Module 3: Liquid Fossil Fuel (Petroleum) Lecture 25: Refinery Equipments

Transcription

1 1 P age Module 3: Liquid Fossil Fuel (Petroleum) Lecture 25: Refinery Equipments

2 2 P age Keywords: Pipe still heater, heat exchangers, distillation tower, pumps Refinery equipments Introduction In refinery, there are different types of equipments used for different purposes. It is difficult to include the description of all the equipments. Here, some of the important equipments are briefly discussed. Pipe still heater Crude oils are heated in pipe still heater before entering into the atmospheric distillation column. This heater is a special type of furnace which heats crude oil upto about C depending on the type of feed. A large number of tubes connected through bends are housed within the furnace in multiple rows. The still is built with two distinct heating sections, a radiant section, which can receive heat directly from the flame and a convection section, which takes heat from the hot gases travelling to the stack. The hot flue gases arising in the radiation section flow into the convection section where they circulate at high speed through a tube bundle before leaving the furnace through the stack. Shield or shock section separates the two major heating sections. The tubes in this section are close to the radiation section that protects the convection section tubes from direct radiation. The shield section normally consists of two to three rows of bare tubes that are directly exposed to the hot gases and radiation flame. Fig 1. depicts a pipe still heater. There are two major types of fired heaters, such as vertical cylindrical or box-type heaters depending on the geometrical configuration of the radiant section. In box-type heaters, the radiation section usually is of a square or a rectangular cross section. The tubes in the radiation section may be arranged horizontally or vertically along the heater walls and the burners are located on the floor or on the lower part of the longest side wall where there are

3 3 P age no tubes. Box-type furnaces are mainly used where large capacities and large heat duties are required. Fig. 1. Pipe still heater In the cylindrical-type furnace, the radiation section is of the shape of a cylinder having a vertical axis and the burners are situated on the floor at the base of the cylinder. The vertical walls of the furnace are the heat transfer area and therefore exhibit circular symmetry with respect to the tube bundles. In the radiation section, the tubes may be arranged in a circular pattern around the walls of the furnace or they may have a cross or octagonal design which will expose them to firing from both sides. Crude oil is pumped into the furnace through the tubes at the convection section. At this section the crude is preheated and then goes to the radiation section to be heated upto C. Heat transfer at a high rate is obtained by passing the crude with a high flow rate.

4 4 P age The fired heaters have corrosion and material problems due to the elevated temperatures experienced both on the process side and in the fire-box. The Atmospheric Heater receives flashed crude at about 260 C and sends it to the atmospheric column at about 350 C. For sweet crude, the radiant tubes and lower rows of convection tubes are typically 5% chrome with carbon steel in the up section. Heat exchangers Different types of heat exchangers are used in refinery. If the hot stream is a vapour and is cooled without much condensation, the heat exchanger is called a vapour heat exchanger. If the vapour is condensed in the exchanger, then it is called a vapour denser exchanger. The vapour condensed in this way is then subsequently cooled in a exchanger which is called cooler or after-cooler. The heating equipment used to heat the bottoms of a fractionator is called a reboiler. Double pipe heat exchangers are common in refinery. Fins, either in spiral form or as longitudinal plates on the inside pipe in the double pipe Viscous or low-heat transfer rate fluids can be passed through the finned tube to get higher heat transfer area. Jet type condenser has application in refinery. Barometric jet type condenser is used in vacuum distillation to condense steam as well as create some vacuum. The working principle of this type of condenser is to mix the vapour and the cooling medium intimately by high pressure jets and heat exchange is done by mixing of the two streams or conduction heat transfer. In heat exchangers, the decision of which material (fluid/slurry) is to be passed through tube is dependent on different factors. Usually dirty streams are passed through tubes as the tubes in tube bundles can be cleaned, but sometimes, if the stream contains lots of coke or debris, then that should be passed through shell and shell may be disposed after certain time. High pressure fluids are passed through tubes as they can withstand high pressure but shell does

5 5 P age not. Corrosive fluids and water are usually sent through tubes, as corrosion resistant tubes are cheap and the cleaning of deposition or water scale is easier in tubes than shell. Normally large volume liquids are passed through shell. The metallurgy of heat exchanger varies with temperature and the composition of stream. The material of construction for majority of the exchangers is 100% carbon steel. Admiralty brass tubes have been used in fresh water cooled exchangers to prevent water-side fouling and corrosion. Due to the high cost of the brass bundles and improvements in cooling water treatment, many brass tube bundles are replaced with carbon steel. In hot hydrocarbon service, 5% chrome materials in heat exchangers are mostly used. As the sulphur content in the crude increases, the use of high chrome tubes and 12% chrome shell and channel linings are required. Fractionating tower Fractionating towers are the equipments which separate the components from a mixture. Crude oil atmospheric distillation tower is the most important fractionating unit in refinery. Distillation tower with bubble-cap trays is the most conventional and familiar design. The other types of trays used are sieve tray, perforated tray etc. Small bubble caps are best for large liquid loads and large caps are for large vapour load. Some designers use small cap for small tower and large cap for towers of 10 to 20 ft in diameter. Cast ion is largely used for bubble cap as it is more corrosion resistant than steel. In vacuum towers, pressed alloy steel caps are becoming important. Many caps are of removable types, i.e, the vapour uptake and cap are properly assembled on the plate, but sometimes this arrangement is not satisfactory as bolts rusts and cast iron caps may break. The atmospheric column is commonly lined more extensively than the preflash column because the feed, which is at about C, is not only hotter but also contains larger amounts

6 6 P age of HCl and H 2 S. The top of the column is often lined with Monel 400 (about 67% Ni 23% Cu) to protect against condensing HCl. Normally, the lower 2/3 or ¾ of the column will be lined with 12% chrome cladding to protect against high temperature sulfur corrosion. In the area of the feed inlet, or flash zone, 316 stainless steel may be required in those plants processing crudes high in naphthenic acid content. Pumps Different types of pumps are used in the refinery as per requirements. Asphalts or semi-solid materials are handled with reciprocating steam pumps. For handling large amounts of medium viscosity oil, centrifugal pumps may be used. Centrifugal pumps are largely used for pumping non-viscous fluids because of their high capacity low head characteristics. But more viscous materials are handled by rotary pumps.

7 7 P age Reference 1. Petroleum refinery engineering, W. L. Nelson, 4 th edition, McGraw-Hill Book Company, Fired Process Heaters, Matlab - Modelling, Programming and Simulations, Hassan Al-Haj Ibrahim, Emilson Pereira Leite (Ed.), , Sciyo, 2010 ISBN: Fuels and combustion, S. Sarkar, 2 nd edition, Orient Longman Ltd., 1990.