(539d) Energy Saving In Crude Oil Atmospheric Distillation Columns by Modifying Vapour Feed Inlet Tray From Pre-Fractionation Train

The conventional procedure for reducing Crude Distillation Units (CDU) energy demand is based on increasing the enthalpy of the feed. The process can begin by preheating the crude oil with the hot tower overhead and product side streams of the crude unit. Following by desalting, the feed is introduced to a flash drum (surge drum), where light components are vapourized and discharged off from the top and by gravity force the liquid settles at the bottom of the vessel, where it is withdrawn. The liquid is further preheated with the hot bottom stream and side products in an array of several heat exchangers followed by heating using an external heater (a furnace) to increase the feed enthalpy as much as possible and achieve a perfect separation in the tower. The vapour product of the flash drum will meet up again with the preheated liquid upon entrance to the tower where they are inserted as one single stream. Nevertheless what has been failed to recall during all past efforts of crude oil optimization, is the probable revision of such an outline or topology given and simple re-streaming of the process without causing cardinal changes in the process. The approach adopted in this work is the first to focus on the possibility of an improved energy efficient design of new towers and optimization of existing units considering minimum expenditure, through re-streaming of the process and making use of light components from the pre-fractionation unit in conventional crude oil distillation topologies to conserve energy in the column. Optimizations have been performed herein with a detailed crude oil distillation case study and thereby disclosing the proposal to be more beneficial in terms of process economy. The approach presented is simple and practical, and is attained through slight re-routing of the vapour feed product of the surge drum upon the insertion stage. The vapour product of the flash drum contains light end hydrocarbons which correlate more to the top of the column components and products, rather than the middle or lower section (where feed is usually inserted). It is theoretically illustrated and represented by simulation means that introducing the vapour product of the flash drum into upper stages of the column separately rather than same stage insertion with the liquid exiting the bottom of the flash drum can lead to several advantages in the unit. With a composition of mainly light hydrocarbons corresponding more to higher stages of the column, inserting this feed to upper trays results in an increase of purity of side stream products or column's main products (Distillate and Bottom). This is due to less contact of the vapour entering and the liquid falling down from upper trays. Eventually the rising vapour is less likely to experience condensation and be presented in the side stream products. Therefore the tower heat duty would decrease as no further boiling of light components would be required and consequently can maintain the same purity of products with a lower duty. In other words, the condenser requires less work to pull up the light hydrocarbons as they are closer in terms of stages to the top of the column. Moreover, the principal factor that determines a column diameter is the vapour flow-rate, hence an additional superiority of this solution would be an increase of Liquid to Gas flows (L/G) between the two feed stages. The ultimate result is a decrease in column diameter between the two feeds with further decrease in the investment costs. However, if one also desires same pervious energy utilization in reboiler (or steam) and condenser can be considered, whereas in this case, higher purity of products will be achievable in compare to the basic common topology case. The first advantage of reducing reboiler energy consumption can be in an operating column while the second benefit of reducing investment costs can be made use of in the design phase of new distillation processes. In-column composition profile of applied mixture can be considered to obtain an initial estimation of the composition corresponding stage to that of the vapour feed exiting the flash drum. Therefore an early standpoint would be to insert the vapour feed between the early stages of the column where correspondency is observed. However relying only on composition similarity would not be adequate for any act of modifying the topology, hence other means for further confirmation and finding the minimum condenser duty is essential. Consequently, results form a computer simulation can be plotted to determine the optimum feed stage and examine the precision of above approximation. Simulation runs are performed at several different feed points, keeping the material balance and total number of stages constant. For this case, predetermined specifications from the case study process description will be held as constants for the simulation to observe the variations in the condenser duty through change in the feed stage. AspenHysysTM, a steady state simulator is used to find such an optimum tray. Considering such an optimum feed stage, it is possible to save 12.6% of condenser duty with respect to the base case. Needless to say, that this energy conservation is achieved at no additional cost of implementing any sort of additional equipment, difficult and expensive rerouting of the streams or unlikely possible fundamental changes. Finally, due to the fact that by insertion of vapour feed to upper stages, L/G will be increased at certain sections (withdrawal and return stages of side streams) of the column, one can expect to design new columns having lower associated section dimension than for using the ordinary topology, thus reducing capital or investment costs of new designs. Therefore at these points, same process conditions and product purities can be achieved using smaller in diameter and lower in cost towers. Compared with the base case, a decrease of approximately 0.7 million USD/yr can be achieved with the proposed topology for new designs or around 0.6 million USD/yr for modification of online towers. In other words, one can conserve approximately 9% in the annual cost through modification of the current topologies. If one also desires, simulation can be performed by use of a reboiler for heat generation instead of steam with the proposed topography to vividly see the advantages and also observe the energy conservation in both reboiler and condenser or behold that inserting this feed to the upper trays results in an increase of products purity with the same condenser or reboiler duties.