Cycle steps

A Cycle is a list of steps. Each step opens the devices of one bed that it needs, found by their roles (see add_device!), and keeps the others closed. Pressures carry over from step to step: a step that changes the pressure starts where the step before left the bed, unless given from.

The step types, with the ends of the bed each one opens by default. The bottom of each bed is its feed end.

Step	Opens	Pressure	Default name
Pressurise	the feed device, or the light inlet with source = :light_product	rises to to, by default the feed pressure	pressurisation, or LPP
Adsorb	the feed device and the product device at the other end	held at pressure, by default the feed pressure	adsorption
Blowdown	an outlet at its through end, by default the product device at the top	falls to to	blowdown
Evacuate	an outlet at its through end, by default the extract device at the bottom	falls to to	evacuation
Purge	the light inlet and the extract device at the other end	held at pressure, by default where the step before left it	purge
Equalise	the link between two beds	both meet between their start pressures	equalisation A to B
Idle	nothing	unchanged	idle

Pressures in steps are in Pa; Mocca.Units has bar, so 0.2bar is 20000 Pa. While the pressure changes, the pressure on the far side of the valve follows an exponential ramp (see Model equations).

Pressurise

Raises the bed from where the step before left it to the feed pressure, or to to, through the feed device. With source = :light_product it takes light product through the light inlet at the top instead: light product pressurisation (LPP), as in the wet flue gas process of Krishnamurthy et al. (2014).

Pressurise(15.0)                                  # feed, to the feed pressure
Pressurise(20.0; source = :light_product)         # LPP

{"type": "pressurise", "duration": 15}
{"type": "pressurise", "duration": 20, "source": "light_product"}

Adsorb

Feeds the bed at constant flow, with the light product leaving the other end at the feed pressure, or at pressure. Give the flow as one of an interstitial velocity [m/s], a superficial_velocity [m/s] or a flow_rate [m³/s at the bed inlet].

Adsorb(15.0; velocity = 1.0)

{"type": "adsorb", "duration": 15, "velocity": 1.0}

A bed without a feed device of its own is fed through a link from the bed before it, as in the dual-adsorbent process, and takes no flow setting.

Blowdown

Lowers the bed to to through an outlet: by default the product device at the top (co-current). With through = :bottom it goes through a waste or extract device at the bottom instead (counter-current).

Blowdown(30.0; to = 0.2bar)

{"type": "blowdown", "duration": 30, "to": "0.2 bar"}

Evacuate

Lowers the bed to to through the extract (vacuum) device at the bottom, which takes the CO₂ product. Like blowdown, through picks the other end.

Evacuate(40.0; to = 0.1bar)

{"type": "evacuate", "duration": 40, "to": "0.1 bar"}

Purge

Light product enters through the light inlet at a constant flow and leaves through the extract device at the other end, sweeping the CO₂ out. The flow is given as in Adsorb.

Purge(20.0; velocity = 0.2)

{"type": "purge", "duration": 20, "velocity": 0.2}

Equalise

Opens the link between two beds, such as their tops, so that gas flows from the bed at the higher pressure into the other. The size of the link valve, its conductance, sets how far the pressures come together in the time given. Afterwards both beds are taken to be at the mean of their start pressures, where the next ramps start.

Equalise(10.0; beds = (:A, :B), conductance = 1e-6)

{"type": "equalise", "duration": 10, "beds": ["A", "B"], "conductance": 1e-6}

The Two-bed VSA with pressure equalisation example builds a process with it.

Idle

Keeps every device of the bed closed, for example while another bed finishes its steps.

Idle(10.0; bed = :B)

{"type": "idle", "duration": 10, "bed": "B"}

Several beds

With several beds, each step names its bed. Steps run one after the other; Concurrent runs steps on different beds at the same time, as one stage:

Concurrent(Pressurise(15.0; bed = :A), Evacuate(15.0; bed = :B, to = 0.1bar))

Beds can also follow sequences of their own, side by side, each a list of steps with a label. A shorter sequence waits with its devices closed until the longest one ends:

Cycle("silica gel" => [...], "13X" => [...])

{"sequences": [{"label": "silica gel", "steps": [...]}, {"label": "13X", "steps": [...]}]}

The Wet flue gas: dual-adsorbent, two-bed VSA example runs a cycle like this. New kinds of step can be added with register_step!; see Extending Mocca.