Quench Control - An advanced approach for temperature control

 

Quench control is used in closed loop refrigerant applications where cooling of the hot recycle gas stream is achieved by direct mixing of a cold multi-phase gas/liquid. The liquids used for quenching are already produced by operation of the refrigeration compressor, so a separate cooler is not needed.

Also included is a cooler vessel and a quench fluid line for cooling; the quench fluid maintained in a liquid state through the entirety of the quench fluid line. Yet further included is a quench control valve disposed downstream of the cooler vessel to control a flow rate of the quench fluid routed therein. Also included is a refrigerant suction drum located downstream of the quench control valve and configured to receive the quench fluid from the quench fluid line, the refrigerant suction drum in fluid communication with at least one component for cooling.

Figure 1 shows a piping schematic for quench control on the first side-stream of a four-stage refrigeration compressor. In its simplest form, a quench controller is a standard P&ID loop that maintains a given temperature at a set point by opening and closing a control valve to maintain the controlled temperature. However, this simple P&ID loop strategy has its shortcomings. The main issue is that the set point temperature of the quench controller depends on the pressure of the cooled stream. Since the pressure changes continuously, the set-point needs to be continually adjusted as well. The control system must be designed to ensure that the thermal/carbon content and the quench system can operate asynchronously. This allows one product to be completing its process cycle in the quench tank while a new product is being treated in the furnace.

Figure 1: Typical anti-surge piping that utilizes quench flow to cool the stream

Avoid Slow Loop Reaction

One of the shortcoming of the standard P&ID loop approach is its slow response. The quench loop dynamics will be slow relative to that of the pressure and flow controllers. The hot recycle flow demand will change rapidly as flow stability decreases near the surge control line. Therefore, P&ID control alone is not sufficient to keep pace with the changes in recycle flow.

Controlling Temperatures Logically

Another limitation of the standard P&ID approach is that the quench controller will continue its attempt to maintain a temperature set point even when the hot recycle valve is closed. As the quench controller tries to control an unpredictable temperature in a pipe with little or no flow, the quench valve can open spuriously.

A crucial element with quench control is allowing quench control only when it is necessary. If the hot recycle valve is closed or only has marginal amount of flow, then the best control strategy is to keep the quench valve completely shut. This control strategy applies even if the measured quench stream temperature strays from what would be the controlled set point. The quench valve should only be used to chill the hot recycle gas. Thus, the quench valve should only be open and controlling when a significant amount of hot recycle gas is flowing.

Set Point Calculation

The setpoint temperature depends on the pressure of the cooled stream because the drums located at the inlets of a multi-stage compressor contain a two-phase mixture. The separated gas in the upper portion of the drum is saturated, so the temperature of the gas is a function of the drum pressure. If the quench controller setpoint is too low, an increase in the suction pressure can cause the setpoint of the temperature controller to be in the saturation zone or liquid zone of the refrigerant. If the quench controller setpoint drops below the saturated vapor temperature at the current pressure, the controller will not be able to satisfy the setpoint, causing the control valve to open. This excess liquid will create level problems in the suction drum and put unnecessary demands on the inventory of the refrigerant accumulator. If the quench controller setpoint is much greater than the saturation temperature, the hot gas will potentially overheat the compressor. If the quench controller setpoint is too close to the saturation temperature, significant amounts of liquid may be drained and wasted from the main accumulator, leading to compression inefficiency. Additionally, excess liquid in the inlet drum has no chance to evaporate and may be carried over into the compressor, threatening impeller damage. Either scenario greatly disturbs the refrigeration process and needs to be avoided. Thus, the setpoint for the quench controller needs to be maintained near, but slightly above the saturation point of the inlet drum. See Figure 2 below:

Figure 2: P-h diagram illustrating the quench control line. If the control line is too far to the right, the inlet to the stage overheats. If the control line is too far to the left, the inlet drum accumulates liquid and threatens damage.

Temperature Control Loop

Attempting to tune a temperature P&ID loop to keep pace with a changing flow control loop will certainly prove to be a wasted effort. The temperature control loop will continually struggle to maintain a temperature which is highly influenced by a rapidly changing hot recycle flow. The solution to this problem is to open the quench control valve, in addition to its closed loop response, directly proportional to the hot recycle valve opening (i.e. feed forward control). At any given drum pressure a ratio metric quench flow is needed to bring the flow mixture temperature to the quench control like. Feed forward control quickly sets the required quench to hot recycle ratio while P&ID control is only required to maintain zero steady state error.

Quench Control Strategy

The quench control corrects the deficiencies of the standard P&ID loop approach through the following methods:

1. Calculation of the gas dew point to determine the temperature set point
2. Utilizing a feed forward signal from the antisurge controller to improve response time
3. Automatic sequencing the quench valve via coordination with the antisurge controller