst such as a Zeigler-Natta and/or metallocene catalyst, but the reactor can also be used for free radical polymerizations. Reaction vessel2is configured for receiving reactants, for example via inlet feed stream6. Reaction vessel2may be equipped with one or more traditional safety systems such as a safety relief valve37, for example a model #26XA23 relief valve available from Teledyne Fluid Systems, Farris Engineering an Allegheny Teledyne Company, for venting to a flare via line38when pressure in the reaction vessel exceeds a desired operating pressure typically caused by fire exposure. In an embodiment of the present invention, this pressure could be set at a higher pressure than the pressure at which the emergency safety injection kill system125activates, e.g., higher than the pressure required to rupture the rupture disk10discussed below. In another embodiment of the present invention, this pressure could be set at a lower pressure than the pressure at which the emergency safety injection kill system125activates. Safety relief valve37may be coupled to reaction vessel2as known to those skilled in the art, for example via line9.

In an embodiment of the present invention, each leg of a bulk loop reactor contains an injection port, similar to injection port7shown inFIG. 1, that is coupled to the emergency safety injection kill system125, for example via jumper lines13and14extending from line15and coupling (as described herein in detail with reference to line15) to an injection port. The injection ports for jumper lines13and14are not shown inFIG. 1. During an emergency such as a runaway reaction, the emergency safety kill injection system injects a reaction kill agent into multiple points around the base or other lower elevation points such that the kill agent may readily mix with the reactants to kill the reaction. For example, a gaseous kill agent may bubble up through the reaction vessel2to assist with mixing and killing the reaction. Also, pumps (not shown) in each leg of the reaction vessel2may help to speed the killing action. For example, even if power is lost, remaining wind milling of the pump may assist dispersion of the kill agent and speed the killing action.

A mechanical barrier isolates the reaction vessel2from the remainder of the emergency safety injection kill system125, which, during normal reactor activity, prevents flow from entering into line15from the reaction vessel2. An example of a suitable mechanical barrier is a non-reclosing pressure relief safety device such as a burst or rupture disk. Referring toFIG. 1, a rupture disk10is disposed within line15, for example in close proximity to tee8or in another suitable location between tee8and modified pilot operated relief valve20. In an embodiment of the present invention, to ensure that the emergency safety injection kill system does not accidentally activate due to a leak of pressure across the rupture disk, a dual rupture disk may be utilized and the pressure monitored between the two disks to check for such leaks. Suitable rupture disks conform to the safety requirements of ASME Sections VIII and III and are available from BS&B Safety Systems, Inc. located in Tulsa, Okla. In an embodiment of the present invention, the rupture disk is designed to rupture at a pressure of about 750 psig. Therefore, upon the event of a runaway reaction, once the internal pressure of the reaction vessel2reaches about equal to or greater than 750 psig, the rupture disk10will fail allowing line15to pressurize. During normal operation of the reaction vessel2with the closed rupture disk10in place, line15would be shut at both ends (i.e., via the closed rupture disk10and by the closed modified pilot operated relief valve20) and remain at a very low pressure of about 0 psig. The pressure in line15may be monitored with the pressure indicator and transmitter30to check for leaks across the modified pilot operated relief valve20or closed rupture disk10.

Line15is coupled to the modified pilot operated relief valve20having a main valve body23controlled by a pilot valve29.FIG. 2is a more detailed diagram of the modified pilot operated relief valve20, wherein an ASME approved pilot operated safety relief valve is modified and installed to inject a fluid, e.g., a catalyst kill agent, into the reaction vessel2rather than to relieve fluid flow out of the reaction vessel. Typically, in a normal pilot operated safety relief valve installation, the outlet line from the pressurized vessel, e.g., line15, is connected to the contained side55(normal inlet side) of the pilot operated relief valve and the normal location of the pressure sensing line27is also in the contained side (normal inlet side) of the pilot operated relief valve. However as modified for use herein, line15and pressure sensing line27are coupled to the relief side60(normal outlet side) of pilot operated relief valve20. Pressure sensing line27may connect directly to line15or may be connected to relief side60of the main valve body23, as shown inFIG. 2, for example via known means such as threads, drill and tap, welding, and the like. The pilot valve29continuously senses the pressure in line15, as shown inFIG. 1, via the pressure sensing line27.

