Condensate receiver level control

Condensate receiver level controls top-product drawoflf —Base level controls bottom-product drawoflF. 

Tliere are many columns operating today with condensate receiver level controlling reflux and base level controlling bottcHn-product flow. There are other columns in which condoisate receiver level adjusts top-product flow while base level manipulates steam flow. How do we choose between them, assuming that we cannot, for some reason, have both levels adjust drawoff flows ... 

Consider Figure 19.2 where top-product flow is set by flow control, reflux flow is set by condensate receiver level control, boilup is fixed by flow control of steam or other heating medium, and bottom-product flow is determined by column-base level control. As shown by the dotted line, we wish eventually to control column top composition by manipulating distillate flow. Let us assume that feed rate, feed composition, feed enthalpy, and boilup are fixed and that we wish to find the changes (i.e., gains ) of top and bottom compositions in response to a change in D, the top-product rate. 

Condensate receiver level control cascaded to distillate flow ccmtrol... 

Condensate receiver level controlled by distillate flow. 

Our example system has a flow-controlled feed, and the reboiler heat is controlled by cascade from a stripping section tray temperature. Steam is the heating medium, with the condensate pumped to condensate recovery. Bottom product is pumped to storage on column level control overhead pressure is controlled by varying level in the overhead condenser the balancing line assures sufficient receiver pressure at all times overhead product is pumped to storage on receiver level control and reflux is on flow control. 

In the evaporator, temperature is controlled by steam, level by liquid product, and pressure by auxiliary cooling or vapor to the reboiler. Level in the condensate receiver is controlled by condensate. 

Once the basic concept of material-balance control has been selected for a process, one must apply the same concept to all process steps. It is for this reason that the first step in designing column controls is to determine the material-balance control arrangement. Control in the direction of flow is the most commonly used concept (although the least desirable), and a frequently encountered arrangement is shown on Figure 1.5. Here level in the condensate receiver (also commonly called reflux drum or accumulator) sets the top product, or distillate flow, while the level in the base of the column sets the bottom product flow in other columns base level sets steam or other heat-transfer media to the reboiler, in which case the condensate receiver level sets top product flow. 

If, during normal operation, the bottom- and top-product valves of a column are closed manually or are held closed by a total reflux switch, the overhead and base inventories build up until the feed valve is shut off. The column is then on total reflux with the steam valve controlled by AP and reflux flow set by condensate receiver level. 

Let us look at a material-balance control scheme that is in the direction of flow. Let feed rate be set by averaging level control of the feed tank, let condensate receiver level set top-product flow, and let column base level set bottom-product flow. We will assume that each level controller is cascaded to the appropriate flow controller. 

As an example let us choose the case of Figure 6.5 where top-product flow is the demand flow, condensate receiver level sets steam flow, base level sets feed flow, and both reflux and bottom-product flow are ratioed to top-product flow. As before, level controllers are cascaded to flow controls with linear flow meters. 

Condensate Receiver Level Cascaded to Steam Flow Control... 

Considerable controversy has existed on the question of whether to have the condensate receiver level adjust the r ux flow or the top-product flow. One well-known author argues strongly for the former. Controversy also exists as to whether it is better to have column-base level control bottom-product flow or the reboiler heating mediiun, usually steam. Another expert recommends the second. It is probably apparent that we cannot follow both reccnimendations at least one of the two levels must control a diawofir flow. 

Another commonly encovinteted control scheme is that of Figure 19.3. This is similar to that of Figure 19.2 except that overhead condensate receiver level is controlled by throttling distillate, while reflux is flow controlled, perhaps eventually cascaded from top composition control. 

Condenser tail pipes, used tvith any condenser, are sealed with a 34-foot leg into a sump, or with a condensate pump operating under vacuum on suction. With surface-type condensers, the level may be sealed in a receiver with a float or other type of level control. 

Where the MU water supply to cast-iron boilers does not precisely keep up with steam generation demands, the water level can quickly decrease and the problems become even more acute. Conversely, where MU does precisely keep up with steaming rates and is supplied to a common condensate receiver-FW tank via automatic level control, the tank can easily overfill when condensate finally drains back under on-off operating conditions. This gives rise to a loss of valuable hot, treated water from the system and the start of another chain of cause and effect problems. 

The reactor effluent passed through porous stainless steel filters at the top of D-1 to a water-cooled condenser and then to the high pressure separator, F-4. The liquid from the high pressure separator was removed through a level control valve at the bottom of FA to a low-pressure receiver, F-7, which was surrounded by ice water. 

Second, changes in column pressure have other impacts such as changes in the off-gas rate, the amount of reboiler duty, and hydraulic profile of the plant. In the case of partial condensation, pressure control can interact with the overhead receiver level. While these effects are real, their magnitude is sometimes exaggerated and cited as reasons for not making any changes. 

Pressure gauges for compressor inlet and outlet, condenser inlet, liquid receiver inlet, evaporator inlet, lubrication oil pressure gauge Level gauge and level controller for flow of liquid refrigerants to evaporator. Provision to evacuate the unit by reversing suction/dischaige sides. 

Level Control of Condensate Receiver and Required HMup... 

Level control in condensate receivers or reflux drums is commonly achieved by manipulating either top product flow or reflux flow. Less commonly, overhead level control is accomplished by adjusting boilup or by adjusting condenser cooling water. For the first two cases, a relatively simple control system can be used. 

As indicated by Figure 9.9, this column has a horizontal condenser and vertical, cylindrical condensate receiver or reflux drum. We will assume that the level controller is of the PI type with set point at midscale of the level transmitter span and that gain 2 auto overrides are employed. Hie level overrides then function as described in the following. 

Level Control of Overhead Condenser Receiver via R ux Man ulation Then... 

If a column has automatic control ofil = L /D (it does not matter whether condensate receiver is level controlled via reflux or top product), the following prep equations apply for constant F, ... 

They assumed a control system in which distillate is flow controlled, reflux is set by condensate receiver (reflux drum) level control, and bottom-product rate is controlled by column-base level. Overhead composition is controlled by boilup. 

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