A - CONTACTOR REACTOR SHELL B - TUBE BUNDLE ASSEMBLY C - HYDRAULIC HEAD D - MOTOR E - IMPELLER F - CIRCULATION TUBE

Transcription

1 STRATCO Contactor TM Reactor Performance Upgrade: Alkylation Tube Insert Technology Pam Pryor, STRATCO, Inc. Randy Peterson, STRATCO, Inc. Tom Godry, UOP LLC Yung Lin, UOP LLC The current trend towards elimination of MTBE has resulted in increased attention on alkylation technology. As refiners search for means to replace the lost octane and volume associated with MTBE, alkylation s role has historically never been more significant. For those refiners with existing STRATCO Effluent Refrigerated alkylation capacity, a common question is how to cost effectively expand the capacity of the unit. This paper presents the commercial application of patented tube insert technology in a Contactor reactor and its ability to afford cost effective incremental alkylation capacity to a refiner. Introduction At the heart of STRATCO s Effluent Refrigerated Alkylation Technology is the STRATCO Contactor reactor (Figure 1). The Contactor reactor is a horizontal pressure vessel containing an inner circulation tube, a tube bundle to remove the heat of reaction, and a mixing impeller. The hydrocarbon feed and sulfuric acid enter on the suction side of the impeller inside the circulation tube. As the feeds pass across the impeller, an emulsion of hydrocarbon and acid is formed. The emulsion in the Contactor reactor is continuously circulated at very high rates. The superior mixing and high internal circulation of the Contactor reactor allows for efficient removal of the heat of reaction and minimizes the temperature difference between any two points in the reaction zone (less than 1 F (0.6 C)). This reduces the possibility of localized hot spots that lead to degradation of the alkylate product and increased chances for corrosion. The intense mixing in the Contactor reactor also provides uniform distribution of the hydrocarbons in the acid emulsion. This prevents localized areas of non-optimum isobutane to olefin ratios and acid to olefin ratios, both of which promote olefin polymerization reactions. Figure 1 STRATCO Contactor Reactor EMULSION TO SETTLER COOLANT OUT A - CONTACTOR REACTOR SHELL B - TUBE BUNDLE ASSEMBLY C - HYDRAULIC HEAD D - MOTOR E - IMPELLER F - CIRCULATION TUBE A F B HC E C D COOLANT IN Page 1

2 Figure 2 shows the typical Contactor reactor and acid settler arrangement. A portion of the emulsion in the Contactor reactor, which is approximately 50 LV% acid and 50 LV% hydrocarbon, is withdrawn from the discharge side of the impeller and flows to the acid settler. The hydrocarbon phase (reactor effluent) is separated from the acid emulsion in the acid settlers. The acid, being the heavier of the two phases, settles to the lower portion of the vessel. It is returned to the suction side of the impeller in the form of an emulsion, which is richer in acid than the emulsion entering the settlers. Figure 2 Contactor Reactor/Acid Settler Arrangement SPENT FRESH TO REFRIGERATION SECTION FROM REFRIGERATION SECTION OLEFIN AND ISOBUTANE REFRIGERANT RECYCLE PC TO FEED/EFFLUENT EXCHANGERS The STRATCO alkylation process utilizes an effluent refrigeration system to remove the heat of reaction and control the reaction temperature. With effluent refrigeration, the hydrocarbons in contact with the sulfuric acid catalyst are maintained in the liquid phase. The hydrocarbon effluent flows from the top of the acid settler to the tube bundle in the Contactor reactor. A control valve located in this line maintains an acid settler back pressure of about 60 psig (4.2 kg/cm 2 G). The pressure of the hydrocarbon stream from the top of the acid settler (reactor effluent) is reduced to about 5 psig (0.4 kg/cm 2 G) across the back pressure control valve. A portion of the effluent stream is flashed, reducing the temperature to about 35 F (1.7 C). Typically, the vapor in this stream will be in a range between 3 to 10% by weight and 90 to 98% by volume. Additional vaporization occurs in the Contactor reactor tube bundle as the net effluent stream removes the heat of reaction. Opportunity for Improvement While Contactor reactors have been used successfully for many years, there are opportunities to improve performance. Neutron backscatter tests show that the liquid portion of the refrigerant is unevenly distributed across the tube sheet, when the effluent refrigerant enters the channel head of existing Contactor reactor tube bundles. This is shown in Figure 3. Specifically, the desirable liquid refrigerant preferentially flows through the lower tubes while the upper and outer tubes see mostly vapor. Commercially, the result is reduced heat transfer yielding a less than optimum overall heat transfer coefficient for existing Contactor reactors. Also, the tubes that contain mostly vapor will have warmer tube wall temperatures, that can potentially lead to higher corrosion rates than the tubes receiving more liquid refrigerant. Page 2

3 Figure 3 STRATCO Contactor Reactor Channel Head Distribution Concerns One approach to improving flow distribution in a heat exchanger tube bundle is to use tube inserts. The purpose of the tube inserts is twofold. First, the use of inserts will improve the uniformity of the liquid to vapor ratio of the fluid that enters each tube by introducing some additional pressure drop at the inlet of each tube. In addition, when properly designed, the tube inserts will have a secondary benefit that is even more critical to performance. By shifting the throttling of the refrigerant stream from the upstream control valve to the inlet of each Contactor reactor tube, the refrigerant stream flowing into the Contactor reactor channel can be maintained as all liquid rather than the two-phase mixture characteristic of the unmodified operation. This will further insure that good distribution to all tubes in the Contactor reactor tube field is achieved. By doing so, every tube will provide equal cooling and this will maximize the overall heat transfer. Commercial Experience STRATCO has been continually working to enhance the boiling side heat transfer efficiency of the Contactor reactor tube bundle. UOP had independently developed and patented a tubing insert technology that was well suited for use in STRATCO s Contactor reactor. STRATCO and UOP have jointly worked to commercialize the patented tube insert technology to optimize the Contactor reactor overall heat transfer coefficient and minimize tube bundle corrosion. Working with an undisclosed North American refinery, the tube inserts were placed in service in early December This particular refiner has eight Contactor reactors and four acid settlers, as depicted in Figure 4. Figure 4 North American Refiner Alkylation Reaction Section SETTLER SETTLER SETTLER SETTLER FRESH INTERMEDIATE SPENT Page 3

