11.5.1 Hydrocarbon Feed Storage
The vapour pressure of the hydrocarbon that was obtained from Kwinana Refinery was estimated to be 200 kPag at 25°C. Therefore, in order to contain it as a liquid, it must be held under pressure or continuously chilled. The normal boiling point of -7°C makes chilling difficult so that pressurised storage was preferred. Compressed gas cylinders (e.g. butane, LPG, etc.) are typically rated to 3.3 MPa and are available in various volumes. These vessels were considered suitable for the present application and two 45 kg cylinders were leased to store the hydrocarbon. Unfortunately, the cylinders are equipped to discharge vapour only so that additional equipment is required to provide a liquid feed for the pilot plant. This is described in Section 11.5.2.
A dual feed tank system was installed. The first, smaller tank operates at above atmospheric pressure (up to 220 kPag) and is used to process raw feed through the column. The second, larger tank, operates at atmospheric pressure and is used when sufficient ETBE has been produced to increase the boiling point of the feed mixture to above the ambient temperature. Both feed tanks can be isolated with ball valves and connected to the feed pump suction.
The composition of the feed must be controlled to ensure the ethanol excess is within acceptable limits. This is achieved by measuring the weight of components as they are added to the feed tank using a set of accurate (±100 g) bathroom scales. Hydrocarbon is measured by difference while ethanol can be weighed directly. The ethanol must be introduced to the feed tank first to allow the hydrocarbon vapour to be bubbled through the mixture (the vapour inlet is located at the bottom of both feed drums). A cross-over line was added to the system to permit the hydrocarbon vapour to be passed through the bottoms cooler and partially condensed before being introduced to the feed tank. This increases the rate at which hydrocarbon can be added to the feed tank(s) since the flow rate is dependent on the pressure difference between the cylinder and the tank, and the pressures are direct functions of the temperature and composition in each vessel. Where the hydrocarbon concentration in the feed tank is high, the vapour pressure in the tank is similar to the pressure in the cylinder so that a temperature differential is required to provide an adequate flow rate.
A helical rotor feed pump was selected to transport the feed mixture to the column. The helical rotor pump provides a continuous flow (rather than a surging flow from a diaphragm pump) and was significantly cheaper than a centrifugal pump for the combination of small flow and medium head required. A speed controller provides accurate flow control. The NPSIIR of the pump is negligible but the feed tanks are elevated to ensure the suction is alw ays flooded. An air bleed (used to prime the pump) and a drain (to empty the feed tanks) were also installed in the suction line. A non-return valve was installed in the discharge line so that the pump can be stopped and started while the column is operating under pressure.
The discharge line includes a pressure gauge to detect blockages between the pump discharge and the column. Since the pump operates via positive displacement, very high pressures are achievable. Therefore, a serious blockage has the potential to result in a pump failure if not detected promptly. Shortly after commissioning, rubbery organic solids were detected in the feed tank and at various downstream locations. These are tolerable with respect to the pump but might lead to blockages in the feed distributor or elsewhere. A strainer was installed to collect particles larger than 270 ^xm.
Was this article helpful?