The pilot valve29is coupled to main valve body23via control line31for actuation, e.g., opening and closing, of the main valve body23. The main valve body23has a piston72with a chamber74disposed over the valve member72. When the pressure in line15increases to a set pressure as sensed by sensing line27, the pilot valve29opens to vent the chamber74back through line31exiting through the outlet33of the pilot valve29and reduces the chamber pressure thereby providing for opening of the piston72. The set pressure for the pilot valve29can be set to operate at any desired sensed pressure between about 0 psig and the set pressure of the rupture disk. Preferably the set pressure should be set high enough so as not to trip the system unnecessarily, for example due to a small leak into line15or other non-emergency event. The desired set pressure for the pilot valve29is about between 100 and 250 psig, (0.791 and 1.83 MPa) and the most desired set pressure for the pilot valve29is about 200 psig (1.48 MPa). Additionally, the modified pilot operated relief valve20installation desirably should include a back flow preventer so that the valve will not backflow if a reaction pressure becomes higher than the injection source pressure. In the embodiment shown inFIG. 2, pilot valve29is normally biased in a closed position by a pressurized gas source95such as nitrogen or air connected via line96. In an alternative embodiment of the present invention, line96could connect pilot valve29to line22as shown inFIG. 1, such that gas from container25is used to bias pilot valve29. While inner walls77and78of relief side60are shown necked down as they approach piston72, walls77and78may have alternative alignments such as substantially parallel, flared, etc. Likewise, while inner walls81and82of contained side55are shown flared inward as they approach piston72, walls81and82may have alternative alignments such as substantially parallel, flared outward, necked, etc.

An example of a pilot operated valve suitable for modification and installation as described herein is disclosed in U.S. Pat. No. 4,445,531, issued May 1, 1984 and entitled “Pilot for Safety Valve”, hereby incorporated herein in its entirety. Another example of a pilot operated valve suitable for modification is the Iso-Dome Series 400 available from Anderson, Greenwood & Co. of Stafford, Tex. Sizing of the modified pilot operated safety relief valve should be made to accommodate transfer and injection of the amount of kill agent required to kill the reaction in reaction vessel2. The amount of kill agent required is expected to be relatively small for typical reactors and therefore the smallest valve commercially available may be sufficient for this application.

The modified pilot operated relief valve20is coupled to a pressurized source of reaction kill agent. Referring toFIG. 1, a volume bottle100is connected via an outlet and line22to the contained side55(normal inlet side) of the modified pilot operated relief valve20. An inlet of volume bottle100is connected to a pressurized source of kill agent25via line26. The volume bottle100and the pilot operated relief valve20are desirably located in close proximity to reaction vessel2to minimize the pressure drop, thereby maximizing the speed and amount of injection of kill agent into the reactor (providing an instant ‘shot’ of kill agent). Desirably, the volume bottle100is sized to hold an amount of kill agent sufficient to be injected through all injection ports and quickly kill the reaction based upon the amount of reactants therein. Also desirably, the kill agent is under a pressure sufficient to be higher than the pressure of the reactor under runaway conditions.