4 For this particular refinery, there is one common back pressure control valve for each pair of Contactor reactors. As a result, the testing of the inserts required installation into a pair of Contactor reactors rather than installation into a single Contactor reactor. Data collection was extensive, both before and after the inserts were installed, as there are many variables that impact the heat transfer coefficient. When evaluating data taken upstream and directly downstream of the back pressure control valve of Contactor reactor 1 A/B as shown in Figure 5, it can be seen that prior to installation of the inserts, a temperature gradient did occur across the control valve on the order of F. This was expected based on the original design criteria. The temperature gradient after installation of the inserts effectively goes to zero indicating that the effluent refrigerant going to the tube bundle inlets is now completely liquid, as desired. Figure 5 Contactor Reactor 1 A/B Back Pressure Control Valve Temperature Gradient Temperature, F Settler 1 Refrigerant Temperature (D/S CV) Contactor Reactor 1A/B Temperature (U/S CV) 30.0 For comparison purposes, the same data is presented for Contactor reactors/acid settler 3 during that same time in Figure 6. Page 4

5 Figure 6 Contactor Reactor 3 A/B Back Pressure Control Valve Temperature Gradient 55.0 Settler 3 Refrigerant Temperature (D/S CV) Contactor Reactor 3 A/B Temperature (U/S CV) 50.0 Temperature, F As with normal refinery operations, the reported olefin feed composition fluctuated throughout the test period. The degree to which this accurately reflected actual operation is somewhat suspect. We chose to average the feed composition over the test period and corrected for the sensible heat of the feeds. The results of the calculated heat transfer coefficients, before and after installation of the tube bundle inserts, are presented in Figures 7 and Figure 7 Heat Transfer Coefficient for Contactor Reactors 1 A&B Before and After Tube Inserts U' Value Contactor Reactor 1A Contactor Reactor 1B 40 Page 5

6 Figure 8 Heat Transfer Coefficient for Contactor Reactors 1, 3 and 4 A&B Before and After Tube Inserts Contactor Reactor 1A Contactor Reactor 1B Contactor Reactor 3A Contactor Reactor 3B Contactor Reactor 4A Contactor Reactor 4B U' Value Contactor reactor 1 A/B tube bundle heat transfer coefficients are increased by approximately 20%. There are further optimization studies ongoing. Initial data from the optimization program suggests that the heat transfer coefficient improvement is even greater at increased throughput. Testing is currently ongoing to determine the impact of higher feed rate loadings on the heat transfer coefficient with the tube inserts in place. Economic Analysis The value of the improvement in the Contactor reactor tube bundle overall heat transfer coefficient will vary from location to location. In either case, we expect to see an improvement in tube bundle life based on improved flow distribution to the tubes. One can envision several scenarios for economically justifying installation of the inserts. Two possible cases bracketing the options available are: Case 1: Lower Contactor Reactor Temperatures In this case, the existing alkylation unit equipment, outside the reaction section, is not capable of handling additional throughput without a major revamp. One would expect the refiner to see a reduced Contactor reactor operating temperature which leads to both higher alkylate octane and lower acid consumption. Case 2: Increased Contactor Reactor Capacity In this case, the existing alkylation unit equipment is capable of handling additional throughput without a major revamp. The refiner would be able to process additional olefin feed through the unit resulting in increased alkylate production. Page 6

7 Sufficient data was not available from the commercial test site to prepare an economic analysis of the tube insert benefits for this particular location. We expect such data to be presented once all Contactor reactors have the inserts installed. For purposes of this paper, we assumed a standard STRATCO Effluent Refrigerated sulfuric acid alkylation unit with an MTBE raffinate feed. Assuming a 20% increase in the heat transfer coefficient, the economics for each case are presented in Table 1. Table 1 Economic Benefits of STRATCO/UOP Tube Bundle Inserts Case 1 Case 2 Number STRATCO Contactor Reactors 8 8 Reactor Temperature Before Inserts, F Reactor Temperature After Inserts, F Whole Alkylate, bpsd Octane Improvement Acid Usage Before Inserts, sh ton/day Acid Usage After Inserts, sh ton/day Acid Regeneration Costs, $/sh ton Alkylate Value, $/octane-barrel 0.30 na Alkylate Margin, $/bbl na 5.00 Total Benefit, $/day 2,000 15,000 Total Benefit, $/year $700,000 $5,250,000 Summary STRATCO and UOP have commercialized UOP s patented tube insert technology that optimizes the Contactor reactor overall heat transfer coefficient and minimize corrosion. These inserts are placed in the inlet of each tube within the STRATCO Contactor reactor tube bundle inlet to evenly divide the refrigerant flow to the tubes by keeping the refrigerant in the liquid phase until it passes through the inserts. Recent field tests have proven that these inserts increase the heat transfer coefficient of the bundle by at least 20% and we believe that avoiding warmer tubes will extend tube bundle life in H 2 SO 4 alkylation service. In addition, we expect the following process benefits can be achieved: Lower reactor temperatures Higher alkylate octane Lower acid consumption Lower corrosion rates Increased Contactor reactor capacity Page 7