Appelpolscher has just left IGC’s Monday morning status meeting with Smarly, Quirk, and the operations personnel. Over the weekend, a major unit in the hydrogen production process, a centrifugal compressor supplying carbon monoxide to a reactor, failed. The operations people were able to switch in a standby unit, an old reciprocating compressor that can barely meet minimum requirements for throughput and discharge pressure. Smarly has questioned the possibility that discharge pressure oscillations from the temporary unit may damage the reactor product during the interim operating period by causing how rate change-s in the feed to the reactor. Because Appelpolscher has suggested the idea of putting one or two surge tanks between the compressor and reactor to damp out any oscillations, his group was chosen to come up with a quick design. Some assumptions can be made:
(i) The proposed piping arrangement would be similar to that shown in Exercise 2.5. If two tanks are used, they should be sized identically.
(ii) The valves before the surge tanks exhibit approximate linear pressure/flow relations.
(iii) The ideal gas law holds approximately.
(iv) Pressure perturbations are caused by reciprocating action of the pistons in the compressor. These perturbations arc approximately sinusoidal.
(v) Because of the present low-pressure limitations, no more than 10% of the compressor’s nominal discharge pressure Pd can be dissipated in the surge system, excluding the valve at the entrance to the reactor. If two surge tanks are included in the design, the allowable drop across each tank is 5% of Pd.
(vi) The pressure controller for the reactor is able to maintain its pressure essentially constant.
Available Data
(i) The nominal discharge pressure P4 is 200 psig (gauge). The nominal throughput of carbon monoxide is 6000 lb/h.
(ii) The compressor contains four cylinders (pistons) driven by a common shaft that rotates at 600 rpm. The cylinders are spaced equally around the shaft to balance shaft loading.
(iii) The maximum (estimated) pressure fluctuation is 2 psig; that is, the amplitude of the fluctuation caused by reciprocation of the pistons is 2 psig.
(iv) Operating personnel want no more than a 0.02-psig variation in pressure before the valve at the entrance to the reactor.
(v) The nominal discharge temperature of the compressor is 300oF. The surge tanks probably will operate isothermally. Appelpolscher is always worried about losing his year-end bonus; this time he thinks it will disappear if any of the reactor product is out of spec and has to be burned. He asks you to make a two-step analysis of the situation:
(a) Would a single tank or two equal-sized surge tanks be better to damp the pressure fluctuations? (If two tanks ate used, they would each have to be about one-fourth the volume of a single tank to keep total system costs roughly equivalent.)
(b) How large would the tank or tanks have to be so reduce pressure fluctuations to an acceptable level? Your answers should deal with how the proposed design can incorporate a 20-ft-long, 3-in.-i.d. pipe that now connects the compressor to the reactor.