Wet-gas drum, pressure

For example, let s say that the molecular weight of the gas increases by 20 percent. The AP, developed by the compressor, would then increase from 90 to 108 psi. If the absorber pressure is fixed, then the wet-gas drum pressure would drop by 18 psi. To prevent this excessive decrease in the drum pressure, we throttle on the suction PC control valve. The pressure drop through this valve would increase by 18 psi. But the pressure at P1 will drop by 18 psi, just as if we never moved the suction throttle valve. 

Of course, we may also control the wet-gas drum pressure very nicely by the spillback PC valve, shown in Fig. 28.5. But this mode of control causes N, the number of moles, in Eq. (28.2), to rapidly increase. And the motor amps will increase just as fast as the number of moles. 

As the ACFM increases, we run out to the right on the compressor curve, shown in Fig. 28.3. As we move away from the surge point, the polytropic feet of head decreases. As the polytropic feet of head is reduced, the compressor AP comes partially back down to its initial value, until a new equilibrium is established. But because the initial disturbance of the equilibrium—the increased molecular weight— moved us away from surge, the new equilibrium will be established farther away from surge than the initial equilibrium. Not only will the new equilibrium be established farther away from surge, but the pressure in the wet gas drum will also wind up lower than the initial pressure in the drum. 

But, suppose we must maintain a constant pressure in the wet-gas drum. The pressure in this drum may be controlling the pressure in an upstream distillation column. To hold a constant pressure in the drum, we will have to resort to spillback suction pressure control, illustrated in Fig. 28.5. 

Why, then, in ordinary process plant practice, do we see an increase in the amps on a motor driving a centrifugal compressor as the gas becomes heavier Does it take more work to compress a mole of propane [44 MW (molecular weight)] than it does to compress a mole of methane (16 MW) Certainly not. It s just that compressing a heavier gas forces the spillback to open, to prevent the pressure from falling in the wet-gas drum. This extra gas recirculating through the compressor is the factor that increases the amp load on the motor driver. 

I hope you all understand the mistake Jane made. She trusted John. But if she had it all to do over again, what could Jane have done differently She has to drive the compressor by a fixed-speed motor. The gas molecular weight is going to be unpredictable. The pressure in the wet-gas drum has to be kept constant. 

If the molecular weight of the gas increases, the gas density will increase. The AP will increase. The pressure in the wet-gas drum will drop. The new suction throttle PC valve will start to close. This will restore the pressure in the wet-gas drum, without increasing the flow of gas through the spillback valve. But what happens to Px How does closing the suction throttle valve affect the actual compressor suction pressure ... 

The suction throttle valve, shown in Fig. 28.6, is analogous to the dam on the River Yeo. When we close this valve, the pressure in the wet-gas drum will increase. But the pressure at Pv the compressor suction pressure, is completely unaffected by the movement of the suction throttle valve. 

Let s look at Figure 25-2. The discharge pressure of the compressor is fixed by the absorber fuel gas pressure control (PC). If we start to raise the absorber pressure by closing this PC, the compressor will have to develop more differential pressure. That means it will have to produce more feet of polytropic head (Hp). As shown in Figure 25-1 the compressor will be backed up on its curve. The volume of gas compressed will be reduced. The pressure in the wet gas drum will then rise. The other PC on the spill-back line will then start to close in order to reduce the volume of gas flowing back to the compressor s suction. 

If the molecular weight of the gas increases, the gas density will increase. The AP will increase. The pressure in the wet-gas drum... 

Vapors from the coke drum must pressure their way through to the combination tower reflux drum. Any restriction to their flow will increase the operating pressure of the coke drum. To avoid exceeding the coke drum relief valve pressure, some operators vent the reflux drum to a flare. This makes it appear as if the wet gas compressor is limiting the resid feed rate. Most often, though, the problem lies with upstream pressure drop. 

Before contacting the day-shift foreman, 1 reviewed the unit log sheet (Fig. 3-3). It was apparent from the log sheet that the high coke drum pressure was caused by an excessively high pressure loss upstream of the wet gas compressor, combined with limited capacity of the compressor to handle the uncondensed coker vapors. 

Other causes of excessive coke drum back pressure are badly fouled combination tower overhead condensers, partially plugged trays, or insufficient tower pumparound heat removal. Fouled condensers and lack of pumparound heat removal overload the wet gas compressor. Plugged trays are best identified with a pressure drop survey. 

This phenomenon is commonly encountered in high-pressure separator drums, absorbers, and strippers of vapor recovery sections in FCCUs. The presence of cyanides (due to incomplete water washing of cracked wet gas) activates steel surfaces and increases the rate at which atomic hydrogen enters the steel surfaces. 

The methods you select, whether instrumental or wet chemistry, must provide data that helps you answer a question related to the hypothesis. Ideally, the methods go right to the heart of the problem, to detect or measure an analyte central to the hypothesis. Ideally, the problem has a handle you can try to measure. (We use the term handle to describe the problem s key attribute. If a sample is offcolor, the color is the handle you want to use to solve the problem. If you have bulging drums, the gas causing the pressure is the handle. )... 

For the ground-hold condition, the pressure in the test chamber was maintained at atmospheric, while the insulation was evacuated to within the range of 5 to 10 /i Hg. The pressure within the test tank itself was kept at 0.3 5 psig by means of a back-pressure regulator. The flow rate of the hydrogen gas was measured by an 0.1955-in.-diameter Venturi meter and data recorded for a period of 10 hr before the test was terminated. For the second run, the test chamber w as evacuated to a pressure of about 10 atm and the insulation to 0.5 /it Hg. Continuous boil-off data was recorded for a period of 220 hr, with a wet-drum gas meter used in place of the Venturi to measure this lower flow rate. Finally, a series of tests was conducted at several test chamber pressures from 0.29 to 5 psia, which provided corresponding compressive loads on the insulation. The boil-off rate was allowed to stabilize for at least 8 hr at each external pressure condition before data were recorded. Each test lasted 12 to 24 hr. 

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