Glycol Dehydration Instrumentation and Controls Most glycol dehydration units are sufficiently automated that they can operate unattended. The degree of automatic control of the equipment can vary considerably and depends largely on the specifications by the owner company. The discussion in this section highlights the main control points, which may be considered as the minimum control of a dehydration unit. The controls relate mainly to gas flow, temperatures, pressures, glycol circulation, and lean glycol concentration. Pressure gauges should be installed o n all vessels, including the reboiler, and on the discharge side of the pump. p ump. Similarly, thermometers should be installed on all vessels, as well as ahead and after all heat-exchange equipment on both cold and hot lines.
Control of Gas Flow Gas flow is usually controlled with a flow control valve upstream of the inlet separator. The operator can set the flow to a certain rate. If the set rate is not met, then the valve opens fully a nd allows the available gas flow to enter the separator and the contactor. Downstream Down stream of the contactor, there may be a meter, which meters the gas flow, or the meter may also be located upstream of the separator. At some point down stream of the contactor, there usually is a back pressure valve. This valve ensures that the pressure in the contactor is steady without abrupt changes. The pressure is set above the downstream line pressure to ensure steady operation of the contactor.
Lean Glycol Circulation Rate To achieve the required water dewpoint depression, it is necessary to circulate a certain amount of lean glycol per pound of water to be removed from the gas. The rate of glycol circulation depends on several conditions, which are all interrelated. These conditions are lean glycol purity, after regeneration, which depends on the reboiler temperature and whether or not stripping gas is used, with zero or one stage contacting for the stripping gas; water content of the gas, which depends on gas temperature and pressure in the inlet separator; number of actual trays (or equivalent packing height) in the contactor; and the design approach temperature in the contactor. In general, a circulation rate of 3 to 5 gal of lean glycol per pound of water to be removed from the gas is required. If the glycol purity is not sufficiently high, any larger circulation rate might not give the necessary dewpoint depression. Usually there is an attempt to match the circulation rate to near the minimum required rate to achieve the necessary drying. Overcirculation has disadvantages: the heat load on the regenerator is increased, requiring more fuel gas consumption; the lean glycol returning to the contactor con tactor is at a higher temperature because of less efficient heat transfer; more hydrocarbons are absorbed, especially compounds such as benzene, toluene, ethyl benzene, and xylene (BETX), if these compounds are present in the gas; and additional acid gas is absorbed, if sour gas is being dehydrated. Because there is concern about the absorption of other compounds besides water, as well as for
energy efficiency, the glycol circulation rate should be set to remove the required requ ired water only. In field installations, using gas driven pumps, the pumps are set to the required pump rate by b y a gas control valve. This is usually a manually operated needle valve. The manufacturer of the pump provides a chart for the pump that shows the pump rate in volumetric units per time vs. the number of strokes of the plunger per p er minute.
Reboiler Temperature The temperature of the glycol in the reboiler determines largely the purity to which the glycol is regenerated. However, there is a limit on the temperature to which the glycol can be heated. This limit is a few degrees below the decomposition de composition temperature, as shown in Table 5.3, because above this temperature, the glycol molecule breaks down. In light of this, the normal temperature in which TEG is heated in the reboiler is about 380 to 390°F. This temperature range results in a lean glycol purity of just under 99% 9 9% on a mass basis, the other 1% being water. Thus, it is very important to control the reboiler temperature to the range of 380 to 390°F or some other range that provides adequate regeneration of the rich glycol. In most glycol dehydration units, the heat for regeneration is supplied by burning a small amount of the gas in a fire tube in the reboiler vessel. The size of this vessel is determined by the maximum design rate of glycol circulation, and the size of the fire tube itself is designed for a limit on the heat flux from the fire through the steel tube to the glycol on the shell side of the fire tube. The larger the fire tube, the lower is the heat transfer rate per unit area. The flame should be burning along most of the tube, as opposed to an intense flame at the front of the burner. The fire tube should be [30] designed for a heat transfer rate per square foot of fire tube no greater than 7,000 Btu/h. A thermowell located in the shell of the reboiler and immersed in the glycol is equippe d with a temperature regulator that controls the instrument gas supply to a control valve on the fuel gas supply line to the burner. By setting the regulator at the desired temperature, the gas flow to the burner is automatically controlled, resulting in a narrow operating temperature range for the reboiler. A pilot light ignites the gas to the main burner when the controller co ntroller allows the gas to flow. The reboiler controls also include a high-temperature shutdown and a shutdown of the fuel supply in case of pilot-light failure. Most glycol reboilers are equipped with a flame arrestor at the air inlet to the burner. The flame arrestor consists of a tightly wound metal sheet, with sufficient space between the wound metal to allow sufficient air through the arrestor into the burner. If an external source of flammable vapors is sucked in with the air a ir through the flame arrestor, such vapors will not ignite outside of the flame arrestor, as the temperature of the g as is cooled below the ignition point, thus preventing a backflash or explosion.
Liquid Level Controls The main liquid level of concern is the level of the condensed liquids in the inlet separator. This vessel can be a two-phase or a three-phase separator. It is very important that no condensed liquid flows with the gas into the contactor. If condensate or salt water gets into the contactor, the result could be foaming or deposits of salt occurring on the fire tube. Heavy hydrocarbons
will eventually gum up the packing in the reboiler column or plug the filter. Flashing of hydrocarbons in the still could damage the packing in the still column. In light of this, most glycol units are equipped with high-level alarms and shutdowns, which activate when the liquid level is exceeded in the inlet separator. The glycol level in the contactor c ontactor is also important, as any increase in level above the gas inlet pipe can result in interruption of circulation of glycol because of insufficient glycol returns. Both liquid levels, in the inlet separator and the contactor, are controlled by conventional liquid level floats and outlet valves. Where a flash tank is employed, it is again important to ensure that the leve l of the glycol be maintained at a set level. The liquid outlet valve must be the throttling type, as opposed to snap acting, to ensure a smooth and steady flow of rich glycol to the regenerator. The glycol level in the surge drum should be at a t about 2/3 to 3/4 full. In small units, the heat-exchanger coil is in the surge drum and has to be totally submersed to be effective in heat transfer.
Pressure and Temperature Indicators All vessels are usually equipped with pressure gauges, as well as pressure relief valves. An operator checking the operation of the glycol unit can quickly see the pressure at which each vessel is operating. The same cannot, in many instances, be said about the temperature, especially in the glycol lines upstream and downstream of each heat exchanger. Ample installation of dial thermometers on the glycol lines is helpful but lacking in many cases. Thermometers are usually installed in the inlet separator and the reboiler. As a minimum, additional thermometers should be installed on the glycol line ahead of the contactor after the heat exchanger and ahead of the reboiler still. Ideally, dial thermometers are installed on all lines entering and leaving the heat exchange equipment.