Any suitable reaction killing agent having a desirable quick killing action may be used, and desirably is a fluid such as a liquid or gas. In an embodiment of the present invention the reaction is a free radical polymerization and the killing agent is any known to be useful to those of ordinary skill in the art, such as water or ethylbenzene. In another embodiment of the present invention, the reaction killing agent may be a catalyst killing agent that quickly kills a needed catalyst, thereby halting the reaction. Examples of kill agents useful with the present invention include alcohols, e.g., methanol, isopropanol; water; water and alcohol; carbon monoxide, pure or mixed with diluent gases such as nitrogen; a N,N-Bis(2-hydroxyethyl) alkylamine, such as ATMER 163 available from ICI Americas Inc.; a polymeric nitrogen and sulfur compound known as STADIS 425 available from Dupont; and the like. Mixtures of these materials can also be used. Any compound or material that is known to those of ordinary skill in the art of killing a reaction to be useful can be used with the present invention. In one embodiment of the present invention, the reaction is a poly alpha-olefin polymerization, specifically propylene polymerization, and the killing agent is carbon monoxide, for example dilute carbon monoxide in nitrogen, more specifically about 3 weight percent carbon monoxide in nitrogen. The carbon monoxide may be supplied, for example, via a pressurized bottle25, and is desirably supplied at a pressure substantially higher, e.g., from about 1500 to 2000 psig (10.4 to 13.9 MPa) than the rupture pressure of the rupture disc, e.g., about 750 psi (5.27 MPa). The carbon monoxide can be supplied directly from a pressurized container of kill agent25or more desirably from an intermediate source such as volume bottle100as shown inFIG. 1.

Pressures may be monitored at various points in lines22and26, for example with pressure indicator/transmitter24and/or a local gauge28. For example, line22connecting the volume bottle100and the safety valve23may be monitored for pressure changes to ensure a sufficient supply of carbon monoxide is available to kill a catalyzed reaction in the event of an over pressurized reactor due to a runaway reaction. When the pressure transmitter24in line22indicates that the pressure has dropped below a desired minimum pressure, for example about 1500 psig (10.4 MPa), the carbon monoxide bottle25is desirably replaced.

In an emergency such as a runaway reaction, the pressure in reaction vessel2increases until the rupture disk10fails allowing line15to pressurize. The modified pilot operated relief valve20senses the increased pressure in line15, and if greater than the set pressure, opens the main valve body23, as described previously. Upon opening, pressurized kill agent from volume bottle100passes through the opened modified pilot operated relief valve20via line22, reversing the direction of flow in line15(the line connecting the reaction vessel2to the modified pilot operated relief valve20), through the burst rupture disk10, through the flush line5, through the injection nozzle7, and thus injecting the killing agent into the reaction vessel2to kill the reaction therein.

In an embodiment of the present invention, the mechanically operated emergency safety injection kill system125is a stand-alone safety system. In alternate embodiments, the mechanically operated emergency safety injection kill system125may be used as a redundant and/or backup system working in conjunction with traditional safety systems and/or other kills systems. Examples of traditional safety systems include relief valves, such as valve37inFIG. 1, and/or burst disks. Example of other kill systems include engineered control kill systems, such as second kill system4inFIG. 1, that are electronically controlled and rely on availability of energy sources. Various embodiments may include the staging of two or more safety systems such that the systems activate at progressively increasing pressures. In an embodiment of the present invention, a polypropylene bulk loop reactor comprises three systems staged to activate progressively: a first system comprising an engineered control system designed to activate at a relatively low pressure compared to the other systems; a second system comprising the mechanically operated emergency safety injection kill system described herein designed to activate at a relatively medium pressure compared to the other systems; and a third system comprising a traditional safety system such as a relief valve and/or rupture disk designed to activate at a relatively high pressure compared to the other systems and typically used to relieve a vessel being exposed to an external fire. In alternative embodiments, a polypropylene bulk loop reactor may be equipped with the first and second systems (set to activate near simultaneously or staged, and if staged in order of first then second or alternatively second then first), or alternatively the second and third systems (set to activate near simultaneously or staged, and if staged in order of second then third or alternatively third then second).

While the embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the teachings of the invention. Safety relief design criteria, pendant reaction kill system processing equipment, and the like for any given implementation of the invention will be readily ascertainable to one of skill in the art based upon the disclosure herein. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. Use of the term “optionally” with respect to any element of a claim is intended to mean that the subject element is required, or alternatively, is not required. Both alternatives are intended to be within the scope of the claim.