Patent Publication Number: US-2018031338-A1

Title: Reactor liquid cooldown with heat exchanger

Description:
BACKGROUND 
     In order to shorten downtime for turnarounds, refineries use cold nitrogen injected into reactor recycle loops to cool down reactors quicker than with simply using the hydrocarbons in the system. This system will reduce the amount of nitrogen required for most cool-down cycles by almost ⅔—increasing the value to the customer drastically. 
     Systems outfitted with piping of incompatible metallurgy are not able to use liquid nitrogen and the nitrogen must be vaporized and brought to an acceptable temperature before injecting into the customer&#39;s system (using mobile nitrogen vaporization units). Systems outfitted with stainless steel piping are able to inject liquid nitrogen directly, which requires far less nitrogen—usually around ⅓, but the majority of cool downs are with cold gas. Both technologies are mature, although direct injection generally requires a higher level of safety consciousness. Some customers with stainless piping are further reluctant to pursue liquid cooldowns because of the risk of recycle compressor failure, or other failures that could result in liquid nitrogen reaching the reactor itself. The major drawback of cold gas systems is that the time it takes to perform the cool down to the customer&#39;s satisfaction, and the nitrogen usage—both of which offer an opportunity to create value for the customer through novel solutions. 
     SUMMARY 
     One embodiment of a reactor liquid cool down method includes obtaining a warm recycle stream ( 102 ) from a reactor ( 101 ) and compressing the warm recycle stream ( 102 ), thereby producing a compressed warm recycle stream ( 104 ); cooling a compressed warm recycle stream ( 104 ) with a controlled liquid cryogen stream ( 110 ) in a heat exchanger ( 107 ), thereby producing a cool recycle stream ( 112 ), wherein the cool recycle stream has a mean fluid temperature, monitoring the mean fluid temperature and comparing the mean fluid temperature to a predetermined control valve set point, thereby defining a temperature deviation; modulating a temperature control valve ( 109 ) to vary the controlled liquid cryogen stream ( 110 ) in order to produce a temperature deviation that is less than a predetermined value, and returning the cool recycle stream ( 112 ) to the reactor ( 101 ). 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein: 
         FIG. 1  is a schematic representation of one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     The proposed solution may include a stainless piping skid that may be mounted on a non-DOT trailer (capable of being pulled by non-DOT pickup trucks). The customer&#39;s entire recycle stream is redirected through temporary piping into the skid, where liquid nitrogen could be injected without risk to the customer&#39;s piping. 
     The skid would include automatic bypass and isolation valves as well as a temperature control valve and multiple thermocouples (for voting purposes). The customer&#39;s stream would enter the skid through the first isolation valve. After passing through the first isolation valve, the customer&#39;s steam enters a heat exchanger. A cooling stream (such as a cryogen, in particular liquid nitrogen) enters the heat exchanger and removes heat from the customer&#39;s stream. 
     The thermocouples would be used to isolate and bypass the skid in the event a predetermined low temperature limit was reached (to be agreed upon with the customer—2 out of 3 voting). The liquid nitrogen would enter the piping via a temperature control valve—the thermocouples would also be used as the control point (also to be agreed upon with the customer). Liquid nitrogen pressure would be provided by a small mobile nitrogen pumping and vaporization unit or simply the centrifugal pump on the liquid nitrogen transport. The skid would be controlled by a simple PLC. Power and air would be provided by the transport or pumper. In this manner, customers with incompatible piping in their existing system would be able to enjoy the benefits of liquid cooldown. 
     Turning to  FIG. 1 , reactor  101  is to be cooled down. In one embodiment of the present invention, warm recycle stream  102  is removed from reactor  101  and introduced into compressor  103 . Compressed warm recycle stream  104  may pass through first isolation valve  106 , after which it enters heat exchanger  107 . Liquid cryogen stream  108  enters temperature control valve  109 , thus generating controlled liquid nitrogen stream  110 , which then enters heat exchanger  107 . Heat exchanger  107  is an indirect contact heat exchanger. Liquid cryogen stream  108  may be any compatible cryogen known in the art. Liquid cryogen stream  108  may be liquid nitrogen. 
     The heat exchanger  107  may be any appropriate type known in the art. The heat exchanger  107  may be of a shell-and-tube design. The heat exchanger  107  may be of a plate-and-frame design. The heat exchanger  107  may be made of stainless steel. 
     Compressed warm recycle stream  104  and controlled liquid nitrogen stream  110  do not mix within heat exchanger  107 . A cool recycle stream  112  is produced, which exhibits a mean fluid temperature. If the temperature of warm recycle stream  102  deviates from a predetermined temperature, compressed warm recycle stream  104  may be bypassed through line  118  and normally closed valve  105 . 
     Temperature sensor  119  senses the mean fluid temperature, and transfers this temperature information to temperature control valve  109 . In one embodiment of the present invention, three temperature sensors ( 119 A,  1198 ,  119 C) are used, thereby allowing the voting of two out of three, in order to improve reliability and accuracy. The mean temperature is compared to a predetermined temperature control valve set point. Temperature control valve  109  then adjusts controlled liquid nitrogen stream  110  in order to bring the mean temperature closer to the predetermined temperature control valve set point. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